NUMERICAL INVESTIGATION ON A BASELINE SHAPED FILM COOLING HOLE: ONE-AND TWO-DIMENSIONAL COOLING PERFORMANCE

A recently proposed baseline shaped film cooling hole is numerically investigated on a flat plate at four blowing ratios (M), 0.5, 1.0, 2.0 and 3.0 and two density ratios (DR), 1.2 and 1.5. Results show that there exists an optimal blowing ratio and a best coolant operating condition in the current simulation set. The cooling effectiveness shows insensitivity to changes in density ratio for the same blowing ratio no higher than 1.0. A high density ratio effectively compensates for the negative impacts brought by a higher blowing ratio coolant than the optimal. It is also summarised that a velocity ratio (VR) around 1 should be a division point where the coolant flow behaviour differentiates and thus decides the cooling performance characteristics. Specifically, this paper proposes the idea of the quantification of two-dimensional film cooling performance, through three additional parameters: area-averaged adiabatic cooling effectiveness, standard deviation of adiabatic cooling effectiveness distribution and a defined distortion factor measuring the uniformity of adiabatic cooling effectiveness distribution. Different conclusions concerning the optimal coolant operating condition are drawn based on whether these two-dimensional cooling performance parameters or the commonly employed one-dimensional parameters, centreline and lateral-averaged adiabatic cooling effectiveness, are applied for evaluation. When using one-dimensional parameters, the DR=1.5 M=2.0 case is the best coolant operating condition, while using the two-dimensional parameters, the optimal condition changes to DR=1.5 M=1.0 and the optimal blowing ratio also changes from 2.0 to 1.0. Besides, the DR=1.2 M=3.0 case becomes the worst cooling performance working condition instead of the two M=0.5 cases. In addition, VR is found to better scale and predict area-averaged cooling effectiveness. Some new information on the uniformity and the variation level of cooling effectiveness distribution under various coolant operating conditions is also revealed. Visualisation of the flow field structures shows that the two-dimensional cooling performance is decided by the offsetting results of the negative and positive influences from the counter-rotating vortex development in lateral, spanwise and streamwise directions.