| EHD heat transfer enhancement technique,nano-refrigerant heat transfer enhancement technique and microcavity structure heat transfer enhancement technique are applied to flow boiling in mini-channel,and the effects on heat transfer and pressure drop characteristics of three enhancement techniques and their combinations with each other are studied.The main research work is as follows:(1)Si O2-R141b nano-refrigerant of 0.01wt%,0.05wt%,0.1wt%,0.2wt%mass concentration are prepared by two-step method for flow boiling experiment under 200V?800V DC electric field.Both DC electric field and Si O2 nanoparticles can improve flow boiling heat transfer performance in mini-channel.The heat transfer performance under the action of electric field is better at higher voltages and lower heat flux,with the maximum enhancement factor being 1.45.For Si O2 nanoparticles,its strengthening effect on heat transfer shows a nonlinear relationship with concentration with the optimal concentration being 0.1wt%and the maximum enhancement factor being 1.21.The heat transfer enhancement effect is more significant when the electric field is combined with nanoparticles and the maximum compound enhancement factor is 1.58.(2)Mini-channels featuring microcavity surface of three kinds of arrays on heated surface are designed and manufactured and their saturation boiling heat transfer performance is discussed.The combined effect of 3 techniques on flow boiling heat transfer is discussed and the optimal match of voltage,concentration and cavity array for heat transfer is investigated.The maximum compound enhancement factor of electric field and microcavity structure reaches 1.69.The maximum enhancement factor is 1.82 for the combination of electric field,nanoparticles and microcavity structure,and the effective enhancement heat flux range is expanded.(3)The total pressure drop characteristics and flow instability of flow boiling is studied when the 3 heat transfer techniques are applied,and the comprehensive performance of all the techniques applied in this paper is evaluated.When the three techniques are integrated,the total pressure drop,two-phase flow pressure drop and two-phase friction pressure drop per unit length of the flow boiling in the mini-channel are increased by 19.28%?23.65%?22.54%respectively.The effect on comprehensive performance of EHD heat transfer enhancement technique relies on heat flux and comprehensive performance of the combined techniques of EHD,nanoparticles,and microcavity structure is the best.(4)A high-speed camera is used to record the process of flow boiling in mini-channel and COMSOL software is used to calculate the electrical stress on the liquid-gas interface to analysis the compound impacting mechanism.The number of bubbles in mini-channels under the action of electric field is increased,and the number of small bubbles in mini-channel with microcavity surface is larger than that in ordinary mini-channel at the same voltage.It is found that the bubble is stretched by the electric field in the direction orthogonal to the electric field line and pressed against the heated surface,which increases the wetting area of the heated surface and improves the heat transfer coefficient. |
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