Experimental investigation on the effective thermal conductivities of different hydrate-bearing sediments

The natural gas hydrate has been regarded as an important future green energy.Significant progress on the hydrate exploitation has been made,but some challenges are still remaining.In order to enhance the hydrate exploitation efficiency,a significant understanding of the effective thermal conductivity(ETC)of the hydrate-bearing sediment has become essential,since it directly controls the heat and mass transfer behaviors,and thereby determines the stability of hydrate reservoir and production rate.In this study,the effective thermal conductivities of various hydrate-bearing sediments were in-situ measured and studied.The impacts of temperature,particle size and type of sediment were investigated.The effective thermal conductivities of the quartz sand sediments before and after hydrate formation were in-situ measured.The results show the weak negative correlation of effective thermal conductivity of the quartz sand sediment on the temperature before and after the hydrate formation.The effective thermal conductivity of the hydrate-bearing sediment decreases with the increase of particle size of the sediment.The dominant effect of the type of porous medium on the characteristics of the effective thermal conductivity of hydrate-bearing sediment was highlighted.The results indicate that both the effective thermal conductivities of hydrate-bearing quartz sand sediment and hydrate-bearing silicon carbide sediment are weakly negatively correlated with temperature,but the effective thermal conductivity of hydrate-bearing clay sediment is weakly positively dependent on the temperature.In addition,the values of the effective thermal conductivities of various hydrate-bearing sediments are in the order of hydrate-bearing silicon carbide sediment>hydrate-bearing quartz sand sediment>hydrate-bearing clay sediment.These findings could suggest that the intrinsic thermal conductivity of porous medium could control the characteristics of effective thermal conductivity of hydrate-bearing sediment.

Enhanced thermal and electrical properties of poly (D,L-lactide)/ multi-walled carbon nanotubes composites by in-situ polymerization

Biodegradable poly （D,L-lactide） （PLA）/carboxyl-functionalized multi-walled carbon nanotubes （c-MWCNTs） composites were achieved via in-situ polymerization. These as-prepared composite materials were characterized with FT-IR, XRD, TG, DSC, SEM, and high insulation resistance meter. The results demonstrate that the multi-walled carbon nanotube was carboxyl functionalized, which improved the collection between c-MWCNTs and PLA, and further realized the graft copolymerization of c-MWCNTs and PLA. There is a higher glass transition temperature and a lower pyrolysis temperature of PLA/c-MWCNTs composites than pure PLA. The c-MWCNTs gave a better dispersion than unmodified MWCNTs in the PLA matrix, and an even coating of PLA on the surface of c-MWCNTs was obtained, which increased the interfacial interaction. High insulation resistance analysis showed that the addition of c-MWCNTs increased the electric conductivity, and c-MWCNTs performed against the large dielectric coefficient and electrostatic state of PLA. These results demonstrated that c-MWCNTs modified PLA composites were beneficial for potential application in the development of heat-resisting and conductivity plastic engineering.

Laboratory and field experiment on measurement of soil thermal conductivity by probe method

The authors presented a new measuring method of the soil thermal conductivity,the probe method,which is designed and made based on the theory of line heat source. This method is used to measure thermal conductivity of coarse sand,fine sand and silty clay in different water contents. The results that measured by the probe method are well consistent with those of QTM-D_2. The soil thermal conductivity increases in different levels with the increase of the water content. Compared the soil thermal conductivity measured by the probe method in laboratory with in-situ experiment,it shows that the measuring gap gradually increases with the increase of the depth. The reason is that the in-situ measuring thermal conductivity can reflect the actual situation of the soil mass.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat ex changer.

原位热传导修复过程中热量传递的数值模拟

目前原位热传导修复技术存在热量传递机理不明、主要影响因子作用关系不清的问题,通过模拟室内土柱实验实现对土壤内部热湿耦合迁移机理的验证,并应用到室外场地尺寸,明确场地尺寸下热源温度、初始含水率对原位热传导修复的影响作用。建立了原位热传导修复耦合模型,利用小试实验对其进行了数值模拟验证,在场地尺寸下探究了热源温度、初始含水率对原位热修复的影响。结果表明,原位热传导修复耦合模型准确度较高,模拟结果与实验结果平均相对误差为1.30%。沸腾阶段持续时间与热源温度成反比,过热阶段升温速率与热源温度成正比,在工程实践中应以去除目标为评价标准而不是冷点温度。土壤初始含水率在15%~35%范围内,相同温度影响下含水率越低导热系数越高,原位热传导修复技术适用于低含水率场地,初始含水率高于15%,在进行修复之前应进行适当排水或设置止水帷幕。该研究结果可为原位热传导修复技术工程实践应用提供理论参考。

回填材料对土壤原位热传导修复的影响及数值模拟

原位热传导修复技术是一种有机污染土壤高效修复手段。由于施工过程中加热井与土壤间会存在一定空隙,关于是否使用回填材料以及回填材料的选取原则,尚未有明确的指导意见。利用实验和数值模拟方法,对原位热传导修复过程中回填材料的影响进行了研究,分析了不同加热温度(200、400、600、800℃)、回填材料(空气、原土)、回填厚度(40、100、150 mm)对传热的影响。结果表明,基于实验数据所建立的原位热传导数值预测模型是可靠的,模拟计算值与实测值最小平均相对误差为6.69%;当加热温度高于450℃时,无回填料时传热效果更好;当加热温度小于300℃时,用土壤回填较好;在300~450℃时,有无回填料传热效果相差不明显;原位热传导修复技术工程在应用过程中,回填材料厚度100 mm时传热效果最佳。本研究结果可为污染土壤原位热传导修复的工程实践提供参考。