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Title: An experimental and numerical study on the impact of wind induced turbulence on gaseous dispersion in porous media
Author: Pourbakhtiar, Alireza
ISNI:       0000 0004 7658 5180
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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This research focuses on how wind turbulence influences gas transport in the porous media. It can be useful in measuring the amount of greenhouse gasses from subsurface to atmosphere or a hazardous gas like Radon emission into buildings. It can also be important in other fields of research, anywhere that gas transports through porous media. A novel experimental arrangement is demonstrated for measuring wind turbulence-induced gas transport in dry porous media under controlled conditions. This equipment was used to measure the effect of wind turbulence on gas transport (quantified as a dispersion coefficient) as a function of distance to the surface of the porous medium exposed to wind. Two different methods for the measurement of wind-induced gas transport were compared. In one of approaches, which is a modified version of other one, five sensors are placed inside the sample of porous material at same intervals which can measure the oxygen concentration values. Approaches are used for measuring diffusion and wind-induced dispersion. Tracer gases of O2 and CO2 with average vertical (perpendicular to the surface of porous medium) wind speeds of 0.02 to 1.06 m s-1 were applied at room temperature condition. Five different fractions of soil are utilized to find out how the particle size can affect the gas transport in a specific wind condition at the surface of soil as the porous media. It is shown that gas dispersion was 20-100 times higher due to wind action. Ten wind conditions (plus calm condition with zero wind speed) are selected and three perpendicular components of wind as well as wind fluctuations are characterized. Oxygen breakthrough curves as a function of distance to the wind-exposed surface of the porous medium were analysed numerically with a finite-difference based model to assess gas transport. Potential relationships between breakthrough time and wind speed characteristics in terms of average wind speed, wind speed standard deviation and wind speed power spectrum properties in three dimensions were investigated. Statistical analyses indicated that the wind speed had a very significant impact on breakthrough time and that the characteristics for the wind speed component perpendicular to the porous medium surface were especially important. For the experiments, the penetration depth (Z50) is introduced. Linear inverse relation between penetration depth and empirical factor is determined. Wind characteristics can affect the gas transport speed and penetration depth inside porous media for particle sizes above 1mm. At particle size below 0.5 mm the effect of wind on gas transport is negligible. The relation between different wind speed characteristics such as wind speed or its power spectrumand particle shape and size on gas transport is analysed. The main component of wind which affects the gas transport was found to be the vertical one. An expression (Eq. 26) for calculating the wind-induced dispersion coefficient has been developed which is dependent on wind speed. The direct calculation of the empirical factors and wind induced dispersion coefficient of porous media at the surface is more accurate by fitting the empirical and numerical parameters.
Supervisor: Papadikis, Konstantinos ; Bridge, Jonathan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral