Charles J. Mueller

On the adequacy of certain experimental observables as measurements of flame burning rate

Abstract This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO∗2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C∗2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C∗2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO∗2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C(1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.

Development of an Experimental Database and Kinetic Models for Surrogate Diesel Fuels

Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel. The essential components of such a program are discussed and include: (a) surrogate component selection; (b) the acquisition or estimation of requisite elementary chemical kinetic, thermochemical, and physical property data; (c) the development of accurate predictive chemical kinetic models, together with the measurement of the necessary fundamental laboratory data to validate these mechanisms; and (d) mechanism reduction tools to render the coupled chemistry/flow calculations feasible. In parallel to these efforts, the need exists to develop similarly robust models for fuel injection and spray processes involving multicomponent mixtures of wide distillation character, as well as methodologies to include all of these high fidelity submodels in computationally efficient CFD tools. Near- and longerterm research plans are proposed based on an application target of premixed diesel combustion. In the near term, the recommended surrogate components include n-decane, iso-octane, methylcyclohexane, and toluene. For the longer term, n-hexadecane, heptamethylnonane, n-decylbenzene, and 1-methylnaphthalene are proposed.

An Experimental Investigation of the Origin of Increased NOx Emissions When Fueling a Heavy-Duty Compression-Ignition Engine with Soy Biodiesel

It is generally accepted that emissions of nitrogen oxides (NOx) increase as the volume fraction of biodiesel increases in blends with conventional diesel fuel. While many mechanisms based on biodiesel effects on in- cylinder processes have been proposed to explain this observation, a clear understanding of the relative importance of each has remained elusive. To gain further insight into the cause(s) of the biodiesel NOx increase, experiments were conducted in a single- cylinder version of a heavy-duty diesel engine with extensive optical access to the combustion chamber. The engine was operated using two biodiesel fuels and two hydrocarbon reference fuels, over a wide range of loads, and using undiluted air as well as air diluted with simulated exhaust gas recirculation. Measurements were made of cylinder pressure, spatially integrated natural luminosity (a measure of radiative heat transfer), engine-out emissions of NOx and smoke, flame lift-off length, actual start of injection, ignition delay, and efficiency. Adiabatic flame temperatures for the test fuels and a surrogate #2 diesel fuel also were computed at representative diesel-engine conditions. Results suggest that the biodiesel NOx increase is not quantitatively determined by a change in a single fuel property, but rather is the result of a number of coupled mechanisms whose effects may tend to reinforce or cancel one another under different conditions, depending on specific combustion and fuel characteristics. Nevertheless, charge-gas mixtures that are closer to stoichiometric at ignition and in the standing premixed autoignition zone near the flame lift- off length appear to be key factors in helping to explain the biodiesel NOx increase under all conditions. These differences are expected to lead to higher local and average in-cylinder temperatures, lower radiative heat losses, and a shorter, more-advanced combustion event, all of which would be expected to increase thermal NOx emissions. Differences in prompt NO formation and species concentrations resulting from fuel and jet-structure changes also may play important roles.

Investigation of the impact of biodiesel fuelling on NOx emissions using an optical direct injection diesel engine

Abstract The impact of biodiesel fuelling on NO x emissions was investigated using an optically accessible diesel engine. A soy-based biodiesel (B100) and three separate primary reference fuel (PRF) blends were evaluated over a range of loads at an engine speed of 800 r/min. Experimental operating conditions were carefully controlled to maintain a constant start of combustion (SOC), and a PRF blend was identified that would eliminate differences in premixed-burn fraction. A load-averaged NO x increase of ∼10 per cent was observed for B100 relative to the PRF blend with matched premixed-burn fraction. The results indicate that factors other than SOC and premixed-burn fraction affect the tendency for biodiesel to increase NO x . Equilibrium calculations reveal no significant differences in stoichiometric adiabatic flame temperature between the test fuels; however, experimental data suggest that actual flame temperatures may be influenced by differences in soot radiative heat transfer. The effect of biodiesel on mixture stoichi-ometry at the lift-off length may also play an important role in increasing NO x emissions.

Vorticity generation and attenuation as vortices convect through a premixed flame

Abstract A sequence of PIV images shows the time history of both the vorticity field and the velocity field as vortices of different strength convect through a premixed flame. The vortices represent individual eddies in turbulent flow; the goal is to understand how each eddy wrinkles the flame and how the flame also may alter the eddy. It is found that weak vortices are completely attenuated primarily due to volume expansion. Strong vortices do survive flame passage, but only if they can weaken the flame due to stretch effects. Intense flame-generated vorticity is measured which has a magnitude that exceeds that of the incident vortex in some cases. The flame-generated vorticity in the products induces a velocity field that tends to reduce the amplitude of flame wrinkling; thus it acts as an additional flame-stabilizing mechanism. This mechanism affects the wrinkling process and should be included in models. A new nondimensional vorticity enhancement parameter ( E ) is suggested as a way to estimate the effect of vortex size, strength, Reynolds number, and Froude number on vorticity attenuation and production. Measurements are made for E approximately equal to 0, −1, and −2, corresponding to no change in vorticity, total attenuation of the vortex, and flame-generated vorticity, respectively. Buoyancy forces are important in one case that is considered, but not in other cases. The results can be used to quantify the size of the small eddies that can be neglected in large eddy simulations; the role of small eddies is estimated in one example.

Effect of unsteady stretch rate on OH chemistry during a flame-vortex interaction: To assess flamelet models

Some basic assumptions of flamelet models are assessed by comparing profiles of OH mole fraction measured during an unsteady flame-vortex interaction to the OH profiles computed for a steady, planar counterflow flame (SPCF) with full chemistry. It is important to make such comparisons for both the same local three-dimensional stretch rate, which is measured instantaneously at locations along the flame front, and the same heat loss, as characterized by the product temperature T2. The fundamental experimental procedure consists of interacting a laminar, premixed flame with an impinging vortex ring of reactants. The OH flame chemistry was quantified using planar laser-induced fluorescence (PLIF) techniques, while the three-dimensional stretch rate measurements were made possible by the use of particle-imaging velocimetry (PIV) diagnostics on the repeatable, axisymmetric experiment. Whereas previous comparisons have been limited to steady counterflow flame experiments, the present study considers a flame which is unsteady, freely-propagating, curved, far from walls, and has realistic heat losses; thus, it contains the physical processes present in turbulent premixed flames. It was found that there are significant (25%) differences between measurements and steady counterflow flame computations of peak OH mole fractions and OH reaction zone widths. Even where the stretch rate was constant along the flame, the OH profiles showed variations, indicating that the OH profile is not a unique function of the instantaneous local stretch, but depends on the time history of the flowfield. Such history effects may be better modeled using unsteady counterflow flame simulations. Large differences (a factor of two on centerline) occur between the measured three-dimensional and two-dimensional stretch rates, indicating the importance of experimentally determining the full three-dimensional stretch rate for meaningful comparisons with models. Sensitivity analysis shows that heat losses must be realistically modeled, especially if flame extinction is to be simulated. The present type of comparison represents a first step in the assessment of flamelet models.

Effects of unsteady stretch on the strength of a freely-propagating flame wrinkled by a vortex

The objective of this study is to experimentally examine the magnitude and rate at which local flame chemistry responds to unsteady changes in imposed stretch rate. To achieve this objective, a time series of velocity field images was obtained using particle image velocimetry (PIV) diagnostics during the unsteady stretching, wrinkling, and local extinction of a laminar premixed flame by a counter-propagating toroidal vortex. The ∼10,000 velocity vectors per image in both the burned and unburned gases enable the measurement of local flame stretch rates and dilatation rate fields. Dilatation rates peak in the flamefront where the gas expands as its temperature increases. A new method is employed wherein the measured peak dilatation rate at each flame segment is used as an indicator of the local flame strength. It is found that the flame requires a relatively long time to be weakened by positive stretch, yet it is rapidly strengthened by negative stretch. Specifically, in regions where positive stretch rates are several times greater than that required to extinguish the steady flame, flame strength remains above 90% of its unstretched value for 1.2 laminar flame times, after which it drops quickly as extinction occurs. Interestingly, no such time lag is observed in regions of negative stretch, even though negative stretch magnitudes are always smaller. Flame strength is found to increase as soon as negative strain is applied, rising linearly to 240% of the unstretched value within one laminar flame time. This result indicates a significant dependence of flame chemistry on negative strain, a phenomenon that has not previously been experimentally quantified, and that may help explain observed increases in turbulent burning velocity above those produced by flame surface density increases alone.

The Quantification of Mixture Stoichiometry When Fuel Molecules Contain Oxidizer Elements or Oxidizer Molecules Contain Fuel Elements

The accurate quantification and control of mixture stoichiometry is critical in many applications using new combustion strategies and fuels (e.g., homogeneous charge compression ignition, gasoline direct injection, and oxygenated fuels). The parameter typically used to quantify mixture stoichiometry (i.e., the proximity of a reactant mixture to its stoichiometric condition) is the equivalence ratio, Φ. The traditional definition of Φ is based on the relative amounts of fuel and oxidizer molecules in a mixture. This definition provides an accurate measure of mixture stoichiometry when the fuel molecule does not contain oxidizer elements and when the oxidizer molecule does not contain fuel elements. However, the traditional definition of θ leads to problems when the fuel molecule contains an oxidizer element, as is the case when an oxygenated fuel is used, or once reactions have started and the fuel has begun to oxidize. The problems arise because an oxidizer element in a fuel molecule is counted as part of the fuel, even though it is an oxidizer element. Similarly, if an oxidizer molecule contains fuel elements, the fuel elements in the oxidizer molecule are misleadingly lumped in with the oxidizer in the traditional definition of Φ. In either case, use of the traditional definition of Φ to quantify the mixture stoichiometry can lead to significant errors. This paper introduces the oxygen equivalence ratio, Φ Ω , a parameter that properly characterizes the instantaneous mixture stoichiometry for a broader class of reactant mixtures than does Φ. Because it is an instantaneous measure of mixture stoichiometry, Φ Ω can be used to track the time-evolution of stoichiometry as a reaction progresses. The relationship between Φ Ω and Φ is shown. Errors are involved when the traditional definition of Φ is used as a measure of mixture stoichiometry with fuels that contain oxidizer elements or oxidizers that contain fuel elements; Φ Ω is used to quantify these errors. Proper usage of Φ Ω is discussed, and Φ Ω is used to interpret results in a practical example.

An Experimental Investigation of In-Cylinder Processes Under Dual-Injection Conditions in a DI Diesel Engine

Fuel-injection schedules that use two injection events per cycle (dual-injection approaches) have the potential to simultaneously attenuate engine-out soot and NO x emissions. The extent to which these benefits are due to enhanced mixing, low-temperature combustion modes, altered combustion phasing, or other factors is not fully understood. A traditional single-injection, an early-injection-only, and two dual-injection cases are studied using a suite of imaging diagnostics including spray visualization, natural luminosity imaging, and planar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO). These data, coupled with heat-release and efficiency analyses, are used to enhance understanding of the in-cylinder processes that lead to the observed emissions reductions. Results show that combustion of the early-injected fuel occurs in two phases: a cool-flame phase characterized by very weak chemiluminescence, followed by a premixed-burn phase characterized by localized regions of bright soot incandescence. Combustion of the early-injected fuel liberates only a fraction of its chemical energy. Spray visualization images show that this low combustion efficiency could be due at least in part to liquid fuel penetrating to and wetting in-cylinder surfaces, but NO PLIF images of the early-injection-only case also show strong interferences from unburned fuel vapor and/or condensed fuel droplets, suggesting that incomplete bulk-gas combustion and quenching in crevices also may play roles. The traditional single-injection case produced the highest NO PLIF signal levels. Both dual-injection cases reduced NO PLIF signal levels, with the reduction being most dramatic for the retarded-main-injection case.

Using Carbon-14 Isotope Tracing to Investigate Molecular Structure Effects of the Oxygenate Dibutyl Maleate on Soot Emissions from a DI Diesel Engine

The effect of oxygenate molecular structure on soot emissions from a DI diesel engine was examined using carbon-14 ({sup 14}C) isotope tracing. Carbon atoms in three distinct chemical structures within the diesel oxygenate dibutyl maleate (DBM) were labeled with {sup 14}C. The {sup 14}C from the labeled DBM was then detected in engine-out particulate matter (PM), in-cylinder deposits, and CO{sub 2} emissions using accelerator mass spectrometry (AMS). The results indicate that molecular structure plays an important role in determining whether a specific carbon atom either does or does not form soot. Chemical-kinetic modeling results indicate that structures that produce CO{sub 2} directly from the fuel are less effective at reducing soot than structures that produce CO before producing CO{sub 2}. Because they can follow individual carbon atoms through a real combustion process, {sup 14}C isotope tracing studies help strengthen the connection between actual engine emissions and chemical-kinetic models of combustion and soot formation/oxidation processes.