A novel time strip flow visualisation technique for investigation of intermittent dewetting and dryout in elongated bubble flow in a microchannel evaporator

A flow visualisation study of flow boiling of R245fa in silicon multi-microchannels at low mass flux and moderate heat flux has been carried out with a high speed digital camera. The micro-evaporator had 67 channels of length 20 mm, width 223 mu m, and height 680 mu m while the fin width between adjacent channels was 80 mu m. The base heat flux ranged from 2 to 26W cm(-2) for a mass velocity of 100 kg s(-1) m(-2), resulting in exit vapour qualities ranging from 10% to 70%. In particular, a novel time strip technique was developed to analyse the recorded image sequences and significantly highlight the various phenomena occurring along given channels. Notably, this technique was able to reveal profound details regarding the intermittent dryout mechanism of liquid films trapped between the elongated bubbles and the heated channel walls. The results show that the intermittent dryout of the evaporating liquid film is comprised of four stages with distinct time scales and dynamics: (i) the growth of liquid film thinning perturbations to a critical amplitude causing the rupture of the metastable liquid film, (ii) a dewetting stage involving expanding dry spots leading to a rivulet flow regime, (iii) evaporation of the rivulets leading to full dryout, and (iv) a rewetting stage. This intermittent dryout mechanism appears to explain the many seemingly contradictory heat transfer coefficient trends observed with changes in vapour quality in microchannels, thus resolving an important heat transfer dilemma. Furthermore, since dryout is an undesirable event during the practical application of a microchannel evaporator, it is important to delay or even suppress the initial rupture of the liquid film that leads to dryout. This can be achieved by manufacturing or treating the channel surfaces to be highly wettable with the chosen refrigerant. (C) 2010 Elsevier Ltd. All rights reserved.