Numerical Analysis of the Effect of Groundwater Seepage on the Active Freezing and Forced Thawing Temperature Fields of a New Tube–Screen Freezing Method

Abstract

To more comprehensively explore the mechanism of the active freezing and thawing process of a new tube–curtain freezing method in construction, the temperature field of the new tube–curtain freezing process is analyzed using finite element software to establish a numerical model. Six paths were set up upstream and downstream of the model and around the top steel tube to analyze the development of frozen soil curtains during active freezing and forced thawing. The results show that, due to the effect of seepage, the cold energy generated by the upstream frozen pipe will be carried to downstream by water, which leads to the asymmetry of the frozen soil curtain. A greater seepage rate leads to a more pronounced the influence on the development of the temperature field. During the process of forced thawing, the first 15 days of the frozen soil curtain heating rate are fastest; thus, it is necessary to monitor the thawing settlement intensively during this period. By comparing different heads of water and different forced thawing temperatures, it was found that a bigger head of water results in a longer thawing time. At a constant head of water, a higher thawing temperature results in a shorter thawing time, with the thawing time at 50 °C being about 0.5 times that at 5 °C. Low-temperature thawing can be chosen to control the cost; however, when the head of water is large, high-temperature thawing can significantly shorten the thawing time. In addition, the new tube–curtain freezing method has little influence on the surrounding environment, along with a short construction period and low construction cost, in accordance with the concept of sustainable development.

Numerical simulation of temperature field of a new type of freezing device under seepage effect

“…During freezing, seawater seepage conforms to Darcy’s law. Its equation is expressed as [ 21 ]: where Q denotes the seepage volume; K denotes the hydraulic conductivity of porous media in m/s. A denotes the cross-sectional area perpendicular to the seepage direction, Δ H denotes the head of water, L denotes the seepage path.…”

“…In COMSOL software, Darcy’s law is used to analyze groundwater seepage with its own partial differential equation as [ 21 ]: Where ϵ p denotes porosity rate; Q m denotes various source terms; v denotes the speed field in m/s; K denotes the hydraulic conductivity of porous media in m/s.…”

“…Therefore, the coupling of the temperature and permeability fields can be expressed as a functional expression. The Heaviside step function is used for the water-ice phase change during freezing [ 21 ]: where K f and K u denote the permeability coefficients of unfrozen sand and frozen sand, respectively, in m/s; in H(T) = flc2hs(T-T d , δT) , T denotes the temperature of the sandy soil, T d denotes the temperature of the phase change point, and δT denotes the radius of the phase change zone.…”

“…It is assumed that the sand is saturated, homogeneous, and isotropic during the freezing process, and the temperature field is affected by the heat transfer from the porous medium and the seepage heat transfer from the seawater. In the freezing process, the volume ratio of seawater and porous media particles can be expressed as (1 - θ S )/θ S , and the differential equation of the temperature field in the heat transfer process is [ 21 ]: Where T denotes temperature; τ denotes time; C eq denotes the equivalent volumetric heat capacity of saturated sand; λ eq denotes the equivalent thermal conductivity; ρ w denotes the density of water; C w denotes the specific heat of water; V x , V y and V z denote the percolation velocity of water.…”

“…The pore medium heat transfer module in COMSOL software is used to analyze the heat transfer problem, and its own equation is [ 21 ]: where ∇ denotes the Nabla operator; k eff denotes the equivalent thermal conductivity; and Q and Q vd denote various source terms.…”