A simple methodology to incorporate flashing and variation of thermophysical properties for flow boiling pressure drop in a microchannel

Abstract

This paper proposes a simple approach to model the complex interplay among the various thermophysical phenomena occurring in flow boiling. In order to assess the need for such a model, flow boiling pressure drop of FC-72 in a single trapezoidal microchannel with a hydraulic diameter of 111 μm is measured by varying heat flux and mass flux. The pressure drop obtained is in the range of 10–45 kPa, which does not compare well with the existing models based on constant properties. Therefore, a new predictive approach is developed for meticulous evaluation of pressure drop across a microchannel with flow boiling. It uses the separated flow model with evaluation of thermophysical properties at local pressure, thus incorporating the effect of flashing on thermodynamic quality, and the effect of heat flux on the two-phase multiplier. Relations based on a modified form of Clausius-Clapeyron equation are employed for evaluation of local thermophysical properties for fluids. This methodology is combined with different empirical correlations from the literature to predict pressure drop. The proposed predictive methodology, comprising close form equations, is physically sound and yet easy to implement, and reproduces large pressure drop experimental data better than the existing methods.