Analysis of turbulent transport characteristic in hydrogen diffusion flames using direct numerical simulation

Non-premixed hydrogen flame is solved to investigate turbulent transport characteristic using direct numerical simulation with768 computing cores. Local and statistical transport behaviors are observed in nonbuoyant and buoyant flames. The absence of buoyancy results in a simplified flow field which can serve as a "cheaner" test to provide an improved understanding of transport process and assess the numerical models. Results show that a wider flame front and a lateral shift of temperature are observed in nonbuoyant flame, where diffusional transport is dominant to mixing process. While, both diffusion and convection are the principal factors in a buoyant flame, which makes flame wrinkle and results in a thinner flame. Besides, statistical information indicates that both gradient transport and counter-gradient transport are observed in hydrogen non-premixed flames, and a transition from gradient transport to counter-gradient transport is captured. A large pressure gradient associated with thermal expansion is responsible for counter-gradient transport in diffusion-controlled flames. However, convective transport is inferred to promote the occurrence of counter-gradient transport in buoyancy-driven flames. It is worth noting that traditional gradient transport model fails to completely describe the intricate scalar transport behaviors in non-premixed hydrogen flames, whether diffusion-driven or buoyancy-driven flames.