Prediction of the fluid flow, heat transfer and inactivation of microorganism at medical devices in modern steam sterilizers using computational fluid dynamics

Medical devices, which are wrapped in pouches, are generally sterilized using steam sterilizers (autoclaves) in order to achieve a certain level of sterility. During the sterilization process, the medical devices are heated up to a predefined temperature of 134 C at a pressure level of 3.1 bar. Steam is commonly used in the sterilizers because of the high heat transfer rate to the medical devices due to wall condensation effects. The sterility level of the devices is represented by the Safety Assurance Level (SAL) which depicts the amount of inactivated microorganisms at the surface. Since the entire sterilization process consists of several different transport phenomena, such as fluid flow, heat transfer, phase change etc., the prediction of the medical devices’ SAL is a complex task. In the present paper, Computational Fluid Dynamics (CFD) was used to calculate the temperature, pressure, steam quality and, consequently, the medical devices’ SAL temporarily and spatially resolved. For that purpose, model approaches to predict the steam penetration of the pouches and the heat transfer due to wall condensation of the steam were developed and implemented in the CFD code. Therefore, an accurate simulation of the temperature and steam quality in the sterilizer is possible. The CFD results were compared to the measured temperatures of the steam and the devices. The comparison corresponded well for the entire sterilization process and provided a sound basis for a precise simulation of the SAL which was done by a first order reaction kinetics based on the Arrhenius approach. The results presented in this paper show that the developed CFD model is able to predict the temperature, pressure, steam quality as well as the progress of the sterilization process in an accurate manner, and is beneficial for future developments of steam sterilizers in the medical industry.