The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

为探究分段结构对管内流动沸腾过程的影响,采用试验方法分析了制冷剂R245fa在光滑管、内肋管以及采用前肋-后光滑组合管中的流动沸腾换热特性.以质量流速为100～350 kg/(m2·s)、热流密度为9.36～55.84 kW/m2、试验管进口过冷度约为2 ℃进行试验,并以光滑管的流动换热特性为基准,对比不同试验管的换热系数增强比例(enhancement factor,EF)和价值因子(merit factor,MF).研究结果表明:内肋管和前肋-后光滑组合管的换热性能均优于全光滑管;在低质量流速时,内肋管的EF高于前肋-后光滑组合管,即内肋管换热效果更好;而随着质量流速的增加,前肋-后光滑组合管的EF逐渐高于内肋管,达到1.7左右;内肋管的MF在低质量流速时高于前肋-后光滑组合管,但在较高质量流速以及较高热流密度条件下,前肋-后光滑组合管的MF要高于内肋管,即前肋-后光滑组合管的综合性能更好.

Two-phase flow boiling visualization and heat transfer characteristics in the 1 mm microchannel are experimentally investigated. The experiments are carried out at mass fluxes of 300 to 700 kg/m2s and heat fluxes of 10 to 94 kW/m2. Five flow patterns are observed, of which the annular flow is the dominant flow pattern, which is already present in the low vapor quality region and has a significant influence on the heat transfer. In addition, dynamic flow processes such as bubble coalescence in bubbly and slug flow, the rupture of the elongated bubble and the chasing of droplet mist and liquid film wave in churn flow are also recorded. For the heat transfer coefficient profile, the trend is determined by three main regimes: bubble/slug regime, liquid film evaporation regime and intermittent dryout regime. Moreover, the thin liquid film evaporation along with the intermittent dryout is found to dominate the boiling heat transfer of the microchannel in the present study. Finally, the experimental heat transfer coefficients are compared with five predictive correlations, and the prediction accuracies are analyzed. Some modifications are made to improve the accuracy, and the modified three-zone model [41] presents a satisfactory accuracy with MAE=26.29% and θ=88.02%.

The flow boiling heat transfer of environmentally friendly refrigerant HFO-1234yf in a horizontal circular microchannel is experimentally investigated. The inner diameter and heating length of the test channel are 0.5 mm and 300 mm, respectively. Experiments are conducted at relatively high mass flux ranging from 750 to 1500 kg m(-2) s(-1), heat flux from 25 to 85 kW m(-2) and inlet pressure between 0.604 and 0.634 MPa. The heat flux is stepwise imposed to the test channel until dryout occurs. The flow boiling heat transfer characteristics are analyzed in detail. In the pre-dryout region, the heat transfer coefficient (HTC) is found to be mainly determined by the heat flux, while the effect of mass flux is weak. Two different HTC behaviors are observed at low and high mass fluxes, which can be attributed to the prominent effect of thin liquid film evaporation on heat transfer and the occurrence of partial dryout. Moreover, the oscillation of mass fluxes and wall temperatures are observed at high mass flux conditions, resulting in heat transfer deterioration. In the post-dryout region, the critical heat flux (CHF) is found to increase with mass flux, while the critical vapor quality would decrease. Finally, the experimental data of HTC and CHF are compared with correlations from the literature and good agreements are presented.

通过实验研究了环境友好型制冷剂R1234yf在内径为0.5mm的水平圆形微通道内的流动沸腾换热特性,测量了不同工况下R1234yf的沸腾换热系数(HTC),并与传统制冷剂R134a进行了对比,分析了质量流速、热流密度和干度对换热系数变化规律的影响.实验条件为:饱和温度(17±1)℃,质量流速1000～2500kg/(m2·s),热流密度25～143kW/m2.实验结果表明:R1234yf的换热系数随着热流密度的增大而显著增大,而质量流速和干度的影响较小,核态沸腾为其主导换热机制.对比R1234yf和R134a在相同工况下的换热特性,发现两种工质的平均换热系数差别较小,并均随着热流密度增大而逐渐增加,但是R1234yf发生干涸(Dryout)时的热流密度小于R134a.将实验数据与已有文献中的核沸腾主导的经验关联式的预测结果进行了对比,得到了较好的吻合.

通过实验研究了制冷剂R134a在两种微通道尺寸下(0.5 mm和1 mm)的两相压降特性.其中,质量通量G范围为500～2500 kg/(m2·s),热通量q范围为15～147 kW/m2,饱和温度Tsat范围为15～37℃.研究评估了5种已有压降模型,与实验数据进行了比较.结果表明,摩擦压力梯度随干度的升高呈现出先增大后减小的趋势.其中,1 mm管实验结果与Kim和Mudawar的模型吻合较好,但0.5 mm管预测值高于实验值.相比于1 mm管,0.5 mm管压力波动较大,并导致压降数值分布更分散.

In this study, we conduct pool boiling experiments on pure copper and bi-conductive surfaces using sodium dodecyl sulfate as the surfactant and focus on the bubble behaviors and heat transfer performance of the solutions at the boiling crisis. In comparison with the deionized water, the surfactant solutions are found to effectively enhance the boiling heat transfer at low and medium heat flux. This could account for more active nucleation sites and less bubble coalescences, which can accelerate the formation and departure of bubbles. However, as mushroom bubbles are more easily formed in the surfactant solutions with the increase of the heat flux, the rewetting effect of the heater surface is inhibited, and the CHF is lower than that of the deionized water. For the bi-conductive surface, the CHF is found to be improved in the boiling of the surfactant solution compared with the smooth cooper surface, and the heat transfer deterioration are slowed down during the boiling crisis. Moreover, it is found that the low-thermal-conductivity material can hinder the lateral heat transfer, ensuring the generation of small bubbles and inhibiting the expansion of the dry-out area. (C) 2020 Elsevier Ltd. All rights reserved.