Separate effects experiments for air-ingress in helium filled vessel

•Separate effects experiments on gravity-driven air-ingress in helium filled vessel.•Effect of pipe-breaks’ geometric parameters on air-ingress is investigated.•Gravity-driven air-ingress demonstrates three characteristic stages.•A unique non-dimensional density ratio characterizes the transition between stages.•Predictive models are developed for gravity-driven exchange between air and helium. This study performs scaled separate-effects experiments to investigate the gravity-driven ingress of air into a helium filled vessel. Experiments are performed under both adiabatic and heated conditions up to 100°C. An oxygen analyzer is employed to measure the transient oxygen concentration inside the test vessel during the ingress event. The air-ingress transient is found to be characterized by three distinct stages, namely: (a) The initial stage where the exchange flow rate linearly increases with time until it reaches its maxima and makes a transition; (b) The intermediate stage where the exchange flow rate decreases from its maximum value and varies non-linearly with time; and (c) The final stage of ingress where the exchange rate decreases asymptotically to zero towards the end of the ingress. An extensive oxygen concentration transient database is established accounting for various effects including the pipe-break size (or length-to-diameter ratios), break location, break orientation and the initial helium temperature. Predictive models are developed for the mixture density transient and the exchange flow rate during entire ingress process. Additionally, a non-dimensional critical density ratio is defined that determines the point of transition between the initial and the intermediate stages of the ingress process.