This thesis examined the effect of sub-boiling temperatures on subsurface flow and mass transport, as a result of electrical resistance heating (ERH). Low temperature ERH was simulated using a newly developed two-dimensional, electro-thermal, flow and transport model (ETM). To capture the non-isothermal processes in the subsurface during low temperature ERH, the ETM included temperature dependent equations for density, viscosity, and electrical conductivity. The model was validated with laboratory experiments in which voltage distribution, instantaneous power, temperature, and tracer transport were measured. Both the tracer experiments and the simulation results indicated that flow and contaminant movement could be significantly impacted by low temperature ERH due to temperature induced buoyant flow. In the first part of the thesis, the ETM was used to study the onset of buoyant flow in the subsurface and its effect on contaminant transport. Buoyant flow was predicted to occur when the ratio between the Rayleigh and thermal Peclet numbers (buoyancy ratio), was greater than 1. The buoyancy ratio was expressed in terms of subsurface temperature, thermal expansion coefficient and hydraulic gradient, thus facilitating its application to subsurface thermal activities. The effect of buoyant flow on contaminant transport was found to be dependent on the buoyancy ratio and Rayleigh number. The second part of the thesis examined the effect of soil heterogeneity, electrical conductivity and applied groundwater flux on energy and mass transport. To examine soil heterogeneity effects, random permeability fields for two aquifers with varying levels of heterogeneity were generated. Higher soil electrical conductivity values increased the power dissipated and resulted in shorter heating times and quicker onset of buoyant flow. Consequently, electrical conductivity had a statistically significant effect on the subsurface energy distribution. The applied groundwater flux had a strong effect on heat and mass transport with lower velocities resulting in upward plume movement due to buoyancy effects. In addition, buoyant flow was observed to dominate over flow through high permeability zones. The last chapter of the thesis investigated the formation and movement of discrete gas bubbles during ERH by combining ETM with a macroscopic invasion percolation (MIP) model. The model simulated soils with different permeabilities and entry pressures at various operating temperatures and groundwater velocities. It was observed that discrete bubble formation occurred in all soils, with upward mobility being limited by lower temperatures and higher entry pressures. By including the MIP model, the resulting aqueous concentrations were significantly different from results obtained with a conventional advective-dispersive model, especially in high permeability soils. This was due to bubbles moving to cooler areas, collapsing, and contaminating previously clean zones. The results of this thesis demonstrated that sub-boiling temperatures affect subsurface flow and mass transport, especially when temperature-induced buoyant flow occurred. Although this study focused on ERH applications, the results may be applicable to other subsurface thermal activities such as geothermal heating.

This thesis examined the effect of sub-boiling temperatures on subsurface flow and mass transport, as a result of electrical resistance heating (ERH). Low temperature ERH was simulated using a newly developed two-dimensional, electro-thermal, flow and transport model (ETM). To capture the non-isothermal processes in the subsurface during low temperature ERH, the ETM included temperature dependent equations for density, viscosity, and electrical conductivity. The model was validated with laboratory experiments in which voltage distribution, instantaneous power, temperature, and tracer transport were measured. Both the tracer experiments and the simulation results indicated that flow and contaminant movement could be significantly impacted by low temperature ERH due to temperature induced buoyant flow. In the first part of the thesis, the ETM was used to study the onset of buoyant flow in the subsurface and its effect on contaminant transport. Buoyant flow was predicted to occur when the ratio between the Rayleigh and thermal Peclet numbers (buoyancy ratio), was greater than 1. The buoyancy ratio was expressed in terms of subsurface temperature, thermal expansion coefficient and hydraulic gradient, thus facilitating its application to subsurface thermal activities. The effect of buoyant flow on contaminant transport was found to be dependent on the buoyancy ratio and Rayleigh number. The second part of the thesis examined the effect of soil heterogeneity, electrical conductivity and applied groundwater flux on energy and mass transport. To examine soil heterogeneity effects, random permeability fields for two aquifers with varying levels of heterogeneity were generated. Higher soil electrical conductivity values increased the power dissipated and resulted in shorter heating times and quicker onset of buoyant flow. Consequently, electrical conductivity had a statistically significant effect on the subsurface energy distribution. The applied groundwater flux had a strong effect on heat and mass transport with lower velocities resulting in upward plume movement due to buoyancy effects. In addition, buoyant flow was observed to dominate over flow through high permeability zones. The last chapter of the thesis investigated the formation and movement of discrete gas bubbles during ERH by combining ETM with a macroscopic invasion percolation (MIP) model. The model simulated soils with different permeabilities and entry pressures at various operating temperatures and groundwater velocities. It was observed that discrete bubble formation occurred in all soils, with upward mobility being limited by lower temperatures and higher entry pressures. By including the MIP model, the resulting aqueous concentrations were significantly different from results obtained with a conventional advective-dispersive model, especially in high permeability soils. This was due to bubbles moving to cooler areas, collapsing, and contaminating previously clean zones. The results of this thesis demonstrated that sub-boiling temperatures affect subsurface flow and mass transport, especially when temperature-induced buoyant flow occurred. Although this study focused on ERH applications, the results may be applicable to other subsurface thermal activities such as geothermal heating.

A laboratory scale study was conducted to evaluate the thermal remediation of trichloroethylene (TCE) in a saturated groundwater system using electrical resistance heating (ERH). Two experiments were conducted using a two-dimensional polycarbonate test cell, the first consisting of a single pool of TCE perched above a capillary barrier, the second consisting of two pools of TCE each perched on separate capillary barriers. Temperature data was collected during the heating process from an array of 32 thermocouples located throughout the test cell. Visualization of the vaporization of liquid phase TCE, as well as the upward migration of the produced vapour was recorded using a digital camera. Chemical testing was performed 48 hours after experiment termination to measure post heating soil concentrations. A co-boiling plateau in temperature was found to be a clear and evident earmark of an ongoing phase change in the pooled TCE. Temperature was found to increase more rapidly in the second experiment that included a fully spanning barrier. As temperatures increased above the co-boiling plateau, vapour rise originating from the source zone was observed, and was found to create a high saturation gas zone beneath the upper capillary barrier when no clear pathway was available for it to escape upwards. When the source zones had reached the target temperature of 100°C and the ERH process stopped, this high saturation gas zone condensed, leading to elevated TCE concentrations below as well as within the capillary barrier itself. The water table within the experimental cell was also noted to drop measurably when the gas zone collapsed. Post-testing chemical analysis showed reductions in TCE concentrations of over 99.04% compared to the source zone, although due to condensation of entrapped gas and convective mixing, there was a net redistribution of TCE within the experimental domain, especially within confined areas below the capillary barriers. A secondary set of experiments were conducted using a homogenous silica sand pack with no chemical contaminants to determine the effect, if any, of the wave shape of electrical input on the ERH process. It was found that in early time heating, square wave inputs consistently produced a more localized heating pattern when compared to the standard sine wave electrical input. This effect equalized between the two experiments as the ERH process went on, perhaps due to the increased dominance of conduction and convection as the mode of heat transfer in the test cell at higher temperatures. It is believed that the localization of heating in square wave experiments is due to a consistent power supply due to the lack of a sinusoidal ramping in power delivery. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-03-18 14:40:46.019

In situ thermal treatment (ISTT) applications require successful gas capture for the effective remediation of chlorinated solvent dense non-aqueous phase liquid (DNAPL) source zones. Gas production and transport mechanisms during bench-scale electrical resistance heating (ERH) experiments were examined in this study using a quantitative light transmission visualization method. Processed images during water boiling indicated that gas bubble nucleation, growth and coalescence into a connected steam phase occurred at critical gas saturations of Sgc = 0.233 ± 0.017, which allowed for continuous gas transport out of the heated zone. Critical gas saturations were lower than air-water emergence gas saturations of Sgm = 0.285 ± 0.025, derived from the inflection point of ambient temperature capillary pressure-saturation curves. Coupled electrical current and temperature measurements were identified as a metric to assess gas phase development. Processed images during co-boiling of pooled trichloroethene (TCE) DNAPL and water indicated that discontinuous gas transport occurred above the DNAPL pool. When colder zones were introduced, condensation prevented the development of continuous steam channels and caused redistribution of DNAPL along the vapour front. These results suggest that water boiling temperatures should be targeted throughout the subsurface (i.e., from specific locations of DNAPL to extraction points) during ERH applications. Because convective heat loss and non-uniform power distributions have the potential to prevent the achievement of boiling temperatures, a thermal enhancement was developed where dissolved gas delivered to the target heated zone liberates from solution at elevated temperatures and increases gas production. Processed images of ERH-activated carbon dioxide (CO2) exsolution indicated that discontinuous gas transport occurred above saturations of Sg = 0.070 ± 0.022. Maximum exsolved gas saturations of Sg = 0.118 ± 0.005 were sustained during continuous injection of the saturated CO2 solution into the heated zone. Estimated groundwater relative permeabilities of krw = 0.642 ± 0.009 at these saturations are expected to decrease convective heat loss. Discontinuous transport of exsolved gas at sub-boiling temperatures also demonstrated the potential of the enhancement to bridge vertical gas transport through colder zones. In conclusion, sustained gas saturations and transport mechanisms were dependent on the mechanism of gas production and effects of condensation. / Thesis (Master, Civil Engineering) -- Queen's University, 2014-03-27 15:26:30.683