Laser-induced spark ignition of pulsed methane jets in homogeneous and isotropic turbulence without mean flow

The influence of surrounding air turbulence on laser-induced spark ignition of a pulsed methane jet was investigated in an air environment where the turbulence is homogeneous and isotropic without mean flow. The methane jet Reynolds number (Rejet) was set at 160, while the surrounding air turbulent Reynolds number was varied in the range of Reλ = 0 - 220. Minimum Ignition Energy (MIE) was evaluated at four ignition locations by measuring the ignition probability and correlated with the local equivalence ratio (Φ) measured at three ignition locations using Laserinduced Breakdown Spectroscopy (LIBS) technique. The relationship between MIE and the local equivalence ratio obtained in quiescent air environment was similar to that reported in premixed methane/air mixtures. The impact of the surrounding air turbulence on MIE varies for different ignition locations, because the turbulence not only affects the mixing process and thereby the local equivalence ratio, but also increases the heat loss from the ignition point. The MIE decreased with increasing level of air turbulence, when the effect of local mixture composition becoming closer to stoichiometry was more significant than the adverse effect of increasing heat loss. Otherwise, the MIE increased with the level of air turbulence due to the dominance of the enhanced heat loss. The rate of increase in MIE became higher, if the local mixture composition moved further away from stoichiometry when turbulence was present. Successful ignition was also observed at locations where the mixture is relatively difficult to be ignited (Φmean = 2.38 and Φmean = 0.02), which may be attributed to the finite size of the plasma

Conditional analysis of turbulent premixed and stratified flames on local equivalence ratio and progress of reaction

Previous studies on the Cambridge/Sandia stratified burner have produced a comprehensive database of line Rayleigh/Raman/CO LIF measurements of scalars, as well as LDA and PIV measurements of velocity, for flames under non-uniform mixture fraction, under moderate turbulent conditions where the ratio of the turbulent velocity fluctuations to the laminar flame speed is of order 10. In prior work, we applied multiple conditioning methods to demonstrate how local stratification increases the levels of CO and H2, relative to the corresponding turbulent premixed flame, and enhances surface density function (SDF) and scalar dissipation rate of progress of reaction (SDR), based on extent of temperature rise, at a particular location in the flame where the mixing layer and flame brush cross. In the present study, we examine the global features of selected flames at all locations, by obtaining probability density functions (PDFs) for species concentrations, SDRs, and SDFs, conditioned on local equivalence ratio and location in the flame brush throughout the domain. We find that for most cases, species profiles as a function of temperature are well represented by laminar flame relationships at the local equivalence ratio, with some deviations attributable to either differential diffusion near the flame base and local stratification effects further downstream where the flame brush crosses the mixing layer. In particular, CO2 is significantly affected by differential diffusion, and CO and H2 by stratification. However, the stratification effects on the species are relatively minor when conditioned on local equivalence ratio, a simplifying result in the context of modeling. Measurements of the gradient of progress of reaction and scalar dissipation rates, conditioned on local equivalence ratio, show that the thermal zone of the flame is thickened by turbulence: the mean SDF and SDR values are in general lower than those of unstrained laminar flames. The effect is greater under rich conditions, with conditional mean SDR decreasing to less than half of the corresponding laminar value. The extent of flame thickening is the same in the premixed as the stratified case, once the stratified measurements are conditioned on the same equivalence ratio.M. Mustafa Kamal acknowledges funding from University of Engineering and Technology Peshawar (Pakistan). The measurements at Sandia National Labs were sponsored by the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94-AL85000. The authors also thank Dr. Akihiro Hayakawa for his contributions to the laminar flame calculations and Dr. Saravanan Balusamy for his valuable suggestions regarding data processing

Laser-induced breakdown spectroscopy measurements of local equivalence ratio measurement in opposed jet methane and propane flames

The current research reports gas composition measurements, based on Laser Induced Breakdown Spectroscopy (LIBS) in non-reacting flows and flames of premixed streams in an opposed-jet burner using both atomic and molecular emissions. The dependence of spectral intensity ratios of H/O and C2/CN on methane-air and propane- air mixture composition was quantified over a wide range of conditions extending from pure air to pure fuel. The presence of a flame within the laser beam led to significant deterioration of the signal-to-noise ratio of the instantaneous LIBS signal, caused by the variability of the induced plasma, which led to the increased uncertainty of the instantaneous gas composition measurements. The increase of the measurement uncertainty in flames was quantified and corrected by increasing the laser pulse energy, which maintained the measurement uncertainty of instantaneous gas composition below 20%. LIBS was successfully used to measure gas composition in mixtures of methane, propane and air and the results demonstrated its feasibility for instantaneous measurements of local air -fuel ratio

An experimental study of coupling between combustor pressure, fuel/air mixing, and the flame

Fuel-air mixing behavior under the influence of imposed acoustic oscillations has been studied by investigating the response of the fuel mixture fraction field. The distribution of local fuel mixture fraction inside the mixing zone, which is expected to evolve into the local equivalence ratio in the flame zone, is closely coupled to unstable and oscillatory flame behavior. The Experiment was performed with an aerodynamically-stabilized non-premixed burner. In this study, acoustic oscillations were imposed at 22, 27, 32, 37, and 55Hz. Phase-resolved acetone PLIF was used to image the flow field of both isothermal and reacting flow cases and this data along with the derived quantities of temporal and spatial unmixedness were employed for analysis. The behavior of the unmixedness factor is compared with the previous measurements of oscillations in the flame zone. This comparison shows that local oscillations (of order millimeters or smaller) in fuel/air mixing are closely related to the oscillatory behavior of the flame. For each driving frequency, the mixture fraction oscillates at that frequency but with a slight phase difference between it and the pressure field/flame intensity, indicating that the fuel mixture fraction oscillation are likely the major reason for oscillatory behaviors of this category of flames and combustor geometry

Nitric oxide formation in gas turbine engines: A theoretical and experimental study

A modified Zeldovich kinetic scheme was used to predict nitric oxide formation in the burned gases. Nonuniformities in fuel-air ratio in the primary zone were accounted for by a distribution of fuel-air ratios. This was followed by one or more dilution zones in which a Monte Carlo calculation was employed to follow the mixing and dilution processes. Predictions of NOX emissions were compared with various available experimental data, and satisfactory agreement was achieved. In particular, the model is applied to the NASA swirl-can modular combustor. The operating characteristics of this combustor which can be inferred from the modeling predictions are described. Parametric studies are presented which examine the influence of the modeling parameters on the NOX emission level. A series of flow visualization experiments demonstrates the fuel droplet breakup and turbulent recirculation processes. A tracer experiment quantitatively follows the jets from the swirler as they move downstream and entrain surrounding gases. Techniques were developed for calculating both fuel-air ratio and degree of nonuniformity from measurements of CO2, CO, O2, and hydrocarbons. A burning experiment made use of these techniques to map out the flow field in terms of local equivalence ratio and mixture nonuniformity

Predicting toxic gas concentrations resulting from enclosure fires using the local equivalence ratio concept linked to fire field models

Fire field modelling is based on the techniques of computational fluid dynamics (CFD), which provide detailed variable solutions throughout the computational domain quickly, repeatedly and cheaply compared with fire experiments. A main component of fire effluent are toxic gases and one of the main toxic gases responsible for a significant number of fire fatalities is Carbon Monoxide (CO). Current engineering fire field models either ignore the prediction of toxic gas generation or require the use of detailed chemistry to predict toxic gas generation. These detailed chemistry models require considerable data which is not generally available for most common building materials. The main objective of the study presented in this thesis is to develop practical and reasonable engineering fire field models to simulate the production and movement of toxic gases in enclosure fires. The developed toxicity models should be capable of working within the framework of the current popular combustion models in fire safety engineering. In addition, the developed models should not rely too heavily on hard to obtain experimental data. The central idea behind the newly developed toxicity model is the use of the Local Equivalence Ratio (LER). The species yields as functions of the Global Equivalence Ratio (GER) and temperature are input parameters of this model. Correlations for most building materials are available from small-scale fire experiments. Similar approaches to this method are also developed using the CO/CO2 and H2/H2O mole ratios. The LER methodology is further refined by an approach which divides the computational domain for the calculation of toxic gases into two parts, a control region in which the toxic gases are dependent on the LER and temperature, and a transport region in which the toxic gas concentrations are dependent on the mixing of hot gases with fresh air. The toxicity model is then extended to two-fuel cases. In the two-fuel model, the LER is a function of the two mixture fractions, which are used to represent the mixture of the two different fuels, oxygen and combustion products. This model is useful in simulating residential fires, in which wood lining of sidewalls or ceilings is the second fuel. Finally, the transportation of HCI within fire compartments is considered. A mathematical model is developed to simulate the exchange of HCI between gas boundary and wall surfaces and the reaction of HCI with walls. All the toxicity models developed in this study can be integrated into the practical volumetric heat source approach and the Eddy Break-up (EBU) combustion model typically used in practical engineering analysis

Combustion characteristics in the transition region of liquid fuel sprays

A number of important effects have been observed in the droplet size transition region in spray combustion systems. In this region, where the mechanism of flame propagation is transformed from diffusive to premixed dominated combustion, the following effects have been observed: (1) maxima in burning velocity; (2) extension of flammability limits; (3) minima in ignition energy; and (4) minima in NOx formation. A monodisperse aerosol generator has been used to form and deliver a well controlled liquid fuel spray to the combustion test section where measurements of ignition energy have been made. The ignition studies were performed on monodisperse n-heptane sprays at atmospheric pressure over a range of equivalence ratios and droplet diameters. A capacitive discharge spark ignition system was used as the ignition source, providing independent control of spark energy and duration. Preliminary measurements were made to optimize spark duration and spark gap, optimum conditions being those at which the maximum frequency or probability of ignition was observed. Using the optimum electrode spacing and spark duration, the frequency of ignition was determined as a function of spark energy for three overall equivalence ratios (0.6, 0.8, and 1.0) and for initial droplet diameters of 25, 40, 50, 60, and 70 micro m

Using LES to Study Reacting Flows and Instabilities in Annular Combustion Chambers

Great prominence is put on the design of aeronautical gas turbines due to increasingly stringent regulations and the need to tackle rising fuel prices. This drive towards innovation has resulted sometimes in new concepts being prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is estimating the adequate grid, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on three grids, respectively made of 38, 93 and 336 million elements. Results are compared in terms of mean and fluctuating fields. LES captures self-established azimuthal modes. The presence and structure of the modes is discussed. This study therefore highlights the potential of LES for studying combustion instabilities in annular gas turbine combustors

Polydispersity Effects in Low-order Ignition Modeling of Jet Fuel Sprays

Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a timeaveraged cold- ow solution is extended to include local fuel concentration uctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modelling such uctuations is considered, and the e ects of the ow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence a ecting the local ammability factor at the model's cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray ames between 0.2-0.6, which is to be contrasted with values of 1.5 for gaseous fuels.EU Clean Sky 2 project PROTEUS (785349

Impact of Trench and Ramp Film Cooling Designs to Reduce Heat Release Effects in a Reacting Flow

Increasing combustor fuel-air ratios are a recent area of concern in gas turbine film cooling due to the potential for heat release on the surface of film-cooled components. This investigation compared four different cooling designs on their heat release potential: namely fanned, normal and radial trenched, and ramped. Measurements of heat flux to the downstream surface, when subjected to a reacting mainstream flow, provide a qualitative comparison between the four tested configurations. Furthermore, this work studied the effect of multiple injection points in series along the surface of a flat plate. An upstream set of normal holes and an upstream slot are evaluated on their ability to protect the downstream coolant flow from the fuel rich mainstream. Results are presented in terms of heat flux, augmentation of heat flux, and adiabatic wall temperature calculations. Downstream heat release is suspected to be a result of coolant interaction with local free radical concentrations. Concentrations, volume flow rates and jet to mainstream momentum ratio dictate local equivalence ratio and hence, the available local enthalpy generatio