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Prediction of combined heat and mass transfer for a fully developed flow of supercritical water Xu, Li

Abstract

Supercritical water oxidation (SCWO) is a technology for destroying organic wastes. In this process, water, organic waste and oxidant are brought together, and organics are oxidized to carbon dioxide and water. Under supercritical conditions, water has different features compared to normal water. Inorganic salts become insoluble at supercritical condition. Salts dissolved in organic wastes and produced from base and acid reactions in the system precipitate out of solution in the reactor. Fouling from salt is a serious problem for SCWO development. The salt deposition rate is proportional to the mass transfer coefficient and concentration difference between the wall and the bulk solution. In this work, a mass transfer model using concepts introduced by Deissler is developed based on an analogy among momentum, heat and mass transfer. The study focuses on supercritical water flow in a heated horizontal pipe. We ignore entrance length effects and assume that it is fully developed turbulent flow. Buoyancy is neglected in present research, so the flow is axisymmetric. The mixing length model with Van Driest's expression is used to calculate an eddy viscosity. We call this method to calculate heat and mass transfer coefficient the "Deissler- Van Driest" model. The model is validated by comparing laminar parabolic velocity profiles and turbulent velocity profiles for constant properties. The agreement in both cases is very good. Different mesh sizes are tested to check the effect on the results. Further validation is performed for variable properties: results are compared to the velocity profiles and heat transfer coefficients available in the literature, and also heat transfer coefficients predicted by Swenson et al's correlation (Swenson, H.S., Carver, J.R., and Kakarale, C.R., J. of Heat Transfer, November 1965.) Mass transfer coefficients calculated using the Deissler-Van Driest model are very similar to those predicted by the Swenson et al. correlation (with Prandtl number replaced by Schmidt number). Mass transfer coefficient increases steadily with bulk temperature. There is no peak in the pseudocritical region (where heat transfer coefficient peaks). Under supercritical conditions, the diffusion coefficient increases with temperature, but density and viscosity decrease with temperature. Higher diffusion coefficient, lower density and viscosity accelerate the diffusion of molecules and the transport of bulk fluid. This increases the mass transfer coefficient, especially at pseudocritical temperature.

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