Salah S. Ibrahim

Studies of premixed flame propagation in explosion tubes

An experimental and theoretical study of premixed flame propagation in a number of small-scale, cylindrical vessels is described. The study provides further understanding of flame propagation and the generation of overpressure in explosions, and allows the assessment of a mathematical model of explosions through comparisons with the experimental data obtained. Laser sheet images and data gathered on flame location, shape, and overpressures generated during the course of explosions in an empty vessel and obstacle-containing enclosures elucidate the dynamics of the various combustion processes occurring in the different chambers of the vessels. In particular, flame propagation through the vessels, up until flame front venting, is found to be substantially laminar, with significant overpressure only being generated in the later stages of explosions due to rapid turbulent combustion in the shear layers and recirculation zones induced by the obstacles. Comparisons between measurements and predictions also demonstrate that the mathematical model described provides a reasonable simulation of explosions within obstacle containing enclosures of the type investigated, with rapid turbulent combustion being predicted with sufficient accuracy to yield reasonable results for the overpressures generated.

The effects of obstructions on overpressure resulting from premixed flame deflagration

Abstract This paper introduces a new experimental set-up for investigating the effects of obstruction geometry, blockage ratio and venting pressure on overpressures resulting from premixed flame deflagration. Obstructions shaped as cylinders, triangles, squares, diamonds and plates or walls are studied here covering blockage ratios ranging from about 10% to more than 75%. It is found that the deflagration overpressure increases with increasing venting pressure. Also, the maximum overpressure increases, generally with increasing blockage ratio but the rate of increase depends on the obstruction geometry. The wall/plate type obstruction leads to the highest overpressures and the cylindrical obstruction yields the lowest overpressure. The time taken to reach the maximum overpressure decreases with increasing blockage ratio and changes with obstruction geometry implying that the flame accelerates faster due to changed local turbulence levels and length scales.

Experimental study of premixed flame propagation over various solid obstructions

Abstract Accounting for the probability of an explosion is essential in the design of plants and onshore or offshore gas or oil exploration platforms. This, of course, will have major implications on the safety of personnel both in terms of potential loss of life and the possibilities of escalation processes, which could lead to catastrophic consequences. The mechanisms which enhance explosion overpressures, therefore, need to be established with some certainty in order to ensure that all aspects of the safe design of structures and processes and the safe protection of personnel are taken into account. This paper presents an experimental investigation of the interaction of propagating premixed flames with various solid obstructing bodies to quantify the role of generated turbulence in flame acceleration and structure. Three different obstructions with circular, triangular and square cross-sections are studied here covering blockage ratios ranging from about 10% to 78%. High-speed video images were used to track the propagating flame front and the volume of trapped mixture behind the obstacle downstream from the ignition point. Images taken at different times after ignition are presented and discussed in terms of flame structure and acceleration. It is found that obstructions with square cross-sections result in the fastest flame acceleration followed by triangular and circular cross-sections. Also, flame speed is found to increase with increasing area blockage ratio. The volume of trapped unburned mixture is found to be high when square and triangular obstacles are used.

Large Eddy Simulation of a Propagating Turbulent Premixed Flame

A large eddy simulation of a turbulent premixed flame propagatingthrough a chamber containing a square obstruction is presented anddiscussed. The governing equations for compressible, reacting flowsare Favre filtered and turbulence closure is achieved using thedynamic Smagorinsky subgrid model. A simple flame surface densitymodel based on the flamelet concept is employed for the subgrid scalereaction rate. The simulation gives very good agreement with experimentalresults for the speed and the shape of the flame as it propagates throughthe chamber. The peak pressures, however, are underpredicted by20–30%. Reasons for this are discussed and it is concluded that amore sophisticated combustion model is required for complex flowssuch as this one, if a more accurate prediction of the pressureis to be achieved.

Predictions of turbulent, premixed flame propagation in explosion tubes

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 modeled using a {kappa}-{var_epsilon} approach, while 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.

Experimental investigation of flame/solid interactions in turbulent premixed combustion

Abstract An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.

An experimental and numerical investigation of premixed flame deflagration in a semiconfined explosion chamber

A combined experimental and numerical study of turbulent premixed flame propagation over multipleobstacles mounted in a semiconfined explosion combustion chamber is reported. The experimental method used a high-speed laser sheet flow visualization technique to record the progress of the flame front in a stoichiometric methane/air mixture. This allowed calculation of flame speed. Pressure was measured at two locations within the combustion chamber. For the simulations, transient Favre-averaged equations were solved using recent developments of a flamelet model. This model formulates the mean rate of reaction as a function of a transport equation for the flamelet surface density model. Both linear and nonlinear eddy viscosity turbulence models have been used and investigated for the closure of the Reynolds stresses. Excellent agreement of flame structure and pressure impulse was obtained with the use of a nonlinear eddy viscosity turbulence model. It was also found that, irrespective of the eddy viscosity turbulence model, quantitative results of pressure and flame speed were found to be in good agreement with the experimental results. Regimes of combustion covered by the present study have been identified and found to reside in the wrinkled and corrugated (reaction sheets) flamelet regimes.

LES of Recirculation and Vortex Breakdown in Swirling Flames

In this study large eddy simulation (LES) technique has been applied to predict a selected swirling flame from the Sydney swirl burner experiments. The selected flame is known as the SM1 flame operated with fuel CH 4 at a swirl number of 0.5. In the numerical method used, the governing equations for continuity, momentum and mixture fraction are solved on a structured Cartesian grid. The Smagorinsky eddy viscosity model with the localised dynamic procedure of Piomelli and Liu is used as the subgrid scale turbulence model. The conserved scalar mixture fraction-based thermo-chemical variables are described using the steady laminar flamelet model. The GRI 2.11 is used as the chemical mechanism. The Favre-filtered scalars are obtained from the presumed beta probability density function (β-PDF) approach. The results show that with appropriate inflow and outflow boundary conditions LES successfully predicts the upstream recirculation zone generated by the bluff body and the downstream vortex breakdown zone induced by swirl with a high level of accuracy. Detailed comparison of LES results with experimental measurements show that the mean velocity field and their rms fluctuations are predicted very well. The predictions for the mean mixture fraction, subgrid variance and temperature are also reasonably successful at most axial locations. The study demonstrates that LES together with the laminar flamelet model in general provides a good technique for predicting the structure of turbulent swirling flames.

LES Modeling of Premixed Deflagrating Flames in a Small-Scale Vented Explosion Chamber with a Series of Solid Obstructions

In this study, simulations of propagating turbulent premixed deflagrating flames past built in solid obstructions in a laboratory scale explosion chamber has been carried out with the Large Eddy Simulation (LES) technique. The design of the chamber allows for up to three baffle plates to be positioned in the path of propagating flame, rendering different configurations, hence generating turbulence and modifying the structure of the reaction zone. Five important configurations are studied to understand the feedback mechanism between the flame-flow interactions and the burning rate. In LES, the sub-grid scale (SGS) reaction rate should be accounted for by an appropriate model that can essentially capture the physics. The present work has been carried by using the flame surface density (FSD) model for sub-grid scale reaction rate. The influence of the flow on turbulence and flame propagation as a result of the in-built solid obstructions is also examined. The impact of the number and the position of such baffle plates on the generated overpressure, flame speed and structure are studied. Results from the simulations are compared with experimental data for five configurations and they show good agreement.

An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

This paper presents an assessment of Large Eddy Simulations (LES) 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 submodel 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 owing to the calculation of model coefficient by using submodel. 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.