Non－Darcian and Anisotropic Anisotropic Effects on Natural Convection in Horizontal Porous Media Enclosure

Natural convection heat transfer in a horizontal enclosure filled with anisotropic porous media,being isothermally heated at bettom and cooled at top while the vertical walls being adiabatic, is numerically studied by applying the Brinkman model-a modified form of Darcy model giving consideration to the viscous effect. The results show that:(1)a larger permeability ratio (K*) causes a lower flow intensity in the enclosure and a smaller Nusselt number, all Nusselt numbers approach unity in the limit of K* → ∞;a larger thermal conductivity ratio (λ*) causes a stranger distortion of isotherms in the enclosure and a higher flow velocity near the walls, all the Nusselt numbers approach unity in the limit of λ*→0; the permeability and thermal conductivity ratios generally have opposing effects on the Nusselt number. (2) an increasing Darcy number decreases the flow intensity and heat transfer rates,which is more significant at a lower permeability ratio. In particular, with K*0.25, the Nusselt number for Da=10-3 would differ from that of Darcy flow up to an amount of 30%, an analysis neglecting the non-Darican effect will inevitably be of considerable error.

Structural characteristic and ammonium and manganese catalytic activity of two types of filter media in groundwater treatment

Two types of filter media in groundwater treatment were conducted for a comparative study of surface structure and catalytic performance. Natural filter media was adopted from a conventional aeration–filtration groundwater treatment plant, and active filter media as a novel and promising filter media was also adopted. The physicochemical properties of these two kinds of filter media were characterized using numerous analytical techniques,such as X-Ray diffraction（XRD）, scanning electron microscope（SEM）, energy dispersive X-ray（EDX）, X-ray photoelectron spectroscopy（XPS） and Zeta potential. The catalytic activities of these filter media were evaluated for ammonium and manganese oxidation.XRD data showed that both active filter media and natural filter media belonged to birnessite family. A new manganese dioxide（Mn O2） phase（PDF#72-1982） was found in the structure of natural filter media. The SEM micrograph of natural filter media showed honeycomb structures and the active filter media presented plate structures and consisted of stacked particle. These natural filter media presented lower level of some trace elements such as calcium and magnesium, lower degree of crystallinity, lower Mn（III） content and lattice oxygen content than that of active filter media, which were associated with its poor ammonium and manganese catalytic activities. In addition, some γ-Fe2 O3 and Mn CO3 were found in the coating which may hinder the ammonium and manganese catalytic oxidation. This study provides a thorough and comprehensive understanding about the most commonly used filter media in water treatment, which can provide a theoretical guide to practical applications.

Gas hydrate formation in fine sand

Gas hydrate formation from two types of dissolved gas (methane and mixed gas) was studied under varying thermodynamic conditions in a novel apparatus containing two different natural media from the South China Sea. The testing media consisted of silica sand particles with diameters of 150-250 μm and 250-380 μm. Hydrate was formed (as in nature) in salt water that occupies the interstitial space of the partially water-saturated silica sand bed. The experiments demonstrate that the rate of hydrate formation is a function of particle diameter, gas source, water salinity, and thermodynamic conditions. The initiation time of hydrate formation was very short and pressure decreased rapidly in the initial stage. The process of mixed gas hydrate formation can be divided into three stages for each type of sediment. Sand particle diameter and water salinity also can influence the formation process of hydrate. The conversion rate of water to hydrate was different under varying thermodynamic conditions, although the formation processes were similar. The conversion rate of methane hydrate in the 250-380 μm sediment was greater than that in the 150-250μm sediment. However, the sediment grain size has no significant influence on the conversion rate of mixed gas hydrate.