Science Fair Project Encyclopedia
A turbine is a rotary engine that extracts energy from a fluid flow. The simplest turbines have one moving part, a rotor-blade assembly. Moving fluid acts on the blades to spin them and impart 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 focuses and controls the fluid. The casing and blades may have variable geometry that allow efficient operation for a range of fluid flow conditions.
Energy is extracted in the form of rotating shaft power.
Theory of operation
A working fluid contains potential energy (pressure head) and kinetic energy (velocity head). The fluid may be compressible or non-compressible. Several physical principles are employed by turbines to collect this energy;
Impulse turbines change the direction of flow of a high velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid in the turbine blades. Pressure head is changed to velocity head by focusing the fluid with a nozzle, prior to hitting the turbine blades. Pelton wheels and de Laval turbines use this concept. Impulse turbines do not require a pressure casement around the runner, since the fluid jet is prepared by a nozzle prior to hitting the turbine. Newton's second law describes the transfer of energy for impulse turbines.
Reaction turbines develop torque by reacting to the fluid's pressure or weight. The pressure of the fluid changes as it passes through the turbine. A pressure casement is needed to contain the working fluid as it acts on the turbine runner, or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid, and for water turbines maintain suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages may by used to efficiently harness the expanding gas. Newton's third law describes the transfer of energy for reaction turbines.
Turbine designs will use both these concepts to varying degrees whenever possible. Wind turbines use a foil to generate lift from the moving fluid and impart it to the rotor (this is a form of reaction), they also gain some energy from the impulse of the wind, by deflecting it at an angle. Crossflow turbines are designed as an impulse machine, with a nozzle, but in low head applications maintain some efficiency through reaction, like a traditional water wheel.
Classical turbine design methods were developed in the mid 19th century. Vector analysis related the fluid flow with turbine shape and rotation. Graphical calculation methods were used at first. Formulas for the basic dimensions of turbine parts are well documented, and a highly efficient machine can be reliably designed for any fluid flow condition. Some of the calculations are empirical 'rule of thumb' formulas, and others are based on classical mechanics. As with most engineering calculations, simplifying assumptions were made.
Modern turbine design carries the calculations further. Computational fluid dynamics dispenses with many of the simplifying assumptions used to derive classical formulas, and computer software facilitates optimization. These tools have led to steady improvements in turbine design over the last forty years
The primary numerical classification of a turbine is its specific speed. This number describes the speed of the turbine at its maximum efficiency with respect to the power and flow rate. The specific speed is derived to be independent of turbine size. Given the fluid flow conditions and the desired shaft output speed, the specific speed can be calculated and an appropriate turbine design selected.
The specific speed, along with some fundamental formulas can be used to reliably scale an existing design of know performance to a new size with corresponding performance.
Types of turbines
Steam and Gas turbine are sometimes technically referred to as Joule-Brayton or Rankine cycle turbines. These thermodynamic energy conversion cycles have basically 4 stages with minor variations. Wikibook on Joule-Brayton Cycle
- Compressor stage: A working fluid (or liquid or gas) is compressed, this stage is varaible in order.
- Heating Stage: The fluid is heated (or combusted with a fuel) which cause it to expanded.
- Power stage: The expanding fluid turns a turbine which generates workable energy.
- Exhaust stage: The used fluid is either cooled and recycled (closed cycle) or ejected from the system (open cycle).
Water and Wind turbines do not have a thermodynamic cycle and the working fluid is not heated: simply a pressure differential from one side of the turbine to the other cause fluid to spin the turbine.
Uses of turbines
The most common turbine use is electrical power production. Almost all electrical power is produced with a turbine of some type.
Turbines are often part of a larger machine. A Gas turbine, for example, may refer to an internal combustion machine that contains a turbine, compressor, combustor and alternator.
Turbines can have incredible power density (with respect to volume and weight). This is because of their ability to operate at very high speeds. The Space Shuttle fuel pumps, for example, are slightly larger than an automobile engine and produce 25,000 hp (19 MW).
Turbines are also a primary component of jet engines.
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