For ideal gas αT = 1 and therefore:Īt constant pressure, the enthalpy change equals the energy transferred from the environment through heating:ĭH = dQ → Q = H 2 – H 1 → H 2 – H 1 = C p (T 2 – T 1 )Īt constant entropy, i.e., in isentropic process, the enthalpy change equals the flow process work done on or by the system:ĭH = Vdp → W = H 2 – H 1 → H 2 – H 1 = C p (T 2 – T 1 ) Ideal Brayton cycle consist of four thermodynamic processes. Where C p is the heat capacity at constant pressure and α is the (cubic) thermal expansion coefficient. There are expressions in terms of more familiar variables such as temperature and pressure: As can be seen, this form of the law simplifies the description of energy transfer. There are no changes in the control volume. This work, Vdp, is used for open flow systems like a turbine or a pump in which there is a “dp”, i.e., change in pressure. In this equation, the term Vdp is a flow process work. To calculate the thermal efficiency of the Brayton cycle (single compressor and single turbine) engineers use the first law of thermodynamics in terms of enthalpy rather than in terms of internal energy. The net heat rejected is given by Q re = H 4 – H 1Īs can be seen, we can describe and calculate (e.g., thermodynamic efficiency) such cycles (similarly for Rankine cycle) using enthalpies. Isobaric heat rejection – the residual heat must be rejected to close the cycle.The work done by the turbine is given by W T = H 4 – H 3 Isentropic expansion – the heated, pressurized air then expands on a turbine, gives up its energy. ![]() The net heat added is given by Q add = H 3 – H 2 It is a constant-pressure process since the chamber is open to flow in and out.
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