CO₂ flow boiling heat transfer and flow pattern at low temperatures in horizontal smooth tubes

In this study, flow boiling heat transfer coefficients and flow patterns CO₂ are examined in horizontal smooth tubes with inner diameter 6.1 and 3.5 mm at low temperatures. In order to measure the heat transfer coefficients, the test tube was heated by two brass pieces maintained a higher temperature than CO₂ by a secondary fluid. Flow visualization was carried out at adiabatic conditions. This research was performed at evaporation temperatures of -15 and -30°C, mass flux from 100 to 400 kg/m² s, and heat flux from 5 to 15 kW/m² for vapor qualities ranging from 0.1 to 0.8. The CO₂ heat transfer coefficients for the 6.1 and 3.5 mm tubes had nucleate boiling dominant heat transfer characteristics such as the strong dependence on heat fluxes. However, enhanced convective boiling contribution was presented for the 3.5 mm tube at 400 kg/m² s. The presented heat transfer coefficients indicated the reduction of heat transfer coefficient as mass flux increased at low quality regions and also showed that dryout did not occur until the high quality region of 0.8, for mass fluxes of 200 and 400 kg/m² s. The measured heat transfer coefficients were compared with predicted values with some general correlations to predict flow boiling heat transfer coefficients. The pictures of visualized flow patterns were presented and the flow patterns were compared with a flow pattern map. They were used to explain the relation between the flow boiling heat transfer coefficient and vapor quality at the mass flux of 100 kg/m² s.