Carbon Dioxide and R410a Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern in Horizontal Tubes at Low Temperatures

摘要

Carbon dioxide (CO2) has been seriously considered as an alternate refrigerant for HCFC and HFC fluids,due to the increasing interest of environmentally safe refrigerants in air-conditioning and refrigeration systems. Inthis study, CO2 flow boiling heat transfer coefficients and pressure drop are measured in macro-scale (6.1 and 3.5mm) tubes at evaporation temperatures of ???15 and ???30 ??C. The measured results show that the nucleate boiling is amain heat transfer mechanism in the 6.1 mm tube and the contribution of convective boiling becomes greater withthe decrease of tube diameters and the increase of mass fluxes. The surface roughness of the 6.1 and 3.5 mm tubeare presented by SEM and AFM images and surface profiles, and it is shown that the rougher surface of the 6.1 mmtube can affect the flow boiling heat transfer. The CO2 heat transfer coefficients and pressure drop are measured in amini-scale (0.89 mm) multi-ported tube at the evaporation temperature of ???30 ??C. Also, R410A and R22 flowboiling heat transfer coefficients and pressure drop in a macro-scale (6.1 mm) tube were measured, and they arecompared with CO2. This comparison presents that the CO2 flow boiling heat transfer coefficients are higher thanR410A and R22 at low vapor qualities, and CO2 pressure drop is significantly lower than R410A and R22. Thisadvantageous characteristic for CO2 could be explained by properties such as surface tension, reduced pressure, andthe density ratio of liquid to vapor. The prediction of heat transfer coefficients and pressure drop was performed bygeneral correlations and the calculation results are compared with measured values. Two-phase flow patterns werevisualized for CO2 and R410A in the 6 and 3 mm glass tubes, and they are compared with the Weisman et al. andthe Wojtan et al. flow pattern maps. The flow pattern maps can determine the flow patterns relatively well, exceptthe transition from intermittent to annular flow.