AbstractsEngineering

Convection due to the Rayleigh and Marangoni effects is known to enhance heat and/or mass transfer between two phases by as much as an order of magnitude in industrially important processes. In gas-liquid solute transfer applications this type of convection is often driven by a transient penetration type solute concentration profile. The ability to model solute transfer enhancement under such nonlinear transient concentration profiles works towards the objective of optimising such processes. In this thesis, a model of mass transfer enhancement under Rayleigh-Bénard-Marangoni convection is developed from the soluto-hydrodynamics of a gas-liquid solute transfer system. This model is developed under a penetration type base concentration profile and employing a weakly nonlinear approximation to describe slightly supercritical convection. This work improves upon recent related works by retaining a full nonlinear penetration profile throughout the development of the mass transfer enhancement model. A transient mass transfer enhancement model for roll and hexagonal convection patterns, based on the evolution of autonomous amplitude equations having spatially inhomogeneous advective terms is proposed. The model operates under a quasistatic approximation which is applicable for solute transfer in thin liquid layers. An experimental setup with the necessary sensitivity to study the weakly nonlinear mass transfer regime has been developed and used to test the proposed mass transfer enhancement model. Through numerical investigation of the convective amplitudes, it is shown that the spatially nonhomogeneous advective terms can be neglected when estimating the quasistatic amplitudes of roll and hexagonal convection, provided the planform is apriori known. When this is the case, the error introduced by neglecting the spatially nonhomogeneous terms is in the range 1 to 7%. Numerical investigations also indicate that hexagonal structures with vortex motion down the centre flow, similar to the case of Rayleigh-Bénard convection in a rotating fluid are possible with a penetration type base concentration profile. It is also found that for a large range of operating conditions, roll convection gives higher mass transfer rates over hexagonal convection. Under both roll and hexagonal convection, systems dominated by the Marangoni effect are found to produce the highest mass transfer rates. Furthermore, for any specified gas-liquid system it is possible to optimise the mass transfer enhancement by adjusting the ratio of the Marangoni number to the Rayleigh number. The proposed model of mass transfer enhancement under penetration conditions predicts an exponential-power law relationship between mass transfer enhancement and gas-liquid contact time. This is shown to be in good agreement with experimental measurements in the weakly nonlinear regime of the CO2-isobutanol system. The deviations between measured mass transfer rates and predicted mass transfer rates in this system was found to be between 3.4 and 13.5%. The variations of…