Download Studies on the Effect of the Temperature of Intermediate

Studies on the Effect of the Temperature of Intermediate
Fluid on the Effectiveness of Three Fluid Heat Exchangers
Vasudevan T.1,a) and Rakesh S.G.2, b)
1
M.Tech. Student,Dept. of Mechanical Engineering, Amrita School of Engineering, Bengaluru
Amrita Vishwa Vidyapeetham, Amrita University,India.
2
Professor, Dept. of Mechanical Engineering, Amrita School of Engineering, Bengaluru
Amrita Vishwa Vidyapeetham, Amrita University,India.
a)[email protected]
b)Corresponding author:[email protected]
Abstract.Three fluid heat exchangers involving twothermal communications as well as three thermal
communications have been investigated for the effect of change in the intermediate fluid temperature on the
effectiveness of the heat exchangers, using both theoretical and simulation studies. The objective of these heat
exchangers is identified as cooling of hot fluid. The simulations are repeated for hot fluids with different specific
heats. The effect of change in the conducting material on the effectiveness in a three thermal communications heat
exchanger has been investigated. The pressure drop calculations are also done for the heat exchangers.
Keywords: Effectiveness, intermediate temperature fluid, three thermal communications, two thermal
communications
Introduction
Three fluid heat exchangers will have heat transfer taking place between 3 fluids flowing through 3 tubes. The
three fluid heat exchangers are of two types – with two thermal communications (2TC) and three thermal
communications (3TC). Three fluid heat exchanger with 2TC is where three concentric tubes, shown in Fig.1A,are
aligned such that intermediate temperature fluid flows through innermost tube while hot fluid flows through the
intermediate tube and the cold fluid though outermost tube. Three fluid heat exchangers with 3TC, shown in Fig.1B,
has hot and intermediate temperature fluids flowing in the same direction in two smaller tubes placed inside a big
tube throughwhich cold fluid flows.
The three fluid heat exchangers with 3TC has two tubes running parallel to each other inside a big tube. These two
tubes are having a thermal connection between them, longitudinally all along the length, with a conducting material.
The hot and intermediate temperature fluids flow through these parallel tubes and the cold fluid flows through the
outer tube. Here the heat transfer occurs between the hot fluid & cold fluid, hot fluid & intermediate temperature
fluid and also between coldfluid &intermediate temperature fluid. The heat transfer in this type of heat exchangers is
such that the hot fluid dissipates heat directly to the cold fluid. The heat transfer between the hot fluid and
intermediate fluid is due to conduction through the material which connects the two tubes longitudinally. The heat
transfer also occurs between the intermediate temperature fluid and the cold fluid. Thus there are three thermal
communications amongst the three fluids.The application of the heat exchangers that are being studiedhere is
identified as to bring down the temperature of the hot fluid.
(A)
(B)
FIGURE 1.Tubular arrangement of the three fluid heat exchanger with (A) two thermal communications and (B) three thermal
communications
The three fluid heat exchangers have many applications in the field of chemical, petro-chemical and aerospace
industries. They are used in systems to produce ammonia gas, liquefaction & purification of hydrogen gas, air
separation systems. The two fluid heat exchangers with third fluid which is ambient with infinite thermal capacity
are considered as three fluid heat exchangers.
(A)
(B)
FIGURE 2. Flow configuration for three fluid heat exchangers with (A)two thermal communications (B) three thermal
communications
The flow configuration of the three fluid heat exchangers with 2TC and 3TC have been shown in Fig.2. The best
flow configuration has been selected for the application of the cooling of the hot fluid.It has been reported that the
best configurationto decrease the temperature of hot fluid is when hot fluid is flowing in opposite direction to that of
the cold & intermediate temperature fluids in both 2TC and 3TC.[2]
Methodology
The simulation and theoretical studies of the three fluid heat exchangers with 3TC have been done. The
simulation of the three fluid heat exchangers with 2TC has also been done. The simulation and theoretical
calculations have been carried out to find the effectiveness of the heat exchangers with regard to the variation of
intermediate fluid temperature. The pressure drop across the heat exchangers has also been investigated.
Theoretical Studies
Some assumptions had been made for the analysis which are (a) The system works in steady state (b) All the
properties have been taken as constants with time and space (c) There is no heat transfer with the ambient air (d) No
heat source or sink (e) No phase change, and (f) Heat transfer area is constant along length of the tube.
The calculations have been done for various inlet temperature ratios,
𝑋=
𝑇𝑕1 − 𝑇𝑖1
𝑇𝑕1 − 𝑇𝑐1
Various non-dimensional parameters that have been used in the study are:
𝑇 − 𝑇𝑐,𝑖𝑛
Dimensionless Temperature,𝜃 =
Non-dimensional Length, 𝑥 =
Capacitance ratios, 𝑅1 =
𝐶𝑕
𝑙
𝑇𝑕 ,𝑖𝑛 − 𝑇𝑐,𝑖𝑛
𝐿
𝐶𝑖
𝑅2 =
𝐶𝑕
𝐶𝑐
Overall Number of Transfer Units, 𝑁𝑇𝑈 =
𝑈1 𝐴1
𝐶𝑐
The theoretical calculations for finding the temperatures of the intermediate temperature fluid and cold fluid at the
outlet are taken as reported in the literature [1]. The effectiveness is found by the equation
𝜀=
𝑄𝑎𝑐𝑡𝑢𝑎𝑙
𝑄𝑚𝑎𝑥𝑖𝑚𝑢𝑚
The pressure drop in the heat exchanger is also a huge factor other than effectiveness for the selection of heat
exchangers. The pressure drop of the fluids across a heat exchanger is calculated by the Darcy Weisbach equation,
𝛥𝑃 = 𝑓𝐷
𝜌𝑣 2 𝐿
2 𝐷𝑕
The friction factor is calculated by using the Colebrook equation
1
2.51
𝑟𝑕
0.5 = − 2 𝑙𝑜𝑔(
0.5 + 3.7𝐷 )
𝑓𝐷
𝑅𝑒 𝑓𝐷
𝑕
Simulation Studies
The simulation of the heat exchangers has been done using Ansys Fluent software. The simulation of three fluid
heat exchangers with 3TC has been done for various hot fluids having different specific heats and also for different
conducting materials.
The material used for the heat exchanger is taken as copper. The cold and intermediate temperature fluids used here
are water while hot fluids of different specific heats such as water, ethylene glycol and unused engine oil are used.
The simulations have been done for various inlet temperature ratios of X, varying from 0.1 to 0.9. Simulations are
also done for different conducting materials like copper, aluminium and brass, with water as the hot fluid.
The dimensions of the three fluid heat exchangers with 2TC are taken as - length:2m, inner diameters of
intermediate temperature fluid tube: 0.022 m, hot fluid tube: 0.0326 m and cold fluid tube: 0.0616 m. A normal fine
mesh by default is taken with total number of 604647 nodes. The length of the heat exchangers with 3TC is also
taken as 2 m. The inner diameter of the hot and intermediate temperature fluid tubes is taken as 0.022 m while the
inner diameter of the cold fluid pipe is taken as 0.0635 m. A normal fine mesh by default is taken with total number
of 509892 nodes. The simulation has been done for steady state, keeping the energy equations on, with a K-epsilon
model. The mass flow rate of the fluids is kept constant for every condition and the inlet temperature values for hot
and cold fluids are taken as 343 K and 293 K respectively for both the heat exchangers. Meshing used for the three
fluid heat exchangers with 2TC and 3TC are shown in Fig. 3 and Fig. 4 respectively.
FIGURE 3. Mesh of the three fluid heat exchanger with two thermal communications
FIGURE 4. Mesh of the three fluid heat exchangers with three thermal communications
Results and Discussions
The simulations of three fluid heat exchangers with 2TCusing hot fluids having different specific heats such as
water, ethylene glycol and unused engine oil have been carried out.It has been observed that the effectiveness of
heat exchanger decreases as the intermediate fluid temperature at the inlet decreases i.e., when X value increases
(Fig. 5A). It has also been observed that hot fluid outlet temperature difference increases with the increase in the X
value (Fig. 5B). It has been observed that as the specific heat of the fluid increases (water has highest specific heat
while engine oil has the lowest) effectiveness increases and difference in temperature of hot fluid decreases.
The pressure drop along the heat exchanger is calculated using the Darcy Weisbachequation. The Reynold’s number
of the hot fluid has been taken as 25000 and for cold & intermediate temperature fluid it is taken as 20000. The flow
areas are same while the length of tube is taken as 5 m. It has been observed that the pressure drop in the hot fluid
tends to be much higher than other fluids as the area of contact for the hot fluid is high with minimum hydraulic
diameter (Fig. 6).
(A)
(B)
FIGURE 5. Variation of (A) effectiveness (B) hot fluid temperature difference with respect to inlet temperature ratio for three
fluid heat exchangers with two thermal communications with different hot fluids
The pressure drop along the heat exchanger is calculated using the Darcy Weisbachequation. The Reynold’s number
of the hot fluid has been taken as 25000 and for cold & intermediate temperature fluid it is taken as 20000. The flow
areas are same while the length of tube is taken as 5 m. It has been observed that the pressure drop in the hot fluid
tends to be much higher than other fluids as the area of contact for the hot fluid is high with minimum hydraulic
diameter (Fig. 6).
FIGURE 6. Variation of pressure drop along the length of the three fluid heat exchangers with two thermal communications
The simulations of the three fluid heat exchanger with 3TC with different hot fluids like water, ethylene glycol
and unused engine oil have been done. It has been observed that effectiveness of heat exchanger decreases as the
intermediate fluid temperature at the inlet decreases i.e., when X value increases (Fig. 7A). It has also been observed
that hot fluid outlet temperature difference increases with the increase in the X value (Fig. 7B). It has been observed
that as the specific heat of the fluid increases (water has highest specific heat while engine oil has the lowest)
effectiveness increases anddifference in temperature of the hot fluid increases.
(A)
(B)
FIGURE 7. Variation of (A) effectiveness (B) hot fluid temperature difference with respect to inlet temperature ratio for three
Fluid heat exchangers with three thermal communications for different hot fluids.
The simulations of three fluid heat exchanger with 3TC with different connecting materials such as copper,
aluminium and brass have been done. The simulations are done for water as the fluid.
(A)
(B)
FIGURE 8. Variation of (A) effectiveness (B) hot fluid temperature difference with respect to inlet temperature ratio for three
fluid heat exchangers with three thermal communications for different conducting material
From the simulation results it has been found that the effectiveness of the heat exchanger tends to increase with a
connection material that has higher thermal conductivity value (Fig. 8A). That is, in the present case, it has been
found that effectiveness of heat exchanger is more when the conducting material used is copper than in case of
aluminium or brass. The effectiveness and the hot fluid temperature difference tend to increase for increasing values
of conductivity of the conducting material (Fig. 8B).
The pressure drop across the heat exchanger is calculated using the Darcy Weisbachequation. The Reynold’s
number of the hot fluid has been taken as 25000 and for cold and intermediate fluidsit is taken as 20000. The flow
areas are same while the length of tube is taken as 5 m. It has been observed that the pressure drop in the hot fluid is
more than the cold and intermediate fluids as the area of contact for the hot fluids is high with minimum hydraulic
diameter (Fig.9).
FIGURE 9. Variation of pressure drop along the length of the three fluid heat exchangers with three thermal communications
Conclusion
The simulation of the three fluid heat exchanger has been done for various conditions. The simulations have been
done for various inlet temperature ratios of X varying from 0.1 to 0.9. The simulations are done with different hot
fluids for both 2TC and 3TC heat exchangers. The simulations are done with different conducting material for 3TC
heat exchanger. The pressure drop calculations are also done for both 2TC and 3TC heat exchangers.
It has been observed that the effectiveness of the three fluid heat exchanger with 2TC is more than that of a heat
exchanger with 3TC. This is because, the area of heat transfer is more in case of heat exchangers with 2TC. The
large area available for heat transfer also increases the pressure drop in the hot fluid. Thus the pressure drop in case
of the heat exchangers with 2TC is larger than that of heat exchangers with 3TC.
Thus it has been concluded that the selection of heat exchangers with 2TC or3TC has to be done based on the
application for which it is used. If the pressure drop in the hot fluid does not matter in a particular application, and
only aim is to decrease the temperature of the hot fluid, then three fluid heat exchangers with 2TC can be used. But
if the pressure drop is a matter of concern than the effectiveness, then three fluid heat exchangers with 3TC has to be
used.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Dr. V. Krishna, Professor & Head, Dept. of Mechanical
Engineering, PES University, Bengaluru and Dr. S. R. Nagaraja, Professor & Head, Department of Mechanical
Engineering, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Bengaluru for their support and
guidance during the course of this work.
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