Hydraulic turbine Introduction Fluid machines are the machines that convert the fluid energy to mechanical/electrical energy or vice versa. Hydraulic turbine uses the potential and kinetic energy of water and converts into usable mechanical work. In other words, hydraulic turbine is a prime mover that uses the energy of flowing water and converts it into the mechanical energy (in the form of rotation of the runner). Pump adds energy to fluid while turbine extracts energy from fluid. The mechanical energy thus produced is used to run an electric generator which is directly coupled to the shaft of hydraulic turbine. The hydraulic turbines are also known as ‘water turbines’ since the medium used in them is water. Hydro power is a conventional renewable source of energy which is clean, free from pollution and generally has no harmful effect on environment. However following factors are major obstacles in the utilization of hydropower resources. Large investment Long gestation period, and Increased cost of power transmission Reaction turbines Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow. Newton's third law describes the transfer of energy for reaction turbines.
Transcript
Hydraulic turbine
Introduction
Fluid machines are the machines that convert the fluid energy to mechanical/electrical energy or vice versa.
Hydraulic turbine uses the potential and kinetic energy of water and converts into usable mechanical work. In other words, hydraulic turbine is a prime mover that uses the energy of flowing water and converts it into the mechanical energy (in the form of rotation of the runner).
Pump adds energy to fluid while turbine extracts energy from fluid. The mechanical energy thus produced is used to run an electric generator which is directly coupled to the shaft of hydraulic turbine. The hydraulic turbines are also known as ‘water turbines’ since the medium used in them is water.
Hydro power is a conventional renewable source of energy which is clean, free from pollution and generally has no harmful effect on environment. However following factors are major obstacles in the utilization of hydropower resources.
Large investment Long gestation period, and Increased cost of power transmission
Reaction turbines
Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.
Newton's third law describes the transfer of energy for reaction turbines.
Most water turbines in use are reaction turbines. They are used in low and medium head applications. In reaction turbine pressure drop occurs in both fixed and moving blades.
Impulse turbines
Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.
Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.
Newton's second law describes the transfer of energy for impulse turbines.
Impulse turbines are most often used in very high head applications.
TURBINE EFFICIENCY AT DIFFERENT PART FLOW CONDITIONS
Classification
1. According to the head and quantity of water available
Impulse turbine ...requires high head and small quantity of flow.
Reaction turbine ...requires low head and high rate of flow, medium head and medium flow.
2. According to the name of the originator:
Pelton turbine ...named after Lester Allen Pelton of California (U.S.A). It is an impulse type of turbine and is used for high head and low discharge.
Francis turbine ...named after James Bichens Francis. It is a reaction type of turbine from medium high to medium low heads and medium small to medium large quantities of water.
Kaplan turbine ...named after Dr. Victor Kaplan. It is a reaction type of turbine for low heads and large quantities of flow.
3. According to action of water on the moving blades:
Impulse turbine-Pelton turbine
The pressure of liquid does not change while flowing through the rotor of the machine. In Impulse Turbines pressure change occur only in the nozzles of the machine. One such example of impulse turbine is Pelton Wheel.
Reaction turbine- Francis turbine, Kaplan and propeller turbines
The pressure of liquid changes while it flows through the rotor of the machine. The change in fluid velocity and reduction in its pressure causes a reaction on the turbine blades; this is where from the name Reaction Turbine may have been derived. Francis and Kaplan Turbines fall in the category of Reaction Turbines.
4. According to direction of flow of water in the runner:
Tangential flow turbines (Pelton turbine) the water strikes the runner tangential to the path of rotation.
Radial flow turbine (no more used)
Axial flow turbine (Kaplan turbine) water flows parallel to the axis of the turbine shaft.
Mixed (radial and axial) flow turbine (Francis turbine) the water enters the blades radially and comes out axially, parallel to the turbine shaft. Modem Francis turbines have mixed flow runners.
5. According to the disposition of the turbine shaft:
Turbine shaft may be either vertical or horizontal. In modern practice, Pelton turbines usually have horizontal shafts whereas the rest, especially the large units, have vertical shafts.
6. According to specific speed:
The specific speed of a turbine is defined as the speed of a geometrically similar turbine that would develop 1 kW under 1 m head. All geometrically similar turbines (irrespective of the sizes) will have the same specific speeds when operating under the same head
N s=N P
12
H54
Where,
N = the normal working speed,P = power output of the turbine, andH = the net or effective head in meters.
Turbines with low specific speed turbines works under high head and low discharge conditions, while high specific speed turbines works under low head and high discharge conditions.
Difference between impulse and reaction turbine
Aspects Impulse Turbine Reaction turbine
Conversion of fluid energy
All the available energy of fluid is converted into kinetic energy by nozzle
Only a portion of the fluid energy is converted into KE before fluid enters the runner
Changes in pressure and velocity
The pressure is constant (atmosphere) throughout the action of water on runner.
Water enters the runner with an excess of pressure and then both velocity and pressure change as water pass through runner.
Action of water on blades.
Blades are only in action when they are in front of nozzle
Blades are in action all the time.
Admittance of water over the wheel
Water may be allowed to enter a part or whole of the wheel circumference
Water is admitted over the circumference of the wheel
Water tight casing Water tight casing required Not necessaryExtent to which water fills the wheel/turbine
Runner and blades are not completely filled or covered by water
Water completely fills all the passages between the blades throughout the operation of the turbine
Installation unit Always installed above the tail race. No draft tube is used.
Unit may be installed above or below the tail race – use of draft tube is made.
Relative velocity of water
When water glides over the moving blades, its relative velocity either remains constant or reduces slightly due to friction.
Since there is continuous drop in pressure during flow through the blade passages, the relative velocity increases.
Flow regulation Flow regulated by needle valve fitted into the nozzle
Flow regulated by guide vanes.
Comparison of turbines
Pelton Francis Kaplan
Operating head (m) 100-1700 80-500 Upton 400Maximum power output (MW)
55 40 30
Best efficiency 93 94 94Regulation mechanism Spear nozzle and
deflector plateGuide vanes, surge tank Guide vanes, surge tank
Criteria for selection of turbine
S. N Type of Turbine
Head H(m) Specific Speed(Ns)
Speed ratio (Ku)
Max. Hydraulic
efficiency (%)
Remarks
1 Pelton: 1 Jet2 Jets4 jets
Up to 2000Up to 1500Up to 500
12 to 3017 to 5024 to 70
0.43 – 0.48 89 Employed for high head
2 Francis:High headMed. headLow head
Up to 30050 to 15030 to 60
80 to 150150 to 250250 to 400
0.6, 0.9 93 Full load efficiency high, part load efficiency lower than Pelton
Propeller and Kaplan
4 to 60 300 to 1000 1.4 to 2 93 High part load efficiency; high discharge with low head.
Pelton Turbine
Introduction
It is an impulse turbine, named after Lester Allen Pelton who invented it in 1880. Here pressure energy is converted into kinetic energy when water is passed through nozzle and impulse force on the bucket rotates the runner, producing mechanical energy. The runner is fixed on a shaft, and the rotational motion of the turbine is transmitted to a generator by the shaft.
Pelton turbines are suited to high head (100 – 1700 m), low flow applications. Typically to work this type of turbine, water is piped down a hillside so that at the lower end of the pipe it emerges from a narrow nozzle as a jet with very high velocity. The Pelton turbine can be controlled by adjusting the flow of water to the buckets. In order to stop the wheel a valve is used to shut off the water completely. Small adjustments, necessitated by alterations in the load on the generator, are more safely made by a device which deflects part of the water jet away from the buckets.
Construction
Design of Pelton wheel turbine:
The Pelton Turbine has a circular disk mounted on the rotating shaft or rotor. This circular disk has cup shaped blades, called as buckets, placed at equal spacing around its circumference. Nozzles are arranged around the wheel such that the water jet emerging from a nozzle is tangential to the circumference of the wheel. Number of nozzles placed around the wheel depends upon the available water head and operating requirements.
Component Parts: Construction and Operation
Penstock:
It is a large sized conduit which conveys water from high level reservoir to the turbine. It is made up of wood, concrete or steel. Penstock is provided with control valves to regulate the water flow. Trash racks are provided at inlet to prevent debris from entering into it.
Spear and nozzle:
At the downstream end, penstock is fitted with nozzle that converts hydraulic energy into high speed jet. Spear is provided at the nozzle to regulate water flow and obtain a good jet of water at all loads. Spear is so arranged that it can move forward or backward thereby decreasing or increasing the annular area of the nozzle flow passage.
The movement of spear is controlled either manually by a hand wheel or automatically by governing mechanism.
Runner with buckets:
The runner is a circular disk carrying number of cup-shaped buckets which are placed at equal spacing around its circumference. Runner is generally mounted on the horizontal shaft with bearings and the buckets are either casted integrally with the disk or fastened separately. Buckets are made up of cast iron, bronze or stainless steel. Inner surface of the buckets are polished to reduce frictional resistance to the water jet.
Each bucket has a splitter which distributes the striking jet equally into two halves of hemispherical bucket. There is a cut in the outer rim (notch) of each bucket. This notch is provided to make the jet face the bucket only when it has come into proper position with respect to the jet. This position occurs when face of the bucket and axis of the jet are approximately at 90 degree to each other. Maximum driving force will be exerted on the disk when the jet gets deflected through 180 degree. But it practice angular deflection is limited to about 165-170 degree. This is to ensure that the water jet while leaving one bucket does not strike the back of the succeeding bucket.
Casing:
Outflow from the buckets is in the form of strong splash which scatters in all direction. To prevent this and guide water to tail race, a casing is provided around the runner.
Governing mechanism:
Speed of turbine runner is required to be maintained constant so that the electric generator coupled to the turbine shaft runs at constant speed under varying load condition. This task is accomplished by a governing mechanism that automatically regulates the quantity of water flowing through the runner in accordance with any variation in the load.
Working
The Pelton Turbine has a circular disk mounted on the rotating shaft or rotor. This circular disk has cup shaped blades, called as buckets, placed at equal spacing around its circumference. Water is transferred from a high head source through penstock which is fitted with nozzle, through which the water flows out at high speed jet. A needle spear moving inside the nozzle controls the water flow through the nozzle. All the available potential energy is thus converted into kinetic energy before the jet strikes the bucket of runner. The pressure all over the wheel is constant and equal to atmosphere, so that energy transfer occurs due to purely impulse action.
The Pelton turbine is provided with a casing which prevents the splashing of water and guides the water to tailrace.
When the nozzle is completed closed by moving spear, the water striking the runner is reduced to zero but runner due to inertia continues revolving for a long time. In order to bring the runner to rest in a short time, a nozzle(brake) is provided which directs the jet on the back of the buckets; this jet of water is called braking jet.
The speed of the turbine runner is kept constant by a governing mechanism that automatically regulates the quantity of water flowing through the runner in accordance with variation of load.
Each bucket has a splitter which distributes the striking jet equally into two halves of hemispherical bucket. There is a cut in the outer rim (notch) of each bucket. This notch is provided to make the jet
face the bucket only when it has come into proper position with respect to the jet. This position occurs when face of the bucket and axis of the jet are approximately at 90 degree to each other.
Maximum driving force will be exerted on the disk when the jet gets deflected through 180 degree. But in practice angular deflection is limited to about 165-170 degree. This is done to ensure that the water jet while leaving one bucket does not strike the back of the succeeding bucket. The inlet angle of the jet is in between 1 degree and 3 degree but it is assumed zero in all calculations.
Combine spear and deflector operation to control turbine
4. Calculations Velocity D-1:
Efficiency
Hydraulic efficiency
The hydraulic losses due to the liquid friction and local resistances are accounted for by hydraulic efficiency which is defined as the ratio of power developed at the turbine runner to the power supplied by the water jet at the entrance of the turbine.
ηH=power developed by the turbinerunner
power supplied by water jet at the inlet of turbine
¿ρaV 1 (V w 1+V w 2 )u
ρghQ a
= 90~95
Mechanical Efficiency
Power developed by a turbine runner (buckets) is decreased by mechanical losses caused by
friction between the rotating parts(shaft and the runner), friction between the stationary parts(bearing and sealing) and friction in the elements that transmit power.
Due to this power available at the turbine shaft is less than the power developed by turbine runner.
ηM= power available at the turbine shaftpowerdeveloped by the turbine runner
= shaft powerbucket power ( power developedby runner)
¿ PρaV 1 (V w 1+V w 2 )∗u
= 97~99
Volumetric Efficiency
The total quantity of water issued from the jet does not strike the turbine buckets. Some water misses the buckets and passes into the tail race without doing any useful work. The leakage loss is accounted for by volumetric efficiency which is defined as..
ηV=volume of water actually striking therunner
total water supplied by the jet ¿the turbine¿
¿Qa
Q=Q−q
Q
= 97~99
Overall Efficiency
The overall efficiency is defined as
ηo=power available at turbine shaft
power available¿water jet ¿= shaft power
water power
¿ PρgHQ
= 85~90
Where H is the net head in meters and Q is the total discharge in m3/s supplied by jet.
It runs by the reaction force of the exiting fluid. PE and kinetic energy of the fluid come to stationary part of Turbine and partly changes PE
into KE. Moving part (runner) utilize both PE and KE. It works above atmosphere. It will be fully immersed in water. It has draft tube. Eg. Francis turbine.
Francis Turbine Introduction
Francis turbine is an inward flow reaction turbine which was designed and developed by James B. Francis. In earlier stages of its development, Francis turbine had a purely radial flow runner; the flow passing through the runner had velocity component only in plane normal to the axis of the runner. The modern Francis turbine is, however, a mixed flow unit in which the water enters the runner radially at its outer periphery and leaves axially at its center. Francis turbine is most widely used in the hydroelectric power plant where large quantity of water is available at lower and medium head (20 – 700m).
The head acting on the turbine is partly transferred into kinetic energy and the rest remains as pressure head. There is difference of pressure between the guide vanes and the runner which is called the reaction pressure and is responsible for the motion of the runner. That is why Francis turbine is also known as reaction turbine. Water pressure decreases as it passes through the turbine imparting reaction on the turbine blades making the turbine rotate.
In Francis turbine the pressure at inlet is more than that at the outlet. This means that the water in the turbine must flow in closed conduit. Unlike the Pelton type where the water strikes only a few of the runner blades at a time, here the runner is always full of water. The moment of runner is affected by the change of both the potential and kinetic energies of water. After doing work the water is discharged to the tail race through a closed tube of gradually enlarging section known as draft tube. It does not allow water to fall freely to tail race level as in the Pelton turbine. The free end of the draft tube is submerged deep into the tail race. Entire water passage from head race to the tail race is totally closed; does not communicate with the surrounding atmospheric pressure.
Different Parts
Penstock: It is a large sized conduct which conveys water from the upstream reservoir to the turbine runner. Because of large volume of water flow, size of penstock required for a Francis turbine is larger than that of a Pelton turbine. Trash racks are provided at the inlet of the penstock in order to obstruct the entry of debris into the penstock.
Spiral / scroll casing: It constitutes a closed passage whose cross section area gradually decreases along the flow direction area is maximum at inlet and nearly zero at exit.
Guide vanes / wicket gates: These vanes direct the water into the runner at an angle appropriate to design. They direct the flow just as the nozzle of the Pelton wheel. They are fixed in position i.e. do not rotate with the rotating runner. It is operated by hand wheel or automatically by governor.
Governing mechanism: It changes the position of guide vanes to affect the variation in water flow rate, when the load conditions on the turbine change.
Runner and runner blades: The driving force in the runner is both due to impulse and reaction effects. Runner blade no. = 16 ~ 24.
Draft tube: After passing through the runner, the water is discharged to the tail race through gradually expanding tube called the draft tube. The free end of the draft tube is submerged deep into the tail race. Entire water passage from head race to the tail race is totally closed; does not communicate with the surrounding atmospheric pressure.
It is gradually expanding tube which helps easy discharges water, passing through runner to the tail race. Thus it helps to reuse of the exit water.
Construction
Working
Water under pressure (from reservoir and penstock) enters the runner from the G/Vs towards the center in radial direction and discharges out of the runner axially.
Energy of water transfer to rotational energy of runner then rotation of shaft. There is a difference of pressure between the G/Vs and the runner which is called reaction
pressure and is responsible for the motion of the runner. The pressure at inlet is more than outlet. Water flow through the spiral casing.
Water release from a draft tube submersed in water for easy release and reuse of exit water.
Francis Turbines are generally installed with their axis vertical. Water with high pressure enters the turbine through the spiral casing surrounding the guide vanes. The water looses a part of its pressure in the volute (spiral casing) to maintain its speed. Then water passes through guide vanes where it is directed to strike the blades on the runner at optimum angles. As the water flows through the runner its pressure and angular momentum reduces. This reduction imparts reaction on the runner and power is transferred to the turbine shaft.
If the turbine is operating at the design conditions the water leaves the runner in axial direction. Water exits the turbine through the draft tube, which acts as a diffuser and reduces the exit velocity of the flow to recover maximum energy from the flowing water.
4.Calculations Velocity D
Fig shows he runner and the velocity diagrams for an inward flow reaction turbine. The general expression for the work done with usual notations according to the Euler moment equation is given by
Work done=ρQ (V w 1u1±V w 2u2 )
Work done=wQg
(V w 1u1±V w 2u2 )
Where Q= discharge through the runner, m3/s
The maximum output under given conditions is obtained when Vw2=0
Thus the maximum work done is given by:
Work done=wQg
(V w 1u1 )
= 85~90
Efficiency:
Mechanical Efficiency
It accounts for the friction loss at bearings and the power absorbed by the governing mechanism. It is defined as the ratio..
ηM= power available at the turbine shaftpowerdeveloped by the turbine runner
= shaft powerbucket power (power developedby runner )
¿ PρQ (V w1u1±V w 2u2 )
Hydraulic electric plant efficiency
ηo=power available at the turbine shaft
power available¿thewater ¿= shaft power
water power
¿ρQ (V w1u1±V w 2u2 )
ρgHQ
= 85~90
hO = hH ´hM
= 80~90
Volumetric Efficiency:
❑V=Qa
Q=100
Overall Efficiency
It considers both the hydraulic and mechanical losses.
hHE = hG´ hO
hHE = Pout of HE / rgHQ
Advantages and disadvantages of a Francis turbine over a Pelton wheel
Advantages:
Variation in the operating load can be more easily controlled. The ratio of maximum and minimum operating heads can be even two. The operating head can be utilized even when the variation in the tail water level is relatively
large when compared to the total head. The mechanical efficiency of Pelton wheel decreases faster with wear than Francis turbine. The size of the runner, generator and power house required is small and economical if the
Francis turbine is used instead of Pelton wheel for same power generation.
Disadvantages:
Water which is not clean can cause very rapid wear in high head Francis turbine. The overhaul and inspection is much more difficult comparatively. Cavitation is an ever present danger. The water hammer effect is more troublesome with Francis turbine. If Francis turbine is run below 50 percent head for a long period it will not only lose its efficiency
but also the cavitation danger will become more serious.
Propeller turbine
Axial flow reaction turbine:
Water flows parallel to the axis of the rotation of shaft, the turbine is known as axial flow turbine. The shaft of an axial flow reaction turbine is vertical. The lower end of the shaft is made larger which is known as hub. The vanes are fixed on the hub and it acts as runner for axial flow reaction turbine. Two important axial flow reaction turbines are
Propeller turbine Kaplan turbine
In these turbines all parts such as spiral casing, stay vanes, guide vanes, control vanes and draft tube are similar to mixed flow reaction turbines in design. But the water enters the runner in an axial direction and leaves axially. The pressure at the inlet of the blades is larger than the pressure at the exit of the blades. The energy transfer is due to the reaction effect, ie the change in the magnitude of relative velocity across the blades.
In axial flow turbines the number of blades are fewer and hence loading on the blade is larger. Small contact area causes less frictional loss compared to mixed flow turbines.
The water coming out from guide vanes undergoes a whirl which is assumed to satisfy the law of free vortex (Vw =C/r). Whirl is largest near the hub and smallest at the outer end of blade. Hence the blade is twisted along its axis.
Introduction: Propeller turbine
Propeller turbine is an axial-flow reaction turbine used for heads between 4 m to 80 m. Propeller turbine consists of axial flow runner with four to six blades of air-foil shape. The runner is generally kept horizontal, i.e. the shaft is vertical. The blades resemble the propeller of ship and are fixed and non-adjustable. The spiral casing and guides blades are similar to those in Francis turbine.
Construction
Consists of
Shaft Axial flow runner
o Hubo Air-foil shape blades
Spiral casing Draft tube
WORKING
Water enters the runner in the axial direction and leaves axially.
The pressure at the inlet of the blade is larger than at the exit of the blades.
Energy transfer is due to the reaction effect of pressure differences.
EFFICIENCY
Hydraulic Efficiency
ηH=power developed by the turbinerunner
power supplied by water
¿ρQ (V w1u1±V w 2u2 )
wQH
Where, w= weight density of water
Propeller
H = head, m Q = Discharge, m3/s u1= Peripheral velocity of runner
Mechanical Efficiency
ηM= power available at the turbine shaftpowerdeveloped by the turbine runner
= shaft powerbucket power ( powerdeveloped by runner)
¿ PρQ (V w1u1±V w 2u2 )
It is due to bearing friction etc .
Overall EfficiencyThe overall efficiency is defined as
ηo=power available at turbine shaft
power available¿water jet ¿= shaft power
water power
¿ PρgHQ
= 80~90
hO= hH ´hM
FEASIBILITY STUDY
Propeller turbines are generally not used in Nepal.
There is possibility of using this turbine where there is high flow rate and low head such as the plain land area of Nepal.
Since, this turbine has narrow range of output unlike Francis and other turbines , this makes it less usable in changing parameter conditions.
ADVANTAGES AND DISADVANTAGES
Advantages Disadvantages
Fabricated with low cost. Smaller contact area cause less friction.
Less part load efficiency. Absence of adjustable blades.
Low frictional losses since small number of blades.
The Kaplan or propeller type turbines can be mounted at almost any angle, but this is usually vertical or horizontal.
Have fewer blades than of Francis turbine so has less spacing of trash bars and impurities.
The loading on the blades are large due to few number of blades
CONCLUSION
Best suited for full load conditions.
Can be installed when low head and high flow rate is available.
Can be used where low cost and ease of fabrication are priorities.
Full load efficiency is very good than part load efficiency.
Kaplan Turbine
Introduction:
It is an axial flow reaction turbine invented by Prof. Kaplan. A propeller turbine is quite suitable when the load on the turbine remains constant. At part load its efficiency is very low; since the blades are fixed, the water enters with shock and eddies are formed which reduces the efficiency. This defect is removed in Kaplan turbine. In Kaplan turbine the runner blades are adjustable and can be rotated about pivots fixed to the boss of the runner. The blades are adjusted automatically by servomechanism so that at all loads the flow enters them without shock. Thus high efficiency is maintained even at part load. The servomotor cylinder is usually accommodated in the hub.
Kaplan turbine has purely axial flow. Usually it has 4 to 6 blades having no outside rim. It is also known as a variable-pitch propeller turbine since the pitch of the turbine can be changed because of adjustable vanes. It behaves like a propeller turbine at full-load conditions.
Kaplan turbines are now widely used throughout the world in high-flow, low-head power production. It is equivalent to Propeller Turbine at full load.
Kaplan turbine is high speed turbine and is used for smaller heads; as the speed is high, the number of runner –vane is small. Accordingly the runner diameter becomes relatively small and the rotational speed more than two times higher than for a Francis turbine for the corresponding head and discharge.
In this way the generator dimensions as well become comparatively smaller and cheaper.
The comparatively high efficiencies at partial loads and the ability of overloading is obtained by a coordinated regulation of the guide vanes and the runner blades to obtain optimal efficiency for all operation
Kaplan turbine efficiencies are typically about 90%, but may be lower in very low head applications.
Construction:
Runner: 4~6 blades
generally horizontal runner and vertical shaft
Spiral casing
Guide vanes
Draft tube
Governings
Working:
Water enters the runner in the axial direction and leaves axially.
The pressure at the inlet of the blade is larger than at the exit of the blades.
Energy transfer is due to the reaction effect of pressure differences.
Efficiency:
hH = power developed by the T runner /water power
=P/wQH
Where, w= weight density of water
H=head, m
Q=Discharge, m3/s
Mechanical Efficiency
hM = power available at Turbine shaft / power developed by the Turbine runner
= shaft power / bucket power (power developed by runner)
= P / [rQ (Vw1u1 ± Vw2u2)]
It is due to bearing friction etc
Propeller
Overall Efficiency
hO= power available at the turbine shaft / power available from the water jet
= shaft power / water power
= P / rg HQ
hO= hH ´hM
= 80~90%
Feasibility Study:
It is used in Gandak HPP in Nepal.
There is possibility of using this turbine where there is high flow rate and low head such as the plain land area of Nepal.
Since, this turbine has better part flow efficiency it has wide range of scope in for variable load and without reserver.
Advantages Disadvantages
For same power deliver it is more compact.
Part load efficiency is good. Low frictional losses since small number
of blades. Have fewer blades than of Francis turbine
so has less spacing of trash bars and impurities.
High RPM. Good efficiency for high flow and low
head
The loading on the blades are large due to few number of blades
Not suitable for high head and low flow. Very expensive to design, manufacture
and install. Cavitations problems
Crossflow turbines
Cross flow turbine also known as Michell-Banki , have largest market share in small turbines. The design of a crossflow turbine is simple compared to other turbines which make it interesting for developing countries. Turbine can be easily installed with limited constructional expenses. Another major advantage of crossflow turbine is that it can be used for very wide range of head and flow rates.
Unlike most water turbines, which have axial or radial flows, in a crossflow turbine the water passes through the turbine transversely, or across the turbine blades.
Fig. shows the cross section area of crossflow turbine. The rectangular intake to the turbine is fitted with guide vanes to regulate the flow. Water is directed into the full length of the runner and passes through the center striking the blades twice, imparting most of its kinetic energy before leaving the turbine.
The length of the runner can theoretically be increased to any value without changing the hydraulic characteristics of turbine. Doubling runner length doubles the power output at the same speed. At high heads the runner tends to be impact. The lower the head, longer it becomes for given power output.
Reasons for their suitability for Nepal:
No highly trained and skillfull man power needed to manufacture and maintain crossflow turbines.
Easy to repair and maintain Material easily available in market No casted baldes needed Partflow efficiency better than pelton turbine Suitable for medium head for rivers of hilly region.
Advantages:
Effective head can be increased by fitting draft tube Efficiency maintainable at low flow. Suitable for a wide range of head and power Simple in design and fabrication Free of cavitation Runner length can be adjusted
