Impact of Low Temperature Electrical Resistance Heating on Subsurface Flow and Mass Transport

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.

Keywords: electrical resistance heating
remediation
groundwater
mass transport
temperature
Advisor: Sleep, Brent E.