Page 60 - Fluid Mechanics and Thermodynamics of Turbomachinery

Basic Thermodynamics, Fluid Mechanics: Definitions of Efficiency 41
then

(2.39)
t D p R H
which establishes the connection between polytropic efficiency, reheat factor and
turbine isentropic efficiency.

Nozzle efficiency

In a large number of turbomachinery components the flow process can be regarded
as a purely nozzle flow in which the fluid receives an acceleration as a result of a
drop in pressure. Such a nozzle flow occurs at entry to all turbomachines and in the
stationary blade rows in turbines. In axial machines the expansion at entry is assisted
by a row of stationary blades (called guide vanes in compressors and nozzles in
turbines) which direct the fluid on to the rotor with a large swirl angle. Centrifugal
compressors and pumps, on the other hand, often have no such provision for flow
guidance but there is still a velocity increase obtained from a contraction in entry
flow area.

1.08
h p = 0.7

0.75
Reheat factor, R H 1.04 0.8 0.85

0.9

1.0
2 4 6 8 10 12
Pressure ratio, p /p
1 2
FIG. 2.11. Relationship between reheat factor, pressure ratio and polytropic efficiency
(n D 1.3).