Abstract

Autoclaving in the medical domain is the process which uses high temperature pressurized steam to sterilize medical instruments. Ability to predict sterilization performance for a given load and autoclave operating conditions can contribute to maximizing the reliability of a complete work cycle. A Computational Fluid Dynamics methodology incorporating a suitable condensation model leading to accurate prediction of the temperature field in the autoclave chamber is a pre-requisite to be able to predict sterilization performance for a given load. Present work describes CFD modeling of the sterilization phase in an autoclave cycle using the density based wet steam model available in ANSYS FLUENT. Current approach has been arrived after a lot of efforts overcoming numerical problems (e.g. convergence, condensation capture) associated with the direct application of density based wet steam approach. Some problems w.r.t convergence and slight mass imbalance still persist. Efforts are being made to sort out such imbalance related issues. Despite these concerns simulation results show that condensation has been captured both in 2D and 3D models using the present methodology. The report describes results of steady state calculations for prescribed inlet and outlet pressure BCs for a 185L industrial scale autoclave. The chosen boundary conditions are 2.72bar and 403K at the inlet so that the chamber attains a steady state pressure of ~2bars and temperature ~395K. The outlet is a pressure boundary opening to the atmosphere. To attain quick convergence, simulation is carried out with pressure-based algorithm. Once the converged solution is achieved, the setting is then switched to density-based algorithm and subsequently wet steam model of DB algorithm is invoked on the converged solution. Thus the calculation scheme is as follows: PB-WV-SS?DB-WV-SS?DB-WSOFF-SS? DB-WSON-SS? DB-WSON-TR : where (PB:-Pressure-Based; DB:-Density-Based; WV:-Water-Vapor; SS:-Steady-State; WS:-Wet-Steam; TR:-Transient). Such step-by-step implementation of complex physics in the solver results in fairly accurate capture of steam condensation while avoiding the numerical instabilities in the entire simulation cycle.