XXXIV Meeting of the Italian Section of the Combustion Institute 1 SUPERSONIC COMBUSTION REGIME : NUMERICAL AND THEORETICAL STUDY

A three dimensional high order numerical simulations of the H2/air supersonic combustion were performed. Large Eddies Simulation showed to be a promising tool to investigate the fundamental physical phenomena involved in the flame/ turbulence interaction and to support the development of new subgrid scale turbulence and combustion models was investigated using LES simulations. The present LES of the HyShot II supersonic combustor showed that the interactions between the airstream entering the combustor and the H2 sonic jet produce a high turbulence intensity confirmed by an average vorticity of order 10 Hz. The interaction between the hydrogen transverse jets and the supersonic air flow leaded to the bow shock formation and, accordingly, the boundary layer separation. This separation allowed H2 to be convected upstream through the spanwise recirculation vortices created by the baroclinic effect. Once created, the vortices were tilted, stretched, compressed and expanded according to the vorticity transport equation. These vortices are the key structures responsible for the observed fast fuel air mixing. In this context, an analysis of the flame structure was conducted to propose a appropriate kinetic and chemical/turbulence model. The flame structure has been analyzed by means of the Burke and Schumann theory. Introduction The aim of this work is to investigate the validity of combustion models that were developed for low-speed combustion and then traditionally extended to high-speed combustion. In fact, the assumption of fast chemistry, as well as the flamelet chemistry model, may not be valid in supersonic flows, where compressibility may affect the flame structure. Previous numerical [1] and theoretical results [2] by these authors confirmed that the baroclinic term plays a major role in supersonic mixing and combustion and cannot be neglected, having the same order of magnitude of both vortex stretching and compressibility terms. In fact, the spinning acceleration due to the baroclinic term (locally of order 10 ÷ 10 rad/s) indicate how strong pressure and density gradients are in the injection region: there, in fact, vorticity increases 10 to 10 Hz per millisecond. In fact, crossflow H2 injection is responsible for the high vorticity production and injection within the flow field: as the sonic hydrogen jet interacts with the XXXIV Meeting of the Italian Section of the Combustion Institute 2 supersonic air crossflow, coherent structures, that contribute to the fuel-air mixing enhancement, develop quickly: there, supersonic and subsonic vortex speeds alternate. LESs show spanwise vortices generated upstream of the H2 injection are stretched and tilted downstream by vortex stretching. Vortices in between hydrogen jets are also generated by the baroclinic term and are responsible for spreading the fuel over the entire cross section. The intense vorticity generated in the flow field is responsible for nearly complete combustion: chemical kinetics is very fast, in fact, in about 15 orifice diameters (3 cm downstream of the injectors) the fuel fraction is already down to 50%, and only a very small fraction of H2 (YH2) remains unburnt at the end of the combustor. The fast kinetic times are confirmed by the presence of water already upstream of the fuel injection, suggesting the possibility to implement a one-step kinetic mechanism or a Flamelet-Based model to reduce CPU time due to complex chemical kinetics. In order to select an appropriate flame model in these and similar CFD simulations, analysis of the flame structure and morphology after the hydrogen jets are tilted streamwise is reported in the next section. HyShot Combustor Geometry and Simulation Modelling The HyShot geometry (see Fig. 1, where the computational domain is in blue) consists of a rectangular cross section of 75 x 9.8 mm, 300 mm long. Table 1 reports the inlet conditions of hydrogen and air as injected inside the combustor. Figure 1. Hyshot schematic; portion of the combustion chamber simulated in blue. Table 1. Air/Hydrogen inlet flow condition. φ = 0.426 Air Hydrogen Pressure [Pa] 82110 307340