The ignition problem for a scalar nonconvex combustion model

AbstractThe ignition problem for the scalar Chapman–Jouguet combustion model without convexity is considered. Under the pointwise and global entropy conditions, we constructively obtain the existence and uniqueness of the solution and show that the unburnt state is stable (unstable) when the binding energy is small (large), which is the desired property for a combustion model. The transitions between deflagration and detonation are shown, which do not appear in the convex case

Numerical modelling of pressure rise combustion for reducing emissions of future civil aircraft

This work assesses the feasibility of designing and implementing the wave rotor (WR), the pulse detonation engine (PDE) and the internal combustion wave rotor (ICWR) as part of novel Brayton cycles able to reduce emissions of future aircraft. The design and evaluation processes are performed using the simplified analytical solution of the devices as well as 1D-CFD models. A code based on the finite volume method is built to predict the position and dimensions of the slots for the WR and ICWR. The mass and momentum equations are coupled through a modified SIMPLE algorithm to model compressible flow. The code includes a novel tracking technique to ensure the global mass balance. A code based on the method of characteristics is built to predict the profiles of temperature, pressure and velocity at the discharge of the PDE and the effect of the PDEs array when it operates as combustion chamber of gas turbines. The detonation is modelled by using the NASA-CEA code as a subroutine whilst the method of characteristics incorporates a model to capture the throttling and non-throttling conditions obtained at the PDE's open end during the transient process. A medium-sized engine for business jets is selected to perform the evaluation that includes parameters such as specific thrust, specific fuel consumption and efficiency of energy conversion. The ICWR offers the best performance followed by the PDE; both options operate with a low specific fuel consumption and higher specific thrust. The detonation in an ICWR does not require an external source of energy, but the PDE array designed is simple. The WR produced an increase in the turbine performance, but not as high as the other two devices. These results enable the statement that a pressure rise combustion process behaves better than pressure exchangers for this size of gas turbine. Further attention must be given to the NOx emission, since the detonation process is able to cause temperatures above 2000 K while dilution air could be an important source of oxygen

A Numerical Study of H2/O2 Detonation Waves and their Interaction with Diverging/Converging Chambers

Ph.DDOCTOR OF PHILOSOPH

Numerical modelling of inhomogeneous Liquefied Natural Gas (LNG) vapour cloud explosions

The main hazard of Liquified Natural Gas (LNG) is the flammable vapour cloud, which can extend to kilometres as a greenhouse gas or be ignited resulting in fire and explosions. This work aims to carry out a safety study on the vapour cloud explosion of LNG. Since most of the experimental research works are available for Hydrogen/Air mixture, in this present work, the first goal is to extend the existing physical understanding of deflagration-to-detonation transition (DDT), in hydrogen/air mixtures with transverse concentration gradients in closed channels. Explosions in homogenous (uniform) reactive mixtures have been widely investigated, both experimentally and numerically. However, in real accident scenarios, mixtures are usually inhomogeneous due to the localised nature of most fuel releases, buoyancy effects and the finite time between release and ignition. It is imperative to determine whether mixture inhomogeneity can increase the explosion hazard beyond what is known for homogeneous mixtures. Hence, extensive knowledge on these processes has been built up over decades for homogeneous mixtures. The approach is to identify similarities and differences caused by concentration gradients compared to homogenous mixtures with equal average hydrogen concentration. The dynamics of deflagration to detonation transition (DDT), and explosion modelling, have been studied using the newly assembled density-based solvers (VCEFoam) within the frame of OpenFOAM CFD toolbox. In order to evaluate the convective fluxes contribution, Harten–Lax–van Leer–Contact (HLLC) scheme is used for accurate shock capturing. The numerical code is initially verified by four sets of verification test cases. In addition to shock capturing verification, the capability of the current numerical code in capturing the detonation cellular structure has been examined. The CFD results have been compared against both quantitatively and qualitatively with the other previous works as well as an experimental observation. Then, numerical studies have been conducted to investigate flame acceleration and transition to detonation in both homogeneous and inhomogeneous hydrogen-air mixtures in obstructed and unobstructed channel configurations (in medium scale). The developed VCEFoam solver has been used within OpenFOAM, for these simulations. For the considered experiment (Boeck et al., 2016), different sets of configurations and fuel concentration have been studied. Three different geometry configuration such as BR00 (0% Blockage ratio, smooth channel), BR30 (30 % blockage ratio), and BR60 (60% blockage ratio), have been considered in this study. Also, in order to study the effect of a concentration gradient, different mixture concentrations have been investigated in both homogenous and inhomogeneous mixtures. A total of 17 conditions were simulated for different hydrogen concentrations in both homogeneous and inhomogeneous mixtures with and without obstructions. A high resolution grid is provided by using adaptive mesh refinement (AMR) method, which leads to 30 grid points per half reaction length (HRL). The numerical predictions were compared against previous experiments. Overall, the predicted flame tip velocities, overpressures, and locations of detonation onset are in good reasonably agreement with the measurements. It is found that, the transverse concentration gradients can either strengthen or weaken flame acceleration, depending on average hydrogen concentration and channel obstruction. The role of hydrodynamic instabilities and the effect of baroclinic torque and Richtmyer Meshkov (RM) instability have also been studied. The results support that RM instability is the primary source of turbulence generation in the present case. Then vapour cloud explosion study has been carried out for industrial scale scenarios (very large scale). A robust CFD methodology has been presented for modelling very large scale, vapour cloud explosions scenarios. A specific model has been considered for modelling the impact of flame-instabilities, particularly the thermal diffusive instabilities, and Darrieus Landau (DL) instabilities in large-scale models. The numerical model has initially been validated with the largest ever conducted indoor DDT and explosion experiments in the RUT facilities. Good qualitative agreement between the numerical prediction results and experimental measurements of RUT facilities has achieved. After demonstrating the code verification, LNG vapour cloud explosion scenarios, generated from the release of an evaporated liquefied natural gas have been studied. Two different possible incidents in LNG VCE have been studied; explosion modelling in onshore LNG plant and offshore LNG shipping. For the onshore LNG explosion study; an LNG plant has been considered to have fuel leakage from one of its storage tanks. In both onshore and offshore scenarios, the maximum recorded overpressure was below 1.2 bar, which is far below the CJ detonation limit (CJ detonation pressure, for stoichiometric methane/air mixtures, is 16.6 bar). Therefore, in this scenario, LNG flame acceleration was not enough to cause a detonation, and only a flame deflagration has been noticed. The results of the current study can be used in the context of safety to assess the potential risks of explosions in the energy industry

Théorie qualitative et asymptotique des détonations

Shock waves in reactive media possess very rich dynamics: from formation ofcells in multiple dimensions to oscillating shock fronts in one-dimension. Because ofthe extreme complexity of the equations of combustion theory, most of the currentunderstanding of unstable detonation waves relies on extensive numerical simulationsof the reactive compressible Euler/Navier-Stokes equations. Attempts at a simplifiedtheory have been made in the past, most of which are very successful in describingsteady detonation waves. In this work we focus on obtaining simplified theoriescapable of capturing not only the steady, but also the unsteady behavior of detonationwaves.The first part of this thesis is focused on qualitative theories of detonation, wheread hoc models are proposed and analyzed. We show that equations as simple as aforced Burgers equation can capture most of the complex phenomena observed indetonations. In the second part of this thesis we focus on rational theories, andderive a weakly nonlinear model of multi-dimensional detonations. We also show,by analysis and numerical simulations, that the asymptotic equations provide goodquantitative predictions.Les ondes de choc dans les milieux réactifs possèdent une dynamique très riche: de la formationcellules en plusieurs dimensions à fronts de choc oscillant en une seule dimension. À cause del'extrême complexité des équations de la théorie de la combustion, la plupart desla compréhension des ondes de détonation instables repose sur des simulations numériquesdes équations réactives compressibles d'Euler / Navier-Stokes. Tentatives de simplificationthéorie ont été faites dans le passé, dont la plupart sont très efficaces pour décrireondes de détonation régulières. Dans ce travail, nous nous concentrons sur l'obtention de théories simplifiéescapable de capturer non seulement le comportement stable, mais aussi les ondes de détonation instables.La première partie de cette thèse se concentre sur les théories qualitatives de la détonation, oùdes modèles ad hoc sont proposés et analysés. Nous montrons que des équations aussi simples Burgers forcée peut saisir la plupart des phénomènes complexes observés dans lesdétonations. Dans la deuxième partie de cette thèse, nous nous concentrons sur les théories rationnelles, etdérive un modèle faiblement non linéaire de détonations multidimensionnelles. Nous montrons également,par analyse et simulations numériques, que les équations asymptotiques fournissent une bonneprédictions quantitatives

Multiphase extensions to ODE models for detonations of non-ideal explosives

Two streamline, ordinary differential equation (ODE) models for detonation, the Chan-Kirby model and the straight streamline approach of Watt et al., are extended to a multiphase system of equations. These multiphase equations, with realistic equations of state, are used to better capture the heterogeneities in non-ideal explosives used in mining applications. Streamline ODE multidimensional models are normally obtained by reducing the partial differential equations (PDEs) describing the motion of the material to ODEs by making approximations about some of the physics of the problem. These models are referred to as reduced ODE models in this work and are the primary focus of this research into fast, efficient solutions of non-ideal explosives. In the development of these reduced order forms, some terms in the full equations have been removed for analytical convenience. Although this is not always the result of a formal order of magnitude analysis, this somewhat empirical approach is justified by simulation studies. In particular, by demonstrating that in a variety of benchmark problems, the reduced order ODEs give similar results to those obtained from the much more complex, full order PDE models. Further support is obtained by comparing the reduced order solution with experimental results. Comparisons with multiphase direct numerical simulations and experiments are undertaken to investigate the effect of the approximations and assumptions made in the derivation of the models. Both models produce comparable diameter effect curves for two different non-ideal explosives, EM120D and ANFO, in unconfined conditions. Empirical assumptions in the Chan-Kirby model can be eliminated but investigation shows that the straight streamline multiphase extension is based on better approximations for non-ideal explosives. This latter approach also gives better prediction of the diameter effect curve and detonation driving zone shape. The multiphase straight streamline model is then extended to model confined multiphase detonations, with realistic equations of state for the confining material, and predicts most strong confinement examples well. Future work of extending to curved streamlines and including confinement other than strong or weak is discussed

PSA 2016

These preprints were automatically compiled into a PDF from the collection of papers deposited in PhilSci-Archive in conjunction with the PSA 2016

PSA 2016

These preprints were automatically compiled into a PDF from the collection of papers deposited in PhilSci-Archive in conjunction with the PSA 2016