What is "discharge coefficient"

In a nozzle or other constriction, the discharge coefficient (also known as coefficient of discharge) is the ratio of the actual discharge to the theoretical discharge, 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.

Mathematically the discharge coefficient may be related to the mass flow rate of a fluid through a straight tube of constant cross-sectional area through the following equation:

$C_d A = \dfrac{\dot{m}}{\sqrt{{2}{\rho}{\Delta} {P}}}$

Where:
C = Discharge Coefficient through the constriction (unit-less).
A = Cross-sectional area of flow constriction (unit length squared).
$\dot{m}$ = Mass flow rate of fluid through constriction (unit mass of fluid per unit time).
ρ = Density of fluid (unit mass per unit volume).
ΔP = Pressure drop across constriction (unit force per unit area).

This parameter is useful for determining the irrecoverable losses associated with a certain piece of equipment (constriction) in a fluid system, or the "resistance" that piece of equipment imposes upon the flow.

This flow resistance, often expressed as a unit-less parameter, k, is related to the discharge coefficient through the equation:

$k = \dfrac{1}{C_d^{2}}$

which may be obtained by substituting ΔP in the aforementioned equation with the resistance, k, multiplied by the dynamic pressure of the fluid, q.