Fine-resolution two-phase flow heat transfer coefficient measurements of refrigerants in multi-microchannel evaporators

The time-averaged two-phase flow heat transfer coefficients were determined with a very fine resolution based on infra-red (IR) temperature measurements conducted over a 1 cm2 heated area of a multi-microchannel evaporator. This experimental investigation included two low-pressure refrigerants, R245fa and R236fa, as well as one medium-pressure new refrigerant R1234ze(E) flowing in four different test sections. The number of microchannels and their cross-sectional areas were the same for all the tested micro-evaporators, respectively, 67 and 100 × 100 μm2. Channel entrances with and without inlet micro-orifices were tested for establishing stable flow. Using 90 × 90 pixel grid (and then averaged width-wise), 90 local heat transfer coefficients were measured from inlet to outlet, yielding a U-shape of the heat transfer coefficient trend, where the descending branch of the curve corresponds to the coalescing elongated bubble flow regime (CB), while ascending one represents the increasing heat transfer coefficient in the annular flow regime (AF). The effects of channel mass flux, wall heat flux, orifice expansion ratio, and fluid properties on the wall heat transfer coefficient were identified for a selected number of data sets. In terms of predicted heat transfer coefficient values, in both the CB and AF flow regimes, the experimental results were found to be in a good agreement with respective existing flow pattern-based heat transfer prediction methods. Whereas, in order to better reflect the obtained experimental U-shaped trend of the heat transfer coefficient close to the CB-AF flow transition (which is a churn flow), the joined model of these two was modified by proposing a new vapor quality buffer for the width of the transition.