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More Temp. in closed combustion chamber

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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More Temp. in closed combustion chamber

Unread postAuthor: jimmy101 » Sun Mar 09, 2008 8:16 pm

Following up on my "Temperature in a closed combustion chamber" post of several weeks back.

I've used the accoustic techniquie to measure the temperature drop in the chamber without the fan running. I get essentially the same result, the t<sub>1/2</sub> is 114mS versus the previous 110mS for combustion with the fan running.

And now for some video fun...

Someone suggested trying to record the gauge's response with a video camera. Folks, including myself, said it wouldn't work because the typical video frame rate of 30 FPS is way too slow. Well, that may be true of a gun firing but for the closed chamber it turns out you can record the pressure gauge's movement and how fast the pressure drops as the gases cool off. Unfortunately, you can't get the peak pressure since it persists, even in a closed chamber, for too short a time. But you can get a pretty good idea of how long the pressure persists.

Here is a link to the video. Once the video has downloaded and played once you can use the progress slider to single step through the frames. The video has sound but most of what you hear is my PC reproducing the signal from the piezo transducer inside the chamber, as opposed to the sound created outside the chamber by the combustion itself.

Below are the sequence of frames from the video. The frame sequence is across the row, then to the next row. The frames are 33mS apart. In the first two frames the chamber hasn't been fired yet. The peak pressure is in the third frame and the needle is a blur from 80 to 100 PSIG. The pressure drops in the next frame to about 80 PSIG. So, the peak pressure lasts for less than 33mS. I believe the chamber's actual peak pressure is somewhat higher than the gauge says, perhaps in the 100~110 PSIG range. This particular gauge appears to read about 10 PSIG low compared to the gauge on my compressor.

Image
In the video the images are a bit better than in the still images above. The major units on the gauge are every 20 PSIG, the minor units every 4 PSIG. The minimum reading of this gauge is 8 PSIG. Reading the gauge values from the video frames and plotting gives;
Image

The pressure half-life appears to be about 130mSec. Pretty good agreement with the ~110mS that the accoustic studies gave.

I leak tested the chamber before doing the video recording, leakage rate is less than 1 PSIG per minute at 90 PSIG.

For more details you can check out my Closed Chamber Temperature Studies page.
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Unread postAuthor: benstern » Sun Mar 09, 2008 8:25 pm

Good work man!! I always love to see science and research used to better our knowledge of spudguns.

Next time try multiple gauges spaced out along the chamber.
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