Effect of hole imperfection on adiabatic film cooling effectiveness

The influence of a discrete imperfection on film cooling is studied by means of thermochromic liquid crystals measurements of the adiabatic film cooling effectiveness. As a benchmark, the effect of a jet ejected through a perfect hole is used. The film cooling effectiveness achieved with an imperfect hole is compared to the benchmark. A half torus plays the role of the discrete imperfection. The influence of the presence of the imperfection, its position and the turbulence intensity of the free stream are analyzed. The measurement data are depicted as two-dimensional plots but also as integral values. It is found that the imperfection, placed one diameter from the hole leading edge, deteriorates the effectiveness at moderate velocity ratios. Under the same conditions, the same imperfection fixed at the hole exit improves the effectiveness. At the velocity ratio of 1.50, the exit imperfection improves the integral effectiveness relatively for more than two times. The turbulence intensity and imperfections placed deeper in the hole do not have a significant influence.