Numerical investigation of impinging-film heat transfer and flow characteristics of turbine inner casing

An efficient cooling method for the turbine inner casing is essential with the increasing of the turbine inlet temperature. The heat transfer and flow characteristics of a coupled cooling system in the turbine inner casing part, i.e., three rows of impingement jets and film holes, have been studied numerically according to the real turbine operating conditions. Seven inclined angles of the film holes along the mainstream direction (90°, ±60°, ±45°, ±35°) and the impingement jets arrangements have been researched. The positive inclination is that the angle between the fluid flow from the film holes and the mainstream is less than 90°. Otherwise, it would be negative. The numerical validation reveals that the selected computational method can provide a good prediction of the reported experimental results of the impinging-film cooling system. Then the method has been applied in the investigation of the local/average temperature, film-cooling effectiveness, and the flow patterns on the film-cooled surface. The results show that the inclined angle can achieve a significant improvement in the film cooling performance. With the positive inclination of film holes, the average temperature of the interaction surface between the mainstream and the turbine inner casing can decrease 50K compared with that of 90°. And the average temperature on the interaction surface with the negative inclined angle can even be reduced by more than 100K. Additionally, the average film-cooling effectiveness can be increased by up to 31.79%. Such results prove that decreasing the value of inclined angle can achieve a better heat transfer performance. Moreover, the negative inclination of film holes can improve the uniformity of the film-cooling effect. On the other hand, the influence of impingement jets arrangements on the film cooling behavior is negligible. Further analysis of the flow streamline illustrates that the coolant jet from the inclined film holes can attach to the interaction surface more firmly, which will achieve a better protection away from the high-temperature turbine gas. The research will provide direct guidance for the cooling design of the turbine inner casing and improve the thermal efficiency of the gas turbine system.