The US Air Force is expected to sanction a new round of studies into combat turbofan engines equipped with a pulse detonation afterburner (PDA) in place of a reheat system after promising studies of a conceptual device with a simple divergent nozzle.
Tests are being carried out at the Turbine Engine Division of the US Air Force Research Laboratory in Ohio and are based on earlier studies of a conceptual PDA in which concentric annular passages have been placed in the afterburner. The PDA consists of a rotary valve attached to the engine low or high pressure shafts through a gearbox, and is based on some of the most promising pulse-detonation engine configurations being studied for aircraft, missile and reusable launch vehicle applications.
Initial studies had disappointing results and the predicted performance of the PDA-equipped engine "fell short" of the same turbofan with a conventional afterburner. These tests were made without an engine nozzle, so a new series tested a divergent nozzle with three exit to throat area ratios. Area ratios of 1.38, 1.71 and 2.49 were tested with nozzle divergent angles of 8°, 14° and 25° respectively.
Using computational fluid dynamics, the test team showed engine performance improved with increasing nozzle area ratio. They also showed that the PDA-equipped engine was only capable of outperforming its conventional counterpart with a minimum nozzle area ratio of 1.71. The team warns, however, that engine weight and volume increase with greater nozzle area ratio and this could "offset the performance gains".
It adds that further experimentation with real hardware is required to quantify the results, and that further studies are expected involving bell and countoured nozzle shapes to improve performance.
Source: Flight International
