Pietro Paolo Ciottoli

Entropy production and timescales

Spatially homogeneous reactive systems are characterised by the simultaneous presence of a wide range of timescales. When the dynamics of such reactive systems develop very slow and very fast timescales separated by a range of active timescales, with large gaps in the fast/active and slow/active timescales, then it is possible to attain a multi-scale adaptive model reduction along with the integration of the governing ordinary differential equations using the G-Scheme framework. The G-Scheme assumes that the dynamics is decomposed into active, slow, fast, and when applicable, invariant subspaces. We derive expressions that reveal the direct link between timescales and entropy production by resorting to the estimates of the contributions of the fast and slow subspaces provided by the G-Scheme. With reference to a constant pressure adiabatic batch reactor, we compute the contribution to entropy production by the four subspaces. These numerical experiments show that, as indicated by the theoretical derivation, the contribution to entropy production of the fast subspace is of the same magnitude as the error threshold chosen for the numerical integration, and that the contribution of the slow subspace is generally much smaller than that of the active subspace. We explicitly exploit this property to identify the slow and fast subspace dimensions differently from the method adopted in the G-Scheme, where the dimensions of the subspaces are defined on the basis of the asymptotic approximations of the contributions of the fast and slow subspaces. Comparison of the outcome of the analyses performed using two types of criteria underlines the substantial equivalence of the two. This property opens the door to a number of possible applications that will be explored in future work. For example, it is possible to utilise the information on entropy production associated with reactions within each subspace to define an entropy participation index that can be utilised for model reduction.

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.

Detached-Eddy Simulation of Shock Unsteadiness in an Overexpanded Planar Nozzle

This work investigates the self-excited shock wave oscillations in a three-dimensional planar over-expanded nozzle turbulent flow by means of Detached Eddy Simulations. Time resolved wall pressure measurements are used as primary diagnostics. The statistical analysis reveals that the shock unsteadiness has common features in terms of the root mean square of the pressure fluctuations with other classical shock wave/boundary layer interactions, like compression ramps and incident shocks on a flat plate. The Fourier transform and the continuous wavelet transform are used to conduct the spectral analysis. The results of the former indicate that the pressure in the shock region is characterized by a broad low-frequency content, without any resonant tone. The wavelet analysis, which is well suited to study non stationary process, reveals that the pressure signal is characterized by an amplitude and a frequency modulation in time.

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.