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In a nozzle or other constriction, the discharge coefficient (also known as coefficient of discharge or efflux coefficient) is the ratio of the actual discharge to the ideal discharge, [1] i.e., the ratio of the mass flow rate at the discharge end of the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressures.
Orifice plate showing vena contracta. An orifice plate is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice [1] but as the fluid is forced to converge to pass through the hole, the velocity increases and the fluid pressure decreases.
Note: the Strickler coefficient is the reciprocal of Manning coefficient: Ks =1/ n, having dimension of L 1/3 /T and units of m 1/3 /s; it varies from 20 m 1/3 /s (rough stone and rough surface) to 80 m 1/3 /s (smooth concrete and cast iron). The discharge formula, Q = A V, can be used to rewrite Gauckler–Manning's equation by substitution for V.
The orifice plate formula in this article is an adaptation of the formula for a Venturi meter, where a pressure tap does exist in a plane of maximum fluid velocity. The formula below the line "Solving for Q:" in this article is the same as the last formula in this section: Venturi effect#Flow_rate .
The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate. Mathematically the flow coefficient C v (or flow-capacity rating of valve) can be expressed as
Paschen's law is an equation that gives the breakdown voltage, that is, the voltage necessary to start a discharge or electric arc, between two electrodes in a gas as a function of pressure and gap length. [2] [3] It is named after Friedrich Paschen who discovered it empirically in 1889. [4]
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For low viscosity liquids (such as water) flowing out of a round hole in a tank, the discharge coefficient is in the order of 0.65. [4] By discharging through a round tube or hose, the coefficient of discharge can be increased to over 0.9. For rectangular openings, the discharge coefficient can be up to 0.67, depending on the height-width ratio.