(SEM VI) THEORY EXAMINATION 2018-19 FLUID MACHINERY

FLUID MACHINERY (RME-601)

B.Tech Semester VI – Section-wise Important Answers

 SECTION A

(Attempt all – 2 marks each)

a) Difference between Fluid Mechanics and Fluid Machinery

Fluid mechanics is the branch of engineering that deals with the behavior of fluids at rest and in motion. It focuses on theoretical aspects such as fluid statics, kinematics, and dynamics.
Fluid machinery, on the other hand, is the applied branch of fluid mechanics that deals with machines like pumps, turbines, compressors, and hydraulic devices which convert energy from one form to another using fluids.

b) Why are turbines more efficient than pumps?

Turbines convert hydraulic energy into mechanical energy with minimal losses because water flows naturally from high head to low head. Pumps require external power to raise fluid pressure and overcome losses like friction, leakage, and mechanical inefficiencies. Hence, turbines generally show higher efficiency compared to pumps.

c) What is whirl velocity in a turbine? Explain its significance

Whirl velocity is the tangential component of absolute velocity of water at the inlet or outlet of a turbine runner.
Its significance lies in the fact that only whirl velocity contributes to work done on the turbine. If whirl velocity is zero, no power is produced even if flow velocity exists.

d) Why do impulse turbines need high head and low discharge while reaction turbines need low head and high discharge?

Impulse turbines operate on kinetic energy of water, which requires high head to generate high jet velocity. Reaction turbines operate on both pressure and velocity energy and hence work efficiently with low head but higher discharge.

e) What is NPSH? Why is it required?

Net Positive Suction Head (NPSH) is the difference between absolute pressure head at pump suction and vapor pressure head of the fluid.
It is required to avoid cavitation, which causes vibration, noise, and damage to pump components.

f) Why is a reciprocating pump called a positive displacement pump?

A reciprocating pump delivers a fixed quantity of liquid for each stroke of the piston, regardless of discharge pressure. Since fluid is positively displaced from the cylinder, it is called a positive displacement pump.

g) Why is an accumulator used in a hydraulic system?

An accumulator stores hydraulic energy in the form of pressurized fluid. It helps in absorbing pressure fluctuations, supplying extra fluid during peak demand, and improving overall system efficiency.

SECTION B

(Attempt any three – long answers)

1) Centrifugal Pumps in Series and Parallel

Centrifugal pumps are connected in series when high head is required. In this arrangement, discharge of one pump enters the next pump, increasing total head.
They are connected in parallel when large discharge is required. Here, discharge increases while head remains nearly constant.
Common operational difficulties include cavitation, priming failure, air leakage, and vibration. Remedies include proper NPSH, correct priming, sealing joints, and balancing impellers.

2) Effect of Acceleration of Piston in Reciprocating Pump

Due to acceleration and retardation of piston, fluid in suction and delivery pipes experiences additional pressure head known as acceleration head.
Acceleration causes pressure to drop during suction and increase during delivery. Mathematical expressions for acceleration head are derived using Newton’s second law and depend on piston acceleration, pipe length, and fluid density.

3) Torque Converter – Construction and Working

A torque converter transmits and multiplies torque using fluid coupling. It consists of an impeller, turbine, stator, and casing.
Working principle involves redirection of fluid flow by stator, increasing torque output at low speeds.
It is widely used in automobiles due to smooth power transmission, though efficiency reduces at high speed ratios.

SECTION C

(Attempt any one from each question group)

1) Jet of Water and Efficiency of Curved Vanes

A jet of water is a high-velocity stream issuing from a nozzle. When it strikes curved moving vanes, change in momentum produces force and work.
Efficiency depends on inlet and outlet velocities, vane speed, and angle of deflection. Maximum efficiency occurs when outlet whirl velocity is zero.

2) Cavitation in Hydraulic Machines

Cavitation occurs when pressure of liquid falls below vapor pressure, forming vapor bubbles that collapse violently.
Effects include noise, vibration, pitting, loss of efficiency, and damage to runner blades.
It can be avoided by increasing suction head, reducing temperature, and proper turbine installation.
Thoma’s cavitation number expresses cavitation tendency of turbines.

3) Priming of Centrifugal Pump

Priming removes air from pump casing and suction pipe before starting. Without priming, centrifugal pumps cannot create suction.
Similarity parameters such as head, discharge, speed, and power ratios help in predicting performance of geometrically similar pumps.

4) Hydraulic Press – Construction and Working

A hydraulic press works on Pascal’s law. It consists of two cylinders with different cross-sectional areas connected by fluid.
Small force applied on small piston produces large force on bigger piston.
Applications include forging, pressing, and metal forming industries.