Experimental and CFD Analysis of Aluminium Heat Sinks for Avionics Applications

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 02 | July 2016 ISSN (online): 2349-6010

Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications Bopanna K D Assistant Professor Department of Mechanical Engineering New Horizon College of Engineering, Bangalore-560103

Nizamuddin Ali khan PG Student Department of Mechanical Engineering New Horizon College of Engineering, Bangalore-560103

Abstract Efficient thermal management in many avionics system is vital because there is a growing need to minimize the weight and volume of the system. Thermal management is a critical and essential component of maintaining high efficiency and reliability of electronic components. Adequate cooling must be provided while meeting the weight and volumetric requirements, especially for passive air-cooling solutions in avionics applications where space and weight are at a premium. With recent advancements in material science and increase in demand for more aggressive cooling solutions, it has become critical to invent, manufacture, characterize, and implement advanced thermal materials for the design of compact and low weight systems. However due to its isotropic properties of the materials, their thermal performance and reliability can be fully understood. Based on physical, mechanical and thermal properties of aluminum 2024 and alumina we incorporate various approaches for finding out the feasibility for the avionics applications. A combined experimental and computational investigation is performed on different composition of heat sink to know the thermal performance i.e. temperature distribution over each area. Graphs of temperature distribution v/s span length are developed for both the methods. The result obtained from the experimental method is verified with the results obtained from the computational method.

Keywords: Alumina, Fin Effectiveness, Heat Flux, Heat Sink _______________________________________________________________________________________________________ I.

INTRODUCTION TO HEAT SINKS

The main objective of the project is to present the heat sink for efficient cooling of devices using Aluminium matrix composite alumina for the following properties such as good thermal conductivity, low coefficient of thermal expansion and high thermal diffusivity. To design and manufacture two heat sinks of Standard dimensions, one made of pure aluminum and other made of alumina 15% and tested under natural convection. Graphs of temperature distribution v/s span length, heat flux are developed for both the specimens.

To verify the results obtained from experimental method with the computational Method using ANSYS 14.5 II. METHODOLOGY Experimental Method: Experimental test is the best method to determine the thermal performance of the heat sink since it gives the actual output. The experimentation is done by natural or forced convection. For natural convection study, the apparatus generally consist of the heat sink, thermocouples at different junctions and the display unit to record the power input and the temperatures. From the experiment, thermal resistance, temperature distribution and fin efficiencies can be determined.

In order to find out all the above mentioned values for ducted or otherwise for forced convection, fluid flow at different velocities is induced by external agencies like the fan or blower.

Computational Method: The thermal performance i.e., temperature distribution, total heat flux of heat sink can be measured experimentally. Because of complex nature of the profoundly 3D stream in present applications, computational system can be utilized. [11] [12] Computational system gives a subjective or even quantitative forecast of liquid streams. Reenactment can give forecast of stream wonders utilizing the ansys14.5 workbench programming for every single sought quantity, with high determination in space and time and basically any issue and practical working conditions. On the other hand, if discriminating, the outcomes should be worked. There are three main steps involved in computational analysis.

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

Fig. 1: Methodology

III. LITERATURE REVIEW The literature is reviewed to gain background knowledge on heat sink analysis by Experimental approach and computational approach by using Ansys workbench 14.5 Seri Lee [1] developed an analytical simulation model has been developed for predicting and optimizing the thermal performance of bidirectional fin heat sinks in a partially Confined calculation. Sample of calculations are carried out, and parametric plots are provided, illustrating the effect of various design parameters on the performance of heat sink. Different types of heat sinks are examined and their relative performance and cost ranges are presented in his research. An analytical simulation model and method of computing the thermal performance of heat sink in a partially confined configuration are described. The model is validated by comparing the results with existing experimental data, and sample cases are presented with discussions on the parametric behavior and optimization of bidirectional heat sink.

AgnihothraSarma O, A Ramakrishna [2] carried out the work on investigating to optimize the selection of pin fin heat sink using CFD analysis of Splayed pin fin heat sinks for electronics cooling is investigated. Based on the results obtained it was concluded that in the sense of junction temperature splayed pin fins are efficient .It was also found that Hybrid pin fin heat sinks have better performance than aluminum and copper pin fin heat sinks Christopher L.Chapman, Seri Lee, Bill L.Schmidt [3] did a series of investigation on the thermal performance of elliptical pin fin heat sink, cross cut pin heat sink and straight fin heat sink. The overall performance of elliptical pin fin heat sink is compared with the Straight and cross cut fin heat sinks.

Table – 1 Mechanical properties of alumina alloy Mechanical property Units of measure Value Elastic modulus GPa 394 Poisson’s ratio ----0.27 Ultimate tensile strength MPa 260-300 Shear modulus GPa 110 -190 Density g/cm3 3.89

The present research involves considerations to enhance the properties of lightness, extraction of heat and thermal expansion in aluminum 2024 and alumina as heat sink thereby improving the rate of heat absorption making it suitable for various applications. Aluminum-beryllium composites’ have extremely high specific stiffness combined with its above average strength

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

to weight ratio makes it an ideal material to increase the performance of weight critical aerospace components.. Besides its attractive mechanical properties, aluminum beryllium composite’s thermal properties are perfect match for the extreme temperatures of space. Its low coefficient of thermal expansion, high thermal conductivity and high specific heat capacity are all favorable traits for variety of aerospace applications. Due to its disadvantages such as toxic in nature, their size limits and maintaining problems this can be replaced with other metal matrix composite such as aluminum 2024 and alumina alloy and so on. If the health concerned issue is properly addressed these material can be used for various applications.

IV. HEAT SINK PERFORMANCE The performance of the heat sink is determined by the various parameters like the heat transfer coefficient, Fin efficiency, Fin Effectiveness etc.

Heat Transfer Coefficient (h): To define the heat transfer coefficient consider the newtons law of convection given by the equation Q  h Q = hAS(TS – Ta) A S (T S  T A ) Fin Efficiency: Fin efficiency is the ratio of heat lost from the fin to the heat that would be lost when the entire surface of the fin is under base temperature. Generally fin efficiency majorly depends on Length, thermal conductivity of the material. For the present work aluminium of k=237.5 is selected and the length of the fin is too short. Therfore efficiencies may go upto 99%.Considering the short fin condition [16] the efficiency is given by

=

tanh mL mL

Where, m =

hP kf A

h = heat transfer coefficient of fin, P = Perimeter of fin = 2(B+D), L = Length of pin fin, W kf = thermal conductivity of Aluminium fin = 237.5 ( ) , A= Area of fin= B x D mK Fin Effectiveness Is the Ratio of Heat Transfer with the Fin to the Heat Transfer Without: fin, It is very important parameter which tells the usability of the fins in the heat sink. It is a factor which tells how much time the tanh( mL ) heat transfer is more in the heat sink with fins compared to the sink without the fins. It is denoted by ε = . h. A k f .P

Where, h = heat transfer coefficient of fin

W

, P = Perimeter of fin ,kf = thermal conductivity of pin fin m K A= Area of fin= B x D , L = Length of pin fin, 2

Experimental Analysis: Preparation of Composite The materials selected for the study are pure aluminium and alumina composite (Al 2O3). In the present investigation large ingots of matrix material weighing approximately 10kgs was cut into small pieces for accommodating into the crucible. The percentage of alumina was taken 15 % by weight percentage. The solid and most practical strategy received viz., Stir casting to manufacture a metal lattice obstruct with 99.6% artificially immaculate alumina (Al 2O3) particles having molecule size of 5µm and aluminium combination 2024(AA 2024) which served as the matrix; were utilized for the composite's generation. Charge counts were used to focus the amounts of aluminium (2024) amalgam and alumina (Al 2O3) particles needed to create composites having 15% by weight alumina. The alumina particles were at first pre warmed at a temperature of 250˚C for 5-10 minutes to assist enhance with wetting capacity with the AA 2024 combination. The AA 2024 ingots were charged into a fired graphite crucible furnace and heated to a temperature of 700˚C ±30˚C and the liquid alloy was then allowed to cool in the furnace to a semi solid state at a temperature of about 580˚C. The preheated alumina was added at this temperature and stirring of the slurry was performed manually for 5-10, minutes the composites slurry was then superheated to 700˚C and a second stirring performed using a mechanical stirrer. The stirring operation is performed for duration if 10 minutes to help improve the distribution of the composite was then cast into prepared sand moulds. Unreinforced AA2024 was also prepared by casting for control experimentation.

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

Table – 2 Heat Sink Specifications Quantity Dimension Footprint (mm2) LxWxT 101.43x101.43 Base plate thickness(mm) tb 5 Overall height of fin(mm) tf 20 Fin transverse length(mm2) L 4.51 Fin longitudinal length(mm2) W 4.51 Horizontal pitch(mm) SL 6.76 Vertical pitch(mm) ST 6.76

Fig. 2: Top view of the model of the heat sink

Fig. 3: Front view of the model of the heat sink

Performance characteristics of aluminium heat sink Heat transfer coefficient (h):

h

Q A S (T S  T A )

= 9.259 W/(m2K)

Efficiency calculation (η) for aluminium heat sink

=

tanh ml ml

=

tanh( 5 . 965  0 . 02 ) 5 . 965  0 . 02

= 0.9953

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

Effectiveness calculation (η) for Aluminium heat sink tanh( mL )

Effectiveness of Fin,

=

tanh( 5 . 965  0 . 02

h. A

9 . 259  2 . 02  10

k f .P

237 . 5  0 . 0184

5

= 18.144

TO DETERMINE THE HEAT TRANSFER COEFFICIENT, FIN EFFICIENCY AND EFFECTIVENESS FOR ALUMINA COMPOSITE 15% HEAT SINK Heat transfer Coefficient (h):

Q

h

A S (T S  T A )

= 8.378 W/(m2K)

Efficiency calculation (  ) of 15% alumina composite

=

tanh ml

=

tanh( 6 . 0906  0 . 02 )

ml

6 .. 0906  0 . 02

= 0.9951

Effectiveness calculation (ε) for Aluminium heat sink: Effectiveness of Fin,

tanh( mL )

=

tanh( 6 . 0906  0 . 02 )

h. A

8 . 3738  2 . 02  10

k f .P

205 . 625  0 . 0184

5

= 18.122

Fig. 4: Temperature Distribution of Aluminium and 15% Alumina Composite

V. COMPUTATIONAL ANALYSIS

Fig. 5: 3D MODEL GEOMETRY

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

VI. MATERIAL DATA Properties of Aluminium Thermal Conductivity Density Specific Heat

205.625 W m^-1 C^-1 2565 kg m^-3 947 J kg^-1 C^-1

Properties of Alumina Thermal Conductivity Density Specific Heat

237.5 W m^-1 C^-1 2689 kg m^-3 951 J kg^-1 C^-1

VII. RESULTS AND DISCUSSIONS

Fig. 6: Temperature distribution of aluminium

Fig. 8: Temperature distribution of alumina

Fig. 10: Graph of Temperature Distribution

Fig. 7: Heat flux distribution of aluminium

Fig. 9: Heat flux distribution of alumina

Fig. 11: Graph of Heat Flux Distribution

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Experimental and CFD Analysis of Aluminum Heat Sinks for Avionics Applications (IJIRST/ Volume 3 / Issue 02/ 066)

VIII. CONCLUSION Two Heat sinks, one made of pure aluminium and other made of alumina is manufactured and tested under natural convection.

Graphs of temperature distribution v/s span length are developed for both the methods. The result obtained from Experimental method is verified with the results obtained from the computational method.

From the above results, the efficiency and effectiveness variations are very minimal for pure aluminium and alumina composites. 8.2% weight reduction of the existing heat sink can be obtained by inclusion of the alumina composite which is significant in avionic industry.

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