Shock tube

A shock tube is a device used primarily to study gas phase combustion reactions. Shock tubes (and related shock tunnels) can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities.

Working

A simple shock tube is a metal tube in which a gas at low pressure and a gas at high pressure are separated using a diaphragm. This diaphragm suddenly bursts open under predetermined conditions to produce a shock wave that travels down the low pressure section of the tube. This shock wave increases the temperature and pressure of the gas and induces a flow in the direction of the shock wave, creating the conditions desired for the testing being done.

The low-pressure gas, referred to as the "driven gas", is subjected to the shock wave. The high pressure gas is known as the "driver gas". The corresponding sections of the tube are likewise called the driver and driven sections. These gases (which do not necessarily need to be the same chemical composition) are pumped into the tube sections or loaded from pressurized gas supply lines, or (if the desired pressure is less than atmospheric) the gas is pumped out of the tube section until the desired pressure is reached. The diaphragm between the tube sections must be strong enough to hold the initial pressure difference but must also burst cleanly to yield good test results.

The test being conducted begins with the bursting of the diaphragm. There are three common methods used to burst the diaphragm.
* A plunger with a bladed cutting edge on the end can be built into the driver tube; actuating the plunger (electrically, hydraulically, or pneumatically) drives the blade through the diaphragm material to burst it. However, this requires a somewhat complex mechanism.
* Another method is to use diaphragms (such as aluminum discs) that have been scored in a cross-shaped pattern to a calibrated depth, designed to rupture when the pressure difference across the diaphragm is the difference specified for the test being conducted.
* A third method is to use a combustible mixture of gasses in the driver; initiating combustion creates a sudden increase in pressure that bursts the diaphragm (shock tubes of this design are said to use a "combustion driver").

After the diaphragm is burst a compression wave travels down the tube into the driven gas, which then rapidly steepens to form a shock front, known as the incident shock wave. This shock wave increases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave (but at lower velocity than the shock wave itself). Simultaneously, a rarefaction wave, often referred to as an expansion fan, travels back in to the driver gas. The circular section that represents the interface separating the experimental (driven) gas and the driver gas is called the contact surface; the contact surface moves rapidly along the tube behind the shock front.

Once the incident shock wave reaches the end of the shock tube, it is reflected back in to the already heated gas, resulting in a further rise in the temperature, pressure and density of the gas. This effectively creates a high temperature and high pressure reaction zone to which the driver gas is subjected. This reaction can be quenched by using a 'dump tank' which swallows the reflected shock wave. The gas samples are then collected from the tube and analyzed.

Applications

Other than study of gas samples at high temperatures and pressures, shock tubes have numerous applications in combustion and aerodynamics studies. Often solid particles may be injected into the driven section of the shock tube prior to the diaphragm burst. The properties of the combustion reaction of these particles resulting from the sudden increase in temperature and pressure due to the shock wave can be analyzed using data collected with pressure transducers and spectrometers.

For aerodynamic testing, the fluid flow induced in the driven gas behind the shock wave can be used much as a wind tunnel is used. Shock tubes allow the study of fluid flow at temperatures and pressures that would be difficult to obtain in wind tunnels (for example, to replicate the conditions in the turbine sections of jet engines). The duration of the testing is limited, though, by the time available between the passage of the shock wave and the arrival of either the contact surface or the reflection of the shock wave off the end of the tube. In practice, this usually limits the available test time to a few milliseconds.

A further development for aerodynamic testing is the shock tunnel, where a nozzle is placed between the end of the tube and a dump tank. As the shock wave reflects off the end of the tube it creates a region of very high pressure and temperature. Since the dump tank is pumped down to a low pressure (near vacuum), there is a very large pressure difference across the nozzle. Using a shock tunnel, very high temperature hypersonic flow can be created in the test section, located immediately behind the nozzle. This allows testing in conditions that can simulate re-entry of spacecraft or hypersonic transport; but again testing time is limited to the order of milliseconds.

See also

* Shock wave
* Expansion fan
* Supersonic wind tunnel
* Hypersonic wind tunnel
* Ludwieg tube
* Light gas gun

External links

* [http://www.eng.rpi.edu/mane/lightcraft/Curriculum/Experimental/ShockTube/elements_1.html RPI Shock Tube page]
* [http://silver.neep.wisc.edu/~shock/tools/gdcalc.html Shock Tube Calculator]
* [http://silver.neep.wisc.edu/~shock/tools/xt.html Shock tube x-t Diagram]

References

* cite book
last = Liepmann | first = Hans W. | coauthors = Roshko, A.
title = Elements of Gasdynamics | origyear = 1957
publisher = Dover Publications | year = 2001
pages = pp. 79 - 83 | id = ISBN 0-486-41963-0

* cite book
last = Anderson | first = John D. Jr.
title = Hypersonic and High Temperature Gas Dynamics | origyear = 1989
publisher = AIAA | year = 2000 | month = August
pages = pp. 368 - 369 | id = ISBN 1-56347-459-X


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