Among the various technologies available for small-scale combined heat and power plants, the ORC process (Organic Rankine Cycle) represents an economically and environmentally effective technology. In place of water, as used in conventional Rankine process, an organic fluid with favourable thermodynamic properties is used as working medium.
This paper describes CFD (Computational Fluid Dynamics) simulations carried out on a 2D stage of an existing ORC turbine. The working fluid has been modelled both as real gas and as ideal gas. In both cases the same boundary conditions were applied. Real gas properties are described by Redlich Kwong equation of state (Aungier form), available in ANSYS CFX-11. The Redlich Kwong equation is considered one of the most accurate two parameter equations of state. It only requires that the user knows the fluid critical temperature and pressure. The Aungier form of the equation employs the acentric factor and the critical volume as additional parameters to improve its accuracy and extend its application range to include the critical point.
A comparison of the computed Mach number, velocity and density distributions as well as mass flows shows that large relative differences exist between the two simulations. The CFX Redlich Kwong implemented model is more suitable than the ideal gas model to describe the behaviour of organic working fluids. |
