Further Evaluation of a Flow Boiling Correlation for Microchannels and Minichannels

Flow boiling in microchannels and minichannels is being investigated for its potential for high heat flux removal in electronics and other devices. The small channel dimensions characteristic of these channels result, under most circumstances, in laminar flow at the flow rates used in these applications. The present work investigates the applicability of the correlation developed by Kandlikar and Balasubramanian [1] to some of the recent data for plain as well as enhanced microchannels using artificial nucleation cavities. The correlation is able to provide insight into the effect of the nucleation cavities on the heat transfer coefficient during flow boiling in microchannels. Modifications to the use of the correlation with deep laminar flows are suggested based on comparison with new data. INTRODUCTION As work continues both in generating experimental data on flow boiling in miniand microchannels, and in designing heat transfer systems making use of small-diameter channels, designers will be well served by empirical correlations spanning a range of data sets and possessed of wide applicability. In his 1991 and 1998 papers, Kandlikar [2, 3] developed a flow boiling map and two-phase heat transfer coefficient correlation for turbulent flow (Re > 2300) in channels 3 millimeters and up in diameter. That work was extended in 2004 by Kandlikar and Balasubramanian [1] to apply to smaller channel diameters by use of a laminar liquidonly heat transfer correlation. The objective of this work is to review Kandlikar and Balasubramanian’s correlation with more recent experimental data on plain and enhanced channels in the miniand microchannel regions. NOMENCLATURE Bo – boiling number Co – convection number D – circular channel diameter Dh – noncircular channel hydraulic diameter f – friction factor Ffl – fluid parameter Frlo – liquid-only Froude number G – mass flux htp – two-phase heat transfer coefficient hlo – liquid-only heat transfer coefficient kl – liquid thermal conductivity Nu – Nusselt number Prl – liquid Prandtl number q” – heat flux Relo – liquid-only Reynolds number x – vapor quality ORIGINAL CORRELATION The two-phase heat transfer coefficient, htp, is determined differently for nucleate boiling dominant (NBD) and convective boiling dominant (CBD) regions: