Thermodynamic Small Scales in Transcritical Turbulent Jets

This article is a technical note, so it does not have an abstrac

Mixing and non-premixed combustion at supercritical pressures

This thesis is devoted to the numerical investigation of mixing and non- premixed combustion of cryogenic propellants at supercritical pressures. These severe conditions are commonly encountered in high pressure combustion chambers, such as those of liquid-fueled rocket engines (LRE), and lead to significant deviations from the ideal gas thermodynamic behavior of the reacting mixtures. The non-premixed laminar flame structure of liquid oxygen (LOx) and methane or liquid natural gas (LNG) mixtures, a recently proposed LRE propellants com- bination, is investigated by means of a general fluid unsteady flamelet solver. Real gas effects are analyzed on prototypical unsteady flame phenomena such as autoignition and re-ignition/quenching caused by strain perturbations. Such effects influence different flame regions depending on pressure, as well as the critical strain values that a laminar flame can sustain before quenching occurs. Moreover the flame structure is also influenced by the composition of the LNG, in particular the early stage soot precursors production and oxidation. In order to shed light on real gas mixing, a low-Mach approximation for real gas reacting mixtures is presented. A single species non-reacting real gas model is implemented in a highly scalable spectral element computational fluid dynamic (CFD) code with state of the art thermodynamic and transport properties. Transcritical and supercritical planar temporal jets, are chosen as representative test cases for investigating high-pressure mixing by means of direct numerical simulations. The pseudo-boiling phenomenon, occurring in transcritical flows, significantly influences the jet development, mitigating the development of shear layer instabilities and leading to a liquid-like jet break-up. Moreover pseudo-boiling is confined in a narrow spatial region suggesting particular care in the turbulent combustion modeling of non-premixed flames when transcritical thermodynamic conditions are encountered. The results of the present thesis, its physical insights as well as the modeling considerations involved, can be of support in the development of future CFD tools capable of simulating real engine operative conditions and configurations

Numerical investigation of unsteady laminar methane/LOx flamelet at supercritical pressures

High-pressure combustion devices, such as liquid rocket engines, are usually characterized by transcritical and supercritical operating conditions. Propellants injected in the combustion chamber experience extremely high den- sity gradients and real fluid effects. In the present study, real fluid effects on flame structure are investigated in the framework of unsteady laminar flamelet equations, a well established representation and diagnostic tool for non pre- mixed combustion transient phenomena. Real fluid thermodynamic properties are taken into account by means of a computationally efficient cubic equation of state written in a general and comprehensive three-parameter fashion. High-pressure conditions for unsteady flame structure calculations and analysis are chosen as a representative range of a methane/liquid-oxygen rocket engine operating conditions. Particular focus is posed on the constant pressure specific heat behavior at low temperature, which influences the time evolution of the flame structure. Moreover time accurate integration of flamelet equations represent the very first building block of a conditional moment closure for supercritical turbulent combustion

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

Mixing under transcritical conditions: an a-priori study using direct numerical simulation

In this study we investigate mixing under transcritical conditions by means of direct numerical simulation. Atemporal jet configuration is used together with thermodynamic conditions matching those of experiments oncryogenic injection of nitrogen at supercritical pressures. Two cases, representing supercritical and transcriticalconditions, are simulated using a spectral element based, low-Mach numberflow solver, coupled with detailedreal-fluid equation of state and transport properties. The objective is to shed light on the transcritical mixingprocess and to concurrently investigate, in a-priori fashion, the sub-grid scale modeling of thermodynamic andtransport properties in the context of large eddy simulation. Relevant qualitative and statistical differencesbetween supercritical and transcritical jets are observed, despite the same initial Reynolds number. The use of apresumedβ-pdf for sub-grid scale mixing is investigated and is shown to significantly improve, with respect tothe largely used“no-model”, the evaluation of thermodynamic and transport properties

Strain rates, flow patterns and flame surface densities in hydrodynamically unstable, weakly turbulent premixed flames

Recent numerical and experimental studies have unveiled a potentially marked difference between the laminar as well as turbulent propagation of premixed flames exhibiting Darrieus-Landau (DL) (or hydrodynamic) instabilities from flames for which instabilities are inhibited. In this study we utilize two-dimensional numerical simulations of slot burner flames as well as experimental Propane-Air Bunsen flames to analyse differences in turbulent propagation, strain rate and induced flow patterns of hydrodynamically stable and unstable flames. We also investigate the effects of hydrodynamic instability on quantities which are directly related to reaction rate closure models, such as flame surface density and stretch factor. A clear enhancement of turbulent flame speed can be observed for unstable flames, generally mitigated at higher turbulence intensity, which is attributed to a flame area increase induced by the characteristic cusp-like DL-induced corrugation, absent in stable flames, which occurs concurrently and in synergy with turbulent wrinkling. Unstable flames also exhibit, both numerically and experimentally, a different correlation between strain rate and flame curvature and are observed to give rise to a channeling of the induced flow in the fresh mixture. Conditionally averaged flame surface density is also observed to attain smaller values in unstable flames, as a result of the thicker turbulent flame brush, indicating that closure models should incorporate instability-related parameters in addition to turbulence-related parameters

Thermal characterization in LRE: a parametric analysis on injector arrangement

The injector layout influence on a gaseous-methane/gaseous-oxygen (GCH4/GOX) flame emanating from a single shear coaxial Liquid Rocket Engine (LRE) injector is presented. More specifically, in this contribution we investigate the flame-flame interaction, by collecting a database of two-dimensional axis-symmetric simulations, modeled with a symmetry boundary condition to represent the presence of a neighbouring identical injector. The geometrical feature varied as parameter is the lateral confinement, representative of the mutual distance between adjacent injectors. The differences between such a flame and a flame bounded by an isothermal wall, on equal geometry and injection conditions, is also presented. The numerical framework chosen for the high-pressure, turbulent, non-premixed flame description is based on a unsteady Reynolds-Averaged Navier Stokes (uRANS) approach, coupled to a flamelet model accounting for non-equilibrium and non-adiabatic effects

Large scale effects in weakly turbulent premixed flames

In this study we numerically investigate large scale premixed flames in weakly turbulent flow fields. A large scale flame is classified as such based on a reference hydrodynamic lengthscale being larger than a neutral (cutoff) lengthscale for which the hydrodynamic or Darrieus-Landau (DL) instability is balanced by stabilizing diffusive effects. As a result, DL instability can develop for large scale flames and is inhibited otherwise. Direct numerical simulations of both large scale and small scale three-dimensional, weakly turbulent flames are performed at constant Karlovitz and turbulent Reynolds number, using two paradigmatic configurations, namely a statistically planar flame and a slot Bunsen flame. As expected from linear stability analysis, DL instability induces its characteristic cusp-like corrugation only on large scale flames. We therefore observe significant morphological and topological differences as well as DL-enhanced turbulent flame speeds in large scale flames. Furthermore, we investigate issues related to reaction rate modeling in the context of flame surface density closure. Thicker flame brushes are observed for large scale flames resulting in smaller flame surface densities and overall larger wrinkling factors

Interplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation

In this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus- Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker, the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness of the flame curvature probability density function is identified as an unambiguous morphological marker for the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability

Flame induced flow features in the presence of darrieus-landau instability

The onset of hydrodynamic or Darrieus-Landau (DL) instability can largely impact on premixed flame morphology, turbulent flame speed and induced flow field. In this work, we focus on the latter induced flow by means of two dimensional direct numerical simulations (DNS) of slot burner flames performed in a parametric fashion. Results from linear stability analysis are used to select the adequate parameter range to be investigated. The presence of DL instability is initially assessed using a recently proposed statistical marker related to flame morphology. The differences between stable and unstable flames are then statistically investigated, utilizing a single, laminar, DL-induced corrugation as a reference state. Such DL-induced effects are investigated at various turbulence intensities, in terms of local propagation, induced strain rate patterns and flow field as well as vorticity production and transformation. Using displacement speed as a measure of local propagation, no noticeable statistical difference is observed between stable and unstable flames while strain rate and vorticity patterns are shown to be largely influenced by the DL induced morphology. From the modeling point view, an enhancement of counter gradient type transport for turbulent scalar fluxes is observed for hydrodynamically unstable flames