Study On The Characteristics Of Two-phase Flow Heat Transfer In The Microchannels

Due to the numerous potential applications in Micro-electro Mechanical Systems(MEMS), bioengineering and biotechnology, aerospace, microelectronics, material processingand manufacturing,microscale heat transfer has drawn significant attention. But because of itscomplexity, related study on microscale heat transfer is insufficient. In this paper, experimentsare conducted to clarify the heat and mass transfer mechanisms about fluid flowing throughmicrochannel, the main work and results are as follows:With the deionized water and Al2O3nanofluid of0.5%mass fraction as the working fluid,boiling heat transfer characteristics and its influence factors in1mm×1mm,0.7mm×1mm and0.4mm×1mm channels are studied, respectively. Compared to that of original pure fluid underthe same condition, heat transfer coefficient in Al2O3nanofluid is bigger, it can be increasedby5%to12%; In the experimental conditions, heat transfer coefficient increases as the size ofmicrochannel decreases and the heat flux increases, while has no significant change with theincrease of the mass flow rate. The experimental results show that the nucleate boilingmechanism plays a leading role in the micro-channel and two-phase flow boiling heat transfer.The enhancement effects of voltage on the heat transfer are investigated. When taking thedeionized water as the working fluid, it is found that the heat transfer coefficient increasesconspicuously with the increase of voltage when the voltage is low, then flattens out with theincrease of voltage, finally, heat transfer coefficient decreases slightly when the voltage getsto a certain point. The results demonstrate that the optimal strength of electric field applied is25kv. When taking Al2O3nanofluid as working fluid, the changing trend of the heat transfercoefficient in nanofluid is similar with that in water while the changing rate of the former isfar less than the latter. It can be indicated that the nanoparticles have weakened theperformance of electric field on the enhanced heat transfer fluid flow.High-speed photography visualization for flow boiling heat transfer in microchannels iscarried out.Flow boiling bubble morphologies and flow patterns in microchannels areobtained at different heat flux. Contrast the bubble behavior figure in low heat flux with thatin high heat flux, it is found that the bubble nucleation point density increases significantlyand the heat transfer performance has been strengthened with the increase of heat flux.