Single-phase and flow boiling cooling in multiple miniature curvilinear channels

Transcription

1 Thermal Challenges in Next Generation Electronic Systems, Joshi & Garimella (eds) 2002 Millpress, Rotterdam, ISBN Single-phase and flow boiling cooling in multiple miniature curvilinear channels R. Scott Downing Hamilton-Sunstrand, Division of United Techn. Rockford, IL, USA T: (825) , F: (815) , E: Gunol Kojasoy Dept. of Mechanical Engineering, University of Wisconsin-Milwaukee, WI, USA T: (414) , F: (414) , E: Keywords: electronic cooling, two-phase flow, cold plate technology ABSTRACT: This paper describes an experimental investigation of heat transfer in miniature helical flow passages under single- and boiling two-phase flow conditions. A series of test sections were designed and fabricated to cover a range of feature dimensions and hydraulic parameters of a typical miniature helical cold plate for electronic cooling. As a baseline for the curved channel experiments, additional linear channel test sections were also built and experimented by using R 134a as a working fluid. Results are discussed in terms of experimental data, and some experimentally based simple correlations are offered for the prediction of heat transfer coefficient in miniature helical channels under single- and two-phase flow boiling conditions. 1 INTRODUCTION The miniaturization of electronic circuits, higher power levels per chip and increased packaging densities have driven the trend in electronics packaging toward higher heat fluxes. Bar-Cohen (1983) provided an excellent review of chip characteristics and heat dissipations of state of the art digital processors and concluded that heat dissipation of future digital processing chips could reach the 100 W/cm 2 level. Mackowski (1991) predicts that the most challenging problems associated with current and near term military electronic devices lie with power controllers, with steady-state heat fluxes in the 100 W/cm 2 and 200 W/cm 2 range. In fact, pulsed transient heat loads of short duration (approximately a second or less) may exceed 400 W/cm 2. In order to accommodate such high heat fluxes, high performance cooling techniques are required to keep junction temperatures low for acceptable electronics reliability. High performance cooling must provide low junction coolant thermal resistance and the ability to absorb high heat fluxes. Additionally, for aerospace applications the cooling technology must be tolerant to adverse g-fields and be able to operate in all attitudes (gravitational orientations). The cooling system should have a minimum impact to the host aircraft, i.e., low coolant pumping power, low initial and operating costs, and be lightweight and compact. The present study was motivated by the need for accurate experimental data in miniature curved flow passages in the design of small high performance heat exchangers devised at Hamilton- Sundstrand. This novel high-performance cooling approach is aimed at providing extremely low thermal resistance (approx K/W-cm 2 ), and operating up to very high burnout heat fluxes of greater than 200W/cm 2, anticipated in future aerospace power electronics. A unique feature of such a cold plate is the use of forced convective flow boiling in multiple miniature curvilinear flow passages providing enhanced heat transfer and higher critical heat fluxes as compared to flow in straight flow channels. A key design feature of this technology is that the centrifugal forces developed in curvilinear flow should be tolerant to external g-fields. This cold plate technology is acronymed the spiral flow compact high-intensity cooler (SFCHIC). Among the several curved flow passages considered in the conceptual design stage, helically coiled channels were chosen since: 1) The effects of curvature can best be experimentally investi- 187

2 gated with helical passages of constant curvature rather than with a spiral flow passage which has a continuously changing curvature, 2) Helically coiled channels provide an advantage in applications where the total liquid stream must be vaporized or nearly vaporized, 3) The centrifugal forces have been shown to maintain wet wall conditions to higher qualities than would persist in a straight tube, 4) The heat transfer coefficient is also improved due to the secondary flows and wet wall conditions, and finally, 5) Helical flow geometry would be a logical choice for two phase flow heat transfers in adverse g-field or in micro gravity. An extensive literature search conducted by Downing (1999) covered the topics of single- and two-phase flow and heat transfer in helical and small channels. This literature provided an understanding of basic principles and predictive tools relevant to predicting flow phenomena, pressure drop and heat transfer in small curved channels needed to design a SFCHIC type cold plate. Specifically, boiling two-phase flow phenomena in small curved channels have not been studied and extrapolating correlation in the current literature base must be done cautiously. It is therefore, important to experimentally confirm that correlations developed outside the current published data range could be extrapolated, or modifications to those experimentally based predictive methods need to be established. Since the fundamentals of single-phase flow in straight channels provide the basis for understanding and modeling two-phase flow phenomena, experimentation was done in miniature straight and curved channels in single-phase liquid flow as the basis for curved channels and two-phase flows. The philosophy of experimentation was to build a database and experimentally based correlation methodology starting with simple flow conditions and modifying models to account for the complicating effects. For example, the first set of tests was performed with single-phase, straight channel flow under adiabatic flow conditions. The effects of curvilinear and boiling two-phase flow were then experimentally evaluated and correlated as modifying factors to the straight channel single-phase results. This paper presents the results of the single- and two-phase pressure drop experiments in both straight and helical channels. 2 EXPERIMENTAL SETUP 2.1 Flow loop The flow loop schematic is shown in Figure 1. The loop contains a pump, two accumulators--one bladder type and the other gravity type--two condensers, a pre-heater, a heater and a heat-flux amplifier. The entire loop is mounted on a two-level uni-strut frame structure except for the pump pallet. The loop is fully instrumented with absolute pressure and differential pressure transducers, thermocouples and high accuracy Coriolis type flow meters. The bladder accumulator provided sufficient back pressure from a nitrogen tank for adequate NPSH to pump the Refrigerant 134a. The refrigerant passed through a throttling valve and a series of flow meters before entering into an electrical preheater. This automatic PID controlled electrical heater regulated the downstream temperature to user specified set point. For the experiments reported here the preheaters provided a subcooled liquid at the test section. The preheated refrigerant then passed through an expansion valve of appropriate size before being led to the test section. These expansion valves finely adjusted the flow and pressure to the test section. Much of the experimentation was carried out under two-phase flow conditions. Because the single-phase portion before the onset of boiling must be analytically removed from two-phase flow data, it was preferred to minimize the inlet subcooling. The expansion valve, after the preheater was also used for fine tuning of the inlet subcooling to the test section. After regulating the degree of subcooling the refrigerant entered the test section. Heat is applied to the test section by a large pyramid-type copper heat flux amplifier. 188 R.S.Downing & G.Kojasoy

3 2.2 Test sections and instrumentation Eight test section patterns were designed to cover a wide range of geometric variables and be compatible with the heat flux amplifier. Two test patterns were combined into one test block, and four test blocks were fabricated. As a baseline to testing of curvilinear flow passages, two test patterns were designed to have straight flow channels. The linear patterns designated L1 and L2, were build into one test block. By design, the linear channels are similar triangles of different size. The flow passage area of the larger passage, L2, is 50% larger than the smaller pattern, L1. The number of parallel flow paths was selected so that the hydraulic resistance of each pattern was similar. As shown in Figure 2, the smaller passage pattern has 20 parallel paths and the larger passage pattern has 12 parallel flow channels. The spacing of the paths was selected to spread the selected number of channels over the width of the heat flux amplifier footprint. Each of the other three test blocks contained two helical flow test patterns. The test pattern pairing was randomized to provide blocking of uncontrolled factors in both fabrication and experimentation stages. The three helical test blocks contained patterns B and F, patterns C and D, and patterns E and G. The nominal geometric characteristics of these helical patterns as well as the linear patterns, L1 and L2, are summarized in Table 1. As an example for the helical sections, a drawing of patterns C and D is illustrated in Figure 3. The final operation in fabricating the test block was to attach thermocouples into both the heated and unheated surfaces of the block. High quality type-t thermocouples with 32 gauge wire attached with thermally conductive epoxy. Surface slots, inches deep by inches wide were milled into the test blocks for the thermocouples. Each test pattern was instrumented by eight thermocouples to evaluate the temperature distributions along each test section. Detailed thermocouple locations for linear and helical test patterns can be found in Downing (1999). Six thermocouples were located on the heated side whereas the other two on the unheated side of the test block. In addition to the test section instrumentation, instrumentation in the flow loop that was directly tied into the data acquisition included the following: Six pressure transducers Eighteen T-type thermocouples on heat flux amplifier and 8 on flow loop, Coriolis flow meter, and Turbine flow motor Figure 1. Schematic description of flow loop Single-phase and fl ow boiling cooling in multiple miniature curvilinear channels 189

4 Figure 2. Linear test section. Patterns, L1 and L2 Figure 3. Hellical test section, C and D Table 1. Test section channel geometries Text Pattern No. of Helices No. of Helices Helix Diameter Hydraulic Diameter Inverse of In Parallel in Series [inch] of Channel [inch] Curvature (D/d) L L B C D E F G RESULT AND DISCUSSIONS The purpose of the present study is to uniquely relate the single-phase flow in helical channels to flow behavior in linear channels providing a basis for a two-phase flow in helical channels. The experimentally based modeling process builds from the basic geometries (straight channels) to helical passages and from single- to two-phase flows. For example, curved tube heat transfer coefficients are modeled with a heat transfer enhancement factor on the established straight tube heat transfer coefficients. In a similar approach, two-phase multipliers are used to relate two-phase heat transfer performance to single-phase performance. 3.1 Single-phase flow The single-phase flow data in straight channels, L1 and L2, provided insight into the heat transfer coefficient behavior and the transition from laminar to turbulent flow. A striking feature of the present single-phase flow data was the trend of a "blended" transition from laminar to turbulent flow. This is a unique characteristic of channel shapes of flow regions bounded by wall at acute angles This was observed by Eckert and Irvine (1969), in their extensive experimentation on isoscelestriangular passages of several apex angles. Single-phase heat transfer results from the linear test sections compared with the Dittus-Boelter correlation. The data is reasonably well represented by the correlation in the turbulent flow region. Although the over-all data shows a blended transition from the laminar to turbulent flow, at the lower Reynolds number the measured heat transfer coefficient falls about 30% under the predicted value. The lower than predicted results at low Reynolds number is due to the fact that the Dittus- Boelter correlation was based on the fully-developed turbulent flow. Following the single-phase flow in circular helical channel studies of Mori and Nakayama (1965 & 1967), Nag and Chakrabarty (1982), Clark (1974), Kalb and Seider (1972, 1974 & 1983), 190 R.S.Downing & G.Kojasoy

5 Xin and Ebadian, (1997), and Kadombi et al. (1986), either of the following form of correlations was adapted for the present study of the single-phase flow in miniature non-circular flow channels. Nu= f ( Re, d D, Pr) or Nu = f ( De, d D, Pr) (1) Here De is the Dean number defined by 1 / 2 De = Re( d / D) (2) In these equations Re is the Reynolds number based on the hydraulic diameter, d, of the flow channel, and D is the helical coil diameter. In fact, d/d represents the relative curvature of helix. It is to be noted here that for tubes of modest curvatures (d/d < 0.1), the Dean number fully characterizes dynamic similarities of fluid flow through a helix. However, for tubes of very tight curvature there is an additional dependence on the relative curvature d/d above that contained in the definition of the Dean number (Kalb and Seider, 1974). The above referenced predictive correlations were compared with the experimental data of the present study covering the helical test patterns of B, C, D, E, F and G. Although there was a large scatter observed between the measurements of heat transfer coefficient and the predictions, a unique feature of this comparison was the fact that the majority of experimental data fell well above the predictions for the range of experimental conditions. In view of this discrepancy, a new correlation was sought in the form of Eq. (1). The single-phase heat transfer in the miniature helical channels of the present study is best correlated by ( De) ( d ) Pr Nu = D (3) for range of 3.6 < D/d < 13.4; 800 < De < 4000 and Pr 3.6 Figure 4 shows the ratio of the measured to predicted Nusselt number at each data point. From this figure it is evident that the experimental data is predicted by Eq. (1) within m 20% margin. Considering the wide variations in curvatures, the Dean number and the Reynolds number, the data scatter is quite reasonable. With high heat transfer coefficients and large surface area densities of the present test articles the overall conductance between the test block and the fluid is high. In heat exchanger terminology the helical test articles have large NTU's and will perform at effectiveness of near unity. The outlet temperature closely approaches the test block temperature, and consequently, significant changes in the heat transfer coefficient results in only minor changes in the fluid outlet temperature. Unfortunately, this situation made it difficult to precisely measure the heat transfer coefficient. Limitations on the experimental accuracy by the poor resolution due to the high efficiency of the test articles as heat exchangers are the primary contributor to the scatter observed on Figure 4. However, in spite of this situation, a general correlation for single-phase flow in helical coils predicts the data with a reasonable accuracy of m 20% range. 3.2 Two-phase flow Recently, Kandlikar (1990, 1998) has published a series of flow boiling correlations that have gained wide acceptance for straight channels. It was observed that this method provides a better representation of the present data, generally within m 40%. However, the method actually over predicts most of the present data, primarily for the higher heat flux data range. The correlation has convective and boiling terms, providing a way to tune the correlation for the relative strength of both mechanisms. The following equation, closely following the method for Kandlikar, was found to predict 90% of the linear channel two-phase flow boiling data to within m 30% h h = 1.18C Fr + 850B (4) tp csp o csp o where h csp is defined as the convective heat transfer coefficient for the single-phase liquid flow as given by the Dittius-Boelter correlation, Fr csp is the single-phase liquid Froude number, B o is the boiling number and C o is the convection number. These are defined as follows Fr G ρ g D; B q& Gh ; and 1 csp f o f C ρ ρ o x (5) x g f Single-phase and fl ow boiling cooling in multiple miniature curvilinear channels 191

6 Figure 4. The ratio of predicted to measured Nusselt numbers for single-phase heat transfer helical channels figure 5. two-phase flow in linear channels The present experimental conditions that were used to arrive at the modified Kandlikar's correlation are: 1000 < Re < 6000, 0.2 < χ tt < 5, 5 < q & < 25 and 50 < G < 500. The modified Kandlikar correlation is compared on Figure 5 to the measured heat transfer for linear test article, L2. The predictions are good within m 30% margin. Figure 6 shows the measured flow boiling heat transfer coefficient as a function of the mass flow rates for three data sets. Each data set corresponds to a constant heat input, 250 W, 350 W and 450 W, respectively. This figure illustrates a very interesting and important trend. At a given heat flux, the integrated heat transfer coefficient is nearly independent of mass flow and, therefore, quality. This would support Wambsganss (1992) conclusion that at high heat flux levels the heat transfer performance is independent of mass flux. The observed effective heat transfer coefficients in helical channels investigated here were nominally 50 to 100% higher than would be obtained in straight channels operating under the same mass fluxes and qualities. Most of this enhancement is expected to be attributed to the curvilinear flow effects. The controlling heat transfer mechanisms can be determined by inspection of the heat transfer performance as a function of various variables. Convective heat transfer component will increase at higher qualities, but at some point the nucleation degrades into film evaporation. A peak in heat transfer at some intermediate quality (and flow) was observed for most runs at constant heat input. It is assumed that this peak is at the location where the convective heat transfer enhancement is overtaken by the loss in nucleation. Figure 7 is representative of this trend. The peak in heat transfer coefficient occurs at outlet qualities between 35% to 55% range, depending on the heat flux level. This phenomena is directly linked to the optimum value of Martinelli parameter. For almost all of the experimental data, including all helical coil test patterns, the highest measured heat transfer performance occurred at inverse Martinelli parameter of 0.5 to 1 range. The measured heat transfer coefficient against the inverse Martinelli parameter for the helical channel test pattern F is shown in Figure 8. The same trend was also observed in parametric studies of the heat transfer versus average liquid Dean number. A phenomenologically based correlation that reflects the convective and nucleate boiling trends was found to be 0.3 h tp = 0.15hcsp( 1 χ ) + 730q & (6) tt where h csp is the single-phase heat transfer coefficient of the liquid, flowing alone, in the coil. The curved channel single-phase algorithm which is presented in the preceding section is used to evaluate h csp. A scatter plot showing the correlative error including all test patterns is shown in Figure 9. Considering the measurement difficulties and the uncertainties associated with the experimentation, an error margin of m 15% observed in this figure encouraging. However, it should be pointed out that although the approach taken in this study was phenomenologically based on the superposition principle, the results should be used with caution when applied to those cases out of the present ex- 192 R.S.Downing & G.Kojasoy

7 perimental range. This is particularly true for the use of proposed correlations for other fluids since the fluid properties have not been scaled properly other than by the Prandl number for the singlephase flow and the Martinelli parameter for the two-phase flow. Figure 6. Measured heat transfer coefficient as a function of mass flow and heat input Figure 7. Measured two-phase flow heat transfer coefficient against outlet quality Figure 8. Measured two-phase flow heat transfer coefficient against inverse Martinelli parameter Figure 9. Comparison of measured and predicted heat transfer coefficient for helical channel 4 SUMMARY AND CONCLUSIONS Experiments were conducted to develop heat transfer correlations valid for the geometrics and flow conditions expected in miniature helical cold plate technology. The heat transfer results for singleand two-phase flow in eight different test sections containing two linear and six curvilinear passages of different sizes and curvatures were correlated. Using both single-and boiling two-phase flow in these eight test sections, curvature effects on the heat transfer performance of helically coiled miniature flow passages were investigated in terms of heat transfer enhancements. Experimental investigation showed that for the range of experimental conditions the Dittius- Boelter correlation represented the single-phase flow data of linear test sections reasonably well. However, a significant enhancement in heat transfer was observed in curvilinear test sections. A new experimentally based correlation was developed in terms of the Dean number. Experiments under two-phase flow conditions in linear test sections revealed the fact that at a given heat flux, the integrated two-phase flow heat transfer coefficient is nearly independent of mass flux and therefore the quality. Although the higher heat fluxes have higher heat transfer coefficients, at high heat flux levels, the heat transfer performance is independent of the mixture mass flux. A slightly modified version of the Kandlikar's correlation represented the data in linear minia- Single-phase and fl ow boiling cooling in multiple miniature curvilinear channels 193

8 ture channels. Influence of the heat flux and the Martinelli number were studied for the helical miniature channels, and a correlation based on the superposition principle was developed in terms of the nucleate boiling and the two-phase flow convective components. REFERENCES Bar-Cohen, A Thermal Design of Immersion Cooling Modules for Electronic Components, Journal of Heat Transfer Engineering, Hemisphere Press. Clark, J. W. G Heat Transfer in Coiled Tubes, ICI Report, CL-B, Research Note, JWG C/74/9. Dean, W. R Notes on the Motion of Fluid in a Curved Pipe, The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, Vol. 15, pp Dean, W. R The Steamline Motion of Fluid in a Curved Pipe, The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, Vol. 45, pp Downing, R. S Analytical and Experimental Study of Single and Two-phase Cooling in Miniature Straight and Helical Channels, Ph. D. Dissertation, Department of Mechanical Engineering, University of Wisconsin- Milwaukee. Eckert, E. R. C. & Irvine, Jr. T. F Pressure Drop and Heat Transfer in a Duct with Triangular Section, ASME Journal of Heat Transfer. Kadombi, V. et al Heat Transfer and Pressure Drop in a Helically Coiled Rectangular Duct, ASME, Journal of Heat Transfer, 83-WA/HT-1. Kalb, C. E. & Seider, J. D Fully-Developed Flow Heat Transfer in Curved Circular Tubes with Uniform Wall Temperature, AICHE J. Vol. 20, pp Kalb, C. E. & Seider J. D Entrance Region Heat Transfer in Uniform Wall-Temperature Helical Coil with Transition from Turbulent to Laminar Flow, Int. J. Heat and Mass Transfer, Vol. 26, pp Kandlikar, S. G A General Correlation for Saturated Two-Phase Flow Boiling Heat Transfer Inside Horizontal and Vertical Tubes, ASME Journal of Heat Transfer, Vol. 112, pp Kandlikar, S. G Heat Transfer Characteristics in Partial Boiling, Fully-Developed Boiling and Significant Void Flow Regions of Subcooled Flow Boiling, ASME Journal of Heat Transfer, Vol. 120, pp Mackowski, M. J Requirements for High Heat Flux Cooling for Future Avionics Systems, SAE Paper No Mori, Y. & Nakayama, W Study on Forced Convection Heat Transfer in Curved Pipes--Part I. Laminar Region, Int. J. Heat and Mass Transfer, Vol. 8, pp Mori, Y. & Nakayama, W Study on Forced Convection Heat Transfer in Curved Pipes--Part II. Turbulent Region, Int. J. Heat and Mass Transfer, Vol. 10, pp Nag, P. K et al Turbulent Forced Convection Heat Transfer in Helically Coiled Tubes with Uniform Wall Temperature, J. Inst. Engrs. vol. 63, pp Wambsganns, M. W et al Boiling Heat Transfer in a Horizontal Small-Diameter Tube, Trans. ASME, Journal of Heat Transfer, Vol. 115, pp Xin, R. C. & Ebadian, M. A The Effects of Prandtl Numbers on Local and Average Convective Heat Transfer Characteristics in Helical Coils, ASME, Journal of Heat Transfer, Vol. 119, pp R.S.Downing & G.Kojasoy