Investigation On Cooling Characteristics And Applications Of Compound Film Cooling With Opposite Spanwise Angle

Compound angle film cooling with opposite spanwise angle is one of new film cooling structures, which is easy to realize. In a full coverage film cooling structure, as jet inserted through a pair of film cooling holes, which are of opposite spanwise angle, a pair of anti-kidney vortices will be formed downstream, and high film cooling effectiveness will be attained as a result. Based on the brief theory mentioned above, a numerical and experimental study on fluid dynamics and heat transfer characteristics of compound angle film cooling with opposite spanwise angle is carried out in this paper, which consists of 3 parts: the first part studied fluid dynamics and heat transfer characteristics of compound angle film cooling with opposite spanwise angle. Furthermore, this part of study revealed how the film cooling effectiveness was enhanced. Impact of film holes, mainstream tunnel and fluid dynamic parameters on film cooling were discussed in the first part as well. In the second part, sensitivity analysis of compound angle film cooling with opposite spanwise angle was conducted; effect of geometric parameters and fluid dynamic parameters on compound angle film cooling structure, who had an array of opposite spanwise angle film hole pair, were discussed in this part too. In the third part, impact of fluid dynamic parameters and geometric parameters on film cooling and fluid dynamic characteristics were studied numerically by using a simplified model of blade, on which compound angle film cooling with opposite spanwise angle was applied, under real operating condition. Besides, this part of research also took out an arrangement of film cooling holes who took either film cooling performance or fluid dynamic characteristics into account.After conducting an investigation on the flow field and cooling characteristics of the compound angle film cooling, which had a row of film cooling holes with opposite spanwise angle, formation of the anti-kidney vortex pair was figured out. Vortex caused by inserted jet from the front film cooling hole met with and then influenced by a vortex formed by the latter film cooling jet. Under the influence of the latter vortex, the front vortex turned into an inducing vortex, what made up an anti-kidney vortex pair together with the latter vortex. The anti-kidney vortex pair pushed the film cooling jet on spanwise. The investigation showed that, either geometric parameters or fluid dynamic parameters could change the degree of influence of the latter vortex on the front vortex, center distance and rotational speed of the vortex pair would be affected in result. With larger vortex pair center distance and higher rotational speed, film covered wider on spanwise. As a result, the film cooling effectiveness rose correspondingly. However, as cooling jet flowed downstream, mainstream would be drawn into the vortex pair, which would reduce film cooling effect in return. Further studies indicated that, either enhancement of the blowing ratio, the spanwise angle, the mainstream Reynolds number, the temperature ratio and the adverse pressure gradient, or reduction of the streamwise angle and the radius of curvature, would contribute to the center distance and the rotational speed of the anti-kidney vortex pair. However, when under an impact of shock wave, situation became a little complicated. As shock reflection point dropped between the front hole and the latter hole, the film cooling effectiveness in this area decreased dramatically.Secondly, the cooling characteristics of an array of compound angle film cooling holes with opposite spanwise angle were analyzed. The study revealed that, the anti-kidney vortex pair was supported by continuously inserted jet, which was where film cooling effectiveness benefitted from. Result of orthogonal experiment showed that, under given conditions, factors in descending order of the degree of influence on the average film cooling effectiveness were, film hole porosity, blowing ratio, aspect ratio and spanwise angle; factors in descending order of the degree of influence on the average heat transfer coefficient were, blowing ratio, film hole porosity, spanwise angle and aspect ratio. While taking real turbine operating conditions into account, compound angle film cooling with a spanwise angle of 30° acquired higher film cooling effectiveness than structures with other spanwise angle. When the condition was put at low Reynolds numbers, and high temperature ratio as well, compound angle film cooling structure with a spanwise angle of 45° exhibited a better performance than structures with other spanwise angle. However, the spanwise angle had little effect on discharge coefficient.The paper also studied the compound angle film cooling structures on turbine blade, and analyzed its film cooling characteristics, in consideration of the real turbine operating conditions. The research indicated that, structures with opposite spanwise angle(which called OSA in short) achieved higher film cooling effectiveness than the structures with the same spanwise angle(which called SSA in short). Besides, the OSA was better suited to the wider spanwise pitch than the SSA. When the film holes were arranged near the trailing edge, the OSA showed a slightly less energy loss coefficient than the SSA. Under the same blowing ratio, higher local Reynolds number brought higher local average film cooling effectiveness. Thus, film holes should not be arrayed at the shock reflection point. It was noted in this paper that, the compound film cooling structures with smaller spanwise angle were preferred in the throat area of blade passage. When position of film holes was settled, as the blowing ratio increased, decreasing spanwise angle was needed at the same time to ensure a relatively high film cooling effectiveness. Furthermore, at a given blowing ratio, a relatively high film cooling effectiveness appeared if only the spanwise pitch and the spanwise angle increased simultaneously. The results also revealed that, as Reynolds number rose from 0.1Res to 0.4Res, the film cooling effectiveness climbed up dramatically, while the aerodynamic loss had a significant decrease.