Flow Boiling, GWP, HFO, R1234yf, R1234ze(E)
The substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact, it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. Both HFO1234yf and HFO1234ze(E) seem to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a HFO1234yf and HFO1234ze(E) during saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, mean vapour quality and saturation temperature (pressure) are investigated separately to rank the superposed effects of different heat transfer regimes (nucleate boiling or/and forced convection boiling). The experimental tests were carried out at three different saturation temperatures (10, 15, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux, mean vapour quality, and saturation temperature (pressure). The refrigerant mass flux ranges from 200 to 600 kg m-2s-1 and the heat flux from 15 to 30 kW m-2. The experimental measurements were reported in term of boiling heat transfer coefficients and frictional pressure drops. Heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop, whereas the effect of heat flux appears marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in present experimental tests. HFO1234ze(E) and HFO1234yf exhibit heat transfer coefficients and pressure drops similar to HFC134a. Present heat transfer coefficients and pressure drops were also compared against different correlations for refrigerant boiling inside tube. The universal correlation proposed by Kim and Mudawar (2014) and the Friedel (1979) correlation show the best performance in predicting heat transfer coefficients and pressure drops, respectively.