Frost and ice formation in the air convection pile permafrost protection device

The problem under investigation is the frost and ice formation in The a permafrost protection device, the air convection pile. The air convection pile is an open system device which utilizes the bouyant force of warm air to subcool and refreeze the permafrost. Permafrost is a moist, unconsolidated soil in which a portion of the soil thaws during the summer and refreezes during winter. The air convection pile is composed of two tubes and a head. The outer tube is embedded in the permafrost and can serve as part of the support of the structure. inner tube is mounted concentrically with the outer tube and is shorter than the outer tube to provide a flow path. The head provides support for the inner tube and prevents animals and the elements from entering the pile. In the winter the air in the annulus existing between the two tubes is warmer than the ambient air and rises up the annulus. The cold ambient air is drawn in through the inner tube to be heated in the annulus. This flow of cold air refreezes and subcools the permafrost. In the summer the pile should remain inactive. However, frost and ice can form on the outer pile wall surfaces. This research was an experimental study to determine: (1) the severity and cause of the ice, (2) its effects on the winter operation of the pile, and (3) a method of stopping the ice from forming.

Tests indicated that a significant moisture concentration gradient existed in the annulus of the pile. This gradient indicated that the ice was forming by diffusion.

Profiles taken of ice established that the ice accumulation present in the annulus would not be sufficient to block the annulus. However, if wind induced flow proves to be a problem, a sufficient ice mass to cause blockage of the annulus could form. Tests showed that if a large ice mass forms, as long as it did not completely close the annulus, the ice would not stop the start up of the pile. Calculations showed that a large ice mass would sublimate out of the pile.

Test results showed that the amount of ice formed was independent of humidity ratio but did depend on the geometry of the pile; as the distance from the annulus outlet to the permafrost is increased the rate of ice formation is decreased. Test results established that the ice formation could be stopped by installing a bladder containing a low boiling point fluid in the annulus of the pile.

The air convection pile is for a number of reasons an attractive alternative as a permafrost protection device. However, wind induced flow could result in failure of the pile due to the increases in ice formation.

Future studies should address the development of an analytical model of the ice formation to verify the experimental results.