Boiling, BPHE, HFO
HFC134a has been probably the most important refrigerant of the two past decades as it dominated the application in domestic refrigeration, mobile air conditioning and large chillers and it took part as component in several refrigerant mixtures such as HFC404A, and HFC407C. Unfortunately HFC134a exhibits a relatively large Global Warming Potential (GWP), and it will be subjected to a gradual reduce in the use up to a complete phase out in the next future according to the different national and international regulations. The HydroFluoroOlefins (HFO) refrigerants HFO1234yf and HFO1234ze(E) seem to be the most promising substitutes for HFC134a as they exhibit very low GWP values (1 or less) together with pressure and volumetric properties closely near to those of HFC134a. The unique drawback of HFO refrigerants seems to be their mild flammability. The Brazed Plate Heat Exchangers (BPHE), which involve a reduction of the refrigerant charge of one order of magnitude as compared to the traditional tubular heat exchangers, are particularly interesting for limiting the risk of flammable or mildly flammable refrigerants such as HFO1234ze(E). In fact the first attempt to reduce the risk of flammable refrigerants is to decrease the refrigerant charge. This paper presents the experimental heat transfer coefficients and pressure drop measured during HFO1234ze(E) boiling inside a small BPHE: the effects of heat flux, refrigerant mass flux, saturation temperature (pressure) and outlet conditions are investigated. The evaporator tested is a BPHE consisting of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65Â° and a corrugation amplitude of 2 mm. The experimental tests have been carried out at three different saturation temperatures (10, 15 and 20Â°C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10Â°C), whereas the inlet vapour quality ranges between 0.2 and 0.3. The refrigerant mass flux ranges from 11 to 31 kg/m2s and the heat flux from 4 to 17 kW/m2. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity to heat flux, outlet conditions and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. The heat transfer and pressure drop measurements are complemented with an IR thermography analysis carried out during the vaporisation tests. The saturated boiling heat transfer coefficients were compared with a new model for refrigerant boiling inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 7.2%. The present data points were also compared with those of HFC134a and HFO1234yf previously measured inside the same BPHE under the same operating conditions: HFO1234ze(E) exhibits heat transfer coefficients very similar to HFC134a and HFO1234yf and frictional pressure drops slightly higher than HFC134a and HFO1234yf.