Lecture 7 Flashcards
Power cycles are ways of
converting an energy transfer in the form of heat into an energy transfer in the form of work
petrol engine
otto cycle petrol
diesel engine
diesel cycle diesel
jet engine and gas turbine
brayton cycle kerosane jet gas/oil for gas
coal power station
nuclear power
rankine
rankine
heat sink
receives energy in the form of heat
heat engine
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)
a heat engine must
work between a high temp and a lower temp
thermal efficiency
work out/heat high
where work = heat high - heat low
for heat engine itself entropy
stays the same since it always remains teh same internally
maximum thermal efficiency =
(Thigh - Tlow)/Thigh
what is maximum thermal efficiency known as
Carnot efficiency
since some heat from heat source ends in the heat sink the second law can be stated as
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
heat engine works between
heat source and a heat sink
area under the T-s diagram
Heat in
why is the work out only the top part of the area under ts diagram
area on the bottom part is work in, amount of you to put back in to keep cycle going
area encoled by the Ts diagram
is the work out for a reversible cycle
if cycle goes clockwise on Ts diagram net work is
out if it goes anticlockwise net work is inwards - no longer power cycle
if cycle goes clockwise on pv diagram net work is
out if it goes anticlockwise net work is inwards - no longer power cycle
carnot cycle comprises of
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
four processes of the carnot cycle
1 isothermal expansion W=Q dH=0
2 isentropic expansion Q=0
3 Isothermal compressor W=Q dH=0
4 isentropic compressor Q=0
why does the ts diagram prove the carnot cycle is the most efficient
made a box
maximum area for given minimum and maximum temperatures
carnot cycle complete cycle is reversible if
isothermal expansion and compression are reversible and
the expansion and compression are isentropic
issues with making a carnot cycle
isentropic is impossible and isothermal costs are huge and extremely difficult to engineer
carnot cycle all heat added at
maximum temperature dS=dQ/T make T as large as possible minimise entropy increase
carnot principle
Q high/ Q low = Thigh/Tlow
given carnot efficiency what would you want to maximise but would struggle to do so
Thigh - Tlow to be as big as possible - issues with operating range of materials
T high melting T low freezing
Brayton cycle contrains
is an open system containing three reversible processes
1 isentropic compressor
2 isobaric heat addition (combuster)
3 isentropic turbine
reversible =
max output
no line between 4 and 1
air taken in fresh each time (not quite a cycle)
% of turbine power used to drive compressor
1 -(net work / total turbine work)
net work
work out of turbine - work needed to drive compressor
add in isentropic efficiency
more work into compressor
less work out of turbine
(though do need to put less heat in is compressor output temp is higher)
additional step taken when dealing with isentropic efficiencies
multiply (or divide) isentropic case temperature difference by efficiency to add (or minus) from starting temperature
effect of combustor pressure
causes pressure drop across combustor making outlet temperature higher
when calculating pressure drop across combustor
compressor same
need to calculate need starting pressure for turbine
starting temp will remain same
compressor outlet pressure =
turbine inlet pressure unless theres pressure drop
brayton cycle equations
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
