Page 183 - Mechanical Engineer's Data Handbook

FLUID MECHANICS 171

Velocity triangles

Radial velocities: V,, =Q/nb,D, (inlet)
V,, = Q/nbzD, (outlet)
Tangential velocities: VI, = xD, N (inlet)
VI, = nD,N (outlet)
Whirl velocities: V,, =gHqh/Vl, (inlet, usually)
Vw2 =O (outlet, usually)
Guide vane velocity: V, =

vanes
Specific speed of pumps and turbines

It is useful to compare design parameters and charac-
0 teristics of fluid machines for different sizes. This is
done by introducing the concept of 'specific speed',
which is a constant for geometrically similar machines.
Vane and blade angles
4.7.7 Specific speed of pumps and
Guide vanes: a=tan-'V,,/V,, turbines
Blade inlet: B1 =tan- Vrl/( Vll - V,,)
Blade outlet: & =tan- V,,/V,, Symbols used:
Overall efficiency q,, = qmqh N = speed of rotation
Shaft power = pgHQq, Q = flow
Available head H = HI,, - H, - Vf/2g H = head
where: V,=draft tube outlet velocity. P = power
Specific speed of pump N, = - N A
Hi
=
s
Specific speed of turbine N NJ7; ~
H2