We were discussing Otto
cycle, an ideal cycle for internal combustion spark ignition reciprocating
engines or simply petrol engines and also Diesel
cycle, the ideal cycle for the operation of internal combustion compression
ignition reciprocating engines in our previous posts. We have also
seen the Brayton cycle, an ideal cycle for gas turbine engine in our recent
post.
Today we will see here the effect of regeneration on
Brayton cycle efficiency with the help of this post.
Effect of regeneration on Brayton cycle efficiency
Let us recall the basic of reversible heat engine
efficiency, as we know that efficiency of any reversible heat engine depends on
the average temperature of heat energy addition and also on average temperature
of heat energy rejection.
Efficiency of any reversible heat engine = 1- T2/T1
T2= Average temperature of heat energy
rejection
T1= Average temperature of heat energy Addition
Efficiency of any reversible heat engine will be
increased with increase in average temperature of heat energy addition and
efficiency of any reversible heat engine will also be increased with decrease
in average temperature of heat energy rejection.
This concept is used here in order to improve the
thermal efficiency of Brayton cycle. We will see here the effect of
regeneration on closed cycle gas turbine engine and in similar way we can also
see the effect of regeneration on open cycle gas turbine engine.
We can increase the thermal efficiency of the
Brayton cycle with the concept of regeneration. Air leaving the compressor is
heated in a heat exchanger before entering to the heating chamber with the help
of exhaust air leaving the turbine.
We can say that thermal efficiency of the Brayton
cycle could be increased by using some part of energy of exhaust air leaving
the turbine for heating the air coming from compressor before it enters to the
heating chamber.
Heat exchanger which is used here, for heating the
air coming from compressor before it enters to the heating chamber with the
help of some part of energy of exhaust air leaving the turbine, will be termed
as regenerator.
Let us see the following figure, basic arrangements
of various components and TS diagram for Brayton cycle with regeneration, which
indicates the effect of regeneration on thermal efficiency of the Brayton
cycle.
Process 1-2: Adiabatic compression of the working
fluid
Process 2-3: Heat energy addition to the working
fluid at constant pressure in regenerator
Process 3-4: Heat energy addition to the working
fluid at constant pressure in heating chamber
Process 4-5: Adiabatic expansion of the working
fluid through turbine or also termed as power stroke
Process 5-6: Rejection of heat energy for heating
the air coming from compressor at constant pressure in regenerator
Process 6-1: Rejection of heat energy at constant pressure
in cooling chamber
We can see from TS diagram that without considering the
effect of regeneration, average temperature of heat energy addition will be
between 2 and 4 and similarly average temperature of heat energy rejection will
be between 5 and 1.
When we have applied the concept of regeneration, we
can easily say that average temperature of heat energy addition will be between
3 and 4. Because working fluid i.e. air will be heated from 2 to 3 in
regenerator with the help of some part of energy of exhaust air leaving the turbine.
Therefore we can say that that average temperature of heat energy addition will
be increased.
Similarly, average temperature of heat energy
rejection will be between 6 and 1 and therefore we can say that that average
temperature of heat energy rejection will be reduced.
So, we have concluded two important points from the
concept of regeneration in Brayton cycle and these two important points are as
mentioned here
Average temperature of heat energy addition will be
increased
Average temperature of heat energy rejection will be
reduced
As we have discussed so many times that efficiency
of any reversible heat engine will be increased with increase in average
temperature of heat energy addition and efficiency of any reversible heat
engine will also be increased with decrease in average temperature of heat
energy rejection.
Therefore Brayton cycle with the concept of regeneration
will have better efficiency
Thermal efficiency of closed regenerative Brayton cycle
Heat energy added from external source in heating
chamber
Q1 = h4-h3 = m CP
(T4-T3)
Heat energy rejected in cooling chamber
Q2 = h6-h1 = m CP
(T6-T1)
Thermal efficiency of the closed regenerative Brayton
cycle will be determined as following
Efficiency = 1-Q2/Q1
η= 1- [(h6-h1)/ (h4-h3)]
η= 1- [(T6-T1) / (T4-T3)]
Do you
have any suggestions? Please write in comment box.
We will
see another topic i.e. "Concept of regeneration in Rankine cycle" 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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