基于单元损伤过程的液/固双相流冲刷腐蚀临界流速机理模型构建及验证

The erosion-corrosion in liquid-particle two-phase media is a vital cause for the premature failure of various flow passage components, such as pumps, valves, pipelines and so on. There is a phenomenon of critical flow velocity (CFV) for the erosion-corrosion of passive materials, which represents the characteristics that the material damage increases rapidly once the flow velocity exceeds a critical value. It is an important criterion to evaluate the resistance to erosion-corrosion. The investigations on the mechanism of CFV and its influencing factors have been far from sufficient up to now, leading to the absence of the quantified mechanism model. This project aims to build a mechanism model of CFV in liquid-particle two-phase flow based on the unit damage process. Firstly, a CFV model is theoretically derived by taking a criterion that the repassivation time equals to the interval time of solid particle impact as the critical condition. Then, the repassivation process is evaluated experimentally to obtain the expression of repassivation time by using the single particle impingement equipment. Accordingly, the semi-quantitative and semi-empirical model of CFV is built based on the unit damage process. Thereafter, the obtained CFV model will be validated and improved by means of CFV data obtained under the conditions with different levels of damage accumulation. Finally, the CFV mechanism model is built by coupling the accumulated damage, which can be used to explain the essential difference in the CFV of various materials and can provide theoretical fundamentals for the material selection of flow passage components and the control of erosion-corrosion conditions.

液/固双相流冲刷腐蚀是造成各种泵、阀、管道等过流部件过早失效的重要原因。钝性材料冲刷腐蚀损伤存在临界流速现象，它反映出材料损伤在流速超过某一临界值后急剧增大的特性，是评价钝性材料耐冲刷腐蚀性能的重要判据。目前，对临界流速机理及其影响因素的研究远不充分，缺乏定量化的机理模型。本项目拟基于单元损伤过程构建液/固双相流冲刷腐蚀临界流速机理模型。首先以材料再钝化时间和颗粒冲击间隔时间相等作为临界条件，理论推导临界流速机理模型；然后采用单颗粒喷射设备测定再钝化动力学曲线，解析再钝化时间表达式，建立基于单元损伤的临界流速半定量半经验模型；再利用不同累积损伤条件下的临界流速数据对模型进行验证和完善，最终建立耦合了累积损伤的临界流速机理模型。利用此模型解释不同材料临界流速存在差异的内在机理，为过流部件选材和冲刷腐蚀工况控制提供基础理论支撑。

液/固双相流冲刷腐蚀是造成管道等过流部件过早失效的重要原因。钝性材料冲刷腐蚀损伤存在临界流速现象，临界流速是过流部件结构优化、工况控制、损伤部位预测等的重要判据，实现临界流速的理论预测意义重大。但是，目前缺乏量化的临界流速机理模型。本项目首先研究了不锈钢点蚀现象和冲刷腐蚀临界流速现象之间的关联性，发现再钝化性能在冲刷腐蚀临界流速现象中具有重要作用。采用离子标记法和颜色标记法，实验证明了钝化膜在远低于临界流速的条件下就已经发生破裂，明确了钝性材料的临界流速现象是由固体颗粒冲击引起的去钝化过程（钝化膜破裂）与电化学再钝化过程（钝化膜自修复）之间的竞争造成的，进一步证实了再钝化性能在钝性材料冲刷腐蚀损伤中起到了至关重要的作用，为建立临界流速机理模型奠定了理论基础。利用单颗粒冲刷腐蚀实验装置，研究了不锈钢、纯钛和镍基合金的去钝化-再钝化过程，通过理论推导、数学近似和实验验证相结合，提出了一种通用且简单的再钝化时间获取新方法，实现了再钝化性能的可靠量化评价。利用这一新方法，系统研究了不锈钢冲刷腐蚀去钝化过程和再钝化过程的关联性，基于去钝化时间和再钝化时间相等的临界条件，建立了冲刷腐蚀临界流速预测模型，成功预测了临界流速随流体参数的变化规律。同时，发现了去钝化损伤能够抑制再钝化过程，在这种情况下，深入研究了累积损伤对再钝化过程的影响，证明了累积损伤引起的残余应力是导致再钝化过程被抑制的根本原因。本项目最终明确了再钝化在钝性材料中冲刷腐蚀损伤的关键作用，对深入理解冲刷腐蚀机理具有重要的理论价值；建立了钝性材料冲刷腐蚀临界流速预测模型，对预测过流部件冲刷腐蚀损伤敏感部位具有重要的实际应用价值。