Onset of Nucleate Boiling in Mini and Microchannels: A Brief Review

Abstract

The present article summarizes the studies on the thermalhydraulic condition under which the onset of nucleate boiling (ONB) is triggered in subcooled flow boiling. Available correlations and experimental data show that the ONB is tended to be delayed in mini and microchannels. It is known that the ONB condition is significantly dependent on the surface condition even in conventional-sized channels. Accumulation of ONB data accompanied by the information on the surface condition is therefore considered of importance to elucidate the mechanisms of boiling incipience in microchannels. Discussion is also made for the bubble dynamics observed in mini and microchannels. It is indicated that the bubble behavior at ONB in mini and microchannels may significantly be different from that in conventional-sized channels and have greater impact on the system performance. Further studies on the bubble dynamics following nucleation at ONB are also requested to improve the design of heat transfer devices using mini and microchannels.

Review of the calculation of the boiling heat transfer coefficient in mini-channels and micro-channels

“…The expression obtained by Kuo [26] is based primarily on the heat flux density 𝑁𝑁𝑁𝑁 = 0.29𝑞𝑞 𝑊𝑊 " 1.4 (7) The boiling cycle is the process of liquid heating, bubble formation, bubble growth and bubble release. The number of bubbles formed per unit time or bubble formation frequency (f) and bubble detachment diameter (Ddb) are factors that have great influence on heat transfer [9] . During forced flow boiling the bubble formation frequency depends on the heat flux density, mass flux density, fluid type, size and nature of the nucleation center (cavity) and the bubble release diameter.…”

“…From the literature reviewed, it can be said that the theory for single-phase flow is applicable for conventional channels as well as for mini-channels and microchannels [9] . However, the theory for twophase flow in conventional channels is not appropriate for mini-channels and microchannels [10] .…”