Research Article EXPERIMENTAL AND COMPUTATIONAL ANALYSIS AND OPTIMIZATION FOR HEAT TRANSFER THROUGH FINS WITH DIFFERENT TYPES OF NOTCH S.H. Barhatte 1, M. R. Chopade 2, V. N. Kapatkar 3 Address for Correspondence 1,2 Department of Mechanical Engineering, MIT College of Engineering, Pune, India 3 Department of Mechanical Engineering,, Sinhgad College of Engineering, Pune, India E Mail shbarhatte@mitpune.com, vnkapatkar@rediffmail.com ABSTRACT Extended surfaces, commonly known as fins, often offer an economical and trouble free solution in many situations demanding natural convection heat transfer. Heat sinks in the form of fin arrays on horizontal and vertical surfaces used in variety of engineering applications, studies of heat transfer and fluid flow associated with such arrays are of considerable engineering significance. The main controlling variable generally available to designer is geometry of fin arrays. Considering the above fact, natural convection heat transfer from vertical rectangular fin arrays with and without notch at the center have been investigated experimentally and theoretically. Moreover notches of different geometrical shapes have also been analyzed for the purpose of comparison and optimization. In a lengthwise short array where the single chimney flow pattern is present, the central portion of fin flat becomes ineffective due to the fact that, already heated air comes in its contact. In the present study, the fin flats are modified by removing the central fin portion by cutting a notch. This paper presents an experimental analysis of the results obtained over a range of, fin heights and heat dissipation rate. Attempts are made to establish a comparison between the experimental results and results obtained by using CFD software. KEYWORDS: Convection, fin arrays, notches, ICEM-CFD, Fluent 6.3. INTRODUCTION electronic equipment and stationary engines need Augmentation techniques are broadly classified as passive methods, which require no direct application of power or as active schemes, which require external power. Practically useful, augmentation techniques are, however, mostly passive ones. Passive techniques are treated and structured surfaces, rough surfaces, extended surfaces, displaced enhancement devices, swirl flow devices, additives for liquids and gases, etc. Extended surfaces are widely used passive techniques to enhance heat transfer. Further enhancement in heat transfer can be obtained by proper selection of form of extended surface or by making some modifications in the geometry of surfaces like dent marks, grooved, or different types of notches etc NEED OF INVESTIGATION Specially designed finned surfaces called heat sink, which are commonly used in the cooling of optimized design with minimum material and maximum heat transfer from them. In the natural cooling of fins, the temperature drops along the fins exponentially and reaches the environment temperature at some length. But heat transfer from the area near the tip is low. It results in wastage of material for small heat transfer rate. Cutting this portion of fins, results in the complete elimination of heat transfer from that region. Also the central portion of the fin flat becomes ineffective due to the fact that, already heated air comes in its contact. In this investigation, the fin flats were modified by removing the central fin portion by cutting a notch of different geometrical shapes and adding it at the arrays entrance on the two sides, where it is more effective and thereby keeping fin surface area same.
The objectives of investigation are as listed below To carry out study on fins with horizontal array and its analysis for natural convection. To study influence of different geometry and dimensions on heat transfer rate To determine the type of geometry and its dimensions for optimum heat transfer rate. Calculations of minimum material for maximum heat transfer and Convective heat transfer coefficient are also done. EXTENDED SURFACES In all the examples discussed above, the major heat transfer takes by two modes i. e. by conduction or by convection. Heat transfer through the solid to the surface of the solid takes place through conduction where as from the surface to the surroundings takes place by convection. Further heat transfer may be by natural convection or by forced convection. The rate of heat transfer from a surface at a temperature T s to the surrounding medium at T 0 is given by Newton s law of cooling as Q= has( Ts To) Where A S is heat transfer surface area, and h is the convection heat transfer coefficient. When the temperatures T S and T 0 are fixed by design considerations, as is often the case, there are two ways to increase heat transfer rates To increase convection heat transfer coefficient h. To increase the surface area A S. Increasing h may require the installation of a pump or fan, or replacing the existing pumps or fans with a larger ones. This approach in some cases may or may not be practical. Besides in some cases, it may not be adequate. The alternative is to increase the surface area by attaching to the extended surfaces called fins made of highly conductive materials such as aluminum, copper etc. COMPUTATIONAL ANALYSIS Temperature distribution and heat flux along the fin surface can be predicted by computational analysis. Apart from temperature distribution and heat flux various other parameters like Nusselt Number, heat transfer coefficient, and changes in other parameters can also be predicted by computational analysis. There are number of software packages like ANSYS CFX or Fluent GAMBIT used for modeling and meshing of the geometry in use today. ANSYS ICEMCFD is used especially in engineering applications such as computational fluid dynamics and structural analysis. Beginning with a robust geometry module which supports the creation and modification of surfaces, curves and points, ANSYS ICEMCFD s open geometry database offers the flexibility to combine geometric information in various formats for mesh generation. Geometry is collected into a common geometry database (tetin file) which can be used by any of ANSYS ICEMCFD s meshing modules. The mesh file is then imported into Fluent 6.3 solver for post processing. Computational analysis involves the application of heat transfer and fluid dynamics principles. Computational analysis involves three steps mainly modeling, preprocessing and post processing. The discretization schemes used for this purpose are structured hexahedral grid mesh. With the hexahedral grid mesh the solution becomes more accurate. Scheme of tetrahedral grid mesh can also be used but it is less accurate. Moreover for regular geometry it is possible to use hexahedral grid mesh more efficiently and get more accurate results.
EXPERIMENTAL SETUP Tf1, Tf2,Tf3, Tf4 & Tf5 are the temperatures of fins in C. C. To find temperature of whole body (Tbody) (3) D. To find temperature difference between body & surrounding temperature (δt) Fig. 1 Meshed model created in ICEM CFD (4) E. To find mean film temperature (TLm) (5) F. To find coefficient of volume expansion (β) Fig. 2 Schematic of the experimental setup G. To find Grashof number (Gr) (6) Fig. 3 Experimental setup PROCEDURE FOR CALCULATION A. To find average temperature of base plate (Tb) (1) where, Tb1 & Tb2 are the temperatures of base plate at two points in C. B. To find average temperature of fins(t f ) (7) Where, Lc= characteristic length of the geometry, m υ = kinematic viscosity of the fluid, m 2 /s Pr = Prandtl number k = thermal conductivity of fluid, W/mk H. To find Rayleigh number (Ra) Ra = Gr. Pr (8) If 10 4 < Gr.Pr < 10 9 then, Nu = 0.59 (Gr.Pr) 1/4 If 10 9 < Gr.Pr < 10 12 then, Nu = 0.59 (Gr.Pr) 1/3 I. To find Nusselt number (Nu) (9) J. To find heat transfer coefficient (h) where, (2) where, h is heat transfer coefficient, W/m 2 k (10)
Table 1. Table showing HTC for different notches at base temp of 60 deg C Sr. No. W/O Notch Rectr Circular Triangular Trapezoidal 1 6.235 6.039 6.114 6.29 6.0983 2 6.1788 6.0707 6.1893 6.291 6.112 3 6.2152 6.0482 6.2281 6.2785 6.1065 4 6.2306 6.0762 6.2152 6.3057 6.1038 5 6.2958 6.1524 6.1971 6.2934 6.0818 Avg 6.23122 6.07746 6.18882 6.29172 6.1004 Graph 1: HTC at 60 deg 6.4 6.3 6.2 6.1 6 5.9 W/O Notch Circular Trapezoidal Graph 2: Comparison of HTC by CFD and HTC by experiment WO notch Rectangular Circular Triangular Trapezoidal Table 2: Comparison of HTC by CFD and HTC by experiment Base temp of Average heat transfer coefficient 60 deg WO notch Rect Circular Triangular Trapez CFD 5.3434 6.039 5.31779 7.03509 7.0260 Exptl 6.231 6.07 6.18882 6.29172 6.1004
CONCLUSION Following points are worth noting from the present investigation work. Computational analysis and subsequent experimental investigations have revealed fins can be used effectively to enhance the rate of heat transfer. It is also revealed that heat transfer coefficient and in turn the rate of heat transfer can further be increased by increasing the surrounding fluid velocity i.e. by forced convection. The performance of heat transfer fins can be analyzed effectively by commercially available CFD software, Fluent 6.3 in specific. It is proved beyond doubt that the heat transfer coefficient is highest for the set of fins with triangular notch. This has been shown by the CFD analysis as well as by the experimental analysis. The same methodology of experimental investigation and computational analysis can be used further for different types of notches and fins. SUGGESTIONS FOR THE FUTURE WORK In the present investigation work and based on the literature review, it was assumed that the rate of heat transfer will increase by considering the use of heat transfer fins. Further from the computational analysis it was understood that the rate of heat transfer is increased by having a notch at the centre of fin. The computational analysis did not indicate of any particular shape of notch which would give maximum heat transfer. It was only our presumption of considering only the commonly available shapes like circular, rectangular, triangular, and trapezoidal. There probably would be different other shapes and sized for which the rate of heat transfer would be maximum. In the future work, the size of notch may also be considered. Computational analysis and experimental analysis can be done for different sizes of a single given notch. The effect of this size on heat transfer can be analyzed. In particular, different sizes of triangular notch should consider for which the heat transfer rate would be maximum. REFERENCES 1. Bejan A. and Morega A. M., Optimal Arrays of Pin Fins and Plate Fins in Laminar Forced Convection, J. Heat Transfer, Vol 115, pp. 75-81. 2. Poulikakos, A. and Bejan, A., Fin Geometry for Minimum Entropy Generation in Forced Convection ASME Journal of Heat Transfer, Vol 104, pp. 616-623. 3. Incropera F. P., DeWitt D. P., 1996, Fundamentals of heat and mass transfer, 4th Edition, John Wiley & Sons, Pg. No. : 147-172. 4. P. K. Nag, 2006, Heat & Mass Transfer, 2nd Edition, Tata McGraw Hill Co. Pg. No. : 86-108 & 425-449 5. J. P. Holman, 2004, Heat Transfer, 9th Edition, Tata McGraw Hill Co, Pg. No. 43-53 & 315-350 6. Yunus A. Çengel, 2004, Heat Transfer- A Practical Approach, SI units 2nd Edition, Tata McGraw Hill Co., Pg. No. : 156-168, 333-352 & 459-500 7. El Hassan Ridouane and Antonio Campo, Heat transfer and pressure drop characteristics of laminar air flows moving in a parallel-plate channel with transverse hemi-cylindrical cavities International Journal of Heat and Mass Transfer, Vol. 50, September 2007. 8. W. W. Lin and D. J. Lee, Second-law analysis on a flat plate-fin array under crossflow International Communications in Heat and Mass Transfer, Volume 27, Issue 2, February 2000. 9. Gustavo Ledezma and Adrian Bejan, Heat Sinks With Sloped Plate Fins in Natural and Forced Convection International Journal of Heat and Mass Transfer, Vol 39, Issue 9, June 1996.
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