Turbulent explosions in hydrogen enriched fuel blends
Creators: name: Li, Tao
Publisher: name: Imperial College London
Year: 2017

description: This thesis presents an experimental investigation of turbulent explosions resulting from hydrogen enriched fuel blends in an obstructed flame tube facility. The influence of mixture reactivity and thermal radiation induced ignition in turbulent explosions were studied followed by a quantification of the flow field development. Fuel lean binary H2/CO, H2/CH4 and ternary H2/CH4/CO mixtures were first studied in a two-obstacle configuration to assess the impact of mixture reactivity on explosion overpressures and flame speeds. The mixture reactivity was varied by introducing different H2 substitutions at equivalence ratios of 0.80, 0.60 and 0.40. The results highlight significant differences in explosion behaviour between the two blending components, with CO mixtures providing substantially higher over- pressures than the corresponding CH4 blends. The results suggest that methane has a mitigating effect up to comparatively high hydrogen blending fractions and that synergistic effects between fuel components need to be taken into account. A new scaling parameter (β) is proposed that successfully linearises the peak explosion overpressure between different fuel blends in response to the hydrogen concentration. A scaling based on acoustic theory shows good agreement with experimental data and a simple method for estimating the overpressure change caused by variations in the mixture reactivity in a fixed geometry is also evaluated. The impact of thermal radiation induced ignition was explored in fuel lean H2/CH4/Air mixtures with a continuous wave laser operating in the near infrared as the radiation source and acetylene black particles as the radiation target due to their relationship with soot emissions. Influences of ignition location and ignition delay time were studied. The results show that the ignition kernels caused by irradiated particles can be successfully entrained into the main flow and/or re- circulation zones formed around obstacles and cause multipoint explosions. The resulting relationship between fuel consumption ahead of the advancing flame and the evolution of the strength of the explosion was shown to be complex and typically lead to increased explosion durations with reduced peak pressures. The complex effect of forward radiation induced ignition on the pressure development stems from the interactions of the two explosion kernels resulting in two sharp pressure rises separated by a quasi-stable stage with a duration depending on the radiation induced ignition time. The total impulse was estimated by integrating the instantaneous pressure over time and the results show different characteristics for different configurations. The mixture reactivity also affects turbulent explosions indirectly via turbulence- chemistry interactions in the critical recirculation zones behind the obstacles. The flow field development was therefore quantified in a two obstacle configuration using high-speed (10 kHz) particle image velocimetry (PIV), time-series PIV and Mie scattering in H2/CO/Air and H2/CH4/Air mixtures with H2 substitution levels of 50%, 80% and 100% for a fixed stoichiometry of 0.60. The time-resolved evolution of the recirculation zone behind the second obstacle was successfully captured with the explosion over-pressure and flame propagation speed also measured. Data is presented for the mean horizontal (u) and vertical (v) velocity components at 24 spatial locations for each mixture along with the translational velocities of the shear driven recirculating eddies. It is shown that, despite large differences in flow velocities and over-pressures, the impact of the mixture reactivity on the temporal evolution of the flow field evolution can be approximately normalised using a dimensionless time scale.