GUY NORRIS / LOS ANGELES
US Air Force technology promises to improve efficiency for high-speed aircraft, but big hurdles remain
As part of ongoing research efforts to develop a practical pulse detonation engine (PDE), the US Air Force Research Laboratory at Wright-Patterson AFB, Ohio, has revealed mixed results from tests of a detonation-driven turbine.
The study, performed by the laboratory's propulsion directorate, is one of several under way to prove the viability of the PDE concept for future propulsion needs ranging from missiles and boosters to high-speed aircraft and reusable launch vehicles. Although the basic PDE cycle promises simplicity and efficiency, it faces several hurdles, some of the most major being the ability to aspirate the PDE at subsonic speeds without significantly decreasing performance, and to extract auxiliary power for accessories.
PDEs are air-breathing jet engines with no moving parts and the potential to operate from stationary to Mach 4. Combustion takes place in an open-ended tube in which fuel is mixed with air and detonated. As the detonation wave travels down the tube at supersonic speed, it draws in fresh fuel and air and the cycle is repeated.
For aspiration and power extraction the USAF has been testing the ability of a compressor and turbine to stand up to the harsh pulsing flow of a PDE. The latest tests were a follow-on to earlier experiments in which a coupled PDE-turbocharger demonstrated shaft power extraction and self-aspiration.
For this series a Garrett T3 automotive turbocharger was attached to a 0.9m (3ft)-long detonation tube. The turbine was spun to over 130,000RPM and evaluated at "virtually all" significant operating conditions, says the study.
Results of several test runs were "surprisingly different" to theoretical predictions, says the USAF. "Even accounting for the 60-75% efficiency of the turbine/compressor in the total work, the overall efficiency of the detonation blowdown drops by a factor of around four when driving a turbine as opposed to making pure thrust." In addition, significant back-pressurisation of the detonation tube was seen.
The results were not all negative, however, and the USAF says the turbine stood up well and was still functioning after more than 50,000 detonations. Saying the result shows "cause for optimism", it adds that no visible pitting or discolouration was seen on the unit.
Source: Flight International
