Capillarity and Gravity

In porous and granular materials, Hquid movement occurs by capillarity and gravity, provided passages are continuous. Capillary flow depends on the hquid material s wetting property and surface tension. Capillarity appHes to Hquids that are not adsorbed on capillary walls, moisture content greater than fiber saturation in cellular materials, saturated Hquids in soluble materials, and all moisture in nonhygroscopic materials. 

Ceaglske and Hougen (C4) showed that the movement of moisture in sand is controlled entirely by capillarity and gravity, and that diffusion is not involved. Diffusion equations cannot be made to apply by using variable diffusivity values. 

Also shown in Fig. 9.3 is the capillary pressure in a water-air system with the mean radius of curvature equal to the particle radius. It is clear that as the particle size spans over many orders of magnitude, the handling of the radiative heat transfer and the significance of forces such as capillarity and gravity also vary greatly. 

Su et al. (27) also quantified the conibined effect of gravity, capillary, and viscous forces on flow in a fi acture model, using Bond (Bo gravity force/capillary force) and capillary (Ca-viscous foree/capillary force) numbers. Figure 11 demonstrates the effect of capillarity and gravity driven flow, and shows fliat for the Bond number from 0.004-0.027, the volume of water per a dripping event remains insensitive to the Capillary number and for the nd number from 0.0S3 to 0.057, die volume of water per a dripping event increases drastically as the Capillary number increases. 

A debris bed module dealing with a solid and a liquid phase in thermal equilibrium is under development. Darcy s law is used to compute molten material velocity in the debris bed taking into account capillarity and gravity effects. The steam flow through the debris bed is calculated by the thermal hydraulics module applying Darcy s law and assuming that the steam is in thermal equilibrium with the compounds of the debris bed. 

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