Extension of the age-of-fluid method to unsteady and closed-flow systems

The age-of-fluid method is revisited to evaluate and extend it from its traditional application domain of steady, open-flow systems to general unsteady and/or closed-flow systems. The most significant change in the approach comes from the definition of the age of the fluid in a purely kinematic way as a tagging variable, and based solely on the velocity field of the underlying fluid, whereas all the other methods proposed to date somehow determine the age indirectly by tracking an injected tracer material in the flow domain. The resulting mathematical formulation makes it possible to specify the age at arbitrary positions within the flow domain, while being formally very similar to the classical age-of-fluid transport equations previously obtained. This has the benefit of clarifying a number of issues such as the treatment of the diffusion coefficient, the definition of suitable boundary conditions, the validation in unsteady flow conditions, and the extension to closed-flow domains. (C) 2004 American Institute of Chemical Engineers.