Ejectors are passive devices that, when placed aft of a thrust-producing jet, entrain ambient air and increase thrust. In recent years there has been renewed interest in the concept of ejectors or thrust augmentors driven by periodic, unsteady propulsion devices (i.e., pulsed propulsion). The reason for this stems primarily from the interest in pulse detonation engines (PDEs), which are decidedly unsteady, and which therefore seem natural candidates on which to use an ejector. It has been demonstrated convincingly in recent experiments that, under the proper operating conditions, and with a well-designed ejector, thrust augmentation levels approaching or even exceeding 2.0 can be achieved with unsteady thrust sources as drivers. Thrust augmentation is defined as the total time-averaged thrust provided by the ejector and driver system divided by the thrust of the driver alone.
Studies have been conducted using a variety of unsteady thrust sources including actual PDEs, simple pulsed valves, Hartmann-Sprenger resonance tubes, synthetic jets, and pulsejets. The results from each of these experiments have helped to identify the factors that optimize unsteady ejector systems.
Despite the many unsteady ejector experiments performed to date and the growing body of understanding associated with them, some results could not be generalized because all the experiments shared a common small scale. The thrust levels were low (less than 15 lbf), and the driver diameters were between 1 and 2 in. Thus, although rules have been suggested relating, for example, the optimal diameter of the ejector as a fixed ratio relative to that of the driver, they were not definitive because all the drivers tested were nearly the same size.
To address this issue, a pulsejet approximately an order of magnitude larger in exhaust cross-sectional area and thrust than most recent tests was operated with a series of ejectors of varying diameter, length, and shape (cylindrical and tapered). The ejector parameters were systematically varied to determine the configuration yielding the highest thrust augmentation. The installed rig is shown in the photograph.
Pulsejet and one of the tested ejectors installed on a thrust measuring rig.
Long description of figure 1.
Typical results are shown in the graph. A peak thrust augmentation value of 1.71 was obtained using ejectors with parallel walls. The optimized ejector diameter was found to be 2.46 times the pulsejet driver diameter of 6.5 in. This ratio was consistent with the small-scale experiments and may, therefore, be considered a sizing rule. The optimal length was found to be 10 times the driver diameter. This result was found to be the same as for another small-scale pulsejet experiment but to be somewhat different from those where another driving source was used, suggesting that other parameters, such as pulsing frequency, determine optimal ejector length. It was found that the tapered profile ejector yielded a higher thrust augmentation than the best of the straight profile series. The value obtained was 1.81. This result was consistent with numerous other unsteady thrust augmentation experiments. Additional research is needed to determine if there is an optimal ejector taper angle and if that angle can be related to parameters of the driver. The large-scale ejector fabrication and testing were performed at the Air Force Research Laboratory in Dayton, Ohio.
Thrust augmentation as a function of ejector length for tapered- and straight-walled ejectors. D/d, ratio of ejector throat diameter to pulsejet exit diameter; R, radius of ejector inlet rounding (as viewed from the side, or axial radial plane).
Long description of figure 2.
Litke, P., et al.: Assessment of the Performance of a Pulsejet and Comparison With a Pulsed-Detonation Engine. AIAA-2005-0228, 2005.
Paxson, D.; Wilson, J.; and Dougherty, K.: Unsteady Ejector Performance: An Experimental Investigation Using a Pulsejet Driver. AIAA-2002-3915 (NASA/TM--2002-211711), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211711.html
Wilson, J.; and Paxson, D.E.: Unsteady Ejector Performance: An Experimental Investigation Using a Resonance TubeDriver. AIAA-2002-3632 (NASA/TM--2002-211474), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211474.html
Wilson, J., et al.: Parametric Investigation of Thrust Augmentation by Ejectors on a Pulsed Detonation Tube. AIAA-2005-4208 (NASA/TM--2005-213823), 2005. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2005/TM-2005-213823.html
Glenn contact:
Dr. Daniel E. Paxson, 216-433-8334, Daniel.E.Paxson@nasa.gov
Author:
Dr. Daniel E. Paxson
Headquarters program office:
Aeronautics Research Mission Directorate
Programs/projects:
Vehicle Systems Program, Constant Volume Combustion Cycle Engine Project
Last updated: December 14, 2007
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