Rupture of thin liquid film based premature critical heat flux prediction in microchannel

Abstract

Accurate prediction of critical heat flux (CHF) is vital for the application and safety of compact heat exchanger with flow boiling in microchannel. In view of this, the current paper presents a criterion model to predict the premature CHF caused by upstream compressible volume instability (or flow reversal) in microchannel, i.e. the premature CHF is affirmed when the period of flow reversal is in excess of the maximum duration of the existence of the thin liquid film. Partial differential equation (PDE) for the thin film thickness is developed to analyze the transient characteristic of thin liquid film thickness, and film breakup occurs when the thin liquid film is evaporated to a critically low thickness. Mass-spring model is employed to predict the period of reversed flow with the upstream compressibility volume acting as the spring and the liquid column constituting the mass. Both of the maximum duration of liquid film existence and periodic flow reversal decrease with an increase of CHF, and increase with the increase of heat-to-mass flux ratio (or Boiling number). Premature CHF can be eliminated by increasing flow rate and pressure drop multiplier parameter, and the CHF increases with the increase of mass flux and the pressure drop multiplier parameter respectively. The periodic flow reversal model can satisfactorily predict the experimental oscillation periods with the maximum relative error of ±25%, and the CHF predicted by the current criterion is in possession of accuracies within the relative error of ±22.5%.