Page 122 - Mechanical Engineer's Data Handbook

THERMODYNAMICS AND HEAT TRANSFER 111

3.5 Flow through nozzles

Nozzles are used in steam and gas turbines, in rocket nozzle’, where the flow is subsonic; and the ‘conver-
motors, in jet engines and in many other applications. gent divergent nozzle’, for supersonic flow.
Two types of nozzle are considered: the ‘convergent

Symbols used:
p = inlet pressure
p, =outlet pressure
p, =critical pressure at throat
u I = inlet specific volume
u2 =outlet specific volume
C, =outlet velocity
C, = throat velocity

P2
r = pressure ratio = -
P1
Note that C, is independent of p2 and that the nozzle
P
rc =critical pressure ratio = 2 flow is a maximum. In this case the nozzle is said to be
P1 ‘choked’.
A, =outlet area
A, = throat area
n =index of expansion 3.5.2 Convergent-divergent nozzle
h=mass flow rate
In this case:
Critical pressure ratio r, = -
(n: I)(*) Throat velocity C, = /m n+ 1

3.5. I Convergent nozzle Throat area A, = -, mu, r, =
CArJ
Outlet pressure p2 greater than p,. i.e. r>r,
Outlet velocity C,= /= -
Outlet velocity C,= /= -
mV
Outlet area A,=L
mv 1 C,(rf
Outlet area A,=7
C,(r).
Outlet pressure p2 equal to or less than pc,
i.e. r<r,

mu
Outlet area A,=+ Values of the index n and the critical pressure ratio r,
C*(rF for different fluids are given in the table.