Flow boiling of a new low-GWP refrigerant inside a single square cross section microchannel

In this work new experimental heat transfer coefficients measured during flow boiling of HFO-1234ze(E) in a horizontally-assembled square cross section microchannel having an hydraulic diameter of 1.23 mm are presented. The test runs have been performed at around 30°C saturation temperature, correspondent to 5.8 bar, with mass flux ranging between 300 kg m-2 s-1 and 500 kg m-2 s-1. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. On this regard the present data is new and original since the large majority of data in the literature is taken by means of Joule effect heating. The heat transfer coefficients are compared against two predicting models available in the literature. Finally, the local heat transfer coefficients measured during flow boiling of R1234ze(E) inside the square cross section microchannel are compared against the values measured with the same refrigerant in a 0.96 mm diameter circular microchannel, with same surface roughness, with the aim of investigating the effect of channel shape on the heat transfer process.