Adiabatic Flow Boiling in Fractal-Like Microchannels

Abstract

Fractal-like branching channels are proposed for a number of microscale applications, including heat sinks, heat exchangers, absorbers, desorbers, and micro-mixers. Based on model predictions, the benefit of fractal-like channel designs is a lower pressure drop than parallel straight channels for a given flow rate, when compared to an equal channel surface area basis with the terminal channel cross-section of the fractal-like network used to define the parallel channel geometry. The fractal-like flow networks are a unique geometry that follows fractal bifurcation patterns, in this case mimicking the flow patterns found in nature. Two-phase flow applications require an understanding of how the geometric constraints impact the flow characteristics during multiphase flow. One-dimensional modeling predictions are used in this study to asses the relative impact of flow network designs on pressure drop and void fraction distributions for adiabatic flow boiling. The characterization of the flow networks includes a specified branching ratio of channel length and channel width (or diameter) and also the number of branching levels, or bifurcations, in a given length. The goal of the present study is to identify the adiabatic boiling characteristics within the fractal-like flow network and compare results to straight parallel channels. The model used is a compilation of two-phase flow models presented in the literature but modified to include a local two-phase flow parameter, flow re-development, as well as variable property effects. Results are compared with straight channels based on flow boiling conditions, pressure drop, and vapor quality distributions for a range of flow rates and subcooling.