Study On The Critical Heat Flux Characteristics And Visualization Analysis Of Saturated Boiling Heat Transfer In Microchannels

The mechanism of boiling heat transfer in microchannel is complicated. In order toclarify the related boiling heat transfer characteristics in microchannel, combining with thecritical constraint parameter: the Critical Heat Flux, the paper carries out experimental studyon analyzing the influence of the working medium’s physical property, mass velocity, importsubcooling, microchannel geometry size and the critical export dry degree to the CHF. Also,Combinig with the visualization technology, the paper analyses the two-phase flow patternchange cycle and the growth and loss characteristics of the boiling heat transfer bubbles.With the working fluids being water and nano-fluids of three different kings of massfraction(0.2%,0.5%and1.0%), experiments are carried out in microchannels of threedifferent sizes to study the variation characteristics of the CHF during the boiling heat transferprocess. The paper mainly focus on the effect on CHF of the impact factor, such asmicrochannel hydraulic radius, mass flow rate, nano-fluid concentration, import subcoolingand critical thermodynamics dryness. Results show that the smaller the hydraulic radius is, theearlier the CHF happens; the CHF increases with the increasing of the mass flow rate，and theincreasing rate is much more obviouse when the channels size is small, and the CHFdecreases with the increasing of the critical thermodynamics dryness; the mass fraction of thenano-fluid doesn’t have much influence on the CHF, but when the mass fraction increases, theCHF has an increasing intendensy. The paper fitting compares the experiment results withexisting theory model whose working condition is similar to the experiment, results show thatprediction model can well predict the experiment, and the mean absolute error between thepredicted results and experiment results is24.0%and7.1%respectively, which is alsoverifying the reliability of the experiments.During the process of the boiling heat transfer experiment, we use visualizationtechnology to research the influence of effective heat flux on two-phase flow patterntransformation, the results reveal that the changing cycle of two-phase flow pattern decreasesas the effective heat flux increases, and what’s more, the paper also analyse the phenomenonof local back flow, and records and analyses the growth of the bubbles in microchannels. Andanalyses how the effective heat flux and the entrance distance effect the growth cycle of the bubble. The result proves that the growth cycle of the bubble decreases as the effective heatflux increases, and it decreases as the entrance distance increases, too.Then, the paper verifiesthe calculation formula of the bubble’s falling off diameter of the Cole model with theexperiment results, and then, revise the formula.