ROTODYNAMCIS Flashcards
how can liquid have pressure
via a column of liquid which due to its weight, exerts pressure on surface
how can you convert mechanical energy into hydraulic energy
using pump, fan or compressor
how to convert hydraulic energy into mechanical
turbine
how can actual performance of rotodynamic be determined
via experimental testing, however different machines have different characteristics, with speed and diameter
is it possible to determine all performance curves via testing
no but can be sorted by using dimensionless groups
what does the method ‘Dimensionless groups’ allow
pump and turbine manufacturers
to test a relatively small number of machines
how many dimensionless groups will be used to describe performance
12 dimensionless groups
what are the variables of rotodynamic machines (8)
fluid pressure temperature at inlet outlet flow to the machine, the rate of flow, speed of rotation, power, thrust, torque,
what do 12 dimensionless groups provide
the similarity laws for a family of geometrically
similar rotodynamic machines.
what is a turbine and what does it do
Rotodynamic machine that converts hydraulic
energy into mechanical energy
Turbines extract energy from a fluid (e.g.water or
gas) and converts it into useful work.
types of water turbines (2)
impluse (pelton wheel) and reaction turbines (Kaplan or francis)
how does a impluse turbine work
hydraulic energy of fluid is converted to kentic energy via movement of impeller
how does a reaction turbine work
develops power from the combined action of pressure and moving water
placed in water stream, flowing over blades
what is cavitation
formation of vapor bubbles of a flowing liquid where the pressure of liquid falls below its vapor pressure
causes of cavitation (2)
low boiling pressure
gas release
or both
negative effects of cavitation (2)
efficiency loss
erosion damage causing failure
how to stop cavitation
pressure inside pump/turbine higher than vapour pressure of liquid
liquid pressure higher than vapour pressure
why is actual energy different from Euler theory
due to non-uniform velocity and frictions
explain flow regime 1
flow commences as subsonic when Pb is decreased but higher than Pc.
Pressure in nozzle dropping in converging section from P0 to minimum value, which is higher than P* at throat.
explain flow regime 2
flow is sonic at throat (M=1) P*
if Pb reduces, throat pressure which is already lower will decrease further
nozzle becomes chocked when pressure is lower than Pb*
explain flow regime 3
from regime 2, shock wave move diverging nozzle when Pb decreases.
regime 3 occurs when Pb drops to a value where shock wave is on exit plane
explain flow regime 4
further decrease in Pb, becoming less than Pe, becoming under-expanded because back pressure lowe than exit plane pressure
how is Euler equation derived, also discuss importance in rotodynamic machine theory
derived from newton’s second law applied to angular momentum.
it provides the theoretical energy transfer between fluid and the rotor
why does actual energy transferred between impeller and fluid differ from theoretical energy predicted by Euler equation
because of the losses due to non-uniform velocity at blade and friction in impeller
what is compressibility, and how to differ from compressible and incompressible fluid
compressibility is a change in density, produced in fluid by chnage in pressure.
to differ: gases can compress due to large variations of density occurring due to pressure changes, whereas liquid cant compress, so has a constant density
for shock waves what do Fannomlines represent
curves that prove continuity and energy equations,
providing a graphical representation of shock waves
difference between venturi and supersonic
venturi = flow everywhere subsonic (m<1) supersonic = M<1 in converging section, M=1 in throat and M>1 in diverging section
