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
