Water Droplets In Air Research Articles

Influences of pressure on reduced-gravity combustion of HAN–methanol–water droplets in air

Reduced-gravity experiments were performed to investigate combustion characteristics of individual monopropellant droplets composed of hydroxylammonium nitrate (HAN), methanol, and water. The experiments were conducted using the 2.2 Second Drop Tower at the NASA John Glenn Research Center at Lewis Field in Cleveland, OH. Gravitational levels were about 10 −4 times normal gravity and experiments were performed in air at about 25 °C and with pressures from 0.1 to 1 MPa. Droplets initially in the 1-mm size range were supported on quartz fibers and ignited using a hot wire that heated the gas phase. Initial droplet compositions varied from zero (initially pure methanol droplets) to a stoichiometric mixture of 69.4% HAN, 15.2% water, and 15.4% methanol by mass. Results indicated that increasing the pressure increased burning rates, delayed extinction, and promoted easier ignition of droplets. Decreasing the initial mass fraction of methanol reduced burning rates, increased the difficulty of ignition, and promoted gas-phase flame extinction. Internal bubbling was observed at certain initial droplet compositions. Aerosol formation was observed for higher HAN loadings at elevated pressures after the visible gas-phase flame had extinguished, which may be indicative of condensed-phase HAN reactions.

Heterogeneous Oxidation by Ozone of Naphthalene Adsorbed at the Air-Water Interface of Micron-Size Water Droplets

The mass transfer of naphthalene vapor to water droplets in air was studied in the presence of ozone (O3) in the gas phase. A falling droplet reactor with water droplets of diameters 55, 91, and 182 μm was used for the study. O3 reacted with naphthalene at the air-water interface, thereby decreasing the mass transfer resistance and increasing the rate of uptake of naphthalene into the droplet. A Langmuir-Hinshelwood reaction mechanism at the air-water interface satisfactorily described the surface reaction. The first-order surface reaction rate constant, ks, increased with decreasing droplet size. Three organic intermediates were identified in the aqueous phase as a result of ozonation of naphthalene at the surface of the droplet indicating both peroxidic and nonperoxidic routes for ozonation. The presence of an organic carbon surrogate (fulvic acid) increased both the partition constant of naphthalene and the surface reaction rate of O3. The heterogeneous oxidation of naphthalene by O3 on the droplet was 15 times faster than the homogeneous oxidation by O3 in the bulk air phase, whereas it was only 0.08 times the homogeneous gas-phase oxidation by hydroxyl radicals under atmospheric conditions.

Numerical simulation of binary liquid droplet collision

A numerical investigation of binary droplet collision has been conducted. The complete process of the collision of two liquid droplets is dynamically simulated by solving the incompressible Navier-Stokes equations coupled with the convective equation of the level set function that captures the interface between the liquid and the gas phases. The simulations cover four major regimes of binary collision: bouncing, coalescence, reflexive separation, and stretching separation. For water droplets in air, the numerical results are compared with the experiments by and Ashgriz and Poo [J. Fluid Mech. 221, 183 (1990)] on collision consequences. For hydrocarbon (C14H30) droplets in nitrogen gas, the simulated results are compared in detail with the time-resolved photographic images of the collision processes obtained by Qian and Law [J. Fluid Mech. 331, 59 (1997)] in every collision regime. The present numerical results suggest that the mechanism of a bouncing collision is governed by the macroscopic dynamics. However, the fact that the present macroscopic numerical model is unable to capture the collision regime of coalescence after minor deformation supports the speculation that its mechanism is related to the microscopic dynamics. Furthermore, the transition from bouncing to coalescence collisions has been predicted and agrees well with the analytical model. The mechanism of satellite droplet formation for head-on collision and stretching separation collision is also studied based on the detailed time-resolved dynamic simulation results. It is then confirmed that end pinching is the main cause of satellite formation in head-on collisions whereas the capillary-wave instability becomes dominant in large impact parameter cases. In the case of an intermediate impact parameter, the effects of twisting and stretching due to the angular momentum and the inertia of the colliding droplets are significant for the satellite formation.

Single light scattering: Bubbles versus droplets

The intensity and polarization characteristics of scattered light beams are studied for bubbles in water and compared to the case of water droplets in air. The size distributions of the scatters in both systems are assumed to be the same. It is shown that spherical polydispersions of water droplets give generally larger depolarization effects than bubbles, leading to larger entropy production by droplets than bubbles.

Optical Trapping of a Growing Water Droplet in Air

A water droplet, which was formed as a nucleation center from an aerosol of ammonium chloride, was trapped by a converged c.w. IR laser (λ = 1064 nm) using a 100× objective lens, and the successive growth of the droplet was observed under supersaturated water vapor. The size of the droplet increased linearly with time and its maximum radius was 5.7 μm at a laser at 5 mW, indicating that the axial trapping efficiency Q was 0.46. This efficiency is much greater than those reported previously; for example, according to Ashkin and Dziedzic [Science 1975, 187, 1073], Q = 0.08 for a glycerol droplet of radius 6 μm at 40 mW (λ = 514.5 nm).

A one-dimensional numerical model of heat and mass transfer in air-water droplet flow

A one-dimensional numerical model of heat and mass transfer in air-water droplet flow

A one-dimensional numerical model of heat and mass transfer in air-water droplet flow

This paper presents a one-dimensional numerical model of the heat, mass and momentum transfer between water droplets and humid air in the disperse process device. The model merges a model of the disintegration of a water sheet presented by Dombrowski et al. [1] and a one-dimensional numerical model of the transport phenomena in the air washer presented by Demirdžic et al. [10]. Numerical results are compared with the available experimental results. Numerical simulations can predict one-dimensional analysis of the geometrical and thermophysical parameters in the disperse process device.

HEAT TRANSFER IN TWO PHASE FLOW OVER A FLAT PLATE

The theoretical model of heat and mass transfer during laminar two phase (air-water) flow along a flat plate is carried out. The governing continuity and energy equations with its boundary conditions are written for the free stream of two phase flow , the air water droplets mixture boundary layer and the water film over the plate. The equations are given for determining;the mass flux of water evaporation from film surface, the convective heat flux and the mass flux of droplets depositing on the flat plate. The analysis shows high flux resulting from the superposition of the film evaporation process from the plate surface and the heat transfer by convection. The convective heat flux appears higher than during dry air flow due to the enhancement effect of the droplets. Experimental work is carried out. The comparison between the results obtained in the theoretical model and those obtained from the experimental results is presented and it was found an agreement.

Theoretical determination of net radiation force and torque for a spherical particle illuminated by a focused laser beam

Series expressions for the net radiation force and torque for a spherical particle illuminated by an arbitrarily defined monochromatic beam are derived utilizing the spherical-particle/arbitrary-beam interaction theory developed in an earlier paper. Calculations of net force and torque are presented for a 5-μm-diam water droplet in air optically levitated by a tightly focused (2 μm beam waist diameter) TEM00-mode argon-ion (λ=0.5145 μm) laser beam for on and off propagation axis, and on and off structural resonance conditions. Several features of these theoretical results are related to corresponding experimental observations.

Internal and near-surface electromagnetic fields for a spherical particle irradiated by a focused laser beam

Theoretical expressions for the internal and external electromagnetic fields for an arbitrary electromagnetic beam incident upon a homogeneous spherical particle are derived, and numerical calculations based upon this theoretical development are presented. In particular, spatial distributions of the internal and near-surface electric field magnitude (source function) for a focused fundamental (TEM00 mode) Gaussian beam of 1.06 μm wavelength and 4 μm beam waist diameter incident upon a 5-μm-diam water droplet in air are presented as a function of the location of the beam focal point relative to the sphere center. The calculations indicate that the internal and near-surface electric field magnitude distribution can be strongly dependent upon relative focal point positioning and may differ significantly from the corresponding electric field magnitude distribution expected from plane-wave irradiation.

Droplet deposition in two-phase, turbulent flow

Theoretical and experimental studies have been carried out to investigate further the turbulent deposition of particles from two-phase droplet flow onto the smooth wall of a vertical tube. The work of Ganić and Mastanaiah [ Int. J. Multiphase Flow 7,401–422 (1981)] for the Stokes regime has been extended by this study to include the Oseen regime ( Re p ≤ 5 ). The proposed theory satisfactorily describes the existing data, as well as the new data taken for air-water droplet flow passing through a 12.95-mm-diameter stainless-steel tube at Re = 27000, 36000, 43000 and 55000.

A stagnation pressure probe for droplet-laden air flow

It is often of interest in a droplet-laden gas flow to obtain the stagnation pressure of both the gas phase and the mixture. A flow-decelerating probe (TPF), with separate, purged ports for the gas phase and the mixture and with a bleed for accumulating liquid at the closed end, has been developed. Measurements obtained utilizing the TPF in a nearly isothermal air-water droplet mixture flow in a smooth circular pipe under various conditions of flow velocity, pressure, liquid concentration and droplet size are presented and compared with data obtained under identical conditions with a conventional, gas phase stagnation pressure probe (CSP). The data obtained with the CSP and TPF probes are analyzed to determine the applicability of the two probes in relation to the multi-phase characteristics of the flow and the geometry of the probe.

Investigation of droplet deposition from a turbulent gas stream

Turbulent deposition of particles from two-phase flow onto the smooth wall of a tube has been studied theoretically and experimentally. A model is proposed for the deposition motion of large particles based on turbulent diffusion in the core followed by a free flight towards the wall. The theory shows that within the Stokes regime, the dimensionless deposition velocity k- d / u * depends on Re and τ + only, where u* is the friction velocity, Re is the tube Reynolds number and τ + is the dimensionless particle relaxation time. Deposition data are obtained for air-water droplet flow through a 12.7-mm i.d. acrylic tubing at Re = 52,500 and 94,600. The proposed theory satisfactorily describes the existing deposition data as well as present measurements, covering a wide range of Re and τ + .

T‐matrix analysis of the resonances in sound scattering from droplets

A T‐matrix code suitable for acoustic scattering has been developed and used to study the resonances present in water droplets in air, whenever they scatter incident sound energy. The numerical determination of the resonance portions of the back‐scattering amplitudes is compared to exact analytical expressions suited to this case [J. George, J. Acoust. Soc. Am (in press)], and excellent agreement is found. Several resonances (i.e., fundamental and overtones) present in several of the initial modes have been computed and displayed for various combination of substances inside and outside the droplet. The analysis is useful in the resonance study of sound scattering by aerosols and fogs. [H. Uberall is also at Catholic University, Washington, DC 20064 and was additionally supported by Code 421 of ONR.]

T‐matrix analysis of the resonances in sound scattering from droplets

A T‐matrix code suitable for acoustic scattering has been developed and used to study the resonances present in water droplets in air, whenever they scatter incident sound energy. The numerical determination of the resonance portions of the back‐scattering amplitudes is compared to exact analytical expressions suited to this case [J. George, J. Acoust. Soc. Am. (in press)] and excellent agreement is found. Several resonances (i.e., fundamental and overtones) present in several of the initial modes have been computed and displayed for various combinations of substances inside and outside the droplet. The analysis is useful in the resonance study of sound scattering by aerosols and fogs. [H. Überall is also at Catholic University, Washington, DC 20064 and was additionally supported by Code 421 of ONR.]

Resonances in sound scattering from water droplets in air

Resonances in the scattering of acoustic plane waves from water droplets in air are studied. A simple equation to locate the resonances is presented and 12 of the low-frequency resonances are plotted.

A simple way to find resonances in sound scattering from water droplets in air

An especially simple equation presented here reduces the task of locating the resonances in sound scattering from water droplets in air to only a matter of looking up the zeroes of spherical Bessel functions from mathematical tables. We demonstrate this technique by predicting and plotting twelve of the resonances, including the lowest in frequency. The use of these resonances for remote sonar determination of the sizes of the droplets is discussed. A complete derivation of the formulas for scattering cross section and for the resonance condition is given. [Supported in part by ONR.]

Experimental determination of collection efficiencies for small water droplets in air: The wake capture of water drops in air

Experimental determination of collection efficiencies for small water droplets in air: The wake capture of water drops in air

Experimental determination of collection efficiencies for small water droplets in air

AbstractThis paper describes measurements of collection efficiencies for gravitational collisions between water drops of radius R = 30 to 55 μ and saline droplets of radius r = 1 to 12 μ. the results agree quite well with those obtained earlier by Picknett for drops of R = 30, 40 μ and also with the collision efficiencies computed by Hocking, Mason and Shafrir and Neiburger except that, for values of r/R < 0·1, the experimental values are consistently higher than the calculated values and fail to show the sharp cut‐off predicted by theory. the experiments support Hocking's prediction that drops of R ⩽ 18 μ will fail to collect droplets of smaller size.

Scattering of Microwaves by Adjacent Water Droplets in Air

THE scattering by a single, homogeneous sphere, located in a parallel beam of electromagnetic radiation, is given by Mie's1 solution of Maxwell's equations.