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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.
c = discharge coefficient (unitless). This is usually 1.0 if using a diffuser. If using a wand to measure the stagnation pressure, the coefficient value depends on the shape of the flow hydrant orifice. A smooth and rounded outlet has c=0.9, a square and sharp outlet has c=0.8, and a square outlet which projects into the barrel has c=0.7.
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
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.
Deviations from theoretical expectation can be assumed under the Coefficient of Discharge. Thus, one can manufacture an orifice meter of known uncertainty with only the measurement standard in hand and access to a machine shop. The need for flow conditioning, and hence, a fully developed velocity flow profile is driven from the original ...
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.
The coefficient of discharge of Venturi meter ranges from 0.93 to 0.97. The first large-scale Venturi meters to measure liquid flows were developed by Clemens Herschel , who used them to measure small and large flows of water and wastewater beginning at the very end of the 19th century.
The proportionality coefficient is the dimensionless "Darcy friction factor" or "flow coefficient". This dimensionless coefficient will be a combination of geometric factors such as π, the Reynolds number and (outside the laminar regime) the relative roughness of the pipe (the ratio of the roughness height to the hydraulic diameter).