Fundamental Form-s

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From http://en.wikipedia.org/wiki/Francis_turbine

Francis_Turbine_complete Francis_Turbine_High_flow

Francis Turbine and generator

Guide vanes at full flow setting (cut-away view)

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.

As the water moves through the runner its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string around in a circle. If the string is pulled short, the ball spins faster. This property, in addition to the water's pressure, helps inward flow turbines harness water energy.

 

Large Francis turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.

Francis type units cover a wide head range, from 20 meters to 700 meters and their output varies from a few kilowatt to 1,000 megawatt.

In addition to electrical production, they may also be used for pumped storage; where a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.

Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world. Grand Coulee Dam uses a Francis Turbine.

COMPARED TO INVOLUTE TURBINE:

  • Requires much higher head (at least 5 times), pipe delivery at high speed
  • water delivered and redirected in complex compressing channels
  • gravity not utilized
  • water pushes only to inside of vanes, not all the way through
  • flow is reversed only 140? degrees compared to 480 degrees of involute canal turbine
  • surface area of vanes much smaller
  • flow not as streamlined and turbulence-free
  • WE SHOULD BE ABLE TO GET 90% EFFICIENCY TOO!

 

Banki turbine

From Wikipedia, the free encyclopedia

 

Banki turbine. Image credit; European Communities, Layman's Guidebook (on how to develop a small hydro site)

A Crossflow turbine, Banki-Michell turbine, or Ossberger turbine is a water turbine developed by the Australian Anthony Michell, the Hungarian Donát Bánki and the German Fritz Ossberger.

Michell obtained patents for his turbine design in 1903, and the manufacturing company Weymouth made it for many years. Ossberger's first patent was granted in 1922, and he manufactured this turbine as a standard product. Today, the company founded by Ossberger is the leading manufacturer of this type of turbine.

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. As with a waterwheel, the water is admitted at the turbine's edge. After passing the runner, it leaves on the opposite side. Going through the runner twice provides additional efficiency. When the water leaves the runner, it also helps clean the runner of small debris and pollution. The cross-flow turbine is a low-speed machine.

Although the illustration shows one nozzle for simplicity, most practical crossflow turbines have two, arranged so that the water flows do not interfere.

Crossflow turbines are often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit (the guide vane system in the turbine's upstream section) provides flexible operation, with ⅓, ⅔ or 100% output, depending on the flow. Low operating costs are obtained with the turbine's relatively simple construction.

Banki turbine

The turbine consists of a cylindrical water wheel or runner with a horizontal shaft, composed of numerous blades (up to 37), arranged radially and tangentially. The blades' edges are sharpened to reduce resistance to the flow of water. A blade is made in a part-circular cross-section (pipe cut over its whole length). The ends of the blades are welded to disks to form a cage like a hamster cage and are sometimes called "squirrel cage turbines"; instead of the bars, the turbine has trough-shaped steel blades.

The water flows first from the outside of the turbine to its inside. The regulating unit, shaped like a vane or tongue, varies the cross-section of the flow. The water jet is directed towards the cylindrical runner by a fixed nozzle. The water enters the runner at an angle of about 45 degrees, transmitting some of the water's kinetic energy to the active cylindrical blades.

 

 

 Ossberger turbine runner

The regulating device controls the flow based on the power needed, and the available water. The ratio is that (0–100%) of the water is admitted to 0-100%×30/4 blades. Water admission is to the two nozzles is throttled by two shaped guide vanes. These divide and direct the flow so that the water enters the runner smoothly for any width of opening. The guide vanes should seal to the edges of the turbine casing so that when the water is low, they can shut off the water supply. The guide vanes therefore act as the valves between the penstock and turbine. Both guide vanes can be set by control levers, to which an automatic or manual control may be connected.

The turbine geometry (nozzle-runner-shaft) assures that the water jet is effective. The water acts on the runner twice, but most of the power is transferred on the first pass, when the water enters the runner. Only ⅓ of the power is transferred to the runner when the water is leaving the turbine.

The water flows through the blade channels in two directions: outside to inside, and inside to outside. Most turbines are run with two jets, arranged so two water jets in the runner will not affect each other. It is, however, essential that the turbine, head and turbine speed are harmonised.

The cross-flow turbine is of the impulse type, so the pressure remains constant at the runner.

 

Advantages

The peak efficiency of a crossflow turbine is somewhat less than a Kaplan, Francis or Pelton turbine. However, the crossflow turbine has a flat efficiency curve under varying load. With a split runner and turbine chamber, the turbine maintains its efficiency while the flow and load vary from 1/6 to the maximum.

Since it has a low price, and good regulation, crossflow turbines are mostly used in mini and micro hydropower units less than two thousand kW and with heads less than 200 m.

Particularly with small run-of-the-river plants, the flat efficiency curve yields better annual performance than other turbine systems, as small rivers' water is usually lower in some months. The efficiency of a turbine determine whether electricity is produced during the periods when rivers have low heads. If the turbines used have high peak efficiencies, but behave poorly at partial load, less annual performance is obtained than with turbines that have a flat efficiency curve.

Due to its excellent behaviour with partial loads, the crossflow turbine is well-suited to unattended electricity production. Its simple construction makes it easier to maintain than other turbine types; only two bearings must be maintained, and there are only three rotating elements. The mechanical system is simple, so repairs can be performed by local mechanics.

Another advantage is that it can often clean itself. As the water leaves the runner, leaves, grass etc. will not remain in the runner, preventing losses. So although the turbine's efficiency is somewhat lower, it is more reliable than other types. No runner cleaning is normally necessary, e.g. by flow inversion or variations of the speed. Other turbine types are clogged easily, and consequently face power losses despite higher nominal efficiencies.

BANKI TURBINE COMPARED TO INVOLUTE TURBINE:

  • all of the advantages of the Banki Turbine are present in the involute turbine
  • surface area of vanes much less than involute
  • gravity not utilized with captured mass of water
  • water pushes on upper vanes, then falls through center, creating turbulence
  • with water falling on inside of lower blades, debris could collect easier than involute, where water stays in vane to bottom
  • flow is not reversed, extracting less power from water
  • double-pass flow through narrow vanes not as streamlined and turbulence-free as involute
  • WE SHOULD BE ABLE TO GET MUCH HIGHER EFFICIENCIES WITH VARYING STREAM FLOW!