Flow Boiling Enhancement From Microporous Surfaces In Mini-and Microchannels

Mini- microchannels have promising prospects in applications such as the cooling of high-power electronic chips, heat removal of intensively exothermal reactions, and aeronautical engineering. Therefore, fluid flow and heat transfer in microscale have been highlighted in their pertinent research fields. The microporous surfaces can remarkably enhance the liquid boiling heat transfer. The combination of the microporous surfaces with mini- and microchannels ensures salient heat transfer performance and equipment compactness simultaneously. In this paper, a study on flow boiling enhancement from the microporous surfaces in mini- and microchannels was conducted. The main work and results are as follows:1. Heat transfer enhancement from the microporous surfaces in mini- and microchannels, as well as its influencing factors, was experimentally studied, with water and FC-72 as the working fluid. (1) The effects of coating structural parameters, including particle diameter and coating thickness, on the heat transfer enhancement were investigated. The optimum particle diameter and coating thickness with respect to a given liquid were obtained. (2) The effects of liquid mass flux on the heat transfer enhancement were studied. Results revealed that the increase in mass flux can facilitate the microporous surfaces' advantage of lowering ONB wall superheat. Reduction in mass flux yielded high flow boiling enhancement from microporous surfaces, but led to the decreased CHF. (3) The effects of liquid inlet subcooling on the heat transfer enhancement were studied. It was found that increasing inlet subcooling brought about enhanced advantage of lowering ONB wall superheat from the microporous surfaces, and higher enhancement factors as well. With the optimum structural parameters and reasonable mass flux and inlet subcooling, flow boiling heat transfer coefficient was around 3 (with water) and 8 times (with FC-72) that of the plain surface, and the boiling wall superheat was lowered by about 10 K.2. High-speed visualization research on flow boiling from the microporous surfaces in mini- and microchannels was carried out. (1) Flow boiling bubble morphologies and flow patterns from microporous and plain surfaces in mini- and microchannels were obtained at various vapor qualities. The flow patterns from the two surfaces included the isolated bubble flow, coalesced bubble flow and annular flow. Heat transfer mechanisms of the two surfaces with different flow patterns were analyzed, in accordance with heat transfer experimental results and bubble morphology images. In the coalesced bubble flow, flow boiling from the microporous surfaces was controlled by nucleate boiling, and hence the highest enhancement factor. (2) The bubble growth and departure process of the microporous and plain surfaces were recorded using high-speed visualization technique. The flow boiling heat transfer enhancement mechanisms of the microporous surfaces were revealed by comparing the bubble departure diameter, frequency and nucleation site density of the microporous surfaces with those of the plain surface. Compared with the plain surface, the microporous surface reduced the bubble departure diameter by 40-50%. The bubble departure frequency was increased by two times at the most, and the nucleation site density was increased by around 500 times at the most. Thereby, the microporous surfaces can realize outstanding heat transfer enhancement by drastically enhancing nucleate boiling.3. The mini- microchannel scale effect on the flow boiling heat transfer enhancement from the microporous surfaces was investigated. (1) The scale effect on flow boiling from the plain surface was studied by comparing the experimental results with predicted values from empirical correlations. It was found that the heat transfer performance deviated from that in a macroscale tube due to the scale effect. The scale effect of mini- microchannel manifested itself in the form of wall confinement on vapor bubbles. The confinement effect enhanced the heat transfer performance from the plain surface at low vapor qualities, while greatly lowered the heat transfer rate at high vapor qualities. (2) The scale effect on enhancement from the microporous surfaces at various vapor qualities and flow patterns was researched. The confinement had an adverse effect on heat transfer enhancement from the microporous surface at low vapor qualities. The microporous surfaces could hinder the adverse effect of the wall confinement and delay the local dryout, by providing a great number of nucleation sites. (3) The experimental results demonstrated that the confinement effect was influenced by liquid mass flux and channel cross-sectional size. Lowering mass flux and channel height led to more intensive confinement effect with both surfaces. The introduction of microporous surface enlarged the range of influence of mass flux on the confinement effect.