Geometric measurement of film-cooling holes using co-axial light-field imaging

Abstract

In a high thrust-weight-ratio aero-engine, turbine blades are exposed to extremely high temperature and pressure which sets higher demands in the blade cooling technology. The mainstream types of cooling include internal airflow cooling, film cooling, and thermal barrier coating. Cooling efficiency is substantially subject to the machining accuracy of blades, more specifically the film-cooling holes on them. A coordinate measuring machine(CMM) is extensively used in the measurement and inspection of turbine blades which enables measuring blade patterns and building geometric models. However, the probes utilized by a CMM are similar in scale to that of a film-cooling hole, making it almost impossible for a CMM to measure film-cooling holes. The probes are prone to damage and the measurement accuracy cannot be guaranteed. This paper proposed a non-contact measuring technique to cope with the inspection of turbine blades. The technique is based on light-field imaging. A light-field camera with a high magnification lens is applied to caperture images of film-cooling holes and their neighborhood which are then inputted to a 3D reconstruction algorithm. A point cloud is derived by the algorithm and fundamental parameters are extracted. The whole inspection requires only one platform and is able to measure shaped film-cooling holes.