Rune Røjgaard Andreasen

Air flow characteristics in granular biofilter media

AbstractPressure drop (ΔP) as a function of air velocity (V) is a key parameter controlling air cleaning biofilter cost efficiency. At present the V–ΔP relationship in biofilter materials must generally be determined experimentally, as no universal link between the V–ΔP relationship and material physical properties have been established. The objective of this study was, therefore, to investigate the link between the V–ΔP relationship, material particle size distribution, and wetting conditions for a common biofilter medium, Leca. Measurements of V–ΔP was carried out for 36 different Leca particle size fractions with different mean particle diameter (Dm) and particle size range (R) under both dry and wet conditions. Measurements showed that increasing Dm and decreasing R together with wetted conditions in the filter generally resulted in decreasing ΔP for the same V. A set of expressions allowing for accurate estimation of the V–ΔP relationship based on Dm, R. and wetting conditions were established, enabl...

Relating Gas Dispersion in Porous Media to Medium Tortuosity and Anisotropy Ratio

Gas dispersion in a set of three different porous materials with similar particle size, as a function of material tortuosity and anisotropy ratio, was investigated. The materials were packed with different spatial orientations of the individual particles so as to create media with different tortuosity and anisotropy ratios. Three different media (slate chips, wood chips, and pebbles) and four particle orientations have been used to generate a total of nine different porous media mimicking single porosity, dual porosity isotropic, anisotropic, aggregated, or granular materials. Resulting values of tortuosity and anisotropy ratio for each medium were determined via measurements of gas permeability and molecular gas diffusion coefficient. These values were then compared to measured values of gas dispersivity for each medium. The results showed that dispersivity is inversely proportional to tortuosity but directly proportional to anisotropy ratio and that the relations were approximately linear within the range of tortuosities and anisotropy ratios investigated. Wood chips (dual porosity material) yielded higher values of gas dispersivity compared to slate chips (single porosity material). A likely reason is in part the difference in pore structure between the materials and in part a difference in particle surface roughness (which was highest for wood chips) both of which affects dispersion.

Relating Water and Air Flow Characteristics in Coarse Granular Materials

This paper investigates the connection between the velocity (V)–pressure drop (ΔP) relationships for air and water flow in coarse porous filter media with the aim of linking the V–ΔP relationships for air and water. Investigations were carried out using a common biofilter medium, Leca® consisting of rounded porous particles of 2- to 18-mm diameter. V–∆P relations for water flow were measured for 14 different Leca® particle size fractions and compared with measurements of V–∆P relations for air flow in 36 different Leca® particle size fractions (including the 14 used for water flow). The measurements showed a strong relationship between the two types of relationships, and further that this relationship could be described using a single constant. An approach for predicting the water flow V–∆P relationship from the corresponding air flow relationship across different particle size fractions from the same material is suggested, tested, and found valid for Re numbers higher than that which is considered in the presently available studies of air/water V–∆P relationships.