Vapor Extraction From Flow Boiling in a Fractal-Like Branching Heat Sink

Abstract

Increases in pressure due to vapor generation during boiling in microchannels can be reduced by extraction of vapor at its point of inception. Ultimately, this local vapor extraction decreases the pressure drop required to drive the flow through the microchannel network within the heat sink. Indeed, by lowering the overall flow rate by vapor extraction, the pressure drop can, in principle, be lowered below that of single-phase flow. In this present study the relative driving forces necessary for vapor extraction and for flow through the microchannels are investigated. The concept also has the potential to separate flow independent of orientation or gravity. The fractal-like flow network used here is one that has been previously shown to reduce pressure drop and yield a more uniform surface temperature distribution for single-phase flows. The disk shaped heat sink was covered with a porous Nylon membrane with an average pore size of 0.45 microns. Water was used as the working fluid with inlet subcooling of approximately 2.5 K and flow rates ranging between 8 and 12 g/min. The vapor extraction pressure was varied and maintained between 0 and 56 kPa below the average pressure between the inlet and exit of the heat sink. Heating varied from 18 to 30 Watts. Actual vapor extraction data are correlated with the exit quality predicted with no vapor extraction, the value of which is dependent upon mass flow rate, heat input and degree of subcooling. Network pressure drop data are correlated with the membrane pressure difference data.