Numerical Study on the Influence of Different Boundary Conditions on the Efficiency of Hydrogen Recombiners Inside a Car Garage

Passive auto-catalytic recombiners (PARs) have the potential to be used in the future for the removal of accidentally released hydrogeninside confined areas. PARs could be operated both as stand-alone or backup safety devices, e.g.in case of active ventilation failure.Recently, computational fluid dynamics (CFD) simulations have been performed in order to demonstrate the principal performance of a PAR during a postulatedhydrogen release inside a car garage.This fundamental study has now been extended towards a variation of several boundary conditions including PAR location, hydrogen release scenario, and active venting operation. The goal of thisenhanced study is to investigate the sensitivity of the PAR operational behaviorfor changing boundary conditions, and to support the identification of a suitable PAR positioning strategy. For the simulation of PAR operation, the in-house code REKO-DIREKT has been implemented inthe CFD code ANSYS-CFX 15.In a first step, the vertical position of the PAR and the thermalboundary conditions of the garage walls have been modified. In a subsequent step, different hydrogen release modes have been simulated, which result either in a hydrogen-rich layer underneath the ceiling or in a homogeneous hydrogen distribution inside the garage. Furthermore, the interaction of active venting andPAR operation has been investigated.As a result of thisparameter study, the optimum PAR location was identified to be close underneath the garage ceiling. In case of active venting failure, the PAR efficiently reducesthe flammable gas volume (hydrogen concentration > 4 vol.%2 for both stratified and homogeneous distribution. However, the simulations indicatethat the simultaneous operation of active venting and PAR may in some cases reduce the overall efficiency of hydrogen removal. Consequently, a well-matched arrangement of both safety systems is required in order to optimizethe overall efficiency.The presented CFD-basedapproach is an appropriate tool to support the assessment of the efficiency of PAR application for plant design and safety considerations with regard to the use of hydrogen inconfined areas.