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Showing papers in "Combustion Science and Technology in 2003"


Journal ArticleDOI
TL;DR: In this paper, the authors derived a reduced-order model for interaction among vortex shedding, chamber acoustics, and combustion process in premixed-type combustors, where vortex burning is assumed to be localized in space and time.
Abstract: Vigorous burning of vortices, formed behind flame stabilizers, can drive significant pressure oscillations inside premixed-type combustors. The goal of this work is to derive a reduced-order model for interaction among vortex shedding, chamber acoustics, and combustion process. A dump combustor is considered a general system configuration. Formation of vortices at the sudden expansion in a chamber is affected by the oscillatory flow. A new quasi-steady model is proposed for determining the moment of vortex separation. Vortex burning is assumed to be localized in space and time. A "kicked" oscillator model is utilized for deriving the appropriate dynamical system. The moment of burning and the corresponding vortex location are dependent on the chamber geometry, velocity field, and characteristic chemical and hydrodynamic times. If Rayleigh's criterion is satisfied, acoustic waves can develop in the chamber. Model and experimental results are compared for a chosen configuration. A study of model performance for a realistic system is carried out by variation of parameters where the mean flow velocity and the number of modes are treated as variables.

155 citations


Journal ArticleDOI
TL;DR: In this article, the swirling flow in a lean premixed prevaporized (LPP) combustor model was studied, making use of high-speed photography, Laser Doppler Velocimetry (LDV), sound probes, fine-wire thermocouples, and suction probes for chemical gas analysis.
Abstract: The swirling flow in a lean premixed prevaporized (LPP) combustor model (LPP chamber) is studied, making use of high-speed photography, Laser Doppler Velocimetry (LDV), sound probes, fine-wire thermocouples, and suction probes for chemical gas analysis. The experimental study, which involved parametric changes of Re , S , and { , extends from the combustor primary zone to the premixing chamber and was conducted with and without reaction, burning either gaseous propane or liquid fuel assisted with preheated air at 300 °C. Measurements show that the swirling flow mixture that enters the primary zone of the combustor chamber shows evidence of a PVC structure, for S > 0.5, that embraces the central recirculation zone (CRZ). Furthermore, reaction test results show that increasing the swirl number decreases CO and NO x concentrations at the combustor exit and reduces flame stability limits to levels close to the lean limit because of flashback. In addition, a detailed study of the reacting condition ( S =1.05, ...

125 citations


Journal ArticleDOI
TL;DR: In this paper, experimental measurements of flame stabilization, operating range, and emissions are reported for several porous burners, which consist of two sections of reticulated porous ceramic: a section of 23.6 pores per centimeter (ppcm) followed by a Section of 3.9 ppcm.
Abstract: A porous burner in which a premixed flame is stabilized within the solid matrix is a promising technology for various applications due to its low emissions. In this article, experimental measurements of flame stabilization, operating range, and emissions are reported for several porous burners. The burner consisted of two sections of reticulated porous ceramic: a section of 23.6 pores per centimeter (ppcm) followed by a section of 3.9 ppcm. Two different porous media materials, yttria-stabilized zirconia/alumina composite (YZA) and zirconia-toughened mullite (ZTM), were used in two separate, but identical, burners. Results reveal that the interface between the two sections of porous ceramic stabilized the flame effectively over a range of conditions in the YZA burner. In the ZTM burner, however, theflamepropagated through the interface and into the upstream section. A third burner was constructed of YZA in which the length of the upstream section was reduced. The operating range for this burner was determ...

110 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of simplified transport models on the burning velocity of laminar premixed flames is compared with results obtained with a number of simplified models and the influence of the Soret and Dufour effects is evaluated.
Abstract: In many flame simulations, simplified transport models are used to reduce computational costs. This article presents the effect of simplified transport modeling on the burning velocity of laminar premixed flames. Results obtained with the detailed transport model are compared with results obtained with a number of simplified models. Furthermore, the influence of the Soret and Dufour effects is evaluated. All the models are tested on one-dimensional adiabatic premixed flames with CH 4 /air, CH 4 /O 2 , H 2 /air, and H 2 /O 2 mixtures. Results show that the approximations may lead to large errors in the prediction of the burning velocity. For example, neglecting thermal diffusion in an H 2 /air flame may lead to an error of 10%. Furthermore, simplified mass diffusion models appear to be inaccurate, especially for the fuel/oxygen flames.

91 citations


Journal ArticleDOI
TL;DR: The most widely used method for the modeling of complicated thermoacoustic systems is based on linear acoustics and a representation of the system as a network of "elements", that is, acoustic multiports.
Abstract: Although combustion instabilities of low-emission combustion processes have become a major problem in recent years, the prediction of the stability limits of confined flows with heat release is still in its infancy. The most widely used method for the modeling of complicated thermoacoustic systems is based on linear acoustics and a representation of the system as a network of "elements," that is, acoustic multiports. Based on such a network, a linear system of equations is constructed from the transfer matrices of the different elements that serves as the basis for investigating the system dynamics. By employing such a network representation of a simple combustor, predictions of thermoacoustic stability obtained with three different methods of stability analysis are compared in this article. It is found that the commonly employed open-loop stability analysis based on a Bode plot can lead to erroneous results. Similar problems may occur with the analysis of the open-loop gain using a Nyquist diagram, as lo...

86 citations


Journal ArticleDOI
TL;DR: In this paper, a 300MW e, front-wall-fired, pulverized-coal utility boiler was used to measure the local mean gas species concentrations of O 2, CO, CO 2, and NO x, gas temperatures, and charburnout measured at several ports in the boiler including those in the burner region.
Abstract: Measurements have been performed in a 300-MW e , front-wall-fired, pulverized-coal utility boiler. New data are reported for local mean gas species concentrations of O 2 , CO, CO 2 , and NO x , gas temperatures, and charburnout measured at several ports in the boiler including those in the burner region. They complement previously obtained data in the same utility boiler before being retrofitted with low-NO x burners and over-fire air ports. During the current experimental work, a considerable effort was made to assure minimum variations on boiler operating conditions and coal chemical and particle size characteristics so that the data presented are especially useful for three-dimensional mathematical model evaluation and development. The main conclusions are as follows: (1) As compared with our previousmeasurementsin this boiler, prior to the retrofitting, the results show lower local mean O 2 and higher CO concentrations and temperatures in the boiler as a consequence of the lower stoichiometry in the m...

77 citations


Journal ArticleDOI
TL;DR: In this paper, a laboratory-scale burner for mild combustion, which is an apparatus characterized by high internal recycle ratio, high back-mixing, and the possibility of also simulating external recycles of burned gases, is described, as well as the main experimental results achieved.
Abstract: Mild oxidation is one of the promising techniques proposed to control pollutant emissions from combustion plants. It is characterized by high preheating of combustion air and massive recycle of burned gases before reaction. These requisites lead to high combustion efficiency and good control of thermal peaks and hot spots, lowering NO x thermal emissions. In this work, a laboratory-scale burner for mild combustion, which is an apparatus characterized by high internal recycle ratio, high back-mixing, and thepossibility of also simulating external recycles of burned gases, is described, as well as the main experimental results achieved. These were carried out to demonstrate the possibility of reproducing the main phenomena observed in real-size flameless burners. Operating conditions for obtaining stable "mild" combustion with methane and ethane as fuels were investigated and operating parameters maps were obtained.

76 citations


Journal ArticleDOI
TL;DR: In this paper, phase-correlated measurements of different species in a turbulent swirl flame with a self-excited instability are presented for the first time, where phase-resolved OH* chemiluminescence and planar laser-induced fluorescence (PLIF) spectroscopy of OH, CH, and H 2 CO are used to follow the temporal evolution of flame structures in a pulsating swirl-stabilized model injector for gas-turbine applications.
Abstract: Quasi-simultaneous phase-correlated measurements of different species in a turbulent swirl flame with a self-excited instability are presented for the first time. Phase-resolved OH* chemiluminescence and planar laser-induced fluorescence (PLIF) spectroscopy of OH, CH, and H 2 CO were used to follow the temporal evolution of flame structures in a pulsating swirl-stabilized model injector for gas-turbine applications. H 2 CO is a suitable indicator for chemical heat release in combination with OH; CH LIF and OH* emission were shown to be suitable indicators for the average shape and location of flame fronts, while OH LIF marked regions with high temperature, both in the flame front and the burned gas regions. The combustor was operated on methane fuel at atmospheric pressure. Lasers and detectors were locked to the phase angle of the self-excited pressure oscillation using a trigger signal derived from a microphone. Measurements at different phase angles were performed by variable delays with respect to the...

74 citations


Journal ArticleDOI
TL;DR: In this paper, an assumed probability density function (PDF) approach is used to account for turbulence chemistry interaction, and the PDFs employed are a clipped Gaussian distribution for temperature and a joint multivariate g -PDF for species mass fractions.
Abstract: This article deals with high-speed turbulent combustion based on finite-rate chemistry. An assumed probability density function (PDF) approach is used to account for turbulence chemistry interaction. The PDFs employed are a clipped Gaussian distribution for temperature and a joint multivariate g -PDF for species mass fractions. The definition of both PDFs is based on higher order moments obtained from additional transport equations. The objective of this work is to compare different variance transport equations (energy, sensible energy, and temperature) from which the required temperaturevariance is recovered. Moreover, the influence of closed and unclosed correlations of these variance transport equations is investigated. It is shown that some higher order correlations as well as commonly used approximations for the temperature variance recovery may result in significant errors. This is the reason for modifications and for the use of a temperature variance transport equation. Numerical results are compar...

73 citations


Journal ArticleDOI
TL;DR: In this paper, a model reaction between formyl radicals and molecular oxygen was employed to model the complete formation kinetics based on a reaction between CH and molecular dioxide introduced by Krishnamahari and Broida (1961 J. Chem. Phys., 34, 1709-1711).
Abstract: Combustion diagnostics in practical devices are difficult due to the harsh environment encountered. Many optical diagnostics are attractive for combustion diagnostics applications due to their high temperature compatibility. Chemiluminescence measurements may allow combustion monitoring at a level of detail traditionally thought impossible. Among chemiluminescence species observed in flames, OH* chemiluminescence is attractive due to its relatively strong emission at lean equivalent ratios. In order to help in the interpretation of OH* chemiluminescence measurements, an accurate OH* chemiluminescence model must be developed. Current models rely on the modeling of the complete formation kinetics based on a reaction between CH and molecular oxygen introduced by Krishnamahari and Broida (1961 J. Chem. Phys. , 34 , 1709-1711). Recent studies as well as the current study are not ableto match experimental results using the customary OH* model. The present study employs a model reaction between formyl radicals a...

64 citations


Journal ArticleDOI
TL;DR: In this article, a dual ignition criteria model (DICM) was used to interpret the HMX results and sucessfully predicted the slope transition trend and the critical ignition energy for HMX to within 10% of the measured values.
Abstract: A current issue important to high explosive safety is deflagration-to-detonation transitions (DDTs) in accident scenarios. In order to better understand the reactive mechanisms involved in DDT and to begin to approach the fast ignition and heating rates seen in DDT, high-irradiance ( h 800W/cm 2 ) CO 2 laser ignition experiments were performed on the common high explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Reported data include time to ignition as a function of laser irradiance, energy, and ignition temperature. A simple dual ignition criteria model (DICM) was used to interpret the HMX results. The DICM requires two basic criteria for ignition: (1) a minimum surface temperature must be reached and (2) a minimum energy concentration must exist within the solid. The DICM sucessfully predicted the slope transition trend and the critical ignition energy for HMX to within 10% of the measured values. TATB had a single dependence on irradiance...

Journal ArticleDOI
TL;DR: In this paper, a planar laser-induced fluorescence of the hydroxyl radical was used as a qualitative indicator for heat release in a flame. But the results indicate that the forcing frequency and burner configuration have pronounced effects on the response of the flame.
Abstract: A technique has been devised that can provide information about the local dynamic response of a flame to an acoustic field. In the experiments, a flame in an acoustic chamber is subjected to pressure oscillations by a pair of low-frequency drivers. The response of the flame is visualized by planar laser-induced fluorescence of the hydroxyl radical, which is used as a qualitative indicator for heat release in the flame. The resulting images are phase-resolved and averaged to yield a qualitative picture of the fluctuation of the heat release. This is correlated with the output of a pressure transducer near the flame. The tendency to cause instability can then be evaluated using Rayleigh's criterion. Results indicate that the forcing frequency and burner configuration have pronounced effects on the response of the flame. Forcing frequencies ranging from 22 to 55 Hz are applied to the jet-mixed burner supplied with a premixed 50/50 mixture of methane and carbon dioxide at a Reynolds number of 20,000. The burn...

Journal ArticleDOI
TL;DR: In this paper, the authors measured the acoustic spectrum of a turbulent, natural gas-fueled flame in an anechoic environment at several burner diameters, flow velocities, and equivalence ratios, and found that the spectrum has a nearly universal shape whose total power is a function of equivalence ratio and flow velocity.
Abstract: Turbulent premixed combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized the total sound power radiated by turbulent flames, few measurements of their spectral characteristics exist. This article presents systematic measurements of the acoustic spectra over a 100 Hz-30 kHz frequency range from a turbulent, natural gas-fueled flame. Measurements were made in an anechoic environment at several burner diameters, flow velocities, and equivalence ratios. The results show that, for a fixed burner diameter, the turbulent flame's acoustic spectrum has a nearly universal shape whose total power is a function of equivalence ratio and flow velocity. The functional dependence of the power spectra on frequency does vary with burner diameter, however, as the frequency of peak acoustic emissions generally shifts to lower frequencies with increases in burner diameter. These trends are quite similar to those exhibited by the cold flow vel...

Journal ArticleDOI
TL;DR: In this article, the effects of gas-solid nonequilibrium on forced filtration combustion (FC) waves were investigated in a two-temperature model with separate temperature fields for the solid and gas particles.
Abstract: To determine the effects of gas-solid nonequilibrium on forced filtration combustion (FC) waves, a two-temperature model is employed, with separate temperature fields for the solid and gas particles. We consider heterogeneous (solid/gas) combustion in a porous sample with a prescribed gas flux at the inlet. If the reaction is initiated at the inlet (outlet) of the sample and the combustion wave travels in the direction of (opposite to ) gas filtration it is referred to as coflow (counterflow) FC. We determine the effects of gas-solid nonequilibrium on various aspects of forced FC waves. First, we consider coflow FC waves, in which case the gas infiltrating through the hot product region significantly enhances the propagation of the combustion wave. For a relatively small gas flux, the infiltrating gas delivers heat from hot product to the combustion layer, thus increasing the combustion temperature and, hence, the combustion rate. Propagation of such waves is controlled by conduction of the heat released ...

Journal ArticleDOI
TL;DR: In this article, the role of water content in coal oxidation was studied using an isothermal flow reactor at atmospheric pressure and temperatures below 100°C, and the results indicated that inherent water plays a role in chemical reactions occurring during coal oxidation.
Abstract: The role of water content in coal oxidation was studied using an isothermal flow reactor at atmospheric pressure and temperatures below 100°C. Transient rates of consumption of oxygen and production of CO 2 and CO were measured during oxidation experiments, by means of an online dual-column micro gas chromatograph and an oxygen analyzer. Experiments were carried out with a bituminous coal at three levels of initial water content, i.e., 0.8, 2.0, and 3.0%. Comparisons of the rates of production of carbon oxides during the oxidation experiments indicated that inherent water plays a roleinchemicalreactions occurring during coal oxidation. It was also found that the rateof oxygen consumption decreases with increasing water content of a sample. The current observations suggest that inherent water present in coal pores may react with carbonyl species to form carboxyl species during the oxidation process.

Journal ArticleDOI
TL;DR: In this paper, an experimental study of a laboratory-scale, premixed, prevaporized, swirl-stabilized combustor operated with preheated air and fed with liquid heptane as fuel is presented.
Abstract: Due to its good performance in terms of low pollutant emissions, the lean premixed, prevaporized combustor design has received a special interest in the field of gas turbines. This article reports an experimental study of a laboratory-scale, premixed, prevaporized, swirl-stabilized combustor operated with preheated air and fed with liquid heptane as fuel. Under certain operating conditions, it exhibits well-established combustion oscillations at frequencies near 400 Hz. To understand and control these instabilities, it is useful to determine the burner response to external modulations. A novel actuator relying on secondary air modulation was designed and integrated in the injector to allow measurements of the combined transfer function of the burner and actuation system. Two different techniques were used for this determination: direct measurement between command and pressure on photo-multiplier signals based on spectral and cross-spectral densities, or identification through a finite impulse response num...

Journal ArticleDOI
TL;DR: In this article, five different flame states are identified in a compact combustion chamber that is fired by a 30-kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure.
Abstract: Five different flame states are identified in a compact combustion chamber that is fired by a 30-kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure. These flame states include a nozzle-attached tulip-shaped flame, an unattached lifted ring-shaped flame suitable for single-digit-ppm NO x emission, and a Coanda flame that clings to the bottom wall of the burner. Flame-state transition is generated by changing the swirl number and may be further modified by premixing the combustion air with 70% of the natural gas flow. Detailed experimental results of two ring-shaped flames are reported. The reactants flow out of the burner in a conical sheet and enter the spread-out flame. Evidence of precessing and oscillatory behavior of the internal recirculation zone (IRZ) as well as the flame envelope boundary is discussed and explained by the bimodal probability density functions of the velocity distributions. Low-pressure regions in the corner of the tight combustion chamber and...

Journal ArticleDOI
TL;DR: In this article, the results of a combined experimental and numerical investigation of the combustion reactivity of sawdust char in a drop tube furnace were presented, and the experimental results indicated that the bulk of the char oxidation reaction for sawdust occurred in Regime II where chemical reaction and pore diffusion rates are comparable.
Abstract: This article discusses the results of a combined experimental and numerical investigation of the combustion reactivity of sawdust char in a drop tube furnace. The work presented here was motivated by the lack of reliable data in the open literature on the combustion reactivity of woody biomass char. This study involved the experimental determination of the global kinetic parameters of sawdust-char oxidation and the subsequent application of these parameters in a computational fluid dynamics (CFD) code. The modified code was employed for mathematical modeling of the sawdust-char combustion in the drop tube furnace. The experiments were conducted at furnace temperatures of 1473 and 1673 K in a 10 mol% O 2 environment and 1473 K in air. Experimental results indicated that the bulk of the char oxidation reaction for sawdust occurred in Regime II where chemical reaction and pore diffusion rates are comparable. In addition, the sawdust char exhibited distinctive near-extinction combustion behavior at the early ...

Journal ArticleDOI
TL;DR: Sattelmayer and Polifke as discussed by the authors showed that the commonly employed open-loop Bode plot stability analysis can lead to erroneous results for a Nyquist diagram, if the interpretation of the openloop gain curve follows the standard rules of traditional control theory.
Abstract: A negative characteristic of low-emission flames is their weak dynamic flame stability, which leads in many cases to periodic pulsations of heat release and combustor pressure. Due to the extremely complex nature of the forcing and feedback mechanisms in flows with heat release, reliable prediction of the stability limits of combustors has not been achieved yet with satisfactory accuracy. The dynamics of complicated thermoacoustic systems, which are modeled on the basis of linear acoustics and a representation of the system as a network of acoustic elements, can be analyzed using methods derived from control theory. It was shown in a companion article (Sattelmayer and Polifke, Combustion Science and Technology , vol. 175, pp. 453-476, 2003) that the commonly employed open-loop Bode plot stability analysis can lead to erroneous results. Similar problems may occur also for a Nyquist diagram, if the interpretation of the open-loop gain curve follows the standard rules of traditional control theory. A lack of...

Journal ArticleDOI
TL;DR: In this article, an optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW using two different nozzle configurations, one consisting of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondary stream was used for active control.
Abstract: An optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW The efficacy of the controller was tested with two different nozzle configurations The first consisted of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondarystreamwasused for active control The second configuration used a single-feed nozzle with two different swirling air streams An LQG-LTR (linear quadratic Gaussian-loop transfer recovery) controller was designed using the SI-based model to determine the active control input, which was in turn used to modulate the secondary fuel stream Using this controller, the thermoacoustic oscillations, which occurred under lean operating conditions, were reduced to the background noise level A time-delay controller was also implemented for comparison purposes The results showed that the LQG-LTR controller yielded an additional pressure reduction of 14 dB compared

Journal ArticleDOI
TL;DR: In this article, the scalar dissipation rate in the region immediately upstream of a lifted jet flame is determined from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering.
Abstract: Measurements of the scalar dissipation rate in the region immediately upstream of a lifted jet flame are presented. The scalar dissipation is determined in this isothermal region from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering. Fields of the scalar dissipation rate are presented in addition to tabulated values for three different liftoff heights ( Re d =4800, 6400, and 8300). Scalar dissipation rates do not reach levels thought to cause extinction of the leading edge based on comparison with extinction data for counterflow diffusion flames. Additionally, results are presented on the axial flame propagation velocities relative to the jet flow. The data indicate that over the three flow conditions, the flame velocity relative to the flow is approximately constant during the case of a quasi-stationary lifted flame. In light of these findings, it is suggested that concepts involving partially premixed flame propagation, rather than those of critical ...

Journal ArticleDOI
TL;DR: In this paper, a generalization of the work of Pope and Anand (1984) is put forward to take into account the consequence of flamelet structure, but only when and where this structure is not perturbed by turbulence.
Abstract: The calculation of turbulent premixed flames in the framework of the Probability Density Function (PDF) approach remains quite a challenging modeling problem because of the strong coupling that exists between micromixing and reaction in premixed flamelets. In the flamelet regime of turbulent premixed combustion, instantaneous progress variable gradients are essentially fixed by these propagating flamelets. Consequently, the PDF evolution due to the micromixing term cannot be handled without taking into account the reactive contribution itself. In this study, a generalization of the work of Pope and Anand (1984; Flamelet and Distributed Combustion in Premixed Turbulent Flames, Proc. Combust. Inst. , vol. 20, pp. 403-410) is put forward to take into account the consequence of flamelet structure, but only when and where this structure is not perturbed by turbulence. In this manner, a criterion based on the ratio of the Kolmogorov length scale m K to laminar flame width i L allows turbulent premixed combustio...

Journal ArticleDOI
TL;DR: In this paper, the co-combustion of pulverized-biomass in a natural gas-fired large-scale laboratory furnace is investigated experimentally, and the results show that cofiring of biomass fuels, compared with pure natural gas firing, has no significant effect on NO x emissions provided that the fuel-bound nitrogen is l...
Abstract: The co-combustion of pulverized-biomass in a natural gas-fired large-scale laboratory furnace is investigated experimentally. The biomass fuels used included pine sawdust, pine shells, pine branches, and olive stones. For comparison purposes, the co-combustion tests have also included the study of natural gas/pulverized-coal flames. For each solid-fuel, flue-gas data were obtained for pollutant emissions and particle burnout as a function of the solid-fuel cofiring ratio. Ratios up to 20% (energy basis) were used in the experiments. Subsequently, in order to improve the understanding of the processes that occur during the co-combustion of natural gas and solid fuels, the study focused on detailed in-flame measurements of major gas species, including NO x , and gas temperatures for three representative furnaceoperating conditions. The results show that (1) cofiring of biomass fuels, compared with pure natural gas firing, has no significant effect on NO x emissions provided that the fuel-bound nitrogen is l...

Journal ArticleDOI
TL;DR: In this article, the ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG.
Abstract: The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the mom...

Journal ArticleDOI
TL;DR: In this paper, a literature review of experimental measurements of mass specific extinction coefficients, † s for combustion-generated particulate has been conducted to obtain an improved value of the constant in the frequently used expression for the Planck mean absorption coefficient through smoke, s m = C 0 f v T.
Abstract: A literature review of experimental measurements of mass specific extinction coefficients, † s for combustion-generated particulate has been conducted to obtain an improved value of the constant in the frequently used expression for the Planck mean absorption coefficient through smoke, s m = C 0 f v T . This expression, which results from the product of the spectral absorption coefficient for smoke particles and the Planck spectral distribution curve integrated over all wavelengths, is used to simplify radiative transport calculations in combustion systems. It is shown that a value of C 0 = 2370 - 240 m m 1 K m 1 is consistent with the available data and that this value is approximately two times the values commonly used. Using the available experimental values of † s to calculate C 0 eliminates the problems associated with the large uncertainties in soot refractive index encountered when Rayleigh theory is used to predict C 0 . It is also shown that a value of K e = 8.8 - 0.9 for the dimensionless extinc...

Journal ArticleDOI
TL;DR: In this paper, a modeling technique is described for estimating the intensity of mechanically induced thermal and combustion fluctuations at the grain scale (mesoscale) of granular energetic solids in a manner compatible with thermodynamics, contact mechanics, and bulk (macroscale) experiments.
Abstract: A modeling technique is described for estimating the intensity of mechanically induced thermal and combustion fluctuations at the grain scale (mesoscale) of granular energetic solids in a manner compatible with thermodynamics, contact mechanics, and bulk (macroscale) experiments. The technique is illustrated for the dynamic compaction, localized heating, and ignition of the commonly used high-explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) due to mild impact by a constant-speed piston (< 150 m/s). Guided by contact mechanics, bulk dissipated mechanical energy is thermalized at localization sites (hot spots) within the material mesostructure that are centered at intergranular contact surfaces. The evolution of the bulk material response is tracked, and the corresponding evolution of hot-spot temperature, mass fraction, and reaction progress is resolved at the grain scale. Importantly, the bulk and grain-scale descriptions are energetically equivalent. Model predictions indicate that the on...

Journal ArticleDOI
TL;DR: In this article, high-resolution transmission electron microscopy (TEM) imaging performed on precursors and mature soot particles reveals the presence of highly organized carbon nanostructures formed inside the tarlike amorphous condensate by carbonization process.
Abstract: Carbon nanostructures formed in the opposed-flow flames of methane- and oxygen-enriched air are studied experimentally using thermophoretic sampling technique and high-resolution transmission electron microscopy (TEM). Reconstructed evolution of soot particles along the burner centerline shows the existence of two characteristic layers. The narrow layer of polydisperse precursor particles is located on the oxidizer side of the stagnation plane. The precursor particles undergo carbonization and agglomeration as they are driven to the stagnation plane forming the layer of mature soot aggregates on the fuel side of the precursor layer. High-resolution TEM imaging performed on precursors and mature soot particles reveals the presence of highly organized carbon nanostructures formed inside the tarlike amorphous condensate by carbonization process. Two characteristic structures are observed. They resemble shapes of carbon onions and carbon nanopolyhedral particles. Multiwalled carbon nanotubes (MWNTs) appeared ...

Journal ArticleDOI
TL;DR: In this article, the attrition behavior of a dolomite during fluidized-bed calcination and sulfation was investigated in a bench-scale apparatus, where the operating conditions of the bed were typical of atmospheric bubbling fluidized bed combustion.
Abstract: The attrition behavior of a dolomite during fluidized-bed calcination and sulfation was investigated in a bench-scale apparatus. Operating conditions of the bed were those typical of atmospheric bubbling fluidized-bed combustion. Batchwise experiments were carried out in order to study the influence of the chemical reactions on the parallel phenomena of particle fragmentation and abrasion in the bed. To this end changes of particle size distribution and fines elutriation rates were followed throughout conversion during both sequential and simultaneous calcination and sulfation. The study was complemented by SEM/EDX analysis of polished sulfated particle cross sections in order to relate the sorbent attrition propensity to the changes of particle morphology upon sulfation.

Journal ArticleDOI
TL;DR: An implementation of active control of thermoacoustic instabilities on a swirl-stabilized spray combustor is presented and the advantages of the model-based controllers over the baseline strategy are clearly presented.
Abstract: An implementation of active control of thermoacoustic instabilities on a swirl-stabilized spray combustor is presented. Loudspeakers were used as control actuators in the closed-loop control scheme. Pressure transducers located in the combustion chamber produced the feedback signal in the control study. Experimental models of the combustor dynamics were developed using a nonparametric identification method. Linear quadratic Gaussian (LQG), LQG/loop transfer recovery, and H ∞ loop-shaping controllers were derived and tested in simulation as well as experimentally. Phase-delay control was used as a baseline method to compare the performance of these different controllers. The advantages of the model-based controllers over the baseline strategy are clearly presented.

Journal ArticleDOI
TL;DR: In this article, the prediction of the autoignition delay time of fresh fuel in hot air is studied, and a linear mixing ignition (LMI) approach is proposed to estimate the ignition delay.
Abstract: The prediction of the autoignition delay time of fresh fuel in hot air is studied. This problem, encountered in many combustion systems, is in practice often calculated using a simple method, hereafter called homogeneous mixing ignition (HMI). This method, however, neglects transport effects and calculates local instead of global ignition properties. A second method is therefore studied called linear mixing ignition (LMI) and consists of the direct simulation of a one-dimensional mixing layer. This method is tested in a hydrogen and methane fuel configuration using detailed and reduced chemistry mechanisms. The results indicate that molecular transport increases the ignition delay for the methane fuel by about a factor three, but this effect is compensated in the hydrogen case by the small Lewis number of the fuel. Such detailed information is not provided by the HMI method and the LMI approach appears, therefore, as a minimum requirement for the correct estimation of the ignition delay time.