On the effects of convecting entropy waves on the combustor hydrodynamics

Entropy waves, as hot spots or density inhomogeneities, can be generated by the flame unsteadiness in combustors. These waves are convected downstream while being annihilated by the flow decay and dispersion mechanisms. This results in the diffusion of the enthalpy of the wave within the base flow. Decaying entropy waves may, therefore, affect the density and viscosity of the base flow and consequently modify the combustor hydrodynamics. Study of such hydrodynamic modifications is the objective of the current numerical study. In particular, the extent of induced changes in the flow is investigated. To do so, some hydrodynamic indices are introduced, in which vorticity magnitude and the angles between the velocity and vorticity vectors are the main parameters. In keeping with the previous studies, entropy waves are inserted at the channel inlet by a linear-increment and exponential-decrement temperature function in a cold flow. A more realistic, and rarely investigated thermal boundary condition of convective type are considered on the walls of the channel. The results show that convection of the entropy waves through the channel noticeably changes the hydrodynamic parameters, such as vorticity vector, helicity and streamlines alignment. This is in contrast with the general notion, which regards entropy waves as passive scalars

A second order description of shock structure

The structure of gas-dynamic shock waves is of interest in hypersonic flow studies and also constitutes a straightforward test for competing kinetic theories. The description of the shock profiles may be obtained from a second-order theory in the Knudsen number. The BGK approximation to the Boltzmann equation introduces additional terms in the transport of momentum and energy. These relations, known as the Burnett equations, improve the agreement between calculated shock profiles and experiment. However, for some formulations of these equations, the solution breaks down at a critical Math number. In addition, certain terms in the Burnett equations allow unphysical effects in gas flow. A modified kinetic theory has been proposed by Woods (An Introduction to the Kinetic Theory of Gases and Magnetoplasmas, Oxford Univ. Press, Oxford, 1993) which eliminates the frame dependence of the standard kinetic theory and corrects some of the second-order terms. This article describes a novel method devised to solve the time-independent conservation equations, including the second-order terms. The method is used to solve the shock structure problem in one dimension. It is based on a finite difference global scheme (FDGS), in which a Newton procedure is applied to a discretized version of the governing equations and boundary conditions. The method is first applied to the Navier-Stokes formulation of the shock equations. It is then successfully used to integrate a modified version of the second-order equations derired by Woods for monatomic gases, up to a Mach number of 30. Results of the calculations are compared with experimental data for Argon gas flows characterized by up-stream Mach numbers up to 10. The agreement is good, well within the data point spread. The FDGS method converges rapidly and it may be used to study other problems of the same general nature

Application de la réduction systematique de schemas cinetiques au calcul de flammes landnaires etirees de premelanges airpropane

The study of strained laminar flames is of fundamental importance and it is also of great interest for turbulent combustion niodeling. ] be calculations are usually performed either with extremely simple global kinetic schemes or with elementary and complex detailed chemistry. A new approach based on the systematic reduction of kinetic schemes has been recently proposed by Peters. Our goal is to evaluate this method in the context of an atmospheric premixed propane-air flame. An augmented reduced scheme involving 9 species and 6 reactions is proposed. All the computations are compared with those of a reference scheme due to VMrnatz. The results obtained are generally satisfactory. It appears that the differences between the reduced and complex schemes concerning the flame position varywith the flame parameters such as the equivalence ratio, while flames structures are always well predicted. The extinction phenomenon is correctly reproduced by the reduced mechanism and he critical values of the strain rate and the chemical composition at extinction are determined with reasonable accuracy.L'étude des flamnies laminaires étirées est importante sur le plan fondamental et elle présente aussi un interêt considérable pour la modélisation de la combustion turbulente. Le calcul est généralement effectué avec des modèles chimiques très simplifiés ou avec des mécanismes cinétiques détaillés. Une nouvelle approche basée sur une réduction systématique des schémas cinétiques a été proposée par Peters. Notre objectif est d'évaluer cette méthode dans l'analyse des flammes de prémélange air-propane à la pression atmosphérique. Un schéma réduit amélioré à 9 espèces et 6 réactions est proposé. Tous les calculs sont comparés à ceux d'un schéma complexe de référence dû à Vârnatz. Les résultats obtenus sont généralement satisfaisants. Il appareit que les différences entre les calculs en chimie complexe et réduite, en ce qui concerne la position de la flamme, varient avec les paramètres de la flamme tels que la richesse, alors que les structures de flammes sont toujours bien décrites. Enfin, le schéma réduit reproduit correctement le phénomène d'extinction et les valeurs du taux d'étirement et de la richesse à l'extinction sont obtenues avec une précision raisonnable

Computer controlled data acquisition and signal processing for continuous combustion gas sampling

This paper is concerned with gas sampling in the continuous mode. In this mode of operation the sampling probe is moved at low speed and gas analysis is conducted on line. Probe displacement, data acquisition and signal processing are performed by a computer. Distortions associated with the continuous mode of operation are described. The sources of distorsion considered are related to the analyser response function, the sampling line time lag, the mixture evolution of the sampled gases in the line, and the variation of the mass flow in the line due to temperature changes of the sampled gases. A system transfer function is then determined. The algorithm used to restore the input signal from the output signal is based on a constrained iteration method. Tests and applications are given to prove that the method is practical. The main advantage of the method is that it considerably reduces the exploration time and allows detailed studies of flow and flame structures.Cet article concerne le prélèvement de gaz en mode continu. Dans ce mode d'opération, la sonde de prélèvement se déplace en continu à basse vitesse et l'analyse de gaz s'effectue en parallèle. Les déplacements de la sonde ainsi que l'acquisition et le traitement des signaux sont assurés par un ordinateur. Les distorsions associées au mode continu d'opération sont étudiées. Les sources de distorsion considérées sont liées à la réponse d'analyseurs, au temps de transfert dans la ligne de prélèvement, à l' évolution du mélange des gaz prélevés dans la ligne et à la variation du débit massique des gaz prélevés, due à la variation de leur température. Une fonction de transfert du système est ensuite déterminée. L'algorithme utilisé pour restituer le signal d'entrée à partir du signal de sortie est basé sur une méthode d'itération sous contrainte. Des tests et des applications sont ensuite donnés afin de démontrer que la méthode est pratique. L'avantage principal de la méthode est qu'elle diminue considérablement le temps d'exploration et ainsi permet une étude détaillée de la structure d'écoulement et de flamme