Francis turbine

Francis turbine is an inward flow reaction turbine having radial discharge at outlet is known as Francis turbine.

In modern Francis turbine, the water enters the runner of the turbine in the radial direction at the outlet and leaves in the axial direction at the inlet of the runner.

Construction and working

Francis turbine is a mixed flow turbine. The test ring consists of Runner A ring of adjustable guide vanes A scroll casing Draft tube

The runner consist of two shrouds with runner of curved vanes in between the guide vanes can be rotated about their axis by means of a hand wheel. Guide vanes fixed out side turbine casing.

A 15 h.P centrifugal pump supplies water under pressure to the Francis turbine. The flow from the pump is controlled by means of a gate valve.

The water is supplied to the runner through the guide vanes while passing through the spiral casing and guide vanes, the portion of the pressure head is converted into viscosity energy.

When the water passes through the vanes of the runner the velocity energy is transformed into the mechanical energy and hence the runner discharges water into the tail race through the conical draft tube.

This conical draft tube is provided for the purpose of regaining the kinetic energy from the exit water and also facilitating easy accessibility of the turbine due to its location at a higher level than the tail race.

A transparent hollow cylinder is provided between the draft bend and the casing for purpose of observation of flow at the exit of runner.

The output of the turbine can be controlled by adjusting the guide vanes for which a hand wheel and a suitable link mechanism is provided. The net supply head on the turbine is measured by a pressure and vacuum gauge and for the measurement of speed a hand tachometer is used.

A belt is provided over a drum to load the turbine tension in the belt is increased with the help of hand wheel and tension is measured by using spring balance.

Experimental setupIt consist of a Francis turbine to which water under

requisite head is supplied by means of a centrifugal pump. A sluice valve is provided near the outlet end of supply pipe connected to the turbine. a pressure gauge and vacuum gauge measures the pressures head at the inlet and outlet end of the turbine simultaneously. A brake drum is coupled with the Francis turbine to which the load is applied by tightening the belt wrapped round the drum. one spring balance on each end of belt measures the load applied. The speed of running of turbine is measured by means of techno meter. The turbine discharges in the tail race channel which is measured by means of a V-notch.

Procedure1. Prime the pump and start the electric motor to run the

pump.2. Open the delivery valve and allow the water to stand just

up to the crest of v-notch and then close the valve. Note the initial readings of gauge over the notch.

3. Set the full gate (col.2) and open the delivery valve.4. Apply different sets of loads W1 and W2 on the turbine

shaft (col.10 and 11)Note: it can be done by adjusting the weights W1 and W2 at the ends of belt.

5. Note the speed n (r.P.M), gauge reading over notch, pressure head at the spiral casing (col.3) and the pressure at the entrance to the draft tube (col.4) every time the loads are changed.

6. Change the gate opening and repeat the procedure.

Calculations

The discharge flowing through the turbine is calculated as Q = CHn (col.8)

The total net head H under which the turbine is workingH = (positive pressure head)- (negative

suction head at the entrance to draft tube)

The output power is then computed as P= 2πRN(W1 - W2)/4500

The input power to the turbine is then computedInput power = wQH/75

The overall efficiency of the turbine is given asN0 = output power/nput power

Finally the unit quantities are calculatedPu = P/H3/2; Qu=Q/H; Nu=NH

Formula used

1. Discharge = 2/3.Cd√(2g)(l-0.1nH)H3/2

Where , Cd = coefficient of discharge H = head over the notch L = length of the notch

2. Total Head = G+V+X

Where , G = pressure gauge V = vacuum gauge X = height difference between pressure gauge

and vacuum gauge

3. Input of turbine

Input = w.Q.H

75In H.P

4. Output :-

Brake drum diameter = 0.3mSpring load on tight side = T1KgSpring load on slack side = T2 KgT1 and T2 should be taken after subtracting readings at no load

Resultant load = T1 = T2 =T KgB.H.P = πDNT 75x60 where N= speed of turbine in r.p.m

H.P

Efficiency = Output = B.H.P Input I.H.P

Unit speed (Nu) = N/√H

Unit power (Pu) = P/H

Unit discharge(Qu) = Q/√H

Specific speed (Ns) = N√P/H5/4

or Ns = N√Q/H3/4

Observation Table

S.no Gate

opening

Head

reading

Discharge

reading

Pressure

gauge

Vacuum

gauge

H

m

Final

gauge

H

m

Q

m3/s

1 2 3 4 5 6 7 8

Diameter of brake drum = 30cm.Length of rectangular notch = 30cm.

N W1

Kg

W2

Kg

P NO Qu Pu Nu

9 10 11 12 13 14 15 16

Presentation of result

In preparing characteristic curves, it is found convenient to determine unit quantities and then plot graphs between Qu, Pu and efficiency N0 versus Nu, each on separate graph paper for different gate openings. These are known as characteristics curves of the given turbine.

Viva voce

Describe main constructional and principal features of a Francis turbine. What way is it different from pelton wheel and Kaplan turbine?When the load on the shaft changes, why does discharge entering the turbine change and how ?What is the purpose of a draft tube? Why should it be divergent? Why can’t we have a greater angle of divergence? Why can’t we extend the length of the tube more and more? What will happen if some air leaks in the entrance of the draft tube?What are characteristics curves of a turbine and what are their uses.?What are unit power and unit speed?What is the function of spiral casing of the Francis turbine?