5. Discussion of Results

Analysing the charts produced as

a result of the experiments conducted for the two turbines (Francis and Pelton

turbines), it can be seen that the first part of the line is missing because the measured data

were not obtained in the required volume This is

explained by the fact that the experiments have been affected by some of the errors

that occurred during the experiment. This affected the data recorded for these

experiments. In these experiments, the interval data load for both springs was large;

in order to get a proper and complete diagram, it is needed to start with only

a small load on the spring and increase this load in small increments.

5.1. Francis turbine

1)The

maximum torque is obtained at the minimum rotor speed. It was observed from the

results of the laboratory work that when the rotor speed increases then torque will

be reduced, as shown in figure 3. At a guide vane setting of 20, the highest

torque is 0.219 Nm while the rotor speed is 65.8 Hz, which is the lowest. This reduction

in torque is due to the fact that the relationship between these two parameters

is one of inverse proportionality. This is evident from the equation given

below (1).

Pb=2?Tn;

T=Pb/(2?n) (1).

2) It can be seen

from figure 4 that the maximum power output is increased with a rising rotor

speed. This continues until the power output reaches its maximum. Any further

rise in rotor speed does not contribute to an increase in the power output. The

maximum power output is 90.54 W at 65.8 Hz rotor speed. It is the lowest rotor

speed at a guide vane setting of 20. However, the power output trend is decreased.

3) The maximum

efficiency (58.1%) is obtained at a 200 guide vane angle with the

same speed (65.8 Hz) at which power output is the greatest. It is evident from

formula (1) that the efficiency of the turbine and the power output have a

proportional relationship. This means that if the power output of the turbine

is increased then the efficiency of the turbine increases accordingly. In addition, from

figure 5 it can be seen that the turbine efficiency is equal to zero when the

rotor speed is at a maximum. This is due to the fact that at the maximum rotor

speed the turbine works in idle, therefore there is no power output from the

turbine. There is a flow of water, however, and the turbine rotor will still

spin. Electricity is not produced because of the absence of torque or the load

of the turbines.

4) When the flow

rate is reduced, the power output and the maximum efficiency are increased,

then both decrease. The maximum efficiency and power output of the turbine are

not directly dependent on the water flow.

5) Analysing the

results obtained from the experiment using the Francis turbine, it can be seen

that the optimum conditions are a rotor speed of 65.8 Hz, a flow rate of 0.014

m3/sec, a torque of 0.219 and a 200 guide vane setting,

which allows for maximum turbine efficiency (58.1%) and maximum power output

(90.54 W).

5.2. Pelton turbine

1) As shown figure 7, the maximum torque is

obtained at the minimum rotor speed. The highest torque is equal to 0.30 Nm at a

rotor speed of 17.83 Hz for the lowest inlet head, which is 10 mH20. There

are two reasons that could explain the relationship between torque and rotor

speed. Firstly, it could be due to the fact that the

relation between these two parameters is one of inverse proportionality. This

is evident from equation (1). Secondly,

according to the equation T = ?Qvr (2) (source data:

Pelton), it can be seen that the determination of the torque that the

rotor speed is not involved. As can be seen from the formula, the torque

depends on the speed of the flow. Therefore, their independence from each other

is clear.

2) From figure 8,

it is noticeable that, in general, the maximum power output increases with rising

rotor speed. This continues until maximum power output is reached. Any further

rise in rotor speed does not contribute to an increase in the power output, but

rather decreases it.

The maximum power

output 34.29 W, which is obtained from the turbine at a 26 Hz rotor speed while

the inlet head is 15 mH20 (this is taken from the results table

because it is not available from the graph).

3) The maximum

efficiency is not obtained at the same rotor speed at which maximum power

output is obtained. The maximum turbine efficiency (61.65%) is obtained at a rotor

speed of 17.83 Hz at the maximum flow rate (0.000556 m3/sec) while

the inlet head is 15 mH20. “Since the input hydraulic power depends

only on the head and the nozzle area and is independent of the Pelton Wheel

speed then the efficiency is directly proportional to the power output and thus

maximum power and maximum efficiency occur at the same conditions” (PeopleRitEdu,

n.d.).

4) When the flow

is reduced, the maximum efficiency and the maximum power output rise at a 10 mH20

inlet head. “Pelton turbines/wheels are suitable for power extraction when the

water energy is available at high head and low flow rate” (Learning

Engineering, n.d.).

5) Analysing the

results obtained from the experiment with the Pelton turbine, it can be seen

that the optimum conditions for the Pelton turbine are a rotor speed of 17.83

Hz, a flow rate of 0.00056 m3/sec, a torque of 0.3 Nm and 10 mH20

inlet head, which is the maximum for obtaining the maximum turbine efficiency

(61.65%).

6. Conclusion

Two experiments were

conducted using the Francis and Pelton turbines. The purpose for conducting

these two experiments was to examine the operation of the Francis and Pelton

turbines and determine their operating characteristics.

In order to reduce

the rotor speed turbine from its maximum speed to its minimum, a Prony brake

dynamometer was used for both turbines. The torque was measured by the turbine

rotor in different stages. A spear valve was used to vary the volume flow rate

through Pelton turbine.

The data obtained from

the experiments were used to plot graphs of torque, brake power and overall

efficiency versus rotor speed to demonstrate the operating characteristics of

the Francis and Pelton turbines.

From analysing the

charts produced as a result of these experiments, it was noted that during the experiments,

the measured data were not obtained in the required volume. This was perhaps

due to the intervals between the stages of measurement, which were perhaps not

optimal. In turn, this prevented us from conducting the complete analysis of

almost all graphs, which were effectively halves due to the lack of relevant

data.

However, using the

available data obtained as measured and calculated, graphs were constructed for

the two turbines and the optimum conditions for their operation were determined.

The optimum operating

conditions for the Francis turbine are a rotor speed of 65.8 Hz, a flow rate of

0.014 m3/sec, a torque of 0.219 Nm and a 200 guide vane

setting, which is the maximum for obtaining the maximum turbine efficiency

(58.1%) and the maximum power output (90.54 W).

For the Pelton

turbine, optimum conditions are a rotor speed of 17.83 Hz, a flow rate of

0.00056 m3/sec, a torque of 0.3 Nm and a 10 mH20 inlet

head, which is the maximum for obtaining the maximum turbine efficiency

(61.65%).