Forced flow boiling of carbon dioxide in horizontal mini-channel

摘要

This paper covers a wide spectrum of thermal flow behavior, including flow patterns, heat transfer, pressure drop, critical heat flux of flow boiling carbon dioxide at high pressure in horizontal mini-channels. The presented experimental data covers relatively wide ranges: tube diameters from 0.51 mm to 3.0 mm, mass flux from 80 kg/m~2s to 900 kg/m~2s, heat flux from 5 kW/m~2 to 40 kW/m~2, pressure/saturation temperature from 4.0 MPa/5.30℃ to 7.0 MPa/28.7℃. The carbon dioxide at high pressure has small density difference between vapor and liquid and low surface tension, and shows a slightly different structure of the flow pattern from so far observed conventional two-phase flow with air and water and/or larger diameter tubes. So far proposed transition criteria of flow pattern are as a whole ineffective in the present range of experiment, and the discrete bubble model developed by the authors demonstrates its high potential in predicting flow patterns. The phase mal-distribution in the cross-section becomes rather significant beyond a critical Bond number, while less significant or almost axi-symmetric below the critical Bond number. This significant phase mal-distribution leads to the intermittent dryout at the upper wall of the tube, while below the dryout heat flux the boiling heat transfer is dominated by the nucleate boiling mode, being well predicted with conventional correlations.