G. M. Faeth

Current status of droplet and liquid combustion

Abstract The present understanding of spray combustion in rocket engine, gas turbine, Diesel engine and industrial furnace applications is reviewed. In some cases, spray combustion can be modeled by ignoring the details of spray evaporation and treating the system as a gaseous diffusion flame; however, in many circumstances, this simplification is not adequate and turbulent two-phase flow must be considered. The behavior of individual droplets is a necessary component of two-phase models and recent work on transient droplet evaporation, ignition and combustion is considered, along with a discussion of important simplifying assumptions involved with modeling these processes. Methods of modeling spray evaporation and combustion processes are also discussed including: one-dimensional models for rocket engine and prevaporized combustion systems, lumped zone models (utilizing well-stirred reactor and plug flow regions) for gas turbine and furnace systems, locally homogeneous turbulent models, and two-phase models. The review highlights the need for improved injector characterization methods, more information of droplet transport characteristics in turbulent flow and continued development of more complete two-phase turbulent models.

Mixing, transport and combustion in sprays

Abstract Recent advances concerning analysis of sprays and drop/turbulence interactions are reviewed. Consideration is given to dilute sprays and related dilute dispersed flows, which contain well-defined dispersed-phase elements (e.g. spherical drops) and have dispersed-phase volume fractions less than 1%; and to the near-injector, dense spray region, having irregularly-shaped liquid elements and relatively-high liquid fractions. Early analysis of dilute sprays and other dispersed flows assumed either locally-homogeneous flow (LHF), implying infinitely-fast interphase transport rates, or deterministic separated flow (DSF) where finite interphase transport rates are considered, but interactions between dispersed-phase elements and turbulence are ignored. These limits are useful in some instances; however, recent evidence shows that both methods are deficient for quantitative estimates of the structure of most practical dispersed flows, including sprays. As a result, stochastic separated flow (SSF) methods have been developed, which treat both finite interphase transport rates and dispersed phase (drop)/turbulence interactions using random-walk computations for the dispersed phase. Evaluation of SSF methods for particle-laden jets; nonevaporating, evaporating and combusting sprays; and noncondensing and condensing bubbly jets has been encouraging, suggesting capabilities of current SSF methods to treat a variety of interphase processes. However, current methods are relatively ad hoc and many fundamental problems must still be resolved for dilute flows, e.g. effects of anisotropic turbulence, modification of continuous-phase turbulence properties by the dispersed phase (turbulence modulation), effects of turbulence on interphase transport rates, and drop shattering, among others. Dense sprays have received less attention and are poorly understood due to substantial theoretical and experimental difficulties, e.g. the idealization of spherical drops is not realistic, effects of liquid breakup and collisions are difficult to describe, spatial resolution is limited and the flow is opaque to optical diagnostics which have been helpful for studies of dilute sprays. Limited progress thus far, however, suggests that LHF analysis may provide a useful first-approximation of the structure and mixing properties of dense sprays near pressure-atomizing injectors. Since dense-spray processes fix initial conditions needed to rationally analyze dilute sprays, more research is this area is clearly warranted.

Fractal and projected structure properties of soot aggregates

The structure of soot aggregates was investigated, emphasizing the fractal properties as well as the relationships between the properties of actual and projected soot images. This information was developed by considering numerically simulated soot aggregates based on cluster-cluster aggregation as well as measured soot aggregates based on thermophoretic sampling and analysis by transmission electron microscopy (TEM) of soot for a variety of fuels (acetylene, propylene, ethylene, and propane) and both laminar and turbulent diffusion flame conditions. It was found that soot aggregate fractal properties are relatively independent of fuel type and flame condition, yielding a fractal dimension of 1.82 and a fractal prefactor of 8.5, with experimental uncertainties (95% confidence) of 0.08 and 0.5, respectively. Relationships between the actual and projected structure properties of soot, e.g., between the number of primary particles and the projected area and between the radius of gyration of an aggregate and its projected image, also are relatively independent of fuel type and flame condition.

Structure and breakup properties of sprays

Abstract Multiphase flow phenomena relevant to spray combustion are reviewed, emphasizing the structure of the near-injector dense-spray region and the properties of secondary and primary breakup. Existing measurements of dense-spray structure are limited to round pressure-atomized sprays in still gases and show that the dispersed flow region is surprisingly dilute, that separated flow effects are significant because the flow is dilute and developing, and that atomization involves primary breakup at the liquid surface followed by secondary breakup, while effects of collisions are small. Available information about secondary breakup emphasizes breakup due to shock wave disturbances at large liquid/gas density ratios and shows that secondary breakup is a dominant feature of dense sprays that must be resolved as a function of time so that secondary breakup can be properly treated as a rate process. Finally, available information about primary breakup has been dominated by effects of disturbances in the injector passage; therefore, while some understanding of turbulent primary breakup has been achieved, more information about aerodynamic primary breakup is needed to address practical spray combustion processes.

Laminar burning velocities and Markstein numbers of hydrocarbonair flames

Effects of positive flame stretch on the laminar burning velocities of hydrocarbon/air mixtures were studied experimentally using outwardly propagating spherical flames. The test conditions included propane, methane, ethane, and ethylene-air flames at various fuel-equivalence ratios and normal temperature and pressure. Karlovitz numbers generally were less than 0.3 so that the flames were remote from quenching conditions. Within this range, the ratio of the unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. In addition, Markstein numbers varied in a roughly linear manner with fuel-equivalence ratios over the range of the measurements, which were somewhat removed from flammability limits where behavior might differ. Effects of stretch were substantial: Markstein numbers varied from -2.5 to 7.2, yielding corresponding laminar burning velocity variations of 0.4-2,7 times the value for an unstretched (plane) flame over the test range. The ranges of fuel-equivalence ratios for unstable preferential-diffusion conditions (negative Markstein numbers) were as follows: propane, greater than 1.44; methane, less than 0.74; ethane, greater than 1.68; and ethylene, greater than 1.95. Fuel-equivalence ratios for maximum flame temperatures and laminar burning velocities are near unity for the present flames; therefore, neutral preferential-diffusion conditions are shifted toward fuel-equivalence ratios on the unstable side of unity, in qualitative agreement with recent approximate theories treating the effects of stretch on laminar premixed flames.

Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure

Abstract Effects of positive flame stretch on the laminar burning velocities of hydrogen/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements were based on motion picture shadowgraphy, while numerical simulations were based on typical contemporary chemical reaction mechanisms. Flame conditions studied included hydrogen/air flames having fuel-equivalence ratios in the range 0.3–5.0 at normal temperature and pressure. Both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers over the test range (Karlovitz numbers up to 0.4), yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. Markstein numbers were in the range −1 to 6, with unstable (stable) preferential-diffusion conditions observed at fuel-equivalence ratios below (above) roughly 0.7. Present stretch-corrected laminar burning velocities were in reasonably good agreement with other determinations of laminar burning velocities at fuel-lean conditions where Markstein numbers, and thus effects of stretch, are small. In contrast, the stretch-corrected laminar burning velocities generally were smaller than other measurements in the literature at fuel-rich conditions, where Markstein numbers, and thus effects of stretch, are large. Finally, predicted unstretched laminar burning velocities and Markstein numbers were in reasonably good agreement with measurements, although additional study to improve the comparison between predictions and measurements at fuel-rich conditions should be considered.

Structure of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times

Abstract The structure of soot was investigated within the fuel-lean (overfire) region of overventilated buoyant turbulent diffusion flames burning in still air. The study was limited to the long residence time regime where characteristic flame residence times are roughly more than an order of magnitude longer than the laminar smoke point residence time and soot generation factors (the mass of soot emitted per unit mass of fuel carbon burned) are relatively independent of flame residence times. Both gaseous and liquid fuels were used to provide a range of H/C ratios (1–2.7) and fuel types (alkynes, alkenes, alkanes, aromatics, and alcohols) as follows: toluene, acetylene, benzene, propylene, ethylene, n -heptane, propane, and isopropanol. Measurements included transmission electron microscopy to find primary particle diameters, the number of primary particles per aggregate and aggregate geometrical and fractal dimensions. The results show that the structure of soot varies with fuel type but is relatively independent of both position in the overfire region and flame residence time for the long residence time regime. Mean primary particle diameters were 30–51 nm and the mean number of primary particles per aggregate were 255–552, with the larger values associated with the more heavily sooting fuels. Aggregate fractal dimensions, however, were less dependent on fuel type, only varying in the range 1.70–1.79. The structure measurements are used to estimate the optical properties of overfire soot, based on a recent approximate theory for polydisperse aggregates, finding significant differences between aggregate and Rayleigh scattering properties in the visible and near-infrared portions of the spectrum, even though the primary particles are well within the Rayleigh scattering regime.

Structure of particle-laden jets - Measurements and predictions

Measurements of mean and fluctuating velocities of both phases as well as particle mass fluxes were completed in turbulent, particle-laden jets containing monodisperse particles with well-defined initial and boundary conditions. The new measurements were used to evaluate a stochastic separated flow model of the process which treated effects of interphase slip and turbulent dispersion using random-walk computations for particle motion. The continuous phase was treated using a modified k-epsilon model allowing for direct contributions of interphase transport to both mean and turbulence properties. The model performed reasonably well over the new data base, with all empirical parameters fixed from earlier work. In contrast, simplified models ignoring either interphase slip or turbulent dispersion yielded poor agreement with the measurements.

Flame/stretch interactions of premixed hydrogen-fueled flames: measurements and predictions

Abstract Fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) were considered both experimentally and computationally for laminar premixed flames. Mixtures of hydrogen and oxygen with nitrogen, argon and helium as diluents were considered to modify flame transport properties for computationally tractable reactant mixtures. Freely (outwardly)-propagating spherical laminar premixed flames were considered for fuel-equivalence ratios of 0.6 to 4.5, pressures of 0.3 to 3.0 atm, volumetric oxygen concentrations in the nonfuel gases of 0.21 to 0.36, and Karlovitz numbers of 0 to 0.5, at normal temperatures. For these conditions, both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding constant Markstein numbers for particular reactant conditions. The present flames were very sensitive to flame stretch, exhibiting ratios of unstretched-to-stretched laminar burning velocities in the range 0.6 to 3.0 for levels of flame stretch well below quenching conditions. At fuel-lean conditions, increasing flame temperatures (by dilution with argon rather than nitrogen) tended to reduce flame sensitivity to stretch whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior. At low pressures, helium-diluted flames had reduced tendencies toward preferential-diffusion instability behavior compared to nitrogen- and argon-diluted flames due to stabilization of flame properties by strong effects of preferential diffusion of heat. Predicted and measured flame properties exhibited encouraging agreement using contemporary reaction mechanisms. Finally, flame structure predictions suggest that H and OH radical production and transport are important aspects of preferential-diffusion/stretch interactions, reflecting the strong correlation between laminar burning velocities and H+OH radical concentrations for present test conditions.

Measured and predicted properties of laminar premixed methane/air flames at various pressures

Abstract Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements based on motion picture shadowgraphs, and numerical simulations based on typical contemporary chemical reaction mechanisms, were used to find the sensitivities of the laminar burning velocities to flame stretch, characterized as Markstein numbers, and the fundamental laminar burning velocities of unstretched flames. Reactant conditions included methane/air mixtures having fuel-equivalence ratios of 0.60–1.35 and pressures of 0.5–4.0 atm, at normal temperatures. Both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied significantly from unity (in the range 0.6–2.3) even though present stretch levels did not approach quenching conditions. Absolute values of Markstein numbers increased with increasing pressure, while the transition from unstable to stable preferential-diffusion conditions with increasing fuel-equivalence ratio shifted from an equivalence ratio of 0.6 at 0.5 atm to 1.2 at 4.0 atm, suggesting increased unstable flame behavior due to preferential-diffusion effects at the elevated pressures of interest for many practical applications. Finally, predictions using two contemporary chemical reaction mechanisms were in reasonably good agreement with present measurements of both Markstein numbers and unstretched laminar burning velocities.