Effects Of Corrugated Wall Structure And Streamwise Pressure Gradient On Effusion Cooling Performances

Film cooling is one of the most important aspects of modern aero engines technology.And it has become one of the main cooling measures for the hot components of aero engines due to its inherently efficient cooling characteristics.Among them,effusion cooling is one of the cooling measures which has attracted wide attention at present.The main feature of effusion cooling is that the film holes on the cooled wall are denser,which can form a relatively complete film cooling layer near the wall of the gas side.This paper is aimed at the characteristics of effusion cooling improving the effusion cooling structures to improve the film cooling effectiveness for obtaining a better cooling structure.The main contents include: for the effusion cooling structure of a flat bottomed transverse corrugated surface,the influence of corrugated structures and film hole structures on the effusion cooling flow and heat transfer characteristics is studied.For the effusion cooling structure under the favorable/adverse mainstream pressure gradient,the influence of the mainstream channel contraction ratio on the effusion cooling flow and heat transfer characteristics is studied.For the combined scheme of slot injection and effusion cooling where the slot outlet is located upstream the effusion holes-array,the influence of the slot injection on the effusion cooling flow and heat transfer characteristics of the effusion film cooling is studied.Firstly,for the effusion cooling structure of a flat bottomed transverse corrugated surface,the influence of the flow and structural parameters such as blowing ratio,corrugation wave height,corrugation wave length,hole diameter,hole arrangement and perforated percentage on the effusion cooling flow and heat transfer characteristics is studied.The results show that: the film cooling effectiveness on the wavy peak region of transverse corrugated surface is relatively lower than that on the wavy valley region compared to a flat surface.Besides,the laterally-averaged film cooling effectiveness increases gradually along the streamwise direction and then tends to be constant at any blowing ratio.And the film cooling effectiveness increases with the increase of blowing ratio.The film cooling effectiveness increases at the junction region of wavy peak and wavy valley with the increase of wave height.However,the film cooling effectiveness decreases both on the wavy peak and wavy valley regions.Therefore,the film cooling effectiveness distribution uniformity becomes worse in the spanwise direction.The variation of wave length has little effect on the laterally-averaged film cooling effectiveness.The distribution uniformity of film cooling effectiveness in the spanwise direction becomes better with the increase of wave length.The film cooling effectiveness can be increased by decreasing hole diameter,decreasing hole spanwise-to-streamwise pitch ratio and increasing perforated percentage under the same coolant usage for a given cooled area.Meanwhile,the increase of the film cooling effectiveness caused by the decrease of hole spanwise-to-streamwise pitch ratio is weaker than that of the decrease of hole diameter and the increase of perforated percentage.In addition,the discharge coefficient can be increased by increasing hole diameter,increasing hole spanwise-to-streamwise pitch ratio and decreasing perforated percentage.And the influence of hole diameter and hole spanwise-to-streamwise pitch ratio on discharge coefficient can be neglected compared with the effect of the perforated percentage.Secondly,for the effusion cooling structure under the favorable/adverse mainstream pressure gradient,the influence of blowing ratio and mainstream channel contraction ratio on the effusion cooling flow and heat transfer characteristics is studied.The results show that: the penetration of film injection to mainstream will be restrained to a certain extent,leading to the film injection more close to the wall,and the film cooling layer thickness will be thinner under the favorable pressure gradient.Besides,the velocity of the film injection is relatively low and film injection from the first few rows even form a countercurrent flow due to the inhibition of the mainstream when the blowing ratio is small.When the blowing ratio becomes larger,the velocity of the film injection increases,and the inhibition of the mainstream to film injection can be improved.However,the adverse pressure gradient aggravates the penetration of film injection to mainstream.The thickness of the film cooling layer is improved owing to the superposition of film injection along the streamwise direction,and the protection to the wall is also improved meanwhile.As a result,the laterally-averaged film cooling effectiveness under the favorable pressure gradient is lower than that of the zero pressure gradient when the blowing ratio is small.And the reduction of laterally-averaged film cooling effectiveness increases with the increase of the favorable pressure gradient.On the contrary,the adverse pressure gradient is beneficial to improve the laterally-averaged film cooling effectiveness.The effect of the mainstream pressure gradient on the film cooling effectiveness is relatively weak under large blowing ratio.Both the favorable pressure gradient and adverse pressure gradient can improve the laterally-averaged film cooling effectiveness.In addition,the favorable pressure gradient increases the convective heat transfer coefficient of the hot side while the adverse pressure gradient leads to the reduction of the convective heat transfer coefficient under small blowing ratio.When the blowing ratio becomes larger,the favorable pressure gradient and adverse pressure gradient can reduce the convective heat transfer coefficient of the hot side in the upstream region of holes array,and the opposite results are obtained in the downstream region of holes array.Lastly,for the effusion cooling structure with a slot injection structure where the slot outlet is located upstream the effusion holes array,the influence of blowing ratio and slot structure on the effusion cooling flow and heat transfer characteristics for the combined scheme of slot injection and effusion cooling is studied.The results show that: for the combined scheme of slot injection and effusion cooling,the film cooling effectiveness is larger in the upstream region of effusion holes array.The film cooling effectiveness decreases gradually along the streamwise direction when the coolant usage is small,while the film cooling effectiveness decreases slightly and then increases gradually when the coolant usage is large.The exist of slot injection has a great effect on the film cooling effectiveness compared pure effusion cooling scheme and combined cooling scheme.The increase of film cooling effectiveness is particularly obvious especially in the upstream region of effusion holes-array.The overall film cooling effectiveness of combined cooling scheme is larger than that of the pure effusion cooling scheme due to the presence of slot injection,which can increase the coolant usage,when the blowing ratio of effusion cooling part is equal.Meanwhile,this combined cooling scheme makes the blowing ratio of effusion cooling part less than the correspondingly pure effusion cooling scheme under the same coolant usage,resulting in a little decrease of the film cooling effectiveness in the downstream region,while the film cooling effectiveness in the upstream region of the combined cooling scheme is significantly higher in relative to pure effusion cooling scheme under small coolant usage.However,under large coolant usage,the film cooling effectiveness for the combined cooling scheme is higher than that of the pure effusion cooling scheme in the whole holes array region because the film coverage has been formed completely.The presence of slot injection has a significant effect on the convective heat transfer coefficient in the upstream region.The convective heat transfer coefficient of the combined cooling scheme is lower than that of the pure effusion cooling scheme when the blowing ratio of effusion cooling part is equal.However,the convection heat transfer coefficient is almost unchanged in the region far away from the slot injection despite the total coolant usage increases.