复合改性表面低沸点工质相变换热机理研究

Surface modification can improve the boiling and condensing heat transfer. However, the tiny nanostructure can easily be covered with condensate and accumulated bubbles, it might degrade the phase change heat transfer. If the macroscopic surface and surface modification techniques are combined, phase change heat transfer of refrigerant with lower surface tension can be further enhanced. However, it is seldomly investigated on the mechaism so far. Through experimental,theoretical and numerical investigation, the major research contents of this study include: (1) wetting, flow and lagging behaviors of droplet outside the surface;(2)phase change heat transfer characteristics of working fluid with lower boiling temperature. This study proposes to address: the nucleation mechanisms of refrigerant outside the surfaces, and the heat transfer enhancement mechanisms of refrigerant outside the modified surface. The study is based on high accuracy measurement and visualization equipment; Pool boiling, falling film evaporation and condensation heat transfer will be investigated. Through the investigation, the model for phase change heat transfer outside basic compound surface like integral-finned tube should be established; 1-2 methods to manufacture the compound surface to enhance the phase changer heat transfer should be proposed; and the general rule of phase change heat transfer outside the modified surface should be obtained.

在沸腾及凝结相变传热过程中，表面改性可强化换热，但其微纳米结构容易被液膜或气泡覆盖，不利于相变传热的持续高效进行。通过机械加工复合改性，将宏观表面和微纳改性相结合，可以改善低沸点工质的相变换热，但目前缺少对其机理的研究。通过实验、理论及数值分析，本课题的主要研究内容包括：机械加工复合改性表面工质的润湿、流动及迟滞行为，和复合改性表面的传热机理。拟解决的关键问题为：揭示低沸点工质改性表面沸腾和凝结换热过程成核机理，以及获得机械加工复合改性表面低沸点工质相变传热的强化机制。课题拟利用高精度仪表及可视化设备进行降膜蒸发、沸腾和凝结换热过程测试与分析。通过研究，建立预测基本复合改性表面如低肋管表面低沸点工质相变换热的分析模型，掌握1-2种强化低沸点工质相变传热的复杂改性表面设计方法,和获得复合改性表面低沸点工质相变传热的一般规律。

为了研究复合改性表面低沸点工质的相变换热特性，本课题设计了两套针对改性工质相变换热研究的实验台，适用于不同低沸点有机工质的沸腾和凝结换热研究，同时在已有的实验系统上也开展了相关研究。本课题通过不同的表面改性手段对不同的强化表面进行了复合改性处理，基于复合改性表面，研究了表面改性对于低沸点工质沸腾和凝结相变换热的影响及强化换热机理。具体包括超疏水改性三维强化肋片表面低沸点工质的膜状凝结换热特性、超疏水改性三维强化肋片表面低沸点工质的池沸腾相变换热特性、粗糙度和润湿性对不同工质池沸腾换热特性的影响、掌握了复合改性传热表面低沸点工质相变换热的强化或削弱机制、机械加工改性表面低沸点工质相变传热的一般规律等。获得了一种通过复合改性强化低沸点工质凝结换热的技术，可使热流密度10-60kW/m2时凝结传热系数提高70%以上。建立了一种预测超疏水表面核态沸腾传热的预测公式。掌握了一种预测低沸点工质在强化表面凝结换热的数值预测方法，可以分析表面张力等因素对凝结传热过程的影响。掌握了一种数值预测低沸点工质核态沸腾换热系数的方法，预测精度和实验结果偏差-14%~9%。研究结果在制冷空调领域有较广阔的应用前景。