非定常内热源放射性废液竖管降膜蒸发动力学研究

Falling film evaporation is the core process of MVR heat pump and multi-effectevaporation energy-saving evaporation technology.However, for the radioactivewaste liquid, due to evaporation and concentration, the radionuclides are enriched which form an unsteady heat source. This unsteady heat source makes theradioactive waste liquid falling film evaporation dynamics significant different from the conventional evaporation. But the basic research is very lack. To reveal the influence mechanism of the unsteady heat source on heat mass transfer of falling film evaporation, a falling film evaporation heat transfer and mass transfer model for the radioactive waste liquid is built based on the unsteady heat source. This model will study the influence of the change of radioactive nuclide concentration on the physical properties such as liquid specific heat, viscosity, thermal conductivity, surface tension and the boiling point temperature rise. Besides, the influence of the change of radioactive nuclide concentration on endogenous decay heat flux will also be studied. By building the radioactive waste liquid falling film evaporation test system, using means such as particle detection and high-speed photograph, the flow pattern change,the law of the heat and mass transfer and fluctuation of the radioactive waste liquid in the evaporation process is studied. Finally, the heat and mass transfer of falling film evaporation criterion and dry criterion of critical conditions of radioactive liquid waste can be obtained. Providing theoretical basis for radioactive waste liquid energy-saving evaporation design, ensuring the safety and efficiency during the process of falling film evaporation for radioactive waste liquid, this research have great scientific significance.

降膜蒸发是MVR热泵及多效蒸发等节能蒸发技术的核心过程，而对于放射性废液，由于放射性核素随着蒸发浓缩的进行而逐步富集，形成了非定常内热源，使得放射性废液的降膜蒸发动力学特性与常规蒸发有着显著差异，但相关基础研究却极为缺乏。针对非定常内热源对降膜蒸发传热传质的影响机理不明的问题，通过研究放射性核素浓度变化对废液比热、粘性、导热系数、表面张力、沸点温升等物性的影响，以及核素浓度变化与放射性内源衰变热通量的关系，建立基于非定常内热源的放射性废液降膜蒸发传热传质模型。通过搭建放射性废液降膜蒸发试验系统，采用粒子探测和高速拍照等手段研究放射性废液在蒸发过程中流型变化、传热传质和波动的规律，获得放射性废液降膜蒸发的传热传质准则和干烧临界条件判据。该研究可为放射性废液节能蒸发设计提供理论依据，对保证放射性废液降膜蒸发浓缩过程的安全与高效具有重要的科学意义。

本项目搭建了竖管外液膜流动与传热实验台研究液膜流动与传热特性，测量了液膜瞬时厚度、液膜流型及液膜温度等参数，拟合特定工况下的流动、换热及液膜破断关联式。构建了竖管外液膜流动与传热过程三维数值计算模型，通过实验结果验证了三维数理模型的可靠性。通过数值模拟计算出液膜厚度、自由界面速度、自由界面内速度、自由界面温度、壁面温度、自由界面内温度、局部传热系数等参数，解决了传热特性与流动特性割裂的问题，阐明液膜流动与换热特性及其耦合规律，强化液膜流动与传热的科学解释。.通过实验研究了不同雷诺数下、不同热流密度以及不同时间液膜流型、液膜厚度沿流动方向分布。液膜流动具有强烈的随机性与三维特性，本文给出了液膜流场从层流向湍流转捩范围，利用统计学手段分析了液膜瞬时厚度，明晰了不同工况液膜流动规律；通过对实验数据的整合与分析，得到了表征液膜厚度、液膜传热系数、液膜破断判据的实验关联式。.在层流区域，自由界面速度与液膜厚度沿流动方向具有相似的发展区域；在湍流区域，由于流动不规则加剧，此趋势不再明显；壁面剪切力与液膜厚度表现出相似的变化趋势，在液膜表面波峰前，壁面剪切力迅速增加并达到最大值；在液膜表面波谷前，壁面剪切力迅速减小并达到最小值，两者之间存在一定的相位差。在液膜流动换热过程中，当导热占优时，液膜厚度与局部换热系数沿流动方向有明显相反的趋势；当对流占优时，此趋势不再明显。加热热流密度因素通过改变液膜流型而改变液膜局部换热系数沿流向方向的分布，但是对局部换热系数平均值影响不大；随着液膜入口流速的增加，液膜局部换热系数平均值先快速增加，随后增加缓慢；一方面入口流速增加会增强扰动从而提高换热系数，另一方面入口流速的增加也会带来液膜厚度增加，从而增加液膜换热热阻；热流密度会在液膜表面形成热毛细对流，热毛细对流一方面可以强化换热，当热流密度过大，会导致液膜流动过程发生破断。