| In order to better investigate the effect of different factors on film cooling,the influence of different factors in the steady crossflow and the synthetic crossflow fields are simulated by using the Large Eddy Simulation(LES)method.When the steady jet is injected,the Reynolds number of the crossflow is fixed at100,000,and the angles of incidence are taken as 30°,60° and 90° respectively to systematically discuss the influence of the incident angle.Similarly,the arrangements of the holes included single hole,single row,and double rows which are divided into aligned and staggered arrangements.The hole types are composed of round hole,square hole,fan-shaped hole and elliptical hole with different aspect ratios(AR=0.618 and AR=1.618),and the film cooling performance under the blowing ratios being set as 0.5,1,2,and 2.5are explored.According to the simulation results,within the limits of set parameters,under the single-hole incidence,the film cooling efficiency with the blowing ratio of 0.5 is the highest.When the angle of incidence is 30°,the film cooling efficiency is higher than that of the other two.In addition,when the steady jet is incident,the cases with fan-shaped hole enjoy the best cooling performance among the five hole shapes.When the porous incidences are performed,the cooling efficiency of the staggered-row incidence is higher than that of the aligned-row and single-row entrance.the film cooling at a low Reynolds number of 100 is also investigated in this paper,and the simulation results are compared with those of the Reynolds number of 100,000 as a further supplement of the investigations.Based on the inquiry of film cooling performance under the steady jet incidence,numerical simulations of the synthetic crossflow field are further carried out and the cooling performance under different blowing ratios,different hole patterns,different Strouhal numbers,and different orders of incidence of the two rows of holes are analyzed.By comparing the flow structures and the cooling effect corresponding to the synthetic jet and the steady jet fields,it is found that within the scope of the survey,the related optimal conditions are separately the incidence of the elliptic hole with the aspect ratio of 0.618,the blowing ratio of 2.5 and the Strouhal number St=0.22.It is worth noting that,for the both cases,the cooling performance of the staggered and aligned incidences are highly complementary to each other,and the combination of the two may further enhance the cooling efficiency of the near and far fields simultaneously.In order to explain the cooling performance from the flow mechanism,the vortex structures under two cases are also drawn as follows: 1.When the steady jet is incident,the vortex structures affect the core and the size of the jet,which in turn affects the cooling effect;secondly,the porous incident vortex structures will cause the attraction between the jets,making the gas film coverage more uniform;in addition,it is also found that the jet is vertical when the two fluids are mixed and the pressure gradient in the straight direction will cause the jet to rise,and the diameter of the counter-rotating vortex pair will gradually expand laterally along downstream,causing the secondary vortex structures to be absorbed by the counter rotating vortex pairs,making the lateral coverage of the cooling film wider.2.When the synthetic jet is incident,the pseudo-sequence structures in the flow field become more complicated,the vortex rings fall off regularly,and their number increases,but as the vortices develop downstream,there is no obvious change in scale,but the interaction between the vortices is enhanced,and the incident depth of the jet along along downstream increases;under the appropriate Strouhal number,the vortex structures in the flow field become more stable,and when the Strouhal number is too small or too large,it results in strong vorticity at the hole exit and a longer vortex core length,as well as lower film cooling efficiency;when the double-row jets are incident at the same time,It can be observed that there is a vortex pair structure corresponding to the jet size near the orifice,afterwards,the vortex rings gradually break down and become smaller in scale,and gradually develops into a more complicated and disordered form in the flow field.The vortex structure in the synthetic jet flow field is less likely to be attenuated or broken by the mainstream,and the frequency of vortex generation is higher,the vortex flow is more continuous,and the shape of the film is better. |
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