George Vourliotakis

Assessment of Chemical Markers for Heat-Release Rate Correlations in Laminar Premixed Flames

There is considerable interest in formulating practical correlations for the quantification of heat-release rate in flames via appropriate chemical markers, mainly due to difficulties in obtaining a direct estimation of the former. Experimental and numerical studies performed in methane-air flames have identified corrections between suitable kinetic information and heat-release rate. The present work investigates the validity of corrections developed for methane to the combustion of other fuel classes (e.g., alcohols, unsaturated hydrocarbons, and aromatics), and proposes a methodological approach for the development of generic correlations. Numerical assessment of such correlations and their underlying methodology is performed on the basis of experimental data from 24 laminar premixed flames, utilizing a single, in-house, recently published C1–C6 detailed chemical kinetic model. It is shown that the proposed correlations are valid for methane-based fuels but they are largely inadequate for unsaturated fuels under rich conditions and for oxygenated fuels under most conditions. It is shown that heat-release rate is correlated with the dominant carbon path and that the oxidation chemistry of the parent alkyl radical is of pronounced importance. Alternatives to the proposed heat-release rate correlations are also proposed and assessed.

Development and Parametric Evaluation of a Tabulated Chemistry Tool for the Simulation of n-Heptane Low-Temperature Oxidation and Autoignition Phenomena

Accurate modelling of preignition chemical phenomena requires a detailed description of the respective low-temperature oxidative reactions. Motivated by the need to simulate a diesel oil spray evaporation device operating in the “stabilized” cool flame regime, a “tabulated chemistry” tool is formulated and evaluated. The tool is constructed by performing a large number of kinetic simulations, using the perfectly stirred reactor assumption. n-Heptane is used as a surrogate fuel for diesel oil and a detailed n-heptane mechanism is utilized. Three independent parameters (temperature, fuel concentration, and residence time) are used, spanning both the low-temperature oxidation and the autoignition regimes. Simulation results for heat release rates, fuel consumption and stable or intermediate species production are used to assess the impact of the independent parameters on the system’s thermochemical behaviour. Results provide the physical and chemical insight needed to evaluate the performance of the tool when incorporated in a CFD code. Multidimensional thermochemical behaviour “maps” are created, demonstrating that cool flame activity is favoured under fuel-rich conditions and that cool flame temperature boundaries are extended with increasing fuel concentration or residence time.

Experimental Investigation on the Influence of Simulated EGR Addition on Swirl-Stabilized CH4 Flames

AbstractLow-temperature combustion concepts for transport and power generation employ mixture dilution techniques, such as exhaust gas recirculation (EGR), that offer the potential of fuel flexibility, reduced pollutant emissions, and improved efficiency. These combustion modes, however, display a higher sensitivity to the compositional changes brought about by dilution, which in turn may have an adverse influence on the overall system performance. A fundamental study on the interactions between methane and simulated EGR was carried out on a swirl-stabilized, stoichiometric flame. The effects of varying levels and composition of diluents and preheating temperatures on flame structure and exhaust emissions, were experimentally investigated. Reductions of up to 90% and over 95% in NOx and CO emissions, respectively, where observed for higher levels of added diluents, whereas an increase in preheating temperature resulted in the opposite trends. It has been further demonstrated that, depending on fuel and th...

Detailed Kinetics as a Tool for Investigating HCCI Conditions on Engine Performance and Emissions

AbstractHomogeneous charge compression ignition (HCCI) combustion encompasses the advantages of both diesel and gasoline fuel engines. However, shortcomings mainly associated with the rapid pressure rise rate need to be studied in detail and account for the inherencies of each fuel mixture. The work numerically investigates the effect of key operating parameters on performance and pollutant characteristics on a widely investigated research engine. An accredited, detailed chemical kinetic mechanism for gasoline surrogate components, coupled with a NOx sub-mechanism, is implemented on a commercial 0D single-zone engine model, in order to identify optimum performance guidelines regarding operating parameters using typical primary reference fuels. The effect of operated fuels, intake charge and exhaust gas recirculation (EGR) levels are discussed based on pressure traces, heat release rate profiles, combustion phasing analyses, and in-cylinder speciation and kinetic information. The work also explores the app...

Allene and Propyne Combustion in Premixed Flames: A Detailed Kinetic Modeling Study

ABSTRACT The combustion chemistry of allene and propyne is critical for the breakdown of higher hydrocarbon fuels and for molecular growth processes. Allene and propyne consumption reactions are primary sources of the key propargyl and allyl radicals, which are closely linked to benzene, polyaromatic hydrocarbons (PAHs), and soot formation paths. However, uncertainties exist concerning aspects of the C3H4 isomers’ chemistry. The article is part of an on-going effort aiming towards the optimization of a comprehensive kinetic mechanism for the high temperature combustion of small (C1–C6) hydrocarbon species. The mechanism is here validated against species data from stoichiometric and fuel-rich laminar premixed allene and propyne flames with considerable success. A critical evaluation of C3H4 consumption pathways has been carried out and the dynamics of benzene formation and destruction are discussed.

Towards Identifying Flame Patterns in Multiple, Late Injection Schemes on a Single-Cylinder Optical Diesel Engine

ABSTRACT The work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.