Numerical study of long-term cooling effects of thermosyphons around tower footings in permafrost regions along the Qinghai-Tibet Power Transmission Line.

Two-phase closed thermosyphons are extensively used for cooling foundation soils of tower footings in permafrost regions along the Qinghai-Tibet Power Transmission Line (QTPTL). In this paper, the working period of these thermosyphons is firstly determined by field observations. Then, a 3-D numerical heat transfer model is constructed to study the long-term cooling effects of thermosyphons and thermal performance of foundation soils. The numerically simulated results show a significant cooling of foundation soils near thermosyphons in cold season approximately from late October to early May, and a rapid shift of the thermosyphon cooling to the footing side due to greater thermal conductivity of the concrete footing. As a consequence, a colder bulb would develop under the footing, increasing the bearing capacity of foundation soils. In warm season from the mid-May to mid-October, the maximum thaw around the footing would be greater than that in the natural ground due to better heat transfer through the concrete footing. During a 50-year operational period, foundation soils under the footing would roughly go through about four thermal stages under a combined effect of the thermosyphon cooling and climate warming: a rapid cooling stage in the first to 5th year, a stable stage in the 6th–15th year, a rapid warming stage in the 16th–35th year and a slow warming stage after the 35th year. During the operational period, the maximum thaw around the footing would be less than the embedding depth of the footing (5.8 m), meeting the design criterion for thermal stability of foundation soils. [ABSTRACT FROM AUTHOR]