Numerical and experimental investigation of backdraft (original) (raw)
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Methane is one of the most common gaseous fuels that also exist in nature as the main part of the natural gas, the flammable part of biogas or as part of the reaction products from biomass pyrolysis. In this respect, the biogas and biomass installations are always subjected to explosion hazards due to methane. Simple methods for evaluating the explosion hazards are of great importance, at least in the preliminary stage. The paper describes such a method based on an elementary analysis of the cubic law of pressure rise during the early stages of flame propagation in a symmetrical cylindrical vessel of small volume (0.17 L). The pressure–time curves for lean, stoichiometric and rich methane–air mixtures were recorded and analyzed. From the early stages of pressure–time history, when the pressure increase is equal to or less than the initial pressure, normal burning velocities were evaluated and discussed. Qualitative experiments were performed in the presence of a radioactive source o...
Numerical simulation of backdraft phenomena
This paper reports preliminary computational fluid dynamics (CFD) simulations of backdraft observed in an experimental rig at Lund University. The analysis was performed with the CFX software using the Detached Eddy Simulation (DES) turbulence model, a hybrid of Large Eddy Simulation (LES) and RANS, in combination with the EDM combustion model. The DES model uses a RANS formulation in wall proximity to avoid computationally expensive grid resolution that is necessary for realistic LES predictions in wall layers. The preliminary results are qualitatively promising. The simulations began at the instant at which the door opens. A stream of fresh and cold air enters the enclosure as a gravity current. In the rig, ignition was triggered by flammable conditions existing at a wire, which was constantly heated. In the CFD model the ignition time is computed automatically when flammability conditions are reached inside the enclosure, at the wire, as part of the analysis. Subsequently, the fire front is formed. The deflagration expels fuel-rich mixture into environment, and the combustion continues outside the enclosure as a typical ‘secondary’ event. Considering that backdraft is a very complex phenomenon, the outcome is considered by the authors to be encouraging.
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.
Process Safety and Environmental Protection, 2021
The formation of explosive gas zones (EGZs) from flammable vapors, gases, or dust pose safety hazards to many industries. In many cases, explosions may occur in confined areas with obstacles in the path of flame expansion. By studying the effects of obstacle shape, turbulence model, and spark location on flame propagation and turbulence, a more complete understanding of the flame and fluid dynamics interaction has been achieved. Reynolds Averaged Navier-Stokes (RANS) models were tested to determine if these simplified turbulence models could capture the flame dynamics and propagation velocities using fewer computational resources compared to the higher fidelity Large Eddy Simulation (LES) turbulence model. Results showed that square obstacles caused faster flame propagation compared to hexagons and circles. The square had an average flame propagation velocity 26 % faster than the circle, and the hexagon was 16 % faster than the circle using a k-model. Modeling results indicate variation of spark location by as small as 10 % of the obstacle diameter can result in a difference of the flame propagation. Findings on turbulence model accuracy and computational time along with shape comparison can be applied in future modeling of large systems, providing crucial information for safety planning and explosion prevention.
Exploratory backdraft experiments
Fire Technology, 1993
This study is a qualitative exploration of backdraft phenomena. Backdraft is defined as a rapid deflagration following the introduction of oxygen into a compartment filled with accumulated excess pyrolyzates. A scenario describing the physical and chemical fundamentals underlying backdraft phenomena is presented. A half-scale apparatus, designed to avoid dangerous overpressures, was used to obtain data from backdraft experiments. A gas burner supplied a 150 kW natural gas fire in a 1.2 m high, 1.2 m wide, 2.4 m long compartment with a small, 25 mm high, 0.3 m wide vent to ambient at floor level. Significant unburnt fuel accumulates in 180 seconds, when a hatch covering a 0.4 m high, 1.2 m wide vent, centered on a short wall, is opened, simulating a window breaking due to thermal stresses. The propagation across the compartment of the cold density-driven flow, which enters through the new opening, is called a"gravity current." This gravity current carries a flammable mixed layer to a spark located near the bumer on the opposite wall. The rapid deflagration that results upon ignition of the mixed layer is the backdraft. A compartment fite model is used to calculate conditions in the compartment before the vent opens. The hypothesized scenario appears to be confirmed by the deflagrations and exterior fire balls observed in these preliminary experiments.
A Large Eddy Simulation of LNG Pool Fire on Board a Chemical/Oil Tanker
Journal of Physics: Conference Series
The air pollution from maritime transportation has become one of the major environmental concerns. The liquefied natural gas (LNG) has better environmental performance compared to conventional ship fuels. Therefore, the use of LNG as ship fuel has recently gained more attention in the maritime industry. On the other hand, LNG as a fuel can have high risks of explosion and fire on ships. Among the various LNG ship fires, the pool fire is the most common phenomenon. Therefore, this study focuses on the LNG pool fire on board a chemical/oil tanker with different pool aspect ratios. Also, the effects of wind speed on the flame characteristics are investigated. The LNG pool fire on board a chemical/oil tanker is studied by using large eddy simulation approach and Fire Dynamic Simulator (FDS) code, numerically. The results show that the flame characteristics are affected markedly by the pool aspect ratio and wind speed. While the aspect ratio increases, the mean flame height reduces, whereas the heat flux values increases. It is also found that the heat flux values increase with the wind speed.
Numerical simulations of the flow field ahead of an accelerating flame in an obstructed channel
Combustion Theory and Modelling, 2010
The development of the unburned gas flow field ahead of a flame front in an obstructed channel was investigated using large eddy simulation (LES). The standard Smagorinsky-Lilly and dynamic Smagorinsky-Lilly subgrid models were used in these simulations. The geometry is essentially twodimensional. The fence-type obstacles were placed on the top and bottom surfaces of a square crosssection channel, equally spaced along the channel length at the channel height. The laminar rollup of a vortex downstream of each obstacle, transition to turbulence, and growth of a recirculation zone between consecutive obstacles were observed in the simulations. By restricting the simulations to the early stages of the flame acceleration and by varying the domain width and domain length, the three-dimensionality of the vortex rollup process was investigated. It was found that initially the rollup process was twodimensional and unaffected by the domain length and width. As the recirculation zone grew to fill the streamwise gap between obstacles, the length and width of the computational domain started to affect the simulation results. Three-dimensional flow structures formed within the shear layer, which was generated near the obstacle tips, and the core flow was affected by large-scale turbulence. The simulation predictions were compared to experimental schlieren images of the convection of helium tracer. The development of recirculation zones resulted in the formation of contraction and expansion regions near the obstacles, which significantly affected the centerline gas velocity. Oscillations in the centerline unburned gas velocity were found to be the dominate cause for the experimentally observed early flame-tip velocity oscillations. At later simulation times, regular oscillations in the unburned streamwise gas velocity were not observed, which is contrary to the experimental evidence. This suggests that fluctuations in the burning rate might be the source of the late flame-tip velocity oscillations. The effect of obstacle blockage ratio (BR) on the development of the unburned gas flow field was also investigated by varying the obstacle height. Simulation predictions show favorable agreement to the experimental results and indicate that turbulence production increases with increasing obstacle BR.
A two-dimensional axis-symmetric numerical model was solved to investigate the effect of four turbulence models on combustion characteristics, such as the velocity, the pressure, the turbulent kinetic energy and the dissipation rate in a methane-air no-premixed flame. Based on the commercial CFD code Ansys fluent 17.0, different turbulence models including the standard k-ε model, the RNG k-ε model, the realizable k-ε model and the standard k-ω model were used to simulate the flow field in a simple burner. The eddy dissipation model with the global reaction schema was applied to model the turbulence reaction interaction in the flame region. A finite volume approach was used to solve the Navier-Stokes equations with the combustion model. Particularly, the effect of these turbulence models on the combustion characteristics was analyzed. The numerical predictions were validated by comparison with anterior experimental results. Moreover, the predicted axial and radial gradients of velocity in the standard k-ε are overall agreement with literature results. Cite This Article: O. Moussa, and Z. Driss, " Numerical Investigation of the Turbulence Models Effect on the Combustion Characteristics in a Non-Premixed Turbulent Flame Methane-Air.
Propagation indices of methane-air explosions in closed vessels
Journal of Loss Prevention in the Process Industries, 2017
The peak explosion pressure, the maximum rate of pressure rise and the time necessary to reach the peak explosion pressure are important flammability indices of fuel-air combustion in closed vessels, characteristic for the laminar propagation stage of the process. In the present paper, these indices were examined using methane of various concentrations within the flammability limits, at variable initial pressure between 50 and 200 kPa and ambient initial temperature. For each composition, the experimental explosion pressures were compared with the adiabatic explosion pressures, computed under the assumption that chemical equilibrium is reached in the flame. The experimental explosion pressures and the rates of pressure rise are examined in comparison with literature data, the fluctuations being attributed to differences of heat lost by the flammable gas to the explosion vessel, during flame propagation. Using the differences between the adiabatic and experimental explosion pressures, the amount of heat lost to the walls during the explosion propagation in a closed vessel and the fraction of the transferred heat from the total released heat have been determined.
Quantitative Backdraft Experiments
Fire Safety Science, 1994
This paper focuses on 17 experiments in a 1.2 m by 1.2 m by 2.4 m compartment. A methane burner, flowing at either 70 kW or 200 kW, was ignited inside a closed compartment and burned until the initially available oxygen was consumed. After the fire self-extinguished, the burner was left on allowing the unburned fuel mass fraction in the compartment to increase. After removing a hatch, covering a 1.1 m wide by 0.4 m high slot opening, a gravity current entered the compartment. It traveled across the floor, mixed with the unburned fuel, and was ignited by a spark near the burner. After mixture ignition, a backdraft occurred as a deflagration ripped through the compartment culminating in a large external fireball. Histories recorded prior to backdrafl included: fuel flow rates, upper layer temperatures, lower layer temperatures, upper layer species concentrations for 0 2 , CO2, CO, and HC. Data collected to quantify the backdraft included opening gas flow velocities and compartment pressures. Results indicate that unburned fuel mass fractions >lo% are necessary for a backdrafl to occur.