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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
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.
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.
API 14.3 (1990) and ISO standards determined the Coefficient of Discharge by completing numerous calibration tests where the indicated mass flow was compared to the actual mass flow to determine coefficient of discharge. In all testing the common requirement was a fully developed flow profile entering the orifice plate. [8]
If G represents stage for discharge Q, then the relationship between G and Q can possibly be approximated with an equation: Q = C r ( G − a ) β {\displaystyle Q=C_{r}(G-a)^{\beta }} where C r {\displaystyle C_{r}} and β {\displaystyle \beta } are rating curve constants, and a {\displaystyle a} is a constant which represents the gauge ...
The volumetric discharge through the stream-bed can be calculated if the difference in hydraulic head is known: = where is the volumetric discharge through the stream-bed ([L 3 T −1]; m 3 s −1 or ft 3 day −1) is the hydraulic head of the river (elevation stage)