Numerical simulation of flame propagation and localized preflame autoignition in enclosures

https://doi.org/10.1016/j.jlp.2011.09.007Get rights and content

Abstract

A novel computational approach based on the coupled 3D Flame-Tracking–Particle (FTP) method is used for numerical simulation of confined explosions caused by preflame autoignition. The Flame-Tracking (FT) technique implies continuous tracing of the mean flame surface and application of the laminar/turbulent flame velocity concepts. The Particle method is based on the joint velocity–scalar probability density function approach for simulating reactive mixture autoignition in the preflame zone. The coupled algorithm is supplemented with the database of tabulated laminar flame velocities as well as with reaction rates of hydrocarbon fuel oxidation in wide ranges of initial temperature, pressure, and equivalence ratio. The main advantage of the FTP method is that it covers both possible modes of premixed combustion, namely, frontal and volumetric. As examples, combustion of premixed hydrogen–air, propane–air, and n-heptane–air mixtures in enclosures of different geometry is considered. At certain conditions, volumetric hot spots ahead of the propagating flame are identified. These hot spots transform to localized exothermic centers giving birth to spontaneous ignition waves traversing the preflame zone at very high apparent velocities, i.e., nearly homogeneous preflame explosion occurs. The abrupt pressure rise results in the formation of shock waves producing high overpressure peaks after reflections from enclosure walls.

Highlights

► Novel approach for numerical simulation of confined explosions is developed. ► It is applied to model turbulent premixed combustion in vessels of different geometry. ► It allows continuous monitoring of both flame front and preflame reactions.

Introduction

Numerical simulation of flame propagation with preflame autoignition in enclosures is a complex problem. The phenomenology of the process includes flame ignition and propagation, unburned mixture compression and heating, as well as formation of hot spots and fast-spreading localized explosions in the preflame region. The localized explosions evolve from the sites with the minimum induction time and traverse the preflame zone as spontaneous ignition waves with the propagation velocity depending on the local instantaneous distributions of temperature and mixture composition. Better understanding of these phenomena is important for the improvement of existing measures aimed at preventing violent accidental explosions in process plants.

The objective of any combustion model in a CFD code is to provide correct values of mean reaction rates in each computational cell regardless the combustion mode (premixed, nonpremixed, partially premixed, homogeneous, inhomogeneous, spontaneous, frontal, etc.). The correct value of the mean reaction rate in the computational cell can be obtained only if one knows the reaction kinetics and instantaneous fields of temperature and species concentrations inside the cell. The development of reaction kinetics is the separate task which is independent of the CFD combustion modeling. The only relevant issue is the CPU time required for calculating instantaneous reaction rates. This issue can be overcome by applying properly validated short overall reaction mechanisms or look-up tables.

The instantaneous fields of temperature and species concentrations inside the cell are usually not known. Therefore, one has to replace this lacking information by combustion models. There exist many combustion models both for laminar and turbulent flows. If combustion chemistry is fast as compared to mixing, the Spalding (1976) Eddy-Break-Up model can be used. It is simple but has a limited range of validity. There is a whole class of statistical combustion models (based on the formalism of probability density functions (PDF)) with probabilistic representation of turbulence and its interaction with chemistry, Pope (1990). This approach is very attractive for treating both flame propagation and autoignition problems, however requires large CPU resources. The other class of models deals with a flamelet approach, Peters (1986). In this approach, the instantaneous flame is assumed to consist of localized reactive sheets, which are transported by the flow and wrinkled by turbulent eddies. The flamelet approach is applicable when the characteristic turbulent scales are larger than a typical flame thickness. This condition is satisfied in many practical situations.

The approach supposed below is a sort of combination of flamelet and statistical combustion models to allow for simultaneous treatment of frontal combustion by explicit tracing of mean reactive surfaces and volumetric combustion by the transported PDF approach. The availability of such an approach makes it possible to attack the problem of flame propagation with preflame autoignition in enclosures.

Section snippets

Flame-tracking method

The FT method deals with the subgrid model of laminar/turbulent combustion. The essence of the model can be readily explained on the example of laminar flame propagation. In the FT method, the flame-surface shape and area are found based on the Huygens principle: Each elementary portion of the flame surface displaces in time due to burning of the fresh mixture at local velocity un (normal to the flame surface) and due to convective motion of the mixture at local velocity V. The local

Particle method

The preflame zone exhibits volumetric reactions of fuel oxidation, formation of intermediate products like alcohols, aldehydes, peroxides, etc. In general, preflame reactions are inhomogeneous due to inhomogeneous distributions of temperature and main species concentrations and due to high sensitivity of reaction rates to these parameters. Therefore preflame reactions can result in localized energy release.

Direct (and CPU time consuming) way to calculate volumetric reaction rates is to solve

Results and discussion

Described below are the results of application of the FTP method to several test cases illustrating method capabilities.

Concluding remarks

A novel computational approach based on the coupled 3D FTP method has been developed and used for numerical simulation of confined explosions with/without preflame autoignition. The method allows continuous monitoring of both flame front and preflame reactions with generation of high-intensity secondary pressure peaks and is applicable to simulations of accidental explosions in various industrial reactors with complex geometry.

Acknowledgments

This work was partly supported by the Russian Foundation for Basic Research (grant 11-08-01297) and AVL LIST GmbH.

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