污染物沉积和热障涂层脱落对气膜冷却效率影响的数值研究

Effects of Deposition and Thermal Barrier Coating Spallation on Film Cooling Effectiveness: a Numerical Study

污染物沉积到有热障涂层的涡轮叶片上之后,会导致叶片表面状况的改变,进而影响气膜冷却效率。为分析影响规律,利用三维数值计算研究了3种典型情况下的气膜冷却特性:污染物沉积、热障涂层脱落及气膜孔堵塞,并得到了吹风比的影响规律。研究结果表明,平板气膜冷却吹风比为0.5时,冷却效果最佳;在发生沉积和堵塞后,气膜孔附近产生的反旋涡对会削弱沉积和堵塞后的气膜冷却效率,沉积高度增加会强化这种削弱效果;由于在沉积情况下高吹风比会诱导出正向旋涡使得气膜贴近壁面,因此随着吹风比增大,气膜冷却效果增强;因涂层脱落而在孔下游产生的裂纹在低吹风比时会降低冷却效率,高吹风比时改进冷却效果,使得最佳吹风比由0.5上升到0.7左右;气膜孔堵塞会降低冷却效率,使得气膜更易脱离壁面。

With the increase of turbine inlet temperature, particulate matter existing in the hot gas path can be more easily deposited around the film hole and block the film flow. This is also the main cause of thermal barrier coating spallation due to the change of surface conditions after deposition. Firstly, increase of roughness enhances the heat transfer. Then, deposition will change the temperature field and the thermal stress around film hole. Therefore, it is important to analyze the effects of deposition, spallation and block- age on the film cooling effectiveness. A numerical analysis was conducted on flat plat by use of commercial computational code CFX. It was found that optimum blowing ratio（M） is around 0.5, which is in line with other studies. In the case of deposition and blockage, counter-rotating vortex pairs （CVP） induced around film hole can decrease the film cooling effectiveness. Height of deposition will reinforce this weakened effect. With the increase of blowing ratio, film cooling area will be enlarged with deposition presented due to high cooling flow and the effect of co-rotating vortex pairs which drives film close to wall. Spallation loca- ted in the downstream of hole can decrease film cooling effectiveness at low blowing ratio （M = 0.25,0.5 ）and augment it at high blowing ratio （M = 1.0,1.5 ）. It also increases the optimum blowing ratio from 0.5 to 0.7. Hole blockage decreases film cooling effectiveness and decreases the blowing ratio under which the film will penetrate the main stream.