Saturation temperature effect on heat transfer coefficient during convective boiling in microfin tubes

Abstract

The growing attention on environmental aspects puts severe constraints on HVAC technology, mainly involving the working fluids and energy efficiency. Both are related to the main limiting factor of HVAC system: the heat transfer process, which, frequently, involves boiling and condensation. To provide suitable tools for the HVAC system design, it is necessary to gather information on the heat transfer characteristics of the new refrigerants. Particular interest is focused on R1234ze(e), because it is one of the viable options to face the R134a phase out. Something similar can be repeated for the low temperature ORC systems using a refrigerant as working fluid. Using a specifically designed test rig, heat transfer coefficient and pressure drop measures were performed during flow boiling of R1234ze(e). The operating conditions were defined by four parameters: the evaporation temperature (5 °C, 35 °C and 45 °C), the mass flux (two values were considered: 160 kg/m²s and 220 kg/m²s), the average thermodynamic quality (which varies between 0.25 and 0.75) and the quality change (which was fixed to 0.2). The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The results highlighted that the saturation temperature strongly affects the heat transfer coefficient and the pressure drop: as a consequence of the saturation temperature increase from 5 °C to 45 °C, it was observed up to 40% heat transfer coefficient increase and 90% pressure drop reduction.