S.H. Chung

On the characteristics of laminar lifted flames in a nonpremixed jet

Abstract Characteristics of laminar lifted flames stabilized in a nonpremixed jet issuing from a small nozzle into ambient air have been studied experimentally. Results show that near the flame base the lifted flame has a tribrachial structure composed of a lean premixed flame, a rich premixed flame, and a diffusion flame extending downstream from the same point. A simple theoretical formula for the liftoff height of nonpremixed jet flames based on a boundary layer type of jet theory predicts that the liftoff height increases with increasing flow rate for Schmidt numbers in the range Sc > 1 or Sc 1) have lifted flames while methane and ethane (0.5

Stabilization of lifted tribrachial flames in a laminar nonpremixed jet

Abstract The stabilization mechanism of lifted flames in a laminar nonpremixed jet has been analyzed and experimentally investigated. An analysis on the flame response by the perturbation from the tribrachial location shows that the lifted flame is unstable for Sc

Soot zone structure and sooting limit in diffusion flames: Comparison of counterflow and co-flow flames

Soot zone structures of counterflow and co-flow diffusion flames have been studied experimentally using the soot extinction-scattering, polycyclic aromatic hydrocarbon fluorescence, and laser Doppler velocimetry measurements. The counterflow flame has been numerically modelled with detailed chemistry. Results show that two different categories of sooting flame structures can be classified depending on the relative transport of soot particles to flames. These are the soot formation-oxidation flame and the soot formation flame. The soot formation-oxidation flame characteristics are observed in counterflow flames when located on the fuel side and in normal co-flow flames. In this case, soot particles are transported toward the high temperature region or the flame and experience soot inception, coagulation-growth, and oxidation. The soot formation flame characteristics are observed in counterflow flames when located on the oxidizer side and in inverse co-flow flames. In this case, soot particles are transported away from the flame without experiencing oxidation and finally leak through the stagnation plane in counterflow flames or leave the flame in inverse co-flow flames. Sooting limit measurements in both flames also substantiate the two different sooting flame structures and their characteristics.

Synergistic effect of ethylene-propane mixture on soot formation in laminar diffusion flames

Sooting characteristics in laminar diffusion flames have been studied experimentally for ethylene–propane mixtures, and a synergistic effect from mixture fuels has been observed. Soot volume fraction and polycyclic aromatic hydrocarbon (PAH) concentration are increased when a small amount of propane is added to ethylene diffusion flames. Considering C2H2 concentrations and adiabatic flame temperatures, the experimental results indicate that H-abstraction–C2H2-addition (HACA) reaction alone is insufficient to explain soot inception and growth processes in diffusion flames of ethylene–propane mixtures. The synergistic effect of fuel mixture is discussed, emphasizing the importance of C3 recombination reaction during the initial ring formation process for incipient PAHs. The results of soot number density and particle size in low-temperature soot growth region also support continuous soot inception via PAH agglomeration. Therefore, it is suggested that PAHs are directly involved in the soot particle inception processes. The synergistic effect is less significant for ethylene–ethane mixtures compared to ethylene–propane mixtures.

Physical Activity and Physical Fitness as Predictors of All-Cause Mortality in Korean Men

We examined the associations between physical activity (PA), fitness and all-cause mortality and compared their contributions, taking smoking status into consideration. A retrospective cohort study of 18,775 men was carried out between May 1995 and December 2003. Fitness was measured by maximum oxygen uptake and regular PA was defined as at least three times a week, for more than 30 min of leisure time PA. During the mean 6.4 yr of follow-up, 547 deaths were recorded. The hazard ratio (HR) (95% confidence interval [CI]) of regular PA for all-cause mortality was 0.63 (0.52-0.76). The HRs (95% CIs) for men with middle and highest tertile levels of fitness were decreased by 0.58 (0.47-0.70) and 0.58 (0.45-0.75) in comparison to men with lowest one. The inverse association between fitness and mortality was significant among the men who did not engage in regular PA, but not among those who did (p for interaction=0.031). Smoking status did not influence on the associations between regular PA, fitness and mortality. Our result suggested that regular PA and fitness predicted mortality in men. The influence of fitness on mortality was pronounced in the men who did not engage in regular PA.

Effects of oxygen and propane addition on soot formation in counterflow ethylene flames and the role of C3 chemistry

The influence of oxygen addition and propane addition on soot formation is investigated experimentally in counterflow ethylene diffusion flames, and numerical calculations with detailed chemical reaction mechanisms are conducted to interpret experimental results. Soot volume fractions and polycyclic aromatic hydrocarbon (PAH) concentrations increase with a small amount of oxygen addition for ethylene diffusion flames but not for propane diffusion flames. This result supports that the reactions between O atoms and C 2 H 2 produce CH 2 and eventually form C 3 H 3 . which are responsible for the enhancement of incipient rings and thereby PAH and soot formation through C 3 H 3 recombination reaction. When a small amount of propane is added to ethylene diffusion flames, PAH concentrations and soot volume fractions also increase. However, the simultaneous addition of oxygen and propane in ethylene diffusion flames shows that the observed soot-formation enhancement with propane addition to ethylene diffusion flames diminishes with oxygen addition to fuel stream and eventually disappears when sufficient amount of oxygen is added. This implies that the mechanisms responsible for the soot-formation enhancement with propane and oxygen addition to ethylene flames can be explained based on the same chemistry. Numerical results on C 3 H 3 concentrations can explain successfully both the effects of oxygen and propane addition in ethylene flames, through which the importance of C 3 chemistry on soot formation has been demonstrated.

Asymptotic structure and extinction of COH2 diffusion flames with reduced kinetic mechanisms

Abstract Models are considered for diffusion flames involving a fuel consisting of mixtures of carbon monoxide, hydrogen, and nitrogen and an oxidizer consisting of mixtures of oxygen and nitrogen. Initially the kinetic scheme is reduced systematically to the two-step mechanism CO + H 2 O ⇄ CO 2 + H 2 , 2H 2 + O 2 ⇄ 2H 2 O, the water-gas shift, and hydrogen oxidation. In a model for low strain rates there is a broad region of water-gas equilibrium, bounded on the fuel-lean side by a thin zone of hydrogen oxidation and on the fuel-rich side by a thin zone of sudden water-gas freezing caused by chain-carrier removal. Structures of each of these thin zones are analyzed with the aid of asymptotic methods, and it is shown that a three-step mechanism is needed to obtain a reasonably accurate description of the water-gas freezing. In these analyses simplified transport descriptions are employed in which only the Lewis number of hydrogen is allowed to differ from unity. In the model for high strain rates the oxidation of carbon monoxide is nearly frozen while hydrogen oxidation occurs in a broad zone throughout which the reaction rate is approximated as constant. Many predictions of these simplified models are found to agree surprisingly well with results of full numerical integrations for flame structure and extinction.

Analysis of growth of non-spherical silica particles in a counterflow diffusion flame considering chemical reactions, coagulation and coalescence

Abstract The evolution of non-spherical silica particles in a counterflow diffusion flame has been studied considering the effects of convection, diffusion, thermophoresis, chemical reactions, coagulation and coalescence. The counterflow geometry provides a one-dimensional flow field along the stagnation point streamline which greatly simplifies the analysis of non-spherical particle growth. Flame analysis of multi-step chemical reactions of hydrogen/oxygen including both oxidation and hydrolysis of SiCl 4 has been done to predict flame temperatures, concentrations of gas species and particle generation. The present prediction of flame temperatures was in good agreement with the previous experimental data. Two-dimensional aerosol dynamics in which both particle volume and surface area are independent variables has been then analyzed to obtain the evolution of non-spherical particles which has been compared with the previous experimental data. Several different models of coalescence of silica particles were studied; viscous flow sintering, atomistic diffusion sintering, fast sintering and hybrid sintering models. The use of hybrid sintering model yielded the best agreement with the previous experimental data. Since the collision cross section of non-spherical particles is larger than that of spherical particles having the same volume, coagulation of particles was obviously shown to be enhanced. The important role of axial particle diffusion has been identified in the counterflow diffusion flame. Bi-modal size distributions were obtained at some flame heights.

On the structure and extinction of interacting lean methane/air premixed flames☆

Abstract Lean methane/air premixed flames are studied numerically, using a detailed chemical model of 74 reaction steps with 28 species, to investigate the flame interaction between two stretched premixed flames with unequal intensities in a counterflow. The finite difference method, time integration and modified Newton iteration are used, and adaptive grid technique and grid smoothing have been employed to adjust the grid system according to the spatial steepness of the solution profiles. The flame structure of the methane premixed flames has four layers, consisting of the radical cutoff layer, the main heat release layer of the methane decomposition, the radical branching layer and the hydrogen and carbon monoxide oxidation layer. Two modes of flame interaction, the strong and weak interactions depending on the differences in the flame intensities, and their respective behaviors are identified. It has been found that heat loss and incomplete reaction are responsible for the flame extinctions in the weak and strong interaction regimes, respectively. The calculated extinction boundaries are in good agreement with the experimental results.

Measurements of temperature and OH radical distributions in a silica generating flame using CARS and PLIF

Distributions of flame temperature in silica generating coflow diffusion flames have been measured using coherent anti-Stokes Raman spectroscopy (CARS) and qualitative measurements of OH radical concentrations have been made utilizing planar laser-induced fluorescence (PLIF). Silica particles have been visualized through the plane images of light scattering from particles. The results show that aerosol generation due to chemical reactions can significantly affect thermal and chemical characteristics of hydrogen–oxygen diffusion flames. When SiCl4 is added to a flame, the temperature in non-reacting zone decreases due to the increases in both specific heat and density of the gas mixture while the flame temperature increases in the particle formation zone due to exothermal chemical reactions of hydrolysis and oxidation of SiCl4. It has also been found that OH concentration decreases dramatically in the particle formation zone, which can be attributed to both the generation of HCl and the consumption of O2 and H2O during silica formation.