Thermal Needle Probe Research Articles

Variations of thermal properties with water content and density of residual and basaltic sands from Chile

Renewable energy has become a relevant alternative to solve energy and environmental problems worldwide. Characterization of geothermal resources is fundamental for an efficient and sustainable extraction of heat. In this study, thermal conductivity, λ , volumetric heat capacity, C , and thermal diffusivity, α , of two soils from Chile, namely Bío Bío sand and Maicillo residual soil, were analysed. Bío Bío sand is a uniform and clean sand that mainly consists of basaltic particles, whereas Maicillo soil is a silty and clayey sand with a high quartz content. Samples were tested with a thermal needle probe at varying water contents and densities. Measurements to obtain λ , C and α were undertaken. These results allowed the patterns and relationships between geotechnical and heat transfer parameters to be analysed. It was found that dry soil conditions led to the lowest values of λ , while saturated soil conditions led to the highest values. Moreover, Maicillo soil has higher capabilities to transfer heat than Bío Bío sand, and can reach up to 50% greater λ for dense soil and saturated conditions. This is due to its quarzitic and clayey mineralogy and non-uniform grain-size distribution. The results from this research represent an important contribution to industrial applications.

Development of the heat flow cone penetration test (HF-CPT)

The heat flow cone penetration test (HF-CPT) provides in-situ values of thermal conductivity (patent pending). The HF-CPT test records include (1) heat flow (HF) measurements acquired by a CPT add-on module, i.e. heating power and temperature versus time and (2) cone penetration test (CPT) measurements, i.e. cone resistance, sleeve friction and pore pressure versus depth and time. The test method requires a short interruption of the continuous CPT penetration phase, to allow stationary HF heating and cooling cycles.
 Values of thermal conductivity are derived similarly to the principles for laboratory thermal needle probes described in common ASTM standards, particularly [1].
 Data processing makes use of an advanced interpretation method that accounts for the short-cylinder effects of the HF module and short timeframe heat fluxes. The novel interpretation method includes inversion of a numerical forward model of the interaction between the heat flow module and the surrounding soil. The interpretation method also integrates standard CPT results, such that both a semi-continuous thermal conductivity profile and a continuous standard CPT profile are obtained.
 Validation of the interpretation method included comparison of thermal conductivity values derived from other test methods, notably laboratory transient plane source tests [1], in-situ thermal needle probe tests (based on [2]) and thermal cone penetration tests [3]. Figure 1 presents an example of validation results for predominantly clay soil. The results are seen to closely match those derived from an in-situ needle probe. As the HF-CPT measurements were taken two metres apart horizontally from the in-situ needle probe measurements, some deviation between the results is to be expected due to heterogeneity of the soil.

Thermophysical Properties of Bentonite–Sand/Fly Ash-Based Backfill Materials for Underground Power Cable

The surrounding (backfill) materials around the underground power cable systems are essential for dissipating the heat away from it, during the exertion phases. The heat dissipation restrains the thermal instability and risk of progressive drying of the backfill materials, thus reducing the thermal stress on the power cable. Thermal instability indicates the reduction in thermal properties (conductivity or diffusivity) due to the migration of moisture because of heat accumulation. Thus, the backfill materials should have adequate thermal properties and water retention capacity to transfer the heat from the heat source to the surrounding area with minimal moisture migration. The bentonite has high water retention capacity, but low thermal conductivity, whereas sand/fly ash exhibits low water retention and has higher thermal conductivity than bentonite. The addition of bentonite promotes the water holding capacity and thermophysical properties of sand and fly ash. Therefore, this study presents the thermal properties of backfill materials, bentonite–fly ash (B–F) and bentonite–sand (B–S), at varying weight-percent of sand and fly ash with bentonite. Various compositions of the mixtures were compacted to varying dry densities, and water contents and thermal properties variation of backfill materials were measured using a dual thermal needle probe ‘KD2 Pro’ at room temperature. The study deals with the systematic evaluation of the volumetric specific heat capacity, thermal conductivity, and diffusivity of backfill materials against varying dry density and water content. The threshold water content (TWC) has been determined from the thermal diffusivity–water content variation curve, and it has correlated with plastic limit (PL) and optimum moisture content (OMC). Thereafter, the efficacies of two thermal conductivity prediction models have also been evaluated statistically with respect to experimental results.

Plasma drilling on Martian ice: Enabling efficient deep subsurface access to Mars' polar layered deposits

Mars' polar layered deposits (PLD) contain the best historical record of the Amazonian time period. Deep subsurface access at Mars' poles is challenging due to the restriction of Mars' atmospheric condition. Traditional terrestrial mechanical drilling methods using drilling fluids are not feasible in these conditions and the melting probe approach is very inefficient due to heat loss. For ice in Mars' polar condition, the increase in surface area due to cracks would greatly reduce its thermal conductivity. This will lead to a higher thermal efficiency and an easier access to polar layered deposits during melting or sublimation. The preliminary results with pulsed plasma ice drilling, using localized electric breakdown with electric energy input per pulse up to 80 ​J, were promising under both Mars and Earth conditions. To simulate Mars' polar condition, a vacuum vessel system was designed and implemented with a pulsed discharge circuit, ice plate, controlled liquid nitrogen cooling, and CO 2 gas feed-in. A simplified heat conduction model was established for estimating the heat conduction at various porosities for homogenous water ice and CO 2 gas mixture using COMSOL under two scenarios including constant thickness of mixture layer and constant thermal mass of the mixture layer under the heat source. The simulation results for both scenarios indicated more reduction of heat loss with an increasing porosity. Experimental studies of thermal conductivity measurement of granular ice under the Earth condition inside a freezer using a custom thermal needle probe with known porosity proved the concept that voids/cracks in ice can reduce the thermal conductivity. Based on simulation and experimental results, the classic melting probe model is extended with porous ice-CO 2 mixture to estimate the power consumption on Martian ice at varying descending speed. Plasma drilling with limited power is promising for accessing Mars' PLD and can also be expanded to planetary exploration in searching for life on other planets. • Plasma drilling method has no moving parts. • Pulsed plasma drilling improves the Rate of Penetration of the melting probe. • The cracks/voids in ice decreases its equivalent thermal conductivity. • Additional power requirement is acceptable within limited solar power access for Martian probe.

Experimental Investigations on the State-Dependent Thermal Conductivity of Sand-Rubber Mixtures

Soil-rubber mixtures are potential construction materials for use in civil engineering applications such as the thermal insulation layer for buried pipelines in the permafrost zone and underground energy storage systems. Currently, however, the thermal conductivity of soil-rubber mixtures in various stress, saturation, and void ratio conditions is not fully understood. In this study, a stress-controlled apparatus based on the thermal needle probe technique was developed. It was employed to study the influence of the aforementioned factors on the thermal conductivity of sand-rubber mixtures. Thirty-four tests were completed with a wide range of net stresses (0–600 kPa), initial degrees of saturation (0%–100%), void ratios (0.45–0.95), and rubber contents (0%, 10%, and 20%). Three replicates were used in each condition. The results showed that the thermal conductivity was strongly dependent not only on void ratio, rubber content, and degree of saturation but also on net stress. For instance, the thermal conductivity of a dry sand-rubber mixture increased by 30% when the stress increased from 0 to 600 kPa. This increase in thermal conductivity can mainly be attributed to an increase in interparticle contact area, accounting for about 25% of the 30% increase; the reduction in void ratio played a minor role. A semiempirical equation was proposed to calculate the thermal conductivity of sand-rubber mixtures at different stress levels.

Effect of Iron Oxide Coatings on Thermal Conductivity of Silica Sands

Engineering properties of soils coated with naturally occurring or artificially manufactured particles can significantly differ from those of uncoated soils. This study aims at evaluating the effect of iron oxide coatings on the thermal conductivity of silica sands. Laboratory thermal needle probe tests were conducted on uncoated, goethite coated and hematite coated Ottawa 20/30 and Ottawa 100/200 sands. Test results demonstrated that the thermal conductivity (k) of all tested materials increases with increasing applied vertical stress, and the k of hematite coated sands was greater than that of uncoated sands under the similar relative density. However, the measured k of both uncoated and coated sands is a single function of porosity regardless of applied stress level, reflecting the limited effect of applied stress and iron oxide mineral coating on the k of tested materials. Simple resistances in series model was employed to explain the observed test results, and the results of this study was compared with k of previous studies on uncoated natural soils.

Evaluation of thermal conductivity for compacted kaolin Clay-Shredded tire mixtures as thermal insulation material

Thermo-active engineering projects have been extensively constructed with the rapid increase in energy consumption, which poses high requirements on the design of thermal insulation materials. The present study investigated the thermal conduction behaviors of kaolin clay mixed with shredded tires with different particle gradations. The thermal conductivity of compacted kaolin-shredded tire mixtures was measured using a thermal needle probe to evaluate the combined effects of shredded tire mixing ratio and mean particle size. Based on the experimental data, an empirical model was developed to describe thermal conductivity behavior of the mixtures. The results showed that thermal conductivity of kaolin-shredded tire mixtures decreased with increasing shredded tire mixing ratio, and a maximum reduction in thermal conductivity (Δk = 66.14%) was detected in the sample with 50% volumetric ratio of shredded tire with mean particle size D50 of 0.20. The heat conduction mode of the mixtures changed from ‘shredded tire-matrix’ to ‘kaolin-matrix’ with an increase in particle size of shredded tires, corresponding to the lower thermal conductivity measured in the sample with a relatively smaller size of shredded tire. Linear increasing trends were found in the plot of percentage difference in thermal conductivity Δk versus shredded tire mixing ratio Rs, and the slope values exhibited a power function relation with the mean particle size of shredded tires. The developed thermal conductivity model of kaolin-shredded tire mixtures perfectly captured the trends in the testing data, with an absolute percentage error δ of less than 3% and root mean square error RMSE value of 0.0087 W/m·K. This empirical model may be extended to other compacted clay-shredded tire mixtures by adjusting/introducing calculation parameters.

Determination of Thermal Conductivity Properties of Coastal Soils for GSHPs and Energy Geostructure Applications in Mexico

The thermal conductivity of soils is a fundamental parameter for the design of ground-source heat pump systems (GSHPs) and energy geostructures. This paper presents a comprehensive evaluation of the physical, mineralogical, and thermal characteristics of typical coastal soils from Tabasco, Mexico. Twenty-five soil samples from four different strata were studied using the thermal needle probe method, X-ray diffractometry, scanning electron microscopy, and standard geotechnical soil classification tests. The results showed a significant correlation between the dry density and porosity with the thermal conductivity of the studied samples, which ranged between 1.17 and 2.32 W m−1 K−1. The performed statistical analyses indicated that coarse-grained soils had larger thermal conductivities and higher variability than fine-grained soils. Additionally, the performance of six models to estimate the thermal conductivity of soils was validated against the experimental data. All models provided accurate estimations for fine-grained soils, but only the effective medium theory (EMT) showed an adequate fit for coarse-grained soils. The results represent one of the first datasets for the thermal properties of Mexican soils. They will contribute to the implementation of GSHPs and energy geostructures in the country and locations with similar subsoil conditions, especially where time and resources are not available for their experimental determination.

Thermal conductivity of steelmaking slag-based controlled low-strength materials over entire range of degree of saturation: A study for ground source heat pump systems

Controlled low-strength materials (CLSMs) with high workability and relatively high thermal conductivity can function as a thermal grout in the heat exchanger of ground source heat pump systems. The degree of saturation of CLSM strongly influences its thermal conductivity and thus affects the heat transfer capacity of the heat exchanger system. This study investigates the degree of saturation–thermal conductivity relationship of steelmaking slag-based CLSM using two sample preparation methods: a modified high-pressure membrane extractor coupled with a thermal needle probe (HP) and air-dried (AD) methods. The effect of grinding steelmaking slag on the mechanical properties and thermal conductivity of CLSM is also evaluated. The engineering properties test results indicate that ground steelmaking slag presents a positive effect on the compressive strength and thermal conductivity of CLSMs owing to the reduction of the inherent porosity and increased specific surface area of the raw steelmaking slag by the grinding. The degree of saturation and thermal conductivity have a linear relationship for both the AD and HP test methods. However, it was found that the AD method overestimates the thermal conductivity owing to the uneven distribution of the water content in the specimen. Finally, predictive equations are proposed to estimate the thermal conductivity of CLSM for the entire range of degree of saturation. Compared to the recently proposed equation, these equations are more appropriate for thermal conductivity estimation of grouting materials based CLSM, especially at low water content.

Interpreting and validating the thermal cone penetration test (T-CPT)

The thermal cone penetration test (T-CPT) allows in situ measurement of the thermal properties of soil during a CPT – that is without withdrawing the CPT tool and introducing another tool such as a thermal needle probe. An interpretation method is presented based on one-dimensional axisymmetric analytical solutions. Numerical verification of the method is given and numerical simulations are presented to derive a calibration factor required for the interpretation model. This factor accounts for the temperature sensor location and two-dimensional geometric effects, particularly internal voids and end effects for the cone penetrometer. Finally field data are presented for the T-CPT and compared with results from in situ needle probe tests and laboratory tests on sampled material. The interpretation model is able to provide thermal conductivity values robustly.

Thermal conductivity of controlled low strength material (CLSM) under various degrees of saturation using a modified pressure plate extractor apparatus – A case study for geothermal systems

Conventionally, controlled low strength material (CLSM) is defined as a self-compacting and self-leveling material. High thermal conductivity CLSM can be, however, used as the heat transfer medium (thermal grout) in geothermal systems due to its good flowability and low shrinkage. Similar to general soils or earlier developed thermal grouts, the degree of saturation of CLSM is strongly related to its thermal conductivity, which is an input parameter in the design of geothermal systems. Hence, this relationship should be explored for the optimum design in these systems. This paper addresses the thermal conductivity of CLSM made with coal ash and excavated soil as a function of the degree. In order to do this, the pressure plate extractor apparatus (SWC-150) was modified by coupling with a single thermal needle probe. In the experimental program, all general properties (e.g. flowability, bleeding, fresh density, initial setting time, and unconfined compressive strength) were performed first. Afterward, thermal conductivity, the degree of saturation, and matric suction were measured simultaneously by the modified apparatus. As a result, all general properties of the prepared CLSM satisfied the specifications of CLSM. In addition, linear relationships between thermal conductivity and degree of saturation of the proposed CLSM mixtures were observed. It is interesting to note that thermal conductivity of CLSM incorporating excavated soil, which works as filler the CLSM mixture, was relatively improved when compared to that of CLSM without excavated soil. Finally, new predictive models for thermal conductivity estimation of CLSM were proposed based on both normalization and regression methods.

Generalized Approach for Determination of Thermal Conductivity of Buffer Materials

Determination of thermal conductivity of buffer materials is an important aspect in the design and characterization of engineered barrier systems (EBS) in deep geological repositories (DGRs) for safe containment of high-level nuclear waste. Several factors, viz. compaction state, particle size distribution, and mineralogical characteristics of buffer materials, influence the thermal conductivity of composite buffer materials. Therefore, it is essential to give due regards to the influence of these factors while estimating thermal conductivity of buffer materials. In view of this, the present study pertains to an extensive laboratory scale determination of thermal conductivity of a wide range of sand-bentonite based buffer materials employing a thermal needle probe. Precision and accuracy of the thermal needle probe are established in relation to a contemporary and widely endorsed thermal property analyzer. Further, the efficacy of several predictive models available in the literature is evaluated to appraise the thermal conductivity of composite buffer materials in relation to the experimental data. Realizing the need of a generalized approach considering the influence of clay mineralogy and particle size fraction present in the geomaterial, the manuscript proposes generalized thermal conductivity prediction models meant for the buffer materials. The developed models are found to deliver satisfactory performance and accuracy in appraising thermal conductivity of sand-bentonite buffers used in DGRs.

Measurement Method of Thermal Properties of Cementitious Bedding Materials and Unsaturated Soils: Hydraulic Influence on Thermal Parameters

Abstract Due to climate-induced variations, changes in the water content of the soil occur, and consequently its hydraulic and thermal properties change as well. Bedding materials for buried energy cables or geothermal installations are usually in capillary contact to the surrounding soil. Thus, they are subject to seasonal changes in the water content caused by natural climatic variations. For the design and construction of structures using thermal energy or dissipating waste heat in the shallow ground, it is important to be able to make reliable forecasts of the thermal and hydraulic properties of partially saturated bedding materials and soils. For this purpose, the climate-related changes of the thermal and hydraulic properties have to be investigated. In this technical note, an experimental setup is described, allowing for the simultaneous measurement of several hydraulic and thermal parameters during dewatering of a specimen. From the data, interrelations between thermal conductivity and water content, water retention characteristics and unsaturated hydraulic conductivity can be derived. This is why, a common evaporation test device was extended using a thermal needle probe. This setup enabled the examination of disturbed and undisturbed soils. A further method, which is also presented in this paper, provides the possibility of investigating cementitious bedding materials in this evaporation test for the first time. This experiment takes only a few days for cementitious bedding materials as well as for undisturbed soil samples. Thus, with only one single experiment, all the basic parameters required for unsaturated hydraulic modeling can be identified. Since thermal conductivity is measured along with the hydraulic parameters, it is possible to assign a value to the thermal conductivity for many hydraulic states of the soil or bedding material.

Use of a Pre-Drilled Hole for Implementing Thermal Needle Probe Method for Soils and Rocks

The thermal needle probe method, which is widely used for measuring the thermal conductivity λ of soils, deploys a long and thin metallic probe that houses a line heater and a temperature sensor. However, inserting such probes into consolidated or densely compacted soils or rocks is difficult, frequently causing buckling of the probe and severe disturbance to the surrounding ground, leading to unreliable measurements. We found that the use of a pre-drilled hole filled with thermally conductive grease for installing a thermal needle probe was feasible to overcome such challenges, and still yielded reliable measurements of thermal conductivity. The proposed method, i.e., the pre-drilling thermal needle probe method, was verified by finite element calculations and laboratory experiments by varying various parameters, such as the pre-drilled hole diameter, probe diameter, and thermal conductivity of thermal grease. It was observed that increases in the pre-drilled hole diameter and probe diameter and a decrease in the thermal conductivity of the thermal grease caused delays in temperature increase owing to the slowed heat transfer. Nevertheless, all the estimated λ values agreed well with the reference λ values with acceptable errors. Thus, the proposed method yields reliable measurements and can be applied for a wide range of soils from compacted soils to hard rocks.

THERMAL PROPERTIES OF MALAYSIAN COHESIVE SOILS

The thermal properties of soils surrounding energy piles are required for the efficient and optimal design of shallow geothermal energy pile systems. In this study, the thermal conductivity, thermal resistivity and volumetric specific heat of two types of Malaysian cohesive soil were obtained through a series of laboratory experiments using a thermal needle probe. This study was conducted to determine the effect of moisture content on the thermal conductivity, thermal resistivity and volumetric specific heat values of the cohesive soil at a given value of soil density. For soils with low to medium moisture content, a linear increase in the thermal conductivity and volumetric heat capacity was observed as the moisture content gradually increased, while the thermal resistivity values of the soil had decreased. Meanwhile, for soils with high moisture content, the thermal conductivity was observed to have decreased, and a marked increase was seen in the thermal resistivity. This is due to the disruption of the thermal flow continuity in the soil matrix with the presence of moisture in the soil which adversely affects the thermal conductivity

Experimental Investigation and Prediction Models for Thermal Conductivity of Biomodified Buffer Materials for Hazardous Waste Disposal

Microbe-aided modification to certain geotechnical properties of soil has garnered appreciable amount of interest among the researchers in recent days. In particular, microbial influence on thermal conduction properties of soil is relevant in engineered-barrier applications as in case of a deep geological repository (DGR). In view of this, in the present investigation, a radiation resistant and extremophilic microbial species Deinococcus radiodurans has been studied for its possible influence on thermal conductivity of several geomaterial samples. Thermal conductivity measurements for varieties of geomaterials with varying moisture content and bacterial dosage are made using an indigenously fabricated thermal needle probe, efficacy of which has been established through calibration. In general, it was observed that microbial addition enhanced the thermal conductivity of soil. This observation can be attributed to any possible biogeochemical reactions in the geomaterials because of the bacterial addition. Furthermore, based on the experimental data, the following three artificial intelligence (AI) techniques are used to develop thermal conductivity prediction models for geomaterials and to quantify the influence of microbial addition on soil thermal conductivity: (1) multi-gene genetic programming (MGGP), (2) multivariate adaptive regression splines (MARS), and (3) functional networks (FN). The models proposed are found to deliver satisfactory performance in terms of statistical parameters and can be used to predict soil thermal conductivity.

Behavioral Thresholds in Mixtures of Sand and Kaolinite Clay

AbstractNearly all soils are comprised of mixtures of coarse and fine particles. Behavior under mechanical and thermal loading of soil is strongly influenced, and in some cases governed, by the ratio of coarse to fine particles. A better understanding of the fundamental behavior of soil mixtures will provide insight to design decisions for new and emerging geotechnologies. In this work, behavioral threshold fines fractions were identified by experimental methods, where the threshold was defined as the point where changes in the coarse/fine mixture ratio result in abrupt behavior changes. Binary mixtures of sand and kaolinite clay ranging from 0 to 100% fines content were subjected to consistency and undrained shear strength testing with the fall-cone apparatus, compressibility tests using an oedometric cell, thermal conductivity tests with a thermal needle probe, and stress-strain-strength testing in undrained triaxial shear. Results indicate that behavioral thresholds exist at a critical fines content wh...

Determination of the Thermal Conductivity in Adobe With Several Models

The thermal conductivity of the earth materials conditions their ability as thermal isolator and its heating capacity, which has a direct impact on the energy consumption of the buildings built with these materials. Two original mathematical models have been developed (models MA-1 and MA-2) to calculate the effective thermal conductivity (λE) of adobes and their results have been compared with other models already known for other materials and with experimental measures done on adobes. The model MA-1 starts from the electric analogy of the transmission of heat in series and in parallel. The model MA-2 is obtained with a regression curve from experimental and literature values of λE in adobes. The λE in adobes has been measured by the thermal needle probe (TNP) procedure using 10 min as the measuring time. For dry adobes, with average environmental conditions of 19 °C and 41% of relative moisture, the values of λE measured were 0.80 W/(m·K) ± 10%. For natural hygroscopic moisture of 1.67% in the same environmental conditions, a λE of 0.90 W/(m·K) ± 10% was measured. Only five of the 18 models analyzed adjust to the values experimentally measured, and their precision depends on the values of λ of the components, which are obtained from the literature. Of the proposed models, the MA-1 fits for the values of the dry and wet material and with some determined values of the literature. The model MA-2 fits in all cases since it does not depend on the values of the literature but on the density of the material and its moisture content.

Thermal conductivity of soils and rocks from the Melbourne (Australia) region

The thermal conductivity of soils and rocks is an important property for the design of thermally active ground structures such as geothermal energy foundations and borehole heat exchange systems. This paper presents the results of a laboratory study of the thermal conductivity of soils and rocks from around Melbourne, Australia. The thermal conductivity of six soils and three rock types was experimentally measured using both a thermal needle probe and a divided bar apparatus. Soil samples were tested at a wide range of moisture contents and densities. The results demonstrated that the thermal conductivity varied with soil moisture content, density, mineralogical composition and particle size. Coarse grained soils were observed to have a larger thermal conductivity than fine grained soils. In addition, the thermal conductivity of soils increased with an increase in dry density and moisture content. Siltstone, sandstone and basalt rock samples were tested dry and water saturated. They demonstrated an increase in thermal conductivity with an increase in density when dry. However, when water saturated, siltstone and sandstone showed no significant correlation between density and thermal conductivity; whereas a linear increase in thermal conductivity with density was observed for the saturated basalt samples. These differences were attributed to both variations in mineralogy and anisotropy of each sample. The thermal conductivity data obtained from this study provides an initial database for soils and rocks from the Melbourne (Australia) region which can serve for the design of thermo-active structures installed locally and in locations with similar ground conditions.

Field determination of shallow soil thermal conductivity using a short-duration needle probe test

The thermal conductivity of shallow subsoil, one of the important input parameters in the design of a horizontal closed-loop ground heat exchange system, can be derived by means of a thermal needle probe survey. The probe is mounted on a 1.5 m handle, allowing short-duration (10 min) thermal response tests to be carried out at various locations and depths within a land area designated for such a heat exchanger. At two study sites in Central England, a minimum of 12–16 such determinations were required to obtain a stable estimate of bulk thermal conductivity (based on the geometric mean of single measurements), although one cannot necessarily assume that this minimum requirement would be generally valid at all sites. At the two sites, the method proved relatively rapid and able to deliver a value of geometric mean thermal conductivity with a typical uncertainty of the order of 15%.