Flow Structures and Unsteady Behaviors of Film Cooling from Discrete Holes Fed by Internal Crossflow

Abstract

The flow structures and unsteady behaviors of a flat plate film cooling flow behind a single row of circular holes fed by internal crossflow were extensively investigated. The investigation was achieved experimentally using fast-response pressure-sensitive paint (PSP) at a high frame rate and numerically using large-eddy simulation (LES). During the experiment, the coolant flow was discharged from discrete holes (i.e., a row of circular holes with 3D spacing, 6.5D entry length, and 35 deg incline angle) via a crossflow channel. Two blowing ratios (M = 0.4 and 0.8) were tested at a density ratio of DR = 0.97. The measured unsteadiness caused by the predicted flow structure over the coolant surface was identified by spatial correlation. The unsteady signatures were decomposed and demonstrated by proper orthogonal decomposition (POD). The results reveal that the flow structure plays the main role in cooling performance and its instability. The internal flow produced a vortex tube structure that was responsible for the shear vortex (i.e., Kelvin–Helmholtz instabilities) between the coolant and the mainstream at the hole exit. The internal crossflow forced the legs of the counter-rotating vortex pair (CRVP) to spread laterally, and the coolant to fluctuate asymmetrically around the discrete holes. This unsteady behavior may potentially cause high thermal stress and leads to blade cracking over a long time.