Horizontal porous layers

STABILITY OF HORIZONTAL POROUS LAYERS HEATED FROM BELOW... 

Heat transfer rate across a horizontal porous layer. 

Hickox, C.E. and Gartling, D.K.. A Numerical Study of Natural Convection in a Horizontal Porous Layer Subjected to an End-to-End Temperature Difference , 7. Heat Transfer. Vol. 103, pp. 797-802, 1981. 

Lai, F.C. and Kulacki, F. A.. "Oscillatory Mixed Convection in Horizontal Porous Layers Locally Heated from Below . Int. J. Heat and Mass Transfer. Vol. 34. pp. 887-890. 1991. 

Stability of Horizontal Porous Layers Heated from... 

FIGURE 9.25 Evaporation from a horizontal impermeable surface coated with a porous layer with 8 < 8,. + 6g(. The speculated intermittent drying of the layer and the associated temperature and saturation distributions are shown. 

Worster [99] employed the non-slip condition based on the assumption that the characteristic length scale of the flow on the melt/mushy interface is much larger than the space between the arms of the dendrite. This in general sense may not be the case since the BLM convection prevails in the system for most of the cases considered in the experiments. An alternative (the second model) was proposed by Beavers and Joseph [105] who systematically developed an empirical relation between the horizontal velocities of the fluid and the porous layer (or a homogenous and isotropic mushy layer) based on a series of experiments. This condition primarily allows a slip between the velocities above and below the melt/mushy interface. This condition can be expressed as... 

Poulikakos D (1984) Maximum density effects on natural convection in a porous layer differentially heated in the horizontal direction. Int J Heat Mass Transf 27 2067-2075... 

Everywhere at (r < L(0) except for a narrow region, close to the moving three-phase contact line, we have h A, and the liquid motion inside the porous layer under the drop can be neglected both in the vertical and horizontal directions (see the Appendix 1 for details). The size of this narrow region close to the moving... 

As mentioned previously, the permeability of the porous layer and the capillary pressure always enter as a product, i.e., as a single coefficient. Additional experiments were carried out to determine this coefficient. For this purpose, the horizontal imbibition of the liquid under investigation into the dry porous sheet was undertaken. Rectangular sheets, 1.5cm 3cm, were used. These porous sheets were cut from the same membranes used in the spreading experiments. Each sheet was immersed to a length of 0.3-0.5 cm into a liquid container, and the position of the imbibition front was monitored over time, hi the case under investigation, a... 

Vacuum filter systems consist of a horizontal cylinder partially submerged in a tank of biosolids. A layer of porous filter media fabric or tightly wound coils covers the outer surface of the cylinder. As the cylinder surface passes through the tank, a layer of biosolids adheres to the cylinder and vacuum is applied.61 The dewatered biosolids cake is then scraped off the fabric at up to 30% solids. 

Consider an essentially infinite horizontal layer of saturated porous medium that is heated from below and cooled from the top. The situation is shown in Fig. 10.33. 

Gavalas et al. [7] prepared ZSM-5 membranes onto porous a-alumina disks by in-situ hydrothermal synthesis at 175°C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 pm thick and consisted of well-intergrown crystals of about 2 pm in size Pure gas permeation measurements of the best preparations yielded hydrogen isobutane and butane isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185°C, respectively. 

Figure 7 shows the morphology of the thermal CVD prepared membrane. The size of deposited silica particles was mostly not exceeding 11 nm, and the thickness of deposited layer was about 2 pm. Figure 8 shows an EDX analysis of the interface between the support and CVD deposited layer. A silica layer was uniformly fonned on the top of the support with or without evacuation through the porous support. It is supposed that the growth of the deposited silica occurred towards the horizontal and the vertical directions in the pore. 

The assumption that the adsorbate has liquid-like properties after the first layer is difficult to reconcile, because both porous and nonporous (planar) adsorbents exposed to a saturated vapor sometimes adsorb strictly a limited amount and not the infinitely large quantity as postulated by BET. Another drawback of the BET model is essentially the coordination number of the molecules in the higher layers. The BET model assumes that each molecule that is adsorbed in any layer after the first layer, gives out its full latent heat of liquefaction, whether it has horizontal neighbors or not, and shall show a coordination number of 12. But in the absence of horizontal neighbors, the coordination number is much less that 12, and, therefore, the heat evolved (i.e., the heat of adsorption) should be only a fraction of the latent heat of condensation. 

In order to obtain the micropore volume and the external surface area of microporous solids, De Boer et al. (39) have developed the t-method. This method is based on the comparison of adsorption isotherm data of a porous sample and of a nonporous sample of identical chemical composition and surface character (reference isotherm). When the adsorbed N2 volume is plotted against the statistical thickness t (in nanometers) of the adsorbed N2 layer (Vads vs. t), a linear relation can be obtained. If both reference and sample isotherms are identical, as is the case for nonporous solids, a sbaight hue passing through or close to the origin is obtained (Fig. 7). Horizontal departures from the straight line indicate the presence of micropores. 

For the production of thin anode functional layers and electrolyte layers when producing planar solid oxide fuel cells, a modified slip casting process is used. Here, the suspension is applied on a planar, porous anode substrate in a special apparatus as shown in Fig. 21.17. The solvent is drawn through the pores by a vacuum. When the substrate is aligned horizontally, the solids of the suspension deposit evenly on the surface. This process to manufacture thin layers has the following advantages ... 

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