ABSTRACT
COMPLEX
ENGINEERING
PROBLEM
Submitted to: Dr. Taqi Cheema
The objective of this study is to analyze different
design modifications of cross-flow heat exchangers
using COMSOL Multiphysics simulations and
determine the best possible design. The paper aims
to provide a comprehensive comparison between the
original basic cross-flow heat exchanger and all the
proposed design modifications. Based on the
comparison, it was concluded that a slight increase in
the flow area of channels involved in fluid flow is the
most practical design modification to achieve an
increased rate of heat transfer with a reasonable
pressure drop.
Adam Surti 2020037
Mudasir Hassan 2020251
Arsalan Khan 2020276
Heat Transfer ME-353
Table of Contents
Introduction ................................................................................................................................................ 4
Literature Review ....................................................................................................................................... 4
Methodology ............................................................................................................................................... 6
Heat Transfer Effect ................................................................................................................................... 6
Effect of channel width on heat transfer ................................................................................................. 6
Effect of increase in number of channels on heat transfer ..................................................................... 7
Effect of spacing between channels on heat transfer .............................................................................. 7
Design variation ...................................................................................................................................... 7
Design Simulation ...................................................................................................................................... 8
Parameters .............................................................................................................................................. 8
Thermophysical properties ..................................................................................................................... 8
Assumptions ........................................................................................................................................... 8
Original design ....................................................................................................................................... 9
CFD Analysis and Results ...................................................................................................................... 9
Design Enhancements............................................................................................................................... 10
Increasing number of channels ............................................................................................................. 11
Increasing channel depth ...................................................................................................................... 12
Decreasing spacing between channels .................................................................................................. 13
Large number of channels .................................................................................................................... 14
Increasing flow area.............................................................................................................................. 15
Results and Discussion ............................................................................................................................. 16
Design selection ........................................................................................................................................ 17
Conclusion ................................................................................................................................................ 17
References ................................................................................................................................................ 18
List of Figures
Figure 1:Original model geometry ................................................................................................................................ 9
Figure 2:Surface temperature and isosurface contour plots for original design ........................................................... 9
Figure 3:Velocity and pressure distribution for original design ................................................................................. 10
Figure 4:Outlet temperatures of both outflows for original design ............................................................................ 10
Figure 5:Geometry and outlet temperatures (both outflows) for increased number of channels ............................... 11
Figure 6: Surface temperature and isosurface contours for increased number of channels ........................................ 11
Figure 7:Velocity and pressure distribution for increased number of channels ......................................................... 11
Figure 8:Geometry and outlet temperatures (both outflows) for increased depth of channels ................................... 12
Figure 9:Surface temperature and isosurface contours for increased depth of channels ............................................ 12
Figure 10:Velocity and pressure distribution for increased depth of channels ........................................................... 12
Figure 11:Geometry and outlet temperatures (both outflows) for decreased spacing between channels ................... 13
Figure 12:Surface temperature and isosurface contours for decreased spacing between channels ............................ 13
Figure 13:Velocity and pressure distribution for decreased spacing between of channels ........................................ 13
Figure 14: Geometry and outlet temperatures (both outflows) for large number of channels.................................... 14
Figure 15:Surface temperature and isosurface contours for large number of channels .............................................. 14
Figure 16:Velocity and pressure distribution for large number of channels............................................................... 14
Figure 17:Geometry and outlet temperatures (both outflows) for increased flow area .............................................. 15
Figure 18:Surface temperature and isosurface contours for increased flow area ....................................................... 15
Figure 19:Velocity and pressure distribution for increased flow area ........................................................................ 15
List of Tables
Table 1: Model Parameters ........................................................................................................................................... 8
Table 2: Thermophysical Properties ............................................................................................................................. 8
Table 3:results of design modifications ...................................................................................................................... 16
Table 4: Final dimensions of selected design ............................................................................................................. 17
Heat Transfer in Cross Flow Heat
Exchanger
Introduction
A cross-flow heat exchanger exchanges thermal energy from one airstream to another in an air handling
unit (AHU). Unlike a rotary heat exchanger, a cross-flow heat exchanger does not exchange humidity and
there is no risk of short-circuiting the airstreams. A cross-flow heat exchanger is used in a cooling and
ventilation system that requires heat to be transferred from one airstream to another. A cross-flow heat
exchanger is made of thin metal panels, normally aluminum. The thermal energy is exchanged via the
panels. A traditional cross-flow heat exchanger has a square cross-section. It has a thermal efficiency of
40-65 %.[1]
The heat exchanger design can be divided into two main categories, thermal and hydraulic design and
mechanical design. In thermal and hydraulic design, the focus is on calculating an adequate surface area
transfer a certain amount of heat, pressure dope, pumping power work, etc. The goal of the mechanical
design is to design the mechanical integrity of the exchanger, as well as designing various pressure and
non-pressure components.[2]
Common appliances containing a heat exchanger include air conditioners, refrigerators, and space heaters.
These devices are also used in chemical processing and power production. Perhaps the most commonly
known heat exchanger is a car radiator, which cools the hot radiator fluid by taking advantage of airflow
over the surface of the radiator.[3]
In this study, such a heat exchanger is simulated on COMSOL Multiphysics, and analyzed for the variation
of different properties across the device. By inputting an average velocity, the inlet temperatures of the
fluids, and some customized material properties, we produced plots of temperature, isothermal contours,
velocity, and pressure.
Then, a number of design modifications are suggested, and their simulations are iterated in an effort to
improve heat transmission through the crossflow heat exchanger. The outcomes of the simulations are
then compared to find the most practical design modification that can be brought into the original heat
exchanger, for a better heat transfer rate and a more manageable pressure drop.
Literature Review
Cross flow heat exchangers are commonly used in various industries for efficient heat transfer between
fluids. The recent advancements in materials, design, and technologies have significantly improved the
performance and efficiency of cross flow heat exchangers. This literature review provides an overview of
the recent research efforts in the field of cross flow heat exchangers.
A review paper by M. A. Al-Sanea and A. M. Al-Rashed (2019) discusses the different types of cross
flow heat exchangers and highlights the recent advancements in materials and design for improved
performance and efficiency. The authors also discuss the various factors that affect the performance of
cross flow heat exchangers, such as fluid velocity, heat transfer area, and geometric parameters. The
review provides insights into the key challenges in the design and optimization of these heat exchangers
and presents potential solutions for these challenges.[4]
In another review paper, M. H. Al-Rashed et al. (2020) provide a comprehensive analysis of experimental
and numerical studies on cross flow heat exchanger performance. The authors investigate the effects of
various parameters, such as flow rate, heat transfer coefficient, and pressure drop, on the performance of
cross flow heat exchangers. They also discuss the recent advancements in numerical simulation techniques
for the design and optimization of these heat exchangers. The review highlights the importance of accurate
modeling of the flow and heat transfer in cross flow heat exchangers for improved performance and
efficiency.[5]
S. S. Bhagat et al. (2021) discuss the various heat transfer enhancement techniques in cross flow heat
exchangers, including passive and active techniques such as turbulators, inserts, and surface
modifications. The authors provide a comprehensive overview of the recent advancements in these
techniques and their potential applications in various industries. The review highlights the need for further
research to optimize the use of these techniques for improved performance and efficiency of cross flow
heat exchangers.[6]
A study by A. A. Al-Sanea et al. (2020) investigates the design and optimization of cross flow heat
exchangers for energy efficiency. The authors used numerical simulations and optimization techniques to
optimize the geometry of the heat exchangers for improved energy efficiency. The study highlights the
potential of numerical simulation and optimization techniques for the design and optimization of cross
flow heat exchangers and their potential applications in various industries.[7]
Finally, M. A. Al-Sanea et al. (2021) present a multi-objective optimization of compact cross flow heat
exchangers. The authors used numerical simulations and genetic algorithm optimization to optimize the
design of the heat exchangers for improved heat transfer and pressure drop performance. The study
highlights the importance of considering multiple objectives in the design and optimization of cross flow
heat exchangers and presents a promising avenue for the improvement of these heat exchangers.[8]
Methodology
Research &
Review
CAD
Modelling &
Calculations
Comparative
Analysis
Results &
Discussion
Figure 1: Methodology schematic
Heat Transfer Effect
An efficient cross-flow heat exchanger is necessary in many technical devices (e.g air conditioners, air
preheaters, car coolers, and boiler economizers), because Such heat exchangers can reduce size and cost.
Cross-flow heat transfer rate can be improved by a number of techniques involving conversion changing
tube shape and tube placement. It is known that the rate of heat transfer and the pressure drop in the crossflow heat exchanger depends on the piping arrangement and tube shape.
It was evident from the previously described studies that the rate of heat transfer and the pressure loss of
cross-flow heat exchangers is determined by three main factors:
1. Geometry of channels
2. Channel arrangement
3. Number of channels
Effect of channel width on heat transfer
The increase of rectangular channel width can effectively increase the heat transfer rate per unit volume
and reduce the pressure drop at the same mass flux. However, the changes of heat transfer rate per unit
volume and pressure drop tend to be gentle with the width increase.[9] This modification was
implemented on our original model to check its effect on the heat transfer rate.
Effect of increase in number of channels on heat transfer
In a cross-flow heat exchanger, increasing the number of channels can have both positive and negative
effects on heat transfer. On the one hand, increasing the number of channels can increase the heat transfer
surface area, which can enhance heat transfer performance. However, this can also result in a decrease in
the flow rate through each channel, which can reduce the heat transfer coefficient. Additionally, an
increase in the number of channels may increase the pressure drop across the heat exchanger, which can
increase the pumping power required to maintain the flow rate. Furthermore, increasing the number of
channels can promote the mixing of fluid streams, which can enhance the heat transfer coefficient.
However, this effect may be offset by the reduction in flow rate and the increased pressure drop. Finally,
an increase in the number of channels can increase the difficulty of cleaning and maintaining the heat
exchanger, which can result in fouling and reduced heat transfer performance over time. Therefore, the
effect of increasing the number of channels on heat transfer in a cross-flow heat exchanger depends on a
variety of factors, and careful consideration should be given to the specific operating conditions and
design parameters of the system.[10]
Effect of spacing between channels on heat transfer
The spacing between channels in a cross-flow heat exchanger can significantly affect heat transfer
performance. Increasing the spacing between channels can decrease the heat transfer coefficient because
it reduces the surface area available for heat transfer. This is because the larger spacing reduces the amount
of time the fluid spends in contact with the heat transfer surface, resulting in lower heat transfer rates. On
the other hand, decreasing the spacing between channels can increase the heat transfer coefficient because
it increases the surface area available for heat transfer. However, this can also increase the pressure drop
across the heat exchanger and lead to a higher pumping power requirement. Therefore, a balance must be
struck between maximizing the heat transfer coefficient and minimizing the pressure drop. The optimal
spacing between channels depends on the specific operating conditions and design parameters of the heat
exchanger, and should be carefully evaluated to optimize heat transfer performance.[11]
Design variation
This report outlines a study where eight design modifications were proposed and simulated for a
rectangular channel flow. The aim was to determine the most efficient and practical design among these
variations. To ensure a fair and reliable comparison between the designs, solid and fluid thermophysical
properties, inlet temperatures, and average velocities were kept constant across all iterations. The report
presents the results of relevant comparisons made between the design modifications to identify the optimal
design.
The design variations are as follows:
1.
2.
3.
4.
5.
Increase in channel depth
Increase in flow area (increasing channel width and decreasing number of channels)
Increasing number of channels
Decreasing spacing between channels
Large number of channels
Design Simulation
The study focused on testing and analyzing multiple design configurations of a cross flow heat exchanger
to determine the best balance between heat transfer rate and pressure drop. To achieve this, the study kept
the inlet and outlet boundary conditions constant while exploring changes in the geometry of the channels,
such as its dimensions and spacing. The goal was to identify the optimal design configuration that provides
the highest heat transfer rate while minimizing pressure drop.
Parameters
Name
Expression
Value
Description
T_cold
300[K]
300K
Temperature, cold stream
T_hot
330[K]
330K
Temperature, hot stream
u_avg
50[mm/s]
0.05m/s
Average inlet velocity
Table 1: Model Parameters
Thermophysical properties
Property
Variable
Unit
Property group
Thermal
k_iso ; kii = k_iso, 15
conductivity kij = 0
W/(m·K)
Basic
Density
7800
kg/m³
Basic
420
J/(kg·K)
Basic
rho
Heat
Cp
capacity at
constant
pressure
Value
Table 2: Thermophysical Properties
Assumptions
•
•
•
•
Steady state condition
Laminar flow
Uniform properties at inlet of each heat exchanger
No heat transfer occurs between heat exchanger and surroundings
Original design
The model used in the study consisted of two layers with a square cross section measuring 800μm by
800μm and both layers having the same thickness of 60μm. Straight rectangular channels with a crosssectional dimension of 100μm by 40μm and a length of 800μm were employed to allow fluid flow. The
channels were initially positioned with a distance offset of 200μm from the origin, and the spacing
between channels was 20μm.
Figure 2:Original model geometry
CFD Analysis and Results
According to the analysis, the heat exchanger was found to be not very effective in terms of the
temperature difference that was cooled. The fluid with the higher inlet temperature (330 K) released its
heat to the fluid that initially had a lower temperature (300 K), which caused the heat exchanger system
to stabilize until it reached a state of thermal equilibrium, with the temperature gradients between the
fluids being balanced out. The temperature graphs have been displayed in Figure 2.
Figure 3:Surface temperature and isosurface contour plots for original design
The velocity profile and pressure distribution along the channels are shown in Figure 3.
Figure 4:Velocity and pressure distribution for original design
The outlet temperatures of the two outflows can be observed in Figure 4.
Figure 5:Outlet temperatures of both outflows for original design
The hot fluid's temperature decreases from 330 K to a minimum of 310 K, while the cold fluid's
temperature increases from 300 K to a maximum of 315 K. The coefficient of heat transfer resulting from
the temperature difference between the fluids is 3144.3 W/m2 K, and the maximum pressure drop across
the heat exchanger is 92.022 Pa.
To improve the heat exchanger's efficiency, the heat transfer rate should be increased while minimizing
pressure drop, requiring minimal fluid flow. This can be accomplished by modifying the heat exchanger's
geometry and analyzing its impact.
Design Enhancements
CFD analysis has been conducted to examine several design modifications, with a focus on the outlet
temperatures of the fluid flow, net heat transfer coefficient, and maximum pressure drops.
Increasing number of channels
Due to the need to maintain a compact structure and constant cross-section of the heat exchanger, there is
a limit to the number of channels that can be increased. To address this, one additional channel has been
added to each side of the original design, and the resulting data has been depicted in Figures below.
Figure 6:Geometry and outlet temperatures (both outflows) for increased number of channels
Figure 7: Surface temperature and isosurface contours for increased number of channels
Figure 8:Velocity and pressure distribution for increased number of channels
Increasing channel depth
The height or depth of the channel tubes has been altered from 40μm to 50μm while maintaining the rest
of the design identical, resulting in the changes depicted in following Figures.
Figure 9:Geometry and outlet temperatures (both outflows) for increased depth of channels
Figure 10:Surface temperature and isosurface contours for increased depth of channels
Figure 11:Velocity and pressure distribution for increased depth of channels
Decreasing spacing between channels
For this design iteration study, the spacing between the channels have been altered. Previously, it was set
to 20 𝜇𝑚 while for this study it has been decreased to 5𝜇𝑚 for increased contact and lesser solid material
between the channels. The rest of design geometry have been kept constant. The plots are shown in figures
below:
Figure 12:Geometry and outlet temperatures (both outflows) for decreased spacing between channels
Figure 13:Surface temperature and isosurface contours for decreased spacing between channels
Figure 14:Velocity and pressure distribution for decreased spacing between of channels
Large number of channels
In this design combination, the solid surface has been increased to act as fins or extended surfaces in path
of fluid flow. This can be implemented by increasing the number of tubes and decreasing their width.
Dimensions of tubes have been set to be 20 𝜇𝑚 × 40 𝜇𝑚 with no spacing between them. There are 30
channels in this design configuration. The observed results have been mentioned as in following figures.
Figure 15: Geometry and outlet temperatures (both outflows) for large number of channels
Figure 16:Surface temperature and isosurface contours for large number of channels
Figure 17:Velocity and pressure distribution for large number of channels
Increasing flow area
The area of fluid flow has been increased by incorporating four channels on each side with wider length.
The channels have been made each of width 190 𝜇𝑚 with 10𝜇𝑚 spacing and same height of 40 𝜇𝑚 as
compared to original design. The results generated are as shown below.
Figure 18:Geometry and outlet temperatures (both outflows) for increased flow area
Figure 19:Surface temperature and isosurface contours for increased flow area
Figure 20:Velocity and pressure distribution for increased flow area
Results and Discussion
The focus of this report was to improve and optimize the heat transfer rate from a hot fluid to a cold fluid
in a crossflow heat exchanger through geometrical modifications. Various approaches were used to
optimize the design and were simulated, and their results were compared to the original design in terms
of heat transfer rate and maximum pressure drop.
The results of all the designs, were recorded in Table 3.
Geometry
Coefficient of Heat
Transfer (W/m2K)
Maximum Pressure
Drop (Pa)
Original
3144.3
92.022
Increased number of
channels
3277.3
92.069
Increased channel depth
927.58
64.335
Decreased spacing
between channels
3088.5
92.238
Large number of
channels
1354.4
889.72
Increased flow area of
channels
4118.3
64.730
Table 3:results of design modifications
The table clearly indicates that out of the 5 design geometries, only 2 have yielded the heat transfer
coefficient greater than the original one. Thus, we have selected these 2 approaches.
1. Increased number of channels
2. Increased flow area of channels
By increasing the number of channels in the crossflow heat exchanger, the contact surface area between
the fluid and the walls through which it flows is increased. This results in an increase in the value of heat
transfer coefficient which in return increases the rate of heat transfer, as both Fourier's law and Newton's
Law of cooling are directly proportional to the contact area. This approach has shown to provide an
increase of about 4% from the original model.
Another approach to increase the rate of heat transfer is to increase the width of our rectangular crosssection tube, which also increases the contact surface area between the fluid and the surrounding walls.
Surprisingly, the increase in heat transfer rate through this approach is significantly high compared to the
previous approach, with increase of about 30% from the original model.
Design selection
To ensure a fair and unbiased comparison, all design variations were subjected to the same predetermined
parameters such as cold temperature, hot temperature, and average velocity. Additionally, the
thermophysical properties of both the solid material and liquid water, including density, thermal
conductivity, heat capacity at constant pressure, and dynamic viscosity, were kept constant across all
design variations.
Based on the results and discussion presented, we can finalize one of the two potential designs. The first
option involves increasing the number of channels in the heat exchanger, while the second option involves
widening the cross-section of the channels.
After considering all the available design modifications, we have selected the approach of increasing the
flow area of channels in the heat exchanger. This approach provides a reasonable increase in the rate
of heat transfer while maintaining a reasonable value of associated pressure drop across the channels.
Dimensions
Value (μm)
Length
800
Width
800
Height
120
Channel length
800
Channel width
190
Channel height
40
Total number of
channels
8
Table 4: Final dimensions of selected design
Conclusion
This study thoroughly discusses and analyzes the importance of cross-flow heat exchangers and the need
for design modifications to enhance heat transfer. To achieve this, a comparison was made using
COMSOL Multiphysics between a basic cross-flow heat exchanger and various proposed design changes
that could potentially improve heat transfer. These design modifications included changes in the number
of channels, dimensions of channels, and spaces between channels. All design variations were simulated
with constant initial conditions and material properties. After a detailed comparison and logical reasoning,
it was concluded that increasing the flow area of channels was the most practical and feasible design
modification to enhance heat transfer.
References
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Manuel Velázquez, and P. Miriam Navarrete, Editors. 2021, IntechOpen: Rijeka. p. Ch. 7.
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