Suresh K. Aggarwal

A review of spray ignition phenomena: Present status and future research

Theoretical and experimental studies dealing with the spray ignition phenomena are reviewed. Two major topics covered are external-source ignition of liquid fuel sprays and spontaneous spray ignition. Experimental and theoretical investigations of external-source ignition of sprays employing different configurations are discussed first. Three major topics included here are: (i) ignition of quiescent and flowing fuel sprays; (ii) ignition of monodisperse and polydisperse sprays; and (iii) ignition of single-component and multicomponent fuel sprays. Then, experimental studies of autoignition of sprays employing constant-volume enclosures, injection in a uniform air flow, and shock tube techniques, are discussed. Theoretical investigations dealing with spray autoignition phenomena range from phenomenological models to one-dimensional numerical models using global one-step as well as detailed multistep chemistry, and to multidimensional simulations with reduced mechanisms. These models are also discussed in the review. Finally, some advanced topics which are common to both external-source ignition and spontaneous ignition are identified and discussed. An attempt is made to provide a common link between the three dominant ignition modes in sprays, namely individual droplet ignition, droplet cluster ignition, and spray ignition. In a similar manner, common features of external-source ignition and spontaneous ignition of sprays are identified. A general spray ignition model along with important numerical and physical issues are presented. The effect of pressure on spray ignition processes is also discussed. Potential topics for further research are suggested.

A comparison of vaporization models in spray calculations

Abstract : The effects of different gas- and liquid-phase models on the vaporization behavior of a single-component isolated droplet are studied for both stagnant and convection situations in a high-temperature gas environment. In conjunction with four different liquid-phase models, namely, d2 law, infinite conductivity, diffusion limit, and internal vortex circulation, the different gas-phase models include a spherically symmetric model in the stagnant case and Ranz-Marshall correlation plus two other axisymmetric models in the convective case. A critical comparison of all these models is made. The use of these models in a spray situation is examined. A transient one-dimensional flow of an air- fuel droplet mixture is considered. It is shown that the fuel vapor mass fraction can be very sensitive to the particular liquid- and gas-phase models. The spherically symmetric conduction or diffusion limit model is recommended when the droplet Reynolds number is negligible compared to unity, while the simplified vortex model accounting for internal circulation is suggested when the droplet Reynolds number is large compared to unity. Keywords include: Spray; Droplet; Evaporation; Combustion; Modeling.

The structure of triple flames stabilized on a slot burner

A triple flame is a partially premixed flame that contains two premixed reaction zones (one fuel-lean and the other rich) that form exterior wings and a nonpremixed reaction zone that is established in between these wings. The three reaction zones merge at a “triple point.” Triple flames may play an important role in the stabilization and liftoff of laminar nonpremixed flames. They are also of fundamental importance in the reignition of turbulent mixtures. Despite their importance, many aspects of triple flames have not been adequately investigated and are, consequently, not clearly understood. Herein, laminar triple flames stabilized on a Wolfhard-Parker slot burner are investigated. The flow consists of a rich mixture of methane and air emerging from the inner slot and a lean mixture from two symmetric outer slots. In this configuration the three reaction zones that characterize a triple flame can be clearly distinguished. The loci of the “triple points” form a “triple line” in this planar configuration. The velocity field is characterized using laser Doppler velocimetry, and the temperature distribution using laser interferometric holography. In addition, C∗2-chemiluminescence images of the three reaction zones are obtained. A detailed numerical model is employed to completely characterize the flame. It is based on a 24-species and 81-reaction mechanism. The numerical results are validated through comparisons with the experimental measurements. Our results focus on the detailed structure, the interaction between the three reaction zones, the dependence of the flame structure on the initial velocities and mixture equivalence ratios, and the dominant chemical pathways. The lean premixed reaction zone (external wing) exhibits different features from the rich premixed reaction zone. In particular, it is characterized by strong HO2 formation and consumption reactions, and by relatively weak methane consumption reactions. Radical activity is higher in the nonpremixed reaction zone than in the other reaction zones. Overall, radicals from the nonpremixed reaction zone are transported to both the rich and lean premixed reaction zones where they attack the reactants. Simplifying the chemical mechanism by removing the C2-containing species produces significant differences in the predicted results only for the inner rich premixed reaction zone.

Flame-vortex dynamics in an inverse partially premixed combustor - The Froude number effects

In this paper we report on a computational and experimental investigation of the transient combustion characteristics of an inverse partially premixed flame established by injecting a fuel-rich (CH4-air) annular jet sandwiched between a central air jet on the inside and coflowing air on the outside. A time-dependent, axisymmetric, reacting flow model is used to simulate the flame dynamics. A global 1-step and a relatively detailed 52-step mechanism are used to model the CH4-air chemistry. Results focus on the dynamic flame structure and flame-vortex interactions at different Froude numbers (Fr), the scaling of the flame flicker frequency, and the global comparison of experimental and computational results. At high Froude numbers (nonbuoyant limit), the computed flame exhibits a steady-state structure, which is markedly different from that of a jet diffusion flame. The flame structure reveals two distinct reaction zones consisting of an inner premixed region followed by two nonpremixed flames at the wings. Methane is converted to CO and H 2 in the premixed reaction zone and these intermediate species provide fuel for the outer nonpremixed flames. Main reaction pathways associated with the double-flame structure are identified. For intermediate Fr, the buoyant acceleration becomes significant, causing a periodic rollup of toroidal vortices. While the rollup process is highly periodic, the flame exhibits steady-state behavior, since vortices are relevant only in the plume region. For Fr < 1.0, the rollup occurs closer to the burner port, resulting in flame-vortex interactions and a dynamic flame. A distinguished characteristic of this flame is the rollup of two simultaneous vortices corresponding to inner and outer diffusion flames, which convect downstream at the same velocity, and interact with the twin flame surfaces, causing flame flicker and stretch. Both numerical and laboratory experiments are employed to obtain a correlation between the Strouhal number (S), associated with the vortex rollup or flame flicker frequency, and the Froude number. Simulations yield a correlation S = 0.56 Fr 038, while measurements yield S = 0.43 Fr -°38, indicating an excellent agreement, considering that the flow conditions in the numerical and laboratory experiments are only globally matched in terms of overall stoichiometry, Fr, and Reynolds number, and not with respect to burner size and jet velocity. Finally, the effects of chemical kinetics on the computed flame structure are examined. Both the time-averaged and the dynamic flame structure, including flame height, peak temperature, and flicker frequency, are found to be influenced by chemical kinetics. However, the scaling of the dominant frequency or Strouhal number with Fr is essentially the same for the two mechanics. In addition, the frequency is found to be independent of the chemical kinetic parameters used in the global mechanism.

Transient supercritical droplet evaporation with emphasis on the effects of equation of state

Abstract This paper reports a numerical investigation of droplet evaporation in a supercritical environment. A comprehensive physical–numerical model is developed to simulate the transcritical and supercritical droplet vaporization phenomena. The model is based on the time-dependent conservation equations for both liquid and gas phases, pressure-dependent variable thermophysical properties, and a detailed treatment of liquid–vapor phase equilibrium at the droplet surface. The numerical solution of the two-phase equations employs an arbitrary Eulerian–Lagrangian, explicit–implicit method with a dynamically adaptive mesh. The first part of the study examines the capability of different equations of state (EOS) for predicting the phase equilibrium and transcritical droplet vaporization behavior. Predictions using the Redlich–Kwong (RK) EOS are shown to be markedly different from those using the Peng–Robinson (PR) and Soave–Redlich–Kwong (SRK) EOS. Results for the phase-equilibrium of a n-heptane–nitrogen system indicate that compared to PR- and SRK-EOS, the RK-EOS predicts higher fuel vapor concentration, higher liquid-phase solubility of nitrogen, lower critical-mixing-state temperature, and lower enthalpy of vaporization. As a consequence, it significantly overpredicts droplet vaporization rates and, thus, underpredicts droplet lifetimes compared to those predicted by PR- and SRK-EOS, as well as compared to experimental data. Furthermore, using RK-EOS, attainment of the critical mixing state at the droplet surface is predicted earlier in droplet lifetime compared with that using the other two EOS. In contrast, predictions using the PR-EOS show excellent agreement with experimental data over a wide range of ambient conditions. The PR-EOS is then used for a detailed investigation of the transcritical droplet vaporization phenomena. Results indicate that at low to moderate ambient temperatures, the droplet lifetime first increases and then decreases as the ambient pressure is increased, while at high ambient temperatures, the droplet lifetime decreases monotonically with pressure. This behavior is in accord with the published experimental results. The numerical model is also used to obtain the minimum pressure required for the attainment of critical mixing state at the droplet surface for a given ambient temperature.

A review of droplet dynamics and vaporization modeling for engineering calculations

The present paper reviews the methodologies for representing the droplet motion and vaporization history in two-phase flow computations. The focus is on the use of droplet models that are realistic in terms of their efficient implementation in comprehensive spray simulations, representation of important physical processes, and applicability under a broad range of conditions. The methodologies available at present to simulate droplet motion in complex two-phase flows may be broadly classified into two categories. First one is based on the modified BBO equation. This approach is more comprehensive, but requires modifications and/or correlations at higher droplet Reynolds number. The second approach aims at developing correlations, using detailed numerical simulations or laboratory experiments, for the effects of flow nonuniformity and droplet relative acceleration on the instantaneous drag and lift coefficients. Recent advances made in the droplet vaporization models are also discussed. The advanced vaporization models include the effects of transient liquid heating, gas-phase convection, and variable thermophysical properties. All of these models are discussed, and recommendations are made for their inclusion in comprehensive two-phase computations.

A molecular dynamics simulation of droplet evaporation

Abstract A molecular dynamics (MD) simulation method is developed to study the evaporation of submicron droplets in a gaseous surrounding. A new methodology is proposed to specify initial conditions for the droplet and the ambient fluid, and to identify droplet shape during the vaporization process. The vaporization of xenon droplets in nitrogen ambient under subcritical and supercritical conditions is examined. Both spherical and non-spherical droplets are considered. The MD simulations are shown to be independent of the droplet and system sizes considered, although the observed vaporization behavior exhibits some scatter, as expected. The MD results are used to examine the effects of ambient and droplet properties on the vaporization characteristics of submicron droplets. For subcritical conditions, it is shown that a spherical droplet maintains its sphericity, while an initially non-spherical droplet attains the spherical shape very early in its lifetime, i.e., within 10% of the lifetime. For both spherical and non-spherical droplets, the subcritical vaporization, which is characterized by the migration of xenon particles that constitute the droplet to the ambient, exhibits characteristics that are analogous to those reported for “continuum-size” droplets. The vaporization process consists of an initial liquid-heating stage during which the vaporization rate is relatively low, followed by nearly constant liquid-temperature evaporation at a “pseudo wet-bulb temperature”. The rate of vaporization increases as the ambient temperature and/or the initial droplet temperature are increased. For the supercritical case, the droplet does not return to the spherical configuration, i.e., its sphericity deteriorates sharply, and its temperature increases continuously during the “vaporization” process.

NOx emissions in n-heptane/air partially premixed flames

Abstract NO x emissions in n-heptane/air partially premixed flames (PPFs) in a counter-flow configuration have been investigated. The flame is computed using a detailed mechanism that combines the Held’s mechanism for n-heptane and the Li and Williams’ mechanism for NO x . The combined mechanism contains 54 species and 327 reactions. Based on a detailed analysis, dominant mechanisms responsible for NOx formation and destruction in PPFs are found to be thermal, prompt, and reburn mechanisms. The dominant reactions associated with these mechanisms are also identified. The effects of strain rate (a s ) and equivalence ratio (φ) on NO x emissions are characterized for conditions in which the flame contains two spatially separated reaction zones; a rich premixed zone on the fuel side and a non-premixed zone on the air side. For most conditions, except for relatively high level of partial premixing, the NO formation rate in the non-premixed zone is significantly higher than that in the rich premixed zone. Within the rich premixed zone, the contribution of thermal NO to total NO x is higher than that of prompt NO, while in the non-premixed zone, the prompt NO is the major contributor. The behavior is related to the transport of acetylene from the rich premixed to the non-premixed zone, and higher concentrations of CH, O, and OH radicals in the latter zone. A notable result in this context is that the existence of CH does not automatically imply that prompt NO will form. The existence of O and OH is also necessary, in addition to CH, to form prompt NO. The relative contributions of thermal and prompt mechanisms to total NO x are generally insensitive to variations in a s , but show strong sensitivity to variations in φ. There is a NO x destruction region sandwiched between the rich premixed and the non-premixed reaction zones. The NO x destruction occurs mainly through the reburn mechanism. The NO x emission index (EINOx) is computed as a function of φ and a s . These results are qualitatively in accord with previous numerical and experimental results for methane-air PPFs.

Unsteady spray flame propagation in a closed volume

Abstract A hybrid Eulerian-Lagrangian method is employed to study a laminar unsteady propagating flame through an air/fuel-vapor/fuel-droplet mixture in a one-dimensional closed constant volume combustor. The gas-phase properties are obtained by using a continuum approach, whereas the computation of liquid-phase properties is based on a Lagrangian approach. A quiescent mixture of air and fuel mist is ignited by providing a hot wall at the end of the combustor. Following ignition, a flame initiates and propagates into the cold spray mixture. The characteristics of the propagating spray flame are presented in terms of the profiles of gas-phase and liquid-phase properties. The effects of initial droplet size, equivalence ratio, and fuel volatility are studied. Propagation rate is not a monotonic function of droplet size for all equivalence ratios; spray flame speeds can be faster than the premixed flame speed at certain equivalence ratios. Both a premixed character, due to prevaporization, and a diffusion flame character is seen to be present in the results.

Gravity effects on triple flames: Flame structure and flow instability

A fundamental difference between a partially premixed flame and an equivalent premixed (or nonpremixed) flame pertains to the existence of multiple synergistically coupled reaction zones. A “triple flame” is a type of partially premixed flame that contains a fuel-rich premixed reaction zone, a fuel-lean premixed reaction zone, and a nonpremixed reaction zone. The objective of this investigation is to examine gravity effects on the flame structure and flow instabilities related to partially premixed triple flames. (An earlier investigation by us dealing with gravitational effects on partially premixed double flames essentially considered steady 0- and 1-g flames.) A detailed numerical model is employed to simulate a methane-air triple flame established on a slot burner. A relatively detailed mechanism involving both C1- and C2-containing species and 81 elementary reaction steps is used to represent the CH4-air chemistry. Validation of the computational model is provided through a comparison of predictions ...