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Modelling two-phase flows using CFD
84
ANNUAl.. LITERATURE SURVEY 1996
Gas-solid mass transfer in a jetloop reactor Moller K.P. & O'Connor C.T., AIChE Journal, 1996, 42/4 (1187-1190). In English. This article presents results of a study of the mass-transfer rates in a new internal recycle rector, the jetloop reactor (JLR) as proposed by Luft. The residence time studies in this reactor have shown that it behaves as a wellmixed reactor, with recycle ratios easily exceeding the minimum value of 20 required for CSTR behavior. In this system, however, the influence of mass-transfer rate on the reaction rate cannot be determined by varying the impeller speed, as the recycle ratio is dependent on the flow rate and must therefore be measured independently. The objective of this work was to symmetrically investigate the effect of particle size and flow rate on the masstransfer rates to catalyst particles with a view to using this reactor for kinetic studies. (from Authors)
Gas-solid and gas-liquid mass-transfer coefficients Dudukovic A., Milosevic V. & Pjanovic g., AICHe Journal, 1996, 42/1 (269-270). In English. In 1934 Gilliland and Sherwood presented the data on the rates of vaporization of nine liquids into air in a wetted-wall column, in order to test the analogy of Colburn (1930). The results obtained were interpreted as gassolid transport and correlated by: Sh = 0.023 Re .... Sc°'44 . However, some of the texts also offer an alternative' relation: Sh = 0.023Re °'s3 Sc°s3 as a general equation for turbulent mass transfer in pipes for both gases and liquids. (from Authors)
Wavy-to-slug flow transition in slightly inclined gas-liquid pipe flow Grolman E., Commandeur N.C.J., De Baat E.C. & Fortuin J.M.H., AIChEJoumal, 1996, 42/4 (901-909). In English. A process-engineering model is presented for the stratified-wavy-to-intermittent (SW-I) flow-pattern transition in slightly inclined gas-liquid pipe flow. The main parameter for predicting (in)stability of wavy flow in inclined pipes is the average liquid holdup, which was found to reach a maximum, critical value at flow-pattern transition. Observed values of the critical liquid holdup vary between 0.07 and 0.42, depending on pipe diameter, angle of inclination and transport properties of the gas-liquid system. Flow-pattern maps are presented for selected angles of inclination, showing excellent agreement between predicted and observed flow-pattern boundaries. (from Authors)
Fickian diffusion in binary mixtures that form two liquid phases Pettier M., Blass E. & Stevens G.W., AIChE Journal, 1996, 42/4 (910-920). In English. New data on the variation of the diffusion coefficient with concentration in binary nonideal liquid mixtures are presented. The diffusion coefficients were measured with laser holography with an improved analysis procedure and are primarily in systems that form two liquid phases. The results show that for such systems the diffusion coefficient is constant if a chemical-potential driving force is used. If, however, the miscibility gap is wide, the Schreiner equations (Schreiner, 1922) is shown to be more accurate than relations that consider the variation of viscosity. Cluster theories developed for diffusion behavior near critical points were found to explain the data only in water-organic systems on the water-rich side. (Authors)
Permeability of gigaporons particles Pfeiffer J.F., Chert J.C. & Hsu J.T., AIChE Journal, 1996, 42/4 (932-939). In English. The volumetric flow rate of liquid and gas through small gigaporous particles was measured by a new method that isolates single particles in a test apparatus. High-performance liquid chromatography particles from 30 to 50 mum in diameter, previously reported to exhibit convection-enhanced intraparticle mass transfer, were studied. Using a CFD model of the test system, the permeability of individual particles was determined from the pressure-drop-flow-rate relationship. The results of this study might imply that the intrapar title structure does not behave like a bed of uniformly packed microspheres, but rather as an inhomogeneous assemblage of microparticles. The measured permeability values offer the possibility of developing better models of the intraparticle flow field under normal operating conditions. (from Authors)
Modelling two-phase flows using CFD Lun I., Calay R.K. & Holdo A.E., Applied Energy, 1996, 53/3 (299-314). In English. Two-phase flows are encountered in a wide range of industrial and natural situations. Due to their complexity such flows have been investigated only analytically and experimentally. New computing facilities provide the flexibility to construct computational models that are easily adapted to a wide variety of physical conditions without constructing a large-scale prototype or expensive test rigs. But there is an inherent uncertainty in the numerical predictions due to stability, convergence and accuracy. The importance of a well-placed mesh is highlighted in the modelling of two-phase flows in horizontal pipelines. (Authors)
Airflow through beds of cereal grains De Ville A. & Smith E.A., Applied Mathematical Modelling, 1996, 20/4 (283-289). In English. The equations used to model the flow of air through beds of cereal grains become nonlinear when the resistance to flow is a function of the velocity of the air. An analytical solution to this problem is obtained for the case where the flow is predominantly in one direction. The problem is defined in terms of a parameter epsilon which for typical cereal grains has values in the range of 0.02-1.0. A perturbation expansion in terms of epsilon is used to obtain a weakly nonlinear solution to flow problem. The solution is valid for small epsilon and this corresponds to small grains and low air velocity. The solution is used to study the effect of the nonlinearity on the flow pattern. A numerical method is used to extend the solution to larger values of epsilon, and this confirms the general effect of the nonlinear term on the airflow pattern which was determined by the analytical solution. (Authors)
ANNUAl.. LITERATURE SURVEY 1996
Gas-solid mass transfer in a jetloop reactor Moller K.P. & O'Connor C.T., AIChE Journal, 1996, 42/4 (1187-1190). In English. This article presents results of a study of the mass-transfer rates in a new internal recycle rector, the jetloop reactor (JLR) as proposed by Luft. The residence time studies in this reactor have shown that it behaves as a wellmixed reactor, with recycle ratios easily exceeding the minimum value of 20 required for CSTR behavior. In this system, however, the influence of mass-transfer rate on the reaction rate cannot be determined by varying the impeller speed, as the recycle ratio is dependent on the flow rate and must therefore be measured independently. The objective of this work was to symmetrically investigate the effect of particle size and flow rate on the masstransfer rates to catalyst particles with a view to using this reactor for kinetic studies. (from Authors)
Gas-solid and gas-liquid mass-transfer coefficients Dudukovic A., Milosevic V. & Pjanovic g., AICHe Journal, 1996, 42/1 (269-270). In English. In 1934 Gilliland and Sherwood presented the data on the rates of vaporization of nine liquids into air in a wetted-wall column, in order to test the analogy of Colburn (1930). The results obtained were interpreted as gassolid transport and correlated by: Sh = 0.023 Re .... Sc°'44 . However, some of the texts also offer an alternative' relation: Sh = 0.023Re °'s3 Sc°s3 as a general equation for turbulent mass transfer in pipes for both gases and liquids. (from Authors)
Wavy-to-slug flow transition in slightly inclined gas-liquid pipe flow Grolman E., Commandeur N.C.J., De Baat E.C. & Fortuin J.M.H., AIChEJoumal, 1996, 42/4 (901-909). In English. A process-engineering model is presented for the stratified-wavy-to-intermittent (SW-I) flow-pattern transition in slightly inclined gas-liquid pipe flow. The main parameter for predicting (in)stability of wavy flow in inclined pipes is the average liquid holdup, which was found to reach a maximum, critical value at flow-pattern transition. Observed values of the critical liquid holdup vary between 0.07 and 0.42, depending on pipe diameter, angle of inclination and transport properties of the gas-liquid system. Flow-pattern maps are presented for selected angles of inclination, showing excellent agreement between predicted and observed flow-pattern boundaries. (from Authors)
Fickian diffusion in binary mixtures that form two liquid phases Pettier M., Blass E. & Stevens G.W., AIChE Journal, 1996, 42/4 (910-920). In English. New data on the variation of the diffusion coefficient with concentration in binary nonideal liquid mixtures are presented. The diffusion coefficients were measured with laser holography with an improved analysis procedure and are primarily in systems that form two liquid phases. The results show that for such systems the diffusion coefficient is constant if a chemical-potential driving force is used. If, however, the miscibility gap is wide, the Schreiner equations (Schreiner, 1922) is shown to be more accurate than relations that consider the variation of viscosity. Cluster theories developed for diffusion behavior near critical points were found to explain the data only in water-organic systems on the water-rich side. (Authors)
Permeability of gigaporons particles Pfeiffer J.F., Chert J.C. & Hsu J.T., AIChE Journal, 1996, 42/4 (932-939). In English. The volumetric flow rate of liquid and gas through small gigaporous particles was measured by a new method that isolates single particles in a test apparatus. High-performance liquid chromatography particles from 30 to 50 mum in diameter, previously reported to exhibit convection-enhanced intraparticle mass transfer, were studied. Using a CFD model of the test system, the permeability of individual particles was determined from the pressure-drop-flow-rate relationship. The results of this study might imply that the intrapar title structure does not behave like a bed of uniformly packed microspheres, but rather as an inhomogeneous assemblage of microparticles. The measured permeability values offer the possibility of developing better models of the intraparticle flow field under normal operating conditions. (from Authors)
Modelling two-phase flows using CFD Lun I., Calay R.K. & Holdo A.E., Applied Energy, 1996, 53/3 (299-314). In English. Two-phase flows are encountered in a wide range of industrial and natural situations. Due to their complexity such flows have been investigated only analytically and experimentally. New computing facilities provide the flexibility to construct computational models that are easily adapted to a wide variety of physical conditions without constructing a large-scale prototype or expensive test rigs. But there is an inherent uncertainty in the numerical predictions due to stability, convergence and accuracy. The importance of a well-placed mesh is highlighted in the modelling of two-phase flows in horizontal pipelines. (Authors)
Airflow through beds of cereal grains De Ville A. & Smith E.A., Applied Mathematical Modelling, 1996, 20/4 (283-289). In English. The equations used to model the flow of air through beds of cereal grains become nonlinear when the resistance to flow is a function of the velocity of the air. An analytical solution to this problem is obtained for the case where the flow is predominantly in one direction. The problem is defined in terms of a parameter epsilon which for typical cereal grains has values in the range of 0.02-1.0. A perturbation expansion in terms of epsilon is used to obtain a weakly nonlinear solution to flow problem. The solution is valid for small epsilon and this corresponds to small grains and low air velocity. The solution is used to study the effect of the nonlinearity on the flow pattern. A numerical method is used to extend the solution to larger values of epsilon, and this confirms the general effect of the nonlinear term on the airflow pattern which was determined by the analytical solution. (Authors)