Quantitative visualization of gas dynamics during electrical resistance heating
Paul R. Hegele and Kevin G. Mumford
Department of Civil Engineering, Queen’s University, Kingston, ON, Canada
Electrical resistance heating (ERH) is an in situ thermal treatment (ISTT) technology used to remediate contaminated soil and groundwater by increasing subsurface temperatures to achieve boiling of water and non-aqueous phase liquids (NAPLs). Once the boiling point is reached, gas bubbles nucleate, grow and connect throughout the heated zone. Mass removal is achieved through transport of the gas phase to recovery points such as soil vapour extraction or multiphase extraction wells. This transport can be driven by pressure gradients through a connected gas phase, or by the buoyant transport of gas bubbles. An understanding of gas bubble nucleation, growth and connection mechanisms before the onset of gas transport is fundamental to the design and operation of ISTT technologies, because mass removal cannot be achieved without gas capture. Despite its importance, there is limited experimental work concerning gas dynamics during ERH. The purpose of this work is to (i) develop an experimental apparatus that allows heating by ERH and visualization using a light transmission method, which is a laboratory technique that allows quantitative visualization of fluid saturations, and (ii) investigate the onset of gas production and gas connection during heating. The apparatus consisted of a thin experimental cell (40 cm x 20 cm x 1 cm) with tempered borosilicate glass walls in a polytetrafluoroethylene (PTFE) frame. This allowed for visualization as well as temperature, electrical and chemical compatibility. A light emitting diode (LED) panel with a diffuser was used as a light source, which runs cooler than incandescent light sources and avoids problems associated with flickering that is typical of fluorescent light banks. Deionized water with dissolved sodium chloride was used as an electrolyte to conduct electricity between two electrodes, and voltage was controlled externally with a variable autotransformer. The cell was outfitted with six thermocouple probes to measure temperature. Because thin cells required for light transmission suffer from significant conductive heat loss through their walls, dissolved carbon dioxide was added to the pore water to generate gas at temperatures below the boiling point of water. Gas saturations in the cell were measured by light transmission throughout the heating process, and showed the development of connected gas regions as well as the onset of bubble flow. Results suggest that buoyancy-driven bubble transport may occur following local connection of the gas phase within a target heating zone, which should be considered in the design of gas capture systems.