Counter-rotating turbine flow mechanism and aerodynamic design

Counter-rotating turbines have a high efficiency, low weight, and gyroscopic torque, which contribute to their wide use in advanced aerojet engines. Currently, counter-rotating turbines continue to be developed for higher efficiency, stage work loading, and better performance under design and off-design conditions. This paper presents velocity diagrams for counter-rotating turbines, showing that reducing the exit swirl of the turbine inlet nozzle and increasing the stage work loading of a high-pressure turbine rotor are main way to satisfy the turbine design constraints of an axial exhaust flow. According to the turbine design condition of an inlet total temperature of 1850 K, inlet total pressure of 2.5 MPa, and expansion ratio of 6, the through flow and 3D blade design for 1+1/2, 1+1, and 1+3/2 counter-rotating turbines were conducted using the same rotating speeds, hub radius, and inlet hub-tip ratio. The output power ratios for the high-pressure and low-pressure stages were 2.0, 1.8, and 1.5, respectively, with efficiencies of 91.7%, 91.8%, and 92.4%, respectively, and working coefficients for the high-pressure rotors of 3.2, 2.7, and 2.4 for the three types of turbines. The high-pressure rotor of the 1+1/2 turbine has an exit relative Mach number of 1.6 at a 50% blade span and a convergent-divergent blade passage, whereas those of the 1+1 and 1+3/2 turbines were 1.28 and 1.10, respectively, and both had convergent blade passages. The flow losses of these turbines were investigated, showing that severe shock losses are present in counter-rotating turbines in contrast to traditional turbines. A shock loss restriction method using a concave suction side curve was consequently proposed to reduce the intensity of trailing edge shocks and the width of the wake. The efficiency of the 1+1/2 turbine is increased by 0.3% to 92.0% by using this method. The turbine performance characteristics are also presented and discussed. The turbine efficiency remained almost unchanged above a rotating speed of 90%, and there was no sharp drop in the off-design performance. A performance optimization approach for the low-pressure turbine rotor of the 1+1/2 turbine is suggested for 20% high-pressure and 10% low-pressure stage design rotating speeds. This increases the efficiency of the low-pressure turbine by 15% and the entire turbine by 5% so that the turbine power output is enhanced during the engine startup process. © 2020, Science Press. All right reserved.