Quantification of gas saturations during bubbly gas flow using a novel calibration technique

An understanding of gas dynamics is important during the remediation of contaminated soil and groundwater by techniques such as in situ air sparging (IAS) and in situ thermal treatment (ISTT). For example, mass transfer rates between dissolved contaminants and gases are governed by gas-liquid interfacial area, relative permeability effects reduce aqueous flow through a gas-occupied treatment zone, bubble flow allows gas phase transport at lower gas saturations in coarse material, and the onset of gas connection allows increased capture and recovery of gas-phase mass during vapor extraction. Visualization using light transmission methods (i.e., transmitting light through thin experimental cells and capturing digital images of the media and fluids in the cell over time) can be used to investigate gas dynamics in laboratory experiments. These light transmission methods often require calibration to representative wet and dry or residual transmitted light intensities in order to quantify gas saturations. In this work, a new calibration procedure was developed and used to quantify gas saturations during bubbly gas flow in coarse sand, which only used data from the water-saturated image. A known gas volume was injected at slow flow rates into the bottom of a thin cell (100 mm × 80 mm × 8 mm) containing water and sand, such that bubbly gas flow occurred. Pixel-wise gas saturation values at multiple points in time during the gas injection were integrated over the volume of the cell and calibrated to the total volume of gas injected. This method was able to overcome experimental difficulties associated with obtaining representative dry or residual images for use in calibration, and was able to calibrate directly to the distribution of discontinuous gas, which resulted in low errors in local gas saturation (i.e., standard deviations of 0.5%-2% with a median filter applied). Calibrated images were used to validate results from a numerical model used to simulate bubbly gas flow in two and three dimensions for use in air sparging applications. In addition, the light transmission method was applied to laboratory experiments of ISTT using electrical resistance heating (ERH) to investigate the development of a gas phase during boiling.