Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

46 Fluid Mechanics, Thermodynamics of Turbomachinery
angle of about 2q = 7deg gives the optimum recovery for both two-dimensional and
conical diffusers.
Diffuser performance parameters
The diffusion process can be represented on a Mollier diagram, Figure 2.12b, by the
change of state from point 1 to point 2, and the corresponding changes in pressure and
velocity from p1 and c 1 to p 2 and c 2. The actual performance of a diffuser can be
expressed in several different ways:
(i) as the ratio of the actual enthalpy change to the isentropic enthalpy change;
(ii) as the ratio of an actual pressure rise coefficient to an ideal pressure rise coefficient.
For steady and adiabatic flow in stationary passages, h01 = h 02, so that
For the equivalent reversible adiabatic process from state point 1 to state point 2s,
Diffuser efficiency, hD, also called diffuser effectiveness, can be defined as
(2.44a)
(2.44b)
(2.45a)
In a low speed flow or a flow in which the density r can be considered nearly constant,
so that the diffuser efficiency can be written
(2.45b)
Equation (2.45a) can be expressed entirely in terms of pressure differences, by writing
then, with eqn. (2.45a),
Alternative expressions for diffuser performance
(i) A pressure rise coefficient Cp can be defined
where q1 = 1 – 2 rc 2 1.
(2.46)
(2.47a)