Impact of local flame quenching on the flame acceleration in H2-CO-air mixtures in obstructed channels

In accident scenarios originating from weak ignition, flame acceleration preconditions the fresh gas ahead of the flame front and provides the necessary conditions for deflagration-to-detonation transition to occur. Strong shear layers, which form at the rear edge of obstacles in the accelerated flow of fast flames, isolate fresh gas pockets. Vortices in the intense shear layer have the potential to locally quench the flame, limiting the integral heat release and delaying the onset of detonation.This study investigates the potential of local turbulent quenching in H${}_2$-CO-air mixtures. First, the presence of locally reduced heat release is visualized in highly resolved simulations for H${}_2$-air and H${}_2$-CO-air flames. Efficient simulation methods are of great importance for risk analysis studies. In connection with the results from highly resolved simulations this justifies a more detailed look at RANS-based combustion models for said flames. Thus, three different treatments of turbulent quenching are investigated, in which the geometrical configuration (blockage ratio and obstacle spacing) and the geometry size is varied.The results indicate that quenching does not need to be considered in RANS-based combustion models for H${}_2$-CO-air flames in explosion scenarios. But since quenching does eventually occur at very high turbulence intensities, the authors suggest limiting the flame turbulence interaction to flame stretch values obtained from 1D counter-flow flame simulations with detailed chemistry.     

Experimental and CFD-DEM study of the dispersion and combustion of wheat starch and carbon-black particles during the standard 20L sphere test

The 20L sphere is one of the standard devices used for dust explosivity characterization. One concern about the effectiveness and reliability of this test is related to the particle size variation due to particles' agglomeration and de-agglomeration. These phenomena are related to the turbulent regime of the dust cloud during the dispersion. This variable must be considered since it determines the uncertainty level of the ignitability and severity parameters of dust combustion. In this context, this study describes the influence of the cloud turbulence on the dust segregation and fragmentation through a study combining both, experimental and computational approaches. The behavior of the gas-solid mixture evidenced with the standard rebound nozzle was compared with that observed with six new nozzle geometries. Thereafter, the time-variation of the Particle Size Distribution (PSD) within the 20L sphere was analyzed for two different powders: carbon-black and wheat starch. On the one hand, the turbulence levels and PSD variations were characterized by Particle Image Velocimetry (PIV) tests and granulometric analyses, respectively. On the other hand, a computational approach described the dispersion process with CFD-DEM simulations developed in STAR-CCM + v11.04.010. The simulation results established that the homogeneity assumption is not satisfied with the nozzles studied. Nonetheless, the particles segregation levels can be reduced using nozzles that generate a better dust distribution in the gas-solid injections. Subsequently, an additional first-approach CFD model was established to study the behavior of the combustion step for a starch/air mixture. This model considers the gas-phase reactions of the combustible gases that are produced from the devolatilization of wheat starch ($CO,C{H}_4,C_2{H}_4,C_2{H}_6,C_2{H}_2,$ and $H_2$) and allowed to establish the approximate fraction of the particle mass that devolatilizes, as well as to confirm that the modeling of the pyrolysis stage is essential for the correct prediction of the maximum rate of pressure rise.     

A comparative study between two ignition sources: Electric igniter versus pyrotechnic igniter

The risk assessment of combustible explosive dust is based on the determination of the probability of dust dispersion, the identification of potential ignition sources and the evaluation of explosion severity. It is achieved in most of cases with the two main experimental normalized devices such as the Hartmann tube (spark ignition) and the 20 L spherical bomb (with two 5 kJ pyrotechnic ignitors).Ignition energy of the 5 kJ ignitor is well calibrated and generates a reproducible ignition. But, on the other hand, this ignition is not punctual and the over pressure produced is nearly 2 bar. Moreover, the pyrotechnic igniter accelerates the combustion with multi ignition points in a large volume and that disturbs the flame propagation. In this way, this ignition source does not allow to analyze the combustion products because the composition of the pyrotechnic igniter was found in the combustion products.This paper deals with the comparison of two ignition sources in the 20 L spherical bomb. Different explosive dusts of great industrial interest are studied with electrical and pyrotechnic ignitors, in order to understand, first, the influence of each type of igniter on the explosion behaviour and then to evaluate the possibility of establishing a correspondence between parameters obtained with these two ignition sources.Severity parameters of nicotinic acid, aluminium powder and titanium alloy were measured by using the two types of ignition system in our 20 L spherical bomb equipped with the Kühner dihedral injector. The explosion overpressure $\mathrm{P}$ and the rate of pressure rise $\left(\raisebox{1ex}{$dP$}\!\left/ \!\raisebox{-1ex}{$dt$}\right.\right)$ were measured in a large range of concentration allowing to propose correlations between electrical and pyrotechnic ignition for each parameter and each type of powder. These correlations aim to link the tests used with two different collections of experimental parameters for the same dust. The relevance of these correlations will be discussed.     

A new methodology to estimate the gas ascending time from underwater gas releases

Accidental subsea gas releases can pose a threat to people, equipment, and facilities since gas can be toxic or flammable at the concentrations in which the leak occurs. The accurate prediction of the behavior of the gas plume formed in the leaks can be fundamental to the development of techniques of accident prevention or, in some cases, remediation measures, avoiding the emergence of more serious consequences. Among the different ways to analyze the behavior of gas plumes formed underwater, the Computational Fluid Dynamics (CFD) tool stands out for allowing the study of plume behavior to be done in a safer, simpler, and less expensive way, if compared to experimental studies. Inspired by the accidental release of the subsea gas scenario, this work validated a CFD setup of a 2D two-phase air–water flow using the VOF method in Ansys Fluent. The use of the VOF method differs this work from other works that use a hybrid Eulerian–Lagrangian methodology to model such types of flow. In this validation, simulations with a 9 m base tank, and 7 m water depth, and 0.050, 0.100, and 0.450 m${}^3$/s gas flow were performed. The simulated data were compared to experimental results available in literature. After the validation of the setup, a study was carried out varying the size of the leak to 0.24 and 0.17 m, and the gas flow from 0.006 to 0.150 m${}^3$/s aiming to verify how some plume characteristics are affected by the changes. Finally, following the directions from literature for analyzing the ascending gas behavior, and combining it with a dimensional analysis of the data, we proposed a mathematical model for calculating the gas ascending time using only properties of the gas leak. With future modifications of the proposed methodology, we hope that soon it will be possible to simulate gas releases under more realistic conditions. Even so, the findings of this work are already a significant step forward in the study of underwater gas releases.