An impact tube is a metal tube with a diaphragm that separates a high pressure gas from a low pressure gas. When the diaphragm ruptures, a shock wave travels through the low-pressure gas. The shock tube can be used to study gas phase combustion reactions and aerodynamic flow.
An impact tube is an instrument used to study gas phase combustion reactions and aerodynamic flow. In its simplest form, an impact tube is a metal tube with a diaphragm, a semi-flexible barrier, that separates a high pressure gas from a low pressure gas. To initiate an experiment, the diaphragm ruptures, sending a shock wave traveling through the low-pressure gas.
In experiments using an impact 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 have the same chemical composition. Gases are pumped into or out of the tube on each side of the membrane until the desired pressure is reached on each side. To initiate an experiment with a shock tube, the diaphragm can be ruptured using a plunger with a blade attached, although the mechanism required is complex. Many experiments use a scored diaphragm, designed to rupture when a certain pressure is reached inside the tube, or use combustible gases in the driver to burst the diaphragm.
When the diaphragm in a pipe ruptures, a shock wave, a sudden propagating disturbance, travels in the guided gas. The temperature and pressure of the guided gas also increase, and the shock wave induces aerodynamic flow in the direction of the shock wave, but at a slower speed or velocity. A propagating decrease in pressure, called a rarefaction wave or expansion fan, flows back into the driver gas. The contact patch, the boundary between guided gas and guided gas, travels in the guided gas, just behind the front of the shock absorber, defining the shock wave boundary.
When the shock wave reaches the end of a pipe, it is reflected back into the driver gas causing an even greater increase in temperature, pressure and density. Using a waste 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 sampled and studied to observe the effects of high pressure and temperature. An impact tube can also be used to study the effects of combustion on solid particles, which are pushed into the tube before the diaphragm ruptures.
A shock tube can also be used to study the aerodynamic flow of gas guided behind the shock wave. Combustion occurs very quickly. As a result, there is limited time to observe aerodynamic flow, usually only a few milliseconds.
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