Fluid mechanics combustion

Physics chemistry electrochemistry thermodynamics heat and mass transfer fluid mechanics combustion materials science chemical engineering mechanical engineering electrical engineering systems engineering advanced energy conversion. 

TurbulentPremixedFlames. Combustion processes and flow phenomena are closely coimected and the fluid mechanics of a burning mixture play an important role in forming the stmcture of the flame. Laminar combusting flows can occur only at low Reynolds numbers, defined as... 

T. Poinsot, A. Trouve, D. Veynante, S. Candel, and E. Esposito. Vortex driven acoustically coupled combustion instabilities. Journal of Fluid Mechanics, 177 265-292, 1987. 

C. Clanet, G. Searby, and P. Clavin. Primary acoustic instability of flames propagating in tubes Cases of spray and premixed gas combustion. Journal of Fluid Mechanics, 385 157-197,1999. 

Hult, J., Josefsson, G., Alden, M., and Kaminski, C.F., Flame front tracking and simultaneous flow field visualisation in turbulent combustion, in 10th International Symposium and Applications of Laser Techniques to Fluid Mechanics, Paper No. 26-2, Lisbon, 2000. 

To understand the difference in stagnation pressure losses between subsonic and supersonic combustion one must consider sonic conditions in isoergic and isentropic flows that is, one must deal with, as is done in fluid mechanics, the Fanno and Rayleigh lines. Following an early NACA report for these conditions, since the mass flow rate (puA) must remain constant, then for a constant area duct the momentum equation takes the form... 

For those who have not studied fluid mechanics, the definition of a deflagration as a subsonic wave supported by combustion may sound over sophisticated nevertheless, it is the only precise definition. Others describe flames in a more relative context. A flame can be considered a rapid, self-sustaining chemical reaction occurring in a discrete reaction zone. Reactants may be introduced into this reaction zone, or the reaction zone may move into the reactants, depending on whether the unbumed gas velocity is greater than or less than the flame (deflagration) velocity. 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

Kennedy, L.A., Fridman, A.A., Saveliev, A.V. 1995. Superadiabatic combustion in porous media wave propagation, instabilities, new type of chemical reactor. Fluid Mechanics Res 22 1-25. 

Appendix B consists of a systematic classification and review of conceptual models (physical models) in the context of PBC technology and the three-step model. The overall aim is to present a systematic overview of the complex and the interdisciplinary physical models in the field of PBC. A second objective is to point out the practicability of developing an all-round bed model or CFSD (computational fluid-solid dynamics) code that can simulate thermochemical conversion process of an arbitrary conversion system. The idea of a CFSD code is analogue to the user-friendly CFD (computational fluid dynamics) codes on the market, which are very all-round and successful in simulating different kinds of fluid mechanic processes. A third objective of this appendix is to present interesting research topics in the field of packed-bed combustion in general and thermochemical conversion of biofuels in particular. 

Smirnov, N.N., V. F. Nikitin, J. Klammer, R. Klemens, P. Wolanski, and J. C. Legros. 1997. Turbulent combustion of air-dispersed mixtures Experimental and theoretical modeling. Experimental Heat Transfer, Fluid Mechanics Thermodynamics 4 2517-24. 

Strykowski, P. J., A. Krothapalli, and S. Jendoubi. 1996. The effect of counterflow on the development of compressible shear layers. J. Fluid Mechanics 308 63-96. Beer, J. M., N. A. Chigier, T. W. Davies, and K. Bassindale. 1971. Laminarization of turbulent flames in rotating environments. Combustion Flame 16 39-45. 

Bhidayasiri, R., S. Sivasegaram, and J.H. Whitelaw. 1997. Control of combustion oscillations in a gas turbine combustor. 7th Asian Congress of Fluid Mechanics Proceedings. 107-9. 

Using these methods, the elementary reaction steps that define a fuel s overall combustion can be compiled, generating an overall combustion mechanism. Combustion simulation software, like CHEMKIN, takes as input a fuel s combustion mechanism and other system parameters, along with a reactor model, and simulates a complex combustion environment (Fig. 4). For instance, one of CHEMKIN s applications can simulate the behavior of a flame in a given fuel, providing a wealth of information about flame speed, key intermediates, and dominant reactions. Computational fluid dynamics can be combined with detailed chemical kinetic models to also be able to simulate turbulent flames and macroscopic combustion environments. 

Stagnation flows represent a very important class of flow configurations wherein the steady-state Navier-Stokes equations, together with thermal-energy and species-continuity equations, reduce to systems of ordinary-differential-equation boundary-value problems. Some of these flows have great practical value in applications, such as chemical-vapor-deposition reactors for electronic thin-film growth. They are also widely used in combustion research to study the effects of fluid-mechanical strain on flame behavior. 

The photograph is included to make two points. First, the particle paths show qualitatively that the flow follows the anticipated streamlines. Even for the relatively small dimensions, the edge effects that could interrupt similarity behavior at the outflow appear to be minor. Second, and more striking, is the fact that the flame zone is extremely flat. Here is a situation that includes a considerable amount of chemistry (methane combustion) and complex heat and mass transfer. The fact that the flame zone shows no radial dependence is is convincing evidence that the fluid mechanical similarity is indeed valid. 

Turbulence theory provides a classical approach to mixing phenomena. This is a natural way for mechanical engineers and specialists of combustion, who are very familiar with the methods of fluid mechanics. However, when complex chemical reactions are involved, the use of the formalism of turbulence alone seems to lead to a deadlock, as has been pointed out by several authors. An excellent presentation of the state of the art can be found in the recent literature especially by Brodkey (16, 2, 27) and Patterson (3). These reviews reveal no major breakthrough, and only slow progress on a difficult road. 

Typically, a fire growth model is evaluated by comparing its calculations (predictions) of large-scale behavior to experimental HRR measurements, thermocouple temperatures, or pyrolysis front position. The overall predictive capabilities of fire growth models depend on the pyrolysis model, treatment of gas-phase fluid mechanics, turbulence, combustion chemistry, and convective/radiative heat transfer. Unless simulations are truly blind, some model calibration (adjusting various input parameters to improve agreement between model calculations and experimental data) is usually inherent in published results, so model calculations may not truly be predictions. 

FIGURE 20.6 Comparison of measured and modeled HRR in room/corner test on plywood. From Moghaddam et al. [96] for ethanol reaction case. (Adapted from Moghaddam, A.Z. et al., Fire behavior studies of combustible wall linings applying fire dynamics simulator, in Proceedings of the 15th Australasian Fluid Mechanics Conference, Sydney, Australia, 2004.)... 

The relative ease or difficulty of incineration has been estimated on the basis of the heat of combustion, thermal decomposition kinetics, susceptibility to radical attack, autoignition temperature, correlations of other properties, and destruction efficiency measurements made in laboratory combustion tests. Laboratory studies have indicated that no single ranking procedure is appropriate for all incinerator conditions. In fact, a compound that can be incinerated easily in one system may be the most difficult to remove from another incinerator due to differences in the complex coupling of chemistry and fluid mechanics between the two systems. 

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