Quiescent Atmosphere Research Articles

Modelling of Atomization and Vaporization Process in Flash Boiling Spray. 1st Report, Change in Atomization Characteristics with Back Pressure.

In this study, the influence of the flash boiling phenomenon on the fuel spray characteristics was in vestigated using both experiments and model analysis. Pure liquid fuel of n-Pentane or n-Hexane is injected into a vessel under quiescent gaseous atmosphere at room temperature through a pintle-type injector. Fuel sprays are observed by taking photographs for variation of ambient back pressure. In particular, changes in spray characteristics with back pressure are examined in detail. The results show that the saturated vapor pressure of fuel is the most significant factor and the spray characteristics can be varied with the pressure difference between the back pressure and saturated vapor pressure. Furthermore, flow field of the fuel inside the nozzle is estimated with regard to bubble initiation inside the film flow.

Observation of nitric oxide rovibrational band head emissions in the quiescent airglow during the Cirris‐1A Space Shuttle Experiment

Band head emissions from highly rotationally excited NO (v,J) (J≈90) have been observed in the quiescent atmosphere at tangent heights between approximately 115 and 190 km for both sunlit and nighttime conditions. The data were obtained by the cryogenic CIRRIS‐1A interferometer which was operated on‐board the space shuttle between 28 and 30 April 1991. Up to ten emission features between 2020 and 1744 cm−1 in the earthlimb spectra have been identified as the R‐branch band heads of the NO Δv=1 sequence for the vibrational states v′ = 1 – 10. The presence of these band heads requires a very high degree of rotational excitation corresponding to rotational energy in excess of 1.4 eV. These are the first observations of emissions from highly rotationally excited NO in the quiescent airglow and they parallel the recent discovery of highly excited pure rotational transitions of OH in the airglow for both nighttime and daytime quiescent conditions.

Enhancement of Fuel-Air Mixing in Diesellike Flames via Flame Impingement on a Turbulence-Generating Plate.

Soot concentration is very high in the periphery near the head of an unsteady spray flame which is achieved in a quiescent atmosphere in a rapid compression machine. To reduce soot concentration in this region and enhance combustion, it was intended to improve fuel- air mixing in the flame head by letting the flame impinge on a turbulence-generating plate. Two types of turbulence-generating plates, one of the donut type and the other of the cross type, were tested. Soot concentration in the flame was imaged using the laser shadow technique. The overall soot concentration was remarkably reduced in the case in which the flame impinged on the cross type plate at 333 nozzle diameters from the nozzle exit.

An approximate model of atmospheric plumes produced by large area fires

We develop a simple analytical model to estimate the thickness of a smoke layer formed by a plume of a large area fire and to account for crosswinds. We take advantage of the dominant flow features in the upper part of the rising plume and in the smoke layer far from the plume to model the transition of a vertical flow to a horizontally spread thin smoke (or dust) layer. A one-dimensional approximation is used to model the buoyancy decay and plume overshoot. That solution is matched and combined with a solution based on the work required to establish our asymptotic horizontal smoke layer. The solution estimates the thickness of the spreading smoke layer in a quiescent atmosphere or in an ambient wind field.

On Drying of Laundry

When wet laundry is hung from a clothesline to dry in a quiescent atmosphere, the fabric first dries at the top, and the dry part subsequently extends downward. In this paper, the drying process is modelled mathematically with special reference to its moving boundary aspect. The model is used to derive an explicit formula for the time needed for a given part of the fabric to dry, and the results are compared with those of a simple experiment.

NLTE processes in ozone: Importance of O and O 3 densities near the mesopause

Laboratory studies and atmospheric experiments have shown that Non Local Thermodynamic Equilibrium (NLTE) processes must be considered to explain the infrared emission of ozone above 70km for a quiescent atmosphere. For these altitudes, in the calculation of ozone limb infrared radiances, a complete kinetic model describing the coupling between radiative and relaxation processes must be used. In this study a narrowband statistical radiative transfer model, combined with a kinetic model, is proposed providing calculations of NLTE infrared limb emission by ozone with good accuracy. Moreover, because the calculations are very fast, contrary to models based on line-by-line calculations, it has been possible to assess the influence of the various atmospheric input parameters which are of importance for modelling available experimental data such as SPIRE (Green et al, 1986).

Visualization of Unsteady Impinging Spray by Means of Exciplex Fluorescence Method.

An unsteady single spray of n-decane which was miscible with a small quantity of exciplex-forming dopants was impinged on a flat wall with high temperature at a normal angle in a quiescent atmosphere in the experiment presented here. It was possible to generate fluorescent emissions from vapor and liquid phase in this evaporating and impinging spray, when a thin sheet of laser light from Nd : YAG laser was passing through the cross section containing the central axis of this spray. Then, clear 2-D images of vapor and liquid in the spray were acquired by this method distance between the outlet of the injection nozzle and the wall.

Characteristics of Spray Injected from Gasoline Injector. 2nd Report. Effect of Back Pressure.

This paper discuses the atomization mechanism of a spray injected into a low-pressure field, as the focus of an injection system in a suction manifold of gasoline engines. Pure liquid fuel, which is n-Pentane or n-Hexame, is injected into a quiescent gaseous atmosphere at room temperature and low pressure through a pintle-type gasoline injector. Fuel sprays are observed through photographs with varying the back presure, and the changes in spray characteristics with the back pressure below atmospheric pressure are examined in detail. In particular, in the case of back pressure below the saturated vapor pressure of fuel, atomization mechanism is discussed from the viewpoint of the flash boiling phenomenon. Spray characteristics can be obtained with the pressure difference between the back pressure and the vapor pressure, corresponding to the intensity of the flash boiling, for various fuels.

Characteristics of Spray Injected from Gasoline Injector. 1st Report. Macroscopic Behavior of Spray.

In the previous studies, the experiments on a spray injected through an electronic control injector were mainly carried out in a practical engine. The condition in this case could not always clarify the phenomenon of atomization. In the experiment presented here, the fuel oil was injected into a constant volume chamber with the quiescent atmosphere to investigate the effect of this surrounding pressure on atomization process. The fuel oil, Shell-LAWS and n-pentane were used. The spray was imaged onto 35-mm film. As a result, the surrounding pressure has great influence on the growth of spray and the pattern of atomization in the spray.

IGNITION DELAY OF DROPLET CLOUDS: RESULTS FROM GROUP COMBUSTION THEORY

Abstract The ignition and evaporation of spherical cloud of droplets in a hot quiescent atmosphere is examined numerically using transient group combustion analysis. Ignition delay times are calculated as a function of cloud radius, ambient temperature, drop size and droplet number density. The ignition temperature for a cloud of drops was found to be less than that obtained from a single drop. The results indicated an interaction between chemical and physical effects resulting in the possibility of an optimal interdrop spacing for ignition of a fuel with a high boiling point. The model results indicate that for interdrop spacing to radius ratio of less than 5, the ignition and evaporation of a cloud of drops is confined to a thin layer at the surface of the cloud. For drops spaced farther apart thermal penetration from the hot ambient is possible resulting in vaporization within the cloud.

Source height determination by ground effect inversion in the presence of a sound velocity gradient

A propagation inversion method has been devised and tested that enables determination of height of a sound source which is above an acoustically soft ground. The horizontal separation between the point source and receivers is assumed to be a priori knowledge. The proposed algorithm is site adaptive since it makes use of the predicted influence of the ground on near-grazing propagation over flat, continuous ground surfaces and of the predicted influence of a stable refracting atmosphere. Stable refraction has been included in the analysis by assuming a linear sound velocity gradient in the otherwise quiescent atmosphere. This represents a first step towards an atmospheric equivalent of the matched field processing technique used in underwater acoustics. Extensive measurements have been carried out over two test sites with different ground impedance characteristics. The spectra of the acoustic signals used in the analysis were between 50 Hz and 2000 Hz and the grazing angle of each measurement varied from 5° to 8°. The algorithm has been shown to allow accurate deduction of the source height from a priori knowledge of the range where the range varies from 50 m to 100 m in one site and from 130 m to 175 m in the other site.

A Study of the Flame Structure and Soot Formation in Unsteady Spray Flames via 2-D Soot-Scattering Image Analysis.

The formation and extinction processes of soot particles in an unsteady spray flame in a quiescent atmosphere were investigated using a 2-D laser sheet technique. The central cross section of a flame was illuminated twice in a short time interval by laser sheets from a double-pulsed YAG laser, and two images of scattered light from soot particles were taken by two different intensified gated cameras. The velocity vectors of soot clouds at various locations in the sooting regions were estimated using the spatial cross correlation between two images. The results of the temporal and spatial variation of velocity and scattering intensity in the evolving soot clusters made it clear that soot is mainly formed in the periphery of the flame tip where the air entrainment is less and the temperature favors soot formation.

A New Method of Modeling Underexpanded Exhaust Plumes for Wind Tunnel Aerodynamic Testing

A new method for modeling hot underexpanded exhaust plumes with cold model scale plumes in aerodynamic wind tunnel testing has been developed. The method is applicable to aeropropulsion testing where significant interaction between the exhaust and the free stream and aftbody may be present. The technique scales the model and nozzle external geometry, including the nozzle exit area, matches the model jet to free-stream dynamic pressure ratio to full-scale jet to free-stream dynamic pressure ratio, and matches the model thrust coefficient to full-scale thrust coefficient. The technique does not require scaling of the internal nozzle geometry. A generalized method of characteristic computer code was used to predict the plume shapes of a hot (γ = 1.2) half-scale nozzle of area ratio 3.2 and of a cold (γ = 1.4) model scale nozzle of area ratio 1.3, whose pressure ratio and area ratio were selected to satisfy the above criteria and other testing requirements. The plume shapes showed good agreement. Code validity was checked by comparing code results for cold air exhausting into a quiescent atmosphere to pilot surveys and shadowgraphs of model nozzle plumes taken in a static facility.

Vertical aerosol structure of Neptune: Constraints from center-to-limb profiles

We present and analyze center-to-limb (CTL) profiles of the equatorial region of Neptune, extracted from images with good spatial resolution. Models of the quiescent atmosphere of Neptune are consistent with at least two distinct cloud layers, a thin stratospheric haze and a deep cloud. The 8900-Å CTL profile requires the presence of some material above the 5-mbar level. Assuming isotropic scattering, the optical depth of the material is less than 0.15 for hazes with single scattering albedos larger than 0.6. CTL profiles at three methane bands could not be fitted simultaneously with a single scattering layer, implying that there may be multiple haze layers. The methane-band data tolerate thin scattering hazes in the pressure level from 0.4 to 1.5 bars, but the optical depths must be less than 0.05 (again, assuming isotropic scattering). Such a thin haze does not provide sufficient reflected flux at 6340 Å (a continuum region), indicating the presence of another scattering layer deeper than 1.5 bars. A bright cloud layer (single scattering albedo near 1) with optical depth 3.0 at 2.6 bars provides a good fit to 6340-Å reflectivity. Superposed on the quiescent atmosphere are discrete bright features. The lack of detectable disk-integrated variability at shorter wavelengths helps constrain the vertical aerosol structure of these bright regions. Specifically, the difference of the bright feature from its surroundings in 1986 was probably caused by an increase in optical depth or height of a stratospheric haze layer.

Liquid supercoolings and droplet cooling rates of remelted argon-atomized Fe–30Ni powder particles

Individual particles of argon-atomized Fe-30Ni powder are electrodynamically levitated and remelted by a CO2laser pulse. The thermal history of each droplet during remelting and solidification is monitored by single-color radiation pyrometry at each of three wavelengths (850, 750, and 550 nm). Experiments are done in an atmosphere of either air or nitrogen. The average supercooling of six experiments performed in nitrogen is 298 K with a standard deviation of 14 K. This value is of the same order as several others reported in the literature using bulk levitation and emulsification techniques. The average supercooling of seven experiments performed in air is 163 K with a standard deviation of 20 K. The difference suggests that oxides are forming in the air-remelting experiments and catalyzing nucleation at relatively low supercoolings. The average cooling rate of the liquid droplets prior to solidification in nitrogen is 1.5 × 105K/s. This measured cooling rate is somewhat higher than that predicted by Newtonian heat flow modeling, and the difference is attributed to radiative losses not considered in the Newtonian model. The measured cooling rate is used to estimate the total heat transfer coefficient characterizing cooling of a small metal droplet in a quiescent gas atmosphere. A lower bound of 1.5 × 106K/s on the droplet heating rate during recalescence and a minimum average liquid/solid interfacial velocity during recalescence of 0.1 m/s are estimated.

非定常気体噴流に関する研究

In the Gas Injection Diesel Engine (GIDE), the mixture formation process between the injected fuel gas and air plays an important role on combustion and consequently on the engine performance. This paper deals with the characteristics of a transient gas jet as a fundamental study of the fuel gas jet in the GIDE. Helium was injected instantaneously into a quiescent atmosphere with constant injection pressure. The instantaneous distribution of static pressure, velocity and concentration in the jet, which is obtained through the statistical data processing, are discussed to express a transient mixure formation process in the jet. Results are also applied to discuss the ignition and combustion phenomena in the GIDE.

Vertical wall fire in a stratified ambient atmosphere

A mathematical model is presented for understanding the characteristics of a burning vertical wall immersed in a quiescent ambient atmosphere having nonuniform vertical distributions of temperature and oxidizer mass fraction. Such a stratified atmosphere occurs, for example, in the case of a room or aircraft cabin on fire. A set of partial differential equations and suitable boundary conditions describing a laminar boundary layer flow of exothermically reacting gases is solved using a finite difference method. Results for the local burning rate and maximum vertical velocity in the boundary layer are presented for many different cases of nonuniform ambient temperature and ambient oxidizer mass fraction. A comparison of these results with the results for the nonstratified atmosphere shows that the predicted burning rate for a thermally stratified case behaves like a linear combination of results for corresponding nonstratified cases; however, this is not true for compositionwise stratified cases. The stratification has a substantial effect on the velocity in the boundary layer.

Free convective combustion with variable properties

This paper treats the problem of free convective combustion of near vertical fuel surfaces in quiescent oxidant atmosphere, both theoretically and experimentally. The theory improves on existing theories in terms of taking into account variable thermodynamic and transport properties. The locally similar solutions obtained numerically are compared with earlier predictions as well as experiments on mass burn rate, flame stand off and other features. While comparison in the case of some fuels seems bettered by the use of variable properties, the not-too good a comparision in other cases is traced to experimental inaccuracies more particularly related to the non-achievement of steady combustion. To remedy this, an experimental apparatus was carefully designed and the results of these experiments show good comparison with theoretical predictions in all cases considered.

Study of fuel distribution in diesel spray. Effects of nozzle diameter, injection pressure and ambient gas density.

The fuel distribution in a diesel spray seriously affects the performance of diesel engines. Nozzle diameter, injection pressure and ambient gas density are important factors, which affect the atomization, penetration and fuel distribution. Therefore, effects of those factors on fuel distribution in a diesel spray were investigated in the cold and quiescent atmosphere. Fuel concentration was deter-mined by the image analysis of shadow photographs of spray. As a result, the following was revealed. In the case of a smaller nozzle diameter and higher injection pressure, the mixing rate of fuel and ambient gas is greater. In similarly designed diesel engines with various power ranges, by normalizing the dimension of engines by nozzle diameter, it is supposed that the shape and fuel distribution of sprays will be similar at same crank angle.

Droplet Size Distribution in Diesel Fuel Spray

Measurments on the behavior of diesel fuel spray from a microscopic viewpoint are important to clarify atomization mechanism and temporal change in droplet size distribution. In this study, diesel spray injected from diesel nozzles into a quiescent atmosphere at room temperature and high pressure is observed by means of a direct microscopical photographing method varying the photographing time and the nozzle type. The results show that the mechanism of spray atomization is divided into 4 processes, and spatial distributions of breakup droplets are assessed. As time passes, spray droplets are distributed in a stratiform shape. And total distribution of droplet sizes and droplet size distribution in local region are expressed in equations as a function of time from injection, and changes in the distribution with the nozzle type are discussed. Subsequently, temporal changes in the uniformity of the distribution and in spatial distribution of droplets volume rates are clarified.