Irregular-shaped nucleated bubbles induced enhancement of subcooled flow boiling heat transfer in leaf vein inspired three-tiered open microchannels (LTOMCs)

Abstract

Nature is an important source of inspiration for developing desired structures for various industrial applications. The efficacy of water transport in leaf vein systems provides important insights for the structural design of microchannels. In addition, open microchannels (OMCs) have been reported to effectively reduce the two-phase pressure drop and suppress flow instabilities by providing extra space for bubble and vapor expansion. Thus, the synergistic cooperation of open microchannels with leaf-vein-like structures is expected to alter the flow patterns and improve the heat transfer performance in the flow boiling process. In this work, two leaf-vein inspired three-tiered open microchannels (LTOMC) with exactly the same structure but opposite orientations of vein-like channels, i.e., LTOMC-Ⅰ and LTOMC-Ⅱ, were manufactured, respectively. Flow boiling experiments were conducted at the inlet temperature of 25 °C under different mass fluxes. It was observed that the flow patterns of the LTOMC were divided into bubbly flow, slug flow, reverse and rewet flow; the shapes and movement trends of nucleated bubbles were mainly dominated by the vein-like structures. The required wall superheat of the onset of nucleated boiling for LTOMCs was 2–6 °C lower than ROMC; the one for LTOMC-Ⅱ was 2–3 °C lower than LTOMC-Ⅰ at various mass fluxes. The average two-phase heat transfer coefficient of LTOMC-Ⅱ was increased by 83.2% compared to ROMC at the mass flux of 234 kg/m2·s; the one of LTOMC-Ⅱ was 54.1% higher than LTOMC-Ⅰ. The pressure drop of three OMCs was comparable; however, the onset of two-phase flow instability occurred earlier for ROMC at the same heat flux. Besides, the liquid rewetting capability of ROMC was easier to get deteriorated at ultra-high heat flux. This work offers an avenue for structural optimization of microchannel heat sinks to enhance heat transfer performance without compromising the pressure loss.