Brayton Cycle And Its Process

A simple gas turbine plant

A simple gas turbine powerplant is shown in the above diagram. Air is first compressed adiabatically in process 1-2, it then enters the combustion chamber where fuel is injected and burned essentially at constant pressure in process 2-3 , then the products of combustion expand in the turbine to the ambient pressure in the process 3-4 and are thrown out to the surroundings. The cycle is open . The state diagram on the P-V diagram is shown below. Open cycles are used in aircraft, automotive (buses and trucks) and industrial gas turbine installations.

State diagram of a gas turbine pant on p-v plot

The Brayton Cycle is the air standard cycle for the gas turbine power plant. Here air is first compressed reversibly and adiabatically, heat is added to it reversibly at constant pressure, air expands in the turbine reversibly and adiabatically, and heat is then rejected from the air - reversibly at constant temperature to bring it to the initial state. The Brayton cycle, therefore, consists of two reversible isobars and two reversible adiabatics.

The flow, p-v and T-s are shown below in diagram. For m kg of air ,

Brayton cycle

Q1 = heat supplied = mCp ( T3 -T2 )

Q2 = heat rejected = mCp ( T4 - T1)

Cycle efficiency , N = 1 - ((T4 - T1) / (T3 - T2))

Now,

T2 / T1 = ( p2 / p1) ^((y-1) / y) = T3 / T4 (Sice p2 = p3, and p4 =p1 )

(T4 / T1) - 1 = (T3 / T2) - 1

or (T4-T1) / ( T3 - T2) = T1 /T2 = ( p1 / p2 ) ^ ( (y-1) / y) = ( v2 / v1 ) ^ (y-1)

If rk = compression ratio = v1 / v2 the efficiency becomes

N = 1 - (v2 / v1 ) ^ (y-1)

N (brayton) = 1 - 1 / (rk^(y-1))

If rp = pressure ratio = p2 / p1 the efficiency may be expressed in the following form also,

N = 1 - 1 / (rp ^ ((y-1)/ y))

N brayton = 1 - 1 / (rp ^ ((y-1)/y))

The efficiency of the Brayton cycle, therefore, depends upon either the compression ratio or the pressure ratio. For the same compression ratio, the Brayton cycle efficiency is equal to the Otto cycle efficiency.

A closed cycle gas turbine plant is used in gas - cooled nuclear reactor plant, where the source is a high temperature gas -cooled reactor (HTGR) supplying heat from the nuclear fission directly to a working fluid (a gas).

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