| In recent years, micro- /mini- channels' compact heat exchanger has been applied more and more extensively in the fields of automotive, aerospace, electronics and cryogenic industry. But many related theoretical problems such as heat transfer mechanism have not been solved eventually. The research on the characteristics of flow boiling heat transfer in (a) micro- /mini- channel(s) is still in an immature stage. This paper analyzes the difference of boiling heat transfer mechanism between pooling, conventional channels and micro- /mini- channel(s) based on the flow patterns. A new opening direct experimental system was devised to study the characteristics of flow boiling heat transfer of the working fluid R134a in a horizontal smooth mini-tube. The new system overcomes the disadvantages of traditional close circulating system, such as complicated operation, not easy to control the experimental parameters, difficult to attain the balance and stability, etc. A setup was designed and established. The test section is a smooth stainless steel tube, with an internal diameter of 1.127mm and external diameter of 2.012mm respectively. Local heat transfer coefficients are measured for a range of mass flow rate G: 200-500kg/m~2s; heat flux q: 4-22 kW/m~2 ; vapor quality x: 0-0.85; boiling temperature: 30±0.7°C. Among analysis, the influence of the variation of saturated pressure on boiling heat transfer coefficient was considered in the dissertation, which improves most of the previous similar research results and makes the results more rational and reliable. The effects of vapor quality, heat flux and mass flow rate to the boiling heat transfer coefficient have been discussed respectively. The experimental heat transfer coefficient under different working conditions have been compared with the correlations developed for fluid boiling heat transfer prediction in conventional channel after modified.It is found that the flow boiling heat transfer coefficient decreases linearly with the changes of vapor quality (such case is not obvious when several exit vapor quality are low) under the experimental working conditions studied in this paper. In relative higher exit vapor quality, the heat transfer coefficient reaches a maximum value at x=0.2~0.3. In lower exit vapor quality, the heat transfer coefficient increases with the heat flux, however, the effects of heat flux is not obvious in higher vapor quality situation. The mass flow rate has an influence on the effect of heat transfer coefficient only when heat flux is low, otherwise,the influence is very weakly. Due to the relatively low vapor quality range in the experiment, heat flux has a stronger influence on heat transfer coefficient than mass flow rate does. It illustrates that the nucleation boiling mechanism has a greater influence on heat transfer than the convective evaporation mechanism does. The phenomenon that the nucleation boiling mechanism or the convective evaporation mechanism is restrained completely was not found. In general, the flow boiling heat transfer is the result of the co-action between the nucleation boiling mechanism and the convective evaporation mechanism, which is similar to the situation of that in conventional tubes.After the comparison of the flow boiling heat transfer coefficient of R134a obtained by experiment in a mini-tube and that calculated by nine known empirical correlations in the literatures, it is found that the calculate accuracy of the revised correlations of Gungor —Win (1986) and Yu(1999) is superior to others, and the absolute mean deviation with the experimental data is 14.37% and 15.25% respectively. The uncertainty analysis is also carried out in the dissertation, and the relative uncertainty of heat transfer coefficient is 22.99%. At last, the next research fields are proposed too.Gao Long (Heating, Ventilation & Air-conditioning) Supervised by Fan Xiaowei (Professor)...
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