Unsteady ejectors are currently under investigation for use in some pulse-detonation-engine-based propulsion systems. Experimental measurements made in the past, and recently at the NASA Glenn Research Center, have demonstrated that thrust augmentation can be enhanced considerably when the driver is unsteady. In ejector systems, thrust augmentation φ is defined as φ = TTotal/Tj, where TTotal is the total thrust of the combined ejector and driving jet and Tj is the thrust due to the driving jet alone.
Ensemble-averaged contours of measured instantaneous vorticity for three pulsed thrust
sources. Measurements were made using digital particle imaging velocimetry. Top: 220-Hz pulsejet. Center: 275-Hz resonance tube. Bottom: 50-Hz speaker-driven jet.
Long description.
The exact mechanism behind the enhanced performance is unclear; however, it is believed to be related to the powerful vortex emitted with each pulse of the unsteady driver. As such, particle imaging velocimetry (PIV) measurements were obtained for three unsteady drivers: a pulsejet, a resonance tube, and a speaker-driven jet. All the drivers were tested with ejectors, and all exhibited performance enhancement over similarly sized steady drivers. The characteristic starting vortices of each driver are shown in these images. The images are color contours of measured instantaneous vorticity. Each image is an ensemble-average of at least 150 phase-locked measurements. The flow is from right to left. The shape and location of each driver is shown on the far right of each image. The rectangle shown in each image represents the ejector diameter that was found experimentally to yield the highest thrust augmentation. It is apparent that the optimal ejector diameter is that which just "captures" the vortex: that is, the diameter bounding the outermost edge of the vortex structure.
Although not shown, it was observed that the emitted vortex spread as it traveled downstream. The spreading rate for the pulsejet is shown as the dashed lines in the top image. A tapered ejector was fabricated that matched this shape. When tested, the ejector demonstrated superior performance to all those previously tested at Glenn (which were essentially of straight, cylindrical form), achieving a remarkable thrust augmentation of 2. The measured thrust augmentation is shown in the following graph as a function of ejector length. Also shown on the plot are the thrust augmentation values achieved with the straight, cylindrical ejectors of varying diameters.
Measured thrust augmentation as a function of ejector length for the tapered and straight ejectors. Each curve represents a different ejector diameter. The driver is a pulsejet.
Long description.
That the cross-sectional dimensions of optimal ejectors scaled precisely with the vortex dimensions on three separate pulsed thrust sources demonstrates that the action of the vortex is responsible for the enhanced ejector performance. The result also suggests that, in the absence of a complete understanding of the entrainment and augmentation mechanisms, methods of characterizing starting vortices may be useful for correlating and predicting unsteady ejector performance.
Paxson, Daniel E.; Wilson, Jack; and Dougherty, Kevin T.: Unsteady Ejector Performance: An Experimental Investigation Using a Pulsejet Driver. AIAA Paper 2002-3915 (NASA/TM-2002-211711), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211711.html
Wilson, Jack; Paxson, Daniel E.; and Dougherty, Kevin T.: Unsteady Ejector Performance: An Experimental Investigation Using Resonance Tube Driver. AIAA Paper 2002-3632 (NASA/TM-2002-211474), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211474.html
John, Wentworth T.; Paxson, Daniel; and Wernet, Mark P.: Conditionally Sampled Pulsejet Driven Ejector Flow Field Using DPIV. AIAA-2002-3231, 2002.
Paxson, D.; Wernet, M.; and John, W.: An Experimental Investigation of Unsteady Thrust Augmentation Using a Speaker-Driven Jet. AIAA-2004-0092, 2004.
Glenn contact: Dr. Daniel E. Paxson, 216-433-8334, Daniel.E.Paxson@nasa.gov
Author: Dr. Daniel E. Paxson
Headquarters program office: OAT
Programs/Projects: Propulsion and Power, Pulse Detonation Engine Technology
Last updated: January 21, 2005
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