Impulse turbine fluid mechanics

IMPULSE TURBINE

SUBMITTED TO- SUBMITTED BY-Mr.ABHISHEK CHAUHAN KANWALDEEP SINGH SG12927 MECHANICAL 5THSEM

IMPULSE TURBINEIMPULSE TURBINEImpulse turbine is the one in which the available hydraulic energy is first converted into kinetic energy by means of efficient nozzle.High velocity jet issuing from the nozzle then strikes a series of buckets fixed around the rim of wheel(runner).The buckets change the direction of jet without changing its pressure.The resulting change in momentum sets bucket and wheel into rotary motion and thus mechanical energy is made available at the turbine shaft.

PELTON WHEEL TURBINEPELTON WHEEL TURBINEA pelton wheel is a free jet impulse turbine

named after the American engineer Lester Pelton (1829-1908) .It is simple and the only hydraulic turbine ,which operates efficiently and is invariably used for heads in excess of 450m . Smooth running and good performance are the other common features of this unit.

DESCRIPTION OF PELTON IMPULSE DESCRIPTION OF PELTON IMPULSE TURBINETURBINE

PENSTOCK- It is a large size conduit which conveys water from high level reservoir to the turbine . The penstock may be of wood ,concrete ,or steel. SPEAR AND NOZZLE-Nozzle is used to convert hydraulic energy into kinetic energy .Spear is so arranged that it can move forward or backward there by decreasing or increasing the annular area of nozzle passage.

CASING-Casing is provided to prevent strong splash of water ,which scatter in all directions and to guide the water to the tail race.

This casing also acts as a safeguard against accidents.

RUNNER WITH BUCKET- The turbine rotor, called the runner , is a circular disk carrying a number of cup shaped buckets which are arranged equidistantly around the periphery of the disk .

For low heads the buckets are made of cast iron ,but for higher heads they are made of bronze ,cast steel,or stainless steel.

BREAKING JET-when the nozzle is completely closed ,the amount of water striking the runner reduces to zero. But the runner due to inertia goes on revolving for long time . To stop the runner in short time , a small nozzle is provided which directs the jet of water on the back of vanes . This jet of water is called breaking jet.

GOVERNING MECHANISMGOVERNING MECHANISM

Speed of the turbine runner is required to be maintained constant so that the electric generator coupled directly to the turbine shaft runs at constant speed under varying load conditions.

Design of Pelton turbinesDesign of Pelton turbines

NUMBER OF JETSNUMBER OF JETS

Generally a pelton wheel has one nozzle or one jet . However a number of nozzles may be employed when more power is to be produced with the same wheel .The nozzles are spaced evenly around the same runner.Theoretically 6 nozzles can be used with one pelton wheel.Practical considerations ,however ,limit the

use of not more than 2 jets per runner for a vertical runner, and not more than 4 per runner when it is in horizontal position.

NUMBER OF BUCKETSNUMBER OF BUCKETS The number of buckets should be few as possible

so that there is little loss due to friction. The jet of water must be fully utilized so that no

water from the jet goes waste i.e no water escapes without striking the buckets .

Z=15+D/2d

DEPTH AND WIDTH OF BUCKETSWidth of bucket =5dDepth of bucket =1.2d

JET RATIOJET RATIO

m represents the ratio of the pitch circle diameter to the jet diameter. (m=D/d)

For maximum hydraulic efficiency ,the jet ratio lies between 11 and 15 and normally a jet ratio of 12 is adopted.

A bulky installation results when a larger value of jet ratio is adopted.

Its value is nearly equal to 12.

VELOCITY TRIANGLESVELOCITY TRIANGLESThe inner velocity triangle is drawn at

the splitter and outlet velocity triangle is drawn at the outer edge of the bucket

H=net head acting on the pelton wheel

=Hg -Hf , where Hg=gross head and hf=4fLV2/D*2g

Where D*=dia of penstock, N =speed of the wheel in rpm

D=dia of wheel, d=dia of the jet THEN V1=velocity of jet at inlet=sq. root2gH

u=u1=u2=3.14DN/60

the velocity triangle at inlet will be a straight line where

Vr1=V1-U1=V1-u

Vw1=V1

POWER POWER WATER POWER-The power supplied by the water jet is

called as water power, WP.Power supplied by jet=wQH=pgQHwhere Q is the discharge supplied by water

jet in cubic meter per sec,and H is the net or effective head in .

BUCKET POWER-The power developed at the turbine

buckets is called as bucket power,BP.The power delivered by the bucket wheel

is=m(Vu1+Vu2)u

EFFICIENCYEFFICIENCYOVERALL EFFICIENCY=power available at the turbine

shaft/power available from the water jet

n0= shaft power/water power=P/wQH HYDRAULIC EFFICIENCY=work done per sec/k.e per secondnh=((Vu1+Vu2)u)/gH

VOLUMETRIC EFFICIENCY=Volume of water actually striking the

buckets/total water supplied by the jet =nv =Qa/Q=Q-q/Q

MECHANICAL EFFICIENCY=shaft power/water power=P/wQaH