A Multi-Zone Model for Hydrogen Accumulation and Ventilation in Enclosures

Due to the small characteristic molecular size of hydrogen, small leaks are more common in hydrogen systems compared to similar systems with hydrocarbons. This, together with the high reactivity, makes an efficient ventilation system very important in hydrogen applications. There are several models available for ventilation sizing that are based on either a well-mixed assumption or a fully stratified situation. However, experiments show that many realistic releases will be neither, and therefore additional models are needed. One possibility is to use CFD-models, but the small release sizes for pinhole releases (<<1 mm) make it difficult to find an appropriate mesh without excessive computational time (especially since the simulations need to be iterated to find the optimum ventilation size). An alternative approach, which is described and benchmarked in the current paper, is to use a multi-zone model where the domain is divided into several large cells where the mass exchange is simplified compared to CFD, and thus simulation time is reduced. The flow in the model is governed by mass conservation and density differences, due to concentration gradients, using the Bernoulli equation. The release of gas generates a plume which is modelled based on an empirical plume model which gives the entrainment and hydrogen source term for each cell. The model has a short run time and will therefore allow optimization in a short time frame. The model is benchmarked against five experiments with helium at the Canadian Nuclear Laboratories (CNL) in Canada and one hydrogen experiment performed at Lodz University of Technology in Poland. The result shows that the model can reasonably well reproduce accumulation in the experiments with small release without ventilation, but appears to slightly underestimate the level of stratification and the interface height for ventilated cases where the source is elevated from the floor level.