Experimental Investigations Of Coolant Injection Effects On Internal And External Flow Characteristics Of Turbine Blade

The consecutively increasing in turbine inlet temperature of modern gas turbine engine requires more effective cooling techniques to protect the components exposed to high temperature mainstream. Turbine blade and end-wall of the first stage are the important components, which have to be cooled by the extracting air from the compressor of the engine. A part of the extracting air flows into a serpentine coolant channel in the inside of the turbine blade, and then is ejected through film holes into the high temperature mainstream. The other part of the cooling air is used to protect the end-wall by discrete film cooling holes. To optimize currently available cooling techniques and develop advanced cooling concepts, it is necessary to understand the detailed flow characteristics in the coolant channel and at the end-wall. In fact, the investigation on these flow characteristics involves a hot topic of fluid mechanics, i.e.,"flow separation and formation of secondary vortices".This dissertation presents two series of experimental investigations on the flow characteristics using modern measurement techniques, Particle Image Velocimetry (PIV), Time-Resolved PIV (TRPIV) and Planar Laser Induced Fluorescence (PLIF). The first series of experiment is carried out in a real coolant channel of the turbine blade with coolant ejection from the exits at the blade tip and trailing edge, and in the second series of experiment, the temporal and time-mean end-wall flow features are captured in a linear turbine cascade with coolant ejection from upstream converging hole-slots. Conclusions obtained from the experiments can provide turbine designers with a relatively comprehensive database.In the first experiment, coolant injection effect on the flow characteristics in the internal coolant channel of turbine blade is experimentally investigated.It is well known that a detailed measurement of the flow features in a real coolant channel of turbine blade is still a challenge, due to complicated geometric configuration, coolant ejection from different holes, and fluctuation of inlet Reynolds number, and so on. However, to understand the real flow characteristics of cooling air in detail, it is necessary to investigate the fluid flow behaviors in an entire coolant channel with a realistic blade cross section and coolant ejection. In comparison with the previous interrelated investigations, in this dissertation, an entire coolant channel with a realistic blade cross section is manufactured by a five-axis digital-controlled milling machine using Plexiglas material with very good transparency. The entire channel is divided into three passes by two dividing walls. The inlet coolant flows into the1st and3rd passes. A part of the fluid in the1st pass is ejected from a tip exit. The other part flows around a sharp180°bend into the2nd pass, and then around a semi-round bend into the3rd pass. The fluid in the3rd pass is ejected from a tip exit and25trailing edge exits.At first, the flow structures in the real blade coolant channel are captured by the PIV system. The experimental results exhibit different secondary flow features from the previous investigations, which were conducted mostly in simplified two-pass channels with square, rectangular or trapezoid cross sections. Downstream of the bends in the real coolant channel, a pair of secondary vortices caused by the bends mixes with the secondary vortex caused by the geometry. This mixing phenomenon has not been discovered by the experiments using simplified two-pass channels. Therefore, keeping a realistic geometry of the channel is helpful to observe the secondary flow variation process in detail. Secondly, through a statistical post-processing of measured instantaneous velocity fields, the Reynolds-averaged Navier-Stokes (RANS) equations are solved to obtain the pressure distributions in the channel. Thirdly, the effect of the tip coolant ejection on the flow characteristics is discussed. Through the comparisons of the flow characteristics, it can be concluded that the tip coolant ejection results in the flow inside the channel more uniform; the sizes of separation bubble and flow impingement regions decrease, when the tip ejection ratio increases; the characteristics of the secondary vortices are changed by the tip coolant ejection; and when the tip exit located at180°bend is full open, the tip coolant ejection can reduce the pressure drop through the entire channel up to35-40%, in comparison with that of the tip exit full closed.In the second section, an experimental study of coolant injection effect on the external flow characteristics of blades in a linear cascade is carried out.Horseshoe vortex as a special form of secondary flow in linear turbine cascades usually causes the generation of passage vortex, and leads to the highest heat transfer rate in the region near end-wall and blade leading edge. It is important to suppress the formation of the horseshoe vortex, weaken the passage vortex, and enhance the end-wall cooling efficiency thereby. In this dissertation, a new cooling method using upstream converging hole-slots is proposed to inject cooling air and suppress the strength of the horseshoe vortex. The converging slot-hole structure is smoothly designed to transit from a circular hole into a slot, by convergence in axial direction and divergence laterally. This structure results in higher cooling efficiency, but lower aerodynamic loss.Firstly, the flow features in the case of without upstream coolant injection are studied using a TRPIV system with a high time resolution rate. The frequency of the leading edge horseshoe vortex movement and fast switch process are successfully captured. The transient characteristics of the corner vortices in the passage are also captured by the TRPIV system. The evolved process of the secondary vortices is demonstrated in detail through the analysis of the time-mean and temporal characteristics. An increasing in free-stream turbulence level slightly results in a movement of the horseshoe vortex toward the blade, causes a large change in the shape of the passage vortex, enhances the fluctuations of the horseshoe and passage vortices, and reduces the frequency of the horseshoe vortex movement.Secondly, at different upstream coolant injection rates from the converging slot-hole, which has an outlet-inlet aspect ratio of1.38and an inclination angle of30°against the mainstream, the influences of free-stream turbulence level on the end-wall flow characteristics are discussed. The results reveal that at various turbulence levels, the upstream coolant injections can suppress the formation of the horseshoe vortex, and a high momentum injection can weaken the passage vortex; for a low momentum injection, the turbulence level effect on the secondary flow features is obvious, and the higher turbulence level induced a larger reduction of the vorticity in the passage vortex core; while for a high momentum injection, the influence of turbulence level on the secondary flow features is slight, since the horseshoe vortex disappear.Thirdly, the converging slot-hole geometry effect on the end-wall flow characteristics is discussed, using TRPIV and PLIF techniques. To analyze the influence of the upstream film-hole exit shape, the cylindrical hole with the same inlet area with the converging slot-hole is also placed upstream of the blade. A comparison of the coolant injections from the two types of holes reveals that the converging slot-hole can provide a better coolant jet attachment to the end-wall, a smaller horseshoe vortex, and a higher adiabatic film effectiveness. To study the influence of the upstream converging slot-hole exit area, two types of holes with different outlet-inlet aspect ratios of1.38and0.69and the same inclination angle of30°are chosen. In the both cases, the horseshoe vortex is suppressed. A comparison of the coolant injections from the two types of holes reveals that the converging slot-hole with a small aspect ratio can provide a higher cooling effectiveness, slightly move the horseshoe vortex toward the blade, cause a larger reduction of passage vortex. In addition, from the comparisons of the flow characteristics obtained from two types of converging slot-holes with different inclination angles of30°and65°, it can be concluded that the injection from30°hole reduces the strength of the horseshoe and passage vortices, while the injection from65°hole enlarges the horseshoe vortex and enhances the passage vortex.