An elementary model for an advancing autoignition front in laminar reactive co-flow jets injected into supercritical water

In this paper we formulate and analyze an elementary model for the propagation of advancing autoignition fronts in reactive co-flow fuel/oxidizer jets injected into an aqueous environment at high pressure. This work is motivated by the experimental studies of autoignition of hydrothermal flames performed at the high pressure laboratory of NASA Glenn Research Center. Guided by experimental observations, we use several simplifying assumptions that allow the derivation of a simple, still experimentally feasible, mathematical model for the propagation of advancing ignition fronts. The model consists of a single diffusion-absorption-advection equation posed in an infinite cylindrical domain with a non-linear condition on the boundary of the cylinder and describes the temperature distribution within the jet. This model manifests an interplay of thermal diffusion, advection and volumetric heat loss within a fuel jet which are balanced by the weak chemical reaction on the jet's boundary. We analyze the model by means of asymptotic and numerical techniques and discuss feasible regimes of propagation of advancing ignition fronts. In particular, we show that in the most interesting parametric regime when the advancing ignition front is on the verge of extinction this model reduces to a one dimensional reaction-diffusion equation with bistable non-linearity. We hope that the present study will be helpful for the interpretation of existing experimental data and guiding of future experiments.Comment: 17 pages, 9 figure

Self-similarity of fluid residence time statistics in a turbulent round jet

Fluid residence time is a key concept in the understanding and design of chemically reacting flows. In order to investigate how turbulent mixing affects the residence time distribution within a flow, this study examines statistics of fluid residence time from a direct numerical simulation (DNS) of a statistically stationary turbulent round jet with a jet Reynolds number of 7290. The residence time distribution in the flow is characterised by solving transport equations for the residence time of the jet fluid and for the jet fluid mass fraction. The product of the jet fluid residence time and the jet fluid mass fraction, referred to as the mass-weighted stream age, gives a quantity that has stationary statistics in the turbulent jet. Based on the observation that the statistics of the mass fraction and velocity are self-similar downstream of an initial development region, the transport equation for the jet fluid residence time is used to derive a model describing a self-similar profile for the mean of the mass-weighted stream age. The self-similar profile predicted is dependent on, but different from, the self-similar profiles for the mass fraction and the axial velocity. The DNS data confirm that the first four moments and the shape of the one-point probability density function of mass-weighted stream age are indeed self-similar, and that the model derived for the mean mass-weighted stream-age profile provides a useful approximation. Using the self-similar form of the moments and probability density functions presented it is therefore possible to estimate the local residence time distribution in a wide range of practical situations in which fluid is introduced by a high-Reynolds-number jet of fluid

Unsteady flamelet progress variable modeling of reacting diesel jets

Accurate modeling of turbulence/chemistry interactions in turbulent reacting diesel jets is critical to the development of predictive computational tools for diesel engines. The models should be able to predict the transient physical and chemical processes in the jets such as ignition and flame lift-off. In the first part of this work, an existing unsteady flamelet progress variable (UFPV) model is employed in Reynolds-averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES) to assess its accuracy. The RANS simulations predict that ignition occurs toward the leading tip of the jet, followed by ignition front propagation toward the stoichiometric surface, and flame propagation upstream along the stoichiometric surface until the flame stabilizes at the lift-off height. The LES, on the other hand, predicts ignition at multiple points in the jet, followed by flame development from the ignition kernels, merger of the different flames and then stabilization. The UFPV model assumes that combustion occurs in thin zones known as flamelets and turbulent strain characterized by the scalar dissipation rate modifies the flame structure. Since the flamelet is thinner than the smallest grid size employed in RANS or LES, the effect of the turbulence is modeled through probability distribution functions of the independent variables. The accuracy of the assumptions of the model is assessed in this work through direct numerical simulations (DNS) which resolves the flame. The DNS is carried out in turbulent mixing layers since the combustion in a diesel jet occurs in the fuel/air mixing layer surrounding the jet. ^ The DNS results show that the flamelet model is applicable but that its implementation in the UFPV model is flawed because the effects of expansion due to heat release and increase in diffusivity due to rise in temperature are not accounted for in the formulation of the scalar dissipation rate. A new diffusivity-corrected flamelet model is proposed which leads to an improved prediction of flame development. Furthermore, it is shown that the most commonly used approach to calculate the scalar dissipation rate in LES of reacting flows leads to large errors when the LES grid size is large. The DNS results are used to determine the best model for the filtered scalar dissipation rate and its PDF under diesel engine conditions. A new model is derived for the variance of the scalar dissipation rate. The DNS results are also used to compare the performance of the UFPV model with the Perfectly Stirred Reactor (PSR) model predictions. It is shown that the UFPV model performance is superior for turbulent intensities and grid sizes encountered in diesel engine application

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

Implementation of a combustion model based on the flamelet concept and its application to turbulent reactive sprays

El modelado CFD se ha convertido en una herramienta aceptada y ampliamente utilizada en el ámbito del diseño de motores de combustión interna alternativos. Los modelos de combustión avanzados ayudan a comprender los fenómenos complejos químicos y físicos del proceso de combustión y aportan información detallada que no se puede obtener con experimentos. Indudablemente, el modelado del proceso de combustión turbulenta parcialmente premezclada característico de los chorros Diesel es particularmente difícil y por lo tanto es un tema de gran interés para la comunidad científica. Los retos más importantes del modelado de este tipo de llamas son la predicción del proceso del auto-encendido, caracterizado por el tiempo de retraso, y la estructura de la llama cuasi-estacionaria con su característica longitud de lift-off. Estos dos parámetros globales de los chorros Diesel son importantes por varios aspectos. Primero, es relativamente sencillo medir estos dos parámetros y por lo tanto utilizarlos para la validación de modelos y segundo, son factores determinantes en el proceso de la combustión en un motor. El auto-encendido marca el inicio de la tasa de liberación de calor y la longitud de lift-off desempeña un papel fundamental en la formación de hollín. El mecanismo de estabilización de la llama en la zona del lift-off todavía no es bien conocido aunque existen diferentes teorías en la literatura, por lo que su modelado es en la actualidad un reto no resuelto. De acuerdo con el contexto descrito previamente, en este trabajo se pretende implementar un modelo de combustión integrado en un solver RANS utilizando la plataforma CFD OpenFOAM de código abierto. El modelo propuesto está basado en el concepto de flamelets usando una química detallada combinado con funciones de probabilidad determinadas a priori (presumed-PDF) para considerar el efecto de interacción entre la química y las características del flujo turbulento, que implica hipótesis importantes. En primer lugar, con el concepto flamelet se asume que una llama Diesel turbulenta quema localmente como un conjunto de llamas laminares de difusión de flujos opuestos. En segundo lugar se asume que las fluctuaciones de las propiedades introducidas por el flujo turbulento, que son las responsables de los fenómenos de interacción entre la química y la turbulencia durante la combustión, siguen un comportamiento estadístico en el tiempo de acuerdo a una distribución de probabilidad conocida a priori. Los fenómenos complejos del auto-encendido de hidrocarburos exigen el uso de mecanismo químicos detallados para recuperar satisfactoriamente los tiempos de retraso del auto-encendido en un rango amplio de condiciones termoquímicas. Una estrategia de interés para mantener los costes computacionales dentro de límites aceptables consiste en pre-tabular los resultados del cálculo de la química en tablas. Los parámetros independientes de estas tablas son la fracción de mezcla, la variable de progreso y la tasa de disipación escalar. Además, la hipótesis de que la distribuciones de probabilidad de las fluctuaciones generadas por la turbulencia sobre las propiedades del flujo son conocidas permite generar una tabla con la información química del problema apta para su aplicación en cálculo CFD en un entorno RANS. Esta aproximación basada en la pre-tabulación de los resultados químicos presenta dos ventajas fundamentales, siendo la primera de ellas la posibilidad de considerar modelos avanzados de interacción química-turbulencia y la segunda el relevante ahorro de tiempo de cálculo. Sin embargo, estas tablas representan un gran espacio de datos cuya gestión eficiente no es trivial. El desarrollo de un almacenamiento adecuado para un acceso de datos rápido y directo así como un esquema de interpolación multidimensional también forma parte del presente trabajo.Winklinger, JF. (2014). Implementation of a combustion model based on the flamelet concept and its application to turbulent reactive sprays [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/48488TESI

Combustion Fundamentals Research

Increased emphasis is placed on fundamental and generic research at Lewis Research Center with less systems development efforts. This is especially true in combustion research, where the study of combustion fundamentals has grown significantly in order to better address the perceived long term technical needs of the aerospace industry. The main thrusts for this combustion fundamentals program area are as follows: analytical models of combustion processes, model verification experiments, fundamental combustion experiments, and advanced numeric techniques

The effects of ozone addition on flame propagation and stabilization

Combustion plays a vital role in transportation and power generation. However, concerns of efficiency, emission, and operations at extreme conditions highlight the needs to enhance combustion process. If the rate-limiting chemical pathways can be modified, the ignition and combustion process could be dramatically accelerated. Following this idea, addition of ozone (O3) is proposed as a potential solution. O3 is one of the strongest oxidizers. It can be efficiently and economically produced in situ at high pressures, and transported to the desired region from an injection location to modify fuel oxidization and control the combustion process. To serve as a basis for future application on practical combustion systems, this dissertation investigates the effects of O3 addition on two fundamental combustion processes: the propagation of laminar premixed flames and the stabilization of non- premixed jet flames in autoignitive environment. Previous studies have shown that O3 addition can enhance flame propagation, stability and ignition, but the dependence on pressure and temperature were not clear. Furthermore, few studies have been conducted on the effects of ozonolysis reactions, which are rapid even at room temperature for unsaturated hydrocarbons. The results presented in this dissertation are an attempt to address these questions. The effects of O3 addition on the propagation of laminar premixed flames are investigated with respect to pressure, initial temperature, O3 concentration and fuel kinetics. For alkane/air premixed laminar flames, high-pressure Bunsen flame experiments in the present work show that the enhancement in laminar flame speed (SL) increases with pressures. This is due to the fact that O3 decomposition, which releases reactive oxygen atoms, becomes a more dominant O3 consumption pathway at higher pressure. Simulations show that adding O3 at higher initial temperature is not as effective as lower initial temperatures. A nearly linear relation between the enhancement and O3 concentration is observed at room temperature and atmospheric pressure. If the fuel is changed from alkanes to C2H4, an unsaturated hydrocarbon species, ozonolysis reactions take place in the premixing process. When the heat released from ozonolysis reactions is lost, decrease in SL is observed. In contrast, if ozonolysis reaction are frozen, either by cooling the reactants or decreasing the pressure, enhancement of SL by O3 addition is observed. The study on flame stabilization with O3 addition is conducted with a non-premixed jet burner in a quartz tube using C2H4 as the fuel. At low-dilution conditions, autoignition events are initiated by ozonolysis reactions. The autoignition timescale is further investigated quantitatively. Overall, this timescale decreases as the inlet velocity increases. At such autoignitive conditions created by ozonolysis reactions, the stabilization of a lifted non-premixed flame is fundamentally different from non-autoignitive conditions. Propagation is enhanced due to the “preprocessing” of fuel by ozonolysis reactions, after which the mass burning velocity of the reactants is increased as shown by simulation. This can increase the propagation speed by several times. In summary, for the premixed laminar flame propagation, the present results explain the pressure, initial temperature, and fuel dependence of enhancement of flame propagation by O3 addition. A more comprehensive understanding is thus contributed. Furthermore, this dissertation explores ozonolysis reactions as an alternative to create a platform to conduct fundamental research on flame in autoignitive environment.Ph.D

Ignition Delay of Non-Premixed Methane-Air Mixtures using Conditional Moment Closure (CMC)

Autoignition of non-premixed methane-air mixtures is investigated using first-order Conditional Moment closure (CMC). In CMC, scalar quantities are conditionally averaged with respect to a conserved scalar, usually the mixture fraction. The conditional fluctuations are often of small order, allowing the chemical source term to be modeled as a function of the conditional species concentrations and the conditional enthalpy (temperature). The first-order CMC derivation leaves many terms unclosed such as the conditional scalar dissipation rate, velocity and turbulent fluxes, and the probability density function. Submodels for these quantities are discussed and validated against Direct Numerical Simulations (DNS). The CMC and the turbulent velocity and mixing fields calculations are decoupled based on the frozen mixing assumption, and the CMC equations are cross-stream averaged across the flow following the shear flow approximation. Finite differences are used to discretize the equations, and a two-step fractional method is implemented to treat separately the stiff chemical source term. The stiff ODE solver LSODE is used to solve the resulting system of equations. The recently developed detailed chemical kinetics mechanism UBC-Mech 1.0 is employed throughout this study, and preexisting mechanisms are visited. Several ignition criteria are also investigated. Homogeneous and inhomogeneous CMC calculations are performed in order to investigate the role of physical transport in autoignition. Furthermore, the results of the perfectly homogeneous reactor calculations are presented and the critical value of the scalar dissipation rate for ignition is determined. The results are compared to the shock tube experimental data of Sullivan et al. The current results show good agreement with the experiments in terms of both ignition delay and ignition kernel location, and the trends obtained in the experiments are successfully reproduced. The results were shown to be sensitive to the scalar dissipation model, the chemical kinetics, and the ignition criterion

Direct numerical simulation of lifted flames in diesel engine conditions

Increasing energy demand and stringent pollution norms necessitate the design of engines with higher efficiency and lower emission levels. A fundamental understanding of combustion in diesel engines can enable the design of such engines. Of special interest is the lift-off height, the distance between the injector orifice and the start of the high-temperature reaction zone. The lift-off height controls the amount of mixing between fuel and oxidiser before combustion and hence combustion efficiency and pollutant levels. A comprehensive understanding of the flame stabilisation mechanism, which determines the lift-off height, is thus critical. Arriving at this understanding is not trivial due to the high-temperature and high-pressure conditions present in diesel engines which limit the capabilities of experiments. Consequently, important aspects such as the flame structure and its stabilisation mechanism are not well understood. With the aim to elucidate the flame structure and stabilisation mechanism in diesel engine conditions, direct numerical simulations are performed in this thesis. The ambient conditions are matched to the Engine Combustion Network’s Spray A flame. A canonical configuration of two-dimensional laminar lifted flame is considered first. The response of the flames to inlet velocity and scalar dissipation rate is studied. The flames transition from attached, to lifted propagation stabilised to lifted ignition stabilised upon increasing the inlet velocity. A complex multibrachial flame structure with up to five branches is observed. The propagation stabilised and ignition stabilised flames exhibit characteristically different structure, transport budget and displacement speeds. These observations are then employed to identify the stabilisation mechanism of a three-dimensional spatially developing turbulent round jet flame. The turbulent flame structure, transport budget and the displacement speed at the flame base closely resemble the characteristics of the two-dimensional propagating laminar flames indicating that the turbulent flame is stabilised by flame propagation. The DNS data are then utilised to a priori assess the chemistry tabulation combustion models, important design tools used in industry. Results show that these models give good performance in a priori comparison with the DNS data when the dimension and the choice of the control variables are appropriately considered

Supersonic Combustion and Mixing Characteristics of Hydrocarbon Fuels in Screamjet Engines

The combustion characteristics of gaseous propane in supersonic airflow using the rearward-facing step that is swept inward from both end sides is studied. The effect of sweeping the step on the flow field features of propane combustion is investigated. The study of the supersonic combustion of ethylene is carried out using different combustor configurations, different main fuel equivalence ratios, and different pilot fuel equivalence ratios. The swept step shows the ability to hold the propane flame in the supersonic air stream without extinction. It was found that the side sweeping of the combustor exhibits the high temperature and combustion products concentration in the far field domain while the area downstream of the normal injection location characterizes lower temperature and products concentration. It is recommended to optimize the combustor length to ensure the complete combustion and consequently the full liberation of the chemical energy stored in the fuel before the fuel exits the combustor. The main findings from the ethylene study can be summarized in the following points. The step configuration with no pilot injection can afford the flame holding mechanism in the supersonic air stream by creating the flow recirculations in the step base area and featuring permanent high temperature regions surrounding the normal fuel injection. The step configuration showed good mixing capabilities in the far field domain. The wedge configuration proved superiority over the generic rearward-facing step configuration in holding the ethylene flame in the supersonic airstreams, producing overall higher temperature medium throughout the combustor, and exhibiting lower flow losses and higher combustor efficiency. The increase in the equivalence ratio of the ethylene normal fuel injection enhances the general flow field features and energy field characteristics in the combustor except in the step base area where the lower equivalence ratio features better temperature distribution and higher combustion efficiency. Although the wedge with no pilot injection configuration presents the highest level of temperature distribution in the cavity and downstream regions, the 0.02-pilot equivalence ratio increases the temperature of the upstream face of the normal injection and enhances the flame holding mechanism. The 0.02-pilot equivalence ratio presented the optimum pilot injection case that can promote the flame holding mechanism and keep good combustion and flow field qualities. While further increase of the pilot injection equivalence ratio quenches the high temperature gases in the cavity region, which leads to the deficiency in the flame holding mechanism, the excessive pilot fuel injection shows its positive effect by increasing the average flow field static temperature and absolute pressure in the far field domain