Fundamentals of Force Balance in Liquid Films and Transition from Macro- to Microchannels in Flow Boiling

Abstract

Microchannel heat exchangers play a critical role in development of more efficient and environmentally friendly energy systems. Thus, understanding the underlying physics that govern their heat transfer performance is of paramount importance. The most fundamental question is what is considered a microchannel, and what channel size, fluid properties, and flow conditions warrant use of a micro- versus macrochannel model. There have been extensive discussions in the literature regarding the appropriate definition of a microchannel; however, a clear, physics-based distinction of micro- from macrochannels has not emerged. This study analyzes this outstanding scientific question from the perspective of confinement effect on liquid films through investigating the effect of gravity on liquid films thickness. A set of experimental studies are conducted to determine the effect of gravitational force on liquid film asymmetry. The experimental findings are utilized to evaluate the existing criteria for transition from macro- to microchannels. Our analysis suggests that confinement numbers greater than 1.0 distinguish micro- from macrochannels in terms of liquid film symmetry. This analysis is supported by a force balance that shows gravitational force on liquid films exceeds the surface tension force in macrochannels.