Natural convection in inherently safe passive emergency systems

Since long time passive systems have been adopted in architecture due to their importance for energy saving, utilization of environmentally friendly components, low costs. The applications of passive techniques to building depend on the availability of design tools and method for the characterization of the performance of components. Therefore, in order to develop simplified design tool and increase confidence in numerical simulation through experimental validation, the aim of the paper is to study a passive system able to work in emergency conditions without any conventional energy source, minimizing their environmental impact without any decrease of reliability, modularity and economy. The paper deals with the preliminary study of the natural convection in a water heat exchanger during the transient phase of start up. The feasibility study is focused on a simple heat transfer loop based on a single closed vertical circuit, heated in the lower section and cooled in the upper one, on the base of the first important experimental results pointed out in recent years (Department of Fisica Tecnica of Genova University). The introduction of simplified design correlations will also lead to simplified calculation guidelines for thermosyphon installations. The evaluation of unsteady behaviour has been carried out characterizing a pulsed thermal source at variable frequencies and different heat flow rates, and for the evaluation of the transient duration and the study of instability, fluid velocity and temperature profiles for different circuit sections have been analyzed. A simulation of thermal-circulation with a pulsed natural convection has been carried out by means of a numerical CFD code. The comparison of the numerical simulation results with the equations governing the physical phenomena involved may lead to the definition of a design correlation for an innovative and inherently safe emergency cooling system able to remove intensive thermal loads rising in a space during accidents (i.e., fire).