Showing papers on "Mass transfer published in 1977"

Heat and Mass Transfer between Impinging Gas Jets and Solid Surfaces

Abstract: Publisher Summary Heating or cooling of large surface area products is often carried out in devices consisting of arrays of round or slot nozzles, through which air impinges vertically upon the product surface. This chapter presents a comprehensive survey emphasizing the engineering applications and empirical equations, presented for the prediction of heat and mass transfer coefficients within a large and technologically important range of variables. The local variations of the transfer coefficients are based on the experimental data for single round nozzles (SRN), arrays of round nozzles (ARN), single slot nozzles (SSN), and arrays of slot nozzles (ASN). The variation of local transfer coefficients is graphically represented. It also explores how to apply these equations in heat exchanger and dryer design as well as in optimization. The flow field of impinging flow is diagrammatically represented. External variables influencing heat and mass transfer in impinging flow depends on mass flow rate, kind and state of the gas and on the shape, size, and position of the nozzles relative to each other and to the solid surface. The design of high-performance arrays of nozzles is also discussed.

Mass Transfer Studies in Sorbing Porous Media: II. Experimental Evaluation with Tritium (3H2O)1

Abstract: A comparison is made between observed tritium effluent concentration distributions and those calculated with a previously published analytical solution for the movement of chemicals through unsaturated, aggregated sorbing media. In the analytical model the liquid phase of the soil is divided into mobile and immobile regions, with transfer between the two regions diffusion controlled. Effluent data obtained from several displacements of tritium through 30-cm long columns of Glendale clay loam were used to determine the different parameters in the analytical solution by curve fitting. The data indicate some adsorption or isotopic exchange of tritium during its flow through the soil columns. The amount of immobile water increases with decreasing flow velocity and increasing aggregate size, and varies between 6 and 45% of the total volume of water in the columns. The analytical solution provides an excellent description of the experimental effluent data, and shows that tailing can be explained satisfactorily by diffusional exchange of tritium between mobile and immobile regions of the soil. Additional Index Words: intra-aggregate diffusion, immobile water, miscible displacement I PART i OF THIS STUDY (van Genuchten and Wierenga, 1976), a mathematical model was developed to describe the movement of chemicals through unsaturated, aggregated sorbing porous media. The medium was thereby analyzed by dividing the liquid phase into mobile and immobile regions. Transfer between the two liquid regions, presumably a diffusion process, was assumed to be proportional to the concentration difference between the mobile and immobile liquid phases. Sorption processes in both the stagnant and dynamic regions of the medium were further assumed to be instantaneous. The resulting differential equations were solved analytically assuming a linear equilibrium adsorption isotherm. The experiments to be discussed in this study were carried out to evaluate the model of van Genuchten and Wierenga (1976), and to obtain estimates of the different parameters in the adsorption model. The effects of flow velocity, bulk density, and aggregate size on the amount of immobile water (and hence, on the amount of tailing and asymmetry) were also studied. In a subsequent paper (van Genuchten et al., 1977) the theoretical model will be used to describe the movement of 2,4,5-T (2,4,5-trichlorophenoxy acetic acid) through the same soil columns. MATHEMATICAL DESCRIPTION We refer to the original paper of van Genuchten and Wierenga (1976) for a more extensive discussion of the physical system. The following general system of equations describes the mobile (Cm) 'Journal article no. 565, Agric. Exp. Stn., New Mexico State Univ., Las Cruces, NM 88003. The work upon which this report was based was supported in part by funds obtained from the U.S. Dep. of the Interior, Office of Water Resources, as authorized under the Water Resources Act of 1964. Received 7 July 1975. Approved 9 Nov. 1976. Formerly Graduate Student, Dep. of Agronomy, New Mexico State Univ.; presently Research Associate, Dep. of Civil Engineering, Princeton Univ., Princeton, NJ 08540. 'Associate Professor, Dep. of Agronomy, New Mexico State Univ., Las Graces, NM 88003. and immobile (Cim) concentration distributions in a semi-infinite column, resulting from a pulse input of solute. [ dCml f VmC0 0 lim vmCm D -r— = \\ z—»oL \"£ J t ^ t -

Heat and Mass Transfer Data Book

Abstract: Heat and mass transfer data book , Heat and mass transfer data book , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Turbulent mass transfer rates to a wall for large Schmidt numbers

Abstract: New measurements are presented on the influence of Schmidt number on the rate of mass transfer between a turbulent fluid and a pipe wall. It is found that for large Schmidt numbers the fully developed mass transfer coefficient is related to the friction velocity and the Schmidt number by the equation The experiments are accurate enough to rule out the Sc−2/3 or the Sc−2/3 relations commonly used, deduced from plausible limiting expressions for the eddy diffusivity close to a wall. It is argued that these expressions arevalid only over a vanishingly small portion of the concentration field as Sc → ∞.

Kinetic models of diagenesis in disturbed sediments: Part 1. mass transfer properties and silica diagenesis

Abstract: The results of chemical analysis of the interstitial water of several samples of sediments from a large muddy zone along the Belgian North Sea coast are reported. When special care is taken to collect the cores without disturbing the water-sediment interface, the vertical concentration profiles display typical patterns that cannot be explained by constant diffusivity models and suggest the existence of two distinct sedimentary layers with different mass transfer properties. A two-layer model is proposed to describe the vertical silica profiles. It assumes that the mass transfer coefficient in the first 3.5 cm of the sediment upper layer is 100 times higher than in the compacted lower layer. This large increase is mainly due to turbulent processes induced by the movement of the overlying water. From this model, the flux of dissolved silica across the water-sediment interface is calculated.

Exact solution of combined heat- and mass-transfer problem during film absorption

Abstract: Exact solutions of the system of equations of heat and mass transfer accompanying absorption of vapor by a liquid film are obtained. Expressions for the main characteristics of heat and mass transfer are obtained.

Forced Convection Heat Transfer at an Inclined and Yawed Square Plate—Application to Solar Collectors

Abstract: Experiments have been performed to determine the average heat transfer coefficients for forced convection airflow over a square plate that is inclined and yawed relative to the oncoming flow. The experiments involved mass transfer and were carried out via naphthalene sublimation technique, with air as the working fluid. By means of the analogy between heat and mass transfer, the results are presented in a form that can be used directly for heat transfer applications. The experiments encompassed a wide range of angles of yaw and angles of attack, and extended over a Reynolds number range from about 20,000 to 100,000. It was found that owing to three dimensional flow effects, the transfer coefficients were remarkably insensitive to both the angle of attack and the angle of yaw. This enabled all the results to be correlated by the equation j=0.931Re/sup -1/2/ (where j= (h/rhoc/sub p/Uinfinity) Pr/sup 2/3/) with an accuracy of +- 2 1/2 percent. The correlation equation was applied to the determination of the heat transfer coefficient for wind-related heat losses from a flat plate solar collector. It was demonstrated that the currently standard computational equation (which is, in reality, not well suited to the application) substantially overestimates the heatmore » transfer coefficient.« less

Combined heat and mass transfer during absorption in drops and films

Abstract: Combined heat and mass transfer during absorption in films and drops is discussed. Using an approximate approach, simple analytic relations are obtained for the basic characteristics of the process.

Heat, Mass, and Momentum Transfer

Abstract: Life depends on heat and mass transfer between organisms and their surroundings. Such processes as carbon dioxide exchange between leaves and the atmosphere, oxygen uptake by microorganisms, oxygen and carbon dioxide exchange in the lungs of animals, or convective heat loss from the surfaces of animal coats are fundamental to the existence of living organisms. A thorough understanding of these exchange processes is therefore a necessary part of the study of physical ecology. In this chapter we will first discuss molecular diffusion. It is by this process that heat and mass are transported in still air or water, as they are in parts of the lungs of animals, in soils, and in the substomatal cavities of leaves. Molecular diffusion is also important in convective heat and mass transfer between surfaces and fluids flowing over them since a thin boundary layer is always formed near the surface through which transport is by diffusion. After diffusion processes are discussed, we will then present convective heat and mass transfer theory as it applies to fluids moving over plates, cylinders, and spheres. Finally, we will discuss momentum exchange and the force of moving fluids on objects in them.

Diffusional boundary‐layer resistance for membranes with low porosity

Abstract: Diffusional mass transfer across membranes with uniform but low pore densities was studied experimentally as a function of stirring rate and pore area fraction. The results were analyzed in terms of a stagnant film boundary-layer model specially formulated for a heterogeneous membrane containing discrete pores. The overall membrane diffusional resistance is linearly related to the inverse of the pore area fraction of the membranes for a constant stirring rate. An equivalent boundary-layer thickness can be defined which is independent of membrane properties but a unique function of stirrer speed. These experimental boundary-layer thicknesses are greater by a factor of 3 than those predicted by a published Sherwood number correlation determined for homogeneous surfaces, but the stirring rate dependence is in excellent agreement with this same correlation.

Analysis of transport processes in vertical cylinder epitaxy reactors.

Abstract: Momentum, heat, and mass transfer processes were studied in a vertical cylinder reactor for the epitaxial growth of Si from SiCl/sub 4/ in H/sub 2/ by chemical vapor deposition. An analytical solution to the problems of heat and mass transfer in a tapered annulus based on constant transport properties and fully developed laminar flow is presented. The mean gas-phase temperature and deposition rate distribution of silicon are calculated within the reactor by using a developing temperature model. Results of experimental studies of silicon deposition from SiCl/sub 4/ in H/sub 2/ at 1200/sup 0/C in a vertical cylinder reactor are compared with the analytical results and with other models of diffusion-controlled chemical vapor deposition. This study provides an analytical method for calculating epitaxial deposition rate distributions in vertical cylinder reactors, and for designing reactors to improve the yield and uniformity of epitaxial growth. 7 figures, 2 tables.

Mass transfer at high Péclet numbers for creeping flow in a packed‐bed reactor

Abstract: An isotropic, homogeneous packed-bed reactor is modeled as an array of sinusoidal periodically constricted tubes (PCT). The effective asymptotic bed-Sherwood number has been calculated for mass transfer at large Peclet number with a constant wall concentration and creeping flow hydrodynamics. The bed friction factor has also been calculated. The results for these macroscopic bed quantities depend upon two ratios of the microscopic PCT period length, average radius, and sinusoidal amplitude.

Influence of Schmidt number on the fluctuations of turbulent mass transfer to a wall

Abstract: Measurements are presented on the influence of Schmidt number on the frequency of the mass transfer fluctuations at a solid boundary. The shape of the spectral function is similar at all Schmidt numbers. A relation between the mass transfer fluctuations and the fluctuating velocity field can be obtained only at high frequencies. A comparison of the scale and the frequency of the mass transfer fluctuations and the velocity fluctuations suggests that the rate of mass transfer is controlled by convective motions in the flow oriented eddies described by a number of previous investigators. However, the concentration fluctuations caused by these convective motions are greatly dampened close to the wall by molecular diffusion. Thus the mass transfer fluctuations reflect only the scale and not the frequency of the convective motions in the flow oriented eddies.

Enhanced mass transfer in electrochemical cells using turbulence promoters

Abstract: Many electrochemical processes suffer in varying degrees from mass transfer limitations. These limitations may require operation at considerably less than economic optimum current densities. Mass transfer to a surface may be considerably enhanced by insertion of turbulence promoters in the fluid flow path near the affected surface. An instrument was developed to measure local current densities in the hydrodynamically very difficult region near the turbulence promoter. A general method for the relative evaluation of hydrodynamic conditions has been developed. Generalization of the data permits optimization of hydrodynamic cell design using the promoter shapes investigated.

Mass Transfer, Equilibrium, and Kinetics in the Chemical Vapor Deposition of Boron from Impinging Jets

Abstract: The deposition mechanisms of boron from boron trichloride and hydrogen vapors are investigated by a stagnation flow technique. The characteristics of gas flow are first deduced from experimental deposition rates. Then, by use of the Stefan--Maxwell equations for diffusion, the interfacial conditions of deposition, equilibrium, or supersaturation are inferred from experimental rates of deposition in the stagnation zone. Three mechanisms of deposition are deduced, mass transfer--interfacial equilibrium, mass-transfer-surface kinetics, and surface kinetics. They occur as a function of three experimental parameters: temperature, mass flow rate, and inlet composition BCl/sub 3//H/sub 2/. A correlation for mass transfer in impinging jets is obtained. 15 figures, 1 table.

Evaporative winds in X-ray binaries

Abstract: Evaporation of gas from the surface of HZ Her by Her X-1 and its implications regarding the mass transfer process are examined further. The powerful soft X-ray flux results in an evaporation rate greater than previous estimates. The evaporative flow is shown to be subsonic at first, with the result that the capture of evaporated gas by Her X-1 may be efficient, and the self-excited wind mechanism is possible. A criterion for stabilization of mass transfer by stellar wind mass loss is derived. Possible mechanisms for the long-period variability of HZ Her are discussed. Evaporative winds are also estimated for Sco X-1 and Cyg X-2 spectra.

The Evolution of Low-Mass Close Binary Systems. V. Transport Processes in the Envelopes of Contact Components: Erratum

Abstract: Starting from an adiabatic, isopotential, irrotational approximation to the hydrodynamic equations, the nature of mass flow in the envelopes of contact components of semi-detached and contact systems is discussed. The local pressure reduction near L/sub 1/ results in mass outflow from the interior as well as surface flow. A component with a deep convective envelope can couple through the inner Lagrangian point well before its photosphere reaches its nominal Roche lobe. The horizontal pressure gradients characterizing the flow have scale heights only a fraction of the geometrical depth of contact. Eddy formation by Coriolis forces is inhibited by the turbulent viscosity of a convective medium, but the net effect of convection is further to impede net mass flow. The thermal diffusion time scale of the common envelope is found to be of the order of the sound travel time between components. Therefore flow between components can significanty perturb the vertical thermal structure of the common envelope only in the region near the inner Lagrangian point. A model of luminothat envelopeity transfer in contact binaries is proposed in which large-scale heat transport is accomplished by the coupling of Eddington-Sweet-type circulation with the dynamical flow near L/sub 1/. The large-scale circulation betweenmore » components absorbs or releases energy in the envelope of each star according to whether or not it is in the same sense as the static vertical entropy gradient in that envelope. An approximate criterion is obtained for good thermal contact in a common envelope, and shown to accord well with observed depths of contact.« less