Concentration fields near air-water interfaces during interfacial mass transport: oxygen transport and random square wave analysis

Abstract

-Mass transfer across a gas-liquid interface was studied theoretically and experimentally, using transfer of oxygen into water as the gas-liquid system. The experimental results support the conclusions of a theoretical description of the concentration field that uses random square waves approximations. The effect of diffusion over the concentration records was quantified. It is shown that the peak of the normalized rms concentration fluctuation profiles must be lower than 0.5, and that the position of the peak of the rms value is an adequate measure of the thickness of the diffusive layer. The position of the peak is the boundary between the regions more subject to molecular diffusion or to turbulent transport of dissolved mass.

One Dimensional Turbulent Transfer Using Random Square Waves – Scalar/Velocity and Velocity/Velocity Interactions

“…Random square waves were also applied by Janzen (2006), who used the techniques of Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) to study the mass transfer at the air-water interface, and compared his measurements with the predictions of employing ad hoc concentration profiles. Further, Schulz & Janzen (2009) confirmed the upper limit for the normalized RMS of the concentration fluctuations by taking into account the effect of diffusion, also evaluating the thickness of diffusive layers and the role of diffusive and turbulent transports in boundary layers. A more detailed theoretical relationship for the RMS of the concentration fluctuation showed that several different statistical profiles of turbulent mass transfer may be interrelated.…”

“…This form is useful to obtain the reduction function from experimental data, using the normalized mean profile and the RMS profile, as shown by Schulz & Janzen (2009). Equation (23) shows that diffusion, or other causes that inhibit the fluctuations and imply in 0 f   , imposes a peak of f' 2 lower than 0.5.…”

“…In this case, the flow between two parallel plates is considered. We follow a procedure similar to that presented by Schulz & Janzen (2009), in which the measured functional form of the reduction function is shown. As a basis for the analogy, some governing equations are first presented.…”

“…As done by Schulz & Janzen (2009), the profile of can be obtained from experimental data, using equation (63).…”

“…Using the effect of diffusion to interpret the new function, values of close to 1 or 0 indicate strong or weak influence of diffusion, respectively. Considering this interpretation, Schulz & Janzen (2009) reported experimental profiles for in the mass concentration boundary layer during airwater interfacial mass-transfer, which showed values close to 1 in both the vicinity of the surface and in the bulk liquid, and closer to 0 in an intermediate region (giving therefore a minimum value in this region).…”