METAL FOAM CONDENSERS FOR MINIATURE-SCALE REFRIGERATION SYSTEMS

Transcription

1 ExHFT-7 28 June 03 July 2009, Krakow, Poland METAL FOAM CONDENSERS FOR MINIATURE-SCALE REFRIGERATION SYSTEMS G. B. Ribeiro, J. R. Barbosa Jr. *, A. T. Prata Federal University of Santa Catarina, Florianópolis, Brazil ABSTRACT. This study consists of an experiental analysis of the theral-hydraulic perforance of icrochannel ensers with open-cell etal foas to enhance the side heat transfer. The ensers were designed to be part of a iniature-scale refrigeration syste. Three different etal foa saples were tested with distinct nubers of pores per inch (10 and 20) and porosity (89.30 and 94.70%). A conventional enser with plain fins was also tested for a perforance coparison. On the refrigerant side, seventeen icro-channels with diaeter of 1.5 were used in all ensers. The experiental apparatus is coposed of an loop (a closed-loop wind tunnel calorieter) and a refrigerant loop which allows the specification of the ass flow rate and the therodynaic state of the refrigerant at the enser inlet. Iso-butane (R-600a) was used as the working fluid. The experients were perfored at the ensing teperature of 90 C, with coolant inlet teperature of 50 C. Air side flow rates ranged fro /s to /s. The refrigerant ass flow rate was aintained at kg/s. The heat transfer rate, the overall theral uctance UA, the Colburn factor j, the friction factor f and the puping power were calculated as part of the analysis of the prototypes. Keywords: enser, etal foa, porous edia, iniature refrigeration syste INTRODUCTION The current generation of transistors used in coputer processors are responsible for severe rates of heat dissipation per unit area which, if not appropriately controlled, can reduce the perforance and coproise the reliability of electronic devices. Thus, the heat anageent of processors has becoe a cornerstone of coputer technology. A large nuber of studies have been devoted to engineering the reoval of heat fro these devices. According to Ortega and Birle [1], natural and forced convection of are no longer capable of aintaining the processor teperature below acceptable values. Cooling technologies as diverse as liquid jet and spray ipingeent, liquid loops and heat pipes have been considered for high heat flux applications. According to Trutassanawin and Groll [2], the vapor copression refrigeration cycle is the only technology capable of reoving high heat fluxes in a highteperature environent. However, space restrictions (which ipose a liit on the size of the refrigeration syste) are aong the ost challenging aspects on this theral solution. Therefore, the design of copact coponents of the cycle, such as copressors and heat exchangers, is essential for advancing the technology. This paper presents an experiental analysis of copact icrochannel ensers for a iniaturescale refrigeration syste. Copper etal foas were used as extended surfaces for enhancing the side heat transfer. Metal foas have several attributes often pursued in high perforance heat * Corresponding author: J. R. Barbosa Jr. Phone: , Fax: E-ail address: jrb@polo.ufsc.br

2 exchangers, such as a large surface area per unit volue, a high theral uctivity of the solid atrix and large heat transfer coefficients. Floyd et al. [3] investigated analytically and experientally the behaviour of copper etal foa heat exchangers in an ipinging water flow arrangeent, for which a perforance siilar to that of a icrochannel heat exchanger has been observed. They also showed that the foa porosity has a high influence on the surface teperature and on the water flow rate. Haack et al. [4] presented an overview of etal foa aterials for high perforance heat exchangers. They showed that, for a fixed coolant flow rate, both the heat exchanger effectiveness and the pressure drop increase with decreasing pore size. Boosa et al. [5] presented an experiental apparatus for evaluating etal foa heat exchangers. In their tests, the perforance of aluiniu etal foa exchangers under forced convection was copared with that of coercial heat exchangers, using water as coolant. The theral resistance of the etal foa heat exchangers was 2 to 3 ties lower than that of the coercial heat exchanger for a fixed puping power. In the present paper, the influence of the nuber of pores per inch (PPI) and of the porosity on the perforance of etal foa ensers is investigated experientally for itions typical of sall scale refrigeration systes for electronic cooling applications. The heat transfer and the side pressure drop were observed to increase with the nuber of PPI and decrease with increasing porosity. EXPERIMENTAL APPARATUS Air loop The loop is presented scheatically in Figure 1. It consists of a therally insulated, fully instruented, closed loop wind tunnel calorieter. Two direct current (DC) blowers with a variable power input control were used to supply through the test section. In order to easure the flow rate, four calibrated nozzles with different diaeters were installed together with two differential pressure transducers; one of the easures the pressure difference between the inlet of the nozzles and the atospheric pressure, and the other easures the pressure drop across the nozzles. A huidity transducer was also placed upstrea of the nozzles. A two-transducer arrangeent identical to the one described above was used in the test section to easure the enser pressure drop. On the whole, fifteen T-type therocouples were installed in the loop. Five of the were installed upstrea of the enser test section and another five were positioned downstrea of the enser. Three therocouples were placed outside the loop for roo teperature control. The two reaining therocouples were used to easure the teperature upstrea of the nozzles. To control the inlet teperature, a set of electric heaters and a cooling coil connected to a therostatic bath were used. The use of diffusion baffles, porous edia and a fan ensures the unifority of teperature and velocity at the tunnel cross section. Refrigerant loop The refrigerant loop is shown scheatically in Figure 2. A variable-speed copressor was used to pup iso-butane (R-600a) through the loop and an oil filter was used to separate residual oil fro the discharged refrigerant. An accuulator was placed downstrea of the enser. The expansion device is a hand-operated needle valve. The refrigerant ass flow rate was easured with a Coriolis-type ass flow eter placed between the accuulator and the needle valve. Resistance teperature detectors (RTDs) were placed upstrea of the copressor and ass flow eter. At the enser inlet, the pressure was easured by eans of an absolute pressure transducer. Two iersion-type therocouples were installed upstrea and downstrea of the enser. The tests itions are controlled through proportional-integral-derivative (PID)

3 icrocontrollers. The copressor inlet teperature, the ass flow eter inlet teperature and the enser inlet teperature were controlled by electric heaters positioned around the connection tubing. The enser inlet pressure was controlled by an electric heater wrapped around the accuulator. The specification of the test itions is such that the refrigerant is in the superheated vapour state at the inlet of the ass flow eter and of the copressor. Figure 1. Scheatic representation of the loop Figure 2. Scheatic representation of the refrigerant loop METAL FOAM CONDENSERS All ensers were anufactured fro copper, and three etal foa saples were used with different nuber of pores and porosity. The nuber of pores is defined as the quantity of pores encountered in an inch of structure, whereas the porosity can be defined as the volue fraction occupied by the fluid. Here, nubers of pores per inch (PPI) equal to 10 and 20, and porosities of and 94.70% were used. The etal foas were brazed at the top and botto of a 3 thick copper plate with seventeen circular parallel icrochannels with a diaeter of 1.5.

4 As illustrated in Figure 3, flows through the porous edia, extracting heat fro the refrigerant fluid that flows (in a cross-flow arrangeent) inside the icrochannels. The length (in the direction of the flow), height and width of the etal foa saples brazed on the plate were 35, 7 and 48, respectively. A enser with sixty 0.5 thick, brazed plain fins was tested for perforance coparison purposes. All prototypes had the sae frontal area. Figure 3. Illustration of a etal foa enser EXPERIMENTAL PROCEDURE To evaluate the perforance of the iniature-scale ensers, a ensing teperature of 90 C was used for all experiental runs. A 2 C inlet superheat was aintained during all tests. The inlet teperature was kept at 50 C, whereas the flow rate was set at three different values: /s, /s and /s. The refrigerant ass flow rate was kept at kg/s. Overall, three different itions were evaluated for each enser. Each test was perfored twice, which resulted in six tests for each heat exchanger. DATA REDUCTION The heat transfer rate easured on the side Q & is calculated through equation (1), where M & is the ass flow rate, c p, is the specific heat, and T in, and T out, are the inlet and outlet teperatures respectively. Q & = M & c p, ( Tout, Tin, ) (1) Siilarly, the heat rejection fro the refrigerant side Q & ref can be evaluated fro Q & ref = M & ref ( hin, ref hout, ref ) (2) where M & ref is the refrigerant ass flow rate, and h in, ref and h out, ref are the enthalpies at inlet and outlet of the enser. The overall heat uctance UA can be obtained fro the ratio of the heat transfer rate Q & and the logarithic ean teperature difference, Δ Tl. UA = Q& / ΔT l (3)

5 As proposed by Boosa et al. [5], the following diensionless quantities can be used to correlate the -side heat transfer and pressure drop f = 2 ( L / D )( ρ u / 2) Δp h (4) ρ = + ( ρ in, ρ out, D = 4A / L h p u = V& / A p ) / 2 (5) (6) (7) where f is the friction factor, V & is the flow rate and A p is the frontal area of the porous edia. In the equations above, ρ are the inlet and outlet densities, respectively, ρ in, and out, L is the heat exchanger length along the flow, and p The Colburn j-factor was calculated as follows Δ is the easured pressure drop. j = h A M& p Pr c 2 / 3 p, (8) where h is the heat transfer coefficient fro the side calculated in the for [5] h = M& c A p, ( out, in, ) con T ( T T ) T, in (9) A con is the interface area between the porous edia and enser plate, and T is the ensing teperature. The Reynolds nuber, as suggested by Kaviany [6], can be obtained fro ρ u Re = μ K (10) μ = + ( μin, μ out, ) / 2 (11) where μ in, and μ out, are viscosity of the at the inlet and outlet, respectively, and K is the pereability of the etal foas. The pereability can be calculated fro a linear regression of Darcy s law as follows V& KA = μ L p Δp (12) The side puping power W & is calculated fro

6 W & = Δ p V & (13) RESULTS As part of the validation procedure of the experiental setup, the energy balance on both sides of the enser, i.e., the side and the refrigerant side, were copared yielding a deviation of ±10%. The ain source of uncertainty was the heat loss fro the loop to the environent through the duct walls. It should be noted that the refrigerant side heat transfer rate was easured only at itions where the refrigerant was a subcooled liquid at the enser outlet. An uncertainty propagation analysis was perfored based on the data reduction equations. The estiated easureent uncertainties of the pressure drop, friction factor, heat transfer, UA and Colburn factor were ±1.2%, ±2.1%, ±5.0%, ±5.7% and ±5.7%, respectively. The effect of the nuber of pores can be analysed by a direct coparison between ensers A and B. Siilarly, the influence of the porosity can be evaluated by coparing ensers B and C. Figure 4 shows the pressure drop as a function of the voluetric flow rate for the three etal foa ensers. As expected, the pressure drop increases with the increasing flow rate. By coparing enser A and B, it can be seen that increasing the nuber of pores increases the pressure drop. Furtherore, decreasing the porosity increases the pressure drop, as seen in Figure 4. Results for the friction factor are shown in Figure 5. The friction factor decreases with increasing Reynolds nuber. Like the pressure drop, the enser with the higher nuber of pores and the lower porosity presents the highest friction factor. The etal foa enser with lower porosity and higher nuber of pores presented a ore copact solid atrix and ore tortuous flow passages, aking the fluid flow ore difficult. Figure 4. Pressure drop versus flow rate Figure 5. Friction factor versus Reynolds nuber Figures 6 and 7 show the heat transfer rate and the overall theral uctance UA as a function of the flow rate. As expected, the heat transfer rate increases with increasing flow rate. In Figure 6, enser A presented higher heat transfer rates than enser B. Thus, a higher nuber of pores resulted in higher heat transfer rates. Siilarly, lower porosity also resulted in higher heat transfer rates. It can be seen in Figure 7 that, as expected, UA increases with increasing flow rate. Like the heat transfer rate, the etal foa ensers with a higher nuber of pores and a lower porosity presented higher heat transfer coefficients. The increase in the nuber of pores for a fixed porosity

7 results in a higher interface area between the solid atrix and the. The decrease in porosity for a fixed nuber of pores results in a saller area available for heat transfer. However, it also results in a larger volue of the solid atrix. This larger volue has becoe ore significant for the heat transfer due to a low theral resistance between the refrigerant fluid and the coolant. Figure 6. Heat transfer versus flow rate Figure 7. UA versus flow rate Figure 8. Colburn factor versus Re Figure 9. UA versus puping power The results of the Colburn j-factor as function of the Reynolds nuber are shown in Figure 8. Like the friction factor, the j-factor decreases with increasing Reynolds nuber. It can be concluded that the increase in the nuber of pores and the decrease in porosity results in higher Colburn factors. Hence, a higher nuber of pores and a lower porosity iproved the theral perforance of the etal foa ensers investigated here. Figure 9 presents UA as a function of the required puping power. For the lower values of puping power, ensers A and B presented a siilar behaviour. Higher values of theral uctance could be obtained with enser A, but at the expense of a ore elevated puping power. By coparing ensers B and C, it can be seen that a lower porosity resulted in a higher theral uctance, for approxiately the sae puping power. In addition, Figure 9 also shows that the required puping was lower in the conventional (plain fin) enser than in the etal foa ensers. So, at least for the itions investigated, the use of a etal foa enser in a iniature-scale refrigeration syste ay not be advantageous.

8 CONCLUSIONS In order to consider the feasibility of their application as part of a sall scale refrigeration syste, a series of tests was perfored to assess the theral-hydraulic perforance of three etal foa ensers using as coolant. The nuber of pores per inch (PPI) and the porosity were varied and their influence on the ensers perforance was analysed. A refrigerant loop facility that uses iso-butane as refrigerant and a wind tunnel calorieter to control the flow itions were utilized for the iniature-scale heat exchangers perforance evaluation. The ensing teperature used in the experients was 90 C and the inlet teperature was set at 50 C. Three different flow rates and one refrigerant ass flow rate were tested. It was observed that the increasing the nuber of PPI and decreasing the porosity resulted in a higher -side pressure drop and in a higher overall theral uctance. For a fixed puping power, the overall theral uctance of the etal foa ensers was lower than that of a plain fin enser with siilar characteristics. ACKNOWLEDGEMENTS This study was perfored under a joint research prograe between the Federal University of Santa Catarina and EMBRACO. The support fro Eng. Augusto P. Zierann (EMBRACO) is greatly appreciated. Financial support fro CNPq and FINEP is also acknowledged. REFERENCES 1. Ortega, A., and Birle, J. R., The evolution of Air Cooling in Electronic Systes and Observations about its Liits, Proc. 18th National & 7th ISHMT-ASME, Guwahati, pp K114-K126, Trutassanawin, S. and Groll, E., Nuerical analysis of a iniature-scale refrigeration syste (srs) for electronic cooling, International Refrigeration and Air Conditioning Conference, Purdue, R173, pp 1-10, Floyd, D. E., Topolski, M. J and Darabi, J., Design and evaluation of copper etal foas in cold plates, ASME International echanical engineering congress and exposition, Chicago, Haack, D. P., Butcher, K. R., Ki, T., Lu, T. J., Novel lightweight etal foa heat exchangers, ASME International echanical engineering congress and exposition, Orlando, Boosa, S. S., Poulikakos, D., Zwick, F., Metal foa as copact high perforance heat exchangers, Mechanics of aterials, Vol. 35, pp , Kaviany, M. Principles of Heat Transfer in Porous Media, 2 nd Edition, Springer, 1995.