Impact of Detailed Molecular Transport on Ammonia/Hydrogen Ignition Delay Time Measurements in Rapid Compression Machines

The influence of detailed molecular transport, notably, of thermo-diffusion, in the compressed fuel/air mixture in rapid compression machine experiments is studied by detailed numerical simulations. The evolution of a one-dimensional layer of fuel-air mixture subjected to an RCM process was simulated using detailed treatment of transport and chemical reaction. The simulation includes compression work and wall heat losses as essential RCM processes, and adds a detailed treatment of molecular transport processes and chemical reaction within the gas mixture to the description. The model outcome reveals how detailed transport can create inhomogeneities in the initially homogeneous mixture composition, well before chemical reaction sets in. The effect is pronounced in hydrogen-containing mixtures, where temperature gradients at the near-wall boundary layer can cause notable “un-mixing” of hydrogen by thermo-diffusion. This can effectively lead to inhomogeneous fuel-air ratio fields, which are present already when the RCM compression phase is finished. Under these circumstances, the ignition delay and temperature assigned to a RCM experiment correspond to a physically different auto-ignition event than nominal. ... mehrNeglecting these effects may lead to a bias in reported experimental ignition delay time curves.