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Saturday, 10 December 2016

BRAYTON CYCLE WITH REHEATING

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 intercooling 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 fluid 

Process 2-3: Heat energy addition to the working fluid at constant pressure in heating chamber.  

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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