Stereoscopic Particle Image Velocimetry Measurements in Scramjet Combustors

Abstract

The prospect of atmospheric flight at five times the speed of sound, or greater, has captivated researchers for many years. One type of hypersonic propulsion engine capable of providing such astounding flight-speeds is the Dual-mode scramjet (DMSJ). This engine is especially attractive because a single flowpath is used over a wide range of flight Mach numbers. However, the complex combustion process in a DMSJ is not well understood. The flowfield of a DMSJ combustor is highly turbulent and highly three-dimensional due to fuel injection schemes and massive flow separation. These characteristics, along with temperatures in excess of 1200K, make experimental measurements extremely challenging. Therefore, three-component velocity measurements for the combusting flow of a scramjet are currently lacking in the literature. Velocity measurements in a Dual-Mode Scramjet combustor would further understanding of this complicated flow by providing quantitative in-stream information and a benchmark data set that is needed to facilitate computational model validation.
Classic probe-based measurement techniques are unacceptable for measuring the three-component velocity field of a DMSJ because these techniques alter the flowfield. However, a non-intrusive optical-based technique, such as Stereoscopic Particle Image Velocimetry (SPIV), is a candidate for velocity measurements in scramjet combustors. SPIV experiments have been undertaken at the University of Virginia’s Aerospace Research Laboratory in order to measure the flow inside a DMSJ for the case of fuel-air mixing prior to ignition and also for the case of fuel-air combustion. These experiments were conducted at several different measurement locations in two DMSJ combustors with different flowpath geometries, each containing a ramp fuel injector. Because of opposing constraints for SPIV experimental configurations and physical limitations of the supersonic combustion facility, a major accomplishment of this work was the design of appropriate SPIV configurations at each of the desired locations. These SPIV configurations consisted of a laser sheet (between 2.6 and 2.4mm thick) placed at cross-planes in the combustor with two cameras to view the flow illuminated by the laser sheet. Each camera was set at angles ranging from 30 to 35-degrees for different experiments and submicron-diameter seed particles were added to the flow. These particles track the flow and consecutive images of the particles show displacements over a short time interval (400ns to 600ns) and therefore velocity information is obtained. Experiments conducted using the SPIV configurations produced the first SPIV measurements to provide instantaneous, three-component velocity measurements in a DMSJ during fuel-air combustion.
SPIV measurements generated both instantaneous and averaged velocity fields and quantified for the first time the two counter-rotating vortices induced by the ramp fuel injector. Flow quantities such as fluid rotation and turbulence intensity were then extracted from the velocity measurements. Moreover, because the measurements were obtained for both fuel-air mixing and fuel-air combustion at different combustor locations, these results provide insight into the effect of combustion on the fuel-air mixing process. Turbulent mixing length scales were also found in one measurement plane and are reported along with an estimate of the time scale of turbulence. Qualitative comparisons between SPIV measurements and current Computational Fluid Dynamics (CFD) results have also been made. The SPIV measurements will be available so that more detailed quantitative comparisons can be conducted in the future. While the current application of the SPIV technique was quite successful, geometric complications limited the SPIV measurements at certain locations. Using insight gained from the current experiments, recommendations have been made to improve future SPIV measurements.