Convective boiling inside a single circular microchannel

The present paper reports the heat transfer coefficients measured during flow boiling of HFC-134a and HFC-32 in a 0.96 mm diameter single circular channel. The test runs have been performed during vaporization at around 30 °C saturation temperature, corresponding to 7.7 bar for R134a and 19.3 bar for R32. 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. Therefore, with respect to this aspect, the present data are new and original since the large majority of data in the literature are taken by means of Joule effect heating. Since the surface characteristics can affect the boiling heat transfer process, the surface roughness of the test tube has been measured. The flow boiling data taken in the present test section are discussed, with particular regard to the effect of heat flux, mass velocity, vapor quality and fluid properties. The heat transfer coefficients are compared against some predicting models available in the literature.