3-Tube PDE rig shadowgraph
In my last blog I talked about a valved multi-tube PDE called the 3-Tube PDE rig and hinted at some of the exciting experiments that we have run using it.
In our research on PDEs, one of the areas of focus is the effect of the exit nozzles on the operation and performance of PDEs. Not much is known about this so was one of the first areas we decided to dive into. We studied two exit nozzle geometries; a baseline straight nozzle and a converging nozzle with an area ratio of 0.25 (which means that the minimum area is only 0.25 the area of the tube).
By ‘looking’ at the detonation wave using shadowgraph, we discovered that exit nozzles have a big impact on the wave as it leaves the PDE. In a previous blog, my colleague discussed the blowdown from the exit of a PDE tube without a nozzle and showed a shadowgraph sequence.
Shadowgraph is a flow visualization technique that lets you see changes in density. Since the detonation wave moves really fast (~ Mach 5), a high speed digital camera running at nearly 40,000 frames per second was needed to capture these shadowgraph images! Inside the PDE tube the detonation travels as a nearly flat disk, however, after leaving the tube, the detonation changes shape (diffracts).
For the straight nozzle, the detonation transitions into a spherical shape. The dark black band in the images shows where the powerful leading shock wave is… It is this strong shock (pressure) wave that makes the loud “bang” every time the PDE fires.
After the shock wave has traveled out of the image, there is a strong jet of hot gas that continues to flow out of the PDE during the blowdown phase. Near the end of the blowdown, the pressure in the tube actually drops below the outside air pressure, which causes reverse flow. That is, surrounding air and exhaust products are sucked back into the tube. This should give you some sense of the design challenges we are facing with PDEs – The cycle is unsteady, with velocity swings from Mach 5 down to negative velocities!
For the converging nozzle, the leading shock wave has a pronounced oblong shape (as opposed to spherical), and the blowdown lasts a lot longer than with the straight nozzle. There is another interesting feature of using the converging nozzle. The reverse flow phenomenon that was seen with the straight nozzle does not occur with the converging nozzle. This finding could be an important PDE nozzle design consideration. So that’s a quick snapshot of what happens outside of the PDE tube, stay tuned until next time and we will talk about some of the interesting things happening inside the PDE tube.
Experimental and Numerical Investigation of a Valved Multi-tube PDE, AIAA Paper 2008-0110.
