Assumptions For Analysis
In the real world the Brayton cycle is a complex process that is difficult to model. For instance; the working fluid consists of a fuel/air mixture that is undergoing a combustion process. Furthermore, compressors and turbines are not perfect; energy is lost in the friction of the bearings and heat transfer to the environment. So how can we take such a complex process and model it in a way that it is easy to understand. The following assumptions simplify the cycle to allow for an energy balance analysis.
1) The air standard assumption: This assumption states that the working fluid during the cycle can be modeled by air alone. The air can then be treated as an ideal gas. Because the air/fuel mass ratio of most Brayton cycles is typically large, this assumption has proven to be accurate for most real world applications.
2) Isentropic compressor and turbine: The compressor and turbine components of the cycle can be assumed to be isentropic, meaning that they are perfectly insulated and lose no energy to the environment. This also means that there is no irreversible energy loss due to friction.
3) Constant Pressure Heat Addition/Rejection: This assumption states that the heat addition and rejection steps occur at a constant pressure.
4) Closed System: The Brayton cycle typically operates on an open system: Starting with the intake of air from the atmosphere and rejecting it back at high velocity and temperature. However, the atmosphere acts as part of our closed system by dissipating the heated exhaust during the constant pressure heat rejection stage.
Text reference (Thermodynamics 5th Ed., Cengle, Y. p 507-508)
1) The air standard assumption: This assumption states that the working fluid during the cycle can be modeled by air alone. The air can then be treated as an ideal gas. Because the air/fuel mass ratio of most Brayton cycles is typically large, this assumption has proven to be accurate for most real world applications.
2) Isentropic compressor and turbine: The compressor and turbine components of the cycle can be assumed to be isentropic, meaning that they are perfectly insulated and lose no energy to the environment. This also means that there is no irreversible energy loss due to friction.
3) Constant Pressure Heat Addition/Rejection: This assumption states that the heat addition and rejection steps occur at a constant pressure.
4) Closed System: The Brayton cycle typically operates on an open system: Starting with the intake of air from the atmosphere and rejecting it back at high velocity and temperature. However, the atmosphere acts as part of our closed system by dissipating the heated exhaust during the constant pressure heat rejection stage.
Text reference (Thermodynamics 5th Ed., Cengle, Y. p 507-508)