In order to decrease the fuel consumption, a new flight mode is being considered for twin-engine helicopters, in which one engine is put into sleeping mode (a mode in which the gas generator is kept at a stabilized, sub-idle speed by means of an electric motor, with no combustion), while the remaining engine operates at nominal load. The restart of the engine in sleeping mode is therefore deemed critical for safety reasons. This efficient new flight mode has raised the interest in the modeling of the restart of a turboshaft engine. In this context, the initial conditions of the simulations are better known relative to a ground start, in particular the air flow through the gas generator is constant, the fuel and oil system states are known and temperatures of the casings are equal to ambient. During the restart phase of the engine, the gas generator speed is kept at constant speed until the light-up is detected by a rise in inter-turbine temperature, then the starter torque increases, accelerating the engine towards idle speed. In this paper, the modeling of the acceleration of the gas generator from light-up to idle and above idle speeds is presented. Details on the light-up process are not addressed here. The study is based on the high-fidelity aero-thermodynamic restart model that is currently being developed for a 2000 horse power, free turbine turboshaft. In this case, the term high-fidelity refers not only to the modeling of the flow path components but it also includes all the subsystems, secondary air flows and controls with a high level of detail. The physical phenomena governing the acceleration of the turboshaft engine following a restart — mainly the transient evolution of the combustion efficiency and the power loss by heat soakage — are discussed in this paper and modeling solutions are presented. The results of the simulations are compared to engine test data, highlighting that the studied phenomena have an impact on the acceleration of the turboshaft engine and that the model is able to correctly predict acceleration trends.
Unsteady, non-isentropic, discontinuous flows with energy exchange, during solar heating transients of air turbine towers are approached through a proprietary computational front method, initially developed for the study of ignition in solid propellant rocket motors. Its application in the discontinuous flows with energy exchange also proves highly efficient. Computational efficiency is demonstrated by CFD simulation of transients in the air accelerator of the SEATTLER solar mirror, turbine tower. This is a typically unsteady flow simulation for slender channels. A 1-D computational scheme was developed to simulate the interference between zones with different flow conditions. Given values for the thermochemical properties of the working gas are considered and two zones of different flow characteristics are identified. The first zone is the heat exchanger, where a nonisentropic flow develops. At the aft end of this heating zone a second zone of the channel is encountered after a blunt passage, where an isentropic expansion of the gas begins and extends along the tower up to the upper exit. Into the 1-D, unsteady flow scheme of computation, the discontinuity of equations of motion at the interface between the two zones induces very specific precautions and this methodology is detailed into the paper. Consequently, the computational front scheme covers the dual behavior of the fully non-isentropic flow with mass addition and mixing in the heater and of the fully isentropic flow at the exhaust of the gravity draught tall tower, typical for the solar-gravity draught power plants. Small perturbations of the flow, in the form of developing weak shocks, and blunt discontinuities are simultaneously covered. Code robustness is demonstrated and revealed through diagrams. The 1-D numerical scheme is based on the enhanced method of the computational front with resolution of the expansion wave development.
Despite its intricacy the numerical method applied within the TRANSIT code proved successful in describing discontinuous, non-isentropic flows in rocket engines and solar-gravitational towers for green energy. A number of 0-D approaches are known to render some results in demonstrating the feasibility of the solar tower concept, or in unsteady simulation of transient phases in rocket engines. Computational efficiency is demonstrated by CFD simulation of the starting transients in ADDA solid rocket engines and in the SEATTLER solar mirror tower. The code is exclusively directed to unsteady flow simulations in slender channels. The wave front model scheme covers the dual behavior of fully non-isentropic flow with mass addition and mixing in the thrust chamber or blunt heat addition in a heater and fully isentropic through the exhaust nozzle or gravity draught in a tall tower. Along the tower of the solar-gravity draught power plants small perturbation discontinuous flows are covered. Code robustness is demonstrated during runs on the PC.
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