Study On The Flow Boiling Heat Transfer And Enhancement Mechanism In Channels With Micro Fin And Groove

Facing the increasing international energy crisis,heat transfer enhancing has been widely concerned.Among the existing passive heat transfer enhancement technologies,micro-structure channel enhancing is one of the main research directions.The heat transfer is enhanced by adding micro enhancing structure and separating the smallest flow unit.This can enhance the flow and phase transition process while obtaining a larger specific surface area.However,there is still a lack of research on the dominant heat transfer mechanism and the corresponding theoretical explanation,which limits the development of the heat transfer enhancement technology.The present thesis studied the flow boiling heat transfer in microfin tube and narrow rectangular micro-channel.The minimum flow units in the two channels are geometrically similar,and the vapour-liquid interface distribution are both dominated by surface tension.The flow pattern,heat transfer and frictional pressure drop of the channels were experimentally studied,and the numerical calculation method was utilized for quantitative analysis.Firstly,the visualized test section with controllable heat flux was designed to study the two-phase flow patterns of R245fa in the smooth and the microfin tubes.The time-series RGB analyzing method was utilized to identify each flow pattern.Six dominated flow patterns were observed in smooth and microfin tubes.The effect of the operating parameters on the flow patterns transition were analyzed.The results show that there are absorbing thin film in the grooves,reducing the flow patterns transition quality and inhibiting the contraction and rupture of the thin film in the microfin tube.These effects can improve the wettability of the inner wall surface and improve the evaporation performance.The flow pattern transfer criterion was proposed based on the experimental data.In this thesis,the heat transfer and frictional pressure drop characteristics in smooth and microfin tubes were studied comparatively.The effect of operating parameters were discussed.The heat transfer characteristics and mechanism were analyzed.The results show that the heat transfer mechanism in the smooth tube is the superposition of nucleate boiling and forced convection.The thin film evaporation mechanism under high vapour quality is also combined in the microfin tube.The friction pressure drop in the tube increases with the increasing of mass flow rate and vapour quality,but is almost independent with heat flux.the comparison of EF and AR shows that the enhancement mechanism of the microfin tube is the joint action of the expanding surface,the increasing disturbance and the thin film evaporation.The performance of the existing models was evaluated.The recommended correlations for predicting heat transfer and pressure drop were proposed.Based on the visualization and experimental results of the heat transfer coefficients,a theoretical model of annular thin film flow evaporation in microfin tubes is developed.The change of the heat transfer coefficient in the tube with the actual operating parameters was analyzed with the coupling calculation of the film shape and the mainstream flow.The numerical results show that the heat transfer coefficient increases with the increasing of vapour quality and mass flux.HTC reaches the peak value before dry-out.The numerical results were compared with the existing experimental data,and the predicting error of most data is less than 30%.The flow boiling heat transfer characteristics of new working fluids in a narrow rectangular microchannel were studied.The visualization results show that the churn-annular flow is the dominant flow pattern.The nucleation point in the liquid film is observed to be activated for long time.The results show that the heat flux and saturation pressure have significant effects while the mass flux and vapour quality are insignificant.It can be concluded the combined effect of nucleate boiling and film evaporation is the dominant mechanism.A new dimensionless parameter combination prediction model is proposed in this thesis,with MAE=14.93%and?_30%=93.16%.this work is beneficial for optimization of surface dominated enhancing channels.