An experimental study of a pulsed jet ejector

The additional thrust required to accelerate a Vertical/Short Take-Off and Landing (V/STOL) aircraft to flying speed within short distances can be obtained by channeling the engine exhaust through a thrust augmenting ejector. In order to have a viable ejector-augmented powered-lift for these types of aircraft, two most prevailing challenges need to be addressed - they are high performance and compactness of the ejector. Toward this end, an experimental investigation was carried out using a pulsed jet as the driving source for the ejector configuration. The objectives of the investigation were threefold: first, to demonstrate that a suitably sized ejector with the pulsed jet as the driving source can give substantially higher levels of thrust augmentation (total thrust/primary nozzle thrust) than the steady jet driven ejector of comparable dimensions; second, to identify the characteristics of the pulsed flow that could be helpful in determining the optimal ejector flow conditions for maximizing thrust augmentation. And third, to understand the flow physics behind the thrust augmentation mechanism of the pulsed jet ejector.; The first objective was addressed by carrying out direct thrust measurements on the free steady jet, free pulsed jet and the pulsed jet ejector configurations. Within the range of parameters investigated, it has been demonstrated conclusively that for an incompressible pulsed jet (Mj = 0.30) operating at a Strouhal number of around 0.1, thrust augmentation values as high as 1.9 can be obtained with a compact ejector (L/ D ≈ 3) at an area ratio (ejector inlet area/primary nozzle exit area) of about 11.0.; The second objective was addressed by studying the detailed spatio-temporal evolution of the free pulsed jet flow field using phase-locked Particle Image Velocimetry (PIV). The investigation suggests that maximum jet entrainment is achieved when the primary jet Strouhal number is within a range of 0.2 to 0.25. The results further show that the entrainment is mainly dependant on the strength as well as the number of vortex rings present in the flow field. These characteristics helped to define the optimal flow conditions under which maximum mass flow rate into the ejector (and hence maximum ejector thrust augmentation) could be achieved. The results indicate that the thrust augmentation ratio can be improved to about 2.3 with the pulsed jet operating at an optimum Strouhal number of 0.24.; Finally, the third objective was addressed by investigating the flow field characteristics of the pulsed jet ejector using phase-locked PIV. Experiments were carried out at three different area ratios that help to define the conditions for maximum thrust augmentation. The results show that in the presence of the ejector duct, the pulsed jet primary vortex induces a secondary vortex on the wall and the strength of the induced vortex depended strongly on the proximity of the ejector wall. At the optimal location where maximum thrust augmentation was observed, the strength of the induced vortex was found to be highest. This pair of the primary and induced vortex establishes an axial pressure gradient within the duct, that convects downstream thus causing an enhanced mass flow rate into the ejector and hence resulting in higher thrust augmentation.