Fluid Mechanics 2 Assignment Reaction Turbine

reaction turbine

Important Components and Working Functions.

MAY 16, 2016civil engineering department

UCE&T, BZU, MUTAN

SUBMITTED TO

SUBMITTED BY

ROLL NO.

ASSIGNMENT NO. SESSION

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Reaction turbineA type of turbine that develops torque by reacting to the pressure or weight of a fluid; the operation of reaction turbines is described by Newton's third law of motion (action and reaction are equal and opposite).

In a reaction turbine, unlike in an impulse turbine, the nozzles that discharge the working fluid are attached to the rotor. The acceleration of the fluid leaving the nozzles produces a reaction force on the pipes, causing the rotor to move in the opposite direction to that of the fluid. The pressure of the fluid changes as it passes through the rotor blades. In most cases, a pressure casement is needed to contain the working fluid as it acts on the turbine; in the case of water turbines, the casing also maintains the suction imparted by the draft tube. Alternatively, where a casing is absent, the turbine must be fully immersed in the fluid flow as in the case of wind turbines. Francis turbines and most steam turbines use the reaction turbine concept.

In Reaction turbine major portion of the pressure loss takes place in the rotating wheel. Fluid entering

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the rotor around its entire circumference is in action. So its rotor need to be so large as compared to the impulse turbine of same power.

In reaction turbine, water enters the wheel under pressure and flow over vanes. As the water, flowing over the vanes, is under pressure, therefore wheel of the turbine runs full and may be submerged. The pressure head of water, while flowing over that vanes, is converted into velocity head and is finally reduced to the atmospheric pressure, before leaving the wheel.

Main Components of a reaction turbine

A reaction turbine has the following main components.

Spiral casingGuide mechanismTurbine runnerDraft tube

Spiral casing The casing of reaction turbine is designed in such a way that it’s cross sectional area goes on reducing uniformly around the circumference. The cross sectional area is maximum at the entrance and minimum at the tip. Due to this, the casing will be of spiral shape. That is why it is called spiral casing or scroll

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casing. The water from a pipeline is distributed around the guide ring in a casing.

The material of the casing depends upon the head of water. If head is up to 30 meter then concrete should be used. If head is up to 100 meter then welded rolled steel plate should be used. If it is more than 100 meter then cast steel should be used for casing.

Guide mechanism This is the arrangement of blades and vanes which will guide the water to move towards runner.

The guide vanes are fixed between the two rings in the form of a wheel. This wheel is fixes in the spiral casing.

Functions of guide vanes

Guide vanes allow the water to enter the runner without shock. Guide vanes keep relative velocity at inlet of the runner, tangential to the vane angle and thus results in entering of water without shock.Allow the water to flow over them without forming eddies.Allow the required quantity of water to enter the turbine. This is done by adjusting the vanes.All the guide vanes can rotate about their respective pivots. Pivots are connected to the guided ring or regulating ring. This ring is connected to the regulating shaft by means of two regulating rods. Guide vanes may be closed or opened by rotating the regulating shaft, thus allowing required quantity of water to flow.

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The regulating shaft is operated by a governor whose function is to govern the turbine. Governor function is to keep the speed constant at varying loads.

Guide vanes are generally made of cast steel.

Turbine Runner

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The runner consists of a hub, a shroud and runner blades connecting them. The runner converts the energy in the water to rotating motion and torque. The torque is transferred to the turbine shaft

through a bolted friction joint or a combined friction/shear joint. The runner can either be casted or welded. For a welded design the hub and shroud is usually cast and welded together with hot pressed plate vanes. The number of blades depends upon the operating head. Runners with higher head will require a higher number of blades, this is mainly because of strength consideration. Increasing the number of blades reduce the pressure loading on the blade which will help to avoid cavitation and also prevent separation at the runner inlet during low loads. An increase in the number of blades also leads to more contact surface through the runner and thereby an increase in the friction losses. The thickness of the runner blades has to be large enough to withstand the hydraulic forces the blade is exposed to. For high head Francis turbines it is preferred to shape the blade in such a way that the main part of the hydraulic energy is utilized at the beginning of the blade. In

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this area the pressure difference from the pressure to the suction side will be large and thereby also the forces on the blade. It is therefore usual to have an increased thickness of the blade near the inlet and let the blade get thinner towards the outlet.Draft tube

The water, after passing through the runner, flows down through a tube called draft tube.The draft tube is the water conduit from the runner to the outlet gate. Its purpose is to convert the kinetic energy at the runner outlet to pressure energy at the draft tube outlet. This is

possible by leading the water through a channel with increasing cross section. The draft tube consists of a cone and a plate shroud. The draft tube cone is of welded plate design and normally consist of two parts, upper and lower cone. The upper part of the cone is mounted to the lower cover. The lower cone is normally designed as a dismantling piece. This is connected to the draft tube shroud by a flange.It increases the head of water equal to the height of the runner outlet above the tail race.It increases efficiency of the turbine.Function

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Water flows from the penstock into the spiral casing. In the spiral casing the water is distributed around the complete periphery. The water is then guided by the stay vanes and guide vanes in the correct angle towards the runner. The guide vanes are adjustable and can change the angle depending on the inlet and outlet conditions of the turbine, they are controlled by a governor servo motor. The runner transfers the energy from the pressure and

velocity in the water to a rotational momentum. The water exits through a draft tube that extracts the remaining energy in the water.The torque produced in the runner is transferred to a power producing generator through a shaft.

Applications Francis turbines(reaction turbine) 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. Francis type units cover a head range from 40 to 600 m (130 to 2,000 ft), and their connected generator output power varies from just a few kilowatts up to 800 MW.Large Francis turbines are individually designed for each site to operate with the given water supply and water head at the highest possible efficiency, typically over 90%.In addition to electrical production, they may also

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be used for pumped storage, where a reservoir is filled by the turbine (acting as a pump) driven by the generator acting as a large electrical motor during periods of low power demand, and then reversed and used to generate power during peak demand. These pump storage reservoirs, etc. act as large energy storage sources to store "excess" electrical energy in the form of water in elevated reservoirs. This is one of only a few ways that temporary excess electrical capacity can be stored for later utilization.

REFRENCES:http://www.civilengineeringterms.com/fluid-mechanics-2/reaction-turbine-main-components-of-a-reaction-turbine/http://www.daviddarling.info/encyclopedia/R/AE_reaction_turbine.htmlhttp://waterturbines.wikidot.com/main:types-of-water-turbineshttps://en.wikipedia.org/wiki/Francis_ turbine

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