| Film cooling is a necessary mean to achieve efficient cooling,has been widely used in advanced gas turbine engines.One of the main developments in the film cooling technology is the application of shaped holes,which is mainly realized by the innovation of film hole outlet shape.The shaped holes can not only weaken the shear mixing of the main stream and cooling jet and the normal penetration of the cooling jet into the mainstream,but also enhance the spanwise coverage and flow direction extension of the cooling jet.In this paper,the fan-shaped hole and the round-to-slot hole were selected as two typical shaped holes,and the multi-objective optimization method was used to optimize the geometry of the fan-shaped hole and the round-to-slot hole with the discharge coefficient and film cooling effectiveness as the objective functions.The main contents and conclusions are as follows:Firstly,the multi-objective optimization of the fan-shaped hole was carried out,and the large eddy simulation(LES)was carried out for the typical optimal design points.Under the blowing ratio of 0.5,compared with the reference film hole,the two objective functions of all optimal design points are significantly improved.The three optimal design points all have larger lateral expansion angle and hole length,and the main difference lies in the obvious difference of hole inclination angle.Opt-A(corresponding to the maximum film cooling effectiveness)has a relatively small inclination angle.Opt-B(corresponding to the maximum discharge coefficient)and Opt-C(corresponding to the compromise between the two objective functions)have larger and moderate inclination angles.Under the blowing ratio of 1.5,Opt-A has a smaller inclination angle,larger lateral expansion angle and hole length.Opt-B has the largest inclination angle and hole length,and smaller lateral expansion angle.While,Opt-C has a moderate hole inclination angle and hole length,and a larger lateral expansion angle.For the fan-shaped hole,the horseshoe vortex,shear layer vortex and hairpin vortex can be observed.Under the low blowing ratio of 0.5,the horseshoe vortex is relatively weak.Due to the Kelvin Helmholtz instability of the interaction between the secondary flow and the mainstream,the shear layer vortices are formed,which aggravate the mixing of the secondary flow and the mainstream,and develop into hairpin vortices downstream.With the increase of blowing ratio,the obvious horseshoe vortex structure is observed.The vorticity in the fan-shaped hole is enhanced,and the separation of hairpin vortices is promoted.A large number of hairpin vortices distribute in disorder downstream of the fan-shaped hole outlet,called "Hairpin Vortex forest".Secondly,the multi-objective optimization of the round-to-slot hole on the flat surface was carried out,and the LES is carried out for the typical optimal design points.Under the blowing ratio of 0.5,the transition section of Opt-A from the round inlet to the slot exit still has the converging geometric characteristic,and has a small inclination angle and film hole height.According to other optimal options,in order to reduce the aerodynamic loss of cooling jet,the transition section of the diffusing geometric characteristic is needed.Under the blowing ratio of 1.5,all the optimal design points have diffusing geometric characteristic.Compared with the reference film hole,they can enhance both film cooling effectiveness and discharge coefficient.Opt-A has a slightly diffused exit(s/d=0.44),while other optimal options have relatively large diffusion exit.The LES results show that the leading edge vortex pair and the trailing edge vortex pair are observed upstream and downstream of the round-to-slot hole exit.A large number of hairpin vortices are disorderly distributed downstream of the film hole outlet,and the scale of these hairpin vortices is smaller than that of the cylindrical hole.Thirdly,four kinds of converging round-to-slot holes with different transition surfaces were designed.The influence of transition surfaces on film cooling performance was studied,and the comparison with the reference film hole was carried out.The film cooling effectiveness downstream of film hole outlet is significantly affected by different transition surfaces.Under the blowing ratio of0.5,the film cooling effectiveness of inner-arc-shaped and anti-S-shaped converging round-to-slot holes is higher than that of reference film hole,while the film cooling effectiveness of outer-arc-shaped and S-shaped converging round-to-slot holes decreases sharply.Under the blowing ratio of 1.5,only the film cooling effectiveness of the inner-arc-shaped converging round-to-slot hole is significantly improved.The effect of different transition surfaces on the discharge coefficient is also significant.Under the two blowing ratios,compared with the reference film hole,the outer-arc-shaped and S-shaped converging round-to-slot holes have larger discharge coefficient,which means smaller aerodynamic loss.The inner-arc-shaped and anti-S-shaped converging round-to-slot holes have smaller discharge coefficient,which means larger aerodynamic loss.Finally,the multi-objective optimization was carried out for the round-to-slot holes on the surface of turbine vane under the experimental conditions and the shaped holes on the suction surface of the turbine vane under the real working conditions.For the multi-objective optimization of round-to-slot holes on the surface of turbine vane under the experimental conditions,all the optimal design points have large slot length(close to or equal to the upper limit of the current slot length range).For the multi-objective optimization of round-to-slot holes on the suction surface of turbine vane,the optimal design point of film cooling effectiveness is the same as the optimal design point of discharge coefficient.That is to say,the optimal design point has both optimal film cooling effectiveness and discharge coefficient simultaneously.Compared with the reference film hole,the optimal design point has a smaller film hole inclination angle and larger slot width.For the multi-objective optimization of round-to-slot holes on the pressure surface of turbine vane,compared with the reference film hole,the film cooling effectiveness and discharge coefficient of all optimal design points are improved in different degrees.Opt-A has a larger inclination angle and smaller slot width.Opt-B has a smaller inclination angle and relatively larger slot width.And Opt-C has a larger inclination angle and relatively moderate slot width.For the multi-objective optimization of fan-shaped holes on the suction surface of the turbine vane under the real working conditions,all the optimal design points have large lateral expansion angle.Opt-A has a large inclination angle and moderate forward expansion angle.Opt-B has a small inclination angle and moderate forward expansion angle.Opt-C has a small inclination angle and small forward expansion angle.For the multi-objective optimization of round-to-slot holes on the suction surface of turbine vane under real working conditions,the multi-objective optimal results are highly consistent with the optimal results under experimental conditions. |
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