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What is a Shock Tube?

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  • Written By: Niki Foster
  • Edited By: Andrew Jones
  • Last Modified Date: 14 November 2016
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A shock tube is an instrument used to study gas phase combustion reactions, and aerodynamic flow. At its simplest, a shock tube is a metal tube with a diaphragm, a semi-flexible barrier, separating a high pressure gas from a low pressure gas. To begin an experiment, the diaphragm is ruptured, causing a shock wave to travel through the low pressure gas.

In experiments using a shock tube, the high pressure gas is referred to as the driver gas, while the low pressure gas is the driven gas. The gases need not be of the same chemical composition. The gasses are pumped into or out of the tube on each side of the diaphragm until the desired pressure is reached on each side. To begin an experiment with a shock tube, the diaphragm may be ruptured using a plunger with a blade attached, though the required mechanism is complex. Many experiments either use a scored diaphragm, designed to rupture when a certain pressure is reached inside the tube, or use combustible gasses in the driver to burst the diaphragm.

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When the diaphragm in a tube is ruptured, a shock wave, an abrupt, propagating disturbance, travels into the driven gas. The temperature and pressure of the driven gas also increase, and the shock wave induces aerodynamic flow in the direction of the shock wave, but at a lower velocity, or speed. A propagating decrease in pressure, called a rarefaction wave or expansion fan, travels back into the driver gas. The contact surface, the border between driver gas and driven gas, travels into the driven gas, just behind the shock front, defining the border of the shock wave.

When the shock wave reaches the end of a tube, it is reflected, traveling back into the driver gas, and causing an even greater increase in temperature, pressure, and density. Using a dump tank to absorb the reflected shock wave can prevent this reaction. After the shock wave has been created, the gas in the shock tube is withdrawn and studied to observe the effects of high pressure and temperature. A shock tube can also be used to study the effects of combustion upon solid particles, which are inserted into the tube before the diaphragm is ruptured.

A shock tube can also be used to study the aerodynamic flow of the driven gas behind the shock wave. Combustion happens very quickly. As a result, there is limited time to observe the aerodynamic flow, usually just a few milliseconds.

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hamje32
Post 4

@Charred - I imagine that they conduct these kinds of experiments for aeronautical applications or any application where you would see a sudden burst of explosive gas in a controlled environment.

I suppose it could be useful in automotive applications. I think that you would want a car engine not to rattle and shake during combustion. Using these shock tube experiments you can improve the efficiency of the engine by controlling the pressure of the combustion. I don’t know a lot about cars but that’s what I guess would happen.

Charred
Post 3

@allenJo - I doubt it. I think a pressure tap is used to measure pressure whereas it appears that the dump tank acts as a kind of release valve, allowing the reflected air to escape.

To continue with the balloon analogy (however imperfect) it would be like having a balloon with holes in it. It wouldn’t pop, but it would seep out air.

allenJo
Post 2

@Mammmood - Yeah, the low pressure gas does appear to give some kick back to the high pressure gas, doesn’t it?

I think that’s just as useful for the scientific analysis as to observe how much pressure the low pressure gas receives. What’s the dump tank? Is that something like a pressure tap?

Mammmood
Post 1

It’s probably not a perfect analogy, but if you’ve ever blown into a balloon and then accidentally let the balloon deflate while still holding it in your mouth, you’ve probably done something similar to creating a shock tube. You blow out carbon dioxide and inhale it back into your mouth, sending you into a coughing spell no doubt.

I think of this picture when I imagine the exchange of gas that takes place between the high pressure chamber and low pressure chamber of the shock tube. In that case, however, we are dealing with two separate gases, unlike the balloon example, so like I said it’s not a perfect analogy but it helps.

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