Refrigerant flow boiling in microchannel evaporators

Stefan Sulpiz Bertsch, Purdue University

Abstract

This dissertation focuses on the investigation of refrigerant flow boiling heat transfer in multi-microchannel copper cold plate evaporators. A comprehensive review of recent studies related to saturated flow boiling in minichannels and microchannels is first presented. The review focuses on the functional dependence of heat transfer characteristics on thermodynamic vapor quality. Results from recent investigations in the literature are compiled and analyzed. Correlations for flow boiling are quantitatively assessed by comparing them against independent data sets from the published literature. Flow boiling heat transfer data for the refrigerants R-134a and R-245fa in multi-microchannel copper cold plate evaporators are obtained using a carefully designed and fabricated experimental setup. The first test section contains 17 channels with hydraulic diameters of 1.09 mm. The second test section contains 33 channels with 0.54 mm hydraulic diameter. The heat transfer coefficient is measured locally for a thermodynamic vapor quality range from -0.2 to 0.9 (negative quality indicates subcooled inlet conditions), a saturation temperature range from 8 to 30°C, a mass flux range from 20 to 350 kg m-2 s-1 and heat fluxes ranging from 0 to 22 W cm-2. Measurement results were repeatable and showed low measurement uncertainty. The heat transfer coefficient is found to vary significantly with heat flux and vapor quality, but only slightly with saturation pressure and mass flux for the range of values investigated. Finally, a composite correlation including nucleate boiling and convective heat transfer terms is developed. The values predicted by the correlation are compared to a database of 3899 measured heat transfer coefficients from 14 studies covering 12 different wetting and non-wetting fluids and a range of hydraulic diameters from 0.16 to 2.92 mm. The database encompasses mass fluxes from 20 to 3000 kg m-2 s-1, heat fluxes from 0.4 to 115 W cm-2, vapor qualities from 0 to 1, and saturation temperatures from -194 to 97°C. The correlation is able to predict the measured heat transfer coefficients with a mean absolute error of 28% even though several measured data sets show opposing trends to the predictions with respect to some operating parameters.

Degree

Ph.D.

Advisors

Garimella, Purdue University.

Subject Area

Mechanical engineering

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