Jeffrey M. Bergthorson

Combustion of Aluminum Suspensions in Hydrocarbon Flame Products

Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane–air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to determine the flame temperature via both the continuous and aluminum monoxide spectra. The flame burning velocity is measured and the condensed flame products are collected and analyzed for unburned metallic aluminum content. It was observed that, below a critical concentration of about 120  g/m3, aluminum demonstrates incomplete oxidation with a flame temperature close to the methane–air flame. Below the critical concentration, the flame burning velocity also decreases similar to a flame seeded with inert silicon carbide particles. In contrast, at aluminum concentrations above the critical value, an aluminum flame front rapidly forms and is coupled to the methane flame. The flame temperature of the coupled methane–aluminum flame is close to equilibrium values with aluminum as...

An Evaluation of Numerical Models for Temperature-Stabilized CH4/Air Flames in a Small Channel

Flames stabilized in a heated tube with a diameter on the order of the flame thickness (i.e., small Peclet number) are investigated with numerical models of differing formulations. Providing a benchmark for comparison, a two-dimensional detailed model that solves the full, elliptic Navier–Stokes equations assuming axisymmetry is implemented with detailed chemical kinetics. The solutions are compared to those obtained from a simpler one-dimensional volumetric model that relies on a constant Nusselt number assumption to account for the heat transfer between the gas and the wall. This volumetric model presents poor agreement with the detailed results with an average error of 231 K (18%) in wall temperature at the stabilization position. In an attempt to improve the modeling accuracy, the volumetric model is extended to account for the variations in the thermal boundary layer inside the reaction zone and the resulting enhanced flame-wall heat transfer. This extended volumetric model demonstrates significant improvements, given the considerable savings in computational time when compared to the detailed model, with errors smaller than 55 K (4.2%) in stabilization wall temperatures. The leading order effects influencing these flames are inferred based on the dissimilarities in the respective model formulations and on the accuracy of their predictions. Deviations between this improved model and the detailed model are further investigated to determine the influence of the neglected two-dimensional effects. The deviations are attributed to the differences in treatment of radial momentum and H species transport between the two models.

NOx Emissions Modeling and Uncertainty From Exhaust-Gas-Diluted Flames

Oxides of nitrogen (NOx) are pollutants emitted by combustion processes during power generation and transportation that are subject to increasingly stringent regulations due to their impact on human health and the environment. One NOx reduction technology being investigated for gas-turbine engines is exhaust-gas recirculation (EGR), either through external exhaust-gas recycling or staged combustion. In this study, the effects of different percentages of EGR on NOx production will be investigated for methane–air and propane–air flames at a selected adiabatic flame temperature of 1800 K. The variability and uncertainty of the results obtained by the gri-mech 3.0 (GRI), San-Diego 2005 (SD), and the CSE thermochemical mechanisms are assessed. It was found that key parameters associated with postflame NO emissions can vary up to 192% for peak CH values, 35% for thermal NO production rate, and 81% for flame speed, depending on the mechanism used for the simulation. A linear uncertainty analysis, including both kinetic and thermodynamic parameters, demonstrates that simulated postflame nitric oxide levels have uncertainties on the order of ±50–60%. The high variability of model predictions, and their relatively high associated uncertainties, motivates future experiments of NOx formation in exhaust-gas-diluted flames under engine-relevant conditions to improve and validate combustion and NOx design tools.

Thermal Structure and Burning Velocity of Flames in Non-volatile Fuel Suspensions

Abstract Flame propagation through a non-volatile solid-fuel suspension is studied using a simplified, time-dependent numerical model that considers the influence of both diffusional and kinetic rates on the particle combustion process. It is assumed that particles react via a single-step, first-order Arrhenius surface reaction with an oxidizer delivered to the particle surface through gas diffusion. Unlike the majority of models previously developed for flames in suspensions, no external parameters are imposed, such as particle ignition temperature, combustion time, or the assumption of either kinetic- or diffusion-limited particle combustion regimes. Instead, it is demonstrated that these parameters are characteristic values of the flame propagation problem that must be solved together with the burning velocity, and that the a priori imposition of these parameters from single-particle combustion data may result in erroneous predictions. It is also shown that both diffusive and kinetic reaction regimes can alternate within the same flame and that their interaction may result in non-trivial flame behavior. In fuel-lean mixtures, it is demonstrated that this interaction leads to certain particle size ranges where burning velocity increases with increasing particle size, opposite to the expected trend. For even leaner mixtures, the interplay between kinetic and diffusive reaction rates leads to the appearance of a new type of flame instability where kinetic and diffusive regimes alternate in time, resulting in a pulsating regime of flame propagation.

Development of a nanosecond pulsed HV atmospheric pressure plasma source: preliminary assessment of its electrical characteristics and degree of thermal nonequilibrium

This paper discusses the development and characteristics of a distributed nanosecond-pulsed glow-like discharge plasma in air at atmospheric pressure. The produced pulse is of 6.4 kV with duration at half maximum of less than 80 ns, and an average pulse repetition frequency of 150 Hz. The discharge operates in air in a concentric electrode configuration. Spectroscopic studies are presented in order to assess the thermal characteristics of the plasma as well as its spatial characteristics. Electrical diagnostics are presented along with time averaged ICCD imaging of the radially distributed plasma. Although variations occur, it is found that the plasma has uniform vibrational and rotational temperatures across the inter electrode gap illustrating a high degree of disequilibrium in the plasma. Band head intensity analysis proves the existence of a negative glow in the near cathode region. Finally, the sensitivity of individual vibrational bands to vibrational and rotational temperatures is presented as a means to most accurately evaluate the uncertainty of spectrally determined temperatures.

High-Voltage, High-Frequency Pulse Generator for Nonequilibrium Plasma Generation and Combustion Enhancement

This paper outlines the design and implementation of a solid-state high-voltage, high-frequency pulse generator used to drive capacitive loads and particularly, nonequilibrium plasmas at atmospheric pressure. The generator is capable of producing open circuit pulses of 0-12 kV with a 300 ns duration (full-width at half-maximum) at a repetition frequency of 0-25 kHz. The working principles of the generator are presented, along with its electrical diagnostics to illustrate its operation with capacitive loads. The generator was applied to a pin-to-plane electrode configuration to generate a diffuse nonequilibrium plasma discharge with peak voltage of 12 kV. Energy deposition and average power required to drive the discharges in open air at 25 kHz were calculated to be 112 μJ/pulse and 2.80 W. The generator was also applied to lean, stagnation-plate stabilized V-shaped flames to increase their blowoff velocity. A 28%-51% increase of the blowoff velocity is observed using a discharge with the peak voltage of 6.2 kV, and the repetition rate of 25 kHz.

Comparison of laminar flame speeds, extinction stretch rates and vapor pressures of jet A-1/HRJ biojet fuel blends

An emerging goal within the aviation industry is to replace conventional jet fuel with biologically-derived alternative fuel sources. However, the combustion properties of these potential fuels must be thoroughly characterized before they can be considered as replacements in turbomachinery applications. In this study, seven candidate alternative fuel blends, derived from two biological feedstocks and blended in different quantities with Jet A-1, are considered. For each blend, the laminar flame speed, non-premixed extinction stretch rate, and vapor pressure are experimentally determined and compared to numerical simulations and to Jet A-1 data. Hydrodynamically-stretched flame speeds are determined by applying particle image velocimetry (PIV) to an atmospheric pressure, preheated jet-wall stagnation flame, and the unstretched laminar flame speed is inferred using a direct comparison method in conjunction with a binary jet-fuel surrogate, with results spanning a wide equivalence ratio range. Extinction stretch rates were measured using particle tracking velocimetry (PTV) in a non-premixed counterflow diffusion flame, over a range of fuel mass fractions diluted in nitrogen carrier gas. Finally, the vapor pressure of the seven biojet/Jet A-1 fuel blends was measured using an isoteniscope over a wide temperature range. The results of this study indicate that moderate blends of hydrotreated renewable jet (HRJ) fuel with Jet A-1 have similar combustion properties to conventional jet fuel, highlighting their suitability as drop-in replacements, while higher blend levels of HRJ fuel, regardless of the crop source, lead to definitive changes in the combustion parameters investigated here.Copyright

Comparative Analysis of Chemical Kinetic Models Using the Alternate Species Elimination Approach

A major thrust in combustion research is the development of chemical kinetic models for computational analysis of various combustion processes. Significant deviations can be seen when comparing predictions of these models against experimentally determined combustion properties over a wide range of operating conditions and mixture strengths. However, these deviations vary from one model to another. It would be insightful in such circumstances to elucidate the species and sub chemistry models which lead to the varying prediction ability in various models. In this work we apply the Alternate Species Elimination (ASE) method to selected mechanisms in order to analyze their predictive ability with respect to propane and syngas combustion. ASE is applied to a homogeneous reactor undergoing ignition. The ranked species of each model are compared based on their Normalized Change. We further provide skeletal versions of the various models for propane and syngas combustion analysis. It is observed that this approach provides an easy way to determine the chemical species which are central to predictive performance of a model in their order of importance. It also provides a direct way to compare the relative importance of chemical species in the models under consideration. Further development and in-depth analysis could provide more information and guidance for model improvement.Copyright

Increased Flame Reactivity of a Lean Premixed Flame Through the Use of a Custom-Built High-Voltage Pulsed Plasma Source

High-voltage (HV) nonthermal pulsed plasma sources at atmospheric pressure are currently studied for combustion enhancement applications. The sub-microsecond electrical discharges produce reactive species that reduce the global activation energy of the combustion processes and increase the flame propagation speed while limiting gas heating. Current pulses of duration <;100 ns are produced through the discharge of a capacitor-charging HV dc power supply. Time-integrated intensified charge-coupled device images, filtered at 430 nm, of a lean premixed CH4/air laminar flame under the influence of pulsed HV electrical discharges are presented as clear evidence of plasma-assisted combustion.

Scale-Adaptive and Large Eddy Simulations of a Turbulent Spray Flame in a Scaled Swirl-Stabilized Gas Turbine Combustor Using Strained Flamelets

In this paper, the three-dimensional (3D) reacting turbulent two-phase flow field of a scaled swirl-stabilized gas turbine combustor is numerically investigated using the commercial CFD software ANSYS FLUENT™-v14. The first scope of this study aims to explicitly compare the predictive capabilities of two turbulence models namely Scale-Adaptive Simulation (SAS) and Large Eddy Simulation (LES) for a reasonable compromise between accuracy of results and global computational cost when applied to simulate swirl-stabilized spray combustion. The second scope of the study is to couple chemical reactions to the turbulent flow using a realistic chemistry model and also to model the local chemical non-equilibrium effects caused by turbulent strain. Standard Eulerian and Lagrangian formulations are used to describe both gaseous and liquid phases respectively. The fuel used is liquid jet-A1 which is injected in the form of a polydisperse spray and the droplet evaporation rate is calculated using the infinite conductivity model. One-component (n-decane) and two-component fuels (n-decane + toluene) are used as jet-A1 surrogates. The combustion model is based on the first and second moments of the mixture fraction, and a presumed-probability density function (PDF) is used to model turbulent-chemistry interactions. The instantaneous thermochemical state necessary for the chemistry tabulation is determined by using initially the partial equilibrium assumption (PEQ) and thereafter, the detailed non-equilibrium (NEQ) calculations through the laminar flamelet concept. The combustion chemistry of these surrogates is represented through a reduced chemical kinetic mechanism (CKM) comprising 1 045 reactions among 139 species, derived from the detailed jet-A1 surrogate model, JetSurf 2.0. Numerical results are compared with a set of published data for a steady spray flame. Firstly, it is observed that, by coupling the two turbulence models with a combustion model incorporating a representative chemistry to account for non-equilibrium effects with realistic fuel properties, the models predict reasonably well the main combustion trends, with a superior performance for LES in terms of trade-off between accuracy and computing time. Secondly, because of some assumptions with the combustion model, some discrepancies are found in the prediction of species slowly produced or consumed such as CO and H2. Finally, the study emphasizes the dominant advantage of an adequate resolution of the mixing characteristics especially with the more demanding simulation of a swirl-stabilized spray flame.Copyright