Research On Engineering Simulation And Management Mechanism Of The Aero-Thermal Environment Of Cooled Turbine

Gas turbine blades must be cooled to prevent ablation immersed in high temperature, in addition to using super alloy and TBC(thermal barrier coating). The introduction of cooling, with the3D viscous flow field of the mainstream, leads to a much more complex aero-thermal environment. Currently, the prediction and management of the aero-thermal environment of the cooled turbine is important basis of improving design capability for cooled turbine. However, for the complex flow and heat transfer caused by the highly complex cooling configuration, full3D fine simulation is unlikely to fulfill fast prediction for engineering requirement, let alone predicting the unsteady cooling caused by pulsed bleeding air under off-design conditions; on the other hand, advanced management technology such as Blended Blade/EndWall(BBEW) was proposed for flow and heat transfer in the junction of blade and endwall which is the most important part of aero-thermal organization, but the mechanism research is not enough, and the effectiveness and mechanism in turbine stage environment and thermal environment is unclear. For these two aspects, this paper carries out numerical and theoretical research, with contents and results as follows:1. Study engineering simulation based on flow network method of internal cooling of cooled turbine. Develop the flow network method which take into account unsteadiness, variable cross-section, non-ideal gas and various kinds of boundary conditions such as pressure boundary, massflow boundary, temperature boundary and heat flux boundary. Real-time switch of virtual inlet&outlet is used to simulate backward flow of gas at the boundary. To apply the flow network method to calculation of flow and heat transfer of cooled turbine, build flow network branch models of various kinds of cooling configuration, such as film holes, impingement holes, turbulent ribs, turbulent pin-fins, bend passages, trailing edge slots and so on. A computer program is developed based on this method and checked by an example, which indicate the feasibility, quickness and stability of the method.2. Study the coupling of the internal flow network and blade external3D flow. Calculation result of internal flow network provides the source intensity of external cooling, calculation result of blade external flow provides some boundary condition of internal flow network, and by iteration of these two processes the fast method of coupling simulation of blade internal/external flow is founded. A computer programs is developed based on the method and the adiabatic wall model of a full-film cooling turbine vane is taken as example for numerical simulation. First, a calculation of steady cooling is carried out, which improves the feasibility and quickness of the method. Then begin to simulate turbine aero-thermal environment under pulsed inlet coolant. The results indicate that pulse of inlet coolant massflow will lead to a significant change of blade wall temperature downstream and reduction of period-average cooling effectiveness with the same period-average coolant massflow; the pulsing amplitude of each hole with the same cooling chamber are not all equal to that of the inlet of the cooling chamber, and those holes with lower massflow often have larger pulsing amplitude, which leads to higher tendency of negative massflow, i.e. backflow; longer pulsing period,.i.e. slower pulse, leads to smaller pulsing amplitude, for the pulse can be transported far more away and more uniform; pulse from film holes with less massflow will submerge after mixing downstream with flow from film holes with less massflow, and so it It has negligible impact on wall temperature of bladewall downstream. Results above provide references for design of turbine cooling margin.3. Study the management mechanism of the aero-thermal environment by blade/endwall blending in the junction of the leading edge of the blade and endwall. Take a wing-on-plate model as example for the research in which blending surfaces with different sizes are added to the junction, under the guidance of the Blended Blade/EndWall technology(BBEW). The results shows:leading edge blending surface succeeds in managing the aero-thermal environment in the junction of the leading edge of the blade and endwall by weakening or eliminating the horseshoe vortices(HSV), on the one hand, weakened HSV can reduce the flow separation at the leading edge junction and get more uniform flowfield, on the other hand, weakened HSV can reduce fluid in mainstream with high temperature running to the platewall which reduce the wall temperature, and can reduce the velocity near the junction which reduce the heat transfer coefficient. The mechanism of weakening HSV by leading edge blending surface goes like this:the leading edge blending surface weakens the pressure gradient from the stagnated boundary layer by negative pressure gradient generated by shift forward of blade section near the plate. The design rules of leading edge blending surface are obtained by series of numerical simulation as below: increase the streamwise length while keeping the blending surface high enough(higher then the boundary layer). At last, a theoretical derivation finds that he best shape of the blending surface in the centplane for eliminating the HSV is the same as the profile of the coming boundary layer, that the mechanism can be explained like this: the forward shift blade sections near the plate causes the same velocity profile as that of the boundary layer.4. Study the management mechanism of the aero-thermal environment by blade/endwall blending in the whole junction of the blade and endwall. Take a turbine stage as example, based on which various kinds of blending models are designed by the guidance of Blending Blade/EndWall technology(BBEW). The simulation results show that BBEW makes effective managing on the aero-thermal environment of the turbine, with details as such:BBEW compensates through-flow loss caused by introduction of the blending surface with a slight sink of the endwall. The side blending surface weakens the flow separation in the suction corner of the vane; the leading edge blending surface weakens the HSV and the flow separation at the leading edge corner; the blending surface of the vane reduce the height of the wake, which reduce the height of the boundary layer of total temperature at the inlet of the blade. Then the passage vortex moves the fluid with high total temperature at the bottom to the suction corner, while the fluid with lower total temperature expose itself to the hub wall, which reduce the adiabatic wall temperature of the hub evidently.