Operation and noise transmission of an axial turbine driven by a pulse detonation combustor

Pulse Detonation Combustors (PDC's), as part of a hybrid PDC-turbine engine, have potential thermodynamic benefits over existing Brayton-cycle gas turbines. The form of combustion is a cyclic, controlled series of detonations. These systems apply a near or quasi-constant volume combustion process that provides both heat addition and pressure. In a hybrid PDC-turbine engine, the goal of incorporating a pulsed detonation chamber upstream of a turbine is to extract more mechanical energy in a turbine that receives the products of a repeating, pressure-rise detonation process versus the constant pressure, steady-flows available in conventional gas turbines. A rig was built to investigate PDC-turbine interactions and was operated to gather data on performance, operability, and noise levels. The rig consists of a single pulsed detonation combustor firing into a partial-admission, two-stage axial turbine. This paper reports findings of critical risk areas including turbine response to PDC operation, mechanical robustness, noise and system control. At a PDC operating frequency of 5 Hz, the acoustic level near the rig was approximately 3 dB higher than from the turbine operating at the same speed with steady flow input. The noise level is 28 dB lower than a PDC with no turbine downstream operating at the same frequency and discharging directly into the room. Insights into the mechanism for noise reduction were gained via imaging experiments and CFD simulation. High speed video imaging in a 2D PDC-turbine cascade configuration showed significant shock reflection from the cascade. An unsteady, reacting flow computational study showed similar shock reflection as well as a shock system that forms downstream of the cascade. Together, these results show that shock waves are both transmitted and reflected by the turbine stages in proportions that are dependent upon turbine stage design.