SUBCOOLED FLOW BOILING HEAT TRANSFER CHARACTERISTICS OF R134A IN HORIZONTAL HELICALLY COILED TUBES

Subcooled flow boiling heat transfer of R134a in helically coiled tubes was investigated experimentally. Experiments were carried out at pressure ranging from 412 to 653 kPa, inlet subcooled temperature from 5.0 to 11.0° C, heat flux from 0.11 to 15.4 kWm−2 and mass flux from 147 to 249 kg m−2s−1. The wall temperature distribution was analyzed. In the case of single-phase flows, the wall temperature distribution is attributed to the secondary flow and the velocity profile of the main flow. In the case of two-phase flows, the temperature distribution is explained based on the buoyance and drag forces acting on the bubble. An increase of the inlet subcooling leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, the pressure has an opposite function on heat flux at the onset of nucleate boiling (ONB) and heat transfer coefficient. Besides, raising the mass flux can cause a substantial increase in the heat flux at ONB, but the effect on the heat transfer coefficient was negligible. The correlations of heat flux at ONB and the subcooled boiling heat transfer coefficient in a horizontal, helically coiled tube were developed based on the experimental data.