Modeling turbulence in landfill gas flow: Ingress into a horizontal well

Landfill gas collection is a salient component of waste management, energy recovery and environmental protection. The objective of the study is to improve the design and efficiency of operation of horizontal wells from the aspect of flow turbulence. Flow within the porous media (waste matrix, gravel) is coupled to the free pipe flow. Past studies performed this coupling without addressing the turbulence at the ingress to the well apertures. However, investigations at the apertures showed local fluctuations in pressure indicative of the importance of proper turbulence modeling. COMSOL Inc. Multi-physics simulation software is used to couple Darcy flow in the porous media with Reynolds averaged Navier-Stokes equations within the well using two turbulence models: k - e and k - w closure models. The results were compared to semi-analytical solutions implemented in GNU Octave (Eatonet al.,2020). Thequasi-1Dgeometryemployedin the semi-analytical model only accounted for the radial flow of gas and excluded the ingress turbulence effects. This study addresses such effects and the comparison plots from two solutions show the importance of including ingress turbulence. A significant discrepancy in head losses within the well and across the landfill was observed between the two approaches. The cumulative ingress impact diminishes the suction strength required for adequate gas extraction as well as pinpoints the proximity of the intake apertures as the salient locus of head loss in the landfill mass. These effects are shown to impact the design and operational parameters of the horizontal well. Differences in the Reynolds number (found from the turbulence modeling results) showed the limitation of the quasi-1D geometry employed in the semi-analytical solutions.The velocity profiles at the ingress revealed abrupt changes indicative of adequate mesh refining and careful ingress flow modeling. Friction factor variation within the well showed the importance of careful ingress flow modeling. The study conclusively proved the importance of modeling turbulence and its underlying structures at the ingress for a realistic representation of flow in the horizontal wells