Two-Phase Void Fraction in a Microscale Fractal-Like Branching Flow Network

Under fully developed flow conditions, the convective heat transfer coefficient is inversely proportional to the hydraulic diameter of the channel through which a coolant flows. In addition, inside a heat sink several smaller channels can be used in place of a single larger channel as a means of increasing the convective heat transfer area per unit volume. Based on this rationale, microchannels were proposed for high heat flux cooling applications. Unfortunately, for a given flow velocity the pressure drop is also inversely proportional to the square of the diameter of the flow channel. To minimize the increase in pressure drop accompanying a decrease in channel diameter, a bifurcating flow network was proposed and designed with fixed length and width ratios between the upstream wider branch and the two narrow downstream branches. To date, microscale fractal-like flow networks have been studied at OSU for the following applications: single-phase heat sinks, two-phase heat sinks, heat exchangers, passive microscale mixers, ammonia desorbers, and liquid-vapor phase separators. The optimal flow geometry for a given application depends upon size and fabrication constraints and operating conditions. Optimization of flow geometries requires validated one-dimensional models. To be presented are results of two-phase void fraction studies recently conducted at OSU. The objective of these studies is to aid in the phenomenological understanding and physical modeling of two-phase flow in microscale branching geometries.