Friday, 9 December 2016

EFFECT OF REGENERATION ON BRAYTON CYCLE EFFICIENCY

EFFECT OF REGENERATION ON BRAYTON CYCLE EFFICIENCY

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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