Mariko Watanabe

Coaxial jet spray flames of kerosene and oxygen are experimentally studied over a pressure range of 0.1-1.0 MPa to determine the relationship between flame structure, droplet behavior, and soot formation region, which varies with changes in pressure. The direct images and chemiluminescence spectra show that the spray flames have three regions: the blue flame region, which has a peak of CH* and C-2* radical chemiluminescence, luminous flame region caused by soot emission, and blue emission region caused by CO2 emission. With increase in ambient pressure, the flame length shortens drastically, the luminous flame region envelopes the blue flame region, and the blue emission becomes more intense. The result of phase-Doppler anemometry shows that a large number of small droplets evaporate and disappear near the burner, and the evaporation of large droplets also occurs rapidly under high pressure. The result of temperature measurements shows that high-temperature regions appear near the burner. The flame temperature drastically decreases along the central axis, and a minimum temperature point appears. This point moves upstream with increase in ambient pressure because evaporation of the droplets occurs further upstream. A laser-induced incandescence measurement shows that the soot volume fraction does not monotonously increase or decrease with increase in ambient pressure. The soot volume fraction at the central axis becomes low upstream and high downstream. As pressure increases, the vertical position at which the peak of soot volume fraction appears at the central axis moves upstream. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Coaxial jet spray flames of kerosene and oxygen are studied experimentally in the ambient pressure range of 0.1-1.OMPa. The luminescence spectra of the flames are measured by spectroscopic measurement, and the flame temperature is measured by a two-color method. Soot production region and soot volume fraction are measured by laser-induced incandescence (LII). Spectroscopic measurement results show blue chemiluminescence in the combustion reaction region and soot emission in luminous flame is observed downstream of the combustion reaction region. CO2 emission is also observed downstream of the luminous flame. The temperature measurement results show high-temperature regions near the burner. The flame temperature decreases drastically along the central axis and a minimum temperature point appears. This point moves upstream with increase in ambient pressure because vaporization of droplets occurs further upstream due to the change in droplet dispersion. LII measurements show that soot concentration increases till 0.5MPa and decreases above 0.7 MPa. This trend agrees with the spectroscopic and temperature measurements, because the fuel vapor concentration distributions change with increase in ambient pressure due to the change in droplet dispersion and mixing between fuel vapor and oxidizer. These results show that the droplet behavior changes with increase in pressure and affects flame structure and soot production.

This study conducts two-dimensional direct numerical simulations of a planar spray jet to investigate the effects of pressure on droplet behavior, especially evaporation. For the gaseous phase, Eulerian mass, momentum, energy, and species conservation equations are solved. For the disperse phase, the fuel droplets are tracked individually in a Lagrangian manner. Gaseous properties and liquid/vapor equilibrium are estimated based on ideal gas assumption because of the pressure being less than 0.5 MPa. The results show that the lifetime of a droplet changes with changes in pressure. This is because the evaporation rate of each droplet is affected by the ambient pressure and the dispersion of droplets changes as the ambient pressure increases. Moreover, the trend of the change on droplet lifetime changes with change in the ambient temperature.

Coaxial jet spray flames of kerosene/oxygen are studied experimentally in the pressure range of 0.1-1.0 MPa. The flame shapes are observed directly, and the spray cross-section is visualized using laser sheet imaging. The droplet size distributions and axial velocity components are measured by phase Doppler anemometry. Direct observation of flames indicates that, as the ambient pressure increases, the flame length decreases, the luminous flame region moves upstream, and the blue flame region at the top of the flame expands. Mie scattering images show that under high pressure, the spray region becomes narrower and shorter. The droplet mean velocity decreases as the droplets move downstream for each pressure condition however, under high pressure, a region in which the droplet mean velocity decreases moderately appears near the burner port. The droplet mean diameter increases as the distance from the burner port increases, due to a decrease in the number of small droplets and an increase in the ratio of small to large droplets caused by evaporation of the small droplets. In addition, at high pressure, a region appears in which the droplet mean diameter does not change significantly. These results show that spray flame shapes and droplet behavior are strongly affected by ambient pressure.

Two-dimensional direct numerical simulations are applied to a planar jet spray flow in order to investigate the effects of pressure on droplet behavior. For the gaseous phase, Eulerian mass, momentum, energy, and species conservation equations are solved. For the disperse phase, the fuel droplets are tracked individually in a Lagrangian manner. Concerning the vaporization of droplets, a nonequilibrium Langmuir-Knudsen evaporation model is adopted. The results show that the droplet mean diameter increases as the distance from the nozzle increases; however, because evaporation does not progress rapidly at normal temperature, the change in the droplet mean diameter with the increase in the distance from the nozzle is small for each pressure condition. The droplet mean velocity decreases as the droplets move downstream for each pressure condition, and the gradient of the droplet mean velocity toward the axial direction differs with each pressure condition.

Coaxial jet spray flames of kerosene/oxygen are studied experimentally in the pressure range of 0.1-1.0 MPa. The flame shapes are observed directly, and phase Doppler anemometry (PDA) is used to measure the size distributions and axial velocity components of droplets. Direct observation of flames indicates that the flame length decreases and the luminous flame region moves upstream with increase in ambient pressure. The droplet mean diameter increases and the droplet mean velocity decreases as the droplets move downstream under each pressure condition. In addition, the droplets mean diameter increases with increase in ambient pressure. The gradient of the mean velocity increases with increase in ambient pressure. These results show that spray flame shapes and droplet behavior are strongly affected by ambient pressure.

The numbers of ammonia-oxidizing bacteria (AOB), Nitrospira and Nitrobacter in a municipal wastewater treatment plant were examined for five months using a real-time PCR quantification technique. The numbers of AOB and Nitrospira were in the ranges of 3.8×10¹⁰-2.0×10¹¹ and 4.7×10¹⁰-1.6×10¹¹ cell · l^-1, respectively. Additionally, the fractional percentages against the number of eubacteria were in the ranges of 2.1-7.6 and 2.6-7.0 %, respectively. Nitrobacter was less than 1 % as common as Nitrospira. On the other hand, the maximum ammonia- and nitrite-oxidizing rates obtained from aerobic batch tests ranged from 0.08 to 0.41 and from 0.10 to 0.27 mmol-N · l^-1 · hr^-1, respectively. No correlation between cell number and maximum rate was observed. The maximum cell-specific ammonia- and nitrite-oxidizing rates were then estimated to be in the range of 0.53-5.6 and 1.2-5.4 fmol-N·cell^-1 · hr^-1, respectively. In other words, even in the same wastewater treatment plant, these maximum cell-specific rates were not unique. To explore the factors controlling the maximum cell-specific ammonia-oxidizing rate, the relationship with in situ ammonia-oxidizing activity per cell was investigated. A fairly good correlation was obtained. The result indicates that the amount of ammonia oxidized per cell controls the maximum cell-specific ammonia-oxidizing rate and is the primary contributor to the variation. Meanwhile, the maximum cell-specific nitrite-oxidizing rate responded to the increase in the maximum cell-specific ammonia-oxidizing rate when the number of Nitrospira was less than that of AOB.

Experimental observations and numerical simulations are conducted on combustion processes of n-decane polydisperse spray entering a gaseous flat flame stabilized in a laminar 2D counterflow configuration. For the gaseous phase, Eulerian mass, momentum, energy, and species conservation equations are solved. For the disperse phase, all individual droplets are tracked without using a droplet parcel model. The experimental results show that blue flames and luminous flames are observed and there are unsteady changes in the behavior. The numerical results show that the spray flame structures vary depending on the supplied quantities of liquid fuel spray. Furthermore, the instantaneous flame structures are consistent with the typical flame structures observed with the experiment.

Two-dimensional direct numerical simulation is applied to spray flames stabilized in a laminar counterflow, and the detailed behavior is studied in terms of the droplet group combustion. The stretch ratio of the laminar counterflow is 40 1/s. n-decane (C10H22) is used as a liquid spray fuel, and a one-step global reaction is employed for the combustion reaction model. The results show that with increasing the issued liquid fuel mass fraction, two types of spray combustion appear in front of and inside the high gaseous temperature region, i.e., "premixed-like combustion" and "diffusion-like combustion," respectively. A droplet group combustion behavior is observed in the diffusion-like combustion region. This diffusion-like combustion, however, disappears when the issued droplet size becomes small, because the droplets complete their evaporation before entering into the high gaseous temperature region. The droplet group combustion tends to reduce the gaseous temperature. This is caused mainly by the suppression of combustion reaction due to the lack of oxygen and partially by the energy exchange through the convective heat transfer between droplets and gaseous phase. The gaseous temperature reduction is promoted by the latent heat of vaporization of the droplets. The use of the parcel approach has a risk of causing a delay of combustion reaction, since the partial fuel vapor pressure increases at limited locations, which suppresses the global droplet evaporation rate.

Experimental observations and numerical simulations were conducted on combustion processes of n-decane polydisperse spray entering gaseous flat-flame stabilized in laminar 2D counterflow configuration. The experimental burner restrained the flow from fluctuating to investigate the effects of spray characteristics. Concerning the calculations, for the gaseous phase, we used Eulerian mass, momentum, energy, and species conservation equations. For the disperse phase, all the individual droplets were tracked without using a droplet parcel model. Firstly, we observed blue and luminous flames experimentally and the intensity of these flames changed unsteadily. Secondly, we examined the spray flame structure numerically should the supplied quantity of liquid fuel changed. Both timeaveraged and instantaneous spray flame structures varied depending on the quantities of spray. Furthermore, the instantaneous structures were consistent with the typical flame structures observed by the experiment. Consequently, these results show that the difference of the supplied liquid fuel spray can cause the variation of spray flame structures.

A three-dimensional numerical simulation of an isothermal flow past a solid sphere with outflow in a linear shear flow is performed to investigate the effects of the outflow on drag and shear lift. In addition, the effects of the outflow and the fluid shear on diffusion and reaction of reactant from the surface of the sphere are also discussed. The results show that the outflow reduces the drag, and, in the linear shear flow, acts to push the sphere to the lower fluid velocity side and promote the negative lift for the high particle Reynolds numbers. The diffusion and reaction of the reactant from the surface of the sphere are strongly affected by the outflow and the fluid shear because these factors cause the deformation of vortices appearing behind the sphere. (C) 2003 American Institute of Physics.