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.
Highlights
Due to the increasing environmental concern the phase out of refrigerants commonly used in HVAC devices is taking place
Experimental results The experiments focused on R1234ze(e) during flow boiling and mainly aimed to check the saturation temperature effect on the heat transfer performances. They were performed tuning the mean quality in the range xm [0.25;0.75], three inlet saturation temperatures were tested (Tri=5 °C, which was chosen a reference in the comparisons, Tri=35 °C and Tri=45 °C), two mass fluxes were taken into account (G=163 kg/m2s and G=220 kg/m2s) and the quality change was fixed ( x=0.2) while the heat flux is in the range q [10;17] kW/m2, because the phase change enthalpy is a decreasing function of the saturation temperature
About the selection of the temperatures it has to be highlighted that: the reference temperature (Tri=5 °C) is the standard evaporating temperature for HVAC systems, while, the others represent a compromise between the lower evaporation temperatures in ORC systems (55 ÷ 65 °C) [1] and the upper limit of the experimental facility to prevent the damaging of the test section, which has parts made of polymethyl methacrylate
Summary
Due to the increasing environmental concern the phase out of refrigerants commonly used in HVAC devices is taking place. The manuscript aims to rate the performance of R1234ze(e) and reports the experimental investigation concerning the heat transfer coefficient and the pressure drop during flow boiling inside microfin tube J60.
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