Experimental investigation on heat transfer law of multiple freezing pipes in permeable stratum with high seepage velocity

Abstract In order to explore the causes of slow development of the frozen wall in permeable strata with high seepage velocity, the heat transfer law of freezing pipes in such strata was studied based on the hydrothermal coupling model test system. The test results showed that when v = 0 and 3 m/d, the earliest closure position of the frozen wall was the intermediate point between two freezing pipes, and the closure time was 740 and 840 min, respectively. When v = 6 and 9 m/d, the earliest closure position of the frozen wall moved from the intermediate point downstream by 50 mm, and the closure time increased to 1770 and 4250 min, respectively. The seepage caused uneven development of the thickness of the frozen wall, when v = 3, 6, and 9 m/d, the asymmetry coefficients (Rd/Ru) of the frozen wall were 1.17, 1.21, and 1.81, respectively. When the distance between adjacent freezing fronts was reduced to a critical value Lc, the "group-pipe-effect" would occur. This effect would accelerate the expansion speed of the freezing front and shorten the closure time of the frozen wall. Since the convective heat transfer effect of the water flow would offset part of the "group pipe effect", Lc would decrease as the seepage velocity increased. When the seepage velocity was less than 3 m/d, Lc was 400 mm, and when the flow velocity reached to 6 m/d and 9 m/d, Lc was reduced to 154 mm and 130 mm, respectively. Additionally, the formula for calculating the steady-state temperature field of single-row pipes under the action of seepage was proposed using the piecewise equivalent method based on the result obtained from the model tests.