Numerical Investigation of Forced Convection cooling of Electrical enclosure using CFD

Lakshminarasimha. N Int. Journal of Engineering Research and Applications www.ijera.com
ISSN: 2248-9622, Vol. 5, Issue 11, (Part - 5) November 2015, pp.62-66
www.ijera.com 62 | P a g e
Numerical Investigation of Forced Convection cooling of
Electrical enclosure using CFD
Lakshminarasimha. N., IGBC AP
Assistant Professor, Department of Mechanical Engineering, MVJ College of Engineering, Bangalore, India
Abstract
Electrical enclosures consist of high heat generating electrical components, so removal of heat generated
remains as our primary aim. To achieve this, cooling the electrical equipment is always an economical and
optimum solution to keep the electrical components to their operating temperature limits. Placing the cooling
components in the enclosure is another important parameter to be considered. This parameter can be judged
using a simple CFD analysis.
Therefore in the present work CFD simulation has been carried out by considering a typical Aluminum
Electrical enclosure of volume (300mm X 300mm X 300mm) with total internal heat dissipation of 150W. With
those values into consideration the surface area of enclosure, enclosure temperature rise, air flow requirement in
an enclosure is calculated and based on which the fan is selected.
Keywords- Enclosure, cooling, temperature rise, CFD
Terminology
CFD- Computational Fluid Dynamics
CFM- Cubic Feet per minute
∆T- Temperature rise
3D- Three dimensional
TSA- Total surface area
I. Introduction
Any electrical system without an enclosure is
incomplete. As enclosure protects the electrical
components from environmental hazards and it helps
to provide safe cooling mechanism for electrical
components. Any Enclosures can be cooled through
following cooling mechanisms: Natural convection,
Forced convection and closed loop cooling.
Electrical enclosure cooling becomes a necessity
because research has shown that enclosure
temperature rise on every 100
C rise above normal
room temperature decreases life of electrical
components and its reliability is cut by half [1]
. Hence
maintaining enclosure temperature rise becomes a
preliminary criterion.
Present work deals with both flow and thermal
analysis using FLUENT (CFD) on an electrical
enclosure consisting heat generating source
dissipating heat of 150 W. Study comprises on
calculating surface area of enclosure, Internal
temperature rise in an enclosure, air flow
requirement, selecting a fan for an enclosure,
determining maximum velocity and temperature
through numerical simulation and graphical
representation of the results.
CFD is a powerful tool for investigating complex
internal flow problems and in predicting the flow and
temperature in an enclosure and representing results
through color post-script.
II. Literature Survey
Literature survey has been conducted based on
available journals and Industrial data sheets.
Summaries of few important surveyed literatures are
as below:
Hoffman, Pentair Company, [1], [2003], this
technical manual is a ready reckoner for designing an
electronic enclosure. Also this manual is helpful for
engineers in preliminary design stage of any
electronic enclosure in evaluating any kind/type of
design aspects.
MahendraWankhede, et al, [2], [2010], Paper deals
with CFD analysis of Aluminium enclosure.
Enclosure consists of 100W heat generating PCBs.
Paper concludes that use of internal fans reduces
enclosure internal air temperature by 20- 25%
compared to enclosure with no fans.
Through the Literature survey, found that many
literatures are available on cooling of Chip and PCBs.
However from the available resource and data’s,
effort has been made to carry out this work.
III. 3. Methodology
ANSYS products have flexibility in modeling,
meshing and analysis, all combined in single
software. Present analysis work carried using
ANSYS FLUENT.
CFD process involves following steps:
i. Preprocessing- involves Geometry, Meshing
and Boundary conditions
RESEARCH ARTICLE OPEN ACCESS

ii. Solver- involves Discretization by
substitution and solution for algebraic
equation
iii. Post processing- involves color postscript
Similarly the present analysis on electrical enclosure
has been carried out following above steps
3.1 Modeling
Figure 1, shows the 3D model of electrical enclosure
consisting Inlet, Exhaust fan/outlet and Heat block.
The enclosure is of volume 300mm X 300mm X
300mm. Inlet is provided at mid of the front wall and
exhaust fan is at mid of rear wall. The enclosure
material is aluminum.
Figure 1: Electrical enclosure
3.2 Meshing
Figure 2 shows the hexa mapped mesh. Model has
been meshed with 13512 elements and 14902 nodes.
Figure 3 shows mesh convergence plot.
Figure 2: 3D Mesh model
Figure 3: Mesh convergence plot
3.3 Boundary Conditions
Before applying Boundary conditions to the domain
few parameters need to calculated and is followed
below:
3.3.1 Calculating TSA of enclosure
Enclosure size: A x B x C= 300 mm X 300 mm X
300 mm
In feet,
TSA= 2(A x B) + 2(A x C) + 2(B x C)
TSA= 5.81 ft2
3.3.2 Calculating air flow requirement in CFM and
Fan selection
Enclosure internal heat dissipation is 150W. Hence
from Figure 4, for 150 W heat dissipation: 4”
fan is
selected and enclosure ∆T is 6.50
F (30
C).
Figure 4: Fan selection chart [1]
Air flow requirement in CFM [1]
= (3.16 x heat
dissipated in Watts) / Temperature rise in 0
F

Therefore, Air flow required = (3.16 X 150)/ 6.5 =
72.92 CFM
Considering 25% safety margin [1]
,
= 1.25 X 72.92= 91.15 CFM= 92 CFM
Hence, CFM required for 4” fan is 92 CFM
Now Boundary conditions are applied based on
above calculations:
Outlet: Exhaust fan with 92 CFM
Wall: Stationary and No slip condition
Block: Heat load of 150W
Thermal Condition: Coupled
3.4 Solution Controls
Flow is considered to be steady and turbulent.
Turbulent model selected is standard K- epsilon
model hence two equations: turbulent Kinetic energy
and turbulent dissipation rate are solved. SIMPLE
discretization scheme is used for solving momentum
and turbulent equations.
3.5 Mathematical Models
Fluent code solves governing equations for each
and every cell in the domain. The governing
equations are:
i. Mass conservation equation
ii. Momentum conversation equation
iii. Energy conservation equation
Since flow is turbulent, another equation is solved
that is K-epsilon model, it consists of two equations:
turbulent Kinetic energy (eqn. a) and turbulent
dissipation rate (eqn. b)
∂
∂t
ρk +
∂
∂xi
ρkui =
∂
∂xj
μ +
μt
σk
∂k
∂xj
+ Gk +
Gb − ρϵ − YM + Sk ……….. (eqn. a)
And
∂
∂t
ρϵ +
∂
∂xi
ρϵui =
∂
∂xj
μ +
μt
σϵ
∂ϵ
∂xj
+
C1ϵ
ϵ
k
Gk + C3ϵGb − C2ϵρ
ϵ2
k
+ Sϵ …… (eqn. b)
Where, turbulent or eddy viscosity, μt
= ρCμ
k2
ϵ
and
Gk& Gb represents the generation of turbulence
kinetic energy due to mean velocity gradients and
buoyancy. YM represents the contribution of the
fluctuating dilation in compressible turbulence to the
overall dissipation rate [5]
.
IV. Discussion of Results
The results obtained from the analysis are
graphically represented through velocity and
temperature contours plot which helps us to know
maximum flow velocity and maximum temperature
zones in an enclosure. The results are discussed
below:
a) Velocity contour
Figure 5 shows the cut plane velocity contour.
From the figure it is clear that the maximum velocity
is found near inlet. The maximum velocity was found
to be 36.51 m/s.
b) Temperature contour
Figure 6 shows the Temperature contour cut
plane. Maximum temperature was found at
surrounded surface of heated block and its value
obtained is 82.06 0
C.
c) Vector and Stream line plots
Figure 7 shows the vector plot and stream line
plots of velocity and temperature. This signifies the
distribution of air and direction of flow in the
domain. The air flow is from left to right.
3.7 Results validation
Results obtained for the problem are converged
results. However results obtained from analysis is
compared with analytical calculations.
a) Temperature rise (∆T):
Figure 8 shows the snap shot of FLUENT (CFD)
result. It shows the temperature values at exhaust fan
and inlet. The difference in those value gives ∆T and
it is found to be 3 0
C.
Figure 8: Temperature rise- CFD result
From Figure 4, ∆T = 6.5 0
F i.e. 30
C. Hence CFD
result is matching analytical approach for ∆T in an
enclosure.
b) Checking for heat balance
Heat balance in here means heat input to the
enclosure should be equal to heat lost from the
enclosure. The total heat dissipated in an enclosure is
150 W, this is the amount of heat to be removed and
heat balance to be maintained. Figure 9 shows the
CFD result and it is evident that heat balance is
maintained. Negative sign indicates that heat is
removed.
Figure 9: Total heat transfer rate- CFD result

c) Validating maximum velocity
Considered flow to be incompressible and steady
We know 92 CFM is at Outlet,
Considering, Velocity inlet = Velocity outlet
= [92 x (0.305)3]
/ 60= 0.0435m3
/s
Now, 0.0435 / (0.03 x 0.03) = 48.33 m/s
From Figure 5, CFD result for Maximum
velocity was found to be 36. 51 m/s whereas
analytically obtained maximum velocity at inlet is
48.33 m/s. The obtained result is validating and
signifying that the CFD result is converging towards
analytical result.
From all the above comparisons it is evident that
the domain is maintained for both mass and heat
balance and also for ∆T in an enclosure.
V. CONCLUSIONS
CFD analysis has been carried on Electrical
enclosure consisting heat block, dissipating heat of
150 W. The heat block is cooled through forced
convection by providing adequate amount of air flow
in an enclosure. The following conclusions that can
be drawn from the results obtained are:
i. CFD is the powerful and effective tool for
Thermal management of Electronic & Electrical
enclosures
ii. Work is a ready reckoner for engineers in
helping them on how to: select a fan, calculate
air flow requirement and temperature rise for an
enclosure based on both analytical and CFD
methods
iii. Temperature rise with fan placed at the optimum
location was found to be 3 0
C which is well
below the threshold limit.
Present work is no means exhaustive; always
there is a scope for further optimization.
REFERENCES
[1] Hoffman- A Pentair Company, “Heat
dissipation in Electrical enclosure”,
“Technical information on Thermal
Management of Electrical enclosures”,
©2003 Hoffman Enclosures Inc.
[2] MahendraWankhede, VivekKhaire, Dr.
AvijitGoswami and Prof. S. D. Mahajan,
“Evaluation of cooling solutions for outdoor
electronics”, Journal on electronics cooling
from electronics-cooling.com, Volume 16,
No. 3, Fall 2010
[3] Bud Industries, Inc., “Enclosure Design Tips
Handbook”, July 2007, © Bud Industries
Inc.
[4] “A practical formula for air-cooled boards in
ventilated enclosures”, from link:
http://www.electronics-
cooling.com/1997/09/a-practical-formula-
for-air-cooled-boards-in-ventilated-
enclosures/
[5] ANSYS Fluent User Guide, Release 15.0,
Nov. 2013 and ANSYS Fluent 12.0, Theory
guide, April 2009
[6] H K Versteeg and W Malalasekara,
“Computational Fluid Dynamics- Finite
Volume Method”, Text book of CFD,
edition 1995
[7] Younus. A. Cengel, “Text book of Heat
transfer- chapter 15- cooling of electronic
equipments”
Figure 5: Velocity contour plot

Figure 6: Temperature contour plot
Figure 7: Vector and stream line plots

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