A Computational Modelling Approach to the Design of Stable Ventilation Systems

Abstract

The well-known glove box arrangement used in the nuclear industry maintains a pressure depression within a contaminated volume while allowing passage of a small purge flow to the filtration plant. Conventionally, if a glove tears for example, an electromechanical control system restores the depression by opening and regulating an extra connection to the filtration duct. However, faster response and lower maintenance overheads result from replacing the conventional system with the fluidic vortex amplifier (VA) but coupled ventilation system and VA dynamics can be unstable. Designers needed a tool to predict oscillatory performance and, ideally, to advise on the strengths of influence of network parameters on stability. A system modelling approach is presented in which pipework dynamics, including steady state friction, are presented by transmission line modelling (TLM). A semi-empirical feedback model of linearized VA dynamics has been developed and shown to be substantially correct. The model's restriction to small signals does not adversely affect investigation of perturbation growth or decay. Partial transmission line representation of the VA model with special measures for feedback allowed time-domain simulations of network perturbations. However, this approach is indirect and it was possible to convert the whole VA model to transmission lines and then reformulate the TLM scheme into a direct, quantitative stability analysis. Extension of the technique provided a means of assessing the sensitivity of the stability to simple network alterations. The method has been programmed on an 80386 PC and successfully used on a large ventilation system. Success in the present context of dynamic stability for fluidic nuclear ventilation networks should be repeatable in other areas since the approach is applicable to any TLM-amenable system, be it fluid, mechanical, electrical or mixed.