Diffusion-convection process

The Schmidt number is defined as the ratio of molecular momentum to mass difiusivity. It is used to characterise fluid flows in which there are simultaneous momentum and mass diffusion convection processes. It is named after Ernst Schmidt and expressed as... 

Equation [21] shows water transport as a diffusion-convection process. If the diffusive term V.(/)V0) is neglected, Eq. 121 ] reduces to Eq. IK with r(0,r) = 0A7r)O), . This type of kinematic approximation to wilier transport has been used... 

Steady-State Diffusion—Convection Process of Reactant 54... 

In a similar way for Eqns (2.55) and (2.56), the current density for the steady-state diffusion—convection process of oxidant in Reaction (2-II) (in.o) can be expressed as ... 

Normally, if the electron transfer kinetics of oxidant reduction in Reaction (5-1) is very fast so that the diffusion-convection process could not catch up the speed of this electron-transfer process, the oxidant s surface concentration is quickly exhausted to zero, and the obtained Levich plot of /dc,o vs according to Eqn (5.14a) will be a straight line. From the slope (= 0.62nFDQ r / CQ) of the straight line, the parameter either as n, Do, v, or Cq can be estimated if the other three are known. 

Due to the defined hydrodynamic conditions, the diffusion-convection process near the disk/ring electrode surface can be calculated based on the experiment conditions and the measured data, providing the information of the electrode kinetics as a function of the applied potential, which has been described in Chapter 5. 

Amatore C, Oleinick A, Svir 1 (2004) Simulation of diffusion-convection processes in microfluidic channels equipped with double-band microelectrode assemblies approach through quasi-conformal mapping. Electrochem Commun 6 1123-1130... 

The gas-phase mass flux of species k at the surface is a combination of diffusive and convective processes. 

The theory on the level of the electrode and on the electrochemical cell is sufficiently advanced [4-7]. In this connection, it is necessary to mention the works of J.Newman and R.White s group [8-12], In the majority of publications, the macroscopical approach is used. The authors take into account the transport process and material balance within the system in a proper way. The analysis of the flows in the porous matrix or in the cell takes generally into consideration the diffusion, migration and convection processes. While computing transport processes in the concentrated electrolytes the Stefan-Maxwell equations are used. To calculate electron transfer in a solid phase the Ohm s law in its differential form is used. The electrochemical transformations within the electrodes are described by the Batler-Volmer equation. The internal surface of the electrode, where electrochemical process runs, is frequently presented as a certain function of the porosity or as a certain state of the reagents transformation. To describe this function, various modeling or empirical equations are offered, and they... 

If, as is normal, the solution is not stirred, then the conditions of laminar (uniform) diffusion characterising the above description will hold only for a short time. For longer periods, thermal and concentration gradients induce random convection processes and the resultant currents show sizeable fluctuations. 

To know that the overall process of mass transport occurs via three mechanisms, namely convection, migration and diffusion. Convection is the physical movement of solution, migration is the movement of charged analyte in response to Coulomb s law and diffusion is an entropy-driven process. In terms of mass transport, the order of effectiveness is as follows convection migration > diffusion. 

In many processes involving reactive flows different phenomena are present at different order of magnitude. It is fairly common that transport dominates diffusion and that chemical reaction happen at different timescales than convection/diffusion. Such processes are of importance in chemical engineering, pollution studies, etc. 

Transport of gas to the surface. Assuming mixing occurs by molecular diffusion rather than by mechanical or convective processes, the characteristic times for gas-phase diffusion to the surface are in the range 10 l(l-10-4 s for droplets with radii from 10 5 to 10 2 cm, respectively. 

Chemical vapor infiltration (CVl) is similar to CVD in that gaseous reactants are used to form solid products on a substrate, but it is more specialized in that the substrate is generally porous, instead of a more uniform, nominally flat surface, as in CVD. The porous substrate introduces an additional complexity with regard to transport of the reactants to the surface, which can play an important role in the reaction as illustrated earlier with CVD reactions. The reactants can be introduced into the porous substrate by either a diffusive or convective process prior to the deposition step. As infiltration proceeds, the deposit (matrix) becomes thicker, eventually (in the ideal situation) filling the pores and producing a dense composite. 

The aim of the article is to introduce new observations of diffusive-convective phenomena in polymer chemistry. The processes discussed are of significance to those interested in transport phenomena. 

Nucleation of the chalcogenide is much simpler in this process, since a solid phase—the metal hydroxide (or other solid phase)—is already present and the process proceeds by a substitution reaction on that solid phase. In this case, the initial step in the deposition is adhesion of the hydroxide to the substrate. This hydroxide is then converted into, e.g., CdS, forming a primary deposit of CdS clusters. More Cd(OFI)2 and, as the reaction proceeds, CdS and partially converted hydroxide diffuses/convects to the substrate, where it may stick, either to uncovered substrate (in the early stages of deposition) or to already deposited material. This is essentially the same process as aggregation, described in Chap-... 

A PVD-type reactor can be one in which molecules reach the surface directly in a molecular beam from some source or sources in which raw materials are vaporized. At the pressures commonly used (<10 6 Pa), the vaporized material encounters few intermolecular collisions while traveling to the substrate. Historically, higher pressure processes, such as sputtering and close-spaced vapor transport, have been classified as PVD (I). These processes also use physical means to generate the gas-phase species. However, the transport phenomena that need to be modeled for such higher pressure processes are more similar to CVD than PVD because of the diffusive-convective nature of transfer from the gas phase to the substrate. 

Permeation of mAbs across the cells or tissues is accomplished by transcellular or paracellular transport, involving the processes of diffusion, convection, and cellular uptake. Due to their physico-chemical properties, the extent of passive diffusion of classical mAbs across cell membranes in transcellular transport is minimal. Convection as the transport of molecules within a fluid movement is the major means of paracellular passage. The driving forces of the moving fluid containing mAbs from (1) the blood to the interstitial space of tissue or (2) the interstitial space to the blood via the lymphatic system, are gradients in hydrostatic pressure and/or osmotic pressure. In addition, the size and nature of the paracellular pores determine the rate and extent of paracellular transport. The pores of the lymphatic system are larger than those in the vascular endothelium. Convection is also affected by tortuosity, which is a measure of hindrance posed to the diffusion process, and defined as the additional distance a molecule must travel in a particular human fluid (i. e., in vivo) compared to an aqueous solution (i. e., in vitro). 

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