Research of Boiling Heat Transfer and Flow Pattern of Refrigerant in Horizontal Small-Diameter Tubes

在内径为0.51mm、1.12mm和3.1mm的水平小管径管内，对HFC-134a的沸腾传热和流型进行了实验研究。获得了质量流速为150~400 kg/m^2 s，热通量为5~30 kW/m^2时的沸腾传热系数。在质量流速为100~400 kg/m^2s的范围内，在入口质量为0~0.7的范围内，观察到塞状流、段塞流、波浪流和环状流的流型。当入口干度较低、热流密度较高时，管内制冷剂流动发生振荡，管内壁面温度和压力出现振荡。沸腾换热系数受流型的影响，振荡流和弹状流的沸腾换热系数显著降低。结果表明，随着管径的减小，强制对流蒸发对沸腾换热的贡献逐渐减小。提出了传热恶化模型和干涸预测模型，预测值与实验值具有较好的相关性。在3.1 mm管中的压降与Lockhart-Martinelli关联式的预测结果吻合较好，而在0.51 mm管中的压降与均相模型的预测结果吻合较好。

Boiling heat transfer to HFC134a and flow pattern were investigated in horizontal small-diameter tubes, 0.51 mm, 1.12 mm and 3.1mm I.D. Boiling heat transfer coefficients were obtained for mass velocity of 150 〜 400 kg/m^2s, and heat flux of 5〜30 kW/m^2. The flow patterns of plug, slug, wavy and annular flows were observed at inlet qualities ranging from zero to 0.7 for the range of mass velocity of 100〜400 kg/m^2s. For the low inlet quality and high heat flux, the oscillated refrigerant flow occurred and oscillations of wall temperature and pressure were observed in the heated tube. The boiling heat transfer coefficients were affected by the flow patterns, and were considerably reduced for the oscillated and slug flow. It was shown that the contribution of the forced convective evaporation to boiling heat transfer decreases, as the tube diameter decreases. A heat transfer deterioration model and a dryout prediction model were proposed, and predictive value and experimental value are correlated well. The pressure drop in 3.1 mm tube agreed well with the prediction by the correlation of Lockhart-Martinelli, but in the case of the tube diameter 0.51 mm, it agreed well by the homogenous model.