Forced-convection mass-transfer

Fig. 10. Numerical solutions of the forced-convection mass-transfer equation for the case of irreversible first-order chemical reaction [after Johnson et al. (J4)] (Solid lines— rigid spheres dashed lines—circulating gas bubbles).

In the second example, let the case of forced convective mass transfer in pipe flow be considered. Let it be assumed that the turbulent flow of the fluid, B, through the pipe is accompanied by a gradual dissolution of the material, A, of the pipe wall. Experimental... 

Thus, the case of forced convective mass transfer in pipe flow, one has Sh — f (Re, Sc)... 

The effective diffusivities determined from limiting-current measurements appear at first applicable only to the particular flow cell in which they were measured. However, it can be argued plausibly that, for example, rotating-disk effective diffusivities are also applicable to laminar forced-convection mass transfer in general, provided the same bulk electrolyte composition is used (H8). Furthermore, the effective diffusivities characteristic for laminar free convection at vertical or inclined electrodes are presumably not significantly different from the forced-convection diffusivities. 

Experimental results obtained at a rotating-disk electrode by Selman and Tobias (S10) indicate that this order-of-magnitude difference in the time of approach to the limiting current, between linear current increases, on the one hand, and the concentration-step method, on the other, is a general feature of forced-convection mass transfer. In these experiments the limiting current of ferricyanide reduction was generated by current ramps, as well as by potential scans. The apparent limiting current was taken to be the current value at the inflection point in the current-potential curve. 

Thomas, D.G. Forced convection mass transfer in hyperfiltration at high fluxes. Ind. Eng. Chem. Fundam. 1973, 12, 396. 

The Sherwood. Re noldx, and Schmidt numbers are used in forced convection mass transfer currctalions. 

When flow occurs about a sphere the solution to tins forced convection mass transfer problem is quite complex because of the complexity of the flow field. At low flow rates (creqiiiig flow) a laminar boundary layer exists about the sphere which separates from die surface at an at ular porition and moves lowani the forward stagnation point as the flow rate increases. Wake fimnation occurs st the tear of the sphere. At still higher flow rates transition to a turbulent boundary layer occurs. Solutions to the problem of mass transfer during creeping flow about a sphere (Re < 1) have been developed by a nombw of authors with the numerical solutions of Brian and Hales being perhaps the most extensive. Their result is... 

Common Correlations for Forced Convection Mass Transfer at Supercritical Conditions... 

In addition, the forced convection mass transfer coefficient for transfer of the specie j to or from the electrode interface can also be defined by [5]... 

Free convection heat transfer as a source of forced convection mass transfer. It has been demonstrated on numerous occasions that the Chilton-Colburn analogy appearing in Table 2.3 is applicable for converting a forced-convection Nusselt number to a forced-convection Sherwood number as a means of converting the imbedded HTC into its equivalent MTC. In the present situation, the thermal buoyant forces provide the momentum source, which in effect provides the forced-convective flow that drives the mass transfer process. In addition, Grj Gta and Sc > Pr. For this case the alternative equation is... 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

Lin Q, Jiang F, Wang X-Q, Han Z, Tai Y-C, Lew J, Ho C-M (2000) MEMS Thermal Shear-Stress Sensors Experiments, Theory and Modehng, Technical Digest, Solid State Sensors and Actuators Workshop, Hilton Head, SC, 4—8 June 2000, pp 304-307 Lin TY, Yang CY (2007) An experimental investigation of forced convection heat transfer performance in micro-tubes by the method of hquid crystal thermography. Int. J. Heat Mass Transfer 50 4736-4742... 

Wang BX, Peng XF (1994) Experimental investigation of liquid forced-convection heat transfer through micro-channels. Int J Heat Mass Transfer 37 73-82 Wasekar VM, Manglik RM (2002) The influence of additive molecular weight and ionic nature on the pool boiling performance of aqueous surfactant solutions. Int J Heat Mass Transfer 45 483-493... 

Shah, R. K., Laminar flow friction and forced convection heat transfer in ducts of arbitrary geometry, Int. ). Heat Mass Transfer 18 (1975) 849-842. 

In free-convection mass transfer at electrodes, as well as in forced convection, the concentration (diffusion) boundary layer (5d extends only over a very small part of the hydrodynamic boundary layer <5h. In laminar free convection, the ratio of the thicknesses is... 

Dne to the macroporons strnctnre of monolithic stationary phases (flow channels), the solvent is forced to pass the entire polymer, leading to faster convective mass transfer (compared to diffnsion), which provides for analyte transport into and out of the stagnant pore liqnid, present in the case of microparticulate columns. 

Shah, R.K., Laminar Row Friction and Forced Convection Heat Transfer in Ducts of Arbitrary Geometry , Int. J. Heat Mass Transfer, Vol. 18, pp. 849-862, 1975. 

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