研究目的
To better understand free convection and conduction heat transfer mechanisms during thermal response tests with heating cables to estimate subsurface thermal conductivity and its uncertainty, and to assess the effect of perforated plastic disks to reduce free convection when heating cable sections are used.
研究成果
The continuous heating cable method provides more accurate thermal conductivity estimates with lower uncertainty (2.14%) compared to heating sections (15.18% uncertainty), which are affected by free convection. The method reduces power requirements and simplifies field operations, making it a promising alternative to conventional tests. Future work should focus on deeper boreholes and improved convection control.
研究不足
Free convection can occur in heating cable section tests despite mitigation efforts, potentially affecting accuracy. The analysis assumes conductive heat transfer only, which may not fully capture convective effects. Tests are limited to specific borehole depths and geological conditions, and further validation in deeper boreholes is needed.
1：Experimental Design and Method Selection:
Two thermal response tests were conducted using heating cable sections in Quebec City and a continuous heating cable in Orléans. The tests involved heat injection into ground heat exchangers with temperature monitoring using submersible sensors and fiber optic distributed temperature sensing (FO-DTS). Analytical solutions (finite and infinite line source models) were used for data analysis.
2：Sample Selection and Data Sources:
Tests were performed in boreholes of approximately 100 m depth in different geological settings (shale in Quebec City and limestone in Orléans). Temperature data were collected from sensors placed at various depths.
3：List of Experimental Equipment and Materials:
Heating cables, junction box with power supply components, submersible temperature sensors (e.g., DST-centi loggers), FO-DTS systems (e.g., Silixa XT-DTS and Orxy DTS), perforated plastic disks for convection reduction, and calibration baths.
4：Experimental Procedures and Operational Workflow:
For each test, undisturbed temperature was monitored, followed by heat injection at specified rates (e.g., ~40 W/m for sections, ~10 W/m for continuous), and thermal recovery monitoring. Data were collected at regular intervals (e.g., every 10 min for sensors, every 60 s for FO-DTS).
5：Data Analysis Methods:
Thermal conductivity was estimated using finite line source equation for heating sections and infinite line source for continuous cable, with uncertainty analysis based on error propagation methods. Rayleigh number stability criterion was applied to assess free convection.
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