Experimental Analysis of Natural Convective Heat Transfer Performance From Rectangular Fin Array with Perforations

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1 Experimental Analysis of Natural Convective Heat Transfer Performance From Rectangular Fin Array with Perforations #1 A. V. Zoman, #2 D. D. Palande #1 Department of Mechanical Engineering, PG Student, Savitribai Phule Pune University, MCOERC Nashik, India #2 Department of Mechanical Engineering, Associate Professor, Savitribai Phule Pune University, MCOERC Nashik, India Abstract Steady-state natural convection heat transfer from vertical base rectangular perforated fins is investigated experimentally. The experimental set-up was employed in order to take measurements from different samples of rectangular perforated fin array to compare with the sample of rectangular solid fin array. The 7 test samples of aluminum rectangular fin array are produced by machining then different perforations are made on lateral surfaces on the fins of 6 test samples. First, 5 number of perforation with diameter 12 mm on each fin were selected for study then decreasing these number of perforation to 4 then 3 and decreasing diameter to 10 mm was tested to investigate its effects on heat transfer rate. Among 7 test samples higher enhancement of thermal performance was observed with 5 number of perforation of 12 mm diameter. Also, a result shows that perforated fin array increases heat transfer rate compared to the solid fin array. Keywords: Rectangular fin array, Natural convection, Perforation, Heat transfer rate, Lateral surface. I. INTRODUCTION Many engineering systems during their operation generate heat. If this generated heat is not dissipated to surrounding atmosphere, this may cause rise in temperature of the system components. This causes serious overheating problems and leads to system failure, so the generated heat within the system must be dissipated to its surrounding to maintain the system at recommended temperature for its efficient working (S. Kale and V. Bhatkar, 2015). When vertical rectangular fins are heated, the buoyancy force causes the surrounding fluid to start moving which result in development of thermal boundary layers at the bottom edges of the opposing surfaces of the neighboring fins. These boundary layers eventually merge if fins are sufficiently long, creating a fully developed flow. Perforated fins disrupt and ideally reset the thermal boundary layer growth, maintaining a thermally developing flow regime, which in turn, leads to a higher natural heat transfer coefficient. Due to demand of lightweight, compact, and to achieve the required heat dissipation rate, with the least amount of material, perforations are made to the fins. There have been many studies on natural convection heat transfer on optimizing the geometries of fins. Most of experimental findings related to the thermal performances on solid rectangular fins was reported in the literature. (B. Yazicioglu and H. Yuncu, 2007) performed experiments over thirty different fin configurations. It was observed that larger the fin height higher the convective heat transfer rate. But at low temperature difference, increase in the convective heat transfer was not very significant. (B. Yazicioglu and H. Yuncu, 2009) developed a new expression for prediction of the optimal fin spacing for vertical rectangular fins. (S. H. Barhatte et. al, 2011). investigated heat transfer through horizontal base fins experimentally by cutting a notch at center of fin and by computational analysis of different geometrical shapes such as circular, rectangular, triangular and trapezoidal. They concluded that heat transfer coefficient is highest for triangular notch followed by trapezoidal, circular and rectangular respectively. (M. Ahmadi, et. al, 2014) investigated numerically and experimentally steady state natural convection heat transfer from vertically mounted inline interrupted fins. They have done 2D numerical simulation to find fin interruption effects by using fluent software. They developed a custom design test bed to verify theoretical results. They performed comprehensive experimental and numerical parametric study to investigate the effects of fin spacing and fin interruptions. The results show that the interruptions increased the heat transfer rate by resetting the thermal and hydrodynamic boundary layer. They tested 12 heat sink samples for validation of numerical study. They observed that the heat flux from heat sink increased when interruptions were added. They developed new compact correlation to calculate optimum fin interruption for targeted rectangular heat sink. The first perforation on rectangular fin was reported in the literature of (W. Hussain, 2011) experimentally studied the natural convection heat transfer in a rectangular fin plate by circular perforations heat sinks. The patterns of circular perforations ranging from 24 for first fin to 56 for fifth fins were used. They observed that the temperatures drop along the non-perforated fin was less than perforated fin. (R. R. Jassem, 2013) experimentally investigated the heat transfer by natural convection in a rectangular perforated fin plates. Five fins used in this work with first fin non-perforated and others fins perforated by different shapes these fins perforation by different shapes such as circle, square and hexagon keeping same cross section area. The results show that the drop in the temperature of the non-perforated fin was less compared to perforated fins, at the same power supplied for shapes hexagonal, square and circular 2015, IERJ All Rights Reserved Page 1

2 respectively. (U. V. Awasarmol and A. T. Pise, 2011) studied experimentally natural convective heat transfer enhancement of perforated fin array with different perforation diameter ranging from 4mm to 12mm inline. They tested the four configuration from 0⁰ to 90⁰ inclination angle from which they found optimum perforation diameter as 12mm. (A. Khosnevis et. al, 2009) investigated numerically the effect of lateral surface perforation on thermal enhancement of a 3-D channel with a ground attached heater. The result showed for same open area ratio for slot and hole perforation, thermal performance was achieved for hole perforation. (G. Chaudhari and I. Wankhede, 2015) experimentally showed that for certain percentage of perforation, the perforated fin enhanced the heat transfer. The total heat transfer was maximum for 30% perforated fin array. It revealed that the experimental data were in a good agreement with the correlations with average relative errors were less than 20 % for Churchill and Chu s and McAdams correlations. (A. Ganorkar and V.M. Kriplani, 2012) studied whole performance of forced convection heat transfer through lateral perforated fin in 3-D channel. Effect of rectangular fin with circular perforation having diameter 6, 8, 10 mm for different Reynolds no. is analyzed and compared with solid one. They came to conclusion Also due to perforation laminar boundary layer is broken and more turbulence is produced which in turn increased the convective heat transfer coefficient. (A. H. M. AIEssa and N. S. Gharaibeh, 2014) experimentally examined natural convective heat transfer enhancement from a horizontal rectangular fin with triangular perforation and compared with solid one. Magnitude of enhancement is directly proportional to fin thickness and thermal conductivity of material. II. EXPERIMENTAL STUDY The objective of the experimental work is to investigate the effects of perforation size and number of perforations in vertical base rectangular fins. As such, two sets of rectangular fin array samples were prepared and tested: i) 1 solid and ii) 6 perforated rectangular fin array. Base plate dimensions for the samples are kept same for all 7 samples. The rectangular fin array are machined with same fin spacing and fin height and then perforations are drilled on lateral surfaces of fins as listed in Tables 1. 7 different samples are used for testing are shown in Fig Fin Array Dimensions Base plate = 200 mm x 95 mm. Height of fin = 25 mm. Thickness of fin = 3 mm. Spacing of fin= 7 mm No. of fin=7 Diameter of perforation = 12 and 10mm. No. of perforation =5, 4, 3. Table 1 Calculation of surface area of test samples Sr. n d mm A r mm 2 A mm 2 A r % No Solid fin The total area removed from the fin is determined considering the work on interrupted fins [5] which is equal to the area removed in our first perforated fin sample i.e. 5%. This area is selected initially for first perforation sample and then investigations are made when this removed from area is decreased and compared with the geometrically similar solid fin. The area removed is decreased in the terms of number of perforation from 5, 4 and 3 and diameter of perforation 12mm to 10mm. Fig.1 a) 5 perforations of dia. 12 mm perforated fin 2015, IERJ All Rights Reserved Page 2

3 Fig.1 b) 5 perforations of dia.10mm perforated fin Fig.1 c) 4 perforations of dia.12mm perforated fin Fig.1 d) 4 perforations of dia.10mm perforated fin Fig.1 e) 3 perforations of dia.12mm perforated fin 2015, IERJ All Rights Reserved Page 3

4 Fig.1 f) 3 perforations of dia.10mm perforated fin 2.2 Experimental set-up Fig.1 g) solid fin The tested fin arrays are machined from aluminum 6063 material with a thermal conductivity of 130 W/m. K and emissivity of 0.2 at 20 ⁰C. A new set-up is fabricated in workshop for measuring natural convection heat transfer from fin array as shown in Fig.3. The set-up includes an enclosure made of three pieces of plywood having k=0.12 W/m.K each thickness of 19 mm. The front surface of enclosure is covered with glass of 5 mm thick, which has arrangement to replace fin arrays. A heater base plate of aluminum made of nicrome wire sandwiched in mica sheets of 200 W, 50 Hz capacity is used to heat fin array. Concrete block, thermal conductivity, k~0.15 W/m.K. is mounted on enclosure supported on MS frame has in-built 4 bolts to tight fin array over heater plate vertically. Air gap between heater plate and fin array is consider to be negligible. Also heat loss through excess portion to the fin arrays to be assumed negligible. Experimental set-up is shown in fig. 2 and Dimensions of each component is given in Table 2. Table 2 Dimensions of components Sr Component Dimension (mm) No. 1 Enclosure Concrete Heater plate During the experiments, in addition to the power input to the electric heater, surface temperatures are measured at various locations at the back of the fin base plate. Electrical power is supplied through an AC power supply. The voltage and the current are measured to determine the power input to the heater. 5 calibrated K-type thermocouples range 0⁰C - 400⁰C are installed in various locations on the surface of the base plate. One more thermocouple is used to measure the ambient temperature. Total 6 thermocouple wires are connected to multi point temperature indicator to get the readings at 6 different positions. The average of 5 readings T 1, T 2, T 3, T 4, and T 5 other than ambient T 6 is taken as the mean temperature. After supplying power to the heater for 3 hrs, system is allowed to reach a steady-state condition. For each power input value, 6 temperatures, input current and voltage are recorded. This procedure is repeated for each of the 7 samples, for 5 different power inputs; 20, 40, 60, 80 and 100 W. Dimmer stat is used for setting the voltage input. The set-up was kept in room to set the natural convection. 2015, IERJ All Rights Reserved Page 4

5 Fig. 2 Experimental Set-up III. DATA REDUCTION Heat input supplied to fins is given by, Q in = V.I....(1) The most of the heat lost by radiation to surrounding air which is calculated as, Q rad = ε. σ. A (Ts 4 Ta 4 ) (2) Heat lost by convection Q C = Q in - Q rad This rate of heat transfer by convection can be given by, Q C = h. A. (Ts Ta)...(3) Heat transfer coefficient is calculated as, h = Qc/A. (Ts Ta).(4) Nusselt no. as Nu=h.L/k...(5) 3.1 Calculation of base and open area of fins: Area of base plate (A b ) =L. W Area of one solid fin= (2.H.L) + (2.H. t)+ (L. t) Area of N solid fins (A f )= N {(2.H.L) + (2.H. t)+ (L. t)} Area of solid fin array (A s )= A b + A f (6) When a perforation is made in fin, a plane surfaces are removed but at the same time new curved surface is generated. So, that curved area is added and plane area is deducted from area of solid fin array to get the perforated area of fin array Plane area due to one perforation (A d ) = Curved area due to one perforation (A c ) = Total deducted area due to one perforation (A d1 )= A d - A c Total deducted area due to n perforation (A dn )= n(a d - A c )..(7) Total deducted area due to n perforation for N fin(a r )= N[n(A d - A c )]..(8) Area of perforated fin array(a p )= A b + A f - {N[n(A d - A c )]} (9) Surface area of perforated fin(a)=a s -A r...(10) IV. RESULTS AND DISCUSSIONS A study is performed to investigate the effect of perforation parameter on natural convection heat transfer. For this, perforation parameter is varied while the geometric parameters of fins are kept constant. Subsequently, for the case of perforated samples, with different perforation diameter and number of holes are studied experimentally and compared the performance with solid one. In fig. 3 effect of number of perforation is investigated. As the number of perforation increases, temperature difference decreases. For solid fin it is 89⁰C for lower input while for 5 perforation and 12mm diameter is 46⁰C. This is due to the resetting of thermal boundary layer imposed by adding the perforation. If we considered the flow between two vertical parallel fins having perforation i.e. channel, the air flow get disrupted at the perforation region. When air moves along the perforation region, velocity and temperature profile become more distributed. This causes the decrease in thickness of thermal and hydrodynamic boundary layer due to which heat dissipation increases. 2015, IERJ All Rights Reserved Page 5

6 Fig.3 Variation of Temperature difference vs. Heat flux Fig.4 shows the heat transfer coefficient which is maximum for highest perforation diameter i.e.12 mm which is 3.17, 2.69, 1.95 for lower heat input and 8.8, 7.8, 6.3 for higher heat input for number of perforation 5, 4 and 3 respectively compared to 10mm. This is due to the fact that air circulates more through 12mm perforation diameter and small turbulence is created which enhances heat transfer compared to 10 mm diameter and the solid one. Fig.4 Variation of heat transfer coefficient vs Temperature difference Fig. 5 shows comparison of heat flux vs Nu. no. for fins with different perforations. Fins with 5 number of perforation show maximum heat flux than 4 and 3 number of perforations. As more area is removed, so the area in contact with fin is less so heat flux increases. Difference in surface area of these all perforated samples is lesser, so the curves are closer to each other shows the increase in heat transfer. But the difference in surface area of these perforated fin and solid fin is greater which shows deviation of solid fin curve than perforated fin curves. Fig.5 Variation of Heat flux vs. Nu.no. Though area in contact with air is less for 5 number of perforation than for 3 and solid respectively but disruption of boundary layer which is more in case 5 number of perforation than for 4, 3 and solid respectively which enhances heat transfer and shows higher value of Nu. no. 2015, IERJ All Rights Reserved Page 6

7 As we go on decreasing the removed area for 12 mm for 5, 4, 3 number of perforation heat transfer rate decreases. And for nonperforated fin i.e. for solid fin heat transfer rate is least among all fin samples. Hence heat transfer coefficient is less for solid fin as compared to perforated fin for given heat input as shown in fig. 6 and greater for more number of perforation. Also heat transfer coefficient increases with increase in heat input for all fin arrays. Same is the case for 10 mm diameter as shown in fig. 7. Among the two diameters, 12 mm diameter shows higher heat transfer rate. Fig.6 Variation of heat transfer coefficient vs. Heat Input 12 mm diameter Fig.7 Variation of Heat transfer coefficient vs. Heat input for 10 mm diameter Fig. 8 shows the comparison of experimental Nu no. (Nu Exp) with the Nu no. obtained from Churchill and Chu s first correlation for laminar and turbulent flow (Nu ch1), second correlation for laminar flows only (Nu ch2) and Mc. Adams (Nu Mc) relation, which indicate the validity of the experimental set-up. It reveals that experimental data are in agreement with correlations. With increase in heat input the heat transfer rate from perforated fin array increases compared to solid fin. Fig.8 Variation of Nu no. vs. Ra no. 2015, IERJ All Rights Reserved Page 7

8 V. CONCLUSION The effects of perforations on vertical base rectangular fin array were studied experimentally. 1. Though the area is reduced due to perforation, but the reduction difference is very less compared to the boundary layer disruption. And hence the increase in perforations the heat transfer rate by resetting or interrupting the thermal and hydrodynamic boundary layers. 2. All rectangular fin array with circular perforation gives more heat transfer rate than solid one. 3. All rectangular fin arrays with circular perforation of 12 mm diameter gives more heat transfer rate than 10 mm diameter for same number of perforation. 4. Highest heat transfer rate is achieved using 5 number of perforation of 12 mm diameter. 5. Less is the value of temperature difference more is heat transfer coefficient. 5. Fins with more number of perforations increase the heat transfer rate reducing the area as well as weight of system. ` VI. NOMENCLATURE Nu Nusselt number Ra Rayleigh number h Heat transfer coefficient(w/m 2 k) L Fin length(mm) W Width of plate(mm) S Fin spacing(mm) H Fin height (mm) t Fin thickness (mm) A Surface area (m 2 ) d Perforation diameter (mm) N Total no. of fin n No. of perforation on each fin k Thermal conductivity(w/m.k) ΔT Temperature difference = Ts-Ta (⁰C) Ts Surface temperature (⁰C) Ta Ambient temperature (⁰C). REFERENCES S. Kale, V. Bhatkar, (2015), Electronic Equipments Cooling through Rectangular Fin Array by Using Natural Convection: A Review, International Journal of Current Engineering and Scientific Research, Volume.2, Issue.2, pp B. Yazicioglu, H. Yuncu, (2007), Performance of rectangular fins on a vertical base in free convection heat transfer, Heat Mass Transfer 44, pp B. Yazicioglu, H. Yuncu, (2009) A Correlation For Optimum Fin Spacing of Vertically-Based rectangular Fin Arrays Subjected To Natural Convection Heat Transfer, J. of Thermal Science And Technology,, pp S. H. Barhatte, M. R. Chopade, V. N. Kapatkar, (2011), Experimental and computational analysis and optimization for heat transfer through fins with different types of notch, Journal of Engineering Research and Studies, vol.2, Issue 1,pp M. Ahmadi, G. Mostafavi, M. Bahrami, (2014), Natural convection from rectangular interrupted fins, International Journal of Thermal Sciences, vol. 82, pp W. Hussain Abdul Razzak Al-Doori (2011), Enhancement of Natural Convection Heat Transfer From The Rectangular Fins By Circular Perforations, International Journal Of Automotive And Mechanical Engineering, Volume 4, pp R. R. Jassem, (2013), Effect the form of perforation on the heat transfer in the perforated fins, Academic Research International, Academic Research International, Vol. 4 No. 3, pp U. V. Awasarmol, A. T. Pise, (2011), Experimental Study of Effect of Angle of Inclination of Fins on Natural Convection Heat Transfer through Permeable Fins, International Conference on Thermal Energy and Environment, pp.1-11 A. Khoshnevis, F. Talati, M. Jalaal, E. Esmalzadeh, (2009), Heat transfer enhancement of slot and hole shape perforations in rectangular ribs of a 3-D channel, 17th Annual International Conference on Mechanical Engineering, pp.1-5. G. Chaudhari, I. Wankhede, M. Patil, (2015), Effect of Percentage of Perforation on the Natural Convection Heat Transfer from a Fin Array, International Journal of Engineering and Technical Research, Volume-3, Issue-2, pp A. Ganorkar, V. Kriplani, (2012), Experimental Study of Heat Transfer Rate by Using Lateral Perforated Fins in a Rectangular Channel, MIT International Journal of Mechanical Engineering Vol. 2, No. 2, pp A. H. M. AIEssa., N. S. Gharaibeh, (2014), Effect of triangular perforations orientation on heat transfer augmentation from a fin subjected to natural convection, Advances in Applied Science and Research, 29, pp , IERJ All Rights Reserved Page 8