Confined bubble growth during flow boiling in a mini/micro-channel of rectangular cross-section Part I: Experiments and 1-D modelling

Abstract

Heat sinks using evaporation in arrays of parallel microchannels have potential for the removal of high heat fluxes from small areas. They suffer from flow instabilities and uneven distribution between channels that may cause local dryout and overheating. The current state of the art is reviewed critically. A simple 1-D model for bubble growth in a single channel with a compressible volume in its upstream plenum is developed as a tool for the rational design of measures known to reduce flow instabilities, namely inlet resistance and enhanced nucleation in every channel. The model considers two stages of partially and fully confined bubble growth in a single channel of rectangular cross-section, suggested by experimental observations, followed by venting of vapour to the downstream plenum. The experiments also show the influence of apparently minor changes in rig design and operation on upstream compressibility and flow reversal. The model considers upstream compressibility due to subcooled boiling in a preheater or trapped non-condensable gas and the reduction of flow reversal by inlet resistance. The feasibility of measuring transient axial variations in pressure within small channels using inexpensive transducers is demonstrated.