Page 149 - 48Fundamentals of Compressible Fluid Mechanics

6.1. NOZZLE EFFICIENCY
111

Approaching the shock location from the upstream (entrance) yields

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3

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3( !(

Note, as “simple” check this value is larger than the value in the previous example.

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6.1 Nozzle efficiency

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Obviously nozzles are not perfectly efficient and there are several ways to define
the efficiency of the nozzle. One of the effective way is to define the efficiency as
the ratio of the energy converted to kinetic energy and the total potential energy
could be converted to kinetic energy. The total energy that can be converted is
during isentropic process is
(6.1)

is the enthalpy if the flow was isentropic. The actual energy that was

used is
where
(6.2)

The efficiency can be defined as
(6.3)

/

The typical efficiency of nozzle is ranged between 0.9 to 0.99. In the literature

some define also velocity coefficient as the ratio of the actual velocity to the ideal

(6.4)

There is another less used definition which referred as the coefficient of discharge

velocity,
as the ratio of the actual mass rate to the ideal mass flow rate.
(6.5)

6.1.1 Diffuser Efficiency