Boundary layer control

V. TRANSMONOLAYER AND AQUEOUS BOUNDARY LAYER CONTROLLED KINETICS... 

Case 1 Aqueous Boundary Layer-Controlled Permeants... 

Combining the models for membrane-controlled and boundary layer-controlled uptake (Eqs. 3.40 and 3.47) for nonpolar compounds yields (Booij et al., 2003a)... 

Sampling rates for the case of total boundary layer-control can be expected to be nearly independent of temperature, since both the diffusion coefficients in air, and the kinematic viscosity of air are only weak functions of temperature (Shoeib and Harner, 2002). This leaves the air-flow velocity as the major factor that can be responsible for the seasonal differences among sampling rates observed by Ockenden et al. (1998). The absence of large R differences between indoor and outdoor exposures may be indicative of membrane-control, but it may also reflect the efficient damping of high flow velocities by the deployment devices used for SPMD air exposures (Ockenden et al., 2001). 

Comparing these results with the half-equilibration time of the aqueous phase, tm (see table above) we conclude that the aqueous concentration reaches its saturation value well before the exchange process switches from the boundary-layer-controlled to the NAPL-diffusion-controlled regime. Thus, diffusive transport of the diesel components from the interior of the NAPL to the boundary never controls the transfer process. Consequently, the simplex box model described in answer (a) is adequate. 

Two limiting situations may be identified r (1) the rate constant K is very small compared to aD, hence the process occurring in the interaction forces boundary layer controls the deposition rate, and (2) the rate constant K is very large hence the convective diffusion is the controlling factor. The first limiting case was treated by Hull and Kitchener (except for the variation of the diffusion coefficient) while the second was treated by Levich. In the present paper an equation is established which is valid for all values of the rate constant thus also incorporating both limiting situations. 

This concept was demonstrated at the Institute of Process Engineering and Cryogenics at ETH (Switzerland) during the last two years with the FilmCooled Hydrothermal Burner (FCHB). The FCHB operated at pressures of 25 MPa and temperatures up to 2000 K, cf. [1], Experiments and detailed analysis led to the basic design approach for SCWO reactors discussed herein which is based on wall boundary layer control and internal recirculation. 

Other reactor concepts show similar features (see [1] and [4]), which include boundary layer control and internal recirculation. Reference [5] proposes boundary layer control utilizing transpiration cooling. 

G Boundary/shear layer, W Wall of pressure vessel The wall boundary layer control system must protect the walls, while the whole reactor must match the specified performance and stability. 

Prevention or minimization of fouling and concentration polarization represents one of the main challenges that confronts membrane processing in general and membrane filtration of beer in particular. Various approaches have been developed to control membrane fouhng and increase the permeate flux in CMF, including membrane selection and modification, boundary layer control, use of turbulence inducers, or pretreatment of the feed. The two main strategies that are currently used in beer CMF are proper membrane selection and boundary layer control. 

Several approaches have been developed to control membrane fouling. They can be grouped into four categories (a) boundary layer control " ]... 

Figure 9.22A illustrates the purely diffusion-controlled process, in which the effects of boundary layers and interfacial reaction rates are negligible. In this case, the concentrations of the complex at the interfaces are the equilibrium concentrations. Figure 9.22B illustrates the partially boundary-layer-controlled case. Here, prior to steady state, the permeant diffuses across the membrane faster in the feed-side boundary layer and accumulation of permeant in the product-side boundary layer. The consequence of this concentration polarization is a reduction in the net concentration gradient across the membrane, and a reduced flux compared with the diffusion-controlled case. The last case is that of partially reaction-rate-controlled flux, illustrated by the concentration profile in Figure 9.22C. Here, either the permeant initially diffuses away from the feed interface faster than it can be replenished by the interfacial reaction, or the dissociation reaction is not fast enough to prevent accumulation of the complex at the product interface. Again, the net result is a decrease in the concentration gradient compared with that in the purely diffusion-controlled case. In all three cases, the flux is proportional to the slope of the concentration profile across the liquid membrane.

Figure 9.22 A schematic diagram showing the concentration profile of permeant in a liquid membrane when the Process is (A) diffusion-controlled, (B) boundary-layer-controlled, and (C) reaction-rate controlled. The concentration profile in the membrane in absence of boundary-layer and reaction-rate effects is shown for comparison.17...

Wall jets are bounded by a solid surface, the wall, on one side while the outer region of the flow is in contact with the ambient fluid. Wall jets find many applications such as cooling of surfaces, boundary layer control, building ventilation, energy dissipation etc. Jets emanating from sluices and other hydraulic structures close to the bed of channels need special attention to provide the necessary protective works. Prediction of the velocity profiles in the case of a wall jet is more difficult compared to the free jet. After a short distance downstream of the outlet, all jets tend to behave in a similar fashion. The schematic diagrams of a plane wall-jet and the boundary conditions used in the simulation are shown in Figure 1. 

V. G. Bogdevich and A. G. Maluga, Investigation of Boundary Layer Control, Thermophysics Institute Publishing, Novosibirsk, Siberia, 1976, p. 62. 

Here the actuator acts like a constant flux source without any fluctuations content. This mode acts on the main flow and is effective in turbulent boundary layer control for drag reduction. 

Here is the parabolic rate constant. Another common observation is linear weight loss, which generally indicates that gas phase diffusion of volatile products through a boundary layer is rate controlling. For a flat plate coupon, this boundary layer-controlled flux, J, is given by (Geiger and Poirier, 1973) ... 

Overall, it appears evident that fast-swimming sharks, such as the shortfin mako, have developed a skin for optimal boundary layer control to increase peak swimming speeds and contragiUty for pursuit of prey. The future will determine if biomimetic adaptations of the shark skin in applications including aircraft and marine vessels, among others, will lead to increased levels of boundary layer control that can decrease drag and increase maneuverability for modern technologies. 

Lang A, Motta P, Hidalgo P, Westcott M. (2008) Bristled shark skin A microgeometry for boundary layer control Bioinspir Biomim 3(4) 046005. 

The oxidation of graphite at different temperatures is controlled by three mechanisms (or modes) the chemical mechanism is observed at temperatures below 500° C, the in-pore diffusion-controlled mechanism at temperatures between 500 and 900° C and the boundary layer controlled mechanism at temperatures over... 

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