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Inflight shutdown of one engine for twin-engine helicopters have proven beneficial for fuel consumption. A new flight mode is then considered, in which one engine is put into sleep mode (the gas generator is kept at a stabilized, sub-idle speed by means of an electric motor, with no combustion), while the second engine runs almost at nominal load. The ability to restart the engine in sleep mode is then critical for safety reasons. Indeed, the certification of this flight mode involves ensuring a close-to-zero failure rate for in-flight restarts as well as a fast restart capability of the shutdown engine. In this paper, the focus is made on improving the restart time of the shutdown turboshaft engine. Fast restart capability is necessary for flight management reasons. Indeed, in case of a failure of the engine operating close to nominal load while the other one is in sleep mode, there is no more power available and the helicopter can lose up to 15–20 meters per second during autorotation. The restart time becomes a critical parameter to limit the loss of altitude. In the configuration studied, the fast restart is achieved thanks to the electric motor designed to deliver a high torque to the gas generator shaft. This electric motor is powered by an additional battery, more powerful than the conventional one dedicated for standard restarts. The aim of the paper is to assess the potential restart time saving using an approach combining test rig data analysis and numerical results generated by a thermodynamic model able to simulate at very low rotational speed. A gas turbine engine starting process is composed of two main phases: the light-up phase and the acceleration phase. It is important to understand the detailed phenomenology of these two phases as well as the various sub-systems involved, first to highlight the influencing parameters of both phases and then to establish an exhaustive listing of the possible time optimizations. From the test rig campaign, conducted at Safran Helicopter Engines on a high power free turbine turboshaft engine, we are able to accurately break down the phases of the start-up sequence, which helps us to identify what steps of the sequence worth shortening. With the engine performance thermodynamic model, we can then use the information gathered from the test rig analysis to further predict how to save time and to give guidelines for developing new control strategies. The results of this study show that a fast restart going from sleep mode to max power speed can be up to 60% faster than a conventional restart going from sleep mode to idle speed. This is significantly faster, especially if one takes into account the higher final speed targeted by the fast restart.

This paper reports three-dimensional direct numerical simulations of ignition of a composite AP (ammonium perchlorate)–HTPB (hydroxyl-terminated polybutadiene) propellant subjected to a constant flux ϕ. The model includes solid heat transfer, gas-phase combustion with global kinetics and explicit description of propellant microstructure. Simulations show that ignition starts from AP particles because of primary AP/binder flame. Go/no-go computations reveal an unreported intermediate regime between go and no-go with apparent quenching followed by a delayed ignition. This delay is linked to a slow flame spreading from localized scattered hot spots on surface. In accordance with experiments, ignition delays deviate from classical ϕ − 2 scaling for high flux and low pressure conditions. For intense flux levels, simulations attest deradiation extinction upon flux termination meaning that there is a critical flux above which ignition is no longer possible. The role of AP particle shape and size distribution on ignition delay is studied and predicted to be limited. The effect of propellant surface conditions is also investigated and can lead to substantial effects on ignition delay. Finally, semi-transparent propellants are also considered and low absorption materials result in longer ignition times, reduced ϕ-dependence, and absence of deradiation extinction. This work eventually highlights the importance of microstructure-based details in the physics of composite propellant ignition and opens the way towards better understanding of the role of AP particles.

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.