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Application |
GeoStudio |
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Version |
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Primary Analysis |
TEMP/W |
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Keywords |
GeoStudio, TEMP/W, Thermosyphons, End, View |
Introduction
Thermosyphons are used to harness cold weather to maintain frozen conditions in the soil. A thermosyphon essentially comprises two parts: an evaporator installed below ground and a condenser installed above ground. The condenser is maintained at a lower temperature than the evaporator due to the flow of cold air past the apparatus. Fluid in the evaporator changes phase and rises up the pipe towards the condenser as heat is extracted from the ground. As the fluid rises, it cools, condenses, and then returns to the evaporator. The objective of this example is to determine if a permafrost zone can be maintained beneath a heated building for an average climate year. The example also demonstrates the use of cycling climate to establish a long-term thermal regime. Numerical Experiment A two-dimensional analysis was conducted to model the effect of thermosyphons installed in the foundation soil beneath a heated structure. The reader should review the GeoStudio file for details on the problem definition. The soil is assumed to be silty sand and is defined using a fullthermal constitutive model. The volumetric water content is assumed constant at 0.35. The unfrozen and frozen volumetric heat capacity values are input as 3145 kJ/m3/C and 2413 kJ/m3/C respectively. Both the thermal conductivity versus temperature and unfrozen volumetric water content functions were generated using the estimate routine. The left edge of the domain represents the line of symmetry. The thermosyphon pipes extend in the out-of-plane dimension and angle up to the ground surface where the condensers are exposed to the climate. Accordingly, the analysis assumes that heat flow is within a plane perpendicular to the pipes, which is a reasonable assumption if the pipes are relatively long. There are a number of boundary conditions applied in this analysis. At the bottom of the domain is a constant heat flux of 8 kJ/day/m2. The inside of the building is maintained at a constant temperature of 15C. A surface energy balance boundary condition was applied to the ground surface line outside of the building footprint. The data used for the air temperature and wind speed functions are representative of Fairbanks, Alaska. The albedo was assumed constant for the duration of the analysis. The data is commensurate to a start date of about May 1st. There are two thermosyphon boundary conditions in the domain: one having a surface perimeter of 0.0393 m for the half-pipe at the line of symmetry and the other with a perimeter of 0.0785 m. A surface perimeter input is required when the thermosyphon boundary condition is applied to a geometric point such that TEMP/W can convert a flux rate (q) into a flow rate (Q). Both boundary conditions use the air temperature and wind speed functions for Fairbanks. The convective coefficient versus wind speed function is based on Haynes and Zarling (1988). The maximum operating air temperature and minimum temperature difference for vaporization were assumed to be -0.5C and 1C, respectively. The initial temperature condition is established using the material activation at 0C. Recall that the goal of this analysis is to determine if permafrost conditions can be maintained for a typical climate year. Accordingly, the model is solved for a number of successive years until a repeatable temperature cycle is established. For illustrative purpose, the example file is only solved for a period of 10 days to reduce file size. The time step size was 6 hours and every 20th time step is saved (i.e. 5 days).
Downloads
▪ 📗 GeoStudio - TEMP/W - Thermosyphons in End View - Information [PDF]
▪ 💻 GeoStudio - TEMP/W - Thermosyphons in End View - Project [gsz]
See also