1 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA A study on the pressure variation in a ducted heat exchanger using CFD Amresh Gyanathan 1 University of New South Wales at the Australian Defence Force Academy Effective cooling system design is important in the endurance event of the FSAE-A competition. Research into sidepod design and radiator fan optimisation requires further attention. To provide a further understanding on the aerodynamic effects of a sidepod two investigations were performed using CFD. The first investigation identified that high turbulence intensity levels created more uncertainty in predicting the radiator pressure drops. Velocity of the airflow vary around bends in the sidepod. The airflow is accelerated around the convex corners and decelerated around the concave corners. Turbulence levels dissipate slower around convex corners. The deviation of radiator pressure drops obtained using CFD from MUR's experimental results was more significant if inlet flows were more turbulent. The second investigation illustrated that high degrees of curvature in a sidepod may lead to internal flow separation. The latter will occur if the diameter of curvature is too small or the separation length is too large. Validating using data obtained from MUR may not be comprehensive, hence, raising the importance for experimental testing to validate CFD results. RNG k-ε model proved its resilience in all cases simulated. This thesis forms the foundation for future research and design of sidepods in FSAE cars. Contents I. Introduction 2 II. Background Information 2 A. Contribution to cooling system design 2 B. Sidepod and radiator flowfield discussion 3 C. Heat exchanger modelling in FLUENT 4 D. Turbulence modelling in FLUENT 4 III. Methodology 5 A. Effects of inlet turbulence 5 B. Effects of varying curvature 5 IV. Results 6 A. Effects of inlet turbulence 6 B. Effects of varying curvature 7 V. Modification & further results 7 A. Effects of inlet turbulence 7 B. Effects of varying curvature 8 VI. Verification & validation of results 9 A. Effects of inlet turbulence 9 B. Effects of varying curvature 10 VII. Extension to current work 11 VIII. Conclusion 11 Acknowledgements 12 References 12 Nomenclature FSAE-A = Formula Society of Automotive Engineers-Australasia 1 LTA (RSAF), School of Engineering & Information Technology. ZEIT4500/4501.
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A study on the pressure variation in a ducted heat exchanger using CFD
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1 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
A study on the pressure variation in a ducted heat
exchanger using CFD
Amresh Gyanathan1
University of New South Wales at the Australian Defence Force Academy
Effective cooling system design is important in the endurance event of the FSAE-A
competition. Research into sidepod design and radiator fan optimisation requires further
attention. To provide a further understanding on the aerodynamic effects of a sidepod
two investigations were performed using CFD. The first investigation identified that high
turbulence intensity levels created more uncertainty in predicting the radiator pressure
drops. Velocity of the airflow vary around bends in the sidepod. The airflow is
accelerated around the convex corners and decelerated around the concave corners.
Turbulence levels dissipate slower around convex corners. The deviation of radiator
pressure drops obtained using CFD from MUR's experimental results was more
significant if inlet flows were more turbulent. The second investigation illustrated that
high degrees of curvature in a sidepod may lead to internal flow separation. The latter
will occur if the diameter of curvature is too small or the separation length is too large.
Validating using data obtained from MUR may not be comprehensive, hence, raising the
importance for experimental testing to validate CFD results. RNG k-ε model proved its
resilience in all cases simulated. This thesis forms the foundation for future research and
design of sidepods in FSAE cars.
Contents
I. Introduction 2
II. Background Information 2
A. Contribution to cooling system design 2
B. Sidepod and radiator flowfield discussion 3
C. Heat exchanger modelling in FLUENT 4
D. Turbulence modelling in FLUENT 4
III. Methodology 5
A. Effects of inlet turbulence 5
B. Effects of varying curvature 5
IV. Results 6
A. Effects of inlet turbulence 6
B. Effects of varying curvature 7
V. Modification & further results 7
A. Effects of inlet turbulence 7
B. Effects of varying curvature 8
VI. Verification & validation of results 9
A. Effects of inlet turbulence 9
B. Effects of varying curvature 10
VII. Extension to current work 11
VIII. Conclusion 11
Acknowledgements 12
References 12
Nomenclature FSAE-A = Formula Society of Automotive Engineers-Australasia
1 LTA (RSAF), School of Engineering & Information Technology. ZEIT4500/4501.
2 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
CFD = Computational Fluid Dynamics
CAD = Computer Aided Design
OEM = Original Equipment Manufacturers
MUR = Melbourne University Racing
RNG = Renormalized Group
FVM = Finite Volume Method
kL = Pressure loss coefficient
1/α = Viscous inertial resistance factor
C2 = Inertial resistance factor
I. Introduction The FSAE-A competition is organised annually for all competing universities in the Australasia region. The
Academy Racing team is a regular participant of this competition. In this competition there are several events
that allow judges to assess and rank teams accordingly. The endurance event is assessed by pushing the car to its
limits by surviving 34 laps of the autocross track. This event forms 35% of the overall score and is of paramount
importance for all competitive teams [1]. In the 2010 FSAE-A competition the Academy Racing team placed
7th overall and 8th in the endurance event [2]. To improve this score, further design considerations can be
implemented into improving the cooling system of the car. In the FSAE-A competition in 2010, cost
optimization of the cooling system was part of the cost judging criteria. This was included in the combined score
awarded to the participating teams. In the 2010 FSAE-A competition, Academy Racing's WS10 car achieved a
score of 71.2 out of 100 [2]. This result can also be further improved upon. Thus, improving the cooling system
design will be necessary to perform better at subsequent FSAE-A competitions. This project aims to provide an
understanding into the aerodynamic considerations associated with sidepod flows, thus, contributing in part to
the overall cooling system design for the Academy Racing team's car.
II. Background Information
A. Contribution to cooling system design
In a FSAE cooling system design, three main aspects are usually considered. The first aspect is identifying
the heat loss and generation coefficients for each component in the cooling system and integrating them in a
heating circuit. The rate of heat generation and loss of various components in the cooling system have to be
studied along with the coolant flow rates under different conditions. Test driving under adverse atmospheric and
lap conditions may be conducted to give a worst-case scenario that accounts for factors such as slipstream of
leading cars, enforced slow running at high engine power, etc. This test drive will provide the measurements for
a thermally stable lap of the race-circuit, whereby total engine heat generated in one lap is equal to the total heat
dissipated by the radiator in that same lap.
The second aspect is to study the heat transfer characteristics of the radiator. This has to be done by both
experimental testing and CFD analysis for more complex experimental procedures. data obtained from the
former will be used to validate the latter. Coolant of various operating temperatures, pressures and velocities are
also modelled to identify the heat transfer characteristics of the radiator.
The third aspect is the integration of a radiator into a duct and testing the aerodynamic performance of it.
The design of the duct will factor in considerations such as varying cross-section areas throughout the duct,
varying degrees of curvature of the duct, varying inlet and outlet sizes and their locations with respect to the
aerodynamic interaction with the rest of the car. Test data can be used to validate the CFD analysis of this
ducted flow. In motorsport, such ducts are often termed as sidepods and are either situated front-on (ram air
ducted) or side-on (side air ducted) [3]. This project produces an end state that contributes towards the design of
an FSAE cooling system. It should be noted that such a contribution is made in part of the aerodynamic study of
ducted heat exchanger flows; one of the aspects of the FSAE cooling system design.
B. Sidepod and radiator flowfield discussion
The FSAE car is a racing car of a rear engine design. As such, the best ways to fit the radiator with the rest
of the car would be to have it mounted above the engine or in front of the engine. However, having the radiator
at these two locations comes with severe disadvantages. For a radiator situated above the engine, there is an
absence of consistent flow of air through the radiator as the region just above the engine has negligible airflow
(dead-air). This results in ineffective heat dissipation by the radiator. Furthermore, if the radiator is located at
the front of the engine (i.e. side of car or front of car), the acceleration and deceleration of the car will disrupt
the flow in the cooling ducts, thus, making it more intermittent. This is due to the inertia of the moving coolant
3 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
failing to accelerate or decelerate in time. This leads to other problems associated with intermittent pipe flows
(e.g. cavitation of coolant, coolant backflow, etc.). Sidepods are designed to overcome these problems. The
design of sidepods will be able to provide a dedicated air intake for heat dissipation of the radiator [4].
Radiator efficiency is affected by the pressure variation across it. This is in turn determined by the velocity
of the airflow through it. The sidepod has an overall pressure difference between the ends (i.e. inlet and outlet).
This pressure difference varies during different external flow conditions. If there is a pressure drop large enough
across the sidepod, then there will be a flow velocity through the radiator that maximises its heat dissipation. If
the pressure difference across the sidepod is not large enough, there will be inadequate airflow through the
radiator. To overcome this, a radiator fan can be modelled to provide the necessary airflow through the radiator
in all stages of operation. The radiator fan provides the necessary driving force by creating a pressure gradient
that pushes or pulls airflow through the radiator even in adverse situations. [5] states that equation 1 can be used
to compute the pressure drop across the radiator solely by considering the pressure loss coefficient, kL, and
normal airflow velocity on radiator surface, v.
(1)
It is found that a large kL value and a large increase in airflow velocity from the inlet of the sidepod to the
plane surface of the radiator produce the best cooling performance [6]. The latter can be achieved by intelligent
optimisation of the sidepod geometry by creating regions that speed up the airflow. [6] also states that the inlet
of the sidepod should not be made to be of a converging nature. This is to prevent the generation of backflow
and vortices upstream of the sidepod and flow separation on the exterior near the inlet of the sidepod. The speed
of natural airflow at the inlet of the sidepod is limited by the velocity of the race car along the autocross track. In
a typical track, the front velocity of the car is not expected to exceed 100 km/h, owing to the lack of straights in
the track [7]. As such the airflow at the inlet of the sidepod does not exceed 30 m/s, and hence, is
incompressible. This allows for commonly known equations (i.e. steady flows, Bernoulli’s equation &
continuity equation) to be used and model the airflow of a FSAE sidepod.
Sidepods often have varying degrees of curvature and cross-section areas to allow for optimised airflow
through the radiators. By varying the inlet, duct and outlet geometries of the sidepod, the internal airflow can be
studied and used to a design engineer’s advantage [5]. To achieve best cooling performance, a higher air mass
flow rate into the sidepod has to be obtained. However, from the perspective of vehicle aerodynamic drag, a
minimum amount of airflow should be diverted from the main flowfield around the car into the sidepod [8]. [8]
also mentions that considerations have to be made for outlet geometries of sidepods to prevent the loss of
downforce on the car. Outlet airflow design should also be considered as this airflow must not increase the
pressure of the low pressure region underneath the car to an unacceptable level.
The free-stream natural airflow entering a FSAE car sidepod is disturbed by several effects. One of them is
the turbulence generated from the rotation of the front wheels. The front wheels of the car of the Academy
Racing team are situated just to the front of the inlet of the sidepod. The effect of a rotating wheel on the inlet
turbulence of a side-pod varies with the velocity at which the free stream airflow is acting on the rotating wheel
[9]. This leads to a variety of inlet turbulence generation at the inlet. This can be modelled in ANSYS FLUENT
and further details can be found in the turbulence modelling section.
C. Heat exchanger modelling in FLUENT
The Academy Racing team uses a Borland Racing radiator used previously in the Formula Ford race cars.
No data for this radiator or other similar types of radiators within the same class could be obtained readily from
OEM. This means the only way to obtain heat transfer and pressure loss coefficients would be from the
experimental testing of the Borland radiator. From this testing, we can identify the pressure loss coefficient, kL,
and heat loss coefficient, h. Using equation 1, the pressure variation can then be estimated for a variety of
velocities. A quadratic curve of best fit can then be used to model the pressure variation distribution to be
analysed for porous media parameters. Section 7.2.3.6.11 of the ANSYS FLUENT user guide recommends the
use of two equations, 7-2 and 7-25 of the user guide (reflected here as equations 2 and 3 respectively) [10].
These two equations, when combined, culminate in equation 4. In these equations, Si represents the momentum
source term, Δn represents the radiator thickness, represents the viscous inertial resistance factor and C2 is
the inertial resistance factor. The coefficients found from the quadratic curve of best fit can now be used to find
and C2, which are used as inputs for the porous media region.
(
| | ) (2)
(3)
4 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
Figure 1. Fourth order
polynomial fit for kL from
MUR.
Figure 2. Extrapolation
of polynomial.
Figure 3. Power-law model
for kL versus velocity
showing conformal to trend
obtained from experimental
results.
Figure 4. Plot of
quadratic curve of best-fit
using cftool function in
MATLAB.
(
| | ) (4)
The FSAE team from MUR has performed experimental testing on their car radiator (similar class of
radiators) and have obtained a fourth-order polynomial fit for their results as shown in figure 1 [6]. Seeing that
the operating range of a FSAE race car exceeds a forward velocity of 5 m/s, This polynomial fit was then
compared for a larger velocity range to ensure that it would follow a similar trend to the experimental results.
This extrapolation of kL is found in figure 2.
From figure 2, it can be seen clearly that for
velocities after 6 m/s the fourth-order
polynomial fit does not follow a similar trend
as the experimental results. In order to obtain
a curve of best-fit that adheres to the trend of
the experimental data, a power-law model
was used. This model is found in equation 5.
This was substituted into equation 1 and a
series of loss coefficient values, kL, were
plotted against the normal velocity. These
values
(5)
(6)
allowed for a quadratic curve-fit to be used
and this is expressed in equation 6. This
quadratic expression is modelled to a 95%
confidence level using a MATLAB function
called cftool. The coefficients of equations 6
were then compared with those obtained from
equation 4 to find and C2, which will
then be used as the porous media inputs. This
comes from assuming that the radiator is
fitted tightly around the walls of the sidepod,
such that all airflow must flow through the
radiator, that the operating pressure was
assumed to be at 101 325Pa (1 standard
atmosphere) and that kL observed the same
trend for all velocities. Figure 3 shows the
power law approximation of kL versus velocity, while figure 4 shows how p is modelled via the cftool
function.
D. Turbulence modelling in FLUENT
The surroundings of the sidepod is exposed to turbulent
flows. Turbulence intensity captures the level of turbulence
in such flows. It represents the ratio of the root-mean-
square of velocity fluctuations to the mean flow velocity
[11]. Airflow through a FSAE car sidepod resemble that of
a low speed flow through large pipes. As such, the
turbulence intensity of such a flow normally does not
exceed 5% [12]. Additional consideration should also be
made for the external turbulence generated by the front
wheels and ground effects around the sidepod inlet when
performing CFD analysis. Sidepods have varying degrees
of curvature depending on the design requirements. As
such, a turbulence model that is most suitable for high
curvature flows is chosen. The RNG k-ε model was found
to be most suitable for such flows [13]. Despite the
effectiveness of the RNG k-ε model for high curvature
flows, a backup model will be considered as a precaution
when the former fails to provide a solution that converges
to an acceptable value. It is also recommended that the realizable k-ε model is more resilient for flows
undergoing high mathematical constraints [14][15]. This model will be used when the RNG k-ε model fails.
Figure 5. Coarse mesh showing
upstream (green), radiator (blue) and
downstream (pink) zones.
Figure 6. Sliced section of mesh showing
upstream, radiator and downstream zones
having a conformal mesh.
5 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
Figure 10. Velocity magnitude (left) and absolute pressure (right)
contour plots for the case of 30 m/s and 10 % velocity and
turbulence intensity respectively.
III. Methodology
A. Effects of inlet turbulence
It is evident that sidepod inlet turbulence and velocities vary during different stages of the
autocross/endurance events. Hence, it was pertinent to conduct an investigation into the airflow effects of
varying sidepod inlet turbulence intensities and velocities. Pressure variations, velocities and turbulence
intensities were measured for various regions downstream of the sidepod. A general design of a sidepod was
created from CATIA and mesh in ANSYS. Adopting the FVM, the initial coarse mesh created had 36, 960
nodes and 34, 338 elements. Figures 5 and 6 show this mesh created and a cut section through the mesh.
Consideration was made to
ensure that the different zones
of the mesh were conformed to
having identical faces along
zone interfaces. This promoted
better solution convergence
rates and more accurate results.
After which, the mesh was
imported into FLUENT. The
inlet conditions of the sidepod were varied
according to four different velocities (5, 10, 20,
30 m/s) and turbulence intensities (1, 3, 6, 10
%) respectively. Temperature variation across
the duct was not measured and as such no pre-
formulation of temperature related coefficients
was necessary. The sidepod pressure variations were not measured, as the results obtained from the CFD
simulations will show that the conditions that were simulated were of an internal flow nature without any
external influences. External influences are crucial in dictating the pressure drop across the sidepod.
B. Effects of varying curvature A second investigation was performed to explore the effects of varying sidepod duct curvature on the
velocity flowfield and pressure variations along the sidepod. With this, a generic design was created using
CATIA similar to the first investigation. In this sidepod design, an S-bend was included upstream of the design
before the airflow reaches the radiator. The dimensions of the
S-bend include the curvature diameter, which was varied for
400mm, 500mm and 600mm, and displacement length, which
was varied for 100mm and 200mm. This setup can be found
in figure 7. It can be seen that the smaller the diameter of
curvature, the tighter the S-bend will be. This will cause
further complications in airflow and will create more
distinctive velocity, pressure and turbulence intensity
variations along the sidepod. The displacement length
determines if the airflow flows through a sharp bend over a
longer distance or a shorter
distance.
As with the first investigation,
a FVM mesh was generated
ensuring zone interface
conformity. During the meshing
phase, an initial coarse grid mesh
was performed to gather a
generalised understanding of the
flowfield properties through the
sidepod. This would then provide the basis for a refined mesh generation to further elaborate the results obtained
from the coarse mesh. The mesh was then imported into FLUENT and the approach taken to set up the controls
and parameters is similar to that of the previous investigation. The sidepod inlet conditions were varied as
shown in figure 8. As with the first investigation, temperature effects and the overall pressure differences were
ignored.
Figure 9. Longitudinal section plane
creation using FLUENT plane tool.
Diameter
Diameter
Length
Figure 7. CAD model (left) designed with S-bend and CAD sketch
(right) showing how the S-bend was varied.
400, 500, 600 100, 200 1, 10 20, 30 24
No. of
cases
Diameter of
curvature (mm)
Separation
length (mm)
Inlet turbulence
intensity (%)
Inlet velocity
(m/s)
Figure 8. Table of parameters used for varying cases.
6 A study on the pressure variation in a ducted heat exchanger using CFD, UNSW@ADFA
Figure 11. Turbulence intensity contour plots at different
locations of the sidepod. Sidepod outlet (left-most), radiator
outlet (2nd
from the left), radiator inlet (3rd
), cross-section plane
at bend (2nd
from the right) and sidepod inlet (right-most) are
reflected. The convex corner of the cross-section plane is
located at the bottom, while the concave corner is located on
top.
Figure 14. Turbulence intensity contour plots along
longitudinal sections at 10% inlet turbulence intensity and
30m/s inlet velocity. Top left plot is for the case of D=400mm,
L=200mm; bottom left is for the case of D=400mm,
L=100mm; top right is for the case of D=500mm, L=200mm;
and bottom right is for the case of D=600mm, L=200mm.
IV. Results
A. Effects of inlet turbulence.
Upon compiling the results of the simulations, the absolute pressure values were found to vary across the
entire cross-section. This comes about due to the presence of velocity changes throughout the interior of the
sidepod. A longitudinal section was taken along the length of the sidepod and along the length of the cross-
section face. Figure 9 shows how this longitudinal section was created. Using this longitudinal section, contour
plots of velocity magnitudes and absolute
pressures were found. These are
expressed in figure 10. It can be seen in
figure 10 that the velocity increases
around the convex (inner) corners and
decreases around the concave (outer)
corners. This can be likened to a
meandering river where the flow of the
water is always fastest on the inside of a
bend owing to the path of least flow
resistance. Since the flow is
incompressible, simple Bernoulli’s
principle can be applied. This leads to a
lower static pressure in the regions of
higher velocities and vice versa. The
values of static pressure outweigh
those of dynamic pressure (a function
of the square of velocity) and hence,
this leads to the absolute pressure
contours as shown in figure 10. Figure
10 shows the contour plots for the
specific case of 30 m/s and 10% inlet
velocity and inlet turbulence intensity
respectively. This can be used to a
cooling system designer’s advantage
by re-orientating the radiator to
receive faster oncoming airflow across
most of the cross section. Moreover,
figure 10 shows insufficient grid
resolution near the boundary layer
region at the sidepod walls. This will
be rectified in the modifications
section where a refined grid mesh will
be used.
Figure 11 shows the turbulence
intensities at various cross-sections for
the case of 10% inlet turbulence
intensity and 30 m/s inlet velocity.
Using FLUENT plane tool, a cross-
section profile was created at the bend
closer to the radiator. Figure 11 also
incorporates this profile with that of
the sidepod inlet, radiator inlet,
radiator outlet and the sidepod outlet.
From this figure, the progressive
variation in turbulence intensities
downstream from the sidepod inlet can
be observed. From figure 11, it can be
found that the turbulence intensity
started off at 10% at the sidepod inlet.
By the time the flow reaches the cross-
section plane, it has dropped to a
Figure 12. Velocity contour plot for the cases of
curvature diameter=500mm, inlet velocity=30m/s,
turbulence intensity=10%.Left plot shows the case for
separation length=100mm, right plot shows for
separation length=200mm.
Figure 13. Velocity contour plots for the case of curvature