Study On Mass Transfer Phenomena Across A Moving Interface Of Gas-Liquid System

Better understanding of the interfacial mass transfer behaviors under following condition of gas-liquid systems, and further developing the corresponding mass transfer theory are of great importance for real production practice in chemical industry. In this thesis, experiments were conducted to investigate the concentration distribution and hydrodynamics character in the vicinity of the interface, and corresponding mechanism of their influences on the interfacial mass transfer.Firstly,the real-time laser holographic interferometry was applied to observe qualitatively the evolutions of concentrations distributions and to messure quantitively the liquid phase concentrations distribution in the vicinity of the interface during the mass transfer process under the conditions of cocurrent gas-liquid flow. A series of optical qualitative experiments showed the existence of phenomena of near-interface turbulence and periodic burst. The formation mechanisms of these two instable phenomena were analyzed, and the dynamic behavior, controlling factors and the influence of the burst on mass transfer were also studied. In a steady-state for the mass transfer process, with the optical measurement, the two dimensional concentration distributions inside the liquid phase and in the vicinity of the free interface were obtained via the interpretation of interference fringes. The steady-state results gained under different conditions demonstrated a sharp exponent increase in concentration with approaching to the interface. The influence of the Reynolds numbers of both gas and liquid phases on the concentration boundary layer thickness and fathomable near-surface concentration were also observed. This quantitative research padded the information shotages about concentration field near a moving interface during mass transfer. In addition, according to above experimental data and film theory, a shortcut method was proposed to estimate the interfacial mass transfer rate, in which Sherwood number representing the mass transfer coefficient was correlated as a function of the Reynolds numbersn of both phases. This method was also verified with experimental data found in the literature and conducted for other system.Secondly, experiments were conducted using Laser Doppler Anemometer (LDA) to investigate instantaneous hydrodynamic behaviors of the channel flow in the experiments. Instantaneous velocity measurements indicated that the fluctuation velocity near the interface is not isotropy as usually expected. The time averaging velocity distribution can be divided into three regions: laminar sub-layer, turbulent bulk flow and as well as the interfacial vicinity. In the interfacial vicinity, a dip phenomenon, which means that the position with the maximal velocity is lower than the interface, was found. Then, the statistical characters, such as correlation function, spectrum function and character scales, which define the flowing structure were obtained. Consequently, the mass transfer coefficients contributed by the flowing structure were calculated from the eddy pool model. The results showed that the mass transfer rate would be very much enhanced by the burst arriving up to the interface. Therefore the burst phenomena should be considered seriously as an influencial factor for mass transfer.Finally, the above data of concentration and velocity fields near the free interface of the same operating condition are coupled to study mass transfer in a gas-liquid flowing system. A mass balance based approach was used for estimating liquid mass transfer coefficients, and average mass transfer coefficients in the liquid phase were obtained for different flowing conditions. The results were compared to those calculated with other models. Then, considering the burst effect, which enhances significantly the mass transfer, a correction was made on a single eddy model and the new model was proposed to predict effectively the interfacial mass transfer rate. The proposed model was verified with our LDA experimental results, at the same time was compared with balance model and film model. The results demonstrated that the proposed model can give good prediction of mass transfer with periodical burst.