Brayton cycle (thing)

In thermodynamics, the Brayton cycle is a thermodynamic cycle consisting of four processes. The processes are identical to those of the Rankine cycle except that in a Brayton cycle, the working fluid is always superheated, unlike the Rankine cycle where the working fluid changes state. The Brayton cycle is the ideal cycle for jet propulsion engines. The processes are:

1-2 Isentropic compression

2-3 Isobaric heat addition

3-4 Isentropic expansion

4-1 Isobaric heat rejection

In practice the cycle is not actually a cycle but is often modeled as one. For jet propulsion in which the working fluid is air (and some amount of fuel during certain processes), the fourth process is not actually present. Since the "cycle" is performed by in-taking air from the surroundings and rejecting it after it expands, there is in reality no heat rejection. However for the sake of thermodynamic analysis, we often consider the cycle to be closed and the final process to be modeled by a heat exchanger which returns the working fluid to the initial state. Remember of course that this is just a cycle and could be used to generate work for any purpose, not just thrust.

For the ideal cycle, no work is done in processes 2-3 and 4-1, and no heat is transfered in processes 1-2 and 3-4. Using air-standard assumptions the analysis is quite easily:

q_in = h₃ - h₂

q_out = h₄ - h₁

w_in = h₂ - h₁

w_out = h₃ - h₄

η_th = w_net/q_in = {(h₃ - h₄) - (h₂ - h₁)}/{h₃ - h₂}

In general, the working fluid fluctuates in temperature too much for cold air-standard analysis(i.e. constant specific heats) to be valid, thus isentropic processes are generally analyzed by comparing relative pressures at various states. The ratio of pressures for the cycle (since there are only two distinct pressures) is often appropriately termed the pressure ratio r_p = P₂/P₁ = P₃/P₄. Relative pressure analysis is done by noting that r_p = P_r2/P_r1 = P_r3/P_r4, where P_ri is the relative pressure, a function of temperature alone for an ideal gas.

The Brayton cycle is reasonably efficient with thermal efficiencies of above 65%¹ being possible, however the compressor work for a Brayton cycle is typically quite large, leaving much room for improvement. To improve efficiency, intercooling, reheating, and regeneration may be introduced.

Intercooling consists of breaking up the process of compression into a number of smaller compression processes between which the gas is cooled. The net effect such that less work will need to be done in order to compress the gas to a necessary pressure. However the necessitates the use of multiple compressors.

Reheating is similar to intercooling except that it consists of dividing the turbine work into multiple turbines with the gas heated between expansions. Like intercooling, the net result is more work extracted but it necessitates the use of multiple turbines which may be expensive.

Regeneration consists of having the working fluid after expansion exchange its waste heat with the fluid before it enters the turbine. The effect is a reduction in the required heat input and thus improved efficiency.

The Brayton cycle may be run backwards to operate as a refrigeration cycle. This reversed cycle is often called the reversed Brayton cycle or the Bell Coleman cycle.

¹ Source: Cengel and Boles, Thermodynamics, an Engineering Approach. Chapter 9: Gas Power Cycles