Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part I – Pressure drop

Etienne Costa-Patry, Bruno Michel, Bogdan Alexandru Nichita, Jonathan Olivier, John Richard Thome
Elsevier, 2011
Article

Résumé

This article is the first part of a study on flow boiling of R236fa and R245fa. This part presents pressure drop measurements obtained on a silicon multi-microchannel evaporator with 85 urn wide and 560 mu m high channels separated by 46 mu m wide fins. The 135 microchannels were 12.7 mm long. Dielectric refrigerants R236fa and R245fa were used as the evaporating test fluids. The inlet saturation temperature was maintained at 30.5 degrees C while the mass fluxes were varied from 499 to 1100 kg/m(2)s and the base heat flux was tested from 130 to 1400 kW/m(2). A new experimental technique was developed to measure the outlet pressure losses, which represented up to 30% of the total pressure drop and thus cannot be neglected. The microchannel pressure drop measurements were very well predicted by the method of Cioncolini et al. (2009). (c) 2011 Elsevier Inc. All rights reserved.

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Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

Etienne Costa-Patry, Bruno Michel, Jonathan Olivier, John Richard Thome

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 mu m wide and 560 mu m high channels separated by 46 mu m wide fins. There were 35 local heaters and temperature measurements arranged in a 5 x 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 degrees C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m(2), the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness. (c) 2011 Elsevier Inc. All rights reserved.

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This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co approximate to 0.99) and 1.03 (Co approximate to 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1]. (C) 2011 Elsevier Inc. All rights reserved.

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