Conditions of liquid film dryout during saturated flow boiling in microchannels

Abstract

During saturated flow boiling in microchannels, two different trends of the dryout vapor quality ( x do ) have been observed experimentally: x do ∝ G - j and x do ∝ G j with j > 0 . In the literature, the first trend is experimentally observed for refrigerants and CO 2 whereas the second trend is detected so far only for CO 2 at high mass velocities. In this paper, the model of Revellin and Thome [A theoretical model for the prediction of the critical heat flux in heated microchannels. International Journal of Heat and Mass Transfer 51, 1216–1225] for the saturated critical heat flux in microchannels is used to analyze and explain this change of trend. Their theoretical model predicts three different trends of dryout vapor quality. Indeed, it is shown here that when increasing the mass velocity, the liquid film at the outlet of the channel passes through two different states: laminar flow ( Re δ < ψ = 2300 ) and transition flow ( Re δ = ψ ) . Turbulent flow ( Re δ > ψ ) has been omitted in this study since no experimental data are available for this regime. Laminar film dryout yields a decrease of the dryout vapor quality with respect to G ( x do ∝ G - j ) , transition film dryout is characterized by an increase of the dryout vapor quality with G ( x do ∝ G j ) . Furthermore, a new criterion is proposed here to identify the laminar-to-transition mass velocity, one for refrigerants and another one for CO 2 . The criterion gives the maximum value (upper limit) of the mass velocity for which the conventional microchannel correlations for the dryout vapor quality may be used. Notably, for a given pressure, the values of the laminar-to-transition mass velocities for refrigerants and CO 2 are approximately the same. This reinforces the idea that CO 2 is not a maverick fluid but just a normal fluid used at a higher reduced pressure in many boiling applications than conventionally used refrigerants.