Development of PLIF for Hypersonic Flows at ALDiR
Palma, McIntyre, Danehy, Houwing
Overview.
Planar laser-induced fluorescence (PLIF) has been used to obtain both qualitative and quantitative information about the hypersonic flows produced from the shock tunnels at ALDiR. Qualitative images of different species distributions (eg NO, OH) can tell us things like where combustion is occuring or where shock waves are. It is much more involved to make the quantitative measurements. However, quantitative measurements are very important for the validation of Computional Fluid Dynamics (CFD) codes. The images presented here document our first effort at producing quantitative temperature distributions in hypersonic flows. Extensive diagnostics work has been performed in lower-enthalpy facilities but this is the first that we know of in shock-tunnel flows. PLIF is very difficult to perform in shock tunnels; the flows contain metallic contaminants which produce a spectrally-broad background which is difficult to eliminate from the PLIF signals.
How it was done.
Nitric oxide is present in the air flows we are studying and it also has well-known spectroscopic characteristics. For these reasons it was chosen as the chemical species to be probed by the laser. LIF involves exciting a transition in the NO molecule with a laser and then recording fluorescence which is produced when the molecule de-excites by spontaneous emission. If a sheet of laser light is used one can get a two-dimensional image of the fluorescence by using a camera at right angles to the sheet. By using different NO transitions that are sensitive to the temperature we can derive a temperature map of the flow. The shock tunnel is an impulse facility producing a flow test time of only several hundred microseconds. The 226-nm laser radiation required to excite NO was produced by Raman-shifting the output of a tunable KrF excimer laser (248 nm). Lenses formed the beam into a sheet 8-cm wide and less than 1-mm thick. An intensified CCD camera (gain approx. 1,000,000) was used to record the fluorecence signal. A computer ensured that all the events occur simultaneously (ie. shock tunnel firing, laser firing, and image recorded by camera).
Hypersonic Flow over a Cylinder: PLIF Thermometry
Hypersonic flow over a cylinder.
The first set of images are for the Mach 7 flow over a 1-inch diameter cylinder. The flow was from left to right and the laser sheet entered the image from the top. A shadow was produced below the cylinder where the laser was blocked. The flow had an enthalpy of 4 MJ/kg, the freestream was 340 K and 3 kPa, with 5% NO. This flow is symmetric about the centreline and a detached shock is clearly visible in front of the cylinder. The first three images show how the fluorescence signal varied when different rotational transitions were excited. The bottom two images were produced by combining pairs of the top images to produce temperature maps. Different temperature sensitivity was acquired depending on which transions were chosen. For example, the image (d) is used to measure low temperatures (250 to 800 K) such as in the freestream flow. Image (e) is used to measure higher temperatures (800 to 4000 K) such as behind the shock wave.
Hypersonic Flow over a Wedge: PLIF Thermometry
Hypersonic flow over a wedge.
The same technique was also used to study hypersonic flow over a 35-degree half-angle wedge. The attached shock can be seen to curve as it interacts with the expansion fan from the corner of the wedge. Comparisons with CFD show very good agreement.
Further work.
Currently we are improving the accuracy of the technique so that we can make more reliable comparisons with the CFD codes. This involves finding ways to reduce the influence of the metallic contamiant emission. We have also exchanged the Raman-shifted laser system for a more flexible excimer-pumped dye laser system.
These measurements and calculations were performed by Phil Palma and Tim McIntyre in the ALDiR lab at the Australian National University.
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