Evaluation of H-atom adsorption on wall surfaces with a plasma molecular beam scattering technique

Abstract

Toward a precise modeling for wall chemical effects in flame-wall interactions, radical adsorption on different wall surfaces are directly evaluated through a newly-developed molecular beam scattering technique using a non-equilibrium plasma–driven beam source as well as an ultra-high vacuum chamber. Firstly, sensitivities of adsorption rates for each radical species to the wall chemical effect are examined through a series of numerical simulations with detailed gas/surface chemistry for a methane-air premixed flame. Since H-atom has a higher diffusivity and its adsorption significantly inhibits a chain branching reaction of H + O2 = O + OH, H is considered to be the most influential radical on the flame characteristics such as heat release rate or CO emission if compared to OH, O and CH3. Based on the sensitivity analysis, H adsorptions are quantified for quartz and SUS321 surfaces with the plasma molecular beam scattering measurements. It is confirmed that H atomic beam can be successfully produced through the plasma dissociation of H2 molecules. Then, the produced H beam is irradiated onto quartz and SUS321 surfaces at different wall temperatures Tw. It is found that H is adsorbed on the quartz and SUS321 surfaces, and reaction probabilities of H PH has its maxima at Tw ∼673 K for both surfaces. This is probably because that the recombination rate increases as Tw increases, while the desorption rate is also promoted and overcomes the recombination rate at Tw > 673 K. The PH for the SUS321 is in agreement with that in our previous combustion experiments and more precise value can be obtained by the present method.