Experimental Study on Heat Transfer from a Perforated Fin Array with Cross Perforations

Transcription

1 ICHMT2014-XXXX Experimental Study on Heat Transfer from a Perforated Fin Array with Cross Perforations H. Saadat 1, M. M. Tavakol 2, M. Yaghoubi 3 1 B.S Student, Department of Mechanical Engineering, Faculty of Engineering, Islamic Azad University Shiraz Branch, Shiraz, Iran; h.saadat@live.com 2 Instructer, Department of Mechanical Engineering, Faculty of Engineering, Islamic Azad University Shiraz Branch, Shiraz, Iran; tavakolmm@shirazu.ac.ir 3 Professor, School of Mechanical Engineering, Shiraz University, Shiraz, Iran; yaghoubi@shirazu.ac.ir Abstract Experimental study is performed to analyze heat transfer characteristics as well as thermal performance of a new type of perforated fin with cross openings. For measurements, an array of these fins over a flat surface is made from aluminum and two cross circular holes are drilled in each fin. The tests are conducted in a wind tunnel which produces uniform air motion upstream of the test case. Temperature measurements are carried out by means of calibrated thermocouples system and surface temperature measurement is determined by a thermograph imager. Results of thermal structure and heat transfer are plotted for Reynolds numbers based on the fin length. Keywords: Convection heat transfer, perforated fins, cross perforation, temperature measurement Introduction One of the primary purposes in the design of modern thermal systems is the accomplishment of more compact and efficient heat exchangers. Decreasing energy loss due to ineffective use and also enhancement of energy transfer in the form of heat has become an increasingly substantial duty for the design engineers of thermal systems. The heat dissipation rate of electronic systems rises as the component s size and compactness goes up. For some electronic devices, it is crucial to obtain the maximum temperature of the systems within a secure level. For remedy, various cooling technologies i.e. cooling with heat pipe, cold plates, impinging jets are introduced. Furthermore, various types of coolants e.g. liquids or gases are utilized to enhance heat transfer performance. Among different coolants, air cooling is very well-known for its availability and easy thermal management. In fact, it is so economical to evaluate thermal performance of such systems and decreasing their weight and enhance their efficiency. In this regard, various numerical and experimental studies have been performed. Diani et al. [1] investigated to find the effects of geometric parameters on thermal performances of such heat sinks. They also obtained optimized fin configurations for a specific cooling application. Sara et al. [2] compared thermal and hydraulic performances of perforated rectangular blocks with a solid block. They observed lower performances for both solid and perforated blocks at higher Reynolds number flows. El- Sayed et al. [3] determined the optimized fin array position to maximize heat transfer enhancement at high Reynolds number flows. In this regard, fluid flow parameters are measured for the variety of flow orientations. Thermal performance of different types of extended surfaces are evaluated numerically and/or experimentally by several authors, Kadle et al. [4], Lau et al. [5], Wirtz et al. [6], Suabsakul [7], Sata et al. [8] and Molki et al. [9]. Jonsson et al. [10-11] improved empirical correlation to anticipate Nusselt number and pressure drop along different types of fin geometry. Sahin et al. [12] concluded that either increasing the inlet air velocity or increasing the surface area, heat transfer performance of extended surfaces can be burgeoned. Sarkar et al. [13] enhanced thermal performance of a channel using perforated star shape fins for various range of perforations. They proposed empirical relations for convective heat transfer coefficient at various Reynolds numbers. Mahmood et al. [14] attained the effect of temperature ratio and flow structures for a dimpled channel at different Reynolds numbers ranging from 600 to 11,000 with inlet temperature ratio of 0.78 to Sparrow et al. [15,16] probed the effects of bypass flow over the tip of pin fin heat sinks. They also performed an experimental study to determine Nusselt number for a perforated surface which was faced upstream. Two major parameters e.g. the Reynolds number and pitch-diameter ratio are utilized in their study. Dorignac et al. [17] developed a Nusselt number correlation for perforated plates according to the experimental observations. Wu et al. [18] developed a model to anticipate the thermal performance of plate fin heat sinks. They proposed correlations to predict the value of friction factor and Nusselt number for different flow regimes for various Reynolds numbers less than Yu et al. [19] conducted combine experimental and numerical studies for thermal performances of plate fin and plate-pin fin heat sinks. They reported 30% higher thermal resistance for a plate fin in comparison with a plate-pin fin. Yaghoubi and Velayati [20] developed new correlations for the Nusselt number and fin efficiency in turbulent flow over an array of cubes. Shaeri and co-workers [21-24] had a series of numerical simulations for various perforated fins in both laminar and turbulent flows. Several perforated fins with various lateral and longitudinal perforations are considered and heat transfer ICHMT2014, November,

2 characteristics are evaluated and compared with solid fins for the same conditions. According to the above review there is limited number of experimental study regarding heat transfer characteristics of perforated fins. Therefore, the aim of this paper is to analyze heat transfer characteristics of a new type of perforated fins. Each fin has one longitudinal opening and one vertical opening with circular cross sections shown in Figure 1. For this consideration, three-dimensional turbulent air flow and convective heat transfer around the array of perforated rectangular fins are analyzed experimentally and results are presented and discussed in the following sections. Experimental setup Experiments are carried out in a blowing type wind tunnel at the School of Mechanical Engineering in Shiraz University. The test section has a square cross section of cm and 120 cm length. The air velocity in the tunnel is adjusted using a frequency inverter system. The wind tunnel is used to measure the velocity profiles around a wall-mounted hemisphere in the previous study of Tavakol et al. [25]. In order to analyze heat transfer characteristics of the perforated fins, an aluminum model is prepared. This material is extensively used in the cooling of heat dissipating devices in various industries. Attempt is made to minimize the heat loss from lateral and lower sides by insulating the walls using an insulating asbestos layer with 2cm thickness and thermal conductivity of 0.1 W/mK. Then, the aluminum and insulation material is equipped with an extra aluminum cover. An electric console equipped with power regulator with a digital read-out is used to control and monitor the power supply to the exchanger. The power control circuit is adjusted to provide a constant electrical output of 22 W. The prepared model is installed horizontally in the tunnel as shown in Figure 1. The geometry and arrangement of perforated fins is shown in Figure 2. As shown in this figure, the model consists of three perforated fins with one longitudinal and one cross opening. Each opening is located at the middle of vertical plane and horizontal top plane of each fin. In order to measure the temperature, 6 small holes are accurately drilled and equipped with K-type thermocouples with the direct digital read. The accuracy of measurement by means of this type of thermocouples is 0.5 C. These locations are selected in order to minimize the disturbance for the incoming flow as well as observing heat transfer characteristics of the perforated fins. As indicated in Figure 2, point 1 and 4 are located below the longitudinal opening in the windward and leeward side of the model. Points 2-3 and 5-6 are drilled near the base plane in the windward and leeward side of the model, respectively. Figure 1. System configuration in the wind tunnel and the model of perforated fins in the tunnel Figure 3 displays the reference lines on various planes to present the result of surface temperature measurement using thermograph system. The horizontal lines (in Z direction) on the top of the fin are indexed with (h) and the vertical lines (in Y direction) are specified with (v). All geometric parameters of the model are listed in Table 1. Surface temperature measurements are conducted using the thermograph system, Testo 881 model. The Testo thermal imager is a handy and robust device with 2 C accuracy in surface temperature measurement [26]. The thermal imager Testo detects quickly and reliably anomalies and weak spots in components and materials. In view of an imaging process, energy losses and cold bridges as well as damage or overheating in industrial systems are revealed devoid of contact. Other methods including cable or pipeline systems must be exposed over a huge zone, while by utilizing the thermal imager Testo, a single glance is enough. It opens the door to contactless determination and illustration of the temperature distribution on each surface [26]. To take thermal image of each surface, the Testo needs surface emissivity and for the aluminum surface it is selected as 0.8. Table 1. Dimensions of model (cm) A 4 B 2 C 7 D 2 E 1 F 1 G 0.5 H 1.5 I 3 J 7 K 0.5 L 6 N 1 ICHMT2014, November,

3 Figure 2. Arrangement of perforated fins and location of thermocouples Figure3. Reference lines over the perforated fins Results and Discussion A series of measurements are made using calibrated thermocouples and thermograph systems. Each test is continued until an approximate steady-state temperature is reached. The steady-state temperature is achieved when all of the thermocouples and thermographs indicate approximately constant temperature. This took between minutes depends on the air velocity in the wind tunnel. During the experiments, the test room temperature is monitored and recorded as the reference temperature. However, approximately constant ambient temperature is observed. To extract reliable data from measurements, each test is repeated 3 times and no considerable difference is observed. Flow Reynolds number is defined based on the length of the fins, Re L=ρvL/μ and it is varied in the range of by adjusting free stream velocity in the tunnel. Figures 4-5 indicate two thermal images from thermograph systems. Figure 4 shows thermal image without air flow in the tunnel and in Figure 5 the airflow is maintained with Re= As illustrated in these figures, the hot air exit the openings because of heat transfer with the solid base. This support the importance of openings in perforated fins which is maintaining temperature gradient in the fins. Temperature gradient along the fin increases the heat dissipating performance of perforated fins in comparison with solid fins. A preliminary test is done when the model is heated by means of heater elements with no fluid flow in the tunnel. Results of surface temperature measurement for this case using thermograph image are presented in Figure 6. The temperature variation is approximately linear in the vertical direction. It should be pointed out that temperature distribution on line P1(v) and P3(v) are nearly the same, while it is hotter on the line P2(v) which is cooled from one side. Small deviation from linear distribution may be due to variable free convection around the fins and inside the openings in the fins which changes the local temperatures. Comparison between measured temperature at different points in windward and leeward sides are presented in Figures 7-9. These figures show that increasing air flow velocity, the fine temperature is decreased in the thermocouple locations. Also the decrease in temperature originates from increasing convection heat transfer from perforated fins. Another observation can be made from Figures 7-9 is the lower temperature at thermocouple locations in the windward side in comparison with leeward side. The reason is laid in the difference of the flow conditions at these regions. Specifically, at the upstream region stagnation flow region is formed with maximum heat transfer coefficient and in the downstream region the reversed flow zone is formed with lower heat transfer coefficient. At higher flow Reynolds numbers, the difference between temperatures at each point is decreased. Another important observation is the considerable decrease in temperatures by increasing flow Reynolds number. Quantitatively, increasing Reynolds number from to caused about 8 C decrease in the temperature at the location of thermocouple 3. In the following, results of surface temperature measurement with thermograph are presented. Each test is made by adjusting surface emissivity of the fins and calibrated with thermocouples. As mentioned, the reference lines are plotted in Figure 3. It should be noted that the ambient temperature has not remained constant; hence the corresponding ambient temperature is also pointed for each experiment in Figures7-9. ICHMT2014, November,

4 Figure 4. Surface temperature from thermograph, without airflow in the tunnel Figure 5. Surface temperature of the fin for Re=46000 (top view) Figure 6. Surface temperature in three lines without airflow in the tunnel Figure 7. Temperature at point 1 and 4 at different flow Reynolds numbers ICHMT2014, November,

5 Figure 8. Temperature at point 2 and 5 at different flow Reynolds numbers Figure 9. Temperature at point 3 and 6 at different flow Reynolds numbers Figures display results of surface temperature measurement by means of thermograph system. The average fin base temperature is determined from the following relation. Subscript refers to the corresponding thermocouple as illustrated in Fig.2. T base ( T T T T )/ (1) According to the temperature s distribution in these figures, it is concluded that the temperature of the fin surface is increased in the streamwise direction due to flow development. This trend is observed for all flow Reynolds numbers. The maximum surface temperatures correspond to the middle fin which is surrounded by the other two fins. Also fin base temperature decreased by increasing flow Reynolds number for the same power supplied to the fins. Using temperature measurement by means of thermocouples and thermograph system, the average convection heat transfer coefficient of the fin array is calculated according to the followings. If one assumes the model is well insulated, then for steady state condition the total heat supplied by the heating element is transferred to the surrounding medium by means of convection heat transfer and radiation. In this situation the energy balance can be expressed as: Q Qloss Qconvection Qrad (2) In this equation Q denotes the total heat supplied by means of electric heater, Q loss is the convection heat loss from the lateral covers, Q convection is the convection heat transfer from the top surface of the model including the fins and Q rad is the radiation heat loss from the model upper surface. Assuming the radiation heat loss is negligible in comparison with the convection heat exchange due to low fin surface temperature, it is possible to calculate the convection heat loss from the lateral covers. In order to calculate the heat loss from lateral covers a thermal circuit is considered assuming one dimensional conduction heat transfer through the insulating layer and the cover. The convective heat transfer from the upper finned surface can be obtained by subtracting the cover heat loss from the total supplied heat. The average convection heat transfer coefficient is calculated as follow: (3) Qconvection ha( Ts Tamb ) where T s is the average temperature of the surface A is the total area of the uncovered upper surface of fin and unfinned surface over the model and h is the average surface heat transfer coefficient for the upper surface with array of fins. Figure 13 reveals the variation of average heat transfer coefficient over the fin arrays. It is clear that the average Nusselt number increased with rising flow Reynolds number. In addition, comparison between average Nusselt number of the present study and previous study of Shaeri and Yaghoubi [21] for perforated fins and solid fins is presented in this figure. As shown in Figure 13, the present perforated fin array has better heat transfer performance in comparison with both solid fin and perforated fin arrays in the study of Shaeri and Yaghoubi [21]. This is in parts due to the difference in the shape of perforations in the present study and previous study of Shaeri and Yaghoubi [21]. In the study of Shaeri and Yaghoubi [21] the fin array included only one longitudinal perforation which produced temperature gradient in one direction. In the present study, a vertical perforation is added to the fin array in order to have temperature s gradient in the upper half portion of the fin. Figure 10. Surface temperature at different locations over the fin tips, Re=27000 ICHMT2014, November,

6 Figure 11. Surface temperature at different locations over the fin tips, Re=35000 Conclusions In this study, an experimental investigation is conducted in order to analyze heat transfer characteristics of a new type of perforated fins. According to the results, the following conclusions are made: 1. The temperatures of the fin at thermocouple locations are decreased by increasing flow Reynolds number. 2. The fin surface temperature increased in the streamwise direction with the maximum surface temperatures correspond to the middle location of the fin. 3. The average convection heat transfer coefficient increased with increasing flow Reynolds number. 4. Some improvements of heat transfer performance are observed with the current perforation arrangement in comparison with previous study [21]. 5. Combined cross and stream-wise perforations can be preferred from those of single direction perforation. 6. Thermal field over the fins shows that, the constructed fin array can be used as a test case to validate any future numerical simulation of fin array. Acknowledgement This study is supported by Iran's National Elites Foundation. Figure 12. Surface temperature at different horizontal locations over the fins, Re=46000 Figure 13. Variation of the average convection heat transfer coefficient However, if one determines more exact heat losses and radiation term, such convection heat transfer coefficient would be reduced and the difference in calculated convection heat transfer coefficient with the study of Shaeri and Yaghoubi [21] would decrease. It should be pointed out that in the study of Shaeri and Yaghoubi [21] the length of simulated fin was 2.4 cm while in the present study it is 6 cm. References [1] Diani, A., Mancin, S., Zilio, C., and Rossetto, L., An assessment on air forced convection on extended surfaces: Experimental results and numerical modeling. International Journal of Thermal Sciences, 67, pp [2] Sara, O.N., Pekdemir, T., Yapici, S., and Ersahan, H., Thermal performance analysis for solid and perforated blocks attached on a flat surface in duct flow. Energy Conversion and Management, 41, pp [3] El-Sayed, S.A., Mohamed, S.M., Abdel-latif, A.A., and Abouda, A.E., Experimental study of heat transfer and fluid flow in longitudinal rectangular-fin array located in different orientations in fluid flow. Exp. Therm. Fluid Sci., 29, pp [4] Kadle, D.S., and Sparrow, E.M., Numerical and experimental study of turbulent heat transfer and fluid flow in longitudinal fin arrays. ASME J. Heat Transf., 108, pp [5] Lau, K.S., and Mahajan, R. L., Effects of tip clearance and fin density on the performance of heat sinks for VLSI packages. IEEE Trans. Comp. Hybrids, Manufact. Technol., 12, pp [6] Wirtz, R.A., Chen, W., and Zhou, R., Effect of flow bypass on the performance of longitudinal fin heat sinks. ASME J. Electron. Packag., 116, pp ICHMT2014, November,

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