An overview of gravity effects on flow boiling instabilities

Abstract

Modern flight vehicles have increasing demands for flow boiling to support onboard thermal management and power generating systems, and many studies on gravity effects on flow boiling instabilities have been reported. This paper presents a review on flow instabilities in boiling systems, with an emphasis on gravity effects, because no reviews of gravity effects on flow instabilities are available. Different flow instabilities and theoretical methods for their analysis are first reviewed. The contents are organized according to gravity level, including Earth's gravity, microgravity, and hypergravity. The majority of the research activities are related to Earth's gravity. All the related studies have shown that gravity affects flow boiling instabilities, and the degree of influence might depend on gravity levels and many other factors such as inlet subcooling, system pressure, heat flux, and mass flux. The results from the studies of different flow orientations under Earth's gravity are consistent for density wave oscillation, but inconsistent for pressure drop oscillation and flow maldistribution. The contradictions might be caused by the different interactions between the gravity and other factors in different parameter ranges. The results obtained from microgravity studies showed that some flow instabilities did not occur under Earth's gravity, but no detailed understanding was reported since the microgravity periods for the experiments lasted only up to about 20 s, too short to reveal any flow instability details. There are very few studies of flow instabilities under hypergravity. The interaction of gravity with other factors makes the flow instabilities more difficult to be understood. Topics worthy of attention for future research in gravity effects on flow instabilities are proposed.