Flow Control Of The Exhaust System On The Turbo-piston Engine

With the advantages of low fuel consumption of diesel engine and high power density of gas turbine engine, the turbo-piston engine which combines Diesel cycle and Brayton cycle is one of the important development trends of high power density engine. When the turbo-piston engine works in the combined cycle mode, the exhaust gas from diesel engine will be injected into the turbine system of gas turbine engine to recover the exhaust energy to improve the fuel economy of the turbo-piston engine. Gas injecting has a direct impact on the outlet pressure of the combustion chamber of gas turbine engine and the exhaust back pressure of diesel engine, and has a great influence on the performance of diesel engine and gas turbine engine.A basic CFD simulation model of two-stage turbine of the gas turbine engine which is a part of a turbo-piston engine was established, and the model was validated with the grid independence analysis. First two kinds of cases with injecting gas before gas turbine and between two stages was studied by using the simulation model. By drilling a hole on the turbine casing, a n injecting hole was formed, which can be used to study the influence of gas injecting at different positions and by different ways on the performance of two turbines. Results show that injecting between two stages will reduce the performance of gas turbine substantially, which means this case is not suitable for the turbo-piston engine. Many variables were considered to study the influence of injecting before gas turbine on the performance of two turbines, such as the injecting hole’s axial position, circumferential distribution uniformity, injecting inlet area, injecting angle and injecting mass flow rate. Some influence laws were carried out.Based on the law study above, four gas injecting structure divided into two categories were designed, and comparison analysis between different injecting cases with different gas injecting structure were carried out. Results show that the effect of injecting on the performance of gas turbine is greater than that of power turbine. When comparing the results in the same kind of injecting structure, it can be found that the performance of gas turbine and power turbine restricted each other. In addition, the injecting inlet pressure can be lower by using the volute type of injecting structure. In summary, the positive swirl volute was considered as the best injecting structure.The internal flow of the injecting structure and two-stage turbine was investigated by numerical simulation. Results show that injecting structure has a great influence on the circumferential uniformity of gas flow in the passageway of two-stage turbine. The first kind of injecting structure can affect the gas flow near the hub while the volute type of injecting structure can only affect the gas flow at high span of blades. Besides, the gas flow-guided part of the 30 degrees injecting structure which can eliminate the separation flow near the outlet of injecting structure is beneficial for gas mixing, but will increase the pressure of injecting inlet. Affected by gas injecting in four cases, the stagnation point near the leading edge of gas turbine guide blade have different position distribution. At 90% span of power turbine rotor blades, the flow into the rotor has large positive angle of attack, which causes some flow losses that can be reduced by optimal designing the rotor blades of the power turbine.