| More than a half century,the aim of designing gas-turbine engines is to achieve as high thermal efficiency and output power as possible.Therefore,the turbine inlet temperature(TIT)is continuously pushed to higher and higher levels.Presently,the gas-turbine engines must operate under high temperatures,which are far beyond melting points of turbine airfoil materials and can tremendously reduce materials'survivability.In order to improve the turbine parts operating under high temperatures and pressures,advanced cooling technology has been developed.The advanced cooling schemes of using both internal cooling and external cooling are effective ways to protect the airfoil against the hostile combustion gases and prolong the life span of the turbine airfoil.Nonetheless,the cooling air for the internal and external cooling techniques is extracted from the compressor and results in a reduction in thermal efficiency.Therefore,thermal barrier coating(TBC)is commonly used in cooling design as an insulator to shield against combustion gases and to reduce heat transfer between high temperature gases to turbine's airfoil.This thesis deals with simulation of real situations happening in modern gas turbines.The numerical simulation presents the use of CHT analysis to investigate combined effects of film cooling and TBC at different turbulence intensities(Tus)simultaneously.Compared with the previous numerical simulations,this work gives benefits as follows:1)The use of CFD with CHT analysis investigates the combined effects of Tu and TBC on the cooling performances of the showerhead film-cooled and full film-cooled vanes under the experimental conditions.2)Results obtained from this thesis can provide gas-turbine engineers and designers with relatively comprehensive references to obtain better understanding of the cooling performances and an optimal thermal barrier coating strategy of the leading edge and its vicinity of the showerhead film-cooled vane,and all regions of the full film-cooled vane.Because the numerical results show the combination of TBC and film cooling under the different Tus in different regions of the turbine vane i.e.the leading edge,trailing edge,pressure side,and suction side.The major objective of this thesis is to numerically investigate the influence of Tu on cooling performances of film-cooled vanes using CFD code,ANSYS FLUENT.The showerhead film-cooled and full film-cooled vanes modified from a real nozzle guide vane(NGV)reported by Timko(NASA CR-168289)are used.First,the capability of five two-equation turbulence models(Standard k-?,RNG k-?,Realizable k-?,Standard k-?,and SST k-co models)to predict adiabatic film effectiveness on the showerhead film-cooled vane without TBC is discussed at Tu=3.3,10,and 20%.The following interesting conclusions are drawn:1)The film effectiveness predicted by the five turbulence models on the pressureside is generally lower than that on the suction side.2)On the pressure side and the leading edge,discrepancies in prediction of the film effectiveness occur due to the different turbulence models.3)The RNG k-?,Realizable k-? and SST k-?models predict relatively close results of the film effectiveness,especially the RNG k-? and SST k-? models.4)On the suction side,the film effectiveness decreases with increasing in Tu,except at a span of 8%.5)On the pressure side near the leading edge,the spreading length of the cooling air is shorten as Tu increases,and the variation of predicting in film coverage exists when different turbulence models are used.Because the SST k-? turbulence model was used by many researchers to study the film cooling on transonic vanes and blades,and this model was able to give acceptable numerical results.In order to simplify the calculation in this thesis,CHT analysis with the SST k-co model is used to predict the overall cooling effectiveness of the modified vane with showerhead film cooling without TBC at Tu=3.3,10,and 20%.The following interesting conclusions are drawn:1)The increment in Tu decreases the cooling effectiveness,and this effect on the pressure side is stronger than the suction side.2)Compared with the adiabatic film effectiveness,the cooling effectiveness in most regions is higher and more uniform than the film effectiveness for all three Tus.3)Near the exits of film holes in the leading edge,the increase in Tu improves the film effectiveness but reduces the cooling effectiveness.Furthermore,the area with relatively lower cooling effectiveness is moving with Tu.Finally,the comprehensive cooling characteristics of TBC combined with film cooling for the showerhead film-cooled and full film-cooled vanes are presented using CHT analysis with the SST k-co model at Tu=3.3,10,and 20%.The following valuable conclusions can be drawn:For the showerhead film-cooled vane with TBC:1)TBC is effective in the leading edge and its vicinity,and becomes more apparent as Tu increases.At Tu=20%,TBC reduces the metal surface temperature at the region near the hub by about 40-42 K,or about 25%in the overall cooling effectiveness.2)At each Tu,TBC gives the most effective thermal protection against thermal load at the region near the hub of the vane,and the ability of TBC decreases in streamwise direction.3)In the vicinity of the leading edge,the ability of TBC on the pressure side is better than that on the suction side,and this phenomenon is more evident at the higher Tus.4)Near the exits of film holes,TBC may play a negative effect,because TBC may block the heat releasing from solid structure to the mixture of cooling air and mainstream.For the full film-cooled vane with TBC:1)TBC significantly increases the cooling effectiveness,and it does not alter the trends of distribution of overall cooling effectiveness for all three Tus.2)For this work,TBC is more effective on the pressure side than on the suction side.3)TBC increases the overall cooling effectiveness in the ineffectively cooled regions,i.e.an increase by 24%or an reduction by 38 K in the tip and hub of the vane at Tu=20%,but the degree is slow in the regions close to the exits of film holes,the downstream of the diffusion shaped-holes on the suction side,and the trailing edge.4)TBC can block heat flux from mainstream into the vane,but it can also block the heat flux transferred from the solid vane into the mixing fluid of cooling air and the mainstream.When Tu increases,these effects become more significant. |
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