Predictions of turbulent, premixed flame propagation in explosion tubes

doi:10.1016/0010-2180(94)00245-N

Combustion and Flame

Volume 102, Issues 1–2, July 1995, Pages 115-128

https://doi.org/10.1016/0010-2180(94)00245-N Get rights and content

Abstract

A mathematical model capable of predicting the overpressures generated by gaseous explosions is described. The model is based on solutions of the fluid flow equations obtained using a second-order accurate, finite-volume integration scheme coupled to an adaptive grid algorithm. Turbulence generated ahead of a propagating flame is modelled using a κ-ε approach, whilst the premixed combustion process is described using a semiempirical method which admits both chemical kinetic and flow field influences on the burning velocity of a flame, while also maintaining realistic flame thicknesses throughout the course of a flame's propagation. Comparison of model predictions and experimental data obtained in a large-scale cylindrical vessel containing turbulence-inducing rings, reported in the literature, demonstrate the ability of the model to provide reasonable predictions of propagating turbulent premixed flames which interact with obstacles, and the resulting generation of damaging overpressures. In total, the modeling techniques described offer the potential for ultimate application to predicting the behavior of explosions in realistic, three-dimensional geometries.

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Citation Excerpt :
The most frequently used CFD techniques include the Reynolds-averaged Navier-Stokes (RANS) approach, direct numerical simulation (DNS) and large eddy simulations (LES) (Versteeg and Malalasekera, 2007). Than RANS approach has been successfully used to investigate the safety aspect of accidents and explosions for applicable structures (Catlin et al., 1995; Efthimiou et al., 2017; Huang et al., 2020; Popat et al., 1996). The RANS technique has also been applied to large scale applications, providing adequate results within the defined applications (Baraldi et al, 2010, 2017; Marangon et al., 2009; Vyazmina and Jallais, 2016).
Accidental explosions are a plausible danger to the chemical process industries. In the event of a gas explosion, any obstacles placed within the path of the flame generate turbulence, which accelerates the transient flame and raises explosion overpressure, posing a safety hazard. This paper presents numerical studies using an in-house computational fluid dynamics (CFD) model for lean premixed hydrogen/air flame propagations with an equivalence ratio of 0.7. A laboratory-scale combustion chamber is used with repeated solid obstacles. The transient compressible large eddy simulation (LES) modelling technique combined with a dynamic flame surface density (DFSD) combustion model is used to carry out the numerical simulations in three-dimensional space. The study presented uses eight different baffle configurations with two solid obstructions, which have area blockage ratios of 0.24 and 0.5. The flame speed, maximum rate of pressure-rise as well as peak overpressure magnitude and timing are presented and discussed. Numerical results are validated against available published experimental data. It is concluded that, increasing the solid obstacle area blockage ratio and the number of consecutive baffles results in a raised maximum rate of pressure rise, higher peak explosion overpressure and faster flame propagation. Future model development would require more experimental data, probably in a more congested configuration.
Numerical studies of premixed hydrogen/air flames in a small-scale combustion chamber with varied area blockage ratio
2020, International Journal of Hydrogen Energy
The increasing use of hydrogen as a renewable source of energy underlines the need to be able to assess the safety risks involved in the event of an accidental explosion. This paper presents numerical studies for hydrogen/air propagating flames at an equivalence ratio of 0.7 in a laboratory-scale combustion chamber equipped with turbulence generating baffles and a solid square cross section obstruction. The large eddy simulation (LES) modelling technique is used with an in-house computational fluid dynamics (CFD) model for compressible flows to study the flow turbulence and the flame propagation characteristics. The study is carried out using four different baffle arrangements and two different solid obstructions with area blockage ratios of 0.24 and 0.5. Results for the generated peak overpressure and the timing at which it occurs following ignition are considered as the primary safety factors. The time histories of the flame speed and position relative to the ignition source are validated against published experimental data. Good agreement is obtained between numerical results and experimental data which enables further predictions where measurements are limited in the study of vented hydrogen explosions. It was concluded that adding successive baffles and increasing the area blockage ratio escalates the maximum rate at which pressure rises and raises the generated peak explosion overpressure.
On the minimum ignition energy and its transition in the localised forced ignition of turbulent homogeneous mixtures
2019, Combustion and Flame
The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane–air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy.
On the mechanism of pressure rise in vented explosions: A numerical study
2018, Process Safety and Environmental Protection
Citation Excerpt :
Thanks to the improvements in computational technology and resources, CFD is becoming a more attractive and reliable tool as an alternative to experiments in process industries. Reynolds-averaged Navier-Stokes (RANS) methods have been applied in studying explosions for safety-related structures (Birkby et al., 2000; Catlin et al., 1995; Popat et al., 1996). While RANS-based method remains as the major numerical tool in explosion-related studies, the accuracy is generally not sufficient and a certain degree of tuning of model parameters is usually required.
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 small-scale 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 obstruction-generated 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.
The role of CFD combustion modeling in hydrogen safety management – IV: Validation based on non-homogeneous hydrogen–air experiments
2016, Nuclear Engineering and Design
The control of hydrogen in the containment is an important safety issue in NPPs during a loss of coolant accident, because the dynamic pressure loads from hydrogen combustion can be detrimental to the structural integrity of the reactor safety systems and the reactor containment. In Sathiah et al. (2012b), we presented a computational fluid dynamics based method to assess the consequence of the combustion of uniform hydrogen–air mixtures. In the present article, the extension of this method to and its validation for non-uniform hydrogen–air mixture is described. The method is implemented in the CFD software ANSYS FLUENT using user defined functions. The extended code is validated against non-uniform hydrogen–air experiments in the ENACCEF facility.
It is concluded that the maximum pressure and intermediate peak pressure were predicted within 12% and 18% accuracy. The eigen frequencies of the residual pressure wave phenomena were predicted within 4%. It is overall concluded that the current model predicts the considered ENACCEF experiments well.
Large Eddy simulation of hydrogen-air premixed flames in a small scale combustion chamber
2015, International Journal of Hydrogen Energy
Citation Excerpt :
The importance to study deflagrating flames came from the fact of their wide applications in industry and accidental explosions. However, there are already a significant amount of studies reporting data in vented explosions in laboratory chambers [3–12]. Different parameters are reported in these studies, mainly pressure and/or velocity, but the information remains limited because of the difficulty of making detailed measurements [13].
While hydrogen is attractive as a clean fuel, it poses a significant risk due to its high-reactivity. This paper presents Large Eddy Simulations (LES) of turbulent premixed flames of hydrogen–air mixtures propagating in a small scale combustion chamber. The sub-grid-scale model for reaction rate uses a dynamic procedure for calculating the flame/flow interactions. Sensitivity of the results to the ignition source and to different flow configurations is examined. Using the relevant parameter from the calculations, the flames are located on the regimes of combustion and are found to span the thin and corrugated flamelet regimes, hence confirming the validity of flamelet modelling. The calculations are compared to published experimental data for a similar configuration. It is found that both the peak overpressure and flame position are affected by the number of baffles positioned in the path of the flame and this is consistent with earlier findings for hydrocarbon fuels. Also, the LES technique is able to reproduce the same flame shape as the experimental images. A coarse study of sensitivity to the ignition source shows that the size of the ignition kernel does not affect the flame structure but influences only the time where the peak overpressure appears while moving the ignition source away from the base plate leads to a decrease in the peak overpressure.

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^☆: The calculations reported in this paper were made using a modified version of the Mantis Numerics Ltd. code, COBRA. This paper is published by permission od British Gas Plc.

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Predictions of turbulent, premixed flame propagation in explosion tubes☆

Abstract

Combust. Flame

Combust. Flame

J. Hazardous Materials

Combust. Flame

Combust. Flame

Combust. Flame

Combust. Flame

Combust. Flame

Understanding Vapour-Cloud Explosions—An Experimental Study

Combust. Sci. Technol.