Showing papers on "Schmidt number published in 1984"

Mixing and combustion with low heat release in a turbulent shear layer

Abstract: Turbulent mixing and combustion are investigated in a gaseous shear layer formed between two streams: one containing a low concentration of hydrogen in nitrogen and the other containing a low concentration of fluorine in nitrogen. The resulting temperature field is measured simultaneously at eight points across the width of the layer using fast-response cold-wire thermometry. The results show the presence of large, hot structures separated by tongues of cool fluid that enter the layer from either side. The usual bell-shaped mean-temperature profiles therefore result from a duty cycle whereby a fixed probe sees alternating hot and cool fluid, which results in the local mean. The adiabatic flame temperature is not achieved in the mean, at any location across the layer. For fixed velocities, it is found that, in general, two different mean-temperature profiles result from a given pair of reactant compositions if the sides of the layer on which they are carried are exchanged (‘flipped’). This finding is a direct consequence of the asymmetric entrainment of fluid into the layer. Results are compared with the predictions of Konrad and discussed in the context of the Broadwell–Breidenthal model. By comparison with the liquid result of Breidenthal, the amount of product formed in the layer at high Reynolds number is found to be dependent upon the Schmidt number. Results for a helium–nitrogen layer are discussed briefly.

The Variation of the Gas Transfer Coefficient with Molecular Diffusity

Abstract: We argue that the boundary conditions at the air-water interface lead to a variation of the transfer coefficient with the 1/2 power of the molecular diffusivity for slightly soluble gases. For wind-driven bodies of water the transfer coefficient should vary with the −1/2 power of the Schmidt number and with the 1/2 power of that part of the wind stress transmitted by viscosity. The boundary conditions are contrasted with those at a smooth wall where the mass transfer coefficient varies with the –2/3 power of the Schmidt number. Measurements of the transfer coefficient for N2O, CH4, and He at moderate wind stress in an 18 meter wind-wave tunnel support our arguments.

Wind/Wave-Tunnel Experiment on the Schmidt Number — and Wave Field Dependence of Air/Water Gas Exchange

Abstract: Gas exchange experiments have been carried out in the Heidelberg circular wind tunnels and the large linear I.M.S.T. wind/wave facility in order to study the wave field and Schmidt number dependence of gas exchange. With a smooth water surface the experimental results agree well with the model treating the water surface as a rigid wall both in the Sc−2/3 dependence and the absolute rates. Gas exchange rates with a rough wavy surface are 3–5 times higher than in the smooth case and the Schmidt number dependence is lower (Sc−n, n = 0.55 ± 0.05). The total square wave slope seems to be a more appropriate parameter to describe the enhanced rates than the capillary wave slope.

Thermosolutal Convection during Directional Solidification

Abstract: During solidification of a binary alloy at constant velocity vertically upward, thermosolutal convection can occur if the solute rejected at the crystal-melt interface decreases the density of the melt. We assume that the crystal-melt interface remains planar and that the flow field is periodic in the horizontal direction. The time-dependent nonlinear differential equations for fluid flow, concentration, and temperature are solved numerically in two spatial dimensions for small Prandtl numbers and moderately large Schmidt numbers. For slow solidification velocities, the thermal field has an important stabilizing influence: near the onset of instability the flow is confined to the vicinity of the crystal-melt interface. Further, for slow velocities, as the concentration increases, the horizontal wavelength of the flow decreases rapidly — a phenomenon also indicated by linear stability analysis. The lateral in-homogeneity in solute concentration due to convection is obtained from the calculations. For a narrow range of solutal Rayleigh numbers and wavelengths, the flow is periodic in time.

Mass-transfer effects on the flow past an impulsively started infinite vertical plate with variable temperature or constant heat flux

Abstract: An exact solution of the flow of a viscous incompressible fluid past an impulsively started infinite vertical plate in the presence of foreign mass is presented under the condition of (i) variable plate temperature, (ii) constant heat flux. It is observed that an increase in the Schmidt number leads to an increase in the skin-friction.

Calculations of turbulent mass transport in a bluff-body diffusion-flame combustor

Abstract: Experimental measurements of turbulent mass transit in a bluff-body diffusion-flame combustor (using CO2 instead of fuel) are analyzed to evaluate the accuracy of physical models used in flow computations. The data of Lightman and Magill (1981), Lightman et al. (1983), and Roquemore et al. (1983) are used to calculate apparent turbulent Schmidt numbers (TSN) for a series of flow conditions by a modified TEACH viscous-flow code. The modeling principles, calculation grid, and boundary conditions are discussed, and the results are presented in graphs comparing calculated and measured values. It is shown that models such as the two-equation (K - epsilon) model which use a single value of the TSN are inappropriate for conditions typical of gas-turbine combustors: local TSN variations due to turbulence behavior must be taken into account in improved models.

Hall effects on heat and mass transfer flow through porous medium

Abstract: Effects of Hall current on free convection and mass transfer flow through a porous medium bounded by a vertical surface has been analysed The problem is solved analytically The velocity profiles are shown on graphs Effects ofm (Hall parameter)K* (permeability parameter), and Sc (Schmidt number) on velocity are discussed

Effects of hall current on the hydromagnetic free convection with mass transfer in a rotating fluid

Abstract: An exact analysis of Hall current on hydromagnetic free convection with mass transfer in a conducting liquid past an infinite vertical porous plate in a rotating fluid has been presented. Exact solution for the velocity field has been obtained and the effects ofm (Hall parameter),E (Ekman number), andS c (Schmidt number) on the velocity field have been discussed.

Comparisons of Rational Engineering Correlations of Thermophoretically-Augmented Particle Mass Transfer With STAN5-Predictions for Developing Boundary Layers

Abstract: Modification of the code STAN5 to properly include thermophoretic mass transport, and examination of selected test cases developing boundary layers which include variable properties, viscous dissipation, transition to turbulence and transpiration cooling. Under conditions representative of current and projected GT operation, local application of St(M)/St(M),o correlations evidently provides accurate and economical engineering design predictions, especially for suspended particles characterized by Schmidt numbers outside of the heavy vapor range.

Observations on the use of analytical and numerical models for the description of transfer to porous surface vegetation such as lichen

Abstract: Convective deposition of submicron-size aerosol to porous surface vegetation was studied by electrochemical simulation, under Reynolds and Schmidt similarity, to a rectangular array of closely-packed lichen and artificial wire roughness layers. Results, showing an approximate tenfold increase in deposition velocity over that of a flat plate placed at the same position, were compared with predictions made on the basis of various rough-surface transfer models, including those based on statistical eddy renewal, as well as with numerical solutions of the diffusion equation in statistically-renewed surface cavities. Most analytical models could be made to fit the observed data, at least for a limited range of flow velocities, but poorly known and poorly defined parameters limit their usefulness for predictive purposes; and their validity across a large variation in molecular diffusivity (or Schmidt number Sc) is generally not assured. Numerical models also depend on poorly substantiated physical assumptions but the effect of such assumptions on transfer can be calculated for a wider range of conditions than those permitting an analytical solution. This allows more direct feedback between model assumptions and calculated or observed transfer. Numerically calculated values for deposition velocity in air for Sc from 0.7 to 7000 and flow velocities from 0.2 to 5 m s-1 are presented for different model assumptions, with values ranging from 1 cms-1.

Axisymmetric stagnation point flow and mass transfer for power-law fluids. II: Moderate Schmidt number correction

Abstract: The theory for axisymmetric stagnation point flow of power-law fluids has been extended to include the correction terms for convective diffusion at moderate Schmidt numbers. The dimensionless mass transfer rate is expressed as an asymptotic series that is valid for Re(1 − n)/3(1 + n) Sc−13 < 1. The result can be used to predict accurate diffusion coefficients for dilute species in fluids with specified power-law characteristics.

Inertia effects for slow forced scalar transfer from a sphere

Abstract: The effect of small, but nonzero Reynolds number on forced scalar transfer from a spherical particle is considered. The analysis applies to the case of low Peclet number Pe and requires a Prandtl/Schmidt number of order one. For a droplet it is shown that a) inertia does not effect the average transfer before terms of order Pe2 and that b) the indirect effect of inertia, namely that due to drop deformation can be neglected at this order. The way in which these results were obtained makes it possible to apply them for encapsulated droplets and rigid spheres in slip flow, too.

Study of Molecular Diffusivity Influence on Mass Transfer Rate at a Water-Atmosphere Interface

Abstract: In environmental Engineering, it is necessary to construct and improve mathematical models for predicting and controlling pollution, and therefore to study certain basic phenomena such as mass transfer at an interface. For gases of relatively low solubility in water, resistance to penetration occurs at the interface in the viscous sublayer of the liquid phase. To emphasize the importance of molecular diffusivity for mass transfer rate we have worked with wide range of molecular diffusivities using gases like oxygen and carbon dioxide and, in contrast, helium. However, while it is relatively easy to measure oxygen and carbon dioxide concentrations in water, the determination of helium concentrations is very difficult. A solid-gas chromatography method has been used, and problems like pumping, storing ans injecting samples have been overcome. Absorption rates are shown to be strongly related to Schmidt number and wind velocity.