 Open Access
 Total Downloads : 19
 Authors : Lakshminarasimha. N , Dr. M. S. Rajagopal
 Paper ID : IJERTV8IS060478
 Volume & Issue : Volume 08, Issue 06 (June 2019)
 Published (First Online): 24062019
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Transient CFD Analysis of Different CrossSection Fins Under FreeConvection Conditions
Lakshminarasimha. N1
1IGBC AP and Assistant Professor, Department of Mechanical Engineering, New Horizon College of Engineering, Bengaluru
Dr. M. S. Rajagopal2
2Professor and Chairman, Department of Mechanical Engineering,
Dayananda Sagar University, Bengaluru
Abstract Manufacturers of aerospace and defense equipments are presently facing challenges related to both steady state and transient reliability of electronics systems; the continuing reduction in size of electronic components is resulting in higher power density due to which thermal management of electronic components is critical in electronic product development. Among heat transfer augmentation technique, passive cooling technique is more suitable than active cooling for specific applications. Also providing fins can regulate the temperature of the system at optimum levels by providing extended surface area of contact with surrounding cooling medium air. In the present work, the Transient analysis has been carried out for three different cases to determine the transient performance considering different crosssectional fins such as Tapered, Round and Rectangular configurations. The fins are subjected to freeconvection cooling which are placed on plate with four heat sources each dissipating 100W power. Transient analysis is carried out using ANSYS CFD software for time step of 20 seconds and results obtained for different crosssection are compared for optimum temperature levels.
Keywords: Fins, Tapered fin, Round Fin, Rectangular Fin, Transient analysis, ANSYS CFD, Freeconvection.

INTRODUCTION
To increase the heat transfer rates by increasing the surface heat transfer coefficient or increasing the temperature difference between surface and surrounding medium sometimes becomes impossible at particular condition and application; hence at that situation fins can be used for increasing the heat transfer rates from the surfaces [17].
Fins are classified as straight fins with uniform (Fig. 1.1) and nonuniform thickness (Fig. 1.2), annular fins (Fig. 1.3) and spine of constant crosssection (Fig. 1.4) and non uniform cross section (Fig. 1.5).
Fig. 1.1 Fig. 1.2
Fig. 1.3 Fig. 1.4
Fig. 1.5
The knowledge of temperature distribution along the fin is necessary for proper design of fins. Hence in the present work, Transient analysis is carried out using ANSYS CFD for understanding the temperature distribution and heat flow from different crosssection fins such as Tapered, Round and Rectangular fins.

LITERATURE SURVEY
Literature survey is carried out to understand the state of art in CFD transient analysis on natural convection cooling of fins. Here are some literatures as discussed below:
Santosh Kansal et.al., [1], [2015], This paper deals with a comparative study using CFD on Electronic enclosure consisting fins of different configuration. The overall performance of the six different heat sinks with different shaped pinfin structures was studied in this paper for different velocities varying from 5, 10 & 12 m/s. The paper presents simulation and thermal analysis of different shape fins heat sink for an electronic system cooled by natural convection.
Aartee. S. Lokhande, [2], [2018], this article gives overall review on work carried out on Transient analysis fins with different shapes and briefs some of technical details on fins.
Only major journals are discussed in this section, remaining journals, articles and textbooks listed in References.

METHODOLOGY
In the present work, Numerical approach is used to solve the conjugate heat transfer problem. Geometry, Meshing and Analysis are carried out using ANSYS CFD software. The details are given below.
Steps in the analysis involve:

Creation of Geometry

Meshing the Model

Apply Boundary Condition

Physical setup for analysis selecting appropriate Mathematical models

Result visualization and comparison

Geometry
The Geometry model of Tapered fin, Round fin and Rectangular fin is as shown in Fig. 3.1, Fig. 3.2 and Fig.
3.3. The fins are placed on a plate in an enclosure with Freeconvection cooling. The plate is attached with four heat sources. The construction is maintained same for all the enclosures with different crosssection fins.
TOP VIEW SIDE VIEW
Fig. 3.2 Round fin model
Heat Source
ISO VIEW
Plate
Tapered Fin
Enclosure
Fig. 3.3 Rectangular fin model

Meshing
The cut plane mesh model of Tapered fin, Round fin and Rectangular fin is as shown in Fig. 3.4, Fig. 3.5 and Fig.
3.6. The Meshing parameters for three models are maintained same. The Models are meshed with Hexagonal elements.
Fig. 3.1 Tapered fin model
Fig. 3.4 Tapered fin meshed model
Fig. 3.5 Round fin meshed model
20 seconds is set for each heat sources which are with peak power of 100W. The variation of power is considered according to the equation of exponential curve [18], =
Ã— where a and b are constants and t is time. The flow is considered to be laminar and at Tranient condition.

MATHEMATICAL MODELS Conservation equations of mass and momentum for all flows are solved in ANSYS CFD and an additional equation for energy is solved for flows involving heat transfer. Flow inside a Electronic enclosure involves both fluid flow and fluid flow with heat transfer, hence governing equations [14] that are solved in ANSYS CFDare as listed below:
Mass conservation equation:
+. () = m
. (eqn. a) Momentum conservation equation:
() + . () = + . () + +
. (eqn. b)
Where, the stress tensor, is given by
= [ ( + T) 2 . ]
3
Energy conservation equation:
() + . (( + )) = . ( ) + Sh
Fig. 3.6 Rectangular fin meshed model
The convergence plot for the Meshed models is as shown in Fig. 3.7. The Residual monitor or convergence criteria for flow and energy are maintained to be 0.001 and 1e7. Iteration per time step is set for 20 seconds.
Fig. 3.7 Mesh convergence plot


Solution Methodology
In the present analysis, the enclosure consist for four heat sources each dissipating 100W power, attached to a plate on which different crosssection fins are placed. The fins are subjected to freeconvection cooling. The cycle time of
(eqn. c)
The above equations (a), (b) and (c) are a general form of governing equations and are valid for both compressible and incompressible flows.

RESULTS AND DISCUSSION
The velocity and temperature contours results obtained from the Transient analysis on different crosssection fins are discussed in this section. The temperature distribution results obtained for Tapered fin model is dicussed in section 5.1, Round fin model disscssed in 5.2 and Rectangular fin model is discussed in 5.3.

Temperature contours/distribution for Tapered fin model
Fig. 5.1(a)and (b) shows the Temperature contour for Tapered fin model at the time step of 20 second. The maximum temperature of 370C was found at the source. The heat conduction takes place through the thickness of plate from the source. The temperature distribution in detail at different time step is shown in Tabl 5.1 and Plot 5.1.
Fig. 5.1(a) Iso view of Tapered fin model
Time steps
Fin temperature in 0C
Source temperature 0C
0
20
20
1
20.0466
23.6988
2
20.1581
25.2468
3
20.3312
26.2739
4
20.5551
27.1116
5
20.8187
27.8589
6
21.1131
28.5558
7
21.432
29.2225
8
21.7709
29.8707
9
22.1267
30.5078
10
22.4974
31.1386
11
22.8813
31.7666
12
23.2773
32.3942
13
23.6848
33.0232
14
24.103
33.6553
15
24.5314
34.2915
16
24.9698
34.9329
17
25.4177
35.5803
18
25.8749
36.2345
19
26.3414
36.8961
20
26.8168
37.5658
Time steps
Fin temperature in 0C
Source temperature 0C
0
20
20
1
20.0466
23.6988
2
20.1581
25.2468
3
20.3312
26.2739
4
20.5551
27.1116
5
20.8187
27.8589
6
21.1131
28.5558
7
21.432
29.2225
8
21.7709
29.8707
9
22.1267
30.5078
10
22.4974
31.1386
11
22.8813
31.7666
12
23.2773
32.3942
13
23.6848
33.0232
14
24.103
33.6553
15
24.5314
34.2915
16
24.9698
34.9329
17
25.4177
35.5803
18
25.8749
36.2345
19
26.3414
36.8961
20
26.8168
37.5658
Fig. 5.1(b) Side view and Rear side view of plate attached to fin Table 5.1 Temperature distribution for Tapered fin at different time step
Plot 5.1 Temperature vs Time plot for Tapered fin
From the Table 5.1 and Plot 5.1, it is observed that the maximum temperature reached at time step of 20 second by fin was 26.810C and source was 37.560C.

Temperature contours/distribution for Round fin model Fig. 5.2 shows the Temperature contour for Round fin model at the time step of 20 second. The maximum temperature of 360C was found at the source. The heat conduction takes place through the thickness of plate from the source. The temperature distribution in detail at different time step is shown in Table 5.2 and Plot 5.2.
Fig. 5.2 Temperature contour of Round fin model
Table 5.2 Temperature distribution for Round fin at different time step
Time steps
Fin temperature in 0C
Source temperature 0C
0
20
20
1
20.0437
23.1916
2
20.1448
24.7165
3
20.2978
25.765
4
20.4919
26.6187
5
20.717
27.3703
6
20.9656
28.0602
7
21.2323
28.7102
8
21.5137
29.3336
9
21.8074
29.9389
10
22.1118
30.5321
11
22.426
31.1174
12
22.7494
31.698
13
23.0814
32.2762
14
23.4218
32.854
15
23.7703
33.4328
16
24.1267
34.014
17
24.491
34.5984
18
24.863
35.1871
19
25.2425
35.7809
20
25.6297
36.3803

Temperature contours/distribution for Rectangular fin model
Fig. 5.3 shows the Temperature contour for Rectangular fin model at the time step of 20 second. The maximum temperature of 360C was found at the source. The heat conduction takes place through the thickness of plate from the source. The temperature distribution in detail at different time step is shown in Table 5.3 and Plot 5.2.
Fig. 5.3 Temperature contour of Rectangular fin model
Table 5.3 Temperature distribution for Rectangular fin at different time step
Time steps
Fin temperature in 0C
Source temperature 0C
0
20
20
1
20.0437
23.1916
2
20.1448
24.7165
3
20.2978
25.765
4
20.4919
26.6187
5
20.717
27.3703
6
20.9656
28.0602
7
21.2323
28.7102
8
21.5137
29.3336
9
21.8074
29.9389
10
22.1118
30.5321
11
22.426
31.1174
12
22.7494
31.698
13
23.0814
32.2762
14
23.4218
32.854
15
23.7703
33.4328
16
24.1267
34.014
17
24.491
34.5984
18
24.863
35.1871
19
25.2425
35.7809
20
25.6297
36.3803
Comparing Table 5.2 and Table 5.3, the temperature distribution for Round fin and Rectangular fin is found to be same. Hence plotting the temperature distribution and is shown in plot 5.2.
Plot 5.2 Temperature vs Time plot for Round fin and Rectangular fin
Also the temperature distribution along the fin length is plotted for all the fin types and is as shown in plot 5.3. It is found that there is continuous reduction in temperature along the fin length.
Plot 5.3 Temperature vs. fin length plot
Comparing the Maximum temperature obtained for all the fin types, it is found that there is increase in temperature by 3% for Tapered fin compared to Round fin and Rectangular fin.

Velocity streamline plots
Fig. 5.4, Fig. 5.5 and Fig. 5.6 shows the velocity streamline plots for tapered fin, round fin and rectangular fin. The maximum velocity achieved through freeconvection is 0.2 m/s.
Fig. 5.4 Streamline plot for Tapered fin
Fig. 5.5 Streamline plot for Round fin
Fig. 5.6 Streamline plot for Rectangular fin
Streamline plots helps in identifying and studying the circulation zones and flow around fins.


CONCLUSION

Transient analysis is carried out on Electronic enclosure using commercial CFD software ANSYS. The Transient analysis carried out for time step of 20 seconds on Electronic enclosure for three different cases consisting different crosssection fins such as Tapered fin, Round fin and Rectangular fin attached to a plate with four heat sources each dissipating power of 100W. Cooling of fins is through freeconvection. The following are the conclusions drawn from the analysis results are:

In determining Temperature distribution at varied time step in carrying out transient analysis, CFD technique is very much effective at minimum time and cost.

Analysis is carried out for different cross section fins for finding optimum temperature level. It was found that there is increase in temperature by 3% by using Tapered fin compared to Round fin and Rectangular fin and hence tapered are recommended.

The results obtained through the analysis helps as ready reckoner for beginning Engineers in decision making in selection of fins among different crosssections and understanding temperature distribution in fins.

This work showcases determining transient performance of heat sink under natural convection conditions. Further work on varying pitch of the fins can be taken up to optimize the flow.
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