Effect of varying atmospheric conditions on methane boundary‐layer development in a free flow domain interfaced with a porous media domain

Abstract

Mitigation of atmospheric emission of methane from leaky underground infrastructure is important for controlling the global anthropogenic greenhouse gas burden. Overexposure to methane may also cause occupational health problems in indoor/outdoor environments at the local scale. Subsurface soil conditions (e.g. soil heterogeneity) affect methane migration in soils while near‐surface atmospheric boundary conditions (e.g. wind and temperature) affect off‐site emissions across the soil‐atmosphere interface. This study investigated the above‐surface methane concentration boundary‐layer development under different soil conditions (homogenous and layered) and atmospheric boundary controls (wind and temperature). A series of controlled bench‐scale experiments was conducted using an open‐loop boundary‐layer wind tunnel interfaced with a porous media tank inside which a simulated methane point source was embedded to mimic a buried leaky pipeline. Results revealed pronounced effects of wind and, to a lesser extent, temperature on above‐surface methane boundary‐layer development. High atmospheric temperature contributed to the concentration boundary‐layer build‐up whereas high wind velocity caused erosion of the boundary layer. The boundary‐layer methane concentration profiles were adequately simulated within an advection‐diffusion modeling framework combined with a Navier‐Stokes free flow domain by coupling free air flow, heat flow and flow of a multicomponent gas mixture. Simulations further implied that the Fickian diffusion approach may have limited applications when pronounced non‐isothermal conditions prevail within the system. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Gas Component Transport Across the Soil‐Atmosphere Interface for Gases of Different Density: Experiments and Modeling

“…It was not expected to achieve a perfect match to the experimental results, since some of the complexity of the experiments was neglected in the model (variations in atmospheric conditions, possible heterogeneity of the porous medium, and pressure fluctuations at the gas inlet). Although we do not have measurement data to verify the simulated concentrations inside the free flow, we can state that the predicted concentration layers at the soil surface are in agreement with observations made in Chamindu Deepagoda, Oldenburg (2016) andChamindu Deepagoda et al (2018). As expected, surface concentrations increase along the surface in wind direction (see Figure 9b).…”

“…The effect is expected to increase with an increase in wind velocity. Second, gas component concentrations above the soil surface increase in wind-downstream direction due to the accumulation of gas components inside the laminar boundary layer (as has been shown by Chamindu Deepagoda, Smits, & Oldenburg, 2016;Chamindu Deepagoda et al, 2018). Consequently, concentration gradients across the surface are higher at the wind-upstream side of the sand tank than at the downstream side, thereby causing lower concentrations at transect OA than at transect OC.…”

“…Thus, affected locations correspond to the area where diffusive flux is likely to dominate over advective flux, indicating that an increase in wind velocity enhances diffusive fluxes inside the tank. This is likely to be caused by the effect of the wind velocity profile on the concentrations at the soil surface (see, e.g., observations made by Chamindu Deepagoda et al, 2018). With an increase of wind velocity, the thickness of the laminar boundary layer decreases, which reduces soil surface concentrations and increases concentration gradients between soil and atmosphere.…”

“…The setup is adaptable for different gases. It was inspired by previous gas transport and evaporation experiments in coupled systems of porous medium and free flow, where a sand tank is placed underneath a wind tunnel (Chamindu Deepagoda et al, 2018;Chamindu Deepagoda, Smits, & Oldenburg, 2016;Davarzani et al, 2014). Such systems allow for high control over test conditions and possess clearly defined domain boundaries for both the porous medium and the overlying wind field, which is ideal for subsequent numerical studies.…”

Numerical simulation of methane spreading in porous media after leaking from an underground pipe

“…The effects of multi-layer structure (Esposito et al , 2014) and moisture (Deepagoda et al , 2016) on natural gas leak from underground pipeline were also experimentally studied by researchers, with the presence of atmospheric wind near the ground. Researchers (Deepagoda Tkk et al , 2017) also analyzed the effect of ground wind strongness on underground natural gas leak. Similar experimental research was reported by some researchers (Basirat et al , 2015; Gasparini et al , 2015), using a much safer gas, CO 2 , which was safer and easier to make than natural gas experiments.…”