Research on the temperature field of a partially freezing sand barrier with groundwater seepage

To study the distribution characteristics and variation regularity of the temperature field during the process of seepage freezing,a simulated-freezing test with seepage of Xuzhou sand was completed by using a model test developed in-house equipment.By means of three group freezing tests with different seepage velocities,we discovered the phenomenon of the asymmetry of the temperature field under the influence of seepage.The temperature upstream was obviously higher than that downstream.The temperature gradient upstream was also steeper than that downstream.With a higher seepage velocity,the asymmetry of the temperature field is more pronounced.The asymmetry for the interface temperature profile is more strongly manifest than for the main surface temperature profile.The cryogenic barrier section is somewhat"heartshaped".With the increasing velocity of the seepage flow,the cooling rate of the soil decreases.It takes much time to reach the equilibrium state of the soil mass.In our study,seepage flow velocities of 0 m/d,7.5 m/d,and 15 m/d showed the soilcooling rate of 4.35°C/h,4.96°C/h,and 1.72°C/h,respectively.

Numerical simulation of the temperature field within new type of monolayer freezing-shaft lining

Numerical simulation was used to estimate the temperature field within a poured concrete,mono-layer freezing-shaft lining.The affects from various factors were investigated.The maximum temperature within the lining increases as the lining thickness increases,decreases as the soil-side wall temperature decreases,decreases as the air temperature inside the shaft decreases and decreases as the air velocity inside the shaft increases.The compression speed of an insulating foam layer affects the maximum temperature difference between the interior and the sidewalls.The maximum temperature difference between the interior and the sidewalls approaches or exceeds the maximum allowable for the curing of poured concrete structures.Attention should be paid to the question of the lining cracking during the curing period.The temperature gradient in the vertical direction may be minimized by preventing air contact against the steel connection board supporting the base of the freshly poured section.

Experimental study on freezing of liquids under static magnetic field

Freezing processes of several liquids under static magnetic field(SMF) less than 50 mT were investigated. Central temperature of liquid samples held in glass test tubes immersed in a liquid bath was measured and collected. Nucleation temperature and phase transition time were obtained from freezing curves. Normality tests were performed for nucleation temperature of these liquids with/without magnetic field and normality distributions were justified. Analysis of variances was carried out for nucleation temperature of these liquids with magnetic field flux density as the influencing factor. Results showed that no significant difference was found for deionized water with or without SMF. However, differences exist in 0.9% NaCl solution and 5% ethylene glycol solution with and without SMF. Nucleation temperature of 0.9% NaCl with SMF is lower than that without SMF, while its phase transition time is shorter than that without SMF. Nucleation temperature of 5% ethylene glycol with SMF is higher than that without SMF, while its phase transition time is not modified with SMF.

A study on impacts of groundwater seepage on artificial freezing process of gravel strata

Purpose–This paper aims to study the impacts of groundwater seepage on artificial freezing process of gravel strata,the temperature field characteristics of the strata,and the strata process,closure time and thickness evolution mechanism of the frozen wall.Design/methodology/approach–In this paper several laboratory model tests were conducted,considering different groundwater seepage rate.Findings–The results show that there is a significant coupling effect between the cold diffusion of artificial freezing pipes and groundwater seepage;when there is no seepage,temperature fields upstream and downstream of the gravel strata are symmetrically distributed,and the thickness of the frozen soil column/frozen wall is consistent during artificial freezing;groundwater seepage causes significant asymmetry in the temperature fields upstream and downstream of the gravel strata,and the greater the seepage rate,the more obvious the asymmetry;the frozen wall closure time increases linearly with the increase in the groundwater seepage rate,and specifically,the time length under seepage rate of 5.00 m d
1 is 3.2 times longer than that under no seepage;due to the erosion from groundwater seepage,the thickness of the upstream frozen wall decreases linearly with the seepage velocity,while that of the downstream frozen wall increases linearly,resulting in a saddle-shaped frozen wall.Originality/value–The research results are beneficial to the optimum design and risk control of artificial freezing process in gravel strata.

A Mathematical Model of Heat Transfer in Problems of Pipeline Plugging Agent Freezing Induced by Liquid Nitrogen

A mathematical model for one-dimensional heat transfer in pipelines undergoing freezing induced by liquid nitrogen is elaborated.The basic premise of this technology is that the content within a pipeline is frozen to form a plug or two plugs at a position upstream and downstream from a location where work a modification or a repair must be executed.Based on the variable separation method,the present model aims to solve the related coupled heat conduction and moving-boundary phase change problem.An experiment with a 219 mm long pipe,where water was taken as the plugging agent,is presented to demonstrate the relevance and reliability of the proposed model(results show that the error is within 18%).Thereafter,the model is applied to predict the cooling and freezing process of pipelines with different inner diameters at different liquid nitrogen refrigeration temperatures when water is used as the plugging agent.

SIMULATION AND OPTIMIZATION FOR THERMAL SYSTEM IN GROUND ARTIFICIAL FREEZING

The situation of mine development in coal industry of China and the method of shaft freezing in the type of double ring freezing pipe used for shaft sinking under thick alluvium condition are briefly reviewed. The equations of heat conduction are numerically solved for two kinds of artificial shaft freezing-sin-gle and double ring freezing pipe pattern. A two-dimensional finite difference model simulating the temperature field is developed. The paper introduces a way to calculate the harmonic mean of heat conductivety used when dealing with heterogeneous material, The theoretically derived temperature distributions in frozen soil are given and the comparison is made between two types of freezing pipe arrangement. The optimization technique of a general simulation is discussed, and that is in use to improve the thermal system in shaft freezing.

渗流作用下粗粒土冻结壁交圈规律及预测模型探索

由于渗流作用下的冻结壁发育规律不明等原因,部分冻结法施工工程依靠人工经验,缺乏科学的预测模型,与精准化设计和控制还有很长的距离。为探索渗流作用下冻结法施工中粗粒土层冻结壁的演化规律,本文首先建立冻结法施工数值模型,并通过室内模型试验反演验证所建立模型的有效性和适用性。之后,考虑渗流速度、冻结管间距、环境温度、含水率和导热系数等五个因素,开展冻结壁形成正交数值试验,确定冻结壁交圈时间和厚度的主控因素。最后,通过数值模拟研究主控因素作用下冻结壁的发育和演化规律。基于结果建立临界流速、冻结壁厚度、冻结壁交圈时间的预测模型,给出冻结法施工参数的选择方案。结果表明,建立的冻土水热相变耦合模型可有效模拟渗流条件下土体冻结过程,能够满足冻结壁发育过程计算分析的需要。基于正交数值试验结果发现,渗流速度对冻结壁厚度和交圈时间的贡献百分比分别为78%和52%,是影响冻结壁能否交圈和最终厚度的主控因素。当渗流速度超过一定阈值时,冻结壁无法交圈。本文将这一阈值定义为冻结法施工中的临界流速。当冻结壁能够交圈,其最终形成的冻结壁厚度也主要由渗流速度决定,二者呈负指数函数关系。冻结管间距对冻结壁最终厚度影响不大,但是对冻结壁交圈时间影响较大,它们之间存在负指数函数关系。本文给出了0~6 m·d^(-1)条件下的临界流速、冻结壁厚度、冻结壁交圈时间计算模型,它们可为粗粒土地层的冻结法施工参数选择和施工工艺设计提供理论指导。

富水砂卵石地层盆形冻结止水技术研究

为解决全断面砂卵石地层地下工程的地下水控制问题,提出盆形冻结止水技术。首先,采用三轴力学试验研究冻结砂卵石的压缩力学特性和蠕变力学特性;然后,采用大尺度物理模型试验和数值模拟方法研究盆形冻结的温度场扩展规律,并根据研究结果进一步提出盆形冻结积极冻结期的群孔布置技术和维护冻结期的精细化控制技术。结果表明:1)低围压(小于3 MPa)条件下,三轴抗压强度与围压呈正线性相关,然后围压继续增大时,强度基本不变;基于损伤理论建立冻结砂卵石全过程蠕变模型。2)荷载对冻胀率及融沉系数影响最大,细粒土质量分数次之,原状饱和砂卵石是不冻胀材料,并由此建立饱和砂卵石冻胀率及融沉系数三维预测模型。3)基于大型物理模型试验及数值模拟结果,盆形冻结可以在地下水渗流条件下形成完整的盆形结构,起到有效止水的效果。4)盆底冻结管在冻结过程中表现出与单管冻结迥异的“群孔效应”,根据试验结果提出包含冻结管间距、土体导热系数、渗流速度3因素的积极冻结期冻结管合理间距的确定与布置技术。5)为节约维护冻结期的能源,提出隔排切割冻结管、仅保留盆壁冻结管2种维护形式以对冻结区域进行精细化控制,经过分析,2种方式均可以有效满足盆形结构维护期的冷量供给,保持完整的盆形形态。

盾构法联络通道T接头局部冻结加固效果及影响因素研究

为研究盾构法联络通道T接头局部冻结区域的薄弱位置,探究主隧道管片材料、洞门间隙注浆体热导率、是否注浆改良土体和冻结管环数等因素对局部冻结的影响,依托天津地铁10号线某联络通道T接头局部冻结加固工程,采用数值模拟对局部冻结温度场的分布规律进行分析。研究结果表明:1)对于沿洞门处的土体,联络通道顶底部冻结效果差于联络通道侧面;主隧道钢管片向主隧道底部比向顶部延伸的更长,因而联络通道底半环冻结效果略差于顶半环。2)影响局部冻结时间的主要因素为主隧道管片材料和洞门间隙注浆体的热导率。3)主隧道部分采用钢管片工况,钢管片起到通道壁后远近土体导热“桥梁”的作用;当洞门间隙注浆体的热导率较低(如0.93 W/(m·K))时,注浆体不能及时地向主隧道钢管片输送冷量,联络通道壁后近处土体中大量冻结冷量沿钢管片传递至远处土体,使通道壁后较近处土体温度较高,因而比主隧道全部采用钢筋混凝土管片工况冻结时间长。4)随着洞门间隙注浆体热导率的增大,其向主隧道管片输送冷量的效率得到提升,主隧道附近土体的冻结效果得到改善,冻结时长有所缩短;间隙注浆体热导率由0.93 W/(m·K)提升至1.8 W/(m·K)时,可以将主隧道部分采用钢管片工况冻结时间由17 d缩短至12 d,所需冻结时长相比主隧道全部采用钢筋混凝土管片工况反而缩短1 d,即主隧道部分采用钢管片工况的冻结时间对洞门间隙注浆体热导率的改变更为敏感。

冻结法施工环境高强度大体积混凝土温度场演化规律

为了研究冻结法施工环境下大体积高强度混凝土温度场的影响因素和发展规律,以我国西部地区某煤矿主井大体积高强度混凝土井壁为例,采用数值分析软件,研究混凝土强度等级、井壁厚度和环境温度对混凝土温度场的影响,并在施工现场进行温度监测。结果表明:数值模拟结果与现场监测结果温度场变化规律趋于一致;实测值比模拟值温度峰值低,出现的时间早;井壁混凝土厚度和强度等级对温度场影响明显。

水下高速公路盾构隧道联络通道冻结施工模拟分析

公路盾构隧道联络通道建设面临较大的不确定性和风险,通常采用人工地层冻结法进行施工。以孟加拉吉大港卡纳普里河底盾构隧道联络通道为研究对象,分析联络通道施工过程中冻土帷幕和结构衬砌受力情况,研究采用双圈管冻结施工过程中冻土帷幕发展规律。针对施工期和运营期衬砌受力的最不利工况进行验算,分析积极冻结和维护冻结阶段温度场的变化,并研究部分不确定参数的敏感性。分析结果表明,该联络通道设计施工方案合理可行。

单向冻结下渠基土的锅盖效应试验研究

寒旱区刚柔复合衬砌渠道铺设的复合土工膜阻碍土体冻结过程中的水分蒸发,出现复合土工膜下土体增水的现象,即“锅盖效应”。为探究土体的锅盖效应发生机制和规律,制备不同含水率的土柱试样进行单向冻结试验,分析复合土工膜对土柱温度场和水分场的影响。研究表明:有复合土工膜覆盖土柱的降温速率及土层间温差均小于未覆盖复合土工膜的土柱;冻结过程中,未覆盖复合土工膜的土柱底部水分通过土体孔隙通道不断向顶层迁移补充后蒸发,整个土柱含水率降低,且呈不均匀状分布;而有复合土工膜覆盖的土柱,在水分迁移和锅盖效应作用下,底部土体含水率降低,膜下顶层土体含水率显著增加,且膜下结冰,表明复合土工膜使土体在冻结过程中产生明显的锅盖效应。初始含水率不同的土柱受锅盖效应影响后产生的水分迁移量和形态存在差异,初始含水率较小的土柱顶层含水率的增加以气态水迁移为主,初始含水率较大的土柱含水率的增加则以液态水迁移为主。

盾尾刷更换时液氮冻结温度场及冻结参数影响的数值模拟分析

为合理确定高水压下液氮冻结止水更换盾尾刷的冻结设计参数及掌握温度场变化规律,结合某过江通道长距离盾构掘进过程中盾尾刷更换时的液氮冻结止水工程,利用ADINA大型有限元分析软件建立液氮冻结止水及盾尾刷更换数值模型,模拟温度-时间变化曲线与现场实测数据对比,验证模型的合理性,并对土层、去路液氮温度、冻结管长度、冻结管间距、冻结方式进行敏感性因素分析。结果表明:1)冻结效果随土层的不同而发生改变,在卵石层、砾砂层、粉细砂层3种土层中,粉细砂层的冻结效果最差,卵石层最好;2)在有限的冻结时间内,去路液氮温度的不同,只影响土体从开始冻结至越过0℃完成相变这期间的降温速度,不影响完成冻结以后的土体降温速率;3)冻结管长度对土层冻结的影响较小,可以采用较短的冻结管,以降低施工成本和液氮消耗量,但不能过短,应保证其纵向有足够的支撑范围,经计算分析,当冻结管长度为1.52 m时,能在轴面处满足2.0 m的冻结壁厚度要求;4)冻结管间距对冻结效果影响大,冻结管越密,冻结速度越快,效果越好;5)在相同条件下,双环预埋冻结管液氮冻结效果优于管片上直接打孔液氮冻结。

Mathematical models of steady-state temperature fields produced by multi-piped freezing

题目：少量管冻结稳态温度场数学模型目的：少量任意布置冻结管冻结的稳态温度场无解析解。建立任意布置少量管冻结稳态温度场模型，获得解析解，解决人工冻结温度场理论问题。创新点：1．基于势函数叠加原理，确立人工地层冻结中少量管冻结稳态温度场的通用求解方法；2．建立任意布置的3根和4根非等温冻结管下冻结稳态温度场数学模型，获得其解析解通解及特解。方法：1．通过理论分析，将冻结管简化为热汇点源，确定人工地层冻结热势的拉普拉斯方程表述；2．应用势函数叠加原理建立少量管冻结稳态温度场的通用求解方法；3．建立少量管冻结稳态温度场的数学模型，通过理论推导获得温度场解析解；4．通过数值模拟，验证所提方法、数学模型和解析解的正确性和准确性。结论：1．将冻结管简化为点源（热汇），其冻结形成的热势场服从拉普拉斯方程，其解即为热势函数；2．多根冻结管冻结时，将单根冻结管的热势函数叠加，由冻结管的位置决定每根冻结管的热流，再根据边界条件定解。这一方法（即势函数叠加法）可以用于任意布置冻结管冻结稳态温度场解析解的求解：3．将冻结管简化为点源导致获得的解析解存在一定的误差，但误差仅发生在冻结管附近极小的范围内，并且误差微小，完全满足工程上的精度要求。

渗流地层地铁联络通道冻结壁形成过程及其影响因素分析

冻结法凭借封水性好、适应性强和环境影响小等优势成为地铁联络通道施工的主要工法之一,而地下水渗流易引起冻结壁交圈时间延后、厚度和平均温度不及预期,对冻结工程的推进具有不利影响。本文采用频域反射法获取饱和砂砾石未冻水体积分数和温度的函数关系式,基于多孔介质传热和渗流理论,建立考虑相变的饱和地层渗流冻结流热耦合数学模型,利用联络通道冻结模型试验结果验证流热耦合模型的有效性。在此基础上,研究渗流速度、冷媒温度、冻结管直径和渗流温度对联络通道冻结壁形成过程和交圈时间的影响规律。最后,采用正交试验方法对冻结壁形成过程影响因素的显著性进行分析,提出考虑多因素影响的冻结壁交圈时间和标厚时间预测方法。研究结果表明:无渗流时,联络通道冻结温度场呈W型对称分布;当渗流速度为51.2 m/d时,地层温度场不再呈对称分布,上游区域温度高于下游区域温度。渗流速度是影响联络通道冻结壁形成的首要因素,冻结管直径和冷媒温度次之,渗流温度对交圈时间的影响较小。上游侧墙是渗流地层冻结壁扩展的薄弱环节,应加强该区域温度场的监测,跟踪冻结壁的形成过程。

电场辅助冰温和超冰温贮藏对生鲜牛肉品质的影响

为研究电场辅助冰温和超冰温贮藏对生鲜牛肉品质的影响,以牛米龙肉为试验材料,分析电场(设备输出电压3300~4000 V,电流0.04 A)辅助冰温(-1℃)、超冰温(-3℃)和非电场(对照组)条件下贮藏0,7,14,21,28,35 d牛肉pH值、色泽、菌落总数、挥发性盐基氮(TVB-N)含量、硫代巴比妥酸(TBARS)含量、蒸煮损失、质构、T2弛豫时间及挥发性气味的变化。结果表明,在贮藏35 d时,-1℃+电场组的pH值为5.84、TBARS值为0.55 mg MDA/kg、蒸煮损失为31.84%,显著低于-1℃组的5.97、0.60 mg MDA/kg和36.19%(P<0.05);-1℃+电场组a值为10.22,显著高于-3℃组(7.83)和-3℃+电场组(9.63)。贮藏21 d时,-1℃+电场和-3℃+电场组的菌落总数分别为4.87 lg(CFU/g)和5.04 lg(CFU/g),TVB-N含量为10.03 mg/100和10.15 mg/100 g,显著低于各自的对照组【5.36 lg(CFU/g)和5.43 lg(CFU/g),10.85 mg/100 g和10.50 mg/100 g】;同样的,在贮藏第28天,-1℃+电场组P21(86.93)和-3℃+电场组P21(89.69)显著高于各自对照组(84.94和86.87,P<0.05)。-3℃+电场组硬度在贮藏到35 d时为84.95 kg,显著高于-3℃组(77.71 kg,P<0.05)。在同一贮藏时间点,冰温组与超冰温组牛肉间各品质指标差异不显著(P>0.05),电场处理组牛肉色泽、弹性、凝聚性、咀嚼性及恢复性与对照组相比均无显著差异(P>0.05)。结论:电场辅助冰温、超冰温贮藏可有效改善牛肉的品质和新鲜度,延缓品质劣变。

宁夏衬砌渠道渠基冻胀率水热响应试验研究

衬砌渠道周围的水分与温度场分布存在明显差异,此范围内渠基土在水热两场耦合作用下冻胀有明显不同。基于这种差异,开展渠基土在不同水分、温度条件下冻胀率试验研究。结果表明:在不同温度条件下,土体冻胀率随温度的变化过程可分为2个阶段,当温度从0℃降至-5℃左右时,在此阶段产生的冻胀率约占整个冻胀过程的50%~80%,冻胀率与温度线性关系明显。随着温度继续降低,2者非线性关系明显。在无补水情况下,相同温度条件下的土体,初始含水率越大,所产生的冻胀率越大;初始含水率25%和35%的土体产生的冻胀率均为初始含水率15%土体的1倍到2倍以上。当土体在冻胀过程中有水分补充时,其冻胀率与水分变化线性关系明显,初始含水率越大的土体产生的冻胀率增量越大,初始含水率25%、35%的土体冻胀率增量均为15%土体的2倍以上。提出的水热耦合条件与渠基土冻胀率的多元非线性关系与试验数据吻合较好。建立数值模拟模型,对衬砌渠道不同位置处冻胀作用分布与变形进行计算与分析,结果表明:温度越低、水分越大,衬砌渠道所受到的切向冻胀力与法向冻胀力越大。水热变化差异越大时,土体冻胀导致的衬砌渠道冻胀变形越显著。

水泥土地层冻融温度场全过程发展影响研究

为研究水泥改良土地层冻融温度场发展的影响因素,以广州地铁三号线某地铁隧道冻结工程为背景,通过水、热耦合的方式模拟隧道冻融温度场,研究其冻融温度场全过程发展影响规律,并与实测数据对比证明模型的精确性,分析导热系数、容积热容、原始地温和水泥掺量等因素对冻融温度场的影响。结果表明,土体在冻融温度场全过程发展过程中,先是大幅度降温,形成稳定冻结帷幕后,降温速率变缓,有效冻结壁厚度缓慢增加,进入自然解冻阶段后,温度快速上升直至达到相变阶段,在该阶段维持一段长时间后,土体迅速恢复正温;分析不同因素对冻结温度场的影响规律,其中土体容积热容,原始地温对冻结温度场冻结效果呈负相关,对于自然解冻温度场影响不大;土体导热系数的降低会抑制冻结温度场的发展,但是可以缩短自然解冻的周期;在水泥掺量为12%时,土体冻结效果最好,且融沉温度场发展速率最快,自然解冻周期最短。

富水软土基坑组合式冻结加固模型试验研究

近年来随着环保理念的贯彻和对城市水资源保护的重视,许多地区对地下水抽排进行限制,导致大量基坑工程必须寻找新的止水方式。对此,“地下连续墙+水平冻结封底”的组合式冻结加固被提出,利用模型试验对其在不同水压地层中可行性和差异性进行了分析,验证了该支护方式的可行性。结果表明:(1)随着水头高度的增大,冻结壁水分补给速率有所提升,冻结壁交圈时间与地层水头高度呈正相关,而冻结壁厚度与地层水头高度呈负相关关系。(2)临时竖井受地层冻结影响,会有明显冻胀压迫作用,伴随冻结壁发育,冻胀力最终降至某一特定值,并趋于平稳,且其峰值与水头高度成正相关关系;(3)模型试验中冻结壁上部土体的最大位移点位于土体35 cm深度位置,其位移数据为2~3 mm;(4)基坑开挖期间,冻土帷幕的温度随着基坑的开挖和积极冻结的双重作用下能够维持相对的平衡,整个冻结壁保持基本稳定状态。研究成果可为该支护方式在基坑工程中的应用提供理论支撑。