We were
discussing Brayton
cycle, an ideal cycle for gas turbine engine in our recent post. We have
also seen the effect
of regeneration on Brayton cycle and
effect of inter-cooling on Brayton cycle in our previous posts.
Today we
will see here the effect of reheating on Brayton cycle with the help of this
post.
Brayton cycle with reheating
Work output from the turbine, operating between two specific pressure levels, could be increased
by expanding the working fluid i.e. gas or air in stages and reheating the working
fluid between two successive stages. This method of increasing the net work output
from the turbine is termed as multi-stage expansion with reheating.
With technique
of multi-stage expansion with reheating, work output from the turbine could be
increased without increasing the maximum temperature in the cycle.
Combustion in case of gas turbines will typically occur
at four times the quantity of air required for the complete combustion in order
to avoid the excessive temperatures. Hence, the exhaust gases will be rich in O2
and therefore reheating could be accomplished by simply spraying the additional
fuel into the exhaust gases between two expansion states.
Let us see
the following figure, we will have basic arrangements of various components, PV
diagram and TS diagram here to show the effect of reheating on Brayton cycle.
Process
1-2: Adiabatic compression of the working fluidÂ
Process
2-3: Heat energy addition to the working fluid at constant pressure in heating
chamber. Â
Process
3-4: Adiabatic expansion of the working fluid through turbine T1
Process
4-5: Heat energy addition to the working fluid at constant pressure as a result
of reheat in a heat exchanger which is called as re-heater.
Process
5-6: Adiabatic expansion of the working fluid through turbine T2
Process
6-1: Rejection of heat energy at constant pressure in cooling chamber
Process 3-4’:
Adiabatic expansion of the working fluid through the turbine from one specified
pressure level to another specified pressure level without reheat.
Therefore,cycle 1-2-3-4-5-6-1 indicates the gas turbine cycle with reheating having two
stage expansion and cycle 1-2-3-4’-1 indicates the gas turbine cycle without reheating
i.e. ideal closed gas turbine cycle or ideal closed Brayton cycle.
Do you
have any suggestions? Please write in comment box.
We will
see another topic in our next post in the category of thermal engineering.
Reference:
Engineering
thermodynamics by P. K. Nag
Engineering
thermodynamics by Prof S. K. Som
Image
courtesy: Google
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