Reduction of Turbine Blade Tip Leakage Losses and Excitation Forces by Passive Tip Injection

The reduction of turbine blade-tip losses by means of passive tip injection was theoretically investigated. The analysis employed an analytical expression of the blade-tip discharge coefficient. The resulting blade-tip excitation forces (i. e. Thomas-Alford forces) were explicitly evaluated for unshrouded turbines with and without passive tip injection. It was found, that the Thomas-Alford coefficient for cross-force can substantially increase when the blade-tip gap was reduced. This observation can directly be explained on the basis of viscous flow effects through the gap, and it was in excellent agreement with available literature data. Due to passive tip injection, a slight decrease of the blade-tip excitation cross-force was obtained. The potential of the reduction of the cross-force due to passive tip injection was found to be comparable to the corresponding tip loss reduction.