Lecture 7

Question 1

Power cycles are ways of

Answer 1

converting an energy transfer in the form of heat into an energy transfer in the form of work

Question 2

petrol engine

Answer 2

otto cycle petrol

Question 3

diesel engine

Answer 3

diesel cycle diesel

Question 4

jet engine and gas turbine

Answer 4

brayton cycle kerosane jet gas/oil for gas

Question 5

coal power station

nuclear power

Answer 5

rankine

rankine

Question 6

heat sink

Answer 6

receives energy in the form of heat

Question 7

heat engine

Answer 7

device that takes energy in the form of heat from a hot reservoir converts it into work engine then reject heat to somewhere that is a lower temperature (heat sink)

Question 8

a heat engine must

Answer 8

work between a high temp and a lower temp

Question 9

thermal efficiency

Answer 9

work out/heat high

where work = heat high - heat low

Question 10

for heat engine itself entropy

Answer 10

stays the same since it always remains teh same internally

Question 11

maximum thermal efficiency =

Answer 11

(Thigh - Tlow)/Thigh

Question 12

what is maximum thermal efficiency known as

Answer 12

Carnot efficiency

Question 13

since some heat from heat source ends in the heat sink the second law can be stated as

Answer 13

it is impossible to construct a system that will operate in a cycle, extract heat from a reservoir and do an equivalent amount of work on the surroundings

Question 14

heat engine works between

Answer 14

heat source and a heat sink

Question 15

area under the T-s diagram

Answer 15

Heat in

Question 16

why is the work out only the top part of the area under ts diagram

Answer 16

area on the bottom part is work in, amount of you to put back in to keep cycle going

Question 17

area encoled by the Ts diagram

Answer 17

is the work out for a reversible cycle

Question 18

if cycle goes clockwise on Ts diagram net work is

Answer 18

out if it goes anticlockwise net work is inwards - no longer power cycle

Question 19

if cycle goes clockwise on pv diagram net work is

Answer 19

out if it goes anticlockwise net work is inwards - no longer power cycle

Question 20

carnot cycle comprises of

Answer 20

4 reversible processes (can be air or steam)
open system carnot cylce for air
1 isothermal turbine
2 isentropic turbine
3 isothermal compressor
4 isentropic compressor

Question 21

four processes of the carnot cycle

Answer 21

1 isothermal expansion W=Q dH=0
2 isentropic expansion Q=0
3 Isothermal compressor W=Q dH=0
4 isentropic compressor Q=0

Question 22

why does the ts diagram prove the carnot cycle is the most efficient

Answer 22

made a box

maximum area for given minimum and maximum temperatures

Question 23

carnot cycle complete cycle is reversible if

Answer 23

isothermal expansion and compression are reversible and

the expansion and compression are isentropic

Question 24

issues with making a carnot cycle

Answer 24

isentropic is impossible and isothermal costs are huge and extremely difficult to engineer

Question 25

carnot cycle all heat added at

Answer 25

maximum temperature dS=dQ/T make T as large as possible minimise entropy increase

Question 26

carnot principle

Answer 26

Q high/ Q low = Thigh/Tlow

Question 27

given carnot efficiency what would you want to maximise but would struggle to do so

Answer 27

Thigh - Tlow to be as big as possible - issues with operating range of materials
T high melting T low freezing

Question 28

Brayton cycle contrains

Answer 28

is an open system containing three reversible processes
1 isentropic compressor
2 isobaric heat addition (combuster)
3 isentropic turbine

Question 29

reversible =

Answer 29

max output

Question 30

no line between 4 and 1

Answer 30

air taken in fresh each time (not quite a cycle)

Question 31

% of turbine power used to drive compressor

Answer 31

1 -(net work / total turbine work)

Question 32

net work

Answer 32

work out of turbine - work needed to drive compressor

Question 33

add in isentropic efficiency

Answer 33

more work into compressor
less work out of turbine
(though do need to put less heat in is compressor output temp is higher)

Question 34

additional step taken when dealing with isentropic efficiencies

Answer 34

multiply (or divide) isentropic case temperature difference by efficiency to add (or minus) from starting temperature

Question 35

effect of combustor pressure

Answer 35

causes pressure drop across combustor making outlet temperature higher

Question 36

when calculating pressure drop across combustor

Answer 36

compressor same
need to calculate need starting pressure for turbine
starting temp will remain same

Question 37

compressor outlet pressure =

Answer 37

turbine inlet pressure unless theres pressure drop

Question 38

brayton cycle equations

Answer 38

W/Q=mcp(T1-T2)
and polytropic equations
use polytropic equations to find final temperatures and pressure
use isentropic efficiency to correct them
calculate heat in and out