Experimental Investigation On The Characteristics Of Low-boiling Fluid Flow Boiling In Mini/Micro-channel

Based on the heat dissipation for IGBT used for high speed train, aerospace vehicles and high heat flux electronics, experimental studies and theoretical researches about low-boiling fluid flow boiling in micro multichannels were studied. The study included the flow pattern and flow pattern transition, heat transfer characteristics and heat transfer model, the effect of flow instability on friction pressure drop. Theoretical models of flow pattern transition, heat transfer and friction pressure drop were built,correspondingly correlations were proposed.According to visualization experiments, bubbly flow, slug flow, churn flow, liquid slug flow and annular flow were observed successively with the vapor quality increase.The transitions between neighboring flow patterns were investigated theoretically.According to experimental results, the increase rate of active nucleate density with boiling number in slug flow was larger than that in bubbly flow. Therefore the bubbly-slug transition criterion was proposed. By comparing with previous flow pattern criteria and experimental data, the present criterion presented satisfied prediction ability.In the same mass flow rate range, the heat transfer coefficient of flow boiling in microchannel varied as “Saddle-Shaped" curve with the increase of vapor quality.According to the visual experiments, two peak values and valley point were slug flow,initial annular flow and churn flow correspondingly. Since the initial annular flow was close to dry-out region which led to temperature soaring, the slug flow was more suitable for heat dissipation of high heat flux devices. Therefore based on the evaporation of liquid-thin film, Marangoni effect and micro-scale effect, heat transfer model of slug flow was built. The expression of absorption heat for phase change in slug flow was defined, based on it, the slug flow evaporation parameter was proposed to revise Li-Wu heat transfer correlation. The revised correlation could predict 732 slug flow experimental data with MAE of 25.41%, which demonstrated its applicability and practicality.By comparing the friction pressure drop in single channel and multichannel,friction pressure drop in multichannel was higher than that in single channel at the same vapor quality range, but the pressure drop variation range in multichannel was smaller than that in slingle channel. According to the visualization experiemnts and real-time measurements, the instability flow was more obvious in multichannel than that in single channel. The growth equations of slug in instability flow were established based on energy conservation law. The instability flow friction factor was defined based on the slug growth model. As the channel number and ratio of length and diameter had effects on the flow instability, the structure factor was proposed. With the instability friction factor and structure factor, the revised homogenous pressure drop correlation was proposed for flow boiling in multichannel. By comparing with multichannel experiment data, the mean absolute error of the proposed correlation was 17.09%, demonstrated appropriate predictability.