Numerical modelling and simulation of hydrogen and air mixing to prevent ignition delay and flashback

Practically all residential and commercial gas appliances installed within the EU today were designed for operation with natural gas. A clean and efficient solution for heating and hot water generation is the combustion of hydrogen in case of a gas condensing boiler. Safe and stable combustion of hydrogen is a complex issue, and several influence parameters must be understood for the safe design of hydrogen capable gas condensing boilers. In premixed hydrogen-air combustion, there are two physical problems that should be avoided at all costs: flashback through the burner to the premixing duct and ignition delay in the combustion chamber. Well mixing of reactants is therefore, very important to achieve stable and efficient hydrogen combustion. To evaluate the influence of commercialized mixing stages (fan-venturi combination), the impact of the rotational velocity on the degree of mixing in case of a venturi-fan combination used in domestic gas condensing boilers is presented in this paper. Transient, 3-D simulations with different turbulence modelling approaches were performed to assess the degree of mixing upstream the hydrogen capable multi-hole burner. It is shown that the lower angular velocities produce better mixing. It can also be assumed that a local variation of up to 17% in the adiabatic flame speed can be expected due to the mixing processes as the consequence of the local air-fuel equivalence ratio variation.