Modelling of high-speed chemically reactive flows

Thesis

A new numerical Eulerian-Lagrangian solver named AHISsprayDetFoam is developed towards accurate and efficient modelling of high-speed chemically reactive flows within the framework of OpenFOAM. AHISsprayDetFoam integrates modules of built-in OpenFOAM rhoCentralFoam and Lagrangian solvers, the mixture-averaged diffusion model, optimised droplet sub-models, and a sparse stiff chemistry solver based on dynamic adaptive hybrid integration (AHI-S). The optimised droplet sub-models are verified in correct implementation by simulating ammonia droplet acceleration and cooling in the flowing and/or low-temperature air. The accuracy and efficiency gains of AHISsprayDetFoam are examined by simulating 1-D detonation propagation in ammonia droplet-free/laden ammonia-oxygen mixtures. The mixture-averaged diffusion model provides different predictions of pulsating detonation instabilities compared to the built-in OpenFOAM diffusion model. It shows better accuracy in depicting the detonation structure within the droplet-free section attributed to improved multi-component diffusion modelling. The AHI-S chemistry solver reduces the computational cost by around 50% compared to the built-in OpenFOAM EulerImplicit solver. It achieves good accuracy in calculating detonation propagation speed within the droplet-free section under optimal efficiency.

2-D numerical simulations of detonation propagation and extinction in ammonia droplet-laden premixed ammonia-oxygen gas are performed using AHISsprayDetFoam. The detonation propagation/extinction behaviour and two-phase gas-droplet interactions are dependent on initial droplet diameter and number density. The detonative combustion and droplet evaporative, accelerative, and heating effects are higher-intensity in the post-Mach stem region than in the post-incident wave region. The detonation wave degenerates into detonative spots which then decouple into shock and reaction fronts as quenches.

3-D large eddy simulations of a supersonic lifted hydrogen flame are performed using AHISsprayDetFoam. Impacts of Lewis number on autoignition locations and strengths, and flame structures and stabilisation are numerically observed. The impacts of Lewis number are related to mass and thermal diffusions predicted at fuel-coflow and/or coflow-ambient air mixing layers, rationalised by non-negligible mass and thermal diffusions compared to convection. Impacts of turbulent Schmidt and Prandtl numbers are minor as sub-grid scale mass and thermal diffusions are subordinate to filtered diffusion.

Authors: Zhu, R

• DOI: 10.5287/ora-yjrrxmn7j
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: dynamic adaptive hybrid integration; multi-component diffusion; supersonic combustion; sparse ode solver; detonation wave; two-phase reacting flow; eulerian–lagrangian solver
• Subjects: Ammonia; Computational fluid dynamics; Hydrogen; Multiphase flow; Combustion