Download Boiling Heat Transfer - Department of Mechanical Engineering

Boiling Heat Transfer
Introduction:
During the past few decades, great interest has developed in boiling as a method of
increasing heat transfer rates at moderate temperature differences. Boiling heat transfer
is especially effective at high heat fluxes. Thus, it finds application in cooling of nuclear
reactors and rocket engines where heat transfer rates may be of the order of 106 to 107
BTU/hr-ft2 (3 x 106 to 3 x 107 W/m2). See "Boiling", by W.M. Rohsenow, in Handbook
of Heat Transfer Fundamentals, ed. W.M. Rohsenow, J.P. Hartnet and E.N. Ganic,
McGraw-Hill, New York, 1985.
Saturation boiling takes place at a surface situated in a liquid which is at the saturation
temperature. The average liquid temperature may remain well below the saturation
value, producing local boiling at the wall with subsequent condensation of the vapor as it
departs the wall and moves into the colder bulk of the fluid. This is known as subcooled
boiling. When the heated surface is submerged in a container of liquid where there is no
motion except that induced by the boiling process, the process is called pool boiling. If,
on the other hand, a liquid is forced over the heated surface and boils, it is called forced
convection boiling. In this experiment, saturated liquid pool boiling on an electricallyheated, horizontal wire in a saturated liquid is to be studied and observed.
Theory:
Figure 1 shows a representative boiling curve obtained from an experiment involving an
electrically heated horizontal wire submerged in a pool of water at saturation
temperature, Tsat. As the wire surface temperature, Tw is first raised above the saturation
temperature, convective currents circulate the superheated liquid, and vapor is produced
by evaporation at the free surface of water. In this region of the curve, up to point A in
the figure, heat is transferred mainly by single-phase, free convection. The temperature
difference (Tw - Tsat) that drives the heat transfer is given the symbol ∆Te, the wall
"superheat" or excess temperature. The heat flux qs”=f(∆Te) relation up to point A may
be correlated as qs” ~ (∆Te)5/4. Point A is called "the first bubble point" and reflects the
onset of bubble production in the fluid at the heated surface (point A is also called ONB
for “Onset of Nucleate Boiling”). As point A is passed, the effect of boiling manifests
itself, sometimes rather abruptly. In region AB, vapor bubbles are formed at favored
spots on the heating surface and rise to the free liquid surface. The formation cycle and
the size of the bubbles are fairly regular. This is called the region of isolated bubbles for
the bubbles remain isolated from one another as they rise to the free surface. This portion
of the curve is quite steep because, as heat flux is increased, more bubble formation sites
become active and each existing site releases bubbles at a higher rate. Furthermore, the
rising bubbles acts as “pumps” rapidly drawing cold liquid to the wire in their wakes as
they rise. Thus, the heat transfer coefficient rises steeply with rising heat flux.
E
.
∆Te = Tw – Tsat {˚C}
Typical boiling curve for water at one atmosphere; surface heat flux as a
function of excess temperature ∆Te.
Figure 1
Representative boiling curve, saturated pool boiling. [Incropera]
In region BC, larger and more numerous bubbles are formed which begin to interfere
with one another while rising. Further increases in the wire temperature produce
continuous vapor columns above the wire. This is called the region of bubble
coalescence or “jet and columns” to reflect the visual appearance of the bubbles.
Region ABC is called the nucleate boiling regime. Note that the entire boiling curve is
such that the wall temperature is above the saturation temperature. To the right of the
peak of the curve lies the transition boiling regime CD where an unstable vapor film
forms around the wire. If the wire temperature were controlled, the film would collapse
and reform rapidly giving the time-average behavior shown as the CD line. The presence
of this film provides additional resistance for heat transfer and reduces the heat transfer
coefficient with increasing surface temperature. However in our experiment, the power
(heat) dissipation of the wire is controlled. Under this condition, no stable state can be
attained in the CD portion of the curve and there will be an immediate jump from point C
to point E at the same heat flux.
In region ED, the vapor film around the wire becomes stable in the sense that it does not
break up and reform cyclically, but, instead, it always envelops the heating surface,
although its shape is not necessarily smooth. Do to poor conduction through this vapor
layer and the high temperature of the heated surface, radiation may dominated the heat
transfer process. This regime is called stable film boiling.
When an electrically controlled, heated wire is used, the regime CD cannot be obtained.
In region BC, an increase in the electric energy input (and hence q") results in an increase
in Tw due to occasional formation of large vapor patches intermittently on the surface.
When the peak value of heat flux is reached at C, and then exceeded slightly, the boiling
process cannot remove heat as fast as is necessary to maintain a stable wire temperature.
The difference between the energy that must be removed and the energy that can be
removed causes a rapid rise in the temperature of the wire. Unless the electrical input is
quickly reduced the system will proceed toward point E. This temperature, TE, may be
above the melting point of the wire material and, if so, the wire melts before point E is
actually reached. For this reason, the peak heat flux in nucleate boiling is sometimes
called the "burnout" heat flux. A more universal term is the "Critical Heat Flux," since it
represents a critical point in the boiling performance. This burnout can often be avoided
if a high melting point metal such as platinum is used.
Kutateladze (in 1950) proposed the following expression for estimating the burnout, or
critical, heat flux, qb, in saturated pool boiling as:
gL  v  
q"max  Chfgv 

 2


14
v
(1)
where
C = a constant analytically determined by hydrodynamic stability
analysis to be 0.1309 by Zuber
hfg = latent heat
v = density of saturated vapor
L = density of saturated liquid
 = surface tension
g = gravitational acceleration
This expression is obtained by dimensional analysis. Insight was gained by analyses,
which assumed that burnout, or the critical point, can be associated with the instability of
the interface between the departing vapor streams and the liquid streams moving toward
the heating surface. With an electrically heated wire, different types of boiling can exist
simultaneously on different portions of the length of the wire. This, of course introduces
some error in our analysis, since the wire temperature is actually an average wire
temperature.
Experimental Apparatus:
A 0.254mm (0.010in.) diameter, 30 mm (1.181in.) long platinum wire is submerged in a
liquid bath of FC-87 whose temperature is kept at the saturation value (30C) by an
electrical heater and is measured by a thermocouple. The wire is heated electrically using
a programmable D-C power supply. The electrical power input to the wire can be
determined by measuring the voltage drops (Vs, across the shunt resistor and Vwire across
the wire) with a digital data acquisition unit. The average temperature of the wire is
deduced from the electrical resistance of the wire, which is a function of its temperature.
The resistance is computed, knowing the voltage and the current through the wire. The
current is determined from the shunt resistor measured voltage and resistance.
The relation between resistance and temperature of the wire is obtained via a simple
calibration. This calibration is performed in a bath of water that can be varied from room
temperature to near-boiling. As the water is heated, measurements of the water
temperature (which is the same as the wire temperature) are taken using the
thermocouple. The resistance of the wire is also measured at these temperatures. Using a
least-squares fit, a straight line can be fit to this data. (The nice thing about platinum is
that the resistance-temperature relationship is linear.) With this calibration in hand, the
temperature of the wire can later be found by measuring its resistance and calculating
temperature from:
T = mR + b
Objective:
Determine whether the elements of the boiling curve can be reproduced in the lab. As
each of the various processes (single-phase convection, isolated bubble regime, etc.) is
created, document with data and sketches its peculiar characteristics. Describe, in detail,
the heat transfer mechanisms present in each regime of the boiling curve. Determine the
critical heat flux and evaluate the applicability of the Kutateladze correlation to the
conditions of this test.
Procedure:
Data will be taken for the free convection single-phase regime, the nucleate boiling
regime and the early film boiling regime and will be plotted as in Figure 1. Apply a
voltage across the circuit. Then record Vshunt and Vwire. Using these values, calculate T
and the heat flux through the wire surface.
q" 
Pw

A
V w2
Rw
DL
Tw  mRw  b
R w  Rs
Vw
Vs
(2)
(3)
(4)
Then, increase the voltage applied to the circuit. Again, record Vshunt and Vwire. The
water temperature should be monitored to ensure that the liquid stays in the saturated
condition. Determine critical heat flux from the correlation.
Report:
1. Plot heat flux, q", versus temperature difference, (Tw – Tsat), on log-log coordinates.
2. Plot h (W/m2·K) versus (Tw – Tsat) on log-log coordinates.
3. Somewhere in the body of your report, address why boiling doesn't begin when Twire
just slightly exceeds Tsat, but rather when it exceeds Tsat by several degrees?
4. Discuss a method, which would permit the mapping of the boiling curve in region CD.
This is not possible with the current set-up.
5. Does one need to consider heat loss down the wire to the supports of the wire in order
to present accurate data? Why or why not?
6. Does boiling enhance heat transfer relative to single-phase heat transfer?
7. Using a heat transfer correlation, can you estimate the heat transfer coefficient at the
upper-end of the natural convection regime?