Abstract Falling film evaporators offer an attractive alternative to flooded evaporators as the lower fluid charge reduces the impact of leaks to the environment and associated safety concerns. A study was conducted of saturated falling film boiling of two refrigerants on one polished, one roughened and three nanostructured copper tubes in order to evaluate the potential of nanostructures in falling film refrigerant evaporators. Tubes were individually tested, placed horizontally within a test chamber and heated by an internal water flow with refrigerant distributed over the outside of the tubes. Wilson plots were used to characterise the internal water heat transfer coefficients (HTCs). A layer-by-layer (LbL) process was used to create the first nanostructured tube by coating the outside of a tube with silica nanoparticles. A chemical bath was used to create copper oxide (CuO) protrusions on the second nanostructured tube. The third tube was coated by following a commercial process referred to as nanoFLUX. R-245fa at a saturation temperature of 20 °C and R-134a at saturation temperatures of 5 °C and 25 °C were used as refrigerants. Tests were conducted over a range of heat fluxes from 20 to 100 kW/m and refrigerant mass film flow rates per unit length from 0 to 0.13 kg/m/s, which corresponds to a film Reynolds number range of 0 to approximately 1500 to 2500, depending on the refrigerant. Heat fluxes were increased further to test whether the critical heat flux (CHF) point due to a departure from nucleate boiling (DNB) could be reached. The CuO and nanoFLUX tubes had the lowest film Reynolds numbers at which critical dryout occurred at heat fluxes near 20 kW/m2, but as the heat fluxes were increased towards 100 kW/m2, critical dryout occurred at the highest film Reynolds numbers of the tubes tested. Furthermore, in some higher heat flux cases, CHF as a result of DNB for the CuO and nanoFLUX tubes was reached before critical dryout occurred, and DNB became the limiting operational factor. The refrigerant condition that had the worst dryout performance in terms of film Reynolds number was R-134a at 25 °C, followed by R-134a at 5 °C and R245fa at 20 °C. Tests across the heat flux range and refrigerant conditions revealed that compared to the polished tube, the roughened tube had HTCs between 60 and 100% higher, the LbL tube had HTCs between 20% lower and 20% higher, the CuO tube had HTCs between 20% lower and 80% higher and the nanoFLUX tube had HTCs between 40 and 200% higher than the polished tube. The falling film enhancement ratios for the plain and nanostructured tubes were found to be of a similar order of magnitude, typically between 1.3 and 0.8.

中文翻译：

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纳米结构和粗糙管上制冷剂的降膜沸腾：传热、干燥和临界热通量

摘要 降膜蒸发器是溢流蒸发器的一种有吸引力的替代方案，因为较低的流体充注量减少了泄漏对环境的影响和相关的安全问题。为了评估纳米结构在降膜制冷剂蒸发器中的潜力，对一根抛光铜管、一根粗糙铜管和三根纳米结构铜管上的两种制冷剂的饱和降膜沸腾进行了研究。管子经过单独测试，水平放置在测试室内，并通过内部水流加热，制冷剂分布在管子的外部。威尔逊图用于表征内部水传热系数 (HTC)。通过用二氧化硅纳米粒子涂覆管的外部，使用逐层 (LbL) 工艺制造第一个纳米结构管。使用化学浴在第二个纳米结构管上产生氧化铜 (CuO) 突起。第三根管子按照称为 nanoFLUX 的商业工艺进行涂覆。饱和温度为 20 °C 的 R-245fa 和饱和温度为 5 °C 和 25 °C 的 R-134a 用作制冷剂。测试在 20 到 100 kW/m 的热通量范围和每单位长度的制冷剂质量膜流速从 0 到 0.13 kg/m/s 的范围内进行，这对应于 0 到大约 1500 到 2500 的膜雷诺数范围，取决于制冷剂。进一步增加热通量以测试是否可以达到由于偏离核沸腾 (DNB) 而导致的临界热通量 (CHF) 点。CuO 和 nanoFLUX 管具有最低的薄膜雷诺数，在接近 20 kW/m2 的热通量下发生临界干燥时，但是当热通量增加到 100 kW/m2 时，临界干燥发生在测试管的最高薄膜雷诺数处。此外，在一些更高的热通量情况下，由于在发生临界干燥之前，CuO 和 nanoFLUX 管的 DNB 就达到了 CHF，并且 DNB 成为限制操作因素。就薄膜雷诺数而言，干燥性能最差的制冷剂条件是 25 °C 下的 R-134a，其次是 5 °C 下的 R-134a 和 20 °C 下的 R245fa。热通量范围和制冷剂条件的测试表明，与抛光管相比，粗糙管的 HTC 高出 60% 到 100%，LbL 管的 HTC 降低了 20% 到 20%，CuO 管的 HTC 介于 20 % 低和 80% 高，nanoFLUX 管的 HTC 比抛光管高 40% 到 200%。