渗流作用下局部水平冻结体扩展规律试验研究

Experimental study on the expansion law of local horizontal frozen body under seepage

目前采用物理模型试验的方法对渗流作用下的冻结温度场进行研究时,多以均质砂层为地层传热介质,且多为单排管全长冻结,鲜有以砂卵石为地层传热介质的双排管局部冻结。为研究渗流作用下砂卵石地层双排管局部水平冻结体扩展规律,基于相似理论,进行渗流作用下砂卵石地层双排管局部冻结模型试验。研究表明:渗流作用下双排管局部水平冻结体成形过程可分为:第二排冻土柱交圈、形成连续水平冻结体的同时背水面扩展、迎水面再扩展3个阶段,且成形时间及冻结锋面扩展速度均应按时间历程依次划分为4种。试验结果显示,测点交圈时间有随渗流速度的增大而增大的趋势,在冻结时间相同的情况下,渗流速度越小,测点温度越低。渗流速度u对第二排冻土柱交圈速度ve的影响随u的增大逐渐增大,而迎水面扩展速度vm则相反,且受群孔效应影响,vm大于ve。冻结后期,迎水面再扩展速度vu和背水面扩展速度vd均随冻结时间的延长而逐渐减小,水平冻结体上/下游长度逐渐趋于定值。在本模型试验中,通过回归分析,分别拟合得到了第二排冻土柱交圈时间te、迎水面扩展速度vm与渗流速度u的函数关系,迎水面再扩展速度vu、背水面扩展速度vd与冻结时间的函数关系,其中,te,vm分别与u成二次函数关系,vu,vd分别与冻结时间t的平方成反比关系。为控制成形时间及成本,并基于安全考虑,建议取Ed为水平冻结体下游长度,并根据水平冻结体扩展特性提出总成形时间ts计算式。

At present,when the freezing temperature field under seepage is studied by physical model test,the homogeneous sand layer is mostly used as the heat transfer medium in the stratum,and most of them are single-row pipes frozen along the length direction.There are few model tests in which sand and pebble are used as heat transfer medium in formation,and there are few local freezing model tests with double-row pipes.In order to study the expansion law of local horizontal frozen body of double-row pipes under seepage,according to the theory of similarity,the local freezing model test of double row pipes in sandy gravel stratum under seepage is carried out.Research shows that the forming process of local horizontal frozen body of double-row pipes under seepage can be divided into three stages:the second row of frozen soil columns intersecting,the formation of complete horizontal frozen body,and the continuous expansion of the frozen surface,and that the forming time and frontal spreading speed should be divided into four types according to the time history.The experimental results show that the intersection time of measuring points increases with the increase of seepage velocity.Under the same freezing time,the smaller the seepage velocity,the lower the temperature of measuring points.The effect of seepage velocity u on the intersection velocity ve of the second row of permafrost columns increases with the increase of u,while that of expansion velocity vm of water front is the opposite,and affected by the group hole effect,vm is greater than ve.In the later period of freezing,the speed at which the water surface continues to expand vu and expansion speed vd of backwater surface decrease gradually with the extension of freezing time,and the length of upstream and downstream of horizontal frozen body tends to be constant.In this model test,through the regression analysis,the relationship between the second row of frozen soil column intersection time te,the water surface expansion speed vm and the seepage velocity u is obtained.Meanwhile,the relationship between the water surface re-expansion speed vu,the backwater surface expansion speed vd and the freezing time t is obtained.Among them,te,vm and t form a quadratic function relationship,and vu and vd are inversely proportional to the square of the freezing time t,respectively.In order to control the forming time and cost,and based on safety considerations,it is suggested that taking E d as the downstream length of horizontal frozen body,and according to the expansion law of horizontal frozen body,a formula for calculating the total forming time ts is put forward.