Combustion Processes Laboratories

Fast and accurate numerical analysis is not only important for studying Homogeneous Charge Compression Ignition (HCCI) technology but also critical for designing HCCI engines. Chemistry plays the major role in determining Start of Combustion (SOC) and emissions of HCCI engines. The Lawrence Livermore National Laboratory (LLNL) detailed isooctane mechanism contains 857 species and 3,606 reaction steps making the calculation too expensive. This work describes a recent development of isooctane skeletal mechanisms for speeding up numerical simulations of HCCI. By using the rate analysis, two skeletal mechanisms were constructed: one with 258 species and the other with 291 species. The former was developed for accurate predictions of SOC and the latter is an expanded version of the one with 258 species aiming at accurate predictions of both SOC and emissions. Validations of the performances of these two skeletal mechanisms were conducted extensively for the operation regimes anticipated by HCCI engine applications. Both skeletal mechanisms are found satisfactory in predicting SOC with a speeding up factor of 15-20. The expanded version is found necessary for accurate predictions of CO and unburned hydrocarbon emissions.

Monte Carlo simulations of joint PDF approaches have been extensively developed in the past largely with Reynolds Averaged Navier Stokes (RANS) equations. Current interests are in the extension of PDF approaches to Large Eddy Simulation (LES). As LES intends to resolve the large scales of turbulence in time, the coupling between Monte Carlo simulation and the flow field becomes an important issue. It is crucial to ensure some sort of coherency between the scalar field solution obtained via finite-volume methods and that from the stochastic solution of the PDF. In this paper, we first review the advantages and disadvantages of Eulerian and Lagrangian approaches. In order to clarify the coherency feature of a solution method, we introduce the concept of stochastic convergence for hybrid methods. Secondly, we present some preliminary results of an ongoing study with the Eulerian approach that reveals the numerical issues needing to be resolved. Results are presented for simulations of a pure mixing jet and Sandia Flame D using a steady-state flamelet model.

Monte Carlo simulations of joint PDF approaches have been extensively developed in the past largely with Reynolds Averaged Navier Stokes (RANS) equations. Current interests are in the extension of PDF approaches to Large Eddy Simulation (LES). As LES allows to resolve the large scales of turbulence in time and space, a joint LESPDF approach holds the promise to ease the modelling requirements (e.g. mixing models). In the past we have implemented a joint scalar PDF approach into LES with the amelet model using an Eulerian approach. Our preliminary results demonstrated that careful implementation of the Eulerian approach can be fully consistent with the counterpart nite-volume method. In this paper, results of recent LES of a pilot CH4/Air ame (Sandia/TUD ame D) with realistic nite-rate chemistry will be reported using three di erent mixing models including modi ed Curl (MC), Interaction by Exchange with the Mean (IEM), and Eucledian Minimum Spanning Tree (EMST). The calculations were performed with a 12-step reduced chemistry that has been well tested in RANS simulations of Sandia Flame D. In constrast to established RANS results which showed unphysical extinction with selected mixing models, LES results with di erent mixing models all lead to stable combustion and somewhat similar extinction patterns. These results suggest that the requirements of mixing models may be relaxed if large variations in scalar composition are coherently resolved as shown by our implementation of a joint LES-Eulerian PDF approach.

Rural kitchens of solid-fuel burning households constitute the microenvironment responsible for the majority of human exposures to health-damaging air pollutants, particularly respirable particles and carbon monoxide. Portable nephelometers facilitate cheaper, more precise, time-resolved characterization of particles in rural homes than are attainable by gravitational methods alone. However, field performance of nephelometers must contend with aerosols that are highly variable in terms of chemical content, size, and relative humidity. Our investigation of relationships between 24-hour optical and gravitational particle measurements in rural Chinese kitchens depicts that where relative humidity remained below 95%, nephelometric response was strongly linear despite complex mixtures of aerosols. Where 95% relative humidity was exceeded for even a brief duration, nephelometric data were nonsystematically distorted, and neither concurrent relative humidity measurements nor use of robust statistical measures of central tendency offered means of correction. This nonsystematic distortion is particularly problematic for rural exposure assessment studies, which emphasize upper quantiles of timeresolved particle measurements both within and between samples. Precise, accurate interpretation of optically resolved short-term particle concentrations requires short-term gravitational sampling concurrent with optical methods.

Rural kitchens of solid-fuel burning households constitute the microenvironment responsible for the majority of human exposures to health-damaging air pollutants, particularly respirable particles and carbon monoxide. Portable nephelometers facilitate cheaper, more precise, time-resolved characterization of particles in rural homes than are attainable by gravitational methods alone. However, field performance of nephelometers must contend with aerosols that are highly variable in terms of chemical content, size, and relative humidity. Previous field validations of nephelometer performance in residential settings explore relatively low particle concentrations, with the vast majority of 24-hour average gravitational PM2.5 concentrations falling below 40 μg/m3. We investigate relationships between 24-hour gravitational particle measurements and nephelometric data logged by the personalDataRAM in highly polluted rural Chinese kitchens, where gravitationally determined 24-hour average respirable particle concentrations were as high as 700 μg/m3. We find that where relative humidity remained below 95%, nephelometric response was strongly linear despite complex mixtures of aerosols and variable ambient conditions. Where 95% relative humidity was exceeded for even a brief duration, nephelometrically determined 24-hour mean particle concentrations were nonsystematically distorted relative to gravitational data, and neither concurrent relative humidity measurements nor use of robust statistical measures of central tendency offered means of correction. This nonsystematic distortion is particularly problematic for rural exposure assessment studies, which emphasize upper quantiles of timeresolved particle measurements within 24-hour samples. Precise, accurate interpretation of nephelometrically resolved short-term particle concentrations requires calibration based on short-term gravitational sampling.

Results are presented from an experimental study on the ignition of the combustion modified (fire retarded) polyurethane foam Pyrell® (35.3 kg/m3 and 64.0 kg/m3) in elevated oxygen concentrations, ranging from 30% to 60%. The samples are exposed to an external flow and variable radiant heat flux on one face, and insulated on the other faces. The experiments show that Pyrell undergoes a weak smoldering reaction that requires significant assistance in the form of external heat input in order to propagate. The results also show that given sufficient oxygen and radiant heat flux, the smoldering reaction can produce enough volatile fuel and heat to trigger a gas phase ignition, i.e. a transition from smoldering to flaming, in pores in the char region. The experiments also indicate that high-density Pyrell is more ignitable than low-density Pyrell, which could be explained by the greater solid surface area for smoldering reactions to take place.

A computational study has been carried out to investigate smoldering ignition and propagation in polyurethane foam. The one-dimensional, transient, governing equations for smoldering combustion in a porous fuel are solved accounting for improved solid-phase chemical kinetics. A systematic methodology for the determination of solid-phase kinetics suitable for numerical models has been developed and applied to the simulation of smoldering combustion. This methodology consists in the correlation of a mathematical representation of a reaction mechanism with data from previous thermogravimetric experiments. Genetic-algorithm and trail-and-error techniques are used as the optimization procedure. The corresponding kinetic parameters for two different mechanisms of polyurethane foam smoldering kinetics are quantified: a previously proposed 3-step mechanism and a new 5-step mechanism. These kinetic mechanisms are used to model one-dimensional smoldering combustion, numerically solving for the solid-phase and gas-phase conservation equations in microgravity with a forced flow of oxidizer gas. The results from previously conducted microgravity experiments with flexible polyurethane foam are used for calibration and testing of the model predictive capabilities. Both forward and opposed smoldering configurations are examined. The model describes well both opposed and forward propagation. Specifically, the model predicts the reaction-front thermal and species structure, the onset of smoldering ignition, and the propagation rate. The model results reproduce the most important features of the smolder process and represent a significant step forward in smoldering combustion modeling.

This paper presents a generalized pyrolysis model that can simulate the gasification of noncharring, charring, and intumescent materials, as well as smoldering in porous media. Separate conservation equations are solved for gaseous and condensed phase mass and species, solid phase energy, and gas-phase momentum. An arbitrary number of gas-phase and condensed-phase species can be accommodated, each having its own temperature-dependent thermophysical properties. The user may specify any number of solid to gas, solid to solid, or solid + gas to solid + gas reactions of any order. Both in-depth radiation transfer through a semi-transparent medium as well as radiation transport across pores are considered, and melting is modeled using an apparent specific heat. All volatiles generated inside the solid escape to the ambient with no resistance to flow unless the pressure solver is invoked to solve for the pressure distribution in the solid, with the resultant flow of volatiles calculated according to Darcy’s law. Similarly, the user may invoke a gas-phase convective-diffusive solver that determines the composition of the volatiles, including diffusion of species from the ambient into the solid. Thus, in addition to calculating the mass-flux of volatiles escaping from the solid, the actual composition of the vapors can be predicted. To aid in determining the required material properties, the pyrolysis model is coupled to a genetic algorithm that can be used to estimate the required input parameters from bench-scale fire tests, thermogravimetric analysis, or a combination thereof. Model predictions are compared to experimental data for the thermo-oxidative decomposition of a non-charring solid (PMMA) and the thermal pyrolysis of a charring solid (white pine), as well as the gasification and swelling of an intumescent coating, and finally smoldering in polyurethane foam. The predictive capabilities of the model are shown to be generally quite good.

In lean premixed combustion systems, inadequate mixing of the fuel and air, prior to combustion can cause unnecessarily large pollutant emissions. Measuring the extent of mixing of fuel into air is often difficult, since combustion in lean premixed gas turbines takes place at high pressures, often making optical access to the combustion area limited. In addition, the pressure broadening of the molecular absorption lines renders the spectrally narrow line associated with a laser light source less useful. This paper studies some of the problems in determining the extent of mixing of the fuel into air in these lean premixed combustion systems. The focus of this paper is the use of an infrared light emitting diode (IR-LED) to quantitatively measure fuel concentration in a lean premixed gas turbine. The IR-LED emits radiation over a wide wavelength range compared to a laser, meaning that the development of an absorption coefficient to relate the fuel concentration to the absorption of the IR-LED radiation is not as direct as developing the absorption coefficient for the absorption of laser light. Controlled experiments were performed where the pressure, path length and fuel concentration were varied and the effects of these three parameters on the absorption of radiation from the IR-LED were studied. A broad band absorption coefficient was developed relating the absorption of light from the IR-LED to the fuel concentration. This broad band absorption coefficient was found to be in good agreement with calculated coefficient values. Experiments were performed on a lean premixed gas turbine combustor modified for line-of-sight optical access. The concentration profile of this high pressure combustor was found by tomographic reconstruction from line-of-sight absorption measurements using the IR-LED. We demonstrated that the IR-LED can be used for quantitative measurements of the fuel concentration for high pressure systems.

This demonstration system is intended to meet the California Energy Commission’s primary goal of improving California’s electric energy cost/value by providing a low-cost high-efficiency distributed power generation engine that runs on landfill gas. The project team led by Makel Engineering, Inc. includes UC Berkeley, CSU Chico and the Butte County Public Works Department.

The team has developed a reliable, multi-cylinder Homogeneous Charge Compression Ignition (HCCI) engine by converting a Caterpillar 3116, 6.6 liter diesel engine to operate in HCCI mode. This engine utilizes a simple and robust thermal control system. Typically, HCCI engines are based on standard diesel engine designs with reduced complexity and cost based on the well known principles of engine dynamics. Coupled to an induction generator, this HCCI genset allows for simplified power grid connection.

Testing with this HCCI genset allowed for the development of a control system to maintain optimal the inlet temperature and equivalence ratio. A brake thermal efficiency of 35.0% was achieved while producing less than 10.0 ppm of NOx and 30 kW of electrical power. Less than 5.0 ppm of NOx was recorded with a slightly lower brake thermal efficiency. Tests were conducted with both natural gas and simulated landfill gas as a fuel source. This demonstration system has shown that landfill gas fueled Homogeneous Charge Compression Ignition engine technology is a viable technology for distributed power generation.

The influence of the small amounts (1-3%) of the additive di-tertiary butyl peroxide (DTBP) on the combustion event of Homogeneous Charge Compression Ignition (HCCI) engines was investigated using engine experiments, numerical modeling, and carbon-14 isotope tracing. DTBP was added to neat ethanol and diethyl ether (DEE) in ethanol fuel blends for a range of combustion timings and engine loads. The addition of DTBP to the fuel advanced combustion timing in each instance, with the DEE-in-ethanol mixture advancing more than the ethanol alone. A numerical model reproduced the experimental results. Carbon-14 isotope tracing showed that more ethanol burns to completion in DEE-in-ethanol blends with a DTBP additive when compared to results for DEE-in-ethanol without the additive. However, the addition of DTBP did not elongate the heat release in either case. The additive advances combustion timing for both pure ethanol and for DEE-in-ethanol mixtures, but the additive results in more of an advance in timing for the DEE-in-ethanol mixture. This suggests that although there are both thermal and kinetic influences from the addition of DTBP, the thermal effects are more important.

Measurements of line-of-sight laser extinction in a co-annular ethylene-air laminar inverse diffusion flame (IDF) were made to determine soot concentration. Extinction has frequently been used in the literature to measure soot concentration in normal diffusion flames (NDFs), but it has rarely been applied to IDFs. A coflow IDF contains a primary air flow surrounded by a fuel annulus. Soot particles form on the outside of IDFs, advect upward, and eventually quench without being oxidized. It has been proposed in the literature that IDFs will produce less near-flame soot than NDFs because, for flames of comparable fuel, size and flow rates, movement of soot outward into cool regions of an IDF limits its simultaneous exposure to the high temperatures and fuel pyrolysis products needed for soot growth. A two-dimensional soot concentration map of an IDF using experimental data confirms this hypothesis by showing integrated soot volume fractions to be an order of magnitude lower than those reported for NDFs in the literature. Computer simulations of particle temperature histories in an NDF and IDF of similar height lend support to these results.

Experiments were conducted to measure the flame propagation rate of a plug-flow flame through a combustible matrix of randomly oriented cubes of polyurethane foam in microgravity and normal gravity as a function of the forced air flow. The experiments in microgravity were conducted at the Japan Microgravity Center (JAMIC) drop tower, which provides 10s of microgravity. The normal gravity experiments were simulations of the microgravity experiments, and by comparison, were used to determine the effect of gravity on the flame propagation process. The experiment was conducted in a cylindrical geometry. Ignition was accomplished by means of a hot-surface igniter brought into direct contact with the foam at one end of the sample holder. The other end of the sample was sealed to a fan drawing air through the sample, which was adjustable using a variable DC power supply. In this configuration the flame propagation is flow-assisted. The flame propagation rate was determined by means of the temperature histories provided by thermocouples placed along the centerline of the sample. It is found that, both in normal and microgravity, as the air flow rate is increased the flame propagation velocity increases. Comparison between the normal and microgravity experiments shows that the microgravity combustion is greatly influenced by the ignition period. In microgravity the time to initiation of flame propagation is significantly longer than the corresponding time in normal gravity. This is due to the contribution of the buoyant flow that assists the forced flow during the initiation period in normal gravity. A simplified analytical model is presented for correlation of the velocity data.

The structure of laminar inverse diffusion flames (IDFs) of methane and ethylene was studied using a cylindrical co-flowing burner. Several flames of the same fuel flow-rate yet various air flow-rates were examined. Heights of visible flames were obtained using measurements of hydroxyl (OH) laser-induced fluorescence (LIF) and visible images. Polycyclic aromatic hydrocarbon (PAH) LIF and soot laser-induced incandescence (LII) were also measured. In visible images, radiating soot masks the blue region typically associated with the flame height in normal diffusion flames (NDFs). Increased air flow-rates resulted in longer flames. PAH LIF and soot LII indicated that PAH and soot are present on the fuel side of the flame and that soot is located closer to the reaction zone than PAH. Ethylene flames produced significantly higher PAH LIF and soot LII signals than methane flames, which is consistent with the sooting propensity of

Flame heights of co-flowing cylindrical ethylene-air and methane-air laminar inverse diffusion flames were measured. The luminous flame height was found to be longer than the height of the reaction zone determined by planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH) because of luminous soot above the reaction zone. However, the location of the peak luminous signals along the centerline agreed very well with the OH flame height. Flame height predictions using Roper’s analysis for circular port burners agreed with measured reaction zone heights when using values for the characteristic diffusion coefficient and/or diffusion temperature somewhat different from those recommended by Roper. The fact that Roper’s analysis applies to inverse diffusion flames is evidence that inverse diffusion flames are similar in structure to normal diffusion flames.

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phase are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates well the propagation velocities and the overall smoldering characteristics. Direct comparison of the solution with the experimental temperature profiles shows that the model predicts well these profiles at high temperature, but not as well at lower temperatures. The effect of inlet gas velocity is examined and the minimum airflow for ignition identified. It is remarkable that this one-dimensional model with simplified kinetics is capable of predicting cases of smolder ignition but with no self-propagation away from the igniter region. The model is used for better understanding of the controlling mechanisms of smolder combustion for the purpose of fire safety, both in microgravity and normal gravity, and to extend the unique microgravity data to wider conditions avoiding the high cost of space-based experiments.

Results from two forward forced-flow smolder tests on polyurethane foam using air as oxidizer conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions) are presented in this work. The two tests provide the only presently available forward smolder data in microgravity. A complimentary series of ground-based tests were also conducted to determine, by comparison with the microgravity data, the effect of gravity on the forward smolder propagation. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The data consists of temperature histories from thermocouples placed at various axial locations along the fuel sample centerline, and of permeability histories obtained from ultrasonic transducer pairs also located at various axial positions in the fuel sample. A comparison of the tests conducted in normal- and microgravity indicates that smolder propagation velocities are higher in microgravity than in normal gravity, and that there is a greater tendency for a transition to flame in microgravity than in normal gravity. This is due primarily to the reduced heat losses in the microgravity environment, leading to increased char oxidation. This observation is confirmed through a simplified one-dimensional model of the forward smolder propagation. This finding has important implications from the point of view of fire safety in a space-based environment, since smolder can often occur in the forward mode and potentially lead to a smolder-initiated fire. (C) 2004 Elsevier Inc. All rights reserved.

Biodiesel is a notable alternative to petroleum derived diesel fuel because it comes from natural domestic sources and thus reduces dependence on diminishing petroleum fuel from foreign sources, it likely lowers lifecycle greenhouse gas emissions, and it lowers an engine’s emission of most pollutants as compared to petroleum derived diesel. However, the use of biodiesel often slightly increases a diesel engine’s emission of smog forming nitrogen oxides (NOx) relative to petroleum diesel. In this paper, previously proposed theories for this slight NOx increase are reviewed, including theories based on biodiesel’s cetane number, which leads to differing amounts of charge preheating, and theories based on the fuel’s bulk modulus, which affects injection timing. This paper proposes an additional theory for the slight NOx increase of biodiesel. Biodiesel typically contains more double bonded molecules than petroleum derived diesel. These double bonded molecules have a slightly higher adiabatic flame temperature, which leads to the increase in NOx production for biodiesel. Our theory was verified using numerical simulations to show a NOx increase, due to the double bonded molecules, that is consistent with observation. Further, the details of these numerical simulations show that NOx is predominantly due to the Zeldovich mechanism.