Observations of bubble shape and confinement in diabatic two-phase flow in a minichannel

Bubble images were acquired from within the channel for a bubbly gas/liquid flow in large-aspect-ratio minichannel with heat transfer. The purpose of these images is to document measurands that address bubble shape and confinement and to relate these measurands to the heat transfer enhancement. Data were collected under different operation conditions in a laminar flow of Novec 649 in a horizontal rectangular minichannel of 1.29-1.48 mm channel spacing. Air bubbles were injected at either a single point on the lower wall or through a sintered metal plug. Liquid crystal thermography was used to record time-averaged surface temperature data, and a borescope was used to acquire images from a point downstream of the on-coming bubbles. The smallest bubbles were approximately spherical, larger bubbles were asymmetric spheroids, and bubbles that spanned the channel were lozenge-shaped. A relationship between bubble height across the channel and diameter observed in the plane of the channel was demonstrated. Bubbles that spanned the channel had a diameter of at least 1.5 times the channel height. All three bubble shapes displayed an approximately fixed velocity ratio. The velocity scale for this ratio is the liquid velocity, corrected for vapor blockage, at the center of force of the bubble. The mechanisms for heat transfer enhancement in this air/liquid flow are mixing in the liquid phase in the bubble wakes and liquid acceleration due to vapor blockage. We hypothesize that these mechanisms are primarily responsible for the enhancement observed in highly confined vapor/liquid flows where nucleation and microlayer evaporation are present. This hypothesis was supported by a successful correlation that unites the present measurements and data from a flow of naturally nucleated vapor bubbles previously acquired in the same facility.