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Abstract
In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm2 cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m2; the mass flux varied within 200–600 kg/m2-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling heat transfer, considering nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.
NOMENCLATURE
| a | = | channel depth, (m) |
| b | = | channel width, (m) |
| Box | = | local liquid flow Boiling number qw/(G(1-x)hfg), (−) |
| Ca | = | capillary number, (−) |
| Dh | = | hydraulic diameter, (m) |
| E | = | evaporation factor, (−) |
| F | = | forced convection enhancement factor, (−) |
| G | = | total mass flux, (kg/m2-s) |
| hfg | = | latent heat, (J/kg) |
| h | = | heat transfer coefficient, (W/m2-K) |
| l | = | distance from leading edge of heat sink, (m) |
| M | = | molecular mass, (g/mol) |
| Nu | = | Nusselt number, (−) |
| Pr | = | Prandtl number, (−) |
| P | = | pressure, (Pa) |
| pr | = | reduced pressure, (−) |
| pcr | = | critical pressure, (Pa) |
| q | = | heat flux, (W/m2) |
| Re | = | Reynolds number, (−) |
| Ra | = | roughness, (μm) |
| S | = | nucleate boiling suppression factor, (−) |
| We | = | Weber number, (−) |
| x | = | vapor quality, (−) |
Greek Symbols
| α | = | void fraction, (−) |
| δ | = | thickness, (m) |
| δeb | = | elongated bubble film thickness, (m) |
| γ | = | aspect ratio b/a, (−) |
| λ | = | heat conductivity, (W/m-K) |
| μ | = | dynamic viscosity, (Pa-s) |
| ν | = | kinematical viscosity, (m2/s) |
| ρ | = | density, (kg/m3-s) |
| σC | = | flow cross-sectional area expansion ratio, (−) |
| τ | = | shear stress, (Pa) |
| Ψsup | = | boiling suppression factor, (−) |
Subscripts
| a | = | acceleration |
| boil | = | boiling |
| con | = | convection |
| ev | = | evaporation |
| gas | = | vapor |
| i | = | ith position |
| f | = | friction |
| liq | = | liquid |
| liq,0 | = | all flow as liquid |
| st | = | stainless steel |
| tin | = | tin |
| tot | = | total |
| w | = | wall |
Additional information
Notes on contributors
Vladimir V. Kuznetsov
Vladimir V. Kuznetsov is head of the Department of Engineering Thermophysics and Laboratory of Multiphase System at the Institute of Thermophysics Siberian Branch of the Russian Academy of Sciences. He also is a professor at Novosibirsk State University, Russia. He is a graduate of Novosibirsk State University (1972) and he was awarded the Ph.D. (1978) and D.Sc. (1995) at the Institute of Thermophysics Siberian Branch of the Russian Academy of Sciences. His current research interests are concerned with the study of capillarity, fluid dynamics, heat and mass transfer in multiphase and multicomponent flows, including the phase-change phenomena, and catalytic reactions in mini- and microchannel systems. He has published more than 220 articles covering a variety of topics. He is a member of the National Committee for Heat and Mass Transfer of Russian Academy of Sciences, and a member of the editorial boards of Thermal Processes in Engineering and Journal of Engineering Thermophysics.
Alisher S. Shamirzaev
Alisher S. Shamirzaev is a researcher at the Institute of Thermophysics Siberian Branch of Russian Academy of Sciences. He is a graduate of Novosibirsk State University (1993) and he was awarded the Ph.D. (2007) at the Institute of Thermophysics Siberian Branch of the Russian Academy of Sciences. His research interests are concerned with the study of capillarity, fluid dynamics, and heat and mass transfer in multiphase flows, including the phase-change phenomena in mini- and microchannel systems. He has published more then 20 articles in journals and conference proceedings.