Choked Flow

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Choked flow
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When a flowing fluid at a certain pressure and temperature flows through a restriction (such as the hole in an orifice plate or a valve in a pipe) into a lower pressure environment, under the conservation of mass the fluid velocity must increase as it flows through the smaller cross-sectional area of the restriction. At the same time, the Venturi effect causes the pressure to decrease.

Choked flow is a limiting condition which, if the flowing fluid is a gas, occurs when the gas velocity traveling through the restriction increases to the speed of sound in the gas. At that point, the gas velocity becomes independent of the downstream environment pressure
(i.e, lowering the downstream pressure will not increase the gas
velocity any further). The physical point at which the choking occurs
(i.e., the cross-sectional area of the restriction) is sometimes called
the choke plane. It is important to note that although the linear velocity of the gas becomes choked, the mass flow rate of the gas can still be increased by increasing the upstream pressure.

The choked flow of gases is useful in many engineering applications because the mass flow rate is independent of the downstream pressure, depending only on the temperature and pressure on the upstream side of the restriction. Under choked conditions, valves and calibrated orifice plates can be used to produce a particular mass flow rate.

If the fluid is a liquid, a different type of limiting condition (also known as choked flow) occurs when the Venturi effect acting on the liquid flow through the restriction decreases the liquid pressure to below that of the liquid vapor pressure at the prevailing liquid temperature. At that point, the liquid will partially "flash" into bubbles of vapor and the subsequent collapse of the bubbles causes cavitation. Cavitation is quite noisy and can be sufficiently violent to physically damage valves, pipes and associated equipment. In effect, the vapor bubble formation in the restriction limits the flow from increasing any further.[1][2] Received on Fri Jan 12 01:20:00 2007