Design and Analysis of a Heat Exchanger for an Open Circuit Wind Tunnel control system

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Design and Analysis of a Heat Exchanger for an Open Circuit
Wind Tunnel control system
S Puneeth[1]
, N B D Pattar[2]
[1]
Stress Engineer, Abhiyantrana Technologies, Bangalore, Karnataka, India
[2]
Professor, The Oxford College of Engineering, Bangalore, Karnataka, India,
Abstract-This paper presents an account of model design, steady state simulation and a CFD analysis of three heat
exchangers namely, double pipe heat exchanger, shell and tube heat exchanger and transverse fin type heat exchanger in
order to evaluate their performance under a common parameter. The objective of this research work is to select the
appropriate heat exchanger for exchange of excess heat from a liquid rheostat which controls the speed of an AC motor
coupled to the fans of an open circuit wind tunnel. Five models in each of the three heat exchangers were designed and its
pressure drop was calculated. This helped in determining the best model for further analysis. Each model from the three
heat exchangers was analysed and parameters like temperature, pressure and velocity were compared.
Keywords - Heat Exchangers, CFD, wind tunnel, control system.
I. INTRODUCTION
A wind tunnel is an aerodynamic test facility. It is mostly used to study flow patterns around bodies and measure
aerodynamic forces on them. The bodies (called models) are usually scaled down but geometrically similar versions of bodies
of interest like an airplane or an automobile. The results from wind tunnel tests can be ‘scaled’ to the actual velocity and
actual body size using suitable scaling laws. A typical wind tunnel consists of a test section in which the model is kept, a
contraction section and settling section before the test section, and a diffuser after the test section. A fan after the diffuser
creates the wind. The function of the motor in the wind tunnel is to supply the necessary torque to the fan which will convert
the torque into useful thrust. This thrust, which acts as a pressure difference at the fan station, has to supply the various losses
encountered by the moving mass of air across the passage of the tunnel. A heat exchanger is a device that is used to transfer
thermal energy between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at
different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions.
Typical applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single or
multicomponent fluid streams. A major characteristic of heat exchanger design is the relative flow configuration, which is the
set of geometric relationships between the streams.
Su Thet Mon Than et al.[1] (2008) illustrated the general design considerations and general design procedure for a liquid-
liquid heat exchanger. The flow diagram given in their paper gives a general idea about heat exchanger design. The computer
program developed by them was divided into three main steps.
 Calculation of total number of tubes.
 Calculation and Checking of heat load.
 Calculation and Checking of pressure drop for tube and shell side.
The values were compared with the experimental results and were found that the results were almost as same as the existing
design. The computer program developed by them was proven to be highly effective in designing liquid-to-liquid heat
exchanger. Stefano Braccoet al.[2] (2007) worked on both the steady-state and dynamic simulation of a double-pipe heat
exchanger, in parallel-flow or counterflow arrangement and showed results of the steady-state simulation of a counterflow
heat exchanger, as a function of the number of cells, and the effects of some typical transient operating conditions. The
simulation results, in steady-state and transient conditions, show that the model behaves like a real system just considering a
limited number of cells.Prabhat Kumar Gupta, P.K. Kush, AsheshTiwari[3] (2007) showed that a considerable improvement
in the performance of heat exchanger is possible by choosing an appropriate geometrical configuration for a given process
requirement. The thermal and pressure drop performance of a coiled finned-tube heat exchanger depend on the clearance
between the shell and finned tube in addition to other geometric and operating parameters.

_____________________________________________________________________________________________________
J.S. Liaw et al.[4] (2007-2008) studied the airside performance of the fin-and-tube heat exchangers having plain fin geometry.
A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch were made and tested in
a wind tunnel at controlled environment.
MAJOR CONCLUSIONS OF THIS STUDY ARE SUMMARIZED AS FOLLOWS:
 The effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row.
 The influence of tube row on the airside performance is rather small for both heat transfer and frictional
characteristics.
T.N. Krishnaswamy [5] (1955) gave a detailed account of the types of electrical drives for an open circuit wind tunnel. This
paper deals with the calculation of the total air pressure required in the test section of the wind tunnel and the total power
required for generating that air pressure. He gave an account of the different types of electrical drives and their specifications
that can supply the amount of power required for the wind tunnel. It also deals with different losses that can occur in the
motor and the method to calculate them. Heat Exchanger design hand book[6] by D. Brian Spalding and J. Taborek gives a
description of the heat exchanger types, Quantitative relationships for heat exchangers, Analytical and numerical solution
procedure for heat exchanger equations. This book explains the design procedure used for different heat exchangers. It also
gives an account of the properties of different solids, liquids and gases used in heat exchanger design and the parameters to be
considered during the selection of a heat exchanger. Process heat transfer [7] by Donald Q Kern provides fundamental
instruction in heat transfer while employing the methods and language of the industry. The inclusion of the practical aspects
of the subject as an integral part of the pedagogy is intended to serves as a supplement for a strong foundation in engineering
fundamentals. It gives a complete understanding of the tubular heat exchangers available in the world. The procedures given
in this book cover most of the fluids used in heat transfer process.
II. INPUT DATA
A. COLD FLUID INLET TEMPERATURE
The inlet temperature of the cold fluid depends on the atmospheric temperature around the heat exchanger. This temperature
is obtained by recording the atmospheric temperature on a daily basis during the complete project period. The table below
gives the average atmospheric temperature from Sept. 2011 to June 2012.
TABLE I. MONTHLY AVERAGE OF ATMOSPHERIC TEMPERATURE
MONTH AVG. TEMP (O
C)
AUGUST 2011 27.548
SEPTEMBER 2011 28.833
OCTOBER 2011 27.516
NOVEMBER 2011 29.323
DECEMBER 2011 27.355
JANUARY 2012 29.032
FEBRUARY 2012 31.621
MARCH 2012 34.484
APRIL 2012 34.767
MAY 2012 32.645
JUNE 2012 30.967
THE FIG BELOW IS THE GRAPHICAL REPRESENTATION OF THE MONTHLY AVERAGE ATMOSPHERIC TEMPERATURE.
Fig 1.Graphical representation of average atmospheric temperature
The average of the values in table I is 30.28o
C. Hence we can take the inlet temperature of the cold fluid as 30o
C.
0
10
20
30
40
SEPTEMBER
OCTOBER
NOVEMBER
DECEMBER
JANUARY
FEBRUARY
MARCH
APRIL
MAY
JUNE
Avg. Temp

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B. TOTAL HEAT REJECTED
 Power required to run the rotor at 725 rpm (171.69 m/s) is 507.076 kW
 Power required to run the rotor at 190 rpm (45 m/s) is 127.982 kW
 Total heat dissipated is 379.094 Kw
III. DESIGN CALCULATION:
The design of the three heat exchangers is done using a set of formulae which assisted in the selection of the optimum heat
exchanger for the given problem. The selection criterion isdetermined by calculating the pressure drop in each heat exchanger
and selecting the one which is within the minimum and maximum allowable.
The equation for overall heat transfer co-efficient is given as,
Overall heat transfer co-efficient Uc =
( )
(1) Where,
ho= (2)
The equation for pressure drop is given as, for a double pipe heat exchanger,
ΔPa =
[(∆ )
(3) Where,
ΔFa = ′
(4)
Ft =
′
(5)
For a shell and tube heat exchanger,
Total pressure drop in the system,
(ΔP)T= (ΔP)t+ (ΔP) s (6) Where,
Pressure drop on the tube side,
(ΔP) t =
. ∗
(7)
Pressure drop on the shell side,
(ΔP) s = ( )( ′) (8)
For a transverse fin type heat exchanger,
(ΔP)i = (9)
fi = 0.184 .
1 + 3.5 (10)
IV. GRID INDEPENDENCY TEST
In order to minimize the computation time with a desired level of accuracy, the grid independence test is required. The
computational time increases with an increase in the number of elements. On the other hand, a coarser grid may give
misleading results with steep gradients. The computational mesh was chosen so that accurate results can be obtained and the
computational time would be minimum. The following is an overview of the mesh refinement test. A steady state thermal
analysis was conducted on three heat exchanger models for three mesh sizes (ie., 5mm, 6.5mm and 7.5mm) to study the
difference in temperature and pressure. The mass flow rate was kept constant for all the three mesh sizes and the results were
compared. A finer mesh size would be chosen if the percentage change in temperature and pressure is very large or a coarse
mesh is sufficient. The results of the test are given below.
TABLE II. DOUBLE PIPE HEAT EXCHANGER
MESH SIZE (MM) PIPE (K) ANNULUS (K)
5 325.872 312.396
6.5 317.725 307.71
7.5 314.548 306.54

_____________________________________________________________________________________________________
Fig 2. Graphical representation of temperatures at a point for different mesh sizes in DPHE
The Probe values show that the change in temperature in the annulus and the pipe is not more than 3.4% and 1.8%
respectively, which is well below the selected limit of 5%.
TABLE III. SHELL AND TUBE HEAT EXCHANGER
MESH SIZE (MM) TUBE SHELL
5 344.27 312.267
6.5 340.827 309.144
7.5 335.715 307.598
Fig 3. Graphical representation of temperatures at a point for different mesh sizes in ST
The Probe values show that the change in temperature in the Tube and the shell is not more than 2.4% and 1.5% respectively,
which is well below the selected limit of 5%.
TABLE IV. TRANSVERSE FIN TYPE HEAT EXCHANGER,
MESH SIZE (MM) HOT (K) FIN (K)
5 337.006 306.565
6.5 330.265 305.145
7.5 326.963 304.276
295
300
305
310
315
320
325
330
5 6.5 7.5
TEMPERATURE
MESH SIZE
280
290
300
310
320
330
340
350
5 6.5 7.5
TEMPERATURE
MESH SIZE

_____________________________________________________________________________________________________
Fig 4. Graphical representation of temperatures at a point for different mesh sizes in TFHE
The Probe values show that the change in temperature in the Tube and the Fin is not more than 2.9% and 0.7% respectively,
which is well below the selected limit of 5%.
V. RESULTS AND DISCUSSION
Using the formulae given in the previous chapter, the overall heat transfer co-efficient and the pressure drop were
calculated.The details of which are given in the below tables.
TABLE V. DOUBLE PIPE HEAT EXCHANGER
TUBE DIAMETERS M
OVERALL HEAT
TRANSFER CO-EFF W/M
2
-K
PRESSURE DROP NO. OF
UNITSANNULUS N/M
2
PIPE N/M
2
(PSI)
0.0254 0.0508 4935.086 157611.36 18374.060 21
0.0254 0.0635 3497.551 57074.399 18374.060 24
0.0381 0.0762 2291.155 27342.761 2290.863 20
0.0508 0.0762 2201.099 59094.127 682.492 16
0.0635 0.1016 1323.238 11404.987 288.888 17
Table 2 gives the overall heat transfer co-efficient and pressure drop values in a double pipe heat exchanger. Five models with
different diameters were selected to identify the suitable model for the given problem.
TABLE VI – SHELL AND TUBE HEAT EXCHANGER
SHELL DIAMETER
IN (M)
NUMBER OF
TUBES
OVERALL HEAT
TRANSFER CO-EFF W/M
2
-K
PRESSURE DROP
SHELL N/M
2
TUBE N/M
2
0.2032 21 2435.054 11218.24 12505.26
0.254 32 1782.305 5130.22 8664.289
0.3048 48 1401.667 2601.891 6548.789
0.3365 61 1130.802 1801.66 5471.132
0.3874 81 926.602 991.6515 4348.335
Table VI gives the overall heat transfer co-efficient and pressure drop values in a Shell and tube heat exchanger. Five models
with different tube diameters were selected to identify the suitable model for the given problem.
TABLE VII – TRANSVERSE FIN TYPE HEAT EXCHANGER
TUBE DIAMETER IN (M)
OVERALL HEAT
TRANSFER CO-EFF W/M
2
-K
PRESSURE DROP
SHELL N/M
2
TUBE N/M
2
0.0381 2684.564 14678.08 1846.797
0.0508 2048.897 4780.094 1311.675
0.0635 1189.357 2118.405 975.521
0.0762 933.8105 792.348 717.11
0.1016 675.2335 229.321 479.679
Table VII gives the overall heat transfer co-efficient and pressure drop values in a Shell and tube heat exchanger. Five models
with different tube diameters were selected to identify the suitable model for the given problem.
280
290
300
310
320
330
340
5 6.5 7.5
TEMPERATURE
MESH SIZE

_____________________________________________________________________________________________________
VI. ANALYSIS RESULTS
The following figures give the analysis results of three heat exchangers.
Fig 5. Graphical representation of Cold fluid temperature distribution in DPHE
Figure 5 is a graph of the temperature distribution on the cold side of a double pipe heat exchanger. The total heat transfer
length is 3.048 m or 10 ft and the number of heat transfer units are 24. At inlet the temperature is specified at 303.15 K and it
increases along the length of the tube and reaches a maximum of 313 K at the end of the tube length.
Fig 6. Graphical representation of Cold fluid temperature distribution in STHE
Figure 6 is a graph of the temperature distribution on the cold side of the shell and tube heat exchanger. The total heat transfer
length is 1.828 m or 6 ft. At inlet the temperature is 303.15 K and it increases along the path of the fluid inside the shell and
reaches a maximum of 324 K at the end of the tube length.
Fig 7. Graphical representation of fin temperature distribution in TFHE
290
295
300
305
310
315
320
325
330
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
temperature[K]
Chart count
300
320
340
360
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
FINTEMPERATURE(K)
FIN LENGTH (m)
Fin Temperature

_____________________________________________________________________________________________________
Figure 7 is a graph of the temperature distribution on the cold side of the Transverse fin heat exchanger. The total heat
transfer length is 1 m. At inlet the temperature is 303.15 K and it increases along the path of the fluid inside the shell and
Fig 8. Graphical representation of hot fluid temperature distribution in DPHE
Figure 8 is a graph of the temperature distribution on the hot side of the double pipe heat exchanger. The total heat transfer
length is 3.048 m or 10 ft. At inlet the temperature is specified at 353.15 K and it increases along the length of the tube and
Fig 9. Graphical representation of hot fluid temperature distribution
Figure 9 is a graph of the temperature distribution on the hot side of the shell and tube heat exchanger. The total heat transfer
length is 1.828 m or 6 ft. At inlet the temperature is 353.15 K and it decreases along the length of the tube and reaches a
maximum of 320 K at the end of the tube length.
Fig 10. Graphical representation of Cold fluid pressure variation in DPHE
Figure 10 shows a graph of pressure variation on the cold side of the double pipe heat exchanger. The pressure of the tube is
high at the inlet and decreases rapidly as it passes through the small diameter tube in the middle and then a constant pressure
is maintained by the system.
300
320
340
360
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
Temperature[K]
Chart count
Hot Fluid temp

_____________________________________________________________________________________________________
Fig 11. Graphical representation of Cold fluid pressure variation
Figure 11 shows a graph for the pressure variation on the shell side of the shell and tube heat exchanger. The variation is
plotted across the length of the shell and near around the baffles. The negative pressure shows the formation of vortices.
Fig 12. Graphical representation of hot fluid pressure variation in DPHE
Figure 12 gives a graph of the pressure variation on the hot side of the double pipe heat exchanger. The pressure is high at the
inlet and reduces with a constant gradient towards the bend as the temperature of the fluid is reduced and a slight fluctuation
is found and then decreases with a constant gradient till the outlet.
Fig 13. Graphical representation of hot fluid pressure variation
Figure 13 shows a graph for the pressure variation along the length of the tubes in the shell and tube heat exchanger. It shows
a constant pressure gradient from the inlet to the exit.

_____________________________________________________________________________________________________
Fig 14. Graphical representation of Cold fluid velocity profile in DPHE
Figure 14 is a graph of the velocity profile on the cold side of the double pipe heat exchanger. It can be seen from the graph
that the velocity is maximum near the smaller diameter pipe and least after the fluid leaves the middle tube. It is also observed
that a small vortex field is generated near the exit of the small tube due to negative pressure in that area.
Fig 15.Graphical representation of cold fluid velocity profile
Figure 15 gives the graph of the velocity profile on the shell side of the shell and tube heat exchanger. The graph shows the
highest velocity at the inlet and exit since there will be a reduction in the velocity around the baffles. The slight increase in
the velocity in a few regions is because of the formation of vortices.
Fig 16. Graphical representation of hot fluid velocity profile in DPHE
Figure 16 gives a graph of the velocity profile on the hot side of the double pipe heat exchanger. A constant velocity is
observed all through the pipe except near the entry and exit of the bend. These regions show the highest and the least
velocities which are subsided after this region is passed.

_____________________________________________________________________________________________________
Fig 17. Graphical representation of hot fluid velocity profile
highest velocity at the inlet and exit and a constant velocity gradient along the length of the tubes.
Fig 18. Graphical representation of hot fluid velocity profile
highest velocity at the inlet and exit and a constant velocity gradient along the length of the tubes.
V. CONCLUSION
The results of a three dimensional analysis conducted on three heat exchangers, namely double pipe heat exchanger, shell and
tube heat exchanger and transverse fin type heat exchanger, are presented in this report. The results of temperature
distribution, pressure variation and velocity profile were studied and it has been concluded that a transverse fin type heat
exchanger is suitable for the given problem. Furthermore, a grid independency test was conducted with three mesh sizes (5,
6.5, 7.5 mm) to determine whether the parameter (preferably temperature) variation depends on the size of the mesh element.
After analyzing for a constant mass flow rate of 1 kg/s and plotting the probe values for all the heat exchangers it was found
that the variation was less than the limit set (i.e., 5%). Hence it can be concluded that the parameter variation does not depend
on the mesh size for these models and the largest of the mesh sizes can be used for the analysis using the input values. An
analysis was done using a mesh size of 7.5 mm (from the grid independency test) which led to different plots for the cold and
the hot side of the heat exchangers. The analyses performed on the three heat exchanger models, whose results were given in
chapter 7, were compared on the basis of temperature distribution, pressure variation and velocity profile.
Based on the numerical results obtained from this investigation, the following conclusions are made.
 The total area required for installation of the heat exchangers is comparatively larger than the TFHE.
 The pressure gradient in both double pipe and the shell and tube heat exchangers were not constant inside the heat
exchanger whereas the TFHE did not show major variations in pressure inside the heat exchanger although there were
regions with negative pressures outside the heat exchanger.
 The heat transfer surface area of the two heat exchangers are fairly less compared to the surface area of the TFHE, this
is due to the number of fins attached to the tube carrying the hot fluid. These fins assist in increasing the heat transfer
rate while still being compact in design.
 High maintenance in the DPHE and the STHE because of the smaller sizes of tubes when compared to the TFHE.

_____________________________________________________________________________________________________
REFERENCES
[1] Su Thet Mon Than et al. “Heat Exchanger Design” World Academy of Science, Engineering and Technology 46 2008
pg 604 – 611
[2] Stefano Bracco et al. “A Numerical Discretization Method for the Dynamic Simulation of a Double-Pipe Heat
Exchanger” INTERNATIONAL JOURNAL OF ENERGY, Issue 3, Vol. 1, 2007 pg 47 – 58
[3] Prabhat Kumar Gupta, P.K. Kush, AsheshTiwari “Design and optimization of coil finned-tube heat exchangers for
cryogenic applications” Cryogenics 47, 2007 pg 322 – 332
[4] WarakormNerdnoi et al. “HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS IN A DOUBLE-PIPE
HEAT EXCHANGER”
[5] Apu Roy, D.H.Das “CFD analysis of a shell and finned tube heat exchanger for waste heat Recovery applications”
International Journal of Mechanical & Industrial Engineering, Volume-1 Issue-1, 2011 pg 77 – 83
[6] J.S. Liaw, J.Y. Lin et al. “Performance of Plain Fin-and-tube Heat Exchangers-Data With Larger Diameter Tube Under
Dehumidifying Conditions” ASHARE JOURNAL 2007 – 2008
[7] Bergles, A. E., Nirmalan, V., Junkhan, G.H., and Webb, R. L., “Bibliography on Augmentation of Convective Heat and
Mass Transfer II,” Heat Transfer Laboratory Report HTL-31, 2008
[8] T.N.Krishnaswamy, “Selection of electrical drive for the 14’ X 9’ wind tunnel” journal of the aeronautical society of
India, Vol. 7, No. 2, 1955, pg 19 – 28.
[9] Fundamentals of Heat Exchanger Design by Ramesh K. Shah and Dušan P. Sekulic.
[10] Heat Exchanger design hand book by D. Brian Spalding and J. Taborek.
[11] Process heat transfer by Donald Q Kern.
[12] Standards of the Tubular Exchanger Manufacturers Association.
[13] Heat and mass transfer data hand book.

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