Water flow boiling heat transfer in vertical minichannel

Abstract

In recent years, microchannel water flow boiling has demonstrated excellent heat removal capabilities, particular for nuclear reactor safety utilities, ranging from mini-scale cooling devices to chemical industry plant purposes. Of particular interest is cooling technology using water as the primary refrigerant. This paper presents the results of an experimental investigation into the water boiling heat transfer in a minichannel conduit vertically oriented with the upward flow. The test section was a circular minichannel tube made of stainless steel with a 2.12 mm inner diameter. To observe the flow regime, the test section was equipped with sight tubes made of transparent perfluoroalkoxy alkane resin and Pyrex glass at the inlet and outlet, respectively. Water flow boiling experiments were conducted at a saturation temperature of approximately 100 °C, with the mass flux of water as the working fluid from 20 to 120 kg/m2·s, and the average vapor quality in the test section inlet of approximately 0.05 to 0.9. The predicted two-phase flow regimes at the test section were classified as three patterns, namely slug, annular, and churn flows, using a flow pattern map with the vapor quality as the abscissa versus the mass velocity as the ordinate. The flow patterns were confirmed and visualized by observations at the test section outlet sight tube. The characteristics of the heat transfer and fluid flow were analyzed. Moreover, the forced convective heat transfer coefficient and pressure drop in the test section were determined based on the experimental data. The highest mass flux yielded the highest convective heat transfer coefficient. Despite this significant heat transfer coefficient improvement, the pressure loss was higher than that for a higher mass flux, indicating that the pressure loss was more pronounced owing to the presence of the minichannel at a higher mass flux.