摘要:

NOx formation from a methane - air diffusion flame in a two-dimensional jet involving highly preheated air, which has recently become an important topic in industrial furnaces, is investigated numerically using a full chemistry approach including C2, prompt and thermal mechanisms. Effects of increased air temperature on NOx formation are examined. Numerical results show that both NO formation mechanisms increase dramatically with increasing air temperature. A C-shaped production zone of NOx, corresponding to the fuel-lean and fuel-rich regions of triple flame, is identified. It is shown that NO formation with high air temperature can be suppressed efficiently by decreasing the oxygen concentration in the airstream. Production rate analyses of elementary reactions are made. Formation paths of NOx at low and high temperatures are obtained and compared. The results show that the NOx formation path depends strongly on the air temperature. In addition to the thermal route and the HCNNO route, the HCNCN and NOCN recycling routes are greatly enhanced at high air temperature. The results show that the prompt mechanism and the thermal mechanism are strongly coupled at high air temperature. Calculations of prompt NO and thermal NO in a two-dimensional jet and in the counterflow configuration reveal that the conventional method cannot give a correct prediction of prompt NO and thermal NO, particularly at high air temperature. A method using the concept of fixed nitrogen is presented. Numerical results indicate that the formation process of prompt NO and thermal NO can be evaluated properly by the present method.

ISSN:1364-7830    

DOI:10.1080/715695654

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We study the rigorous asymptotic stability of a Chapman - Jouguet (CJ) detonation wave in the limit of small resolved heat release (SRHR). We show that the solution exists globally and that the solution converges uniformly to a shifted CJ detonation wave as t + for initial data which are small perturbations of the CJ detonation wave. A CJ detonation wave is characterized by the property that the speed at the end of it is sonic. A similar phenomenon occurs for a shock profile when the flux function is nonconvex. We use the weighted energy method to overcome the difficulty. The proper choice of the weight cancels the degenerate property of the CJ detonation at the tail. The nonmonotonic part, or the expansive part, of the profile caused by the chemical reaction is treated by the characteristic energy estimate under the assumption of SRHR.

ISSN:1364-7830    

DOI:10.1080/713665323

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Gas combustion, solid combustion as well as frontal polymerization are characterized by stiff fronts that propagate with nonlinear dynamics. The multiple-scale phenomena under consideration lead to very intense computations that require parallel computing in order to reduce the elapsed time of the computation. We develop a methodology to build on the MIMD architecture a parallel numerical method based on the property of the solution, i.e. a stiff quasi-planar two-dimensional combustion front. We illustrate our methodology using two models of the combustion process. The first is a thermo-diffusive model of a two-step chemical reaction exhibiting two transition layers. The second is a thermo-diffusive model of a one-step chemical reaction coupled with a hydrodynamical model using the stream function - vorticity formulation of the Navier - Stokes equations written in the Boussinesq approximation. This methodology makes use of efficient domain decomposition methods, combined with asymptotic analytical qualitative results to adapt the interface position, to solve the transition layer(s) of the solution accurately and operator splitting to take advantage of the quasi-planar property of the frontal process. Then, it provides three complementary levels of parallelism. A first level of parallelism based on the domain decomposition, thus a priori limited to the number of transition layers in the problem. A second based on an explicit parallelism in the orthogonal direction of the front propagation. A third based on the spread of equations on subnetworks of processors. The parallel implementation using the message passing library concept on the Paragon and iPSC860 MIMD computers are discussed. An efficient parallel algorithm to solve the space-periodic stream-function in the second model, based on Fourier modes decomposition combined with the first and second level of parallelism is provided. The direct numerical simulation provided by our numerical method allows us to explore the physical parameter space of the combustion process in order to understand the mechanism of instabilities. Some examples of hydrodynamical and thermal instabilities are given.

ISSN:1364-7830    

DOI:10.1080/713665324

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Fast efficient numerical programs for determining the equilibrium composition of large chemical systems subject to generalized linear constraints are needed for chemical kinetic calculations involving both the conventional local thermodynamic equilibrium (LTE) and the more general rate-controlled constrained-equilibrium RCCE assumptions. For this purpose two callable subroutines based on the well known NASA and STANJAN equilibrium codes have been developed by a simple modification of their input files. To evaluate the performance of these subroutines, test calculations have been made for the hydrogen - oxygen (H - O) and carbon - hydrogen - oxygen - nitrogen (C - H - O - N) systems with various combinations of constraints on the elements, the total moles and the free valence (number of unpaired electrons) in the system. The allowed domain of the constraints was determined and both interior and boundary points were investigated for several temperature and pressure conditions. The results showed that STANJAN was superior to NASA both in convergence and speed under all conditions investigated.

ISSN:1364-7830    

DOI:10.1080/713665325

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Using the combined limits of a large activation energy and a ratio of specific heats close to unity, a dispersion relation has recently been derived which governs the stability of a steady Chapman - Jouguet detonation wave to two-dimensional linear disturbances. The analysis considers instability evolution time scales that are long on the time scale of fluid particle passage through the main reaction layer. In the following, a simplified polynomial form of the dispersion relation is derived under an appropriate choice of a distinguished limit between an instability evolution time scale that is long on the time scale of particle passage through the induction zone and a transverse disturbance wavelength that is long compared to the hydrodynamic thickness of the induction zone. A third order in time, sixth order in space, parabolic linear evolution equation is derived which governs the initial dynamics of cellular detonation formation. The linear dispersion relation is shown to have the properties of a most unstable wavenumber, leading to a theoretical prediction of the initial detonation cell spacing and a wavenumber above which all disturbances decay, eliminating the growth of small-wavelength perturbations. The role played by the curvature of the detonation front in the dynamics of the cellular instability is also highlighted.

ISSN:1364-7830    

DOI:10.1088/1364-7830/1/3/005

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor