Investigation On The Flow And Cooling Performance Of The Film Cooling With Vortex Generators

Turbine is one of the key parts of gas turbine.The high temperature of turbine inlet leads to improved thermal efficiency and power output of the gas turbine.Due to the temperature limitation of the material,advanced cooling technology is essential to further increase the turbine inlet temperature,leading to improved economy and safety of the gas turbine.Film cooling,as an effective external cooling method,is the introduction of a secondary fluid(coolant of injected fluid)at one or more discrete locations along a surface exposed to a high temperature surface to form a layer of coolant film to protect that surface not only in the immediate region of injection but also in the downstream region.To optimize the film cooling effevtiveness,vortex generators are installed upstream or downstream of the cooling hole in the current research.Investigaions of plate file cooling and film cooling in real turbines are carried out to reveal the mechanism of applying vortex generators in film cooling.Plate film cooling with vortex generators are studied by LES to examine the mechanism of cooling effectiveness improvement with applying the vortex generators.Delta winglets and micro ramp are investigated in one row cooling holes and two concecutive cooling holes in plate film cooling models.It indicates that the delta winglets and the micro ramp can generate the downwash anti-CRVP(anti-counter rotating vortex pair),which decay the magnitude of the CRVP(counter rotating vortex pair)and accelerating the reattachment of the injected coolant.The coolant moves downstream closer to the cooling wall,and the mixing of the coolant and the mainsteam is weakened.The anti-CRVP draws the coolant toward the cooling wall and move away from the centerline,leading to a wider coolant film coverage in the spanwise direction.The CRVP is strengthened in the two concecutive cooling hole case,due to the same rotation direction.The anti-CRVP generated by the micro ramp is stronger and moves closer to the wall.Furtermore,the micro ramp is better protected by the coolant from the upstream cooling hole compared with the delta winglets.In addition to the study of passive cooling method of vortex generators,active cooling method of pulsed film cooling is investigated with vortex generators installed.The sine pulsation is added in the coolant inlet to form the pulsed coolant injection.Compared with the steady film cooling,the cooling efficiency in the area near the cooling hole is higher in the pulsed film cooling.In a pulsaton cycle,the coolant is attached on the surface when the blow ratio is low,and injected into the mainsteam when the blowing ratio is high.High blowing ratio leads to the strengthened mixing of the coolant and the mainsteam.Unlike the continuous vortex structures in the steady film cooling,separated vortex wakes are formed in the pulsed film cooling cases.Horseshoe rings are found when the separated wakes move downsteam.Vortex generators impove the film cooling efficiency in both steady and pulsed film cooling,however,they works better in the steady film cooling.In real turbines,film cooling holes are normally applied with compound angles.Compound film cooling with vortex generators has been studied in plate film cooling,considering blowing ratio and pulsation.It indicates that the coolant coverage area is larger and more uniform with increasing compound angle due to the interaction of the two neiboughring holes.The cooling efficiency of compound film cooling increases with employing vortex generators,however,the effect of vortex generators decreases with the compound angle increasing.In terms of vortex structure in the flow field,the CRVP turns into ASV(asymmetric vortex).The strength of ASV becomes stronger with increasing compound angle.The leeward-side branch of anti-CRVP generated by vortex generators disappear due to the influence from ASV.When the blowing ratio increases from M=1 to M=2,the coolant separation away from the surface is strengthened,leading to the intensified mixing of the coolant and the mainstream.The cooling efficiency decreases because of the aggravated mixing.The CRVP generated from the cooling hole becomes stronger with the increased blowing ratio,which decays the anti-CRVP introduced by the vortex generators.With the pulsation added on the coolant inlet in the compound film cooling,the mixing of the coolant and the mainstream is intensified.The massflow of coolant covered on the cooling wall is limited,leading to decreased coolant film coverage.Moreover,the effects of the vortex generators in the pulsed film cooling are not as good as those in the steady film cooling when compound angle applied.Based on the results from plate film cooling,vortex generators applied in real turbine environment are investigated.In endwall film cooling,vortex generators are installed with the cooling holes upstream of the vane leading edge.It shows that the film coverage on the endwall enlarges,especially in the middle of the flow passage.With the influence of the negative pressure gradient upstream of the leading edge of the vane,the mainflow direction deviate seriously from the centerline of the cooling hole,which weakens the effect of vortex generators.When the coolant massflow decreases,the separation of the coolant and the endwall is weakened.Cooling efficiency near the cooling hole increases,while the cooling performance in the flow passage between the vanes decays due to the less coolant massflow.The effect of vortex generators on the pressure side of the vane has been carried out by LES.It indicates that the anti-CRVP generated by the vortex generators decays the CRVP,accelerates the coolant reattachment,and draw the coolant toward the cooling wall.Vortex generators works well in the real turbine condition and on the curved surface,in accordance with the results obtained from the plate film cooling.