CFD analysis of hydrogen and methane turbulent transitional under-expanded jets

Under-expanded jets appear in various processes of engineering interest and, among others, during the injection of gaseous fuels in the combustion chamber. In this context, large eddy simulations of under-expanded hydrogen jets were carried out adopting a low-numerical-diffusion scheme based on the flux-splitting method. Pressure ratios ranging from five to twenty and the influence of chemical species injected were considered. Convergence and uncertainty of the numerical results were preliminarily assessed, and then the comparison with Schlieren acquisitions was exploited to validate the mathematical methodology adopted. Mach disc dimensions were found to be in good agreement with the experiments. Dynamic Mode Decomposition (DMD) was applied to investigate the characteristics of the resultant turbulent flow. A comparison of methane and hydrogen under-expanded jets reveals that the latter penetrates faster and presents a more developed vortical structure that enhances air/fuel mixing.