The present work investigates the numerical simulation of thermal analysis of mixed convection air flow in a CPU Cabinet. The simulation is focused on the non-uniformly heated mother board temperature distribution. In the present work three cases have been studied, 1) Placing the CPU in vertical position, 2) Placing the CPU in horizontal position and 3) Providing exhaust fan on top. The work also includes studies of effectiveness of different inlets provided. The temperature distribution of the components and streamlines were investigated in order to get a clear picture of which case is more effective for cooling of the mother board

1. Proceedings of the 2nd
International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
167
THERMAL ANALYSIS OF AIR FLOW IN A CPU CABINET WITH
MOTHERBOARD AND HARD DISK AS HEAT SOURCES
SUBHAS. L. HUNASIKATTI1
, SUNEEL. M. P2
, P. S. KULKARNI3
, G. S. SHIVA SHANKAR4
1, 4
Department of Mechanical Engineering, S.I.T, Tumkur, Karnataka, India – 572103
2, 3
Department of Aerospace Engineering, IISc, Bangalore, Karnataka, India – 560012
ABSTRACT
The present work investigates the numerical simulation of thermal analysis of mixed convection air flow in a
CPU Cabinet. The simulation is focused on the non-uniformly heated mother board temperature distribution. In the
present work three cases have been studied, 1) Placing the CPU in vertical position, 2) Placing the CPU in horizontal
position and 3) Providing exhaust fan on top. The work also includes studies of effectiveness of different inlets provided.
The temperature distribution of the components and streamlines were investigated in order to get a clear picture of which
case is more effective for cooling of the mother board. The simulation was carried out using a standard commercial CFD
code-ANSYS-Fluent. It is found that horizontal position results in reduction of motherboard average temperature of 0.1
0
C as compared to vertical position. It is also observed that bottom opening has very less effect on motherboard
temperature.
Keywords: CPU Cooling, Electronic Cooling, Forced Cooling of Electronic Devices and Mixed Convection.
1. INTRODUCTION
Every system has a critical temperature and it is very crucial to avoid reaching this critical temperature. When
passing the manufacturer's critical temperature, the system starts to fail or even make the system to work off. As a result,
the heat transfer in the electronic systems must be examined before designing the application [1]. There are many ways in
which possible air-cooling design can fail; inadequate coolant flow, poor flow distribution, poor mixing and low heat
transfer coefficient or power dissipation higher than the expected. To analyze these types of causes CFD approach is
good [2]. The cooling of electronic systems is essential in controlling the temperature and avoiding any hot spots.
Cooling of electronic equipment is one of the most important factors to be considered in designing electronic devices and
in making them energy efficient. Desktop computers are one among the most commonly used devices in the world.
Hence making the desktop computers, energy efficient can effectively reduce the electricity consumption.
The power consumption due to exhaust fans being used in the cabinets becomes significant when considered in
mass. The maximum amount of heat dissipation in the CPU cabinet is because of processor, chipset and hard disk drive.
Many researchers have studied heat transfer effects around the electronic devices. T. Y. Tom Lee, used the CFD
technique to evaluate the temperature and velocity fields of air flow in a computer system enclosure [3]. Yu and Webb
have mentioned total cabinet power dissipation being 313 W [4]. They have also given the details of power dissipation
from individual components such as Memory, Chipset, AGP and PCI cards.
Another computational study has been carried out by Konstantinos-Stefanos P.NIKA’S and Andreas D.
PANAGIOTOU [1]. In this study the investigation has been made regarding the air flow circulation, and the temperature
distribution profiles occurred over non-uniformly heated printed circuit board. Based on the position, geometrical
characteristics and the number of fans have been altered to study the thermal effects. In their study the inlet fans and their
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 9, September (2014), pp. 167-173
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2. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
168
geometrical characteristics have been altered in order to identify which is the best case that produced the maximum heat
transfer results [1]. In past studies some people have tried with structured grid for their simulation [3, 5] only a few of
them have used unstructured grid [1]. The structured mesh has been given better results as compared to unstructured
mesh. Different turbulence models have been tested and compared [1] [6], for these kind of problems k-ɛ Turbulence
model produces the best approach as far as the comparison between numerical and experimental data [1].
From the literature survey it is found that there are no studies on the heat dissipation and fluid flow variations
inside the CPU cabinet in vertical and horizontal positions and there are no studies on which inlet opening is more
effective for cooling of the motherboard. This is the motivation of the present work.
In the present work, we have considered the 3-D, steady state, mixed convection air cooling of a non-uniformly
heated mother board inside the cabinet. Heat dissipation from the mother board and hard disk drive have been studied.
The simulations are carried out using industrial standard commercial CFD code ANSYS-Fluent.
2. PROBLEM DESCRIPTION
The objective of the present work is to analyze the airflow pattern in a CPU Cabinet and temperature
distribution of motherboard for different orientation and different location of openings. The problem essentially needs
geometry simplification to reduce the complexities which may arise in grid generation. It is also required to use
multiblock approach for grid generation since the problem involves many heat sources and boundary conditions. The
orientation of the CPU Cabinet will be altered in order to identify which orientation is the best case that produced the
maximum heat dissipation. The inlet opening of the CPU-Cabinet will be altered in order to identify which inlet opening
is more effective for cooling of the mother board. The detailed geometry is shown in Fig.1.
Figure 1: Basic Geometry of CPU Cabinet
3. NUMERICAL METHODOLOGY
ANSYS-FLUENT 14.5 commercial package software has been used for simulation. The current study uses the
finite volume method and the equations solved are the continuity, momentum and energy equation. From the past studies,
we have considered k-ε is the best turbulence model for this kind of problem as compared to the other turbulence models
and the same has been used in the present work.
The present study is performed under steady state conditions and air is assumed as incompressible. Boussinesq
approximation is used to account buoyancy. SIMPLE algorithm is used for pressure-velocity coupling.
4. GOVERNING EQUATIONS
RANS (Reynolds-averaged Navier-Stroke) equations are solved for steady state flow. The equations for
continuity, momentum and energy are shown below. Where equations 1 is the continuity equation and equations 2, 3 and
4 are the momentum equation in x, y and z direction respectively and the equation 5 is the energy equation:
= 0 (1)
(2)

3. Proceedings of the 2nd
International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
169
(3)
(4)
(5)
Where in equation 3 is a source term which takes gravity effects into account.
5. BASIC ASSUMPTIONS
In order to simplify the present work the following basic assumptions made are.
• The motherboard is considered as non-uniformly heated.
• Radiation heat transfer is neglected because of the domination of forced convection and relatively low
temperature differences inside the cabinet.
• The surrounding air pressure and temperature are considered as 1 atmosphere and 350
C.
• Mother board thickness is considered as 5mm.
• Conduction through the cabinet wall is neglected.
• CPU total heat dissipation is taken as 80 W/m2
.
6. BOUNDARY CONDITIONS
The exhaust fan and processor fan are 80 mm in diameter. As per the manufacturer specifications the velocity of
the 80 mm diameter fan is 2.85 m/s. The cabinet is simulated as zero wall thickness, but only the wall-faces to form the
boundary with no slip condition. The maximum amount of heat dissipation from the CPU is 80 W and the chipset is 10
W. The hard disk drive dissipates 20 W of heat.
7. GRID DEPENDENCE STUDY
A grid dependence study has been carried out successfully based on the following parameters. Motherboard
average temperature, Exhaust fan average temperature, velocity; SMPS exhaust average temperature, velocity and inlet
average velocity.
Figure 2: Mesh Generated by ICEM CFD
In the present case, the multiblock structured grid has been used. Total 9 different grid was tested, out of them 4-
cases with first element height from the wall is 0.5mm and 5-cases with first element height from the wall is 0.1mm with
growth rate at 1.2.

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30
31
32
33
0 5 10 15 20 25AverageTemperaturein
0C Number of Elements in Lakhs
0.5mm first element
height
0.1mm first element
height
Figure 3: Variation of Average Motherboard Temperature for Different Grid Size
The cells were denser in the region near to the boundary surfaces and coarse in the middle region of the domain.
Finally the 5-Lakh element denser grid has been used for further study, since it was found to take less computational
space and time. Fig.3. shows the variation of average motherboard temperature for different grid size. It can be seen that
there will not be much change of the results after increasing the mesh elements above 5-Lakhs.
8. RESULTS AND DISCUSSION
In this work three different cases have been studied by placing the CPU in vertical and horizontal orientations,
the details of the cases studied are listed in TABLE 1. Temperature distribution and corresponding streamlines are used
for the thermal analysis of CPU cabinet.
Table 1: Analysis of Simulated Cases
Cases simulated Description of each case
A Simulation of CPU Cabinet-Vertically oriented.
B Simulation of CPU Cabinet-Horizontally oriented.
C Simulation of CPU Cabinet-Modified based on analysis of case A and case B.
8.1 Case A: Simulation of CPU Cabinet with vertical orientation
The first case refers to simulation of CPU Cabinet with vertical orientation. In general, a Cabinet has two fans,
exhaust fan and processor fan. One is located at the back side of the cabinet and the other one is placed over the
processor heat sink as fluid outlet. The temperature distribution over the surface of the motherboard is illustrated in fig. 4.
The simulation showed that the maximum temperature is up to 65.9 0
C and it is concentrated over the center of the
chipset, around 55.30
C temperature takes place at the center of the processor, this is due to the maximum amount of heat
dissipation is from the processor and chipset. The overall average temperature of the motherboard is 37.7 0
C. The stream
lines shown in Fig.5. is the clear picture about how the air is moving inside the cabinet and also it shows the variation of
motherboard temperature due to variation of local air velocity and temperature.
Figure 4: Temperature distribution contours for Figure 5: Air streamlines for vertical orientation
vertical orientation

5. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
171
8.2 Case B: Simulation of CPU Cabinet- horizontal orientation
The second case refers to the simulation of CPU Cabinet with placing it in horizontal orientation, keeping all
other parameters same as that of case A except acceleration due to gravity in the negative x-direction. The temperature
distribution over the surface of the motherboard is shown in Fig.6. The simulation showed that the maximum temperature
of the chipset is around 63.30
C and that of motherboard is around 55.30
C. Again, this is due to higher amount of heat
dissipation from the chipset and motherboard as compared to other parts (Hard disk drive, DVD, etc.). The overall
average temperature of the motherboard is 37.6 0
C.This shows that 0.10
C less than that of case A, in this case the
maximum amount of fresh air will contact over the surface of the motherboard (refer Fig.7) and also case B shows better
cooling of the motherboard than case A.
Figure 6: Temperature distribution contours Figure 7: Air streamlines for horizontal orientation
for horizontal orientation
8.3 Case C: Simulation of CPU Cabinet-vertical orientation and changing exhaust fan location
In case A and B, the maximum quantity of hot air after hitting the surface of the motherboard and hard disk
drive will move towards the top portion of the cabinet. So we think that instead of placing an exhaust fan at the back side
of the cabinet, placing it at the top portion of the cabinet then the fresh air comes in giving better cooling effect. The
temperature distribution and airflow inside the cabinet in this case is illustrated in Fig.8 and Fig.9 respectively. The total
average temperature of the motherboard that can be obtained is 37.7 0
C.
Figure 8: Temperature distribution contours Figure 9: Air streamlines for exhaust fan on top
for exhaust fan on top
8.1.1 Effectiveness of left side inlet in CPU Cabinet
In the above simulated cases the left side inlet is mainly helpful for entering fresh air into the cabinet. When the
fresh air enters into the cabinet it will directly hit over the surface of the motherboard and extract the heat from the
mother board after that it will spread in different location of the cabinet and finally it will move out through the exhaust

6. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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fan. The Fig.10 shows the distribution of air inside the cabinet. These stream lines show that the left side inlet is mainly
helpful for cooling of the motherboard, not much affecting for cooling of the hard disk drive.
Figure 10: Air streamlines for left inlet opening.
8.1.2 Effectiveness of bottom inlet in CPU Cabinet
As like left side opening the bottom side opening is also helpful for entering fresh air into the cabinet. When the
fresh air enters into the cabinet some portion of the air will hits the bottom surface of the hard disk drive and extract the
heat from that surface, After hitting the surface of the hard disk drive the surface gets cooled, air extract the heat from
that surface, air becomes hot. There is no contact between air and motherboard. Finally the hot air moves out through
the exhaust fan and processor fan. The fig. 11 shows the distribution of air inside the CPU Cabinet. These stream lines
show that bottom side inlet is mainly helpful for cooling of the hard disk drive not much affecting for cooling of the
motherboard. The comparison of temperature at different locations for all simulated cases is shown in TABLE 2.
Fig. 11: Air Streamlines for Bottom Inlet Opening
Table 2: Different Components Average Temperature at Different Cases
Variables Case A*
Case B*
Case C*
Processor average temperature 48.4 0
C 48.5 0
C 48 0
C
HDD average temperature 35.2 0
C 35.20
C 35.20
C
Chipset average temperature 62.60
C 62.40
C 64.40
C
Motherboard average temperature 37.70
C 37.60
C 37.70
C
Processor fan exhaust average temperature 35.30
C 35.30
C 35.30
C
Exhaust fan average temperature 35.20
C 35.20
C 35.30
C
(*A, B, C - refer table 1.)

7. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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CONCLUSION
This case study demonstrates the capability of computational fluid dynamics software in predicting flow field
and heat transfer in an air-cooled computer system. The conclusions from the present case study are:
• Horizontal case shows average motherboard temperature 0.1 0
C, chipset average temperature 0.20
C less than that
in the vertical case. The study suggests less fan power requirements in case of horizontal position as compared to
vertical position.
• Left side inlet is mainly helpful for cooling of the motherboard and not much affecting for cooling of the hard disk
drive. Bottom side inlet is mainly helpful for cooling of the hard disk drive and not affecting for cooling of the
motherboard.
• Top exhaust fan case shows average motherboard temperature is same as in the vertical case and 0.1 0
C more than
that in the horizontal case; processor average temperature is 0.40
C less than that in the vertical case and 0.50
C less
than that in the horizontal case; chipset average temperature 0.20
C less than that in the vertical case and is same as
that in horizontal case. The study suggests less fan power requirements in case of a top exhaust fan as compared to
the existed a vertical case and more fan power requirements in case of a top exhaust fan as compared to the
existed horizontal case.
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