Experimental investigation of radiation transmission through a water spray

Radiation attenuation by a water spray is experimentally investigated. Spectral transmissivity measurements are performed between 1000 and with an experimental device involving a Fourier transform spectrometer. The spray is produced by a so-called Tee-Jet 400 067 nozzle for water pressure between 1.5 and 6 bar. Key features like mean attenuation levels due to absorption and scattering by droplets and complex absorption pattern by water vapor are identified. Known effect of attenuation modification when increasing the water pressure is observed. A simulation is also performed to evaluate a numerical code developed in a companion study. The achieved agreement demonstrates the ability of the simulation to describe the radiation attenuation by the spray.     

Solar absorption by Mie resonances in cloud droplets

Recent studies suggest that resonant absorption of sunlight by cloud droplets may constitute a significant and unaccounted-for solar energy sink in the atmosphere. We spectrally resolve, for the first time, all solar absorption, including sharp resonances, in typical liquid water clouds. Resolving all sharp resonances requires a resolution in size parameter χ=2πr/λ (r—droplet radius, λ—incident wavelength) of about 10^-7. The canonical integration resolution Δχ≈10^-1 produces absorption biases up to 70% over 10 nm spectral bands. Hence, neglecting Mie resonances may cause substantial biases in radiance-based retrievals from sensor channels where atmospheric absorption is particle dominated.The canonical resolution produces broadband solar mean and RMS absorption coefficient biases of about 0.02% and 4%, respectively. Self-cancellation of the pseudo-randomly distributed biases explains why the mean bias is much smaller than the RMS bias. Exceeding 1% RMS accuracy in solar absorption requires Δχ<10^-5. Increased cloud heating due to resolving all resonant absorption is less than 0.1%, equivalent to about global annual mean heating. Overlap of droplet and water vapor absorption within clouds helps diminish the net enhanced absorption by sharp resonances. Hence, the heretofore unrepresented absorption is negligible for global climate, though very important for narrow spectral regions. These results apply to homogeneous liquid water clouds and aerosols.     

Small-angle light scattering symmetry breaking in polymer-dispersed liquid crystal films with inhomogeneous electrically controlled interface anchoring

We have described the method of analyzing and reporting on the results of calculation of the small-angle structure of radiation scattered by a polymer-dispersed liquid crystal film with electrically controlled interfacial anchoring. The method is based on the interference approximation of the wave scattering theory and the hard disk model. Scattering from an individual liquid crystal droplet has been described using the anomalous diffraction approximation extended to the case of droplets with uniform and nonuniform interface anchoring at the droplet–polymer boundary. The director field structure in an individual droplet is determined from the solution of the problem of minimizing the volume density of the free energy. The electrooptical effect of symmetry breaking in the angular distribution of scattered radiation has been analyzed. This effect means that the intensities of radiation scattered within angles +θ _s and–θ _s relative to the direction of illumination in the scattering plane can be different. The effect is of the interference origin and is associated with asymmetry of the phase shift of the wavefront of an incident wave from individual parts of the droplet, which appears due to asymmetry of the director field structure in the droplet, caused by nonuniform anchoring of liquid crystal molecules with the polymer on its surface. This effect is analyzed in the case of normal illumination of the film depending on the interfacial anchoring at the liquid crystal–polymer interface, the orientation of the optical axes of droplets, their concentration, sizes, anisometry, and polydispersity.     

Stagnation-point polydisperse spray flame ignition

A theory of stagnation-point flow polydisperse spray flame ignition by an isothermal hot surface is presented for the first time. The configuration investigated consists of a mixture of fuel droplets and air flowing against an isothermal hot surface (such as a hot ignition probe). The polydisperse spray of droplets is modelled using the sectional approach. A single global chemical reaction is assumed for the case when ignition occurs. The mathematical analysis makes use of a small parameter that is exploited for an asymptotic approach. An analytical criterion for ignition is derived which includes effects of the flow field, the reactants and all the fuel spray-related parameters, including the initial size distribution of the spray's droplets. Numerical calculations disclose how the actual droplet size distribution impacts on the critical stagnation point temperature necessary to promote ignition. Additionally, the analytical estimates are compared with predictions of a numerical finite difference code with very satisfactory agreement.     

Structure and dynamics of the missing-row reconstruction on O/Cu(0 0 1) and O/Ag(0 0 1)

The geometry and the vibrational properties of missing row reconstructed O/Cu(0 0 1) and O/Ag(0 0 1) surfaces are investigated by means of density functional theory and density functional perturbation theory, using the local density and the generalized-gradient approximations. Our results predict very similar structural and vibrational properties for the two reconstructed surfaces. In the case of copper our calculations reproduce quite accurately the experimental results, while for the missing row reconstructed O/Ag(0 0 1) surface the agreement between theory and experiment is less satisfactory.     

On the impact noise of a drop falling on water

An analysis is made of the impact sound generated when a water droplet impinges on a nominally smooth air-water interface. Guo and Ffowcs Williams [Journal of Fluid Mechanics 227 (1991) 345-355] derived a mathematical representation of the sound produced in the initial period of impact of duration . The theory of this paper is a simple extension of their method that covers all of the effective life time () of the impact sound pulse. At the acoustic wavefront the predicted pressure signature in the water consists of a large impulsive peak with the directivity of an acoustic monopole source. Behind this peak lower frequency components of the sound are strongly influenced by the effective pressure-release condition on the free surface of the water, and the radiation acquires the characteristics of a much weaker dipole field (with dipole axis normal to the water interface) accompanied by a rapid decrease in wave amplitude. The dipole pressure exhibits a single cycle oscillation before decaying to evanescence about 0.1 ms after the initial impact. Predictions of the acoustic pressure spectral level are shown to be consistent with measurements available in the literature. Applications of the theory are envisaged to situations where the entrainment of air bubbles by droplets is not important, so that the impact is the dominant source of sound—for example, in estimating the contribution to the self-noise of a supercavitating vehicle from ventilating gas containing a ‘spray’ of small droplets impinging on the cavity gas-water interface.     

Particle Formation from Single Droplets of Aqueous Solutions of Lead Nitrate

The thermal evolution of droplets of aqueous solution of lead nitrate was studied in a drop-tube furnace, which simulates typical conditions for material synthesis, through spray pyrolysis, and for the thermal destruction of liquid-containing waste. The processes of droplet evaporation, precursor precipitation within the droplet and thermolysis of the precipitated particles were followed by means of the spectral analysis of the ultraviolet light scattered by the aerosol produced during the heating of aqueous droplets (100 μm) of lead nitrate, with different salt concentrations, from ambient temperature up to 1200 K. Dimensions and physico-chemical properties of the droplets/particles were obtained in situ by means of ultraviolet spectra of light scattering (UVSLS) and compared with scanning electron microscopy (SEM) of the sampled material. A plasma generated in the air by an optical breakdown induced by a Nd:YAG laser was employed as the light source in the wavelength range 200–400 nm, thus allowing an exceptionally high photon flux in the ultraviolet region where intense and species-specific interactions with metal species take place. The spray drying process was followed by measuring the light transmitted by the droplets in the backward region. As the drying process progresses, the surface concentration reaches a saturation value and solute is deposited as a solid phase forming a surface crust, which grows steadily. At this point in the process of droplet drying, information was retrieved from the light reflected by the particle interface. Two spectral scattering behaviors were detected at temperatures above the salt precipitation within the droplet. On the basis of Mie calculations and SEM measurements, these behaviors were attributed to lead nitrate particles with typical diameters of the residual droplets (about 50 μm) and to micrometer-sized lead oxide particles. The effect of salt concentration on the drying process and the thermolysis of lead nitrate to oxide was investigated by changing the salt concentration from very dilute conditions up to almost the saturation concentration.     

Direct numerical simulation of diluted combustion by evaporating droplets

Diluted combustion has been studied using DNS in a three-dimensional temporally developing reacting shear-layer with the oxidizer stream laden with evaporating droplets. The gaseous phase is described in the Eulerian frame while the discrete droplet phase is treated in the Lagrangian frame, with strong two-way coupling between the two phases through mass, momentum and energy exchange. Grid-resolution-independent results have been obtained in cases without and with droplets. A comprehensive parametric study has been conducted by varying the initial Stokes number (St₀) and mass loading ratio (MLR₀). Detailed field analysis has been conducted to examine the complex nonlinear interactions among droplet dynamics, evaporation, turbulence and combustion, and so on. Effects of evaporating droplets on averaged flow and combustion quantities have also been presented. In particular, the conditional scalar dissipation rate is found to be enhanced by evaporating droplets, which suggests that they can promote micromixing and combustion under certain conditions, in addition to their roles in combustion suppression. The transport equation for the mixture fraction variance has been analyzed, with a focus on the vaporization-related source terms. Such source terms exhibit more complex local variations in the present shear-flow non-premixed flame configuration, compared with the case in the homogeneous decaying turbulence configuration of Réveillon and Vervisch (2000).     

Simulations of a photopumped X-ray laser using the H-like Cl−Li-like Se scheme

Calculations of the modal photon densities and gain in a photopumped Cl XVII-Se XXXII X-ray laser are presented. In this paper we undertake a realistic simulation of the generation of both Cl and Se plasmas, using a high-power optical laser, which includes radiation from both Ly-α fine-structure components of H-like Cl pumping the 2p_3/2-5d_5/2 transition in Li-like Se. The calculations are performed in two dimensions in a realistic geometry taking into account plasma gradients. This gives information about the spatial extent and time evolution of X-ray lasing gain on the 5-4 transitions in Li-like Se. We find that gain (about ) is expected only when the optical laser includes a pre-pulse. Calculations show that the absorption of pumping radiation in the pumped plasma can reduce the gain by 20%. Time-dependent calculations have shown that the gain is reduced by 30% in comparison to the steady-state calculations. The effect of the spectral profile and self-radiation of 5d_5/2-2p_3/2 transition in Li-like Se reduces the gain by about 2%.     

The relationship between absorption coefficient and temperature and their effect on the atmospheric cooling rate

Several approaches are considered to determine the temperature effect on the absorption coefficient within a correlated k-distribution method. Taking in the 610- region for example, the absorption coefficients and atmospheric cooling rates calculated using these approaches are compared with line-by-line integration. It is emphasized in this paper by numerical calculation that the effect of pressure on absorption coefficient is related to temperature and vise versa; the larger the pressure, the larger the effect of temperature on absorption coefficient. Results show that the temperature effect must be considered in radiative calculations although its effect on the absorption coefficient is much smaller than that of pressure.     

The physical parameterization of the top-of-atmosphere reflection function for a cloudy atmosphere—underlying surface system: the oxygen A-band case study

The paper is devoted to the physical parameterization of the top-of-atmosphere reflection function. The accuracy of the parameterization is checked against exact radiative transfer calculations in a cloudy atmosphere for various cloud-top-heights, cloud optical and geometrical thicknesses, solar illumination and surface reflection conditions. It was found that the error of approximation is smaller than 5% for most cases studied at the wavelength interval , which corresponds to the oxygen A-band. This band is routinely used in cloud-top-height retrievals. The model proposed can be used for cloud-top-height and cloud geometrical thickness retrievals. This allows to avoid a standard look-up-table retrieval scheme, involving complex numerical procedures.     

Discharging fused silica test masses with ultraviolet light

A Kelvin probe and tunable light source were used to study fused silica discharging by UV illumination. The discharge rate scales linearly with charge magnitude and illumination intensity. Optimal discharging occurs at 215 nm, likely due to absorption by E^′ centers. 90% discharge corresponds to incident energy.     

Mid-infrared absorption measurements of liquid hydrocarbon fuels near

Quantitative absorption spectra for several hydrocarbon fuels in the liquid phase at are presented. Measurements of toluene, n-dodecane, n-decane, and three samples of gasoline were made over the spectral region 2700–3200 to support the development of mid-infrared laser-absorption diagnostics for measurements of fuel vapor in the presence of liquid films and aerosols. A procedure for quantitative Fourier transform infrared (FTIR) absorption measurements of strongly absorbing liquids is described and the resulting absorption spectra are compared with previously measured absorption spectra in the vapor phase. The measured absorption spectra for liquid gasoline are shown to scale with the volume percent of olefin, alkane, and aromatic hydrocarbons in each sample. Finally, the observed frequency shift of in the spectra of vapor and liquid hydrocarbons is discussed, including the potential for measurements of fuel vapor in the presence of liquid films.     

Radiative heat transfer analysis in pure water heater used for semiconductor processing

A simplified one-dimensional model is presented to analyze the non-gray radiative transfer in pure water heater used in the rinsing processes within semiconductor production lines, and the ray-tracing method is extended to simulate the radiative heat transfer. To examine the accuracy of the simplified model, the distribution of radiation absorption is determined by the ray-tracing method based the simplified model and compared with the data obtained by three-dimensional non-gray model in combination with Monte Carlo method in reference, and the effects of the water thickness on the radiation absorption are analyzed. The results show that the simplified model has a good accuracy in solving the radiation absorption in the pure water heater. The radiation absorption increases with the water thickness, but when the water thickness is greater than , the radiation absorption increases very slowly with the water thickness.     

Attenuation of solar radiation by a water mist from the ultraviolet to the infrared range

A model is developed for the hemispherical transmittance of direct and scattered solar radiation from a cloudless atmosphere by a mist layer of water droplets in order to investigate the potential of water misting systems to serve as a protection from solar irradiation with particular emphasis on harmful UV radiation. The proposed model is based on published spectral experimental data for solar irradiation, Mie theory for interaction of the radiation with single spherical droplets, and radiative transfer theory. Known limiting solutions are employed to simplify the Mie calculations. The modified two-flux approximation is used to account for both direct and diffuse irradiation in lieu of a numerical solution for the full radiative transfer equation in anisotropically scattering media. The role of the governing parameters of a disperse water curtain of water droplets, water content, and droplet size for sample conditions is studied in some detail, particularly in the near-ultraviolet part of the spectrum where radiation can result in human tissue damage.     

Density functional theory study of the near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene on group IV semiconductor surfaces

The near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene adsorbed on C(1 0 0)-2 × 1, Si(1 0 0)-2 × 1 and Ge(1 0 0)-2 × 1 surfaces is studied with density functional theory calculations. Time dependent density functional theory calculations of the near edge X-ray absorption fine structure with a modified exchange-correlation functional agree well with experiment, and show that the spectral features arise from excitation to π^∗, and orbitals, where X represents C, Si or Ge. The excitation energies are dependent on the surface, and for acetylene, the location of the π^∗ band also varies with the surface. Calculations of the vibrational modes show the CH stretching frequencies for carbon atoms bonded directly to the surface vary significantly between the three surfaces, while those for carbon atoms not bonded to the surface do not change significantly.     

Investigation of the water-vapor continuum in the THz region using a multipass cell

Laboratory absorption measurements of the water-vapor continuum in the far infrared region from 12 to (0.4 - 1.83 THz) were obtained using a multipass absorption cell, a Fourier transform spectrometer and a liquid-He-cooled bolometer detector. Measurements were made at a temperature, with water vapor and nitrogen pressures up to 2.2 and , respectively. The effects of the choice of lineshape function and far-wing cut-off factors on the reported continuum absorption are analyzed by modeling the resonant water-vapor spectrum using van Vleck-Weisskopf and Lorentzian lineshapes. Comparisons with available microwave data and model calculations are also presented.