Fluids- Turbines Flashcards
Categories of turbines
Hydraulic turbine: working fluid is water
Wind turbine: working fluid is air and energy extracted from wind
Steam turbine: convert energy from steam into mechanical energy of rotating shaft
Gas turbine: working fluid is compressible gas (like turbojet engine)
How do positive displacement turbines work?
Like pd pump backwards. Fluid pushes into closed volume, turns a shaft or displaces a reciprocating rod. Closed volume of fluid then pushed out as more fluid enters. There is a net head loss. Energy taken from flowing fluid turned into mechanical energy.
What are positive displacement turbines used for?
Not power production but mainly flow rate or volume measurement.
How do impulse turbines work?
Fluid sent through nozzle. High speed jet fired on bucket-shaped vanes that transfer energy to the turbine shaft. Famous one is Pelton wheel.
Formula for power generated by Pelton wheel
P=ωρQ((Vj-rω)(1-cosβ))r
Where Vj is velocity of the jet
β is angle fluid leaves bucket at relative to horizontal (obtuse)
How do reaction turbines work?
Fixed guide vanes (stay vanes) and adjustable guide vanes (wicket vanes) guide fluid to the rotating blades (runner blades). Flow enter tangentially to the circular arrangement at high pressure. Spiral casing called volute.
Formula for efficiency of turbine
Pmech/Pfluid=ωTs/ρgHQ
What do gross and net head mean for hydroelectric power stations?
Gross head: the difference between the upstream and downstream of the dam.
Net head: the difference between the upstream and downstream of the turbine.
What values for power will using gross and net head give?
Gross gives ideal values. Net gives actual values.
How does a gas turbine work?
Draws in and pressurises are using compressor. Adds fuel and combusts to cause rapid expansion which drives turbine. The turbine drives the compressor and the the shaft is connected to an electric generator to produce power.
How does jet engine work?
Basically a gas turbine but the energy directed to the exhaust to generate thrust rather than shaft to generate electricity.
Difference between windmill and wind turbine
Windmill used for mechanical power generation (grinding grain and pumping water, etc).
Wind turbine used for electrical power generation.
Cut-in speed, rated speed and cut-out speed of wind turbine
Cut-in: minimum wind speed at which useful power can be generated.
Rated: wind speed that delivers rate power (often maximum power).
Cut-out: maximum wind speed at which wind turbine is designed to produce power. At greater speeds a breaking mechanism stops blades to avoid damage and for safety issues.
Formula for available wind power
P=0.5ρV^3A
A is disk area swept out by the turbine blades
Wind power density
Available wind power over disk area
0.5ρV^3
