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Water turbine

(waterturbine)

A water turbine is a rotary engine that takes energy from moving water.

Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids.Now they are mostly used for electric power generation. They harness aclean and renewable energy source.

Contents

1 History

1.1 Swirl
1.2 Time line
1.3 A New concept

2 Theory of operation

2.1 Reaction turbines
2.2 Impulse turbines
2.3 Power
2.4 Pumped storage
2.5 Efficiency

3 Types of water turbines

4 Design and Application

4.1 Typical range of heads
4.2 Specific speed
4.3 Runaway speed

5 Environmental impact

6 See also

7 External links

History

Swirl

Water wheels have been used for thousands of years for industrial power.Their main shortcoming is size, which limits the flow rate and head that can be harnessed.They also tend to rotate slower than the machines they power.

The migration from water wheels to modern turbines took about one hundred years. Development occurred during the Industrial revolution , using scientific principals and methods.They also made extensive use of new materials and manufacturing methods developed at the time.

The word turbine was coined by the French engineer Claude Bourdin in the early 19thcentury and is derived from the Latin word for "whirling" or a "vortex". The main difference between early water turbines andwater wheels is a swirl component of the water which passes energy to a spinning rotor. This additional component of motionallowed the turbine to be smaller than a water wheel of the same power. They could process more water by spinning faster andcould harness much greater heads. (Later, impulse turbines were developed which didn't use swirl).

Time line

Ján Andrej Segner developed a reactive water turbine in the mid 1700's. It had a horizontal axis and was a precursor to modern water turbines. Itis a very simple machine that is still produced today for use in small hydro sites. Segner worked with Euler on some of the early mathematical theories of turbine design.

In 1820 , Jean V. Poncelet developed an inward-flow turbine.

In 1826 Bénoit Fourneyron developed an outward-flow turbine. This was an efficient machine (~80%) that sentwater through a runner with blades curved in one dimension. The stationary outlet also had curved guides.

In 1844 Uriah A. Boyden developed an outward flow turbine that improved on the performance of the Fourneyron turbine. Its runner shape was similar tothat of a Francis turbine.

In 1849 , James B. Francis improved the inward flow reaction turbine to over 90% efficiency. He also conducted sophisticated tests and developed engineeringmethods for water turbine design. The Francis turbine, named for him, is the first modern water turbine. It is still the mostwidely used water turbine in the world today.

Inward flow water turbines have a better mechanical arrangement and all modern reaction water turbines are of this design.Also, as the swirling mass of water spins into a tighter rotation, it tries to speed up to conserve energy. This property acts onthe runner, in addition to the water's falling weight and swirling motion. Water pressure decreases to zero as it passes throughthe turbine blades and gives up its energy.

Around 1913 , Victor Kaplan created the Kaplan turbine, a propeller type machine. It was an evolution of the Francisturbine but revolutionized the ability to develop low head hydro sites.

A New concept

All common water machines until the late 19th century (including water wheels) were reaction machines; water with a head acted on the machine and produced work. A reaction turbine needs to fully contain the waterduring energy transfer.

In 1879 by LesterPelton invented a machine that worked off a completely different concept. The Pelton wheel , named for him, works with impulse; water pressure is turned into kinetic energy with a nozzle . The resulting water jet impacts curved turbine blades, reversing the water's flow,causing the runner to spin. Turgo turbine and Crossflow turbines were later impulse designs.

Theory of operation

Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning,the force acts through a distance (force acting through a distance is the definition of work ). In this way, energy is transferred from the water flow to the turbine.

Water turbines are divided into two groups; reaction turbines and impulse turbines.

The precise shape of water turbine, what ever its design, is driven by the supply pressure of water.

Reaction turbines

Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. Theymust 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 ofenergy for reaction turbines.

Most water turbines in use are reaction turbines. They are used in low and medium head applications.

Impulse turbines

Impulse turbines change the velocity of a water jet. The jet impinges on theturbine's curved blades which reverse 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 energy is converted to kinetic energy by a nozzle and focused on the turbine. No pressurechange occurs at the turbine blades and the turbine doesn't require a housing for operation.

Newton's second law describes the transfer ofenergy for impulse turbines.

Impulse turbines are most often used in very high head applications.

Power

Water is very heavy and it's flow energetic. The power available in dammed wateris;

$P=\eta\cdot\rho\cdot g\cdot h\cdot\dot V$

where:

$P =$ Power( J/s or Watts)
$η =$ turbine efficiency
$ρ =$ density of water (kg/m³)
$g =$ acceleration of gravity (9.8 m/s²)
$h =$ head (m, this is the difference in heightbetween the inlet and outlet water surfaces)
$\dot V$ = flow rate (m³/s)

Pumped storage

Some water turbines are designed for Pumped storage hydroelectricity . They can reverse flow and operate as a pump to fill a highreservoir during off-peak electrical hours, and then revert to a turbine for power generation during peak electrical demand. Thistype of turbine is similar to the francis in design.

Efficiency

Large modern water turbines operate at mechanical efficiencies greater than 90% (not to be confused with thermodynamic efficiency ).

Types of water turbines

Reaction turbines:

Impulse turbines:

Design and Application

Turbine selection is based mostly on the available water head, and less so on the available flow rate. In general, impulseturbines are used for high head sites, and reaction turbines are used for low head sites.

Typical range of heads

Kaplan 2<H <40 (H= head in meters)
Francis 10< H<350
Pelton 50< H <1300
Michell-Banki 3< H <250
Turgo 50< H <250

Specific speed

The specific speed, $n s$ , of a turbine characterizes the turbine's shapein a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of knownperformance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type.

The specific speed of a of a turbine can also be defined as the speed of an ideal, geometrically similar turbine, which yieldsone unit of discharge for one unit of head.

The specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point ofmaximum efficiency. These allow accurate calculations to be made of the turbine's performance for a range heads and flows.

$n_s=n\sqrt{P}/H^{5/4}$ (dimensioned parameter), $n$ = rpm

$N_s=\frac{\Omega\sqrt{P/p}}{gH^{5/4}}$ (dimensionlessparameter), $Ω$ = angular velocity (radians/second)

Example; Given a flow and head for a specific hydro site, and the rpm requirement of the generator, calculatethe specific speed. The result is the main criteria for turbine selection.

The specific speed is also the starting point for analytical design of a new turbine. Once the desired specific speed isknown, basic dimensions of the turbine parts can be easily be calculated.

Runaway speed

The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will bedesigned to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.

Environmental impact

Water turbines have positive and negative impacts on the environment.

They are one of the cleanest producers of power, replacing the burning of fossil fuels and eliminating nuclear waste. They usea renewable energy source and are designed to operate for decades. They produce significant amounts of the world's electricalsupply.

On the other hand there are some negative consequences; by their nature, water turbines interrupt the natural ecology ofrivers, killing fish and stopping migrations. Native American Indian tribes in the Pacific Northwest had livelihoods built aroundsalmon fishing. Aggressive dam building destroyed their way of life.

External links

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