Height effect on nucleation-site activity and size-dependent bubble dynamics in microchannel convective boiling

Abstract

Forced convection boiling in microchannels is studied experimentally under the uniform heat flux boundary condition. Several microchannel heat sinks with integrated temperature sensors, spanning two orders of magnitude in height 5-500 µm, have been fabricated with designed nucleation sites on the bottom surfaces. The microchannels are capped by a glass wafer to monitor bubble activity using video microscopy. Distributed micro heater elements on the device backside are used as the heat source, while the working liquid flow rate is adjusted using a syringe pump. The boiling curves of the device temperature as a function of the input power have been measured for various flow rates. The curves for increasing and decreasing heat flux exhibit a hysteresis loop, while the conditions corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. The activity of nucleation sites as well as the ensuing bubble dynamics, from incipience to departure, is found to depend on the channel height. The critical size above which a nucleation site is active, along with three aspects of bubble dynamics, namely growth rate, departure size and release frequency, have been characterized experimentally and proper control parameters have been identified.

Confined bubble growth during flow boiling in a mini/micro-channel of rectangular cross-section Part I: Experiments and 1-D modelling

“…Frictional pressure drops in the liquid slugs upstream and downstream of the bubble are calculated by an expression for the Fanning friction factor f F in steady laminar flow on a channel of cross-sectional aspect ratio g ¼ h/w, derived from White [31] f F Re ¼ 24 1 À 1:3553g þ 1:9467g 2 À 1:7012g 3 þ 0:9564g 4 À 0:2537g 5 ; 0 g 1 ð18Þ combined with a minimum value f F ¼ 0.01 to represent transition to turbulent flow at Re z 1500 at a relative roughness of 10 À2 on a Moody diagram. It is known from [25] that this is only a rough guide to friction for the conditions of rapid accelerations and flow reversals in microchannel boiling.…”

The critical role of channel cross-sectional area in microchannel flow boiling heat transfer

“…The effects of microchannel size and surface characteristics on bubble nucleation and flow patterns have also been investigated . Lee et al (2005) investigated the effect of microchannel height on nucleation-site activity and bubble dynamics.…”

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

“…The contribution of forced convective evaporation to the boiling heat transfer decreased with decreasing tube diameter. Lee et al [19] investigated the effect of microchannel height on nucleation-site activity and bubble dynamics using three fluids: water, methanol and ethanol. The heat sink consisted of ten parallel microchannels fabricated in a silicon wafer, with width ranging from 150 to 9000 μm and height ranging from 5 to 510 μm.…”