On the importance of upstream compressibility in microchannel boiling heat transfer

Experimental data for flow boiling of water and n-hexane in microchannels of diameters 0.90 and 2.00 mm have been studied for mass fluxes ranging from 60 to 180 kg m−2 s−1, heat fluxes from 20 to 465 kW m−2, operating pressures of 100–200 kPa and exit vapour qualities from 0.0 to 0.8. Two different single-channel test sections having very different thermal time constants (<1 s and 200 s) have been employed. Results have been obtained over a wide range of inlet compressibility provided by a specified volume of gas introduced into a side branch upstream of the test section. Introduction of even minute levels of inlet compressibility led to the establishment of steady oscillations in pressure, mass flow and wall temperature. The frequency of these oscillations is very well predicted by a mass-spring model and it is concluded that the oscillations are a resonant phenomenon that say nothing about the boiling process itself.Inlet compressibility is accompanied by a change in the magnitude and behaviour of the boiling heat transfer coefficient. Most strikingly, for a “hard” inlet (no upstream compressibility) the heat transfer coefficient (h) is found to depend on mass flux but not on wall heat flux whereas for sufficiently “soft” inlet, h depends on heat flux but not mass flux. These results shed new light on the diversity of behaviours reported in the literature for microchannel boiling. Control and specification of the upstream compressibility condition is essential to proper characterisation of conditions.