Preliminary Investigation of Transparent Soil for use in Heat Transport Experiments

D. Kucharczuk, K.G. Mumford, G.A. Siemens

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Soil Mechanics

ABSTRACT: Heat transport in the subsurface is important in applications of geothermal energy systems, thermal remediation technologies, and the design of energy foundations. The investigation of these applications at the laboratory scale benefits significantly from an ability to collect temperature data within the porous media system, and at high spatial and temporal resolutions. Temperature measurements made using conventional probes (thermistors or thermocouples) can be limited in their spatial resolution. Higher-resolution methods, such as thermal imaging, are limited to measurements of the outside surface of a laboratory experiment. This paper presents a novel laboratory technique for measuring temperature using a transparent soil, including relationships between the refractive indices of various oils and temperature, as well as results from using these oils as a pore fluid within a transparent soil to investigate the relationship between image intensity and temperature. RÉSUMÉ Le transport de la chaleur souterraine est important à déterminer dans l'application des systèmes d'énergie géothermique, des technologies d'assainissement thermique et de la conception des fondations de l'énergie. L'investigation de ces applications en laboratoire permet des avantages tels que mesurer la température à l'intérieur d'un système de milieu poreux ayant une haute résolution spatiale et temporale. Les mesures de température faites avec des sondes conventionnelles (thermocouples) peuvent être limitées par leur résolution spatiale. Les méthodes à haute résolution telles que l'imagerie thermique, sont limitées à des mesures prises aux surfaces externes en laboratoire. Cet article présente une technique de laboratoire ingénieuse qui permet de mesurer la température en utilisant un sol transparent. Cet article présente les relations entre les indices de réfraction de certaines huiles et la température ainsi que les résultats découverts en utilisant ces huiles comme fluide dans un sol transparent et ceci, dans le but d'étudier la relation entre l'intensité des images et la température. 1 INTRODUCTION The measurement of temperature is essential for the physical modelling of thermal systems such as, for example, energy foundations (Gao et al, 2008), thermal storage in aquifers (Sommer et al, 2013), and thermal convection in porous media (Menand et al, 2013). It is advantageous to collect these data at high spatial and temporal resolutions. However, this can be difficult as conventional probes (thermistors or thermocouples) provide measurements at discrete, often widely-spaced, locations. Higher-resolution data, such as that obtained using thermal imaging cameras, is limited to the outside of a physical model, and may not be representative of temperatures in the porous media. Measuring temperatures using images of a transparent soil system may complement these other techniques by overcoming these limitations. Transparent soil has previously been used in a variety of applications to track a non-transparent phase in porous media. These range from tracking air within a soil system (Sills et al, 2012) to modelling a leaky underground storage tank (Guzman and Iskander 2013). In these applications, the pore fluid was chosen so that the system appeared transparent at room temperature by matching the refractive indices of the pore fluid and the soil particles. Because these applications require that the transparency be maintained despite small temperature fluctuations in the laboratory, the pore fluid in these applications was also selected so that its refractive index was insensitive to changes in temperature. The change in the transparency of a transparent soil system with changes in temperature, however, suggests that digital images can be used to non-invasively measure the temperature of a soil system. The objective of this study was to develop a relationship between the intensity of digital images taken of a transparent soil and its temperature, suitable for calibrating future experiments. Small-scale laboratory tests were conducted between 5°C and 40°C to choose a pore fluid that would allow greater changes in intensity with temperature than a typical pore fluid used for transparent soil testing. The precision of the temperature measurement at different spatial resolutions for the selected oil was also investigated. 2 BACKGROUND 2.1 Refractive Index Transparent soil systems are based on the principle of refraction. Refraction describes the apparent bending of light as it passes through a material. The severity of this bending is described by the material's refractive index. A material's refractive index is a dimensionless number that describes the change in the speed of light as the light passes through a material. It is defined as the ratio of light speed in a vacuum to the light speed in the

RÉSUMÉ: minary Investigation of Transparent Soil for

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D. Kucharczuk; K.G. Mumford; G.A. Siemens (2014) Preliminary Investigation of Transparent Soil for use in Heat Transport Experiments in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper409,author = D. Kucharczuk; K.G. Mumford; G.A. Siemens,title = Preliminary Investigation of Transparent Soil for use in Heat Transport Experiments ,year = 2014}