Experimental investigation of microchannel flow boiling heat transfer with non-uniform circumferential heat flux at various gravitational orientations

Abstract

Flow boiling of Perfluorohexane (FC-72) in rectangular microchannels with one-sided uniform heating was studied experimentally at different rotations (θ). Various rotational orientations were investigated ranging from θ = 0° (bottom-heating) to 90° (side-heating) in increments of 30° as well as 180° (top-heating).

The channels had a relatively high aspect ratio of 10 (5 mm x 0.5 mm), a hydraulic diameter of 909 μm and a heated length of approximately 78 mm. Mass fluxes of 10 kg/m2s, 20 kg/m2s and 40 kg/m2s were considered at several heat flux values at a saturation temperature of 56°C. For these conditions, in-channel flow visualisations and heated surface temperature distributions were recorded; fluid temperature and pressure readings were taken, and heat transfer coefficients were determined from subcooled conditions, through the onset of nucleate boiling, to near dryout conditions within the channel.

A channel at a rotation of θ = 0° produced the optimal results. θ = 0° had the highest heat transfer coefficient at all mass flux and heat flux combinations tested and had the lowest cross-sectional temperature variation of all rotations, minimizing the probability of warping electronic components. θ = 0° was nucleate boiling dominated resulting in an improved heat transfer performance with an increase in heat flux. θ = 180° experienced heat transfer coefficients that were greater than θ = 30°, 60° and 90° at various qualities up to χ = 0.3 where the vapour slug became confined the heat transfer coefficient decreased rapidly. θ = 90° had the lowest heat transfer coefficients at most mass flux and heat flux test cases. θ = 0° had the highest pressure drop while θ = 180° had the lowest pressure drop.