19-11-2012, 02:22 PM
Seminar Report On: - TURBINES
1 TURBINES.doc (Size: 1.14 MB / Downloads: 44)
Abstract
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.
The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and water wheels.
Gas, steam, and water turbines usually have a casing around the blades that contains and controls the working fluid. Credit for invention of the steam turbine is given both to the British engineer Sir Charles Parsons (1854–1931), for invention of the reaction turbine and to Swedish engineer Gustaf de Laval (1845–1913), for invention of the impulse turbine. Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery.
INTRODUCTION
A turbine is a type of engine that can extract energy from a fluid, such as water, steam, air, or combustion gases. It can be contrasted with a piston engine, which uses a piston instead of a turbine to extract energy.
The physical makeup of a turbine is a series of blades, typically made of steel but sometimes ceramic, which can withstand higher temperatures. The fluid goes in one end, pushing the blades and causing them to spin, then gets ejected out the other end. The fluid leaves the turbine with less energy than it had going in - a portion of the difference is captured by the turbine.
Turbines are the core of our civilization. Practically every form of electric power is generated by a turbine. When we say coal power, nuclear power, hydrothermal power, etc., we mean using some energy source to agitate a gas which then drives a turbine and generates power. A turbine is one of the most common types of engines, where an engine is defined simply as something that takes an input and generates an output. Along with heat engines and motors, turbines make up the vast majority of dynamic machinery.
THEORY OF OPERATION
A working fluid contains potential energy (pressure head) and kinetic energy (velocity head). The fluid may be compressible or incompressible. Several physical principles are employed by turbines to collect this energy:
Impulse turbines change the direction of flow of a high velocity fluid or gas jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid or gas in the turbine blades (the moving blades), as in the case of a steam or gas turbine, all the pressure drop takes place in the stationary blades (the nozzles). Before reaching the turbine, the fluid's pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelton wheels and de Laval turbines use this process exclusively. Impulse turbines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor. Newton's second law describes the transfer of energy for impulse turbines.
Reaction turbines develop torque by reacting to the gas or fluid's pressure or mass. The pressure of the gas or fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (such as with wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages are usually used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy
. Pelton wheels and de Laval turbines use this process exclusively. Impulse turbines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor. Newton's second law describes the transfer of energy for impulse turbines.