This session discusses the thermodynamics of adiabatic compressions and expansions.
Much of the presentation being valid for both types of processes, we first deal with compression, the specifics relating to expansion being given at the end.
As the components in which these processes take place are assumed adiabatic (that is usually the case in real machines), their heat exchange with the surroundings is zero.
It follows that the reference process is reversible adiabatic, also called isentropic.
Moreover, at the entrance and exit of these components, the velocities of fluids are always relatively low, so that the kinetic energies are negligible.
Under these conditions the first principle writes simply for heat exchangers: Δh = τ
Calculate the heat brought into play in these components is to determine the enthalpy change of the fluid flowing through them.
To follow the presentation, go to next step.
(Session realized on 06/16/11 by Renaud Gicquel)It is important to keep in mind that the isentropic efficiency of a compressor depends on the compression ratio, whatever the technology used.
For a displacement compressor, the law giving its value must be determined experimentally.
For a multi-stage dynamic compressor, it can be estimated if one knows the polytropic efficiency.
The underlying assumption in the polytropic approach is indeed that irreversibilities are uniformly distributed throughout the compression.
This hypothesis is well respected in the multi-stage dynamic compressor all stages of which have the same compression isentropic efficiency.
The isentropic efficiency of this stage can then be regarded as the polytropic efficiency.
This important issue is discussed thoroughly in the textbook.
This session discusses the thermodynamics of compressions and expansions.
We have shown that: