Gennaro Russo

Using Large Eddy Simulation for understanding vented gas explosions in the presence of obstacles.

In this work, a validated Large Eddy Simulation model of unsteady premixed flame propagation is used to study the phenomenology underlying vented gas explosions in the presence of obstacles. Computations are run of deflagrating flames in a small-scale combustion chamber closed at the bottom end and open at the opposite face. A single obstacle is centred inside the chamber. Methane-air mixtures of various compositions (ranging from lean to stoichiometric and rich), and obstacles with different area blockage ratios (30, 50 and 70%) and shapes (circular, rectangular and square cross-section in the flow direction) are investigated. All cases are initialized from stagnation. The competition between combustion rate and venting rate allows explaining both number and intensity of the overpressure peaks observed.

Numerical simulation of turbulent gas flames in tubes.

Computational fluid dynamics (CFD) is an emerging technique to predict possible consequences of gas explosion and it is often considered a powerful and accurate tool to obtain detailed results. However, systematic analyses of the reliability of this approach to real-scale industrial configurations are still needed. Furthermore, few experimental data are available for comparison and validation. In this work, a set of well documented experimental data related to the flame acceleration obtained within obstacle-filled tubes filled with flammable gas-air mixtures, has been simulated. In these experiments, terminal steady flame speeds corresponding to different propagation regimes were observed, thus, allowing a clear and prompt characterisation of the numerical results with respect to numerical parameters, as grid definition, geometrical parameters, as blockage ratio and to mixture parameters, as mixture reactivity. The CFD code AutoReagas was used for the simulations. Numerical predictions were compared with available experimental data and some insights into the code accuracy were determined. Computational results are satisfactory for the relatively slower turbulent deflagration regimes and became fair when choking regime is observed, whereas transition to quasi-detonation or Chapman-Jogouet (CJ) were never predicted.

Oxidative dehydrogenation of propane over vanadium and niobium oxides supported catalysts

Publisher Summary This chapter discusses the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase TiO2 in ethane oxidative dehydrogenation (ODH). Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components, inducing redox and acid properties, respectively, together with the effect of the preparation conditions on the catalytic performances have been studied. The vanadia–itania catalysts are very active, but with low selectivity, because of their high reducibility. Catalytic performances of VOx/TiO2 systems in ethane ODH are improved by the addition of niobium. When TiO2 is coimpregnated by vanadium and niobium oxides, the presence of niobium enhances the selectivity to ethylene at low vanadium content, whereas it slightly depresses the activity without enhancing the selectivity at high vanadium content. This should be because of the effect of niobium on vanadium reducibility, especially affected at low vanadium content. By changing the order of addition of vanadia and niobia to the support, catalysts with slightly different redox and acid properties are obtained. At low vanadium loading, supporting the two oxides at the same time results in the best catalytic performances, while at high loading a two steps impregnation gives the best results.

Numerical model of ignition processes of polymeric materials including gas-phase absorption of radiation

Abstract A one-dimensional unsteady mathematical model of solid fuel ignition is presented. The solid fuel is heated by an external radiative heat source. Some radiation is absorbed in depth by the solid fuel and some by the decomposition products in the gas phase. Solid fuel degradation occurs according to a zero-order Arrhenius pyrolysis reaction and gas-phase combustion according to a second-order Arrhenius reaction. Gas-phase heat and mass transfer and solid phase heat transfer are described by differential balance equations that are coupled through the boundary conditions at the interface. The solution is computed numerically by an implicit finite difference method. PMMA radiative ignition is simulated by varying the intensity of the radiative heat flux and predictions show quite good agreement with experiments. The ignition process oceurs in the gas phase in a premixed fashion, rapidly followed by the transition to a diffusion flame. As the radiative heat flux is increased, higher surface temperatures and pyrolysis mass fluxes are reached, ignition occurs closer and closer to the fuel surface, and ignition delay times decrease. Gas-phase absorption of radiation plays a fundamental role in the predicted ignition phenomenon and ignition delay times. In particular, with realistic data and no absorption of radiation in the gas phase, ignition does not occur at all. Finally, a parametric study is performed in order to analyze the dependence of the predicted ignition phenomenon on key parameters used to model degradation and combustion processes, such as preexponential factors and activation energies of the reactions.

Nitric oxide decomposition over Cu-exchanged ZSM-5 with high Si/Al ratio

Abstract The catalytic properties of Cu-exchanged H-ZSM-5 ( Si Al = ca. 80 ) in NO decomposition were investigated. It was found that, both in the presence and in the absence of oxygen in the feed, the rate of NO decomposition increased with the copper content up to about 640% Cu over-exchange, while the turnover frequency reached a maximum value at about 500% over-exchange. The reaction order of NO decomposition in the absence of O2 was 1.6 in the range of NO concentration from 1000 to 5000 ppm. Addition of 1 % O2 in the feed resulted in the decrease of NO conversion to N2 depending on the contact time. In all the range of Cu% exchange the catalysts were active for the catalytic oxidation of NO to NO2, reaching the equilibrium conversion both in the absence (above 350°C) and in the presence (above 300°C) of O2 in the feed.

Numerical simulation of gas explosions in linked vessels

The ability of the CFD code AutoReaGas to simulate a gas explosion in two linked vessels was investigated. These explosions present an anomalous destructive power because both peak pressures and rates of pressure rise are much higher than those generated in single vessel explosions. A fair agreement was observed between the computed results and experimental data taken from literature. Moreover, the computed values of the turbulence intensity at varying diameters of the connecting pipe demonstrate that turbulence induced in both vessels represent a major factor affecting the explosion violence.

Sub-grid scale combustion models for large eddy simulation of unsteady premixed flame propagation around obstacles

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., Phys. Fluids 12 (2000) 1843-1863; Flohr and Pitsch, Proc. CTR Summer Program, 2000, pp. 61-82; Kim and Menon, Combust. Sci. Technol. 160 (2000) 119-150; Charlette et al., Combust. Flame 131 (2002) 159-180; Pitsch and Duchamp de Lageneste, Proc. Combust. Inst. 29 (2002) 2001-2008) 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.

Modeling of Transport Phenomena and Kinetics of Biomass Pyrolysis

A mathematical model of biomass pyrolysis accounting for transport phenomena and chemical processes, described according to a two-stage, semi-global reaction scheme, is presented. The kinetic scheme is based on lumping the different pyrolysis products into three groups: gas, tar and char. In addition to primary biomass degration reactions, secondary reactions of tar cracking to light hydrocarbons and tar polymerization to char are modeled. Transport phenomena include heat convection, conduction and radiation, mass convection and pressure and velocity variations described according to the Darcy law. Properties (porosity, thermal conductivity, permeability, gas and solid volume) vary as chemical reactions take place. The effects of application (parallel or perpendicular to solid fuel grain direction) of the radiative heat flux, used to cause thermal degradation, are investigated. The propagation of a primary reaction front through the virgin biomass and of a secondary reaction front through the char layer, due to volatile products escaping towards the irradiated surface, is predicted. Furthermore, while for parallel grain heating no relevant pressure variations are observed, for perpendicular grain heating, a gas overpressure front precedes, in the virgin solid region, the primary pyrolysis front. A fairly good agreement is obtained between theory and experiments for the total gas production as a function of time. Also, the simulated pressure and temperature distributions are in agrement with experiments as long as structural changes do not affect significantly transport phenomena and chemical reactions. Finally a parametric study of the effects of thermal conductivity and permeability of wood and char on the degradation process is presented.

Oscillatory behaviour in nitrous oxide decomposition on over-exchanged Cu-ZSM-5 zeolite

Abstract N 2 O decomposition on over-exchanged Cu-ZSM-5 zeolite was investigated in the range of temperatures from 323 to 400°C. When only N 2 O in helium was fed, regular periodic oscillations of the outlet concentration of N 2 O were observed. Catalyst pretreatment affected the pattern of the oscillations, but not their periods or amplitudes. Also the average conversion of N 2 O to N 2 and O 2 was dependent on the initial oxidation state of the catalytic active sites, being lower when the catalyst was pretreated in oxygen. The addition of NO, O 2 or both in the feed resulted in disappearance of the oscillations and decrease of N 2 O conversion. The experimental observations are in agreement with a scheme of reactions in which N 2 O acts either as oxidizing or as reducing agent for the catalytic sites.

Simulation of VCEs by CFD modelling : an analysis of sensitivity

Abstract The AutoReaGas code developed by TNO and Century Dynamics Ltd has been utilized to simulate the occurrence of a Vapour Cloud Explosion in a large fuel storage area. The sensitivity of the results to the main input variables has been investigated, together with the effect of fuel amount and fuel concentration inside the cloud. The marked sensitivity to several adjustable parameters, such as the turbulent modelling constant Ct, shows that the use of Computational Fluid Dynamics codes is not a straightforward task but still requires an adequate skill. From the whole set of computed results, some suggestions are derived, in order to improve the predictive power of the code.