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of the Int. Heat
Heat Transfer
Transfer Conference
Conference
Proceedings ofProceedings
the 14th International
IHTC14
IHTC14
August 8-13,
8-13, 2010,
2010, Washington,
Washington, DC,
DC, USA
USA
August
IHTC14-23353
IHTC14-23
HISTORY, ADVANCES, AND CHALLENGES IN LIQUID FLOW AND FLOW BOILING
HEAT TRANSFER IN MICROCHANNELS: A CRITICAL REVIEW
Satish G. Kandlikar
Rochester Institute of Technology
Rochester, NY, USA
[email protected]
ABSTRACT
As the scale of devices becomes small, thermal control and
heat dissipation from these devices can be effectively
accomplished through the implementation of microchannel
passages. The small passages provide a high surface area to
volume ratio that enables higher heat transfer rates. High
performance microchannel heat exchangers are also attractive
in applications where space and/or weight constraints dictate
the size of a heat exchanger or where performance
enhancement is desired. This survey article provides a historical
perspective of the progress made in understanding the
underlying mechanisms in single-phase liquid flow and twophase flow boiling processes and their use in high heat flux
removal applications. Future research directions for (i) further
enhancing the single-phase heat transfer performance, and (ii)
enabling practical implementation of flow boiling in
microchannel heat exchangers, are outlined.
Figure 1 – Publication histogram showing papers related to
single-phase liquid heat transfer and fluid flow in microchannels.
INTRODUCTION
Smaller channel size in a heat exchanger provides higher
surface area to volume ratio and results in higher heat and mass
transfer rates and lower equipment size. Automotive and
aerospace industries embraced the use of smaller sized passages
in compact heat exchangers to meet the weight and size
constraints, while high performance requirements in cryogenic
industries, for example, necessitated the use of millimeter-sized
passages in equipment with relatively large heat transfer rates
and higher effectiveness requirements.
Tuckerman and Pease [1] in 1981 demonstrated for the first
time the high heat flux removal capability of up to 800 W/cm2
achieved with microchannels in single-phase and two-phase
flows. After almost three decades, such high heat flux removal
is now being accomplished in practical devices, although much
of the work has been conducted in the last several years. A brief
historical survey highlighting research in the areas of singlephase and two-phase cooling with microchannels, followed by
recommendations for future work, is presented in this paper.
Figure 2 – Publication histogram showing papers related to flow
boiling heat transfer and two-phase flow in microchannels.
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After a slow start, research interest in microchannels has
increased significantly in the last decade. Figures 1 and 2 show
histogram of technical publications in each year related to
single-phase liquid flow and flow boiling, respectively.
Application of microchannels with single-phase liquid flow for
heat removal was investigated at only a few places in the world
until 1996. Starting in 1997, the pace accelerated until 2008. It
seems to have reached a plateau, with 2009 numbers falling
below the 2007 level. The main thrust during this period was on
understanding the fundamental mechanisms. Research is now
focused on further refining the theoretical concepts, generating
experimental data sets, enhancing heat transfer performance
while reducing pressure drop, and extending the use of
microchannels to new applications.
Figure 2 shows the publication histogram for research
related to flow boiling in microchannels. Until 2004, research
was conducted only at a few academic institutions as the basic
questions regarding the viability of flow boiling were being
examined. The interest since 2005 has somewhat leveled off,
indicating some degree of saturation in our efforts in this field.
The focus of research during this period was on the
fundamental
issues,
mechanisms,
and
performance
characterization. A breakthrough is awaited to yield higher heat
transfer coefficients, higher CHF values, and stable operation
before the flow boiling technology in microchannels can be
implemented in practical devices.
This paper is organized to provide a historical perspective
followed by the state-of-the-art developments in the two major
topics in microchannels: single-phase flow and flow boiling.
Condensation is not covered in this paper. A brief overview of
research conducted in the last three decades is presented first,
highlighting some of the important contributions in the
respective fields. This is followed by a more in-depth review of
the research contributions made in the last five years in each
area. With over several thousand publications reported over this
time, it is truly a formidable task to cover all the advancements
published in this field. Although every effort has been made,
the author is painfully aware that there may have been
inadvertent omissions of some important publications.
Excellent review articles covering the developments in
microchannel research, giving the details of experiments, their
uncertainty levels, and model/correlation decriptions have been
published periodically and readers are encouraged to read these
for an exhaustive coverage of the earlier work [2-28].
NOMENCLATURE
Roman Letters
Ca
capillary number
D, Dh Diameter, hydraulic diameter, m
f
friction factor
h
heat transfer coefficient, W/m2 °C
latent heat of vaporization, J/kg
hfg
j
volumetric flux, m/s
q
heat flux, W/m2
Re
Reynolds number
V
mean flow velocity, m/s
We
x
Weber number
quality
Greek Letters
ε
roughness parameter
μ
viscosity, Pa·s
ρ
density, kg/m3
σ
surface tension, N/m
Subscripts
cf constricted flow
cp constant properties
FP floor to mean profile distance
L
liquid
m mean
T
total
t
transition to turbulence
0
corresponding to smooth wall
v, V vapor
w
wall
SINGLE-PHASE LIQUID FLOW
Historical Development Timeline – Single-Phase
Liquid Flow
The developments in single-phase flow in microchannels
may be divided into two aspects: fluid flow and heat transfer.
Major milestones in our understanding in these fields are
summarized in Fig. 3, covering a 25-year period. Starting with
the Tuckerman and Pease [1] work in 1981, the next research
emphasis was on design and implementation aspect during
1986-1988 [29-32]. This was followed by research focused on
fundamental understanding during the eleven year period 19922002 [33-39]. Major emphasis during this period was on
validation of continuum theory for incompressible fluid flow in
microchannels. The period 2003-2004 reflects a renewed
interest in this field, with a number of specialty conferences
dealing with microscale phenomena emerging during this
period. The focus shifted on channel classification and
fundamental understanding of the fluid flow and heat transfer
phenomena, and a recognition of need for further heat transfer
enhancement [40-47]. Finally, during the 2005-2006, attention
was focused on practical implementation, heat transfer
enhancements using offset strip fins and nanofluids, developing
an in-depth understanding of roughness effects and transition to
turbulence, and influence of channel deformity [48-52]. The
timeline is provided to illustrate general historical development
in this field, with individual papers as representative
developments during those periods. The period 2006 onward is
left out as more time may be needed for the research
community to recognize the significance of research reported in
the last four years. However, a review of some of the important
recent papers is presented later in this paper to indicate the
state-of-the-art advancements in this field.
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Figure 3 – Historical development timeline over 25 years highlighting advances in single-phase liquid flow in
microchannels.
Historical Perspective – Single-Phase Liquid Flow
Significant advances have been made in the single-phase
liquid flow research in microchannels in the last three decades.
Microchannels were implemented in heat exchangers in some
of the earlier studies. An excellent summary of early
developments in microchannel heat exchangers, providing a
clear historical perspective, was presented in 1993 by Goodling
[2]. It is a “must-read” article for all researchers in this field. It
is interesting to note that even when microchannel research was
in its infancy, seventy-eight papers were cited by Goodling.
During the period 1981-1993, the main centers engaged in
research in this field were: Stanford University and Lawrence
Livermore National Labs – the pioneering thesis by Tuckerman
[53] investigated for the first time microchannels in singlephase and two-phase modes for high heat flux removal,
Mundinger et al. [54] applied the high performance silicon
microchannel heat exchanger design to laser diode array
cooling; Massachusetts Institute of Technology and Lincoln
Laboratories – Phillips [55], and Phillips et al. [31, 56]
provided the entrance region Nusselt number and friction factor
values in developing flows for three side heated channels under
H1 boundary condition (constant heat flux with uniform
circumferential temperature at any cross-section) along with a
detailed design procedure, and Missaggia et al. [57] and
Walpole et al. [32] demonstrated effective cooling of laser
diodes with 500 W/cm2 heat removal rate and a laser junction
temperature rise of only 40 °C; North Carolina State University
– Turlik et al. [58] applied the microchannel design to multichip
modules covering 25 cm2 area; Auburn University - Knight et
al. [33] provided optimization schemes for designing
microchannel geometrical parameters for laminar and turbulent
flows; Nippon Institute of Technology, Japan – Sasaki and
Kishimoto [29] used microchannels for diode array cooling,
and Kishimoto and Sasaki [30] employed diamond-shaped
interrupted fins, perhaps the first application of enhanced
microchannels. A number of individual contributions were also
seen during this period – Bar-Cohen and Jelinek [59] developed
a geometrical optimization scheme for fin spacing in air cooled
heat exchangers that could be extended to microchannels, and
Bejan and Morega [60] applied pin fins to enhance heat transfer
in a wide, single flow passage (narrow gap) while keeping the
pressure drop low. These developments show that practical
implementation of microchannels with single-phase liquid
cooling generated considerable interest by 1993.
In the later part of 1990s, the focus shifted to
understanding the fluid flow and heat transfer phenomena in
microchannel liquid flows. An excellent summary of the early
developments is given by Mehendale et al. [3]. A number of
authors, as early as in 1983, reported the single-phase studies
for gas flows, which are not covered in this paper. In 1993,
Peng and Wang [61] and Wang and Peng [62] reported the first
study on single-phase flows in minichannel and Peng et al. [34,
35] in microchannels. They suggested possible departure from
the continuum theory. The Poiseuille and Nusselt numbers
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exhibited a dependence on Re, which could not be explained.
The reasons for the discrepancies were subsequently explained
by other investigators over the next ten years.
The author feels compelled to write a word of gratitude to
late Professor X.F. Peng, of Tsinghua University in China,
whose untimely demise at the age of 48 is difficult to accept.
His contributions in exploring the single-phase and two-phase
flows and heat transfer characteristics have made a significant
impact on the research in this field. He is credited as one of the
pioneering researchers to systematically study the single-phase
and two-phase flows in microchannels.
In 2000, several papers focused on the application aspects.
Harris et al. [36] developed a microchannel heat exchanger for
liquid-to-air heat transfer, while Fedorov and Viskanta [37]
studied the conjugate substrate conduction and microchannel
convection problem and identified locations where severe
thermal stresses may occur.
The fundamental understanding of the flow and heat
transfer phenomena in microchannels was a major focus area
for researchers in the first half of the first decade of this
millennium. The experimental work presented by Peng and
coworkers [34, 35, 61, 62] in the mid-90’s and by Mala et al.
[63], Mala and Li [64] and Xu et al. [65] in the late 90s still left
the question unresolved regarding the applicability of
continuum models to liquid flows in microchannels. Xu et al.
[65] reported the validity of the conventional theory for liquid
flow in microchannels using fully developed flow; however
they neglected the entrance region effects. The uncertainties
associated with entrance and exit loss coefficients and other
factors such as non-uniformity in channel cross-sectional
dimensions and experimental uncertainties, especially at low
flow rates, seem to have fortuitously compensated for the
entrance region effects. Palm [4] and Sobhan and Garimella [6]
and indicated that the current research at that time was
inconclusive regarding the microscale effects on liquid flow
and applicability of continuum equations. Judy et al. [38] in
2002 reported the first experimental data that followed the
continuum theory for liquid flows in microchannels. At the
same time, Qu and Mudawar [39] confirmed the applicability
of continuum theory for 231 µm x 731 µm minichannels.
However, the discrepancy among various data sets could not be
resolved. Wu and Cheng [41] indicated that surface roughness
may affect the heat transfer in microchannels. Guo and Li [43]
proposed the surface roughness, axial conduction and
measurement errors as possible sources for discrepancy in
microchannel data. Lelea et al. [46] in 2004 presented data
confirming the continuum theory; however, reviews by Obot
[28] in 2001 and Morini [12] in 2004 left it as an unresolved
open question. Obot [28] asserted that the transition to
turbulence does not occur below Re=1000 in smooth
microchannels and that Nu varies as Re1/2. In 2004, Sharp and
Adrian [47] confirmed that the transition to turbulent flow
occurs at approximately the same Reynolds numbers as in
macroscale tubes. In 2005, Steinke and Kandlikar [66]
presented experimental data confirming the validity of
continuum theory and also presented an analysis of earlier data
showing the entrance region effects, entrance and exit losses
and experimental uncertainties as the major reasons for the
discrepancy in the earlier data reported in the literature. They
also presented a detailed analysis of the uncertainties associated
with fluid flow and heat transfer data available in the literature.
Lee et al. [49] verified the applicability of the continuum theory
and suggested that careful attention should be given to the
boundary conditions applied during the experiments.
The effects of property corrections and dissolved gases on
single-phase diabatic flows were reported by Steinke and
Kandlikar [67] in 2004. Their work indicates that because of
the steep temperature gradients in microchannels, it is
important to apply property correction factors, generally of the
form given below, in evaluating single-phase friction factors.
.
(1)
Figure 4 – Effect of dissolved gases on adiabatic friction
factors in microchannels, redrawn from [63].
The presence of dissolved gases in water during diabatic
single-phase experiments resulted in higher friction factors due
to small bubbles forming on the walls and increasing resistance
to the flow [67]. Figure 4 shows the effect of dissolved gas
content on the friction factors after accounting for the variable
property effects given by eq. (1). It is seen that the friction
factors are higher with a dissolved oxygen content of 8 ppm,
but the results for 1 ppm concentration are in good agreement
with the conventional theory for laminar flow. It is therefore
recommended that degassed water be used during experiments
for evaluating the friction factors in microchannels. Clearly this
poses a challenge in predicting pressure drop in practical
systems using liquids containing dissolved gases. It would be of
practical advantage to avoid bubbles in microscale heat
exchangers.
Defining microchannels has been a subject of considerable
discussion in the literature. Kandlikar and Grande [40] in 2003
reported a channel classification scheme that identified
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microchannels as channels in the hydraulic diameter range of
10 μm – 200 μm. This scheme is followed in the present paper.
The earlier scheme by Mehendale et al. [3] had arbitrarily
proposed 1 mm as the distinction between microchannel and
mesochannel flows. It should be remembered that the channel
classification scheme is intended to serve only as a guide. The
associated flow and heat transfer phenomena in these channels
are dictated by specific operating conditions, fluid properties
and type of flow, such as single-phase or two-phase flow, gas or
liquid, etc. Further discussion on this topic is given by
Kandlikar [68].
At the same time when the issue of the applicability of
continuum theory to microchannel flows was resolved in 2005,
the use of enhanced microchannels to meet the high heat flux
cooling demands was recommended by Kandlikar and Grande
[44]. Peles et al. [69] presented experimental data on micro-pin
fins and obtained a very low thermal resistance value,
comparable to the microchannel flows. Colgan et al. [50]
reported a practical implementation of liquid cooling using
enhanced microchannels with offset-strip-fin geometry to
dissipate very high heat fluxes. Steinke and Kandlikar [70]
measured heat transfer coefficient values of over 500,000
W/m2-K using the same offset strip-fin microchannel geometry
employed by Colgan et al. [50]. A few investigators have
studied optimization of plain and enhanced microchannel
geometrical parameters [71, 72] and modeling of plain
microchannels [73] to provide practical design guidelines. The
effect of entrance region on heat transfer under H1 boundary
condition (circumferentially uniform wall temperature and
axially uniform wall heat flux thermal boundary conditions) has
been studied by Lee and Garimella [74].
Nanofluids were introduced in microchannel heat sinks in
the early part of the 2000s. In 2005, Koo and Kleinstreuer [48]
reported an important finding that lists the conditions under
which nanofluids may provide enhancement for application in
practical single-phase microchannel devices. They reported that
(i) a nanofluid concentration of at least 4 percent, (ii) elevated
thermal conductivity of nanofluids, and (iii) treated channel
walls which prevent nanoparticle adhesion, may contribute in
increasing the heat transfer rate.
The current literature clearly indicates that continuum
modeling is applicable to liquid flow in microchannels. Rosa et
al. [26] indicated in their review article that scaling effects need
to be considered to identify if there are any special effects due
to microscale channel dimensions in single-phase flow. Some
of these issues were identified by the earlier investigators. The
entrance and exit losses for macroscale channels are
empirically correlated based on macroscale experiments. Their
validity in microscale applications is open to question. The
effects due to flow area changes need to be revisited as well.
The property variation effects become important as the
temperature gradients are quite steep near the wall. Parallel
channels experience maldistribution effects which need to be
quantified for the specific header and channel geometry.
The areas where fundamental heat transfer related research
with single-phase liquid flow has been focused in the last five
years are: design analysis for optimization, heat transfer
augmentation techniques, and roughness effects. These topics
are reviewed in the following sections.
Roughness Effect
The effect of surface roughness on single-phase liquid flow
in microchannels has been studied in the literature both
numerically and experimentally. These studies indicate that the
conventional representation of constant friction factor in the
laminar region is not accurate. The classical experiments by
Nikuradse were found to have large uncertainties in the
pressure drop measurements in the laminar region [75]. A
number of different parameters are employed in the literature to
model roughness features [76]. Kandlikar et al. [51] proposed
replacing the root diameter with a constricted hydraulic
diameter Dh,cf:
,
2
(2)
where subscript cf refers to the constricted flow parameters and
ε is the roughness parameter defined as the distance between
the mean floor line and the roughness peak height. The
experimental results for the uniform roughness were correlated
well. Using the constricted flow diameter, a modified Moody
diagram was presented in which the laminar region was
accurately represented by a straight line for all relative
roughness values, and the friction factor reached a plateau
beyond which the increased roughness had no effect on friction
factor [51].
The results of regularly spaced ribs however showed a
dependence on the pitch of the ribs [77]. The friction factor
gradually shifted from the constricted flow diameter at close rib
spacing to the smooth tube value at very high spacing. Rawool
et al. [78] numerically simulated the flow in microchannels
with trapezoidal, rectangular and triangular shaped roughness
structures. The trapezoidal shape yielded the lowest friction
factors in the group. The circulation behind the roughness
elements was seen to cause additional losses leading to an
increase in the friction factor. The numerical work by Croce et
al. [79] on conical-shaped three-dimensional elements of base
diameter b, spaced evenly at a pitch of s in both directions
indicates that the heat transfer and pressure drop results can be
correlated by modifying ε in Eq. (2) to εb/s. Their results show
that the two-dimensional ridge roughness elements are more
effective in providing relatively large heat transfer enhancement
compared to the 3-D elements as the large flow blockage in the
valleys of the 3-D elements more than offsets the enhancement
in the heat transfer at the tips of the conical elements. A review
of different roughness shapes and their effects, and the
instability mechanisms leading to turbulent transition in
microchannels is presented in [22].
Several models were presented in the literature to account
for roughness effects at the microscale level. Kleinstreuer and
Koo [80] proposed a porous medium model for the roughness
region, which is assumed to act in parallel to the main flow in
the core of the channel. The flow resistance in the roughness
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region, modeled as porous region, consists of a constant term
and a velocity dependent power law term. The momentum
equation for the open region (in the core of the channel) is
solved by matching the slopes of the velocity profiles in the
porous region and the open channel region. Bahrami et al. [81]
proposed a model based on a Gaussian distribution of
roughness elements for the prediction of pressure drop in fully
developed laminar flow in microchannels. Earlier, Qu et al. [82]
proposed a roughness-viscosity model in which the fluid
viscosity was modified to include a viscosity term induced by
the surface roughness. The roughness viscosity was considered
similar to the eddy viscosity in the turbulent flow. In a
subsequent paper on heat transfer effects, Qu et al. [83]
modified their roughness viscosity model to include two
components in the apparent viscosity. This included the sum of
the roughness viscosity derived in their earlier paper and a
friction viscosity. The experimental heat transfer results were
correlated with this approach. However this approach has not
been tested with other data from the literature.
The effect of roughness on heat transfer is not as well
studied in the literature. A recent publication by Morini et al.
[84] indicates little effect of roughness on heat transfer in the
laminar region for water and FC-72 in microchannels and
minichannels. The earlier work by Kandlikar et al. [42]
indicated an increase in the heat transfer coefficient with
stainless steel minichannels with different relative roughness
values obtained through acid etching.
Gamrat et al. [85] studied uniformly spaced roughness
structures numerically and proposed a roughness-layer model
for heat transfer in laminar fully developed tubes. They
extended the porous region model of Kleinstreuer and Koo [80]
by using the porosity of the roughness layer and roughness
height as parameters. Their results indicate that the friction
factor increases more than the heat transfer coefficient over the
range of relative roughness up to 11 percent.
The effect of roughness is seen in inducing early transition
to turbulence. This transition is believed to be one of the
reasons for the higher friction factors for rough microchannels
reported in the literature. The instabilities and relaminarization
processes occurring in the flow around roughness elements has
been a topic of intense research in the fluid mechanics
community. The pioneering work of Narasimha and
Sreenivasan [86] provides an insight into the underlying
mechanisms for early turbulence transition. Based on the
uniform roughness as well as ribbed geometries, the following
correlation for transition Reynolds number is based on the
constricted flow diameter defined in Eq. (2) [77]:
(
)
For 0 < εD h,cf ≤ 0.08;
Re t,cf = Re to − 18,750 ε / Dh,cf
For 0.08 < εDh,cf ≤ 0.15
Re t,cf = 800 − 3,270 ε / Dh,cf − 0.08
(
)
(3)
where Ret,o is the transition Reynolds number for smooth tubes
for the given channel cross-section. For the two-dimensional
roughness elements, the experimental results of Brackbill and
Kandlikar [77] indicate that the rib height and spacing both
influence the transition.
Hu et al. [87] numerically studied the effect of threedimensional roughness elements in channels of microfluidic
devices for Reynolds number range of 0.1-10. The channel
height was chosen to be 5 µm and the roughness elements were
0.1-2.1 µm in height and the spacing was varied from 0.1-1.5
µm. The effect of varying channel height from 5-50 µm was
also investigated. They presented equations for the reduction in
the channel height due to roughness elements to be applied in
the conventional pressure drop equations.
Three-dimensional roughness has been studied so far in
microfluidic applications at low Reynolds numbers, mainly to
enhance mixing. It is a promising technique for liquid heat
transfer applications. Further data is needed covering hydraulic
diameters in the range of 10-200 µm in order to assess the
effect of such structures on fluid flow and heat transfer in
microchannels.
Heat Transfer Augmentation of Single-Phase Liquid
Flow
In high heat flux removal applications such as chip
cooling, further increases in heat transfer coefficients over plain
microchannels are required to meet the cooling needs. A
detailed review of various techniques applicable to heat transfer
enhancement in microchannels and minichannels was presented
by Kandlikar and Steinke [45]. The enhancement techniques
are classified into two categories:
surface roughness, flow disruptions,
Passive Techniques:
pin fins, offset-strip fins, channel curvature, re-entrant
obstructions, secondary flows, out-of-plane mixing,
and fluid additives (nanofluids)
Active Techniques: vibration, electrostatic fields, flow
pulsation, and variable roughness structures
Some of these enhancement techniques have been
investigated in the literature. A brief review of the available
studies is presented here. Among passive techniques, surface
roughness has been an area of growing interest as seen from the
review presented in the earlier section. Xu et al. [88] utilized
the concept of a redeveloping thermal boundary layer due to
flow obstructions. These obstructions were created by a series
of intersecting longitudinal (triangular, with Dh = 155 µm, pitch
= 0.45 mm) and transverse microchannels (trapezoidal, sides of
1.015 mm and 0.712 mm, height = 0.212 mm, and pitch =
3.694 mm). In a silicon chip with 21.45 mm flow length, the
heat transfer enhancement was found to be around 16 percent
while friction factor was actually reduced by about 20 percent.
Further performance improvements should be possible through
optimization. Chang et al. [89] studied the effect of height on
the heat transfer in rib-roughened side walls of microchannels.
The heat transfer coefficient decreased immediately behind the
ribs, but an overall performance improvement was noted. More
systematic study is needed to clearly establish the efficacy of
opposing ridges. Wei et al. [90] employed dimpled surfaces and
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obtained 10 to 35 percent improvement in heat transfer
coefficient with very little increase in friction factor.
Optimization of this geometry is needed to fully explore its
potential.
Pin fins are extensively used in macroscale applications.
Kosar and Peles [91] experimentally studied pin fins of circular
cross-section extending over the entire height of microchannels.
They found that the performance of pin fins to be predictable
by macroscale correlations. Significant enhancements, with
heat transfer coefficients comparable to boiling were reported.
They emphasized the need to optimize the geometrical
parameters for obtaining the desired benefits while keeping
pumping penalties at a minimum.
A major milestone was achieved when Colgan et al. [50]
employed offset strip fins in microchannels with water flow to
dissipate heat fluxes of 400 W/cm2. They employed 180 µm
deep channels with fins at a constant pitch of 100 µm and fin
width of 65 or 75 µm. The fin length was either 210 µm or 250
µm with a gap of 40 µm between the fin rows. Significantly
higher heat transfer performance was noted, with an
improvement by a factor of approximately 1.5 to 3. By limiting
the flow length to 3 mm, the pressure drop was limited to below
30 kPa. Steinke and Kandlikar [70] conducted experiments with
fins of 250 µm length and 50 µm thickness, the same
configuration as employed by Colgan et al. [50], and reported
the average heat transfer coefficients to be in excess of 500,000
W/m2 ºC. This is perhaps the highest heat transfer coefficient
value reported for single-phase internal flow. Further
enhancement is possible by reducing the channel width and fin
length. A detailed geometrical optimization of the strip-fin
geometry needs to be performed to account for the
accompanying pressure drop penalty [72].
Other fin geometry studied in the literature is the
longitudinal fins by Foong et al. [92]. The short fins seem to be
promising. Another geometry that holds promise is mircofins or
helical grooves. This geometry has been very successful in
refrigeration applications. Zhang et al. [93] evaluated
aluminum, silicon and metallic foam heat sinks and determined
that the aluminum and foam heat exchangers could achieve a
heat dissipation rate of 100 W/cm2, while silicon microchannels
could dissipate 200 W/cm2. The thermal conduction resistance
within the metal foam was the limiting factor. Tsuzuki et al.
[94] introduced a wavy, S-shaped fin and optimized the
geometry using a CFD solver to study parametric dependence
of fin angle, guiding wing, thickness, length, and roundness.
From these parametric studies, they identified geometrical
parameters that are most suitable in enhancing heat transfer
while keeping the pressure drop low.
Nanofluids are mixtures of nanoparticles and a base liquid
used in a cooling system. Enhancement in thermal conductivity
of nanofluids was reported by Lee et al. [95], followed by a
study showing heat transfer enhancement with nanoluids by
Xuan and Li [96] and Keblinski et al. [97]. Nanofluids have
also been investigated extensively in microchannels.
Introducing nanoparticles in a liquid has shown to enhance the
energy transport in the boundary layer [48, 98-101]. Bergman
[102] showed that, the combination of increased thermal
conductivity and reduced specific heat in nanofluids affects
their performance compared to the base liquid. Although the
exact mechanism is not clear, Bergman introduced a
dimensionless parameter to indicate whether the heat transfer
performance will improve or deteriorate with nanofluids.
Excellent recent reviews on the enhancement of thermal
conductivity of nanofluids have been presented in the literature
and the reader is referred to those publications, e.g. [103-110].
Among active enhancement techniques, synthetic jets have
been studied by Chandratilleke et al. [111]. They introduced
synthetic jets from a neighboring volume through a pulsing
device with a zero net mass flow rate. This technique
introduces very little pressure drop penalty and is suitable for
localized cooling of hot spots. Introducing a vapor compression
refrigeration system to lower the coolant temperature, or a
Peltier cooler for cooling of hot spots on a chip are among
some of the active systems being considered [112].
Challenges and Future Research Needs in SinglePhase Liquid Flow
A number of research needs have been identified in the
discussion presented in the above section on sibgle-phase liquid
flow. Some of the major ones are summarized below:
Roughness Effects: The effects of two-dimensional and threedimensional roughness elements on friction factor and heat
transfer have been studied mostly using numerical techniques
in the literature. Since the effect of recirculation and eddies
generated behind the roughness structures are not accounted
for accurately in the numerical schemes, experimental
validation of the numerical results is essential. Developing
theoretical models to account for these effects, especially on
heat transfer, is an important area. Such models are expected
to provide information regarding the underlying mechanisms
which can help in designing roughness surfaces with higher
enhancement in heat transfer than the associated increase in
pressure drop. There is also a need for developing optimized
2-D and 3-D geometries of the roughness structures that are
specifically tailored for a given range of Reynolds numbers.
In addition, new enhancement techniques need to be
developed that utilize unique fabrication techniques that are
available at microscale, such as etching, for mass production.
Heat Transfer Augmentation, Liquid Flow: Among the passive
devices studied, fins hold the most promise. The offset strip
fin geometry needs further investigation for geometrical
optimization considering the suitable fabrication techniques.
The header configuration is an important consideration in its
design. Different cross sectional geometries need to be
explored for pin-fin configuration. The optimizing procedure
outlined by Tsuzuki et al. [94] for designing S-shaped fins
provides a very promising approach. It is expected that
specific geometrical parameters will be optimized using
numerical simulation in the future for a given liquid under a
specific Reynolds number operation.
7
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From its success at macroscale, microfin or microgroove
geometry seems to be a strong candidate to provide heat
transfer enhancement with relatively low pressure drop
penalty. Micromixer is another type of device that has been
extensively studied in the literature. It seems to be very
promising in augmenting heat transfer as well, since it is able
to effectively induce mixing of fluids from the core of the
flow channel to the heated surface. Among the active
enhancement techniques, pulsed flow and synthetic jets are
attractive for localized cooling. Concurrent research on the
effect of pulsations/vibrations on the system integrity needs
to be investigated before applying them in practical devices.
It is expected that there will be significant research activity in
this area in the coming years.
Numerical Results for Entrance Region: Among other topics,
there is a need for generating numerical results for heat
transfer and friction factors for microchannels of different
aspect ratios under the more practical H2 boundary condition
(constant heat flux axially and circumferentially).
FLOW BOILING
Historical Development Timeline – Flow Boiling in
Microchannels
Figure 5 shows major milestones in flow boiling research
over the 25 years span since the first study on this topic by
Tuckerman and Pease [1] in 1981. There was little research
activity in the period 1981-1991. During 1992-2001, the major
developments included a systematic study in minichannels and
silicon microchannels, local heat transfer coefficient data, and
identifying major obstacles [113-117]. In the period 2002-2004,
research focus shifted to identifying flow patterns and
differences between diabatic and adiabatic flows, modeling,
and continuing work on local heat transfer coefficient data [8,
67, 118-124]. During 2005-2006, new work was presented on
numerically modeling bubble growth, overcoming instabilities,
and use of enhanced structures for heat transfer [14, 125-131].
The developments beyond 2006 onwards are left out, as their
relevance and importance are expected to become clear during
a span of the next five to ten years.
Figure 5 – Historical development timeline over 25 years highlighting advances in flow boiling in microchannels.
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Historical Perspective – Flow Boiling in Microchannels
Flow boiling in microchannels was first studied by
Tuckerman and Pease in 1981, but it took more than ten years
before a major effort was undertaken by other researchers to
explore this topic further. The first major effort on understanding
the thermohydraulic behavior of flow boiling systems in
microscale passages was presented by Moriyama et al. [112] in
1992. They used R-113 refrigerant in 35-110 µm high and 30 mm
wide rectangular passages. This geometry is sometimes referred
to as microgap in the literature. The pressure drop results were
correlated with the available two-phase correlations and the heat
transfer was found to be 3-20 times larger than that for singlephase flow. One of the important findings was that the capillary
number was identified as an important parameter in determining
the liquid film thickness while modeling the slug flow. They also
developed analytical models for the film flow with a vapor core
and slug flow with vapor bubbles separated by the liquid slugs.
The agreement between the model and experimental results was
quite good and the effect of capillary number was shown to be an
important group. Since this article addresses a number of issues
that are being researched currently, a more in-depth review of
their work is presented below.
The experimental data obtained by Moriyama et al. [113] is
shown in Figs. 6 and 7. The heat transfer coefficient is plotted as a
function of quality for the two mass fluxes in Fig. 6. The heat
transfer coefficient increases dramatically with quality near x=0,
levels off in the mid-quality region, and then drops rapidly at
higher qualities as dryout occurs. Figure 7 shows a comparison of
the data with the pool boiling correlation by Armstrong [132],
0.602q . , (or
1.66 . ). A higher heat
given by ∆T
transfer coefficient is obtained during flow boiling, and an
increasing trend is noted with an increase in mass flux. These
trends are actually a combination of pool boiling and flow boiling
characteristics, as the large width of the channels employed (30
mm) reduces the flow effect on nucleation, but causes the flow of
liquid around nucleating bubbles. Heat transfer is thus enhanced
from the convective contribution due to the liquid flow.
In 1993, Peng and Wang [61] reported one of the first studies
with subcooled flow boiling of water in minichannels. The fully
developed boiling region had little influence from subcooling or
flow velocity. These observations were helpful in determining the
nature of flow boiling in microchannels, as will be discussed in
later sections. Although the heat transfer data indicated the
presence of nucleate boiling, they could not observe any bubbles,
perhaps due to the lack of a high speed and high magnification
imaging system.
A more systematic study covering a large range of
minichannel diameters was conducted by Bowers and Mudawar
[114] in 1994. They reported the average heat transfer, pressure
drop and CHF data in these channels. Jiang et al. [115] were the
first researchers to incorporate the microchannel passages (Dh <
200 µm) with hydraulic diameters of 40 and 80 µm in a silicon
heat sink. The thermal performance was unaffected by the flow,
and was dominated by the nucleate boiling, similar to the earlier
studies on minichannels. Hetsroni et al. [116] in 2000 used 103129 µm hydraulic diameter microchannels for cooling silicon
chips and studied the non-uniform temperature distribution on
the chip. They observed single-phase flow followed by annular
flow pattern during boiling. Kandlikar et al. [117] in 2001
observed the flow boiling instability associated with nucleation
and rapid bubble growth in a minichannel. Instability will be
reviewed in a later section.
Figure 6 – Heat transfer coefficient variation with quality in
a narrow channel of height 110 µm by Moriyama et al.
[113], redrawn from the original plot.
Figure 7 – Comparison of the flow boiling plot with
Armstrong’s pool boiling correlation by Moriyama et al.
[113], redrawn from the original plot.
In 2002, Serizawa [119] conducted adiabatic flow
experiments with steam-water and air-water flows in
microchannels and studied flow patterns, which were affected by
the surface roughness. He presented a flow pattern map based on
his experimental observations. The differences between the
adiabatic and diabatic flows were studied experimentally by
Hetsroni et al. [120], who clearly demonstrated the role played
by nucleation and bubble growth in diabatic flows. The main
features being the absence of rapid flow oscillations and
instabilities in adiabatic flows.
Moriyama et al. [113] in 1992 provided a plot showing the
dependence of h on x as seen in Fig. 6. In 2003, Yen et al. [121]
9
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measured the local heat transfer coefficient in single microtubes
as a function of x, q and G and noted a strong decreasing trend in
a single microchannel. They confirmed the contribution from the
nucleate boiling component. In 2004, Steinke and Kandlikar [124]
reported the local heat transfer coefficients as a function of heat
flux, mass flux and quality for water in parallel microchannels.
The microchannels were etched in silicon and were heated with a
copper heater deposited on the back surface. The heat transfer
coefficient was found to decrease dramatically along the length as
quality increased. Considering that the fluid is water at
atmospheric pressure, the deterioration in heat transfer coefficient
with increasing quality indicated a major departure from the flow
boiling characteristics in conventional channel (Dh > 3mm).
Moriyama et al. [113] compared their flow boiling data in
microchannels with gaps of 65 µm and 110 µm with Armstrong’s
[132] pool boiling correlation. Their results indicate a
convergence at high heat fluxes as shown in Fig. 7 (for 110 µm
gap). The reduced dependence on the mass flux points to the
dominance of nucleate boiling, which has been confirmed in
microchannels by a number of later studies. In these studies, the
researchers reported experimental heat transfer data for water and
several refrigerants (see review papers on this topic, e.g. [8-11]).
The experimental data confirmed the earlier observations,
showing a strong decreasing trend with quality at low qualities
and mixed trends at moderate qualities, e.g. [114-116, 120, 121,
124, 131, 133, 134]. A number of correlations have been proposed
in the literature, but none of them are able to predict the heat
transfer coefficient consistently well with the available
experimental data [24, 135-137]. Although attempts are
continuing to correlate the flow boiling data, it is becoming
apparent that the underlying data sets suffer from some of the
shortcomings listed below:
a) Large experimental uncertainty in measurement of
channel dimensions, local wall temperatures, and
estimation of wall heat flux.
b) Differences in wall boundary conditions (a subject not
explored in flow boiling literature)
c) Differences in nucleation cavity sizes and distribution on
the channel walls.
d) Uncertainty related to the presence of instability
condition and its severity.
Until experimental data sets that are free from the above
concerns become available, any correlation scheme cannot be
expected to provide an accurate predictive methodology. The
concerns regarding flow boiling instabilities are being addressed
in some of the recent publications. There is an immediate need to
generate experimental data in flow boiling systems using the
same rigor that the researchers have applied in their quest for
checking the applicability of the continuum models in singlephase liquid flow.
High speed visualization during flow boiling in
microchannels is again receiving considerable attention with the
focus now on flow patterns, e.g. by Martin-Callizo et al. [138],
Schilder et al. [139] and Harirchian and Garimella [140]. These
studies are aimed at linking the modeling effort to the flow
pattern. Although it is difficult to predict the flow patterns with
any degree of certainty, the insight gained through such
visualization is extremely useful.
Modeling of Flow Boiling Heat Transfer
The first model of the flow boiling phenomenon was
presented by Moriyama et al. [113] to predict the pressure drop
and heat transfer coefficient during slug and film (annular) flow
in a large aspect ratio microchannel passage with a gap of 35110 μm as described earlier. Although this paper addresses a
number of fundamental aspects using non-dimensional
parameters, it has not been considered by later researchers in
their modeling effort. A more in-depth review of this paper is
presented to bring out their important contributions in this area.
Moriyama et al. [113] modeled the slug flow as shown in
Fig. 8 for the low quality region, and film flow as shown in Fig.
9 for the high quality region. Since the liquid-vapor interface
plays an important role in the thickness of the film deposited
µL T
was
during the slug and film flows, capillary number Ca
introduced in the modeling; µL is the liquid viscosity, jT is the
total volumetric flux, m/s, and σ is the surface tension, N/m. The
volumetric flux may be replaced by the representative flow
velocity. The capillary number represents the ratio of the viscous
to surface tension forces.
Figure 8 – Schematic of the slug flow model employed by
Moriyama et al. [113], redrawn from the original plot.
Figure 9 – Schematic of the film flow model employed by
Moriyama et al. [113], redrawn from the original plot.
Figure 10 shows the variation of the two-phase to singlephase heat transfer coefficient ratio for 65 µm and 110 µm gap
sizes, two different mass fluxes in each plot, and three different
q/G ratios (shown in the inset table). For a given fluid in the film
flow region, a lower Ca value would indicate a thicker film, and
hence a lower heat transfer coefficient, whereas a higher Ca
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value would indicate a thinner film and correspondingly a higher
heat transfer coefficient. Their modeling seems to capture this
trend well for the smaller gap sizes (see the trend in the right hand
region of the plots). It may be instructive to study and extend the
model by Moriyama et al. [113] incorporating some the recent
findings on instability and liquid film thickness.
Figure 10 – Variation of heat transfer coefficient with
capillary number, Ca, for two gaps, 65 µm (upper plot), and
110 µm (lower plot) obtained by Moriyama et al.[113],
redrawn from the original plot.
In 2002, Jacobi and Thome [118] presented an elongated
bubble model, which was later refined into a three-zone model by
Thome et al. [141]. This model has considerable similarities to the
Moriyama et al. [113] model. The single-phase liquid heat
transfer in the slug was calculated using steady-state laminar flow
equations, thin film evaporation is calculated using conduction
equations in the liquid film, and heat transfer in the dry regions is
considered by gas flow equations separately. Information on the
film thickness is needed in the model. The model was tested with
different fluids and the subsequent work focused on determining
the slug and bubble lengths and frequency. The time-averaged
liquid film thickness was estimated from the heat transfer data.
Liquid film thickness plays an important role in modeling
heat transfer in slug flow as well in film (or annular) flow. A
recent work by Zhang et al. [142] provides useful information on
liquid film thickness measurement using laser extinction method
in a microgap as a function of interface velocity. An interesting
observation made from these experiments was that the film
thickness increased with the interface velocity. For interface
velocities of 3-5 m/s, the film thickness was around 10 μm for
water and 20 μm for ethanol and toluene. The film thickness in
flow boiling in microchannels is expected to be higher during
explosive boiling due to higher interface velocities.
Zhang et al. [143] in 2002 used the available correlations
and developed a model to predict the local heat transfer
coefficient variation along the length of 20-60 μm hydraulic
diameter parallel microchannels. They compared their
predictions with the local wall temperature measurements in the
silicon chip. The variation and the actual heat transfer coefficient
values along the length were predicted well. This model
illustrates the practical implementation of the available
information in the design of microchannel heat exchangers.
Many researchers noted that the flow boiling heat transfer in
microchannels is dominated by the nucleate boiling contribution,
which is mainly influenced by the heat flux. Correlations that are
similar to pool boiling type correlations, e.g. Armstrong [132]
and Cooper [144], with no mass flux term present in them have
been found to predict the microchannel flow boiling data with
some success [24, 113, 145]. Similarities and differences
between pool boiling and microchannel flow boiling was
explored by Kandlikar [146, 147] and it was noted that the
elongated bubbles provide advancing and receding interfaces on
the heater surface that may be compared to the expanding and
contracting interfaces at the base of a bubble during pool boiling.
The heat transfer in the interface region consists of transient
conduction and microconvection. Further guidance could be
derived from the recent research this area on pool boiling, e.g.
[148, 149]. Models based on annular flow [150, 151], annular
flow in triangular microchannels considering the contact angle
effects [152], and bubble coalescence [153] have also been
proposed in the literature. Fogg and Goodson [154] considered
the nucleation phenomenon and linked it to the instability
leading to water hammer (which is similar to the backflow
observed by other researchers.). There are a number recent
papers published on the modeling aspects specific to different
flow patterns. In general, these models perform well with the
parent data sets used in their development. A broader testing
using large number of data sets has not been reported in the
literature. The modeling effort also suffers from the nonavailability of reliable experimental data over a wide range of
conditions.
Flow boiling Heat Transfer Enhancement
Enhancement in flow boiling heat transfer was
experimentally studied by Kosar and Peles [130] by
incorporating fins. A recent study by Krishnamurthy and
Peles[155] indicated significant heat transfer enhancement with
microfins using HFE7000 in 222 µm hydraulic diameter
channels. Nanofluids have been studied with modest
improvements as reported in a review article by Cheng et al.
[156]. Recently Khanikar et al. [157] implemented
nanostructures on the channel walls with modest enhancement.
Repeated testing reduced the enhancement effect due to damage
to the nanostructures. This field is still its infancy and more
research efforts are needed.
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Use of nanofluids for flow boiling enhancement is a
promising area that is currently being explored in pool boiling
applications. A study by Lee and Mudawar [158] indicates that
nanofluids caused surface deposition in microchannels. Recent
finding by Ahn et al. [159] showed that using 0.01%
concentration alumina nanoparticles, CHF was enhanced by about
55 percent. The enhancement was due to deposition of the
nanoparticles on the surface causing the contact angle to decrease
from 65º degrees to about 12º. Ahn et al.’s work confirms that
reducing contact angle of the surface by some other means may
be an effective way to enhance the CHF in microchannels.
Flow Boiling Instabilities
Flow boiling instabilities were observed by a number of
investigators [120, 121, 124, 131, 133, 134, 160, 161]. They have
been identified as a major concern in reliable operation of a
microchannel heat exchanger. The high heat transfer coefficients
during single-phase heat transfer prior to nucleation result in
lower wall temperatures, thus delaying the onset of nucleation. It
also leads to a high liquid superheat at the nucleation location.
The bubble nucleation in this superheated liquid environment
causes an explosive growth of the bubble, causing it to expand in
both downstream and upstream locations [129, 162]. The
resulting backflow in the channels is a major source of instability
in microchannels. Numerical simulation of the bubble growth
phenomenon confirmed the role of local liquid superheat in
causing the unstable condition [125]. The bubble expansion
upstream results in a backflow into the header of the parallel
microchannels. Providing artificial nucleation sites, introducing
inlet restrictors and incorporating heated regions to initiate
nucleation were some of the remedies investigated by a number of
researchers [122, 127, 128]. Although these remedies reduce the
severity of the instabilities, flow oscillations are still present and
may adversely affect the CHF condition. In the last five years, a
number of studies have been reported on identifying the envelope
of operating conditions for stable and unstable operation.
Brutin and Tadrist [163] modeled the instability phenomenon
by considering the pressure forces across a slug formed during
explosive boiling. The overpressure created by the slug was
compared with the channel pressure drop. The ratio of the two
pressure drops was then linked to the applied heat flux and the
resulting ratio of channel pressure drop and heat flux
(dimensional quantity) was used as a parameter indicative of the
flow instability. The constants derived are for their specific setup
geometry, and further generalization is needed to extend this
concept to cover other system configurations and test fluids.
Garrity et al. [164] studied the Ledinegg type instability in a 1
mm x 1 mm minichannel using 56 parallel channels in stainless
steel plate for a fuel cell cooling application using HFE-7100. The
pressure drop vs. flow rate was modeled and the instability region
was predicted, with good agreement with the experimental data.
Zhang et al. [165] presented a detailed model to predict
Ledinegg instability in microchannels using the available
correlations for pressure drop in developing the pressure drop
versus mass flow rate supply and demand curves. Comparing the
slopes of the two curves, onset of instability was predicted. Using
the model, they identified the ratio of kinematic viscosities of
liquid to vapor phases as an important parameter. According to
their findings, lower values of this ratio for HFE-7100 leads to
more stable operation compared to water. Effects of other
parameters on instability were similar to those observed for
larger systems. In another recent paper, Zhang et al. [166]
proposed an active oscillatory mass flow controller to reduce the
flow oscillations. Such dynamic control seems very promising to
control oscillatory system behavior in systems where reliable
operation is of critical importance.
Critical Heat Flux (CHF)
CHF during flow boiling in microchannels is a topic of great
interest as it limits the heat fluxes that can be safely and reliably
dissipated. A number of studies have reported experimental data
for CHF in microchannels. A detailed list of these data sources
may be found in recent papers on CHF modeling using large
databases [167, 168]. The CHF in microchannels is clearly
affected by the presence of instabilities, and large spread is
observed in the data. This scatter is believed to be due to CHF
values being reported under both stable and unstable operating
conditions. As pointed out by Bergles and Kandlikar [14], the
instabilities are responsible for lower CHF values.
A number of correlations have been proposed in the
literature for CHF in microchannels. In general, they were
developed with a small data set, and provided useful insight into
the CHF mechanism [169, 170]. However, they had limited
success in predicting other data sets that were not employed in
the correlation development. Two correlations developed for
macroscale applications by Katto and Ohno[171] and Shah [172]
however were found to predict the CHF generally within 20 to
40 percent MAE. This is quite remarkable considering that the
underlying data sets in their correlation development used tubes
larger than 3 mm in diameter. For example, R-123 data by Kosar
and Peles [173] was correlated within 24.7 percent MEA by the
Katto and Ohno correlation. Park and Thome [174] report errors
between 23-60 percent with this correlation. The success of
these general correlations also shows the importance of utilizing
large number of data sets in correlation development. However,
Roday and Jensen [175] report that the Katto and Ohno
correlation resulted in large overprediction for D=0.430 mm and
0.700 mm, and smaller overprediction with D=0.286 mm tubes.
In general, the Katto and Ohno correlation underpredicted, while
the Shah correlation overpredicted the experimental CHF data.
On the other hand, specific correlations have been developed in
the literature based on the researchers’ own experimental data,
e.g. [170, 173, 174, 176, 177]. They are able to correlate the
parent data set well, but generally do not fare as well with other
data sets.
Recently, utilizing a scaling analysis [178], Kandlikar [168]
proposed a CHF model that shows that the length-to-diameter
(L/D) ratio acts as a step-function to yield high-CHF and lowCHF regions. The CHF was considered to be dependent on the
local parameters, and was a result of the vapor pushing under the
liquid at the wall resulting in a vapor cut-back. The
hydrodynamic instability resulting from this force was proposed
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by Palmer [179] in 1976 and used renently by Sefiane [180] in
modeling the interface instability during pool boiling. The new
CHF model [168] utilizes Weber number, capillary number
(earlier proposed by Moriyama [113] in film thickness modeling),
and a new non-dimensional number K2 representing the ratio of
the force due to momentum change resulting from evaporation at
the interface and the surface tension force:
K
D
V
(5)
Figure 11 – Comparison of CHF predictions by Kandlikar
[168] with parent data in microchannels and minichannels.
The step-function form dependence on the L/D-ratio is
believed to be linked to the instability during flow boiling. A
higher L/D-ratio leads to a lower CHF, while short tubes with a
lower L/D-ratio yields a higher CHF. The CHF correlation [168]
covers a wide range of parameters: hydraulic diameter from 0.127
mm to 3.36 mm, quality from 0.003 to 0.98, We from 1.7 to
86,196, Ca from 0.1×10-3 to 117×10-3, and L/D from 10 to 352.
A comparison of the model prediction with the experimental data
is shown in Fig. 11. The parametric trends among Weber number,
Capillary number and the new parameter K2 reported in [168]
provide a fundamental understanding of how inertia, viscous and
surface tension forces affect the CHF phenomenon at macroscale
and microscale. In general, surface tension forces are dominant at
very small channel sizes and lower mass fluxes. As the channel
size or mass flux increases, viscous terms (Ca) become important,
and finally for larger channel dimensions, or at higher mass
fluxes, inertia forces (We) are dominant.
Challenges and Future Research Needs in Flow Boiling
The main challenges facing flow boiling in microchannels
are in providing reliable system operation and a high value of heat
transfer coefficient. Some of the research needs were presented in
the discussion above. Some of the important areas are outlined
below:
Heat Transfer: Careful experiments to generate reliable data under
stable operating conditions and under controlled uncertainty
conditions are needed. One of the major concerns is the validity
of the flow boiling data. It is strongly recommended that all
flow boiling experimental setups be first validated with the
single-phase experiments before generating flow boiling data.
Further, the experimental data is needed to represent the
realistic operating conditions. For example, microchannels
may be operated with subcooled entry, or two-phase entry
(such as in the evaporator of a refrigeration system). The
experimental data for the latter condition is severely lacking.
Instability: The work on instability modeling in microchannels is
receiving increased attention in the last four years. The
analysis presented by Zhang et al. [165] provides a useful
direction. Future work is suggested to build on these concepts
and integrate the actual system operation in the modeling that
would include the upstream compressibility effects, explosive
boiling and other types of instabilities, including parallel
channel and density wave instabilities. Active control to
provide stable operation has been recently proposed [166],
however research on developing passive techniques would be
highly desirable from operational and cost standpoints.
Modeling: It is recommended to include the transient nature of
flow and conjugate effects resulting from the wall thermal
characteristics. Use of non-dimensional parameters is strongly
recommended to provide an understanding of the underlying
mechanisms.
Enhancement: Enhancement in the flow boiling heat transfer
coefficient is essential before this mode can be implemented in
practical devices. Recognizing that the single-phase flow with
water in an offset strip-fin geometry resulted in h values of
around 500,000 W/m2K [50, 70], it is reasonable to expect
comparable or higher h values to justify the additional
complexities introduced by the flow boiling systems. Effect of
nanostructures on pool boiling is being investigated currently
by many researchers and their application in flow boiling
needs to be examined.
CHF: Unstable operating conditions result in lower CHF values
in microchannel flow boiling. This aspect needs to be further
explored. Controlling unstable operation through active
control for enhancing CHF offers a reliable technique [166]
and should be pursued further.
BOOK PUBLICATIONS AND SPECIALTY FOCUSED
CONFERENCES
A few books dedicated to microchannel fluid flow and heat
transfer have been published in the literature. The first book by
Kim et al. [181] focuses on the minichannel ranges and provides
basic equations that are helpful in designing compact heat
exchangers utilizing these channels. The second book by Zhang
et al. [182] focuses on the silicon heat sinks, providing the reader
with the fundamental aspects of the silicon cooling system
design for applications as well as research. They confirm the
validity of the Hsu and Graham [183] nucleation model and
provide performance characteristics of various coolers in two
phase flow. The third book by Celata [184] provides an
excellent survey of the research reported in the literature on
single-phase and two-phase flows. It includes contributions from
several experts in different fields. The single-phase liquid flow
and two-phase adiabatic flow in minichannels and flow boiling
in minichannels are covered in greater detail. The fourth book
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by Kandlikar et al. [185] is published in 2006 and covers gaseous
flows, liquid flows, boiling and condensation, along with some
biological applications. The treatment in the single-phase liquid
flow is on understanding the fundamentals with specific equations
for entry region flows, roughness effects and microchannel
system design for chip cooling application. The coverage on
boiling deals with the fundamentals of the boiling phenomena.
The fifth book by Yarin et al. [186] provides a detailed coverage
of literature related to single-phase and two-phase cooling
systems. Special focus is on the single-phase and two-phase flows
in heated capillaries.
A number of specialty conferences are devoted to heat
transfer and fluid flow in microchannels. The following list is not
exhaustive, but provides an overview of the scope of activities
worldwide:
• Engineering Foundation Conference on Boiling Heat
Transfer, since 1992, currently under ECI
• ASME International Conference on Nanochannels,
Microchannels and Minichannels, held annually since 2003
• ECI International Conference on Transport Phenomena in
Micro and Nanodevices, since 2004
• ECI International Conference on Heat Transfer and Fluid
Flow in Microscale, since 2005
• ASME Micro/Nanoscale Heat and Mass Transfer
International Conference, since 2007
• Micro and Nano Flows Conference, since 2006 (Initially
sponsored by ImechE/Royal Society of Edinburgh
Colloquium)
• European Conference on Microfluidics, since 2008
In addition to the above specialty conferences, Mechanical
Engineering societies in a number of countries (such as ASME,
JSME, KSME, etc.) have a number of sessions devoted to the
fluid flow and heat transfer at microscale.
CONCLUDING REMARKS
The advantages of using microscale passages in improving
heat transfer has propelled research in this area. This subject will
continue to receive attention as the focus shifts to further
enhancing heat transfer, reducing pressure drop and
implementation of these processes in practical devices. We are
witnessing a paradigm shift that is expected to yield significant
reduction in equipment sizes of some conventional heat transfer
systems as well and introduce new products utilizing microscale
technologies.
In single-phase liquid flow in microchannels, it is now well
accepted that the continuum models are applicable. The reduction
in scale is seen as an important consideration while considering
the near wall region. Implementing nanofluids in the systems
using microchannels is seen to provide significant changes in the
performance. It is becoming clear that microchannels are gateway
to flow modification through such nanoscale surface enhancement
configurations. Enhancing heat transfer in single-phase flow
through novel techniques will continue to be a high priority
research endeavor in this area.
Flow boiling in microchannels is receiving renewed interest
as the challenges for achieving stable operation are being
overcome through a fundamental understanding of the
underlying processes. Developing simple and inexpensive
systems for operating flow boiling systems under stable
conditions will be an important topic of research. Enhancing
flow boiling heat transfer coefficient through novel techniques
will continue to receive attention. Implementing some of the
enhancement techniques that have been proven to be effective in
compact heat exchanger passages (minichannels), will be a high
priority area for research. Application of nanocoatings on the
channel walls is seen to be another area for research in reducing
the wall superheat at incipience and increasing CHF.
Another aspect of microchannel research that cannot be
overlooked is the potential strides that can be made through
cross-disciplinary implementation of ideas from the fields of
micromixers, biological sciences and microfluidic devices used
in drug delivery and diagnostic applications.
Needless to say, we all should be thinking of the nanofluidic
frontier that promises to provide even more exciting
opportunities for heat transfer and fluid flow researchers.
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