Investigation On The Improvement Method For The Overall Performance Of A Counter-Rotating Turbine

As a core technique for gas turbine engines, the counter-rotating turbine has higher stage efficiency and less gyroscopic effects in comparison with conventional co-rotating turbines. Nowadays, the aero-engine industry has witnessed many successful cases in which the counter-rotating turbines have been well applied, such as the F119 engine from P&W. However, the current studies mainly focus on the off-design performance analysis of vaneless counter-rotating turbines and we have not seen published reports concerning the off-design performance analysis and improvement for counter-rotating turbines. In order to enhance the overall aerodynamic performance of the counter-rotating turbines, there exists a positive significance to analyze the loss sources of counter-rotating turbines at off-design conditions as well as propose the profile improvement methods. Flow field analysis and blade profile improvement method are presented through detailed numerical simulations for a counter-rotating turbine. Main contents of this thesis are as follows.1) An aerodynamic design of a counter-rotating turbine is implemented. Numerical simulation for the flow field within this counter-rotating turbine ranging from 50 through 100 percent of design speed and 65 to 130 percent of design expansion ratio is also performed. When the turbine ranges from 80 to 90 percent of design speed, a relatively high efficiency is also achieved. At this condition, the high pressure rotor inlet incidences become positive, which contributes to higher profile loss and secondary loss. Thus the efficiency decreases a bit. When the turbine ranges from 50 to 70 percent of design speed, the comparison between the flow fields of the counter-rotating turbine and a conventional two-stage co-rotating turbine, which have the same design parameters, is implemented. It is found that, the absolute flow angles at the high pressure rotor outlet increase for both counter-rotating and co-rotating turbines under low speeds. As the turning of the second vane within the conventional turbine points to the same direction of the high pressure rotor’s tangential velocity, positive incidences appear at this vane’s inlet, which well explains the boundary layer separation on the suction side nearing the leading edge. By contrast, the turning of the second vane for the counter-rotating turbine is reversed to the opposite direction of the high pressure rotor’s tangential velocity, which results in negative incidences at the inlet. As a consequence, the flow separation occurs on the pressure side near the leading edge. That is the main reason of the decrease in efficiency when turbine ranges from 50 to 70 percent of design speed.2) According to the loss sources of the counter-rotating turbine at part load conditions, the thesis proposes a profile improvement method for the second vane. The profile improvement method, based on a theory of separation angle, helps to accelerate the flow on the pressure side near the leading edge and makes the second vane more adaptable to negative incidences. The redesigned vane significantly eliminates the flow separation on the pressure side near the leading edge.3) Performance parameters of the counter-rotating turbine are analyzed before and after the optimization. Efficiencies of the redesigned counter-rotating turbine at off-design conditions are all increased and the efficiency at the design condition does not deteriorate. When the counter-rotating turbine runs at 70 percent and 50 percent of the design speed, the isentropic efficiencies of the low pressure stage are improved by 1.5% and 2.0%, respectively. Correspondingly, the total isentropic efficiencies of the counter-rotating turbine at these two part load conditions rise by 0.5% and 0.7%, respectively.