Characteristics of Flow Boiling Heat Transfer of Sub-critical CO_2 in Mini-channels with Micro-fins

摘要

The flow boiling heat transfer characteristics of sub-critical CO_2 in mini-channels with micro-fins was investigated. The tests used a 0.6 m long extruded aluminum tube with 8 flow-channels 1.7 mm inner diameter with 0.16 mm high micro-fins. The average heat transfer coefficients during evaporation were measured at qualities from 0.1 to 0.9 for saturation temperatures of 1-15 °C, mass fluxes of 100-600 kg/m~2s and heat fluxes of 1.67-8.33 kW/m~2. The results showed that the CO_2 heat transfer coefficient was mainly a function of the heat flux and evaporating temperature, increasing with increasing heat flux or evaporating temperature, with dry-out occurring earlier with increasing heat flux or decreasing evaporating temperature especially at low heat fluxes. The mass flux had less effect on the heat transfer coefficient compared to the heat flux and the saturation temperature. Applicable flow boiling heat transfer coefficient correlation of sub-critical CO_2 in mini-channels with micro-fins was also developed.