Valeria Di Sarli

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.

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.

Stability and Emissions of a Lean Pre-Mixed Combustor with Rich Catalytic/Lean-burn Pilot

In this work, a reactor network model was developed to study homogeneous gas-phase methane combustion taking place under typical operating conditions of lean pre-mixed combustors piloted by rich catalytic/lean-burn (RCL) systems. In particular, the thermokinetic interaction between the pilot stream (i.e. the stream exiting the RCL stage) and the main feeding stream to the homogeneous reactor was investigated in terms of combustion stability and emissions. The homogeneous combustor was modeled as a perfectly stirred reactor (PSR). The pilot stream was mixed with the main feeding stream prior to entering the PSR. Numerical results have shown that the opportunity to stabilize combustion is strongly linked to the presence of hydrogen in the pilot stream. Combustion stability is highly sensitive to variations in fuel split between catalytic pilot and homogeneous reactor. The increase in pilot fuel split (and, thus, in the inlet hydrogen concentration to the PSR) enlarges the operating window of stable combustion (in terms of higher heat losses, lower preheat temperatures and lower residence times), while still achieving NOx and CO emissions lower than 9 ppm (at 15% O2). These results highlight the potential of the RCL technology as a valuable alternative to conventional diffusion flame-based pilots.

CFD simulations of the effect of dust diameter on the dispersion in the 20 l bomb

Prevention and mitigation measures for dust explosion are based on the knowledge of the thermokinetic parameters which characterise flammability such as Minimum Explosible Concentration, MEC, and explosion behaviour such as maximum explosion pressure, PMAX, and deflagration index, KSt. Measurements of these parameters are performed in spherical vessels (20 L sphere or 1 m 3 sphere). The main issues in performing such measurements are related to the dust dispersion and to the turbulence level reached inside the sphere. Also, the dispersion and then concentration of the dust/air mixture in the vessel significantly affects the flame propagation, if stratification and sedimentation occurs. In this work we use a previously validated CFD 3D model to simulate the dust dispersion inside the sphere at different dust diameters. Results show that on increasing the dust diameter the dust is mainly concentrated at the vessel walls and the dust paths are different from those of the fluid flow.

Explosibility and flammability characteristics of nicotinic acid-lycopodium/air mixtures

Roberto Sanchirico, Paola Russo*, Valeria Di Sarli, Almerinda Di Benedetto Istituto di Ricerche sulla Combustione, CNR, Via Diocleziano 328, 80124, Napoli, Italy Dipartimento di Ingegneria Chimica Materiali Ambiente, Sapienza Università di Roma, via Eudossiana 18, 00184, Roma, Italy Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125, Napoli, Italy paola.russo@uniroma1.it

CFD Modeling and Simulation of a Catalytic Micro-Monolith

In this work, a first step in modeling and simulating the thermal behavior of an entire catalytic micro-monolith was performed. In particular, a Computational Fluid Dynamics (CFD) model was developed for simulating three-channel and five-channel micro-combustors. For both configurations, the operating maps were built as functions of the inlet gas velocity and compared to the operating map of a single-channel configuration. Results show that, due to the relevance of heat losses in micro-devices, it is not possible to extrapolate the behavior of the multi-channel configurations from that of the single channel. Therefore, simulation of the entire catalytic micro-monolith is needed. However, this is computationally demanding: it has been found that the CPU time almost linearly increases with the number of channels simulated. Finally, for a fixed total mass flow rate, it has been demonstrated the opportunity to maximize the overall fuel conversion by means of a non-uniform distribution of mass flow rates among the channels.

Explosion behavior of ammonia and ammonia/methane in oxygen-enriched air

The effect of enriching air with oxygen on the explosion behavior of ammonia and ammonia/methane has been experimentally investigated in a closed 5‐l cylindrical vessel. Results have shown that for both ammonia and ammonia/methane, when moving from air to pure oxygen, a transition takes place from a normal deflagration behavior to one that near or at the end is accompanied by a combustion‐induced Rapid Phase Transition (cRPT) pressure pulse, an explosion mode characterized by strong oscillations in pressure time histories culminating in over‐adiabatic peaks (i.e., values of the peak pressure even higher than 400 bar and, thus, much higher than the thermodynamic value). On the basis of these findings under the experimental conditions applied in our tests, the occurrence of cRPT should be carefully considered in the development of industrial processes based on ammonia reactions in oxygen‐enriched atmosphere. For that, the phenomenon should be investigated in equipment of larger volume.