The importance of Soret effect, preferential diffusion, and conjugate heat transfer for flashback limits of hydrogen-fueled perforated burners

Abstract

Avoiding flashback is a primary challenge in the development of modern burners, which should be capable of substituting natural gas with hydrogen in domestic end-user devices. The heat exchange between the burner plate and the burned and unburned gases, as well as the effects of preferential diffusion, significantly impact the flashback of such burners fueled with hydrogen. For these effects, the design of the burner plate plays a pivotal role. In this study, three-dimensional simulations with detailed chemistry have been performed to investigate the effect of three competing physical mechanisms, namely, preheating of fresh gases, preferential diffusion, and Soret effect, which drive the flame flashback dependence on the holes/slits size. Two different geometries are considered: circular holes with varying diameters and slits with fixed lengths but different widths. Steady-state simulations with decreasing inlet velocities are employed to estimate the critical inlet velocity for flashback. Conjugate heat transfer (CHT) is considered for the heat exchange between the burner plate and the gases. For circular holes, the enclosed geometry promotes more effective heat transfer, leading to a higher influence of preheating effects for small diameters. This results in a non-monotonic dependence on hole size, with a non-trivial optimum diameter to avoid flashback. This behavior is specific to circular holes and differs from that observed in previously studied infinitely long slits, where a linear dependence on the slit width was found. Additionally, the individual influence of non-unity Lewis numbers and Soret diffusion is analyzed. Notably, the Soret effect, in combination with CHT, is found to instaurate a strong, non-linear, self-accelerating mechanism that has a leading-order effect on the flashback propensity of larger holes. This finding underscores the necessity of including both effects in numerical simulations for accurate estimations of the flashback limits in domestic burners.

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