热管及保温材料在青藏直流联网工程中的应用研究

Application of thermosyphons and insulated boards in Qinghai-Tibet DC Interconnection Project in permafrost regions

针对青藏直流联网工程塔基热管措施应用效果,通过现场实测资料确定了热管年内工作周期及混凝土桩基表面热效应,考虑无绝热段热管传热过程组成,建立空气–热管–土体耦合传热数学模型,利用有限元方法系统模拟不同年平均地温分区锥柱式塔基传热过程及气候变暖背景下基础周围多年冻土热状况发展变化趋势。结果表明:冷季热管工作期间,其对周围土体冷却降温效果显著,同时由于混凝土塔基为热的良导体,热管产生的"冷量"通过基础及其底座快速向基础周围传递,使得基础下形成大范围低温冻土。暖季,热管停止工作期间,由于基础埋设较浅,混凝土塔基良好的导热性能使得其周围浅层土体温度升温较快,量值基本与天然地表下同一深度接近,而基础下部深层地温则主要受热管作用控制,温度较低。在单一塔腿4根热管及50 a气温升高2.6℃背景下,-1.0℃、-1.5℃两种年平均地温条件下,桩基础下部多年冻土仍保持冻结状态,满足工程对于冻土地基热状况的要求。-0.5℃年平均地温条件下,运营后期桩基础周围土体季节融化深度已大于桩基埋深。在该地温条件下,通过热管–保温板复合措施的采用,可有效发挥热管的"冷却降温"及保温板的"隔热保冷"效能,在大幅减小基础周围土体的最大季节融化深度的同时降低锥柱式基础底部深层地温,进而满足工程需求。

The working period of thermosyphons and the thermal effect of concrete tower footing are determined using the in-situ measured data. A coupled heat transfer model among air, thermosyphon and soil is established considering heat transfer processes within the thermosyphon without adiabatic section. Based on the model, the heat transfer processes around the tower footing and the temporal variations of thermal regime around the tower footing are studied by numerical simulation under a climate warming scenario. The results indicate that in cold seasons when thermosyphon works, the soils around the thermosyphon are obviously colder than those far away, revealing a remarkable cooling effect. Meanwhile, clod energy from the thermosyphon flows quickly to the base of tower footing due to its larger thermal conductivity. As a consequence, extensive cold permafrost develops beneath the footing. While in warm seasons, the shallow soils around the tower footing warm quickly,but those beneath the tower footing are still colder due to the cooling effect of the thermosyphon. Under the scenario of climate warming, the soils beneath the tower footing installed with four thermosyphons still keep frozen in permafrost region with the mean annual ground temperatures of-1.0℃ and-1.5℃, meeting the need of the project. But for permafrost region with the mean annual ground temperature of-0.5℃, the maximum seasonally thawing depth around the tower footing is already larger than the buried-depth of the tower footing during the later period of operation. Using the combined method of thermosyphons and insulated boards, the maximum thawing depth around the tower footing can be diminished obviously and will be less than the buried-depth of the tower footing. And meanwhile, the deep soil temperatures beneath the tower footing will also be much lower.