Pasquale Eduardo Lapenna

Unsteady Non-Premixed Methane/Oxygen Flame Structures at Supercritical Pressures

ABSTRACT Unsteady non-premixed flame structures of methane/oxygen mixtures are investigated at supercritical pressures, representative of liquid rocket engines combustion chamber operating conditions. A general-fluid formulation of the flamelet equations is used and deviations from ideality of the thermodynamic properties are taken into account by means of a computationally efficient cubic equation of state written in a general three-parameter fashion. The effects of pressure and scalar dissipation rate are investigated in the context of prototypical unsteady laminar flame configurations, such as autoignition and re-ignition/quenching. In auto-igniting flamelets, real gas effects are observed to influence different flame regions depending on the thermodynamic pressure. Moreover, the mixture ensuing from methane oxidation is never observed to reach a saturated (two-phase) thermodynamic region. Re-ignition and quenching phenomena are analyzed using a time dependent forcing function for the scalar dissipation rate, in order to investigate the response of the real gas flame structures to typical turbulent perturbations. The role of pressure on the critical strain values that a real gas laminar flame can sustain without quenching is investigated and compared to its ideal gas counterpart.

Low-Mach number simulations of transcritical flows

This work is carried out with the support of the Italian Ministry of University and Research (MIUR) and of CCRC/KAUST 1975-03 CCF Subaward Agreement. The authors acknowledge the Italian Super-Computing Interuniversity Consortium CINECA for support and high-performance computing resources under Grant No.DL-3D-SC/ HP10C4YS8W.

The effect of fuel composition on the non-premixed flame structure of LNG/LOx mixtures at supercritical pressure

Methane and Liquefied Natural Gas (LNG) have been recently considered as propellants, together with cryogenic oxygen (LOx), for Liquid Rocket Engines (LRE) applications, having shown some advantages over the commonly used propellants. In this framework the aim of the present study is to understand how the composition of LNG, with respect to the pure methane, can influence non-premixed combustion at typical LRE supercritical operating conditions. In the present work, the flame structures of LNG/LOx mixtures at supercritical pressures are investigated by means of a general fluid formulation for the unsteady laminar flamelet equations. Real fluid effects, which are commonly encountered in LRE thrust chambers because of high pressure pressure conditions, are taken into account by means of a comprehensive three-parameter cubic equation of state. Flame structures are analyzed at elevated pressures ranging from near-critical up to largely supercritical, for pure methane and various representative mixtures models of LNG. The LNG/LOx steady state flame structure characteristics are found to show close similarities to those of the pure methane. On the other hand pollutants species, such as early stage soot precursors (i.e. acetylene), are found to be strongly dependent from LNG composition. Sooting tendencies of LNG is further investigated with a more detailed chemical mechanism in order to shed light on Polycyclic Aromatic Hydrocarbons (PAH) presence at supercritical pressures. A single representative mixture for LNG composition is analyzed showing significantly larger production of soot precursors, such as benzene and naphthalene.

Characterization of pseudo-boiling in a transcritical nitrogen jet

This study is devoted to the investigation, by means of direct numerical simulation, of the interaction between turbulent motions and the pseudo-boiling process. To this end, fully resolved data of a transcritical nitrogen jet are used, obtained via high order methods and using detailed thermodynamic and transport properties. A laminar pseudo-boiling process is simulated in a quiescent setting and used as a consistent reference to shed light on the mutual effects of the jet evolution and thermodynamic non-linearities. In the turbulent scenario, pseudo-boiling is shown to be faster, in an average sense, to the laminar reference case. A consistent definition of the pseudo-boiling rate, based on the concept of the displacement speed, commonly used in premixed flame propagation, is introduced and, for a better physical interpretation, split into a normal diffusion component and a curvature component. The pseudo-boiling rate is statistically analyzed to evaluate the rate of mass transfer from the liquid-like state to the gas-like state during the jet evolution. Normal diffusion is found to be the dominant component of the pseudo-boiling rate, while the curvature component is shown to have a role only when warm fluid pockets are deeply entrained in the jet cold core.This study is devoted to the investigation, by means of direct numerical simulation, of the interaction between turbulent motions and the pseudo-boiling process. To this end, fully resolved data of a transcritical nitrogen jet are used, obtained via high order methods and using detailed thermodynamic and transport properties. A laminar pseudo-boiling process is simulated in a quiescent setting and used as a consistent reference to shed light on the mutual effects of the jet evolution and thermodynamic non-linearities. In the turbulent scenario, pseudo-boiling is shown to be faster, in an average sense, to the laminar reference case. A consistent definition of the pseudo-boiling rate, based on the concept of the displacement speed, commonly used in premixed flame propagation, is introduced and, for a better physical interpretation, split into a normal diffusion component and a curvature component. The pseudo-boiling rate is statistically analyzed to evaluate the rate of mass transfer from the liquid-like s...

Darrieus-Landau induced regime of propagation of turbulent premixed flames in Bunsen configurations

Hydrodynamic or Darriues-Landau (DL) instabilities in weakly turbulent Bunsen flames are numerically investigated under low-Mach number assumptions and a simplified deficient reactant thermochemistry. The DL instabilities are responsible for the formation of sharp folds and cusps in the flame front. Varying the ratio between the flame thickness and the Bunsen diameter the cut-off wavelength is modified and the instabilities induced. It is shown that stability criteria developed in the framework of asymptotic theory for planar flames can adequately predict the behavior of turbulent premixed flames in Bunsen configurations. A statistical characterization of flame morphology in the presence/absence of hydrodynamic instability is given in terms of flame curvature. Similar flame conformations were obtained in a recent experimental work. The statistical analysis highlight that the skewness of the flame curvature probability density function is a consistent marker of the instability presence and two different turbulent modes of flame propagation are identified.