Numerical solutions to the unaveraged mass balance equation for the case of a flat mobile interface reveal that both high-and low-frequency velocity fluctuations can contribute to mass transfer. This is in contrast to transport to a solid boundary where only low-frequency fluctuations, normal to the wall, are important. The average mass transfer coefficient is found to depend on Schmidt number to the -0.5 power, in agreement with classical theories. It is related to the velocity field in the liquid primarily through the mean-square value of the gradient of normal velocity fluctuations at the interface. SCOPEThe absorption of a slightly soluble gas in a concurrent, turbulent gas-liquid flow is controlled by flow fluctuations in the liquid through mechanisms that are not yet understood. This is a situation where the Schmidt number is large, and as a consequence the concentration boundary layer in the liquid is confined to a very thin region near the interface. Shear, caused by the gas flow, induces wave formation and also greatly enhances mass transfer rates. Mass transfer coefficients measured by Aisa et al. (1981) and McCready and Hanratty (1 984b) are about one order of magnitude larger than those found for turbulent mass transfer at a solid boundary. The fundamental problem that must be considered in order to understand mass transfer in these complex flows is how the gas and liquid velocity fields interact to control the transport rate. The work presented here addresses this question by numerically solving the unaveraged mass balance equations for a randomly varying velocity.A similar approach was taken previously by Campbell and Hanratty (1983) in their studies of mass transfer between a turbulent fluid and a solid boundary. These yielded the surprising result that mass transfer is controlled by low-frequency velocity fluctuations normal to the surface that contain only a small fraction of the turbulent energy.The principal hydrodynamic difference between the case considered here and the case considered by Campbell and Hanratty is that near a mobile boundary the velocity normal to the boundary varies linearly with distance from the boundary, rather than quadratically. The goal of this work has been to explore the consequences of this difference. CONCLUSIONS AND SIGNIFICANCEMass transfer at a mobile boundary (a clean gas-liquid interface) is found to be fundamentally different from mass transfer at an immobile boundary (a contaminated gas-liquid interface or a solid), in that velocity fluctuations of all frequencies are playing an important role. This is reflected in the equations for the massThe presentaddressof Eleni Vassiliadou is KoninklijkeShell Lab.. Amsterdam, Holland. transfer coefficient that are derived from computer experiments.The velocity normal to mobile and immobile surfaces are given respectively by u = p ( x , z, t ) y, and u = p ( x , z, t ) y', where all terms have been made dimensionless using a friction velocity and the kinematic viscosity. Campbell and Hanratty (1983) T h e importanc...

Electrochemical techniques are used to show that drag-reducing polymers decrease the magnitude and the frequency of mass transfer fluctuations. Measurements of the frequency spectrum of the mass transfer fluctuations allow calculation of the normal component of velocity fluctuations in the immediate vicinity of the wall. This information is used to interpret the observed effect of drag-reducing polymers on the rate of mass transfer. Eleni Vassiliadou SCOPEA paper by McConaghy and Hanratty (1977) presents results of the effect of dilute polymer solutions of Separan AP-30 on measurements of average mass transfer coefficients between a turbulent fluid and a pipe wall for fully developed velocity and concentration fields. They found that the decrease in the mass transfer rate, caused by the addition of polymers, at a given volumetric flow rate, is greater than the percent change in the pressure gradient, and that the use of an analogy between momentum and mass transfer overpredicts the amount by which the mass transfer rate is decreased.This paper shows how drag-reducing polymers affect the fluctuations in the mass transfer coefficient. It is based on measurements presented in theses by McConaghy (1974) and Vassiliadou (1983). The motivation for this paper is the testing of ideas that have emerged from recent numerical experiments that simulated turbulent mass transfer at a wall (Campbell and Hanratty, 1983). CONCLUSIONS AND SIGNIFICANCEDrag-reducing polymers cause a large decrease in_the magnitude of the mass transfer fluctuations, (P)''*/K, and change the character of the function describing the fluctuating mass transfer coefficient, k(t). The function does not have the large pulses found for a Newtonian fluid, and is described reasonably well by a Gaussian distribution. Furthermore, the frequency characterizing k(t), made dimensionless with wall parameters, is less than for a Newtonian fluid.Campbell and Hanratty (1983) transfer is the zero frequency value of the dimensionless spectral density function, W,(O), characterizing the normal component of velocity fluctuations in the immediate vicinity of the wall. This quantity is calculated from the measurements of the spectral function of the mass transfer fluctuations that are presented. It is found to decrease with increasing drag-reduction.The dependence of the time-averaged mass transfer coefficient on W,(O) is found to be the same as predicted in the numerical experiments of Campbell and Hanratty. However (k2)"'/E is found to be more strongly dependent on WB(0). This suggests that other hydrodynamic variables than W,(O) need to be considered.