Experimental Study of Effect of Angle of Inclination of Fins on Natural Convection Heat Transfer through Permeable Fins

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1 Proceedings on International Conference on Thermal Energy and Environment (INCOTEE 2011) Experimental Study of Effect of Angle of Inclination of Fins on Natural Convection Heat Transfer through Fins U. V. Awasarmol 1 and Dr. A. T. Pise 2 1 Assistant Professor, Dept. of Mechanical Engineering, Army Institute of Technology, Pune, Maharashtra, (India) 2 Professor & Head, Dept. of Mechanical Engineering Government College of Engineering, Karad, Maharashtra, 41124, (India) u_awasarmol@rediffmail.com ABSTRACT Natural convection heat transfer can be enhanced by using permeable fins. Natural convection heat transfer from the fins is greatly affected by the angle of inclination of fins. Experimental studies were conducted to analyze the natural convection heat transfer from solid and permeable fins and also the effect of angle of inclination of fins on heat transfer. This paper is outcome of experimental study conducted to compare the rate of heat transfer with solid and permeable fins and the effect of angle of inclination of fins. fins are formed by modifying the solid rectangular fins by drilling three inline holes per fin. Solid and fin block are kept in isolated chamber to study the natural convection heat transfer. Natural convection heat transfer through of each of these blocks was compared in terms of variations in steady state temperatures of base and tip. The steady state temperatures were recorded at constant heat flux condition. At the same time the steady state temperatures were recorded for different angles of inclination of fins. Blocks having solid and permeable fins were tested for different inputs (i.e., ). Also the blocks were rotated through the different angles of inclination of fins (i.e.0 0, 1 0, 30 0, 4 0, 60 0, 7 0, 90 0 ). It is found that using permeable fins, heat transfer rate is improved and convective heat transfer coefficient increases by about 20% as compared to solid fins with reduction of cost of the material 30%. And the optimum angle of inclination of fins is 90 0 i.e. vertical fins. It is also found out that the permeable fins are cooler than the solid fins and the minimum base temperature is recorded at 90 0 angle. Keywords: natural convection, permeable fins, solid fins, inclination of fins, heat transfer enhancement 1.0 INTRODUCTION Fins are the extended surfaces to increase the heat transfer rate from the body by increasing the convective surface area. Fins find their wide variety of applications. The enormous application of the fins makes it an interesting field for the researchers. Optimizing the heat transfer rates results in saving of power supplied and increased efficiency in case of the automobile engines, computer chips etc. Natural convection from block with fins may be used to simulate wide variety of engineering applications as well as provides better insight into more complex systems of heat transfer such as heat exchangers, refrigerators, electric conductors etc. Convection may be enhanced by using the permeable fins instead of the solid fins with the optimum angle of inclination of fins of fins. Bassam and Abu [1, 2] conducted the numerical analysis and found that the heat transfer through permeable fins resulted in significant enhancement over solid fins. They stated that under no condition did the increase of number of permeable fins result in decrease in Nusselt number as opposite to solid fins. They used certain assumptions to make the analysis simple that the fins are made up of highly conducting material. They did not validate their results with experimental work. Ridouane and Campo [3] in their study showed the enhancement in heat transfer using grooved channels. They found that the grooves enhance the local heat transfer relative to flat passage. Jamin and Mohamad [4] quantified and compared the steady state heat transfer from a heated vertical pipe with and without porous medium. They found that the largest increase in Nusselt number was achieved by high thermal conductivity solid carbon foam sleeve, which was about 2. 1

2 Experimental Study Of Effect Of Angle Of Inclination Of Fins On Natural Convection Heat Transfer Through Fins times greater than a bare copper pipe. Ahn et al. [] in their experimentation compared the heat transfer rates with rounded and elongated holes in rectangular holes. They showed that elongated holes enhance more heat transfer rate than rounded holes but at the cost of pressure drop. Layeghi [6] in his numerical analysis also showed that heat transfer can be enhanced using porous media but there is a pressure drop. Abdullatif Ben-Nakhi et al [7] studied the natural convection in open cavity. They found that the heat transfer rate increases with the thin fins attached to the hot surface. Zhnegguo et al., [8] have used three dimensional petal shaped finned tubes to enhance the heat transfer. Povel and Mohamad [9] in their experimental and numerical study, investigated the effect of metallic porous material, inserted in a pipe, on rate of heat transfer. Effect of porosity, porous material diameter, thermal conductivity as well as Reynolds number on heat transfer rate and pressure drop were investigated. Ashok Tukaram Pise and Umesh Vandeorao Awasarmol[11] studied the effect of permeability of fins on natural convection heat transfer. They experimentally found out that the permeable fins perform better than the solid fins. They did not discuss the effect of angle of inclination of fins on the on the heat transfer enhancement through permeable fins. fins are formed by drilling three inline holes of diameter 4mm on each fin. The experimental set up also has an arrangement of changing the angle of inclination of fins. The dimensions of both the types of fins are same. (i.e. length 2mm, thickness 2 mm, width 7 mm). 2.0 EXPERIMENTATION An experimental setup of fins is developed and discussed as below (See Fig. 1). Each fin block is provided with a heater. The same input wattage was supplied to both the fin blocks and the temperatures were recorded at the base and the tip of each solid and each permeable fin. 2.1 Experimental Setup: Fig. 1 shows the experimental set up for the natural convection heat transfer from permeable and solid fins with the effect of angle of inclination of fins of fins. After the extensive literature reviewed, the use of the permeable fins with the replacing solid fins is discussed in the paper. Also attempts are made to find the angle of inclination of fins at which heat transfer rate is optimum. This work is the outcome of an experimental work. So in the proposed work, fin blocks are taken for experimentation. fins are formed by modifying the solid rectangular fins by drilling three holes per fin. The heat transfer rates, average heat transfer coefficients and percentage saving of material for solid and permeable fins with the different angles of inclination of fins are compared. It is found out that natural convection heat transfer rate improves with the use of permeable fins oriented at an angle of 900 (vertical). It consists of two halves of a fin blocks, connected back to back so that the heat transfer from both the sides of the block is same. This back to back arrangement of the fin blocks (twin block) facilitates to measure the exact quantity of heat transfer through the fins. Such two twin blocks were considered for the experimentation. One block has solid fins while the other one has permeable fins. The permeable Fig. 1 Schematic of Experimental Setup 2

3 Proceedings on International Conference on Thermal Energy and Environment (INCOTEE 2011) Fig. 2 Schematic showing dimensions of fins (Left) and angle of inclination of fins (Right) fins are formed by modifying the solid rectangular fins by drilling three holes per fins of 4 mm diameter each inline. Fig. 2 shows the schematic of the dimensions of the fin block and the angle of inclination of inclination given to the fins. Heating coils were inserted in the fin blocks to simulate the internal heat generation. Electric supply is given to the coils through dimmerstat so as to vary the input power. Input power supplied through the coil was measured by wattmeter. Base and tip temperature of the each fins were measured by thermosensors of PT100 having least count of 0.10C. Tests were taken by the varying the input supplied to the heating coils and by changing the angle of inclination of fins. In order to ensure the natural convection, this entire assembly is placed in an enclosure. Enclosure was made up of acrylic sheet. Two thermally isolated compartments were formed with acrylic sheet, keeping an air gap to provide insulation between two compartments. Upper sides of the compartments were kept open in order to ensure proper natural convection. 2.2 Test Procedure Test fin blocks of solid and permeable fins were placed in enclosures. Input power was supplied to each fin blocks. Under steady state conditions, the temperatures at base and h = Convective heat transfer coefficient tip of each solid and permeable fin were recorded. Same procedure was repeated for varying the input power (i.e., ). These readings were also taken by changing the angle. The readings of temperatures were taken at different angles i.e. 00, 10, 300, 40, 600, 70, Data Reduction: Under steady state condition total heat supplied in to the system is equal to the total heat flow out of the system. In this case, the same quantity of heat is supplied to both fins twin blocks A and B. As both the twin blocks are made up of same material and both the blocks are tested under the same conditions, it is assumed that the heat transferred by conduction and radiation heat loss are nearly equal from both the fin blocks. q Q Q 1 conv solid conv permeable h. A. T h. A. T 2 solid permeable Assuming the fin of length infinity, hp ka ( T T ) a Where, P = Perimeter of fin c b 3 3

4 Temperature (deg C) Temperature (deg C) Temperature (deg C) Experimental Study Of Effect Of Angle Of Inclination Of Fins On Natural Convection Heat Transfer Through Fins h = Convective heat transfer coefficient Ac = Area of cross section K = Thermal conductivity of fin material Tb = Base Temperature To = Ambient Temperature temperatures of permeable fins are smaller as compared to those in solid fins for all the angles and for all the inputs. It is clear from the Fig. s that the temperatures of solid fins are more elevated as compared to permeable fins. It means that for the same heat flux the fin with permeable fins is less temperature than solid fins this means that heat transfer rate is more in permeable fins as compared to solid fins. Using equation 1, 2 and 3 avg. heat transfer coefficients for both fins blocks were calculated and the percentage increase in heat transfer coefficients were calculated Fig. 3 Effect of Angle on Base Temperature of Solid Fins 3.0 RESULTS AND DISCUSSIONS After the extensive experimentation, results obtained for solid and permable fins are discussed broadly in two categorize i.e. temperature profile and average heat transfer coefficient. 3.1 Temperature Profile As the basic purpose of the fin is to maintain the base temperature on lower side, the average temperatures of the base versus angle of inclination for each solid and permable fins different input condition were plotted as shown in Figs 3-. Fig. s 3 shows that base temperature of the solid fins decreases with increase in angle of fins. The fins are found to be cooler when angle of inclination 90 degrees (i.e. Vertical). The block is found to be hotter as the fins are made horizontal. This is quit logical since the resistance to the flow of air increases when the angle is changed from vertical to horizontal. For permeable fins also the similar trend as seen in solid fins is observed. It is very important to note that the Fig. 4 Effect of Angle on Base Temperature of Fins fins() fins() Solid fins() Solid fins() Fig. Comparison of Temperatures of Solid and Fins at Different Inputs Fig. shows Comparison of temperatures of solid and permeable fins at different inputs for the various angles of inclination. From the Figs. 3-, it is evident that the temperatures of solid fins are more elevated as compared to permeable fins. It means that for the same heat flux the fin with permeable fins is less temperature than solid fins. It also means that heat transfer rate is more in permeable fins 4

5 h (w/m^2/k) h (w/m^2/k) h (W/m^2/K) Proceedings on International Conference on Thermal Energy and Environment (INCOTEE 2011 as compared to solid fins sold fins () solid fins () permeable fins() Fins() 0 0 Fig. 6 Effect of Angle on Heat Transfer Coefficient (h) for Solid Fins Effect of angle on 'h' for permeable fins Fig. 7 Effect of Angle on Heat Transfer Coefficient (h) for Fins Fig. s 6 and 7 show that the heat transfer coefficients are more for higher input. Thus the rate of heat transfer coefficient is more for higher heat transfers through solid or permeable fins. It is also clear from the Fig. s 6 and 7 that heat transfer coefficient increases as the fins are rotated from horizontal (0 deg) to vertical (90 deg). The curves of the h are diverging with the angle. This shows that at higher and higher angle, the percentage increase in heat transfer coefficient also increases. Thus the heat transfer is more effective for vertical permeable fins as compared to the solid fins. Fig. 8 Comparison of Heat Transfer Coefficient (h) for Solid and Fins at Different Inputs 3.2 Heat Transfer The fin with permeable fins showed encouraging results. It is evident from the results that the enhancement in heat transfer is significant at the different values of input power. It is seen that the orientation of fins also plays important role as far as heat transfer enhancement is concerns. It is obvious that as the heat flux increase, the average heat transfer rate also increases. Fig. 8 shows comparison of heat transfer coefficient (h) for solid and permeable fins at different Inputs for various angles of inclination. The average heat transfer coefficient for permeable fins is higher than that for solid fins. It is also clear from fig 8 that average heat transfer coefficient increases with increase in heat flux and the angle of inclination of fins. The heat transfer coefficient is higher for the permeable fins as compared with solid fins. From the above discussions, it is evident that the heat transfer rate increases with permeable fins as compared to solid fins. When using more than one solid fin, downstream fins have lower contribution to the overall heat transfer than the upstream fins. The use of permeable fins is an excellent passive method for providing higher heat transfer rates. This is logical since the long solid fins tend to significantly suppress the convection currents. The air can flow through the permeable fins without restriction leading to higher convection heat transfer rates. Thus the permeable fins with optimum angle of inclination can significantly enhance the natural convection heat transfer rates.

6 Experimental Study Of Effect Of Angle Of Inclination Of Fins On Natural Convection Heat Transfer Through Fins 4.0 CONCLUSIONS From the above discussion, following conclusions can be drawn: 1. Temperature profiles show that the temperatures of solid fins are more elevated as compared to permeable fins. It means that for the same heat flux the fin with permeable fins runs cool which shows that heat transfer rate is more in permeable fins as compared to solid fins. 2. There is a net increase in heat transfer rate of the block with permeable fins as compared to that of the fin block with solid fins 3. The average heat transfer coefficient and the ratio of heat transfer coefficient of the fin with permeable fins to the fin with solid fins have been increased by significant value. 4. There is a reduction in area, which means the reduction in cost of the material which is about %. For the same heat transfer, the material removed by mass in permeable fins is about 10 to 30%. Thus the cost of the material saved is considered approximately 10 to 30%. And the weight of the fins can also reduced. The similar experimental study can be extended by changing the no. of holes, no. of fins and size of holes. This study can be extended for the forced convection heat transfer also for the various parameters discussed above. NOMENCLATURE P = Perimeter of fin h = Convective heat transfer coefficient Ac = Area of cross section K = Thermal conductivity of fin material Tb = Base Temperature To = Ambient Temperature Q conv = Convective heat transfer rate ACKNOWLEDGEMENT Army institute of Technology, Pune (India) REFERENCES [1] Bassam A/K Abu Hijleh, Natural convection heat transfer from a fin with high conductivity permeable fins, ASME J. Heat Transfer, vol-12, Apr-2003, pp [2] Bassam A/K Abu Hijleh, Enhanced forced convection heat transfer from a fin with high conductivity permeable fins, ASME J. Heat Transfer, vol-12, Oct- 2003, pp [3] EI Hassan Ridouane, Antonio Campo, Heat Transfer Enhancement of air flowing Across Grooved Channels: Joint Effects of channel height and Grooved Depth ASME J. Heat Transfer, vol-130, Feb-2008, pp [4] Yorweart L. Jamin, A.A. Mohamad Natural Convection Heat Transfer Enhancement from Fin Using Porous Carbon Foam ASME J. Heat Transfer, vol-130, Dec-2008, pp [] H.S. Ahn, S.W. Lee, S.C. Lau, Heat Transfer Enhancement For Turbulent Flow Through Blockages With Round and Elongated holes in a Rectangular Channel ASME J. Heat Transfer, vol-120, Nov-2007, pp [6] Mohammad Layeghi, Numerical Analysis of Wooden Porous Media Effects on Heat Transfer From Staggered Tube Bundles ASME J. Heat Transfer, vol-130, Jan- 2008, pp [7] Abdullatif Ben-Nakhi, M.M. Eftekhari,D.I. Loveday, Natural Convection Heat Transfer in a Partially Open Square Cavity With Thin Fin Attached to the Hot Wall ASME J. Heat Transfer, vol-130, May-2008.pp [8] Zhang Zhnegguo, Xu Tao, Fang Xiaoming, Experiental study on heat transfer enhancement of helically baffled heat exchanger combined with three dimensional finned tubes Applied Thermal Engineering 24 (2004), pp [9] Bogdan I. Povel, Abdulmajeed A. Mohamad, An experimental and numerical study on heat transfer enhancement for gas heat exchangers fitted with porous media International Journal of Heat and Mass Transfer 47 (2004), pp [10]Yunus A Cengel, Heat Transfer: A Practical Approach", Tata McGraw Hill publishing company ltd [11]Ashok Tukaram Pise, Umesh Vandeorao Awasarmol, Investigation of Enhancement of Natural Convection Heat Transfer from Engine Cylinder with Fins International Journal of Mechanical Engineering and Technology, Vol. 1 (2010), pp