ROTODYNAMCIS

Question 1

how can liquid have pressure

Answer 1

via a column of liquid which due to its weight, exerts pressure on surface

Question 2

how can you convert mechanical energy into hydraulic energy

Answer 2

using pump, fan or compressor

Question 3

how to convert hydraulic energy into mechanical

Answer 3

turbine

Question 4

how can actual performance of rotodynamic be determined

Answer 4

via experimental testing, however different machines have different characteristics, with speed and diameter

Question 5

is it possible to determine all performance curves via testing

Answer 5

no but can be sorted by using dimensionless groups

Question 6

what does the method ‘Dimensionless groups’ allow

Answer 6

pump and turbine manufacturers

to test a relatively small number of machines

Question 7

how many dimensionless groups will be used to describe performance

Answer 7

12 dimensionless groups

Question 8

what are the variables of rotodynamic machines (8)

Answer 8

fluid pressure
 temperature at inlet 
 outlet flow to the machine,
 the rate of flow,
 speed of rotation, 
 power,
 thrust,
 torque,

Question 9

what do 12 dimensionless groups provide

Answer 9

the similarity laws for a family of geometrically

similar rotodynamic machines.

Question 10

what is a turbine and what does it do

Answer 10

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.

Question 11

types of water turbines (2)

Answer 11

impluse (pelton wheel) and reaction turbines (Kaplan or francis)

Question 12

how does a impluse turbine work

Answer 12

hydraulic energy of fluid is converted to kentic energy via movement of impeller

Question 13

how does a reaction turbine work

Answer 13

develops power from the combined action of pressure and moving water
placed in water stream, flowing over blades

Question 14

what is cavitation

Answer 14

formation of vapor bubbles of a flowing liquid where the pressure of liquid falls below its vapor pressure

Question 15

causes of cavitation (2)

Answer 15

low boiling pressure
gas release
or both

Question 16

negative effects of cavitation (2)

Answer 16

efficiency loss

erosion damage causing failure

Question 17

how to stop cavitation

Answer 17

pressure inside pump/turbine higher than vapour pressure of liquid

liquid pressure higher than vapour pressure

Question 18

why is actual energy different from Euler theory

Answer 18

due to non-uniform velocity and frictions

Question 19

explain flow regime 1

Answer 19

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.

Question 20

explain flow regime 2

Answer 20

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*

Question 21

explain flow regime 3

Answer 21

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

Question 22

explain flow regime 4

Answer 22

further decrease in Pb, becoming less than Pe, becoming under-expanded because back pressure lowe than exit plane pressure

Question 23

how is Euler equation derived, also discuss importance in rotodynamic machine theory

Answer 23

derived from newton’s second law applied to angular momentum.
it provides the theoretical energy transfer between fluid and the rotor

Question 24

why does actual energy transferred between impeller and fluid differ from theoretical energy predicted by Euler equation

Answer 24

because of the losses due to non-uniform velocity at blade and friction in impeller

Question 25

what is compressibility, and how to differ from compressible and incompressible fluid

Answer 25

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

Question 26

for shock waves what do Fannomlines represent

Answer 26

curves that prove continuity and energy equations,

providing a graphical representation of shock waves

Question 27

difference between venturi and supersonic

Answer 27

venturi = flow everywhere subsonic (m<1)
supersonic = M<1 in converging section, M=1 in throat and M>1 in diverging section