Fluid Mechanics and Hydraulic Machines — Practice

Question 1

At what bucket speed is the hydraulic efficiency of an ideal Pelton wheel maximum?

Accepted Answer

When the bucket speed equals half the jet speed.

Question 2

Which turbine suits a low head and a large flow?

Accepted Answer

The Kaplan turbine.

Question 3

A pump gives 30 L/s at 20 m head at 1000 rpm. Find discharge, head and power ratio at 1500 rpm.

Accepted Answer

Q = 45 L/s, H = 20 x 2.25 = 45 m, and power rises by 1.5^3 = 3.375 times.

Question 4

For air at Mach 2, find T0/T and p0/p.

Accepted Answer

T0/T = 1 + 0.2 x 4 = 1.8 and p0/p = 1.8^3.5 = 7.82.

Question 5

A jet of area 0.002 m2 and speed 30 m/s hits a fixed plate at right angles. Find the force.

Accepted Answer

F = rho a V^2 = 1000 x 0.002 x 900 = 1800 N.

Question 6

A jet with V = 40 m/s and Q = 0.08 m3/s strikes a wheel with bucket speed 20 m/s and full deflection. Find power and efficiency.

Accepted Answer

P = 1000 x 0.08 x 20 x 20 x 2 = 64 kW. Jet power is 0.5 x 1000 x 0.08 x 1600 = 64 kW, so ideal efficiency is 100 percent.

Question 7

A 1 to 25 scale spillway model is tested. Which law applies and what is the velocity scale?

Accepted Answer

Froude similarity, since gravity dominates. Velocity scales as the square root of the length scale, so the model velocity is 1/5 of the prototype.

Question 8

State the condition to avoid cavitation in a pump.

Accepted Answer

NPSH available must exceed NPSH required.

Practice — Fluid Mechanics and Hydraulic Machines