CONDENSATION HEAT TRANSFER COEFFICIENT MEASUREMENTS AND FLOW PATTERN VISUALIZATIONS OF R515B AND R450A INSIDE A 3.4 mm DIAMETER CHANNEL

An alternative low Global Warming Potential (GWP) refrigerant that could be used to replace R134a in heat pumps, refrigeration and air-conditioning systems is the hydrofluoroolefin R1234ze(E). As a drawback, R1234ze(E) is classified as a mildly flammable fluid (A2L ANSI/ASHRAE classification) and it presents a lower volumetric cooling capacity compared to R134a. In the search for non-flammable R134a substitutes, hydrofluorocarbon/hydrofluoroolefin binary mixtures can be considered. R515B (R1234ze(E)/R227ea at 91.1/8.9% by mass) and R450A (R1234ze(E)/R134a at 58.0/42.0% by mass) are two alternatives classified as A1 (not flammable). R515B is an azeotropic mixture with GWP100-years = 299, whereas R450A is a near-azeotropic blend (temperature glide 0.6 K at 40 °C) with GWP100-years = 547. In this work, condensation tests are performed with R515B and R450A inside a circular cross-section channel with an inner diameter equal to 3.4 mm. The test section is composed of two copper heat exchangers designed for the measurement of the quasi-local heat transfer coefficient. A glass tube, located between the two diabatic parts of the test section allows the visualization of the two-phase flow patterns by a high-speed camera. Heat transfer coefficients are measured at 40 °C mean saturation temperature and mass flux between 50 and 300 kg m-2 s-1. The prediction accuracy of condensation heat transfer models is then assessed against the experimental results. Measured heat transfer coefficients are also compared with those of R1234ze(E) at the same operative conditions. Regarding the diameter of the present test tube, it is worthy to point out that small diameter tubes are often employed in finned coil heat exchangers and minichannel heat exchangers are a common solution for air-cooled condensers.