Assessment of Approximate Soret Diffusion Models for Hydrogen and Ammonia Combustion

Abstract Detailed modeling of combustion processes involving hydrogen poses challenges due to the high diffusivities of the light hydrogen molecule ( $$\text{H}_2$$ ) and radical (H) compared to other species. Thermodiffusion, also known as the Soret effect, describes the diffusive flux of species induced by gradients of temperature. The Soret effect becomes important if the fuel species is much lighter (or heavier) than the mean molar mass of the mixture. While accurate models for Soret diffusion exist, e.g. the multicomponent diffusion model, they are usually computationally expensive. In this work, modeling strategies for approximating Soret diffusion available in popular software packages as well as additional models from the literature are assessed in terms of their accuracy. Four methods for computing reduced collision integrals are compared and three formulations for the thermodiffusion coefficients are investigated for hydrogen and ammonia combustion. All tested approaches for computing collision integrals are found to yield good results. The approximate Soret diffusion model by Chapman and Cowling has shown the best prediction accuracy for typical hydrogen flames and ammonia/hydrogen blends when compared to the multicomponent diffusion model. Results are also compared to the model by Hirschfelder and Warnatz, implemented in the popular software packages Chemkin and STAR-CD, and the model by Bartlett and coworkers, which is available in Ansys Fluent, using different benchmark cases. This work shall serve as a review of implementation details of common models as well as a guideline for accurate and efficient Soret diffusion modeling in future hydrogen and ammonia combustion simulations.