| Gas turbine is a kind of thermal power device that uses air as the working medium.Raising turbine inlet temperature is an effective way to increase output power of the gas turbine.Nowadays,the inlet temperature of state-of-the-art turbine far exceeds the melting point of materials for making blades.Thus,it is necessary to develop a series of high-efficiency cooling technologies to ensure safe and stable running of turbine blades.Film cooling is one of primary cooling methods applied to the outer surface of turbine blades.The aerodynamic characteristic in the turbine cascade is complicated and the flow is usually unsteady.Wake transport process is one of key factors causing the flow unsteadiness.The periodic pulsation of aerodynamic parameters such as velocity,pressure and turbulence intensity caused by the unsteady wake has significant effects on the flow field in downstream blade cascade,which impacts the film cooling of the blade consequently.The paper provides a comprehensive study on the effect of unsteady wakes on film cooling effectiveness for gas turbine blades with local and full-coverage film holes using pressure sensitive paint measurement technique.Upstream unsteady wakes are simulated using a spokewheel type wake generator.The distribution of the film cooling effectiveness on the blade surface are obtained under different blowing ratios,mass flux ratios,wake Strouhal numbers,film hole shapes and different oncoming free-stream turbulence intensities.The main contents of the study are as follows: Detailed film cooling effectiveness distribution for gas turbine blades with local film hole rows on leading edge,suction surface and pressure surface are obtained.The effects of blowing ratio,wake Strouhal number and film hole position are investigated,and the physical mechanism is revealed by numerical simulation.Detailed fullcoverage film cooling effectiveness distribution for gas turbine blades with dust-pan shaped holes,laid-back fan-shaped holes and W-shaped holes are obtained.The effects of film hole shape,mass flux ratio,wake Strouhal number and oncoming free-stream turbulence intensity are investigated.The main conclusions are as follows: For the turbine blade with three rows of cylindrical holes on the leading edge,an increase in the blowing ratio causes an increase in the film cooling effectiveness over most of the blade surface except for the upstream region of pressure side.As wake Strouhal number increases,the film cooling effectiveness of the pressure side increases,the film cooling effectiveness of the leading edge decreases and that of the suction side decreases.The unsteady wake impacts the film coverage on the surface in two aspects: on one hand,the wake promotes the fluid diffusion,which causes an increase in the coolant coverage area and the film cooling effectiveness;on the other hand,the deficit of the mainstream velocity produced by the wake promotes the coolant to lift off from the wall and causes reduction in the film cooling effectiveness.Both decay and enhancement of the film cooling effectiveness are observed on the leading edge,which is determined by the weight of two factors.For turbine blades with single row of dust-pan shaped holes on suction surface and pressure surface,the film cooling effectiveness decreases with the increase of the blowing ratio on the suction surface,while the film cooling effectiveness increases first and then decreases on the pressure surface as blowing ratio increases.As wake Strouhal number increases,the film cooling effectiveness of pressure surface and suction surface decreases.For turbine blades with full-coverage film holes,the film cooling effectiveness of leading edge and pressure surface increases,while the film cooling effectiveness of suction surface decreases with the increase of mass flux ratio.As wake Strouhal number increases,the effect of mass flux ratio on the film cooling effectiveness decreases.The film cooling effectiveness decreases with the increase of wake Strouhal number in most regions of the blade surface,and the influence of wakes on suction surface is greater than that on pressure surface.The free-stream turbulence superimposed on the unsteady wake decreases the film cooling effectiveness further.The raised profile in the middle of the Wshaped hole changes the structure of counter-rotating vortices at the hole outlet,which is favorable for the coolant attachment to the wall under the condition of unsteady wakes.It can be seen from the study that the influence of unsteady wakes on the film cooling effectiveness of the turbine blade is magnificant and complicated.Therefore,the upstream wake is an important parameter that cannot be ignored in the design of the cooling structure and the thermal analysis of the turbine blade. |
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