Hydrogen/Air Supersonic Combustion Modelling and Validation for Scramjet Applications

Abstract

In the field of air-breathing hypersonic scramjet vehicles design and development, the Italian Aerospace Research Centre -CIRA contributed to an international project, called Stratospheric Flying Opportunities for High-Speed Propulsion Concepts -STRATOFLY, in collaboration with several European organizations, coordinated by Politecnico di Torino under the EC Horizon 2020 programme financial support. The aim of this project was the improvement of enabling technologies for the realization of a commercial hypersonic aircraft, able to fly at Mach 8, at 30÷35 km of altitude, for at least 4 hours with a minimum environmental impact and especially low NOx emissions. Understanding the complex, supersonic, turbulent, combustion processes occurring during scramjet operations is of fundamental importance. For this purpose, a thorough 0D kinetic assessment was carried out by means of the open-source Cantera software for the identification of the most suitable kinetic mechanisms, able to predict with satisfactory accuracy both the ignition delay times and the NOx emissions at the relevant scramjet operating conditions. Several kinetic schemes were investigated and the computed results were compared with the literature available shock tubes experimental data. In the entire investigated operative box, the best agreement, in terms of induction times, was achieved using the kinetic mechanism developed by Zettervall and Fureby with the exception of the ignition tests carried out in presence of argon as diluent bath gas, where the kinetic schemes by Kéromnès and CRECK together with the Aramco-II full mechanism provide the best matching. Moreover, for considering the generation of NOx, the three fundamental thermal route reactions by Zel'dovič, were added. Furthermore, full 3D CFD simulations were carried out by means of Ansys ® Fluent in order to compare experimental data and evaluate the predictivity and accuracy of the chemical/kinetic sub-model conceived in this way when coupled with mixing, turbulence and fluid dynamic interactions arising from the simulations of the whole propulsive system. For the sake of comparison, the experiments carried out on the small-scale scramjet vehicle of LAPCAT-II by the HEG (DLR) were rebuilt.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

“…CIRA is in charge of the assessment of available hydrogen/air reaction mechanisms, in particular with (i) a thorough investigation of detailed and reduced kinetic mechanisms performed through a literature survey, (ii) theoretical investigations and identification of the most performing kinetic schemes for hydrogen/air combustion at selected operative flight conditions, and (iii) 0D/1D simulations of hydrogen/air combustion and comparison of ignition delay times and adiabatic flame temperatures against the available literature experimental data. The work described in the present paper emanates directly from what was recently accomplished in the previous H2020 STRATOFLY project [3][4][5][6] and the present task represents a continuation and extension of it to a wider range of operative conditions. The optimal scheme arises as a suitable trade-off between the accuracy, required for a truthful description of ignition and combustion phenomena, and the computational costs, associated with the available calculation speed and memory storage capacity.…”

“…Based on the author's experience [3,4] and the review of Hu and co-workers [5], only the following eleven available study schemes were considered as the most promising kinetic mechanisms since they were specifically designed and developed to describe not only the hydrogen/oxygen combustion at atmospheric conditions but also at intermediate to high pressures:…”

“…In the experimental study and the related kinetic rebuilding carried out by Hu and coworkers [5], the ignition delay times were defined as time spacing between the arrival of the reflected shock waves at the end-wall of the used test rigs and the linear extrapolation of the steepest rise of OH* emission signal to zero level, which is slightly lower than the time corresponding to the maximum mass fraction of the radical OH or its excited version OH*.…”