Study of Frosting Heat-and Mass-Transfer. Defrosting Mechanisms, and its Control by Impinging Solid Particles.

本研究分为两个部分。首先对平板内湿气流在不同凝结和结霜条件下的传热传质进行了实验研究。二是利用极浅层沸腾床进行了冷却管内霜层除霜及传热传质的实验研究。主要研究结果如下：1.传热传质与霜层生长的实验研究。I)霜层的厚度、质量和密度的增长取决于气流的速度；由于水蒸气在霜层中的扩散和凝固，密度随时间而增加。(2)膜状凝结的总热通量和潜热通量随湿度差的增大而增大。Iii)存在临界质量通量。膜状冷凝换热系数在该点有最大值。…传热传质及除霜机理的实验研究在沸腾床中，冷却管周围有更多的霜层。I)设计了一种专门用于流态化床除霜的新型射流分布器，并开发了一种可视化技术来研究结霜的形成和流态化颗粒的运动。Ii)使用高速摄像系统观察发现，分配器产生的气固撞击射流有效地去除了管子上的霜层。3)测量了冷却温度降至-30ﾟC时的局部换热系数和结霜面积分数，以及DB温度的影响。阐明了管内温度、颗粒静态床层高度、分布器和稳定器的位置对除霜性能和传热特性的影响。4)利用激光多普勒粒子动力学分析仪对剥霜的粒度分布进行了测量，基本达到了本研究的目的，本论文的研究成果和后续的研究成果将对工程和工业具有重要的参考价值。较少

The present research is divided into two parts. The first is an experimental study on heat-and mass-transfer of humid air flow in a parallel plate under various conditions covering condensation and frost formation. The other is an experimental study on defrosting of frost layers on cooled-tubes using a very shallow fluidized-bed and the heat/mass transfer. The obtained results are summarized as follows :1. Experimental Study on Heat/Mass Transfer and Frost Layer Growth. i) Growing thickness, mass, and density of the frost layer depend on the velocity of air stream ; the density increases with time, owing to the diffusion and solidification of water vapor in the frost layer. ii) Total heat flux and latent-heat flox of filmwise condensation are increased with increasing the humidity difference. iii) There exists a critical mass flux. at which the heat transfer coefficient of filmwise condensation has the maximum value.2. Experimental Study on Heat/Mass Transfer and Defrosting Mechanisms … More of Frost Layers around Cooled-Tubes in a Fluidized Bed. i) A novel jet distributor has been designed specially for defrosting in the fluidized bed, and a visualization technique has been developed to investigate the initiation of frost formation and the motion of fluidizing particles. ii) Observation using a high speed video system revealed that gas-solid impinging jets produced by the distributor strip effectively off the frost on the tubes. iii) The local heat transfer coefficient and frost area fraction were measured lowering the coolant temperature to -30^ﾟ C ; the effects of the DB temperature. tube temperature, static bed height of particles, and position of distributor and stabilizers on the defrosting performance and heat transfer characteristics were clarified. iv) The size distribution of stripped frost was measured with a Laser Doppler Particle Dynamics Analyzer System.In summary, the objectives of the present study have almost been achieved, and the results of present and successive research will make great contributions towards engineering and industries. Less