A new approach to model and simulate numerically surface chemistry in rarefied flows

A new approach is proposed to model and simulate numerically surface chemis try within the frame of rarefied gas flows. It is developed to satisfy all together the following points: (i) describe the gas phase at the microscopi c scale, as required in rarefied Rows, (ii) describe the wall at the macros copic scale, to avoid prohibitive computational costs and not only consider crystalline but also amorphous surfaces, (iii) reproduce on average macros copic laws correlated with experimental results, and (iv) derive analytic m odels in a systematic and exact way. The problem is stated and solved in th e general framework of a nonstatic flow in contact with a catalytic and non porous surface, without ageing process. It is shown that the exact and syst ematic solution method based on the Laplace transform, introduced previousl y by the author to model collisions in the gas phase, can be extended to th e present problem. The proposed approach is applied to Eley-Rideal and Lang muir-Hinshelwood recombination, assuming a coverage locally at equilibrium. The models are developed considering one atomic species and extended to th e general case of several atomic species. Numerical calculations verify tha t the models derived in this way reproduce the experimentally observed beha vior accurately. (C) 1999 American Institute of Physics. [S1070-6631(99)008 05-3].