Fluid Machinery - 2 - Mechanical Engineering MCQ

For same specific speed and working under the same head i.e. NS1=NS2&H1=H2

N₂ = 2400 RPM

The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. The water strikes the bucket along the tangent of the runner. The shape of the buckets is of double hemispherical cup or bowl. Each bucket is divided into two symmetrical parts by a dividing wall known as splitter. The jet of water strikes on the splitter. The splitter divides the jet into two equal parts and the jet comes out at the outer edge of the bucket. The buckets are shaped in such a way that the jets gets deflected through 160 ° or 170 °. 

Specific speed: It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW. The specific speed is useful to compare the performance of various type of turbines. The specific speed differs for different type of turbines and is same for the model and actual turbine.

1. The specific speed of Pelton wheel turbine (single jet) is in the range of 10-35

2. The specific speed of Pelton wheel turbine (multiple jets) is in the range of 35-60

3. The specific speed of Francis turbine is in the range of 60-300.

4. The specific speed of Kaplan turbine is greater than 300.

Specific speed: It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW. The specific speed is useful to compare the performance of the various type of turbines. The specific speed differs for the different type of turbines and is the same for the model and actual turbine.

Following are the range of specific speed of different turbines

• The specific speed of Pelton wheel turbine (single jet) is in the range of 10-35
• The specific speed of Pelton wheel turbine (multiple jets) is in the range of 35-60
• The specific speed of Francis turbine is in the range of 60-300.
• The specific speed of Kaplan/propeller turbine is greater than 300.

So the Kaplan turbine has the highest specific speed.

Cavitation is the formation of gas bubbles of a flowing liquid in a region where the pressure of liquid falls below its vapour pressure i.e. low pressure.

The regulating valve is provided on the delivery pipe. It regulates the flow of liquid after its exit from the pump. It is usually a sluice type of valve.

Another form of positive acting rotary pump is the single - screw extruder pump typified by the Mono pump. It has a specially shaped metal helical rotor revolves eccentrically within a resilient rubber or plastic double helix, thus creating a continuous forming cavity which processes towards the discharge of the pump. This type of pump is suitable for pumping slurries and pastes, weather Newtonian or Non - Newtonian in character.

For lifting water to the turbine, A pump similar to a turbine pipe is used but it works by redirecting water back down to the intake to help lift the water.

While, centrifugal pumps are used to transport fluids by the conversion of rotational kinetic Energy to the hydrodynamic energy of the fluid flow. Reciprocating pumps are not used in turbine function.

Centrifugal Pump is a hydraulic machine which converts mechanical energy into hydraulic energy (pressure energy) by the action of centrifugal force acting on the fluid.

Rotation of a centrifugal pump impeller causes the liquid it contains to move outwards from the centre to beyond the circumference of the impeller. The revolving liquid is impelled around the periphery of the impeller by centrifugal effect. Displaced liquid in moving from the casing to the delivery pipe, causes flow in the discharge side of the system.

Impulse turbine: At the inlet of the turbine, the energy available is only kinetic energy

Example: Pelton Turbine

Reaction Turbine: At the inlet of the turbine, the water possesses kinetic energy as well as pressure energy

Example: Francis Turbine, Kaplan Turbine

A screw pump is a type of positive displacement pump that uses two or more screws that intermesh to pressurize fluids and move them in a system. The screws take in fluid then push it out from the other side while increasing its pressure. In offshore and marine installations, a three-spindle screw pump is often used to pump high-pressure viscous fluids.

Axial flow pumps have poor suction capability, so require more starting power. They are started against an open valve to minimize the starting power required.

If the discharge of a centrifugal pump is throttled then its suction lift increases. Because of throttling operation, flow rate of centrifugal pump will decrease. As it is assumed that liquid level inside pump is same so hydrostatic pressure is also same. Due to decrease in flow rate, velocity head loss as well as frictional loss decreases, by applying the Bernoulli’s equation, pump static suction pressure will increase.

In centrifugal pump backward curve vanes are preferred.

Pelton wheel is a kind of impulse turbine. As in case of impulse turbine pressure throughout turbine is constant and equal to atmospheric pressure, so only energy available for turbine is kinetic energy of fluid. There is no change in pressure energy as it takes place in reaction turbine.

Hydraulic ram is used to lift small quantity of water to greater height from large quantities of water available at smaller heights.

= 188.49 m/s

Blade speed ratio = 0.42

Inlet velocity will be = 188.49/0.42 = 448.78 m/s

The fluid or hydraulic coupling is a device used for transmitting power from driving shaft to the driven shaft, running at different speed, with the help of fluid. There is no mechanical connection between the shafts.

The draft tube is a conduit which connects the runner exit to the tail race where the water is being finally discharged from the turbine. The primary function of the draft tube is to convert kinetic head into pressure head  there by increasing effctive head and efficiency. 

Hydraulic efficiency of Pelton wheel=  

Here, k = friction factor

u = blade speed/bucket sp eed

v = jet speed

α = blade angle

For maximum efficiency the bucket speed should be halt of jet speed i.e.

Thus, respective maximum efficiency is 

And, if bucket is frictionless, k = 1