Influence of labyrinth clearance on the flow of rotating orifices in a parallel labyrinth-orifice system

Orifices, especially rotating orifices, are an important flow element of the secondary air system in a modern aero-engine, and their discharge coefficient depends on the geometry, the operating point and the surrounding environment. The influence of Reynolds number, pressure ratio, rotational speed, inlet chamfer, inclination angle, length-to-diameter ratio, etc. on the discharge coefficient of rotating orifices under the assumption of room temperature and adiabatic has been reported in many literatures. However, the rotating speed, the temperature of the gas in the front and rear chambers of rotating orifices and the temperature of the chamber wall change continuously during the actual operation of the engine, especially during the acceleration and deceleration of the engine, which will cause deformation of the chamber wall and the rotating components, resulting in a large change in the labyrinth seal clearance on the periphery of the rotating orifice disk. Although the change of the seal clearance can be evaluated by some methods, it still has a crucial influence on the fluid flow in the front cavity of rotating orifices, which may affect the discharge coefficient of rotating orifices, thereby affecting the amount of cooling air flowing through rotating holes. Therefore, the knowledge of the influence of labyrinth seal clearance should be considered into the discharge coefficient of rotating orifices, which is essential for a reasonable distribution of the cooling air in the second air system under various working conditions and ensures the safety and reliability of the aero-engine in all-inclusive line. This paper presents the relationship between the discharge coefficient of rotating orifices and theoretical velocity ratio in the relative frame of reference (U/Wax) under different labyrinth seal clearance conditions, which is based on the study of the flow state in the front chamber of rotating orifices under different seal clearances, rotating speeds and pressure ratios. The results indicate that with the increase of the labyrinth seal clearance on the periphery of the rotating orifices disk, the discharge coefficient of rotating orifices decrease under the condition of small velocity ratio, while the discharge coefficient is almost unchanged under the large velocity ratio. Comparing the flow field structure and velocity field under the condition of different labyrinth seal clearances, the same pressure ratio and the same velocity ratio, the reasons for the influence of labyrinth seal clearance on the discharge coefficient of rotating orifices are analyzed.