schematic of the model SCRAMJet engine, showing the injector and the locations of the pressure transducers (numbers, 1-13)
The scramjet model consisted of a constant area rectangular duct instrumented along the floor with pressure transducers. Hydrogen was injected into the flow through a centrally mounted plate. A combination schlieren/shadowgraph system was used to examine density gradients within the flow. A shear layer was detected between the co-flowing air and hydrogen streams within which combustion is thought to be occurring. Shock waves were also observed in the flow whose positions and movements were well correlated with surface measurements.
shadowgraph image of supersonic combustion in the SCRAMJet
In parallel with the schlieren work, a planar laser-induced fluorescence (PLIF) system was developed for the next phase of the measurements. This includes direct monitoring of combustion zones via the detection of the combustion intermediary OH and monitoring of unburnt hydrogen zones by the detection of a tracer species. The light source for the experiment is the output of a frequency doubled dye-laser which was pumped by a frequency doubled Nd:YAG laser. This laser light is formed into a sheet and passed through the flow, the wavelength of the light being selected so as to be absorbed by the species of interest. The resulting fluorescence is detected by an image intensified CCD camera operating with a fast gate.
plan view of setup for imaging OH in a SCRAMJet with PLIF
side view of SCRAMJet showing laser beam enter from the top
(L=lens; M=mirror; BS=beamsplitter; CCD=charge coupled devise (digital camera); ICCD=intensified CCD camera; FD=frequency-doubler; HS=harmonic separator; PMT=photomultiplier tube; PDL-2=pulsed dye laser)
The PLIF experiments were initially unsuccessful, because the self-luminosity of the flow was much too high. Special spatial filtering methods and electronic shuttering of the intensified detector used in the PLIF experiment finally overcame this problem, allowing the PLIF method to image regions where measurable quantities of OH were detected. These regions coincided with the thin shear layers on either side of the fuel jet. The presence of OH indicated that intermediate reactions characteristic of ignition were taking place in these layers. These ignition reactions precede the heat release reactions and hence the detection of OH is consistent with the fact that heat release in this region is negligible.
flow visualisation of OH in the SCRAMJet
The two PLIF images shown are from different shock-tunnel runs. Flow is from left to right. Fuel is injected in a streamwise direction at a sonic velocity from a centrally-mounted strut. The fuel stream is surrounded by two supersonic heated air streams. The bright (fluorescing) regions indicate the presence of the OH radical produced during the ignition phase of the combustion processes.
Our measurements indicate that we have been observing the cool potential core of the ignition region of the SCRAMJet engine; thus we would need to look further downstream to observe the combustion zone. Work to do this is currently in progress.
Here are some recent (2/97) scramjet images.