Frost and Ice Formation in the Air Convection Pile Permafrost Protection Device

Experimental studies on frost and ice growth under simulated summer conditions were performed on a 3.0-m (10-ft) model of an air convection pile. The air convection pile is a thermosyphon-type permafrost protection device which has been considered for use in arctic construction projects. The device consists of an outer tube, usually 45.75 cm (18 in.) in diameter, extending 3.05 to 18.3 m (10 to 60 ft) into the permafrost. This outer tube contains a shorter concentric 25.4-cm- (10-in.) dia inner tube. Data was taken for typical arctic temperatures and humidities and for simulated above-ground heights of 0.153, 1.373, and 2.88 m (0.5, 4.5, and 7.5 ft). The results have shown that the ice growth is governed by the concentration gradient in the annulus of the pile.

Heat Transfer in an Air Thermosyphon Permafrost Protection Device

Velocity and temperature profiles were measured in a prototype air thermosyphon permafrost protection device. This device, known as the air convection pile, consists of an 18-in. (0.46-m) outer tube containing a shorter concentric 10-in. (0.25-m) tube extending from 10 to 60 ft (3 to 18 m) into the permafrost. Measurements showed a low frequency oscillating flow in both the annulus and inner tube. Heat removal rates compared favorable with an analytical model and previous experimental results, but the annulus velocity profiles were significantly different, possibly due to the oscillation in the flow.

The Modeling of a Thermosyphon Type Permafrost Protection Device

One promising device for protection of permafrost is the concentric tube thermosyphon. In the winter, the difference in temperature between the annulus and the tube provides a buoyant driving force to move the air down the tube and up the annulus. The resultant heat transfer freezes and subcools the permafrost. The paper describes in detail the flow and heat transfer by solving the boundary layer equations for velocity and temperature considering conduction and radiation at the boundaries. The predicted thermosyphon performance is compared with experimental data. The results for heat removal rate are generally within 10–20 percent.