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Creation of a frozen ground barrier by thermosyphons
2002 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

A frozen ground barrier is a technique that is mainly used for ground stabilisation or groundwater shut off. A new application is to prevent contaminants in the ground from spreading to unwanted areas. When the water in the pores of a saturated soil is frozen, the possibility for contaminants to find a way through is limited. A passive freezing system with thermosyphons was installed in Jukkasjärvi October 31st 2001. The aim was to examine its freezing capacity of the surrounding soil in the northern Scandinavia climate. Jukkasjärvi is placed 12km east of Kiruna, northern Sweden. A passive freezing system is running without any external freezing source. The system is using the phase change of a medium inside a pipe to extract heat from the ground and release it to the air. Its freezing capacity depends strongly on the air temperature, which must be colder than the ground temperature and below freezing to make the phase change occur. Active freezing is another cooling system but is not dependent on air temperature as the passive system. It is using an external freezing medium that is continuously flowing through pipes that is placed in the ground. A combination of these two systems is called the hybrid technique, where a passive freezing method is connected to an active one. The hybrid system runs by the passive technique during time periods when the air temperature is cold enough. Otherwise, the active technique is turned on to maintain the ground frozen. This report is divided in three parts • A description of applications of a frozen ground barrier in contaminated sites • Evaluation of how the thermosyphons in Jukkasjärvi worked during the winter 2001/2002 • Modelling of the temperature distribution in the ambient soil around a thermosyphon with similar conditions as in the test site in Jukkasjärvi. Further, the frost depth between the thermosyphon model is compared with a model with active freezing during the first two weeks. In the model with active freezing the surface layer of snow is replaced to a layer of dry peat. Both models were created by the software TEMP/W. A frozen ground barrier can be created in almost any formation and it can isolate radioactive, heavy metal and organic contaminants. Examples where these contaminants occur are in landfills, waste treatment lagoons and hazardous substance areas. Frozen ground barriers can also be used as a safety system to protect vulnerable areas from an ecological or biological point of view as aquifers and surface water. The temperature was recorded by gauges, type PT 100, at the test site in Jukkasjärvi from November 7th 2001 till May 25th 2002. The seven thermosyphons that were installed showed large differences in heat removal capacity during the test period. Only four of them had a notable cooling influence on the surrounding soil and the heat removal capacity was largest during the two cold periods in the middle of December and the beginning of February. The largest heat transfer, 118W/m2, was recorded at thermosyphon E at December 21st 2001. The differences in cooling capacity for the thermosyphons can be due to spaces between the pipe and the soil. These spaces can be filled with various fractions of air, water, ice and moraine for each thermosyphon. The cooling influence on the ambient soil of the thermosyphons should therefore be affected since the thermal conductivity is different for these materials. Another reason for the differences in the heat removal between the thermosyphons can also be due to the various sizes of blocks in the moraine. It can result in an inhomogeneous heat transporting layer and will therefore affect the temperature gauges in different ways depending on the type of moraine above. The model of a single thermosyphon was not verified by the temperature distribution showed by temperature gauges. The model was too simplified since the layer of clay at the test site in Jukkasjärvi was representing the whole soil profile. The insulating ability of the first layer of moraine was not considered. Thereby, the air temperature had a too large impact on the model. If the moraine and its thermal factors are included in the model, the accuracy will be improved. The model with active freezing and a peat layer at the surface indicates a frost front 7m from the freezing pipe at 1.8m depth, April 18th 2002. The model with a thermosyphon and a snow layer did not have any frost at all this time. Hence, the active freezing model is keeping the frost in the ground for a longer time in comparison to the thermosyphon model.

Place, publisher, year, edition, pages
2002.
Keyword [en]
Technology, thermosyphons, ground freezing, barrier, contaminants, soil, mechanics
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-42676ISRN: LTU-EX--02/358--SELocal ID: 0a6af5fc-0bfa-4517-b592-67648d5fd263OAI: oai:DiVA.org:ltu-42676DiVA: diva2:1015900
Subject / course
Student thesis, at least 30 credits
Educational program
Environmental Engineering, master's level
Examiners
Note
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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