Numerical simulations of high-speed chemically reacting flow (original) (raw)
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Combustion, Explosion, and Shock Waves, 1995
A new numerical algorithm for simulation of nonequilibrium chemically reacting flows of an inviscid muIticomponent gas is described. Application of this algorithm to the numerical solution of several problems of air-hydrogen mixture combustion in oblique detonation waves is demonstrated. Numerical simulation of hypersonic flows of a real multicomponent gas with finite-rate chemical reactions is of both theoretical and practical interest. Extensive investigations to create an aerospace plane have been pursued in recent years. Here the development of a hydrogen-fueled hypersonic ramjet engine (scramjet engine) is a major problem. Nonequilibrium chemical processes occur in all elements of this engine: in an air inlet (air dissociation behind intense shock waves), in a nozzle (combustion product recombination), and, of course, in a combustor, where supersonic mixing and burning of the air-hydrogen mixture take place. The calculation of nonequilibrium hypersonic flows presents a number of severe difficulties. First of all, the dimensionality of the problem increases abruptly because one must monitor the concentrations of all the components of the reacting mixture. Then, the system of equations of chemical kinetics is of a rigid type, i.e., the physical phenomenon involves a few characteristic times differing from one another by a hundred times. The application of explicit numerical algorithms to the simulation of such processes is hardly possible because this requires a time step equal to the minimum characteristic time, whereas the use of implicit algorithms involves problems of inverting large-dimensionality systems of nonlinear equations and adequacy of a numerical solution. In view of the problem complexity it is expedient to begin with the development of efficient numerical methods for calculation of reacting flows of an inviscid gas. To solve this problem, one can use a reliable base, the well-developed numerical simulation of ideal inviscid gas flows. The main problems arising within the framework of the model of an inviscid reacting gas are related to the consideration of shock wave structures with heat release and, first of all, detonation waves. The combustion behind shock waves and in detonation waves can be used in scramjet combustors, while, as noted in [1, 2], an oblique-detonation-wave engine is more promising for flight Mach numbers Moo > 15. That is why the numerical simulation of inviscid reacting flows is of scientific and practical interest. Let us mention some problems that call for solution: the interaction of detonation waves in an air-hydrogen mixture with combustor wails, flame stabilization behind a shock wave, i.e., combustion process localization and its maintenance in different regimes (for instance, under the conditions of a nonpremixed combustible mixture, with changing wedge angle, etc.). The present paper is devoted to the development of an efficient numerical method for calculation of reacting inviscid gas flows and numerical solution of the above problems.
Journal of Mechanical Science and Technology, 2015
An unsteady shock-induced combustion (SIC) is characterized by the regularly oscillating combustion phenomenon behind the shock wave supported by the blunt projectile flying around the speed of Chapman-Jouguet detonation wave. The SIC is the coupling phenomenon between the hypersonic flow and the chemical kinetics, but the effects of chemical kinetics have been rarely reported. We compared hydrogen-air reaction mechanisms for the shock-induced combustion to demonstrate the importance of considering the reaction mechanisms for such complex flows. Seven hydrogen-air reaction mechanisms were considered, those available publically and used in other researches. As a first step in the comparison of the hydrogen combustion, ignition delay time of hydrogen-oxygen mixtures was compared at various initial conditions. Laminar premixed flame speed was also compared with available experimental data and at high pressure conditions. In addition, half-reaction length of ZND (Zeldovich-Neumann-Döring) detonation structure accounts for the length scale in SIC phenomena. Oscillation frequency of the SIC is compared by running the time-accurate 3rd-order Navier-Stokes CFD code fully coupled with the detailed chemistry by using four levels of grid resolutions.
2011
The present study describes the numerical investigation of the effect of incidence shock wave interaction on mixing and combustion of transverse hydrogen injection. Therefore, the second – order implicit upwind TVD scheme in conjunction with local characteristic approach is used for the simulation of unsteady multidimensional chemical reacting flow in a generalized coordinate. The species equations and the convective fluid dynamic equations are solved in a coupled fully implicit form with the LU scheme. The numerical scheme employs BaldwinLomax algebraic turbulence model. Hydrogen and air combustion is simulated by means of a full chemical mechanism. Obtained results indicated that without incident shock wave auto ignition of the hydrogen jet occurs in high temperature airstream. Nevertheless the flame will subsequently quenched downstream of the injector. When the incident shock wave introduced upstream of the injector flameholding could not be achieved. On the other hand, the flam...
A Triangle-Based Unstructured Finite-Volume Method for Chemically Reactive Hypersonic Flows
Journal of Computational Physics, 2001
A triangle-based unstructured finite-volume method is developed for chemically reactive hypersonic calculations. The method is based on a Steger-Warming fluxvector splitting approach generalized to mixtures of thermally perfect gases. Secondorder-in-space and time accuracy is provided by limited flux blending and an implicit multi-stage time marching scheme. The final stiff non-linear problem resulting from discretization presents a very peculiar block diagonal structure. This allows a decoupling of the species and gas dynamic equations in smaller subproblems. A linear algebra argument based on M-matrix theory makes it possible also to show that the method guarantees positivity of species mass densities and vibrational energies under a reasonable CFL-like constraint. Finally, a set of 2-D numerical test cases illustrates the performance of the method.
Detailed simulations of the bifurcation and ignition of an Argon-diluted Hydrogen/Oxygen mixture in the two-stage weak ignition regime are performed. An adaptive meshrefinement (AMR) technique is employed to resolve all relevant physical scales that are associated with the viscous boundary-layer, the reaction front, and the shock-wave. A high-order hybrid WENO/central-differencing method is used as spatial discretization scheme, and a detailed chemical mechanism is employed to describe the combustion of the H2/O2 mixture. The operating conditions considered in this study are p5 = 5 bar and T5 = 1100 K, and fall in the third explosion limit. The computations show that the mixing of the thermally stratified fluid, carrying different momentum and enthalpy, introduces inhomogeneities in the core-region behind the reflected shock. These inhomogeneities act as localized ignition kernels. During the induction period, these kernels slowly expand and eventually transition to a detonation wave that rapidly consumes the unburned mixture. In competition with this detonation wave are the presence of secondary ignition kernels that appear in the unreacted core-region between reflected shock and detonation wave.
In this paper, an algorithm for chemical non-equilibrium hypersonic flow is developed based on the concept of energy relaxation method (ERM). The new system of equations obtained are studied using finite volume method with Harten-Lax-van Leer scheme for contact (HLLC). The original HLLC method is modified here to account for additional species and split energy equations. Higher order spatial accuracy is achieved using MUSCL reconstruction of the flow variables with van Albada limiter. The thermal equilibrium is considered for the analysis and the species data are generated using polynomial correlations. The single temperature model of Dunn and Kang is used for chemical relaxation. The computed results for a flow field over a hemispherical cylinder at Mach number of 16.34 obtained using the present solver are found to be promising and computationally (25%) more efficient. The present solver captures physically correct solution as the entropy conditions are satisfied automatically during the computations.
DSMC Study of Shock-Detachment Process in Hypersonic Chemically Reacting Flow
AIP Conference Proceedings, 2005
Hypersonic chemically reacting flow around a wedge in the near-continuum regime was numerically studied by the DSMC method with the main goal of validation of real gas effect models. The influence of vibration-dissociation coupling on the results of numerical simulations was analyzed. To this end, two models of chemical reactions were used in the computations, the total collisional energy model and a vibrationally favored model. The numerical results were compared with the experimental data of Hornung and Smith on the shockwave stand-off distance in a hypersonic flow around the wedge. Sensitivity of simulation results to chemical reaction rate constants was also estimated.
Interaction between Boundary Layer and Shock in Hypersonic Flows with chemical real gas effects
In this paper, viscous interaction phenomenon in hypersonic flows with chemical reactions is numerically simulated. Two-dimensional Navier-Stokes equations are solved to simulate this phenomenon. Inviscid fluxes are approximated using Van Leer flux vector splitting method and to increase the accuracy of this approximation, MUSCL approach with Van albada limiters is applied. Chemical reactions are considered to be in equilibrium conditions. With this assumption there is no closed form for equation of state for the gas (air) and relation between thermodynamic properties are calculated from thermodynamic tables. In addition, transport properties (viscosity and conductivity) are functions of two independent thermodynamic properties. These functions are calculated using kinetic theory. To evaluate the performance of the model used in this research, some test cases are studied. First test case is flow over a ramp with various angles. The results of this test case are compared with the results of other numerical methods and the effect of geometry on separation length is studied. The second case is a hypersonic flow over a 15-degree ramp. The results are in good agreement compared with experimental data. In addition, there results are compared with the results of ideal gas (nonreacting flow) calculations. It can be seen that ideal gas assumption for air introduces considerable deviation form experimental data.
A computational method for combustion in high speed flows
Computers & Fluids, 2012
A two-dimensional time-accurate numerical model to simulate complex reacting flowfields in chemical non-equilibrium is presented. The aim of this study is to develop a computational tool which permits the analysis and the easy implementation of combustion phenomena for high speed flows. To construct an efficient numerical tool, while maintaining a reasonable accuracy, a semi-implicit numerical method was selected and verified for a hydrogen-air mixture. The numerical approach is based on a time-dependent, finite volume integration of the governing equations suitably modified for chemical non-equilibrium. The evaluation of the reacting constants based on Gibbs free energy and the Van't Hoff equation allows a very easy implementation of the chemical model used, regardless of its complexity. Calculations were performed with adeguate temporal and spatial resolution for modeling the physical process for practical calculation. Comparisons with numerical results are used for a verification of the numerical procedure.