Calculations of explosion deflagrating flames using a dynamic flame surface density model (original) (raw)
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Dynamic flame surface density modelling of flame deflagration in vented explosion
Combustion, Explosion, and Shock Waves, 2012
The propagation of transient, turbulent premixed flames in a vented explosion chamber in the presence of a series of obstacles is numerically investigated by a dynamic formulation for the Flame Surface Density (FSD) with the Large Eddy Simulations (LES) technique. The chemistry is modelled by a one-step overall reaction, which simulates the reaction of a stoichiometric propaneair mixture. The FSD modelling in the reaction rate model is numerically employed with two different sub-grid scale (SGS) models. The first one is based on an empirical correlation of the SGS velocity fluctuations and the second one is based on similarity ideas involved in solving the wrinkled flame front considered as a fractal surface. The numerical predictions are analyzed and compared against an algebraic, simple FSD model together with experimental data. The calculations show that the dynamic FSD models provide superior results as compared with the algebraic FSD model. The comparisons demonstrate the importance of the contributions from the unresolved FSD and provide good agreement with experimental data for the flame structure, overpressure, and burning velocities.
On the mechanism of pressure rise in vented explosions: A numerical study
Process Safety and Environmental Protection, 2018
Accidental gas explosions are a significant concern in process industries. In an explosion event, the promotion of flame acceleration due to turbulence generated from obstacles is responsible for many severe damages. This paper discusses the numerical evaluation and the mechanism of pressure rise in vented explosions in the presence of obstructions using computational fluid dynamics (CFD). The large eddy simulation (LES) technique is employed with a dynamic flame surface density (DFSD) in the combustion model to account for the filtered chemical source term. The experimental test case considered for the validation of simulations is a smallscale explosion chamber with removable baffle plates and obstacles. It is found that the maximum overpressure increases with the baffle plates moved downstream from the ignition source or when additional baffles are placed in sequence. Large separation between baffles and the central obstacle results in lower overpressure due to the relaminarisation of the flame front. The trend of explosion overpressure is related to the competition between the strength of venting and expansion in the explosion chamber. Extensive interactions between the flame and the obstructiongenerated turbulence are found to wrinkle the flame front and increase the burning rate. Satisfactory agreements have been obtained between LES and the experimental data. This confirms the capability of the developed model in predicting essential safety-related parameters in vented explosions. Results reveal the potential of using LES in the selection of design aspects for loss prevention, such as the area of vents and distance between congested regions in chemical processing plants.
An Assessment of Large Eddy Simulations of Premixed Flames Propagating Past Repeated Obstacles
… Theory and Modelling, 2009
This paper presents an assessment of Large Eddy Simulations in calculating the structure of turbulent premixed flames propagating past solid obstacles. One objective of the present study is to evaluate the LES simulations and identify the drawbacks in accounting the chemical reaction rate. Another objective is to analyse the flame structure and to calculate flame speed, generated overpressure at different time intervals following ignition of a stoichiometric propane/air mixture. The combustion chamber has built-in repeated solid obstructions to enhance the turbulence level and hence increase the flame propagating speed. Various numerical tests have also been carried out to determine the regimes of combustion at different stages of the flame propagation. These have been identified from the calculated results for the flow and flame characteristic parameters. It is found that the flame lies within the 'thin reaction zone' regime which supports the use of the laminar flamelet approach for modelling turbulent premixed flames. A sub-model to calculate the model coefficient in the algebraic flame surface density model is implemented and examined. It is found that the LES predictions are slightly improved due to the calculation of model coefficient by using sub-model. Results are presented and discussed in this paper are for the flame structure, position, speed, generated pressure and the regimes of combustion during all stages of flame propagation from ignition to venting. The calculated results are validated against available experimental data.
LES-DFSD Modelling of Turbulent Premixed Flames Past Repeated Obstacles
Proceedings of the 3rd World Congress on Momentum, Heat and Mass Transfer, 2018
This paper presents simulations of propagating turbulent premixed deflagrating flames past built-in solid obstructions in a small-scale combustion chamber. The design of the chamber allows for up to three baffle plates and a central square obstacle to be positioned in the path of the propagating flames in order to generate turbulence and increase the flame propagating speed. The test case considered in this paper uses a stagnant, stoichiometric propane-air mixture in the configuration of three baffles and one central obstacle. Simulations have been carried out with the Large Eddy Simulation (LES) technique. The filtered reaction rate in LES is accounted for using a novel dynamic flame surface density (DFSD) model. Both numerical and experimental results show that the flame is initially laminar and becomes fully turbulent after continuous interaction with the obstacles downstream. Satisfactory agreement made between the LES calculations and the experimental data confirms the capability of the DFSD model in reproducing essential flame characteristic parameters including the maximum overpressure and flame front speed. The interaction between obstaclegenerated turbulence and the flame front is quantified using the sub-grid-scale (SGS) wrinkling factor. Various stages of flame propagation and the dynamic behaviours of the flame are also examined based on the evolution and spatial distribution of the wrinkling factor.
Modeling the initial flame acceleration in an obstructed channel using large eddy simulation
Journal of Loss Prevention in the Process Industries, 2013
The propagation and acceleration of a flame surface past obstructions in a closed square channel was investigated using large eddy simulation. The dynamic Smagorinsky-Lilly subgrid model and the Boger flame surface density combustion model were used. The geometry is essentially two-dimensional with fence-type obstacles distributed on the top and bottom surfaces, equally spaced along the channel length at the channel height. Flame propagation, however, is three dimensional as ignition occurs at a point at the center of the channel cross-section. The effect of obstacle blockage ratio on the development of the flame structure was investigated by varying the obstacle height. Three-dimensional cases were simulated from the initiation of a combustion kernel through spark ignition to the acceleration of the flame front at speeds up to 80 m/s. The transition from laminar flame propagation to turbulent flame propagation within the "thin reaction zone" regime was observed in the simulations. By analyzing the development of the three dimensional flame surface and unburned gas flow field, the formation of several flame structures observed experimentally are explained. Global quantities such as the total flame area and centerline flame velocity were ascertained and compared to the experimental data. High amplitude oscillations in the centerline flame velocity were found to occur from a combination of the unburned gas flow field and fluctuations in the volumetric burning rate.
Journal of Hazardous Materials, 2010
In this work, an assessment of different sub-grid scale (sgs) combustion models proposed for large eddy simulation (LES) of steady turbulent premixed combustion (Colin et al.) was performed to identify the model that best predicts unsteady flame propagation in gas explosions. Numerical results were compared to the experimental data by Patel et al. (Proc. Combust. Inst. 29 (2002) 1849-1854) for premixed deflagrating flame in a vented chamber in the presence of three sequential obstacles. It is found that all sgs combustion models are able to reproduce qualitatively the experiment in terms of step of flame acceleration and deceleration around each obstacle, and shape of the propagating flame. Without adjusting any constants and parameters, the sgs model by Charlette et al. also provides satisfactory quantitative predictions for flame speed and pressure peak. Conversely, the sgs combustion models other than Charlette et al. give correct predictions only after an ad hoc tuning of constants and parameters.
Numerical Investigation of the Effect of Ignition Area on the Subsequent Flame Propagation Behavior
Journal of Thermal Science and Technology, 2009
In this paper, the effect of ignition area on the propagation of a laminar premixed flame is investigated numerically in a two-dimensional channel. A single-step irreversible overall exothermic chemical reaction is applied to model combustion chemistry. The time-dependent system of governing equations for reacting flows is discretized using the finite volume method (FVM) on the hexahedral structure grid cells. The discretized system of equations is solved by adopting Front Flow Red, a multi-scale and-physics computational fluid dynamics (CFD) solver. The computed results show that the flame oscillates during the propagation owing to the strong roll-up of the vortices generated by the strong shear layer originating from the sudden high gas expansion flow at the large ignition area. The instantaneous acceleration of the vortices increases the flame surface area which gives rise to higher propagation speed; consequently, combustion time is shortened. These results suggest that the rapid increase in flame surface, caused by the large ignition area induced strong vortices, could be one of the potential methods in improving combustion efficiency by reducing the burning time in the internal combustion devices.
LES of explosions in venting chamber: A test case for premixed turbulent combustion models
Combustion and Flame, 2017
This paper presents a new experimental and Large Eddy Simulation (LES) database to study upscaling effects in vented gas explosions. The propagation of premixed flames in three setups of increasing size is investigated experimentally and numerically. The baseline model is the well-known laboratory-scale combustion chamber from Sydney (Kent et al., 2005; Masri et al., 2012); two exact replicas at scales 6 and 24.4 were set up by GexCon (Bergen, Norway). The volume ratio of the three setups varies from 1 to more than 10,0 0 0, a variation unseen in previous experiments, allowing the exploration of a large range of Reynolds and Damköhler numbers. LES of gaseous fully premixed flames have been performed on the three configurations, under different operating conditions, varying the number of obstacles in the chamber, their position and the type of fuel (hydrogen, propane and methane). Particular attention is paid to the influence of the turbulent combustion model on the results (overpressure, flame front speed) comparing two different algebraic sub-grid scale models, the closures of Colin et al. (20 0 0) and Charlette et al. (2002), used in conjunction with a thickened flame approach. Mesh dependency is checked by performing a highly resolved LES on the small-scale case. For a given scale and with a fixed model constant, LES results agree with experimental results, for all geometric arrangement of the obstacles and all fuels. However, when switching from small-scale cases to medium-scale or large-scale cases this conclusion does not hold, illustrating one of the main deficiencies of these algebraic models, namely the need for an a priori fitting of the model parameters. Although this database was initially designed for safety studies, it is also a difficult test for turbulent combustion models.