铵盐颗粒在U形换热管中的流动、沉积特性与垢下腐蚀行为

通过数值模拟软件FLUENT，结合Discrete Phase Model模型，对U形换热管中铵盐的流动、沉积特性进行计算流体动力学（CFD）研究。结果表明：铵盐颗粒的粒径和多相介质流速是影响铵盐流动距离和沉积区域的重要因素。颗粒粒径越小、多相介质流速越大，铵盐流动距离越远，沉积区域越大。通过固体颗粒流动沉积模拟实验进一步研究发现，对于粒径小于0.03 mm的固体颗粒，当多相介质流速大于5.50 m/s时，颗粒的流动距离不再增加。通过对失效换热管进行SEM和XRD分析，发现铵盐沉积腐蚀造成管壁减薄，在管壁内外侧高压差作用下产生诱导裂纹，引发应力腐蚀裂纹，最终导致换热管泄漏失效。

Characteristics of Flow,Deposition and Corrosion of Ammonium Salts in the U-Shaped Heat Changer Tube

Simulation software Fluent was employed to perform CFD simulation of ammonium salts flow and deposition in the U-shaped heat exchanger tube with the discrete phase model. It is demonstrated that the flow distance and deposition region of ammonium salts are determined by the particle size of ammonium salts and the flow velocity of multiphase media. The smaller the particle size， the greater the flow velocity, the longer the flow distance, the larger the deposition region. The experimental results show that when the flow velocity of multip hase media is greater than 5.50 m/s, the flow distance of ammonium salts with a size less than 0.03 mm no longer increases. The simulation and experimental results have been verified by a classical failure case of under-deposit corrosion of NH4Cl salts. By the anatomic analysis, it is found that the under-deposit corrosion leads to a decrease for the wall thickness. The stress corrosion cracks are generated in the thinned region under the great pressure difference, resulting in the leakage of the heat exchanger tube.