Investigation On The Film Cooling Mechanism Of Interrupted Slot With Step On The Turbine Vane Endwall

Increasing turbine inlet temperature contributes to improving the overall engine thermal efficiency and thrust-to-weight ratio.While the higher temperature will shorten the lifetime of hot sections.To protect these components,the technology of film cooling is introduced and plays an important role in gas turbine.Recently,film cooling of high pressure turbine endwall receives more attention due to the high temperature and low aspect ratio turbine vane.The higher temperature keeps challenge the limitation of metallic materials.One of the reason people chooses low aspect ratio airfoil is that it helps to reduce the number of blades.As a result,the area of endwall increased and secondary flow was strengthened.The secondary flow complicates film cooling of endwall comparing to blade surface and therefore the cooling jet is more threedimensional.To take advantage of the cooling air and improve the film cooling effectiveness,the current investigation studied the film cooling of high pressure turbine endwall computationally and experimentally.The main jobs and conclusions are following:(1)Introducing a new cooling configuration—interrupted slot,and proposing the cooling scheme of interrupted slot combined with film holes.The interrupted slot aims at optimizing the distribution of coolant,especially improving the film effectiveness near the leading edge and pressure side corner of the vane.Firstly,the characteristic of secondary flow of the vane was numerically investigated using RANS simulation,since the cooling jet is easily affected by the secondary flow.The difference between traditional continuous slot and interrupted slot was analyzed.Both have advantages and disadvantages on film cooling.For the interrupted slot,the coolant mass flow,axial position,circumferential position and incidence angle of the coolant were numerically investigated.The result showed that when the kinetic energy of coolant is lower than the flow inside boundary layer,the secondary flow is strengthened and the film cooling effectiveness becomes worse and vice versa.The secondary flow,the migration of coolant and the distribution of coolant at the exit of interrupted slot deeply affect the cooling performance.The interrupted slot combined with film holes was proposed to save coolant and improves the overall film effectiveness.The film holes were affected not only by the secondary flow in the cascade,but also by the flow produced by the upstream step.The secondary flow in the cascade and the interrupted slot changed the axial and circumferential cooling distribution for the film holes.(2)Exploring the film cooling relationship between cascade endwall and flat plate and the definition of cooling partition.During our investigation,the rig of low-speed wind tunnel was designed and built.The thermal infrared camera was employed to acquire the endwall temperature.Firstly,the cooling characteristic of flat plate was validated by empirical correlation.When the blowing ratio M<1.3,the film cooling coincides with the turbulent boundary layer theory.When the blowing ratio M>1.3,it coincides with jet theory.Then,the cooling performance of interrupted slot was experimentally investigated.Results showed that Comparing to the experiment on the endwall,the computations were able to capture the main feature of endwall film cooling,but the mixing process between coolant and mainstream flow was underpredicted.Finally,the film cooling of the interrupted slot was compared with the two-dimensional slot.The results on an axial line and a streamline was chose for the comparison.The cooling area was divided into four parts.At the upstream region,where horseshoe vortex barely affected,the cooling performance of the cascade endwall was similar with flat plate slot.Inside the passage,the coolant trace converged in the middle of the passage due to the contraction of passage and transverse flow.Close to the leading edge and the corner of pressure side,the coolant flow cannot reach to these regions due to the passage vortex and corner vortex.Therefore,the film cooling was not enough on this area.(3)illuminating the unsteady flow mechanism among the step of interrupted slot,the horseshoe vortex and the potential effect of the vane.As one type of backward-facing step,the flow fields downstream of interrupted slot is unsteady.Besides,the coolant jet of interrupted slot is affected by the vane.DES simulation was utilized to capture the complicated flow.Results showed that the vortex shedding close to mainstream(MSV)is the critical flow structure which affects the film cooling.The MSV rotates in the clockwise direction and entrains the mainstream to the endwall,deteriorating the cooling performance.Increasing the coolant flow rate contributes to the development of the vortex shedding close to coolant(CSV),and the CSV and the MSV gradually detached the endwall in the downstream region.The mixing between mainstream and coolant was then reduced.The sense of rotation was identical for MSV and horseshoe vortex.During the vortex shedding process,the MSV and horseshoe vortex blended together,strengthening the horseshoe vortex,weakening the CSV and deteriorating the film cooling.Due to the potential effect of the vane,the pressure at the exit of the interrupted slot was different.The vortex shedding at different circumferential position behaves differently and therefore results in non-uniform film effectiveness distribution.