Transient CFD Analysis of Different Cross-Section Fins Under Free-Convection Conditions

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Transient CFD Analysis of Different Cross-Section Fins Under Free-Convection 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 cross-sectional fins such as Tapered, Round and Rectangular configurations. The fins are subjected to free-convection 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 cross-section are compared for optimum temperature levels.

Keywords: Fins, Tapered fin, Round Fin, Rectangular Fin, Transient analysis, ANSYS CFD, Free-convection.

  1. 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 non-uniform thickness (Fig. 1.2), annular fins (Fig. 1.3) and spine of constant cross-section (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 cross-section fins such as Tapered, Round and Rectangular fins.

  2. 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 pin-fin 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.

  3. 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:

  1. Creation of Geometry

  2. Meshing the Model

  3. Apply Boundary Condition

  4. Physical setup for analysis selecting appropriate Mathematical models

  5. Result visualization and comparison

      1. 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 Free-convection cooling. The plate is attached with four heat sources. The construction is maintained same for all the enclosures with different cross-section 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

      2. 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], =

        × where a and b are constants and t is time. The flow is considered to be laminar and at Tranient condition.

        1. 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 1e-7. Iteration per time step is set for 20 seconds.

          Fig. 3.7 Mesh convergence plot

      3. Solution Methodology

    In the present analysis, the enclosure consist for four heat sources each dissipating 100W power, attached to a plate on which different cross-section fins are placed. The fins are subjected to free-convection 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.

    1. RESULTS AND DISCUSSION

      The velocity and temperature contours results obtained from the Transient analysis on different cross-section 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.

        1. 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.

        2. 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

        3. 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.

        4. 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 free-convection 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.

    2. 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 cross-section 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 free-convection. The following are the conclusions drawn from the analysis results are:

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

  2. 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.

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

  4. 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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