含气溶胶和不凝气体的蒸汽冷凝回流传热特性研究

The reflux condensation of steam in the tubes of steam generators can remove the residual heat and mitigate the accidents by natural circulation under the certain conditions such as small-break loss of coolant accident without emergency core cooling system. However, aerosols and non-condensable gases released from the reactor core will flow into the tubes of steam generator under the severe accident conditions, reducing the reflux condensation heat transfer capability. It will further deteriorate the accidents and may endanger the integrity of the reactor pressure vessel. Previous investigations focused on the reflux condensation heat transfer without aerosol in high concentration and cannot predict the condensation heat transfer coefficient with high-concentration aerosols and non-condensable gases. Therefore, the proposed project will focus on the reflux condensation heat transfer characteristics of steam in presence of aerosols and non-condensable gases by using experimental and theoretical methodologies. First of all, a database for the reflux condensation will be built based on the visualized reflux condensation experiment in presence of high-concentration aerosols and non-condensable gases. Then the criterion for aerosols deposition on the film-wetted wall will be deduced from the visualized experiments on the interactions between the aerosols and film and the deposition characteristics of aerosols on the wetted wall. After that, this proposed model for aerosol deposition will be compiled into the MELCOR code to estimate the condensation heat transfer characteristics of steam with aerosols and non-condensable gases. This project can provide experimental data, theoretical model and analysis code for the reactor safety analysis under the severe accidents related to the reflux condensation.

特定事故工况下（如丧失应急冷却的小破口事故），蒸汽发生器传热管内的冷凝回流以自然循环的方式导出堆芯余热，缓解事故的严重程度。但在严重事故下，堆芯释放的气溶胶和不凝气体随蒸汽进入传热管，降低冷凝回流传热效率，使事故进一步恶化，危及压力容器的完整性。已有的针对蒸汽冷凝回流的研究未考虑高浓度气溶胶的影响，不能用于含高浓度气溶胶和不凝气体的蒸汽冷凝传热的研究，因此本项目拟对含气溶胶和不凝气体的蒸汽冷凝回流传热进行实验和理论研究。首先通过可视化实验研究含高浓度的气溶胶和不凝气体的蒸汽冷凝传热特性，建立冷凝传热数据库。同时，在可视化实验中观测气溶胶与液膜的作用及其在壁面上的沉积特性，开发气溶胶在被液膜覆盖的壁面上的沉积模型。最后在MELCOR程序中加入本研究开发的气溶胶沉积模型，用于计算含气溶胶和不凝气体的蒸汽冷凝传热特性。本项目为严重事故下涉及冷凝回流的安全分析提供实验数据、理论模型和分析程序。

高压条件下发生余热排出系统丧失事故使堆芯沸腾时，蒸汽进入蒸汽发生器传热管，发生冷凝传热，可将部分堆芯释热量导出，缓解事故进程。然而，高压熔堆等严重事故条件下，堆芯发生熔化，将生成大量不凝气体和气溶胶颗粒，与蒸汽混合后流入蒸汽发生器传热管内发生冷凝。不凝气体会阻碍蒸汽到达冷壁面发生冷凝，降低冷凝传热系数；同时，气溶胶在壁面沉积，形成污垢，会进一步减弱冷凝传热能力，降低通过冷凝回流的堆芯余热导出量，导致堆芯热量无法排出，并有可能导致下封头在高压下的破坏及熔融物喷放到底坑等极其严重的后果，但目前缺乏可以考虑气溶胶沉积的分析模型和程序。.本项目开展气溶胶沉积特性研究，获得气溶胶沉积数据库，以及颗粒直径、入射速度和角度对刺穿液膜和沉积的影响规律；之后开发气溶胶颗粒在干净干表面、污垢干表面和湿表面上的沉积机理模型，建立气溶胶颗粒与冷凝传热耦合作用模型体系；最后将开发的模型植入到商用CFD程序FLUENT中，拓展FLUENT原有不凝气体和液膜模型的应用范围，使其可以准确计算含气溶胶和不凝气体的蒸汽冷凝传热特性。通过研究发现，颗粒直径、入射速度和角度对气溶胶刺穿液膜过程和沉积过程具有重要影响，颗粒直径越大、入射速度越大、入射角度越小，颗粒越容易刺穿液膜；但是当入射速度超过临界值时，颗粒将无法在壁面沉积。通过本项目的研究，获得了气溶胶沉积特性数据、机理模型和分析程序，可支撑严重事故条件下反应堆的安全分析。