Falling film evaporation and pool boiling heat transfer of R1233zd(E) on thermal spray coated tube
Falling film evaporation and pool boiling heat transfer of R1233zd(E) on thermal spray coated tube
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DOI：
10.1016/j.applthermaleng.2021.117329
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发表时间：
2021-09
影响因子：
6.4
通讯作者：
Tsutomu Ubara;H. Asano;K. Sugimoto
中科院分区：
文献类型：
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作者：
Tsutomu Ubara;H. Asano;K. Sugimoto
降膜蒸发器有望作为满液式蒸发器的替代品，以减少制冷剂的使用。保持液膜并促进液膜中的泡核沸腾对于增强薄膜蒸发传热具有重要意义。本研究的重点是通过热喷涂在液膜中进行核沸腾传热，无论传热管的材料如何，都可以应用该涂层。对水平铜管上降膜蒸发的传热性能进行了实验评价。由于泡核沸腾现象，例如成核位点的激活和液膜中的气泡行为，很大程度上取决于制冷剂的物理性质，因此将低 GWP 替代制冷剂之一 R1233zd(E) 的实验结果与 R134a 的实验结果进行了比较。使用两种类型的管：光管和采用电弧丝喷涂方法制造的热喷涂管，外径为19.05毫米，加热长度为50毫米。还进行了池沸腾实验来确认泡核沸腾的壁过热度。传热系数的评估条件为膜质量流量范围为8.5×10−3至6.3×10−2kg/(m·s)、热通量范围为10至85kW/m2、饱和温度为20℃。讨论了受热面结构、热通量和制冷剂热物性对传热性能的影响。结果表明，在足够的热通量和液膜流量的情况下，降膜传热变成了核态沸腾。热喷涂涂层表面不仅增强了泡核沸腾，而且通过液膜中的气泡增强了液体的扩散，因此涂层管产生了更高的传热系数，是光滑管的2.1至4.8倍。 R1233zd(E) 在核态沸腾开始时的壁过热度高于 R134a，因为 R1233zd(E) 具有较高的表面张力和较低的蒸气密度。因此，R1233zd(E) 的池沸腾和降膜蒸发传热系数低于 R134a。
Falling film evaporators have been expected as an alternative to flooded evaporators for reducing the refrigerant usage. It is important for the enhancement of the film evaporation heat transfer to keep a liquid film and promote nucleate boiling in the liquid film. This study focused on nucleate boiling heat transfer in the liquid film by thermal spray coating that can be applied regardless of the material of heat transfer tubes. The heat transfer performance of falling film evaporation on a horizontal copper tube was experimentally evaluated. Since nucleate boiling phenomena, such as activation of nucleation sites and bubble behaviors in the liquid film, strongly depend on the physical properties of the refrigerant, the experimental results for R1233zd(E), one of the alternative refrigerants with low GWP, were compared with those for R134a. Two types of tubes were used: the smooth tube and the thermal spray coated tube fabricated by an arc wire spraying method, with the outer diameter of 19.05 mm and the heating length of 50 mm. Pool boiling experiments were also conducted to confirm the wall superheat for nucleate boiling. The heat transfer coefficients were evaluated with a film mass flow rate range of 8.5 × 10−3to 6.3 × 10−2kg/(m·s), a heat flux range of 10 to 85 kW/m2, and a saturation temperature at 20 °C. The effects of the structure of the heating surface, heat flux, and thermophysical properties of the refrigerant on the heat transfer performance are discussed. Results show that the heat transfer of the falling film became the nucleate boiling dominant with a sufficient heat flux and liquid film flow rate. The thermal spray coated surface enhanced not only the nucleate boiling but also the liquid spreading by the vapor bubbles in the liquid film, and thus the coated tube produced higher heat transfer coefficients of 2.1 to 4.8 times those for the smooth tube. The wall superheat at the onset of nucleate boiling was higher for R1233zd(E) than for R134a, because R1233zd(E) has a higher surface tension and lower vapor density. Thus, the pool boiling and falling film evaporation heat transfer coefficients were lower for R1233zd(E) than for R134a.