Flow And Cooling Mechanism Of Unsteady Pulsed Jets In Turbine

For aero-engines,turbine components must be cooled and film cooling is the most common cooling means.In order to enhance the cooling effectiveness and improve the adaptive capacity to mainstream instability,pulsed film cooling can be a very potential technology.However,there are no clear conclusive outcomes about whether the cooling effectiveness can be improved.Relative studies were mainly carried out in jet-in-crossflow model rather than real turbine blades or vanes.The Research Institute for Gas Dynamics of Engine of Harbin Institute of Technology plan to provide coolant from the oscillation suction in compressor for pulsed film cooling in turbine,in order to improve the overall performance of aero-engines.But,which pulsing form is better and how to choose the blowing ratio are questions that need to be solved immediately.This paper first investigates the pulsed flow field by unsteady numerical simulation method in a 35-deg inclined jet-in-crossflow model.Then,in order to study the effects of different excitation forms on forced film cooling characteristics,common used square wave and sinusoidal wave are selected as excitations to the coolant,and the attachment of the coolant as well as the ingestion of the hot mainstream is compared.The results show that coolant with sinusoidal excitation has stronger attachment to the wall and higher cooling effectiveness,whereas coolant with square excitations has stronger penetration but relatively weaker attachment.Besides,ingestion of hot mainstream exists under square excitation,which can cause ablation to the holes and parts of the wall.However,sinusoidal excitation makes the coolant change gradually,which significantly restrains the ingestion.Further,improving the lowest blowing ratio is not the best way to restrain the ingestion,since the ingestion is mainly decided by the continuity.As for sinusoidal excitation,high-frequency coolant has some kind of uncontinuity like square wave.Considering cooling effectiveness and restraining ingestion,continuous wave with relatively low frequency is recommended for practical use.Second,the sinusoidal excitation was selected and the pulsed film cooling in a real turbine vane was further studied.By unsteady numerical simulation method,this paper mainly explored the blowing ratio selection issue.After determining the best blowing ratio(0.7)of steady jets,pulsed jets based on blowing ratios 0.5,1.0 and 1.5 were investigated.The results show that whether the cooling effectiveness can be improved is mainly determined by blowing ratio rather than pulsing frequency.When impose pulsation to BR=0.5(under the best BR)jets,the cooling effectiveness decreases.And when introduce pulsation to BR=1.0(above the best BR)jets,the cooling effectiveness also decreases.However,imposing pulsation to the further higher BR=1.5 makes cooling effectiveness improved.Further,by adding steady cooling effectiveness according to the pulsing process and averaging the sum,the average effectiveness value can be obtained,which can be used to qualitatively predict whether the pulsed jet can outperformthe steady counterpart.The selection principle of blowing ratio for pulsed film cooling is that it should be higher than the best blowing ratio or further higher to make sure an improvement of cooling effectiveness.