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The trajectories of droplets in the air flowing past an NACA 651-212 airfoil at an angle of attack of 40 were determined. The collection efficiency, the area of droplet impingement, and the rate of droplet impingement were calculated from the trajectories and are presented herein.
Trajectories were determined for droplets in air flowing through 90 deg elbows especially designed for two-dimensional potential motion with low pressure losses. The elbows were established by selecting as walls of each elbow two streamlines of the flow field produced by a complex potential function that establishes a two-dimensional flow around a 90 deg bend. An unlimited number of elbows with slightly different shapes can be established by selecting different pairs of streamlines as walls. The elbows produced by the complex potential function selected are suitable for use in aircraft air-intake ducts. The droplet impingement data derived from the trajectories are presented along with equations in such a manner that the collection efficiency, the area, the rate, and the distribution of droplet impingement can be determined for any elbow defined by any pair of streamlines within a portion of the flow field established by the complex potential function. Coordinates for some typical streamlines of the flow field and velocity components for several points along these streamlines are presented in tabular form.
Trajectories of water droplets about an ellipsoid of revolution with a fineness ratio of 5 (which often approximates the shape of an aircraft fuselage or missile) were computed with the aid of a differential analyzer. Analyses of these trajectories indicate that the local concentration of liquid water at various points about an ellipsoid in flight through a droplet field varies considerably and under some conditions may be several times the free-stream concentration. Curves of the local concentration factor as a function of spatial position were obtained and are presented in terms of dimensionless parameters Re(sub 0) (free-stream Reynolds number) and K (inertia), which contain flight and atmospheric conditions. These curves show that the local concentration factor at any point is very sensitive to change in the dimensionless parameters Re(sub 0) and K. These data indicate that the expected local concentration factors should be considered when choosing the location of, or when determining antiicing heat requirements for, water- or ice-sensitive devices that protrude into the stream from an aircraft fuselage or missile. Similarly, the concentration factor should be considered when choosing the location on an aircraft of instruments that measure liquid-water content or droplet-size distribution in the atmosphere.
Droplet trajectories about a sphere in ideal fluid flow were calculated. From the calculated droplet trajectories the droplet impingement characteristics of the sphere were determined. Impingement data and equations for determining the collection efficiency, the area, and the distribution of impingement are presented in terms of dimensionless parameters. The range of flight and atmospheric conditions covered in the calculations was extended considerably beyond the range covered by previously reported calculations for the sphere.
Trajectories were determined for water droplets or other aerosol particles in air flowing through 600 elbows especially designed for two-dimensional potential motion. The elbows were established by selecting as walls of each elbow two streamlines of a flow field produced by a complex potential function that establishes a two-dimensional flow around. a 600 bend. An unlimited number of elbows with slightly different shapes can be established by selecting different pairs of streamlines as walls. Some of these have a pocket on the outside wall. The elbows produced by the complex potential function are suitable for use in aircraft air-inlet ducts and have the following characteristics: (1) The resultant velocity at any point inside the elbow is always greater than zero but never exceeds the velocity at the entrance. (2) The air flow field at the entrance and exit is almost uniform and rectilinear. (3) The elbows are symmetrical with respect to the bisector of the angle of bend. These elbows should have lower pressure losses than bends of constant cross-sectional area. The droplet impingement data derived from the trajectories are presented along with equations so that collection efficiency, area, rate, and distribution of droplet impingement can be determined for any elbow defined by any pair of streamlines within a portion of the flow field established by the complex potential function. Coordinates for some typical streamlines of the flow field and velocity components for several points along these streamlines are presented in tabular form. A comparison of the 600 elbow with previous calculations for a comparable 90 elbow indicated that the impingement characteristics of the two elbows were very similar.
Uncharged droplets and bubbles can be levitated dielectrophoretically in liquids using strong, nonuniform electric fields. The general equations of motion for a droplet or bubble in an axisymmetric, divergence-free electrostatic field allow determination of the conditions necessary and sufficient for stable levitation. The design of dielectrophoretic (DEP) levitation electrode structures is simplified by a Taylor-series expansion of cusped axisymmetric electrostatic fields. Extensive experimental measurements on bubbles in insulating liquids verify the simple dielectrophoretic model. Other have extended dielectrophoretic levitation to very small particles in aqueous media. Applications of DEP levitation to the study of gas bubbles, liquid droplets, and solid particles are discussed. Some of these applications are of special interest in the reduced gravitational field of a spacecraft.
The fundamental physico-chemical mechanisms governing droplet vaporization and combustion are discussed. Specific topics include governing equations and simplifications, the classical d(2)-Law solution and its subsequent modification, finite-rate kinetics and the flame structure, droplet dynamics, near- and super-critical combustion, combustion of multicomponent fuel blends/emulsions/suspensions, and droplet interaction. Potential research topics are suggested.
An analytic model of SO2 absorption in a falling water droplet is developed and a simulation of SO2 washout is performed. Nonisothermic effects on drop growth, droplet physical parameters, reaction rates, and multicomponent diffusion are treated in the model. The gas-liquid interface is assumed to be at equilibrium, and interfacial resistance is negligible. Raindrops are simulated as falling from a 2 km height through an atmospheric region containing SO2. The droplets decrease in size from evaporation and cooling, and their slightly basic pH aids SO2 absorption. The simulation indicates higher SO2 absorption at higher altitudes, and desorption may occur at ground level. Isothermal effects are concluded to be significant, and quantification of effects will depend on further modelling.
The present investigation provides a confirmation of the dependence of droplet sedimentation on particle concentration. It is shown that it is possible to determine the maximum particle concentration which can remain stable on a given liquid from droplet sedimentation experiments. Droplet sedimentation can be reduced but not totally eliminated by the addition of appropriate amounts of dimethyl sulfoxide (DMSO) to reduce the van der Waals forces to zero. It was found that, at 12% DMSO, a maximum particle concentration of 6.3 x 10 to the 8th cells/ml of glutaraldehyde-fixed human erythrocytes suspended in physiological saline can remain stable on a D2O cushion.
Calculations are presented that simulate the free radical chemistries of the gas phase and aqueous phase within a warm cloud during midday. It is demonstrated that in the presence of midday solar fluxes, the heterogeneous scavenging of OH and HO2 from the gas phase by cloud droplets can represent a major source of free radicals to cloud water, provided the accommodation or sticking coefficient for these species impinging upon water droplets is not less than 0.0001. The aqueous-phase of HO2 radicals are found to be converted to H2O2 by aqueous-phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H2O2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free-radical-produced H2O2 as well as by aqueous-phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.
Photographic and photometric instrumentation was used to study Al burning in air and in O2 and Zr burning in pure O2. The metals, free-falling and of submillimeter size, were ignited by laser. Occasional coalescence of a cooler oxide follower droplet perturbed and then momentarily quenched the spherically symmetric flame emission and condensation fronts of a Zr droplet. An insoluble molten oxide cap consistently formed and grew on the burning Al droplets, blocking local Al vaporization, and providing a site for further condensation with accompanying heat release. Superheating of the liquid metal at the Al/AlO2 interface then occurs, which can presage metal vapor eruptions. Imposing an acoustic field of 90-130 dB, 1-12 kHz transversely on the combustion zone produced no significant changes in the combustion zone geometry.
A discussion is presented concerning the motion of droplets in a surrounding fluid induced by a temperature gradient. Such 'thermocapillary migration' may be characterized as the tangential stress at the interface that is induced by the temperature gradient's variation of interfacial tension, causing motion in the neighboring fluid both inside and outside the droplet. As a result, the droplet moves in the direction of the pole with the lower interfacial tension. An analysis is conducted in the creeping flow limit, for the case of small Marangoni number values. The method of matched asymptotic expansions is used for the solution of the field equations.
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.
A laser was used to ignite Al, Zr, Ti and Hf droplets in separate trials to study metal droplet combustion in air. The burning was recorded with high speed film at high magnification. Al was found to flash vaporize, melt, surface oxidize, revaporize, then combust. All species burned with a vapor diffusion flame surrounded by an oxide condensation front. The Zr, Ti and Hf droplets grew by condensation, coalesced, then convected toward the perimeter of the condensation zone. It is expected that the data will serve as bases for a theoretical vapor diffusion model. Finally, the introduction of a transverse acoustic field was observed to have no effect on either the luminosity or combustion zone geometry.
A two-fluid pneumatic atomizer operating at relatively high liquid and gas pressures produced water sprays that simulated small-droplet clouds of liquid fuel for use in studying vaporization and fuel-air mixing effects on combustor performance and emissions. To characterize the sprays, a scattered-light scanning instrument was developed and measurements of volume median or volume mean diameter, D sub V.5, were correlated with D sub O, W sub w, and W sub n, i.e., orifice diameter, water, and nitrogen gas flow rates, respectively, to give the general expression: D sub v.5 approx. (D sub o sup 0.2) (W sub w sup m) (W sub n sup n), which yields D sub v.5 = 45 (D sub o sup 0.2) (W sub w sup 0.2) (W sub w sup - 1.2). Values of D sub o, W sub w, and W sub n are in centimeters and grams/second, respectively. Farther downstream at an axial distance of 6.7 cm, exponent m increased from 0.2 to 0.4 and exponent n decreased from -1.2 to -1.0 and at a distance of 25 cm downstream of the atomizer, n decreased to -0.8. The increase in exponent m and decrease in exponent n was attributed to a loss of very small droplets from the spray due primarily to vaporization and diffusion effects on clouds of small droplets traveling a distance of 25 cm.
A two-fluid pneumatic atomizer operating at relatively high liquid and gas pressures produced water sprays that simulated small-droplet clouds of liquid fuel for use in studying vaporization and fuel-air mixing effects on combustor performance and emissions. To characterize the sprays, a scattered-light scanning instrument was developed and measurements of volume median or volume mean diameter, D sub V.5, were correlated with D sub O, W sub w, and W sub n, i.e., orifice diameter, water, and nitrogen gas flow rates, respectively, to give the general expression: D sub v.5 approx. (D sub O sup 0.2) (W sub w sup m) (W sub n sup n), which yields D sub v.5 = 45 (D sub o sup 0.2) (W sub w sup 0.2) (W sub w sup -1.2). Values of D sub 0, W sub w, and W sub n are in centimeters and grams/second, respectively. Farther downstream at an axial distance of 6.7 cm, exponent m increased from 0.2 to 0.4 and exponent n decreased from -1.2 to -1.0 and at a distance of 25 cm downstream of the atomizer, n decreased to -0.8. The increase in exponent m and decrease in exponent n was attributed to a loss of very small droplets from the spray due primarily to vaporization and diffusion effects on clouds of small droplets traveling a distance of 25 cm.
This paper describes a lightweight, highly effective liquid droplet heat exchanger (LDHX) concept for thermal management in space. Heat is transferred by direct contact between fine droplets (100 to 300 micron diameter) of a low vapor pressure liquid and an inert working gas. Complete separation of the droplet and gas media in the microgravity environment is accomplished by configuring the LDHX as a vortex chamber. A quasi-one-dimensional, two-phase heat transfer model of the LDHX is developed and used to investigate the potential use of the LDHX for both heating and cooling the working gas in a 100-k W(e) Braytoan cycle. Experimental studies on a small scale LDHX chamber, using air and water as the two media, show excellent agreement with the theoretical model.
This paper describes techniques that have been developed at JPL for visualization and measurements of droplet fields in a moving airstream. The techniques have been applied to a variety of problems which include simulation of heavy rain conditions in a wind tunnel, engine ingestion of wheel spray, analysis of fuel nozzle sprays and aerodynamic breakup of non-Newtonian liquids. The approach pursued here is direct imaging using a pulsed laser illumination source, followed by digital processing of photographic images. A laser double pulse technique has been used for determination of droplet velocities. Laser induced fluorescence has been used to enhance droplet definition in photographic images. The image processing software developed is capable of distinguishing between out-of-focus and in-focus drops, retaining only the latter for size determination.