表面微结构的宽深比与占空比对马氏体不锈钢空化和空蚀的影响

目的 探究表面微结构尺寸变化对空蚀过程的影响以及作用机制。方法 根据点阵型表面微结构的截面形状,改变其宽深比和占空比,用Gambit对其建立二维平面模型。使用Fluent软件对不同尺寸下微结构模型的空蚀过程进行模拟计算,得到绝对压力、含汽率以及气泡运动速度等参数。最后根据模拟计算的结果,在样品表面加工出不同占空比的微结构,使用磁致超声振动空化设备,在与数值模拟相同环境条件下进行空蚀试验,采用失重法对模拟结果进行验证。结果 数值模拟分析显示,微结构的宽深比和占空比对空蚀过程均有影响。当宽深比小于2.5时,微结构底面的含汽率降至0.1以下,沟槽内形成一层“液垫”,减缓了空蚀。相较于其他占空比,当占空比为0.91时,含汽率最小,保持在0.4以下,样品表面同样形成“液垫”,缓冲了气泡溃灭时作用在材料表面的冲击力。此外,随着占空比的增大,微结构表面的绝对压力与液体的饱和蒸汽压差距增大,降低了空化程度,同样起到了减缓空蚀的作用。不同占空比下的微结构样品和光滑样品的空蚀试验结果表明,微结构样品的空蚀质量损失均小于光滑样品的空蚀质量损失,且当占空比为0.91时,空蚀质量损失最小,为5.95 mg,远小于占空比为0.71和0.38时,微结构样品。同样证明了表面微结构占空比越大,材料的耐空蚀性能越好。结论 微结构宽深比主要影响微结构底面的含汽率,而占空比影响样品表面的绝对压力和微结构顶面的含汽率。相较于光滑表面,微结构样品可以有效减缓马氏体不锈钢的空蚀损伤。当宽深比较小、占空比较大时,材料表面的含汽率保持在较低的水平,有利于减缓空蚀损伤。

Effect of Width-to-Depth Ratio and Duty Cycle of Surface Microstructure on Cavitation and Cavitation Erosion on Martensitic Stainless Steels

The work aims to investigate the effects of surface microstructure on cavitation erosion. In this paper, a series of dot array-shaped microstructures were successfully established by Gambit. Ansys Fluent was employed to simulate the cavitation of the flow domain around the surface microstructure with different geometry characteristics. Absolute pressure, volume fraction of vapor phase, and bubble movement velocity were utilized to characterize the cavitation and cavitation erosion. Finally, a cavitation erosion test was performed to confirm the simulation results with a magnetostrictive-vibration cavitation facility. The mass loss method was used to evaluate the cavitation erosion. The results showed that both the width-to-depth ratio and the duty cycle of the microstructure affected the cavitation process. For the width-to-depth ratio of the surface microstructure, when it was less than 2.5, the volume fraction of the vapor phase on the bottom surface of the microstructure was reduced to less than 0.1. A layer of "liquid buffer" was formed in the groove, which could alleviate cavitation erosion. For the duty cycle of the surface microstructure, the volume fraction of the vapor phase was the smallest when the duty cycle was 0.91, remaining below 0.4. The "liquid cushion" was also formed on the surface of the sample, which absorbed the impact energy of the bubbles on the surface of the material as they collapse. As the duty cycle increased, the absolute pressure on the microstructured surface increased, reducing the degree of cavitation and similarly acting to slow down cavitation erosion. The cavitation experiments were performed on microstructured samples at different duty cycles with smooth samples as a control group. It was found that the mass loss and the SEM morphology of cavitation erosion of samples with microstructures were lighter than those of smooth samples, implying that surface microstructures could effectively mitigate the cavitation erosion of materials. Combined with the analyses of the simulation results, it was concluded that the microstructure had different mechanisms to mitigate cavitation erosion under different duty cycles. At the duty cycle of 0.38, the microstructure pushed the bubbles away from the sample surface to mitigate cavitation erosion, and at duty cycles of 0.71 and 0.91, the microstructure mitigated cavitation erosion by reducing the vapor phase volume fraction. When the duty cycle was 0.91, the mass loss of the microstructured samples was 5.95 mg, much smaller than that of the microstructured samples at the duty cycles of 0.71 and 0.38. It was also demonstrated that the larger the duty cycle of the surface microstructure, the better the cavitation erosion resistance of the material. In conclusion, the width-to-depth ratio of the surface microstructure mainly affected the vapor phase volume fraction of the bottom surface of the microstructure, while the duty cycle affected the absolute pressure across the surface and the vapor phase volume fraction at the top surface of the microstructure. It was proved that the surface microstructure could improve the cavitation erosion resistance of martensitic stainless steel. When the microstructure width-to-depth ratio was small and the duty cycle was large, the vapor phase volume fraction was low, which could further mitigate cavitation erosion. In this paper, the effect of the width-to-depth ratio and duty cycle of the surface microstructure on the reduction of cavitation erosion is investigated in terms of numerical simulation as well as experimentally. Reasonable microstructure width-to-depth ratios and duty cycles are determined, and an intrinsic mechanism based on surface vapor phase volume fraction affecting cavitation erosion resistance is proposed. The vapor phase volume fraction is used as an indicator to provide a reference for subsequent quantitative control of the degree of cavitation erosion.