Hot-wire Method Research Articles

Dependence of Thermal Conductivity on Size and Specific Surface Area for Different Based CoFe2O4 Cluster Nanofluids

Enhancing the thermal conductivity of fluids by using nanoparticles with outstanding thermophysical properties has acquired significant attention for heat-transfer applications. Nanofluids have the potential to optimize energy systems by improving heat-transfer efficiency. In this study, cobalt ferrite nanoparticles clusters with controlled mean sizes ranging from 97 to 192 nm were synthesized using a solvothermal method to develop novel nanofluids with enhanced thermal conductivity. These clusters were comprehensively characterized using transmission electron microscopy, X-ray diffraction, Raman spectroscopy, vibrating-sample magnetometry, and nitrogen physisorption. The CoFe2O4 cluster nanofluids were prepared using the two-step method with various base fluids (water, propylene glycol, and a mixture of both). Dynamic light scattering analyses of the average Z-size of the dispersed nanoadditives over time revealed that the stability of the dispersions is influenced by cluster size and the proportion of glycol in the base fluid. The thermal conductivity of the base fluid and nine different 0.5 wt% CoFe2O4 cluster nanofluids was measured using the transient hot wire method at temperatures of 293.15, 303.15, and 313.15 K, showing different temperature dependencies. The study also explores the relationships between the thermal conductivity, cluster size, and specific surface area of the nanoadditives. A maximum thermal conductivity enhancement of 4.2% was reported for the 0.5 wt% nanofluid based on propylene glycol containing 97 nm CoFe2O4 clusters. The findings suggest that the specific surface area of nanostructures is a more relevant parameter than size for describing improvements in thermal conductivity.

Thermal conductivity measurements for n-hexane and n-heptane at elevated temperature and pressure

Thermal conductivity is a very important thermal property parameter in the process of hydrocarbons transportation, storage, combustion and cooling. Therefore, accurate thermal conductivity is important in the utilization of hydrocarbons. In this work, the thermal conductivity of n-hexane and n-heptane in the temperature range of 298.15 K∼523.15 K and pressure range of 0.1 MPa∼15.0 MPa was studied by using transient hot-wire method. In order to facilitate engineering application, function polynomials of temperature and pressure are fitted and correlated with experimental data. The average absolute error of the experimental data and fitting data of n-hexane and n-heptane are 0.72 and 0.61 %, respectively, which proves that the function polynomial can describe the experimental data well. In addition, we compare the collected literature data with our results and find that the literature data is very close to our results. This work is expected to expand the range of available data on the thermal conductivity of n-hexane and n-heptane and contribute to the industrial applications of these two substances.

A double-helix plane winding sensor with wire using transient plane source method

The transient plane source (TPS) method is widely used to measure thermophysical properties of various materials, and the high-precision double-helix sensor is a key technology. A novel sensor using an efficient, simple, and low-cost lacquered micron-wire-wound process is proposed to replace traditional complex micro- and nano-etching processes. The TPS test platform was built based on the Wheatstone Bridge theory. A new sensor sample of diameter 5.377 mm was wound with an enameled copper wire of diameter 0.063 mm. Owing to the characteristics of the process, a 1-mm round hole was bored inside the sensor. An analytical solution for the concentric ring heat transfer model of the sensor with a central circular hole structure was derived, and a two-dimensional concentric circle simulation model of the sensor was established. As the number of turns decreased, the average error between the analytical solution of the concentric ring model with a large center hole (LHAS) and that of the traditional concentric circle model (TAS), as well as LHAS and the simulation model, gradually increased. The maximum relative errors between LHAS and TAS, as well as between LHAS and the simulation model, were 23.69 % and 0.1 %, respectively, and the average errors of the absolute maximum were 11.86 % and −0.06 %, respectively. The LHAS exhibited excellent accuracy because of the structural characteristics of the novel sensor. The accuracy of LHAS has been validated by simulation and hot wire method. Temperature rise data of high-borosilicate glass and polytetrafluoroethylene samples at different powers were obtained. The thermal properties of the samples were determined by fitting LHAS and TAS data. The thermal conductivity errors measured by LHAS were smaller than those measured by TAS, the thermal conductivity and thermal diffusivity errors were less than 3.5 % at different powers, and the measured repeatability errors were less than 1 %, indicating the good application prospect of the proposed model.

Effect of hot-wire method on microstructure and mechanical properties of wire arc additive repaired superalloy

This study shows that the hot wire is an effective way to reduce the amount of undesirable Laves phase during the repair of 718Plus components, which are detrimental to the mechanical properties. The complex relationship between microstructure and hotwire was studied in detail. Five damaged components were repaired by depositing the melted 718Plus wire onto the wrought substrate using directed energy deposition-arc (DED-arc). A finite element model was used to provide clues for the thermal process analysis. The hot-wire method enhanced the cooling rate of the molten pool and suppressed the remelting behaviour of secondary dendritic arms (SDA) in the reheated zone. The resultant microstructure contained refined dendrites with more obvious SDA, which allows the production of tiny reticular Nb-rich liquid spots between dendrites during final solidification stage. These spots beyond a threshold of Nb could cause eutectic reaction, leading to the morphologic transformation from long-stripped to granular Laves phase and a lower volume fraction. Moreover, the hot-wire method hindered the dynamic precipitation of γ' phase due to the shortened precipitation time, which was responsible for the inferior mechanical properties. The heat-treatment re-optimized the precipitation of γ' phase and further dissolved Laves phase to release more Nb for γ' phase, which leads to the improvement of strength about 10 %.

Harnessing nano-synergy: A comprehensive study of thermophysical characteristics of silver, beryllium oxide, and silicon carbide in hybrid nanofluid formulations

Nanofluids, promising to improve heat transfer efficiency, encounter stability and durability challenges, hindering their industrial applications. The emerging concept of hybrid nanofluids, alongside surfactant-driven stability research, presents a promising solution to tackle challenges in heat transfer, potentially revolutionizing thermal management systems and advancing nanomaterial science. This comprehensive study investigates the thermophysical properties of simple and hybrid nanofluid formulations composed of silver (Ag), beryllium oxide (BeO), and silicon carbide (SiC) nanoparticles dispersed in water. Hybrid nanofluids were prepared with varying volumetric ratios of 20:80, 40:60, 60:40, and 80:20 and examined across temperatures ranging from 15 to 45 °C. The influence of surfactants on stability was explored to augment thermal characteristics. Results revealed that the surfactants have a significant effect on stability and the specific mixing ratios can lead to more favourable thermal characteristics. Thermal conductivity enhancements, evaluated via the transient hot-wire method, demonstrated improvements of 7.43 %, 7.17 %, and 5.31 % for Ag/SiC (60:40), Ag/BeO (60:40), and SiC/BeO (80:20) hybrid nanofluids, respectively, compared to water. Viscosity measurements revealed Newtonian behaviour for the Ag, SiC, and BeO nanofluids, with a minimum viscosity enhancement of 3.01 % observed for the BeO nanofluid. Hybrid Ag/SiC nanofluid demonstrated a maximum viscosity enhancement of 4.90 % for 20:80 formulation. Density analysis showed maximum augmentation of 0.25 %, 0.097 %, and 0.0775 % for the Ag, SiC, and BeO nanofluids respectively at 0.025 vol% while hybrid Ag/SiC nanofluid exhibited a maximum of 0.23 % density increase for the 80:20 composition. The statistical models were also developed to predict properties against temperature. Furthermore, the cost analysis identified Ag nanofluid as the most expensive option, while the SiC/BeO hybrid was the most economical. However, the Ag-SiC (60:40) hybrid nanofluid offered a balanced trade-off between properties and cost.

Influence of graphene nanoplatelets on the thermal conductivity of heat transfer oils based on the developed hotwire method

The thermal conductivity measurements of graphene nanoplatelets (GNP) loaded mineral oil suspensions are obtained using an unconventional custom-made Transient Hot-Wire setup using tungsten. The presence of a few layers of graphene in the prepared GNP using a single-step ultrasonication of graphite microparticles without the use of a surfactant is verified by AFM, TEM, Raman Spectroscopy, XRD, and XPS techniques. We measured the thermal conductivity and the specific heat capacity ( Cp ) of GNP-loaded mineral oil suspensions. The experimental result shows a maximum of 45.5% increment in the thermal conductivity of the mineral oil with 1 mg ml−1 (11.481 ×10−4 weight fraction) loading of GNP at 30 °C. Despite thermal conductivity enhancements, the results indicate that the Cp is not compromised, since a low concentration of nanoplatelets was used. The experimental results agree with the Maxwell-Garnett effective medium approach (MGEMA) model for the thermal conductivity of nanofluids which we found to be suitable for lower loading of nanoparticles. We propose GNP incorporated mineral oil suspensions are effective when it comes to heat transfer fluids even with the lower volume and mass fractions.

Assessing the durability of bio-based materials with respect to microbial contamination: Microstructure and macroscopic properties

The aim of this paper was to assess the effect of the microorganism development on the thermal and mechanical properties of building insulation materials (rapeseed straw and lime (RL) and sunflower bark and pith and lime (SL)) and three plant aggregates (rapeseed straw (RS), sunflower bark (SB) and pith (SP)). The samples were exposed to accelerated ageing, in a static environment with a temperature of 30°C and a high humidity of 90 % (±3 %), to favor the proliferation of microorganisms. The paper presents the biological study of the cultivated bacteria and fungi at four ageing times (0, 3, 6, and 12 months) using a counting test. The results showed that these materials were more vulnerable to fungi than to bacteria. Cryo-HRSEM observations were carried out to follow the development of microorganisms at micro scales, at 0 and 6 months of exposure to microbial ageing. The observations showed that the aggregates (RS, SB, and SP) were colonized by microorganisms prior to microbial conditioning for 6 months, but not with the same tendency on the biocomposites (SL and RL) studied which presented weaker development of microorganisms by comparison to the aggregates. The lime has an immediate anti-microbial effect on the aggregates after storage conditions. After ageing, the microorganisms continued to grow on the aggregates, as did the biocomposites, which showed microbial growth of microorganisms of the same type as those found initially on the aggregates. Physical characterization was conducted in order to evaluate the effect of the microbial development. The thermal conductivity was measured with hot wire method for plant aggregates and heat flow meter for the biocomposites. Mechanical characterization was performed by uniaxial compressive test. The studied functional properties (thermal conductivity and compressive strength) of RS, SB, RL, and SL showed a decrease after 3 months, and after 6 months for SP.

Role of aloe vera based nanofluids for cool thermal energy storage system: A comparative study

The present experimental investigation incorporates the preparation of aloe vera gel-based phase change nanofluid (NFPCM) for a cool thermal energy storage (CTES) system. Two sets of NFPCMs were produced by adding graphene nanoplatelets (GNP) in deionized water and aloe vera gel (0.20 wt%, 0.40 wt%, and 0.60 wt%). The zeta potential analysis shows that the NFPCM with aloe vera gel is more stable even after 60 days from the preparation (−44 mV). The thermal conductivity of NFPCMs was analyzed through the transient hot wire method and the enhancement of 42 %, and 37.5 % were measured in the ice phase for NFPCMs with aloe vera gel and DI water (0.60 wt%) respectively at the heat transfer fluid temperature of −15 °C. The NFPCMs with aloe vera gel show that the addition of GNP prepended the peak melting temperature from −16 °C to −10.5 °C, which is due to the quick formation of nuclei as compared to NFPCMs with DI water. Additionally, a slight reduction in the latent heat of only 7 % was observed. The charging characteristics of NFPCMs were studied in a single spherical capsule at −8 °C. The maximum reduction in total charging time of 25 % was recorded for the aloe vera gel-based NFPCM with a maximum concentration of GNP (0.60 wt%). The green synthesis of aloe vera gel-based NFPCM exhibited noteworthy enhancements in thermophysical properties. Additionally, it was observed that the long-term stability of the NFPCM was achieved by excluding surfactants during the preparation process. The incorporation of graphene nanoplatelets (GNP) into aloe vera gel as the foundational phase change material (PCM) facilitates the partial charging of the CTES system with a reduced energy input.

Development of Hot-Wire Laser Additive Manufacturing for Dissimilar Materials of Stainless Steel/Aluminum Alloys

The formation of brittle intermetallic compounds (IMCs) at the interface between dissimilar materials causes considerable problems. In this study, a multi-material additive manufacturing technique that employs a diode laser and the hot-wire method was developed for stainless steel/aluminum alloys. An Al-Mg aluminum alloy filler wire (JIS 5183-WY) was fed on an austenitic stainless-steel plate (JIS SUS304) while varying the laser power and process speed and using paste-type flux and flux-cored wire. The effects of laser power and process speed on phenomena during manufacturing and IMC formation were investigated. Finally, the wall-type multilayer specimens were fabricated under optimized conditions. The suppression of IMC formation to a thickness of less than 2 μm was achieved in the specimens, along with a high interfacial strength of over 120 MPa on average.

Fabrication of silicon nitride based high-Q microring resonators prepared by the hot-wire CVD method and their applications to frequency comb generation

Fabrication of silicon nitride (SiN) based high-Q microring resonators prepared by the hot-wire chemical vapor deposition (HWCVD) method is presented. By the virtue of low-stress HWCVD films, no special precautions against crack propagation were required for high confinement waveguide device fabrication. By using an additional annealing process, the intrinsic Q factor in excess of 5 × 105 was obtained in the telecommunication C band, and which allowed us to observe frequency comb generation. We also investigated into the anneal temperature dependence of the residual hydrogen concentration in the film as well as the optical properties of the microring resonators.

An apparatus to measure thermal conductivity of ceramic pebble beds under uniaxial compressive stress

Pebble beds have various applications in many industrial and research sectors. Lithium ceramic pebble beds have specific application in fusion reactors as one of the most promising candidates for tritium breeding material. During the operation of the fusion reactor, these pebble beds will have to sustain various thermal and mechanical stresses. Therefore, it is necessary to do thermal characterisation of these pebble beds under different stress conditions and generate sufficient data on their thermal properties, which is essential for designing breeder units of fusion reactors. In this work, an apparatus has been designed to evaluate the thermal conductivity of ceramic pebble beds under compressive stress. It can be mounted to any standard universal testing machine with the necessary adjustments. The apparatus is based on the transient hot-wire method, widely used to evaluate the thermal conductivity of porous materials. A platinum wire with a diameter of 0.5 mm was used as a hot-wire. Instead of utilising a thermocouple, the 4-wire approach was used to obtain the temperature. The transient temperature response of the hot-wire was obtained by applying a 7A constant current to it and monitoring the voltage drop along its 40 mm length. The thermal conductivity of the water gel was measured to assess the accuracy of the apparatus; the results showed a less than 1.5 % variation from the reference value. In this study, the thermal conductivity of Li2TiO3 pebbles was measured in an air and helium environment in different stress conditions. In an air environment, measurements were done at atmospheric pressure, whereas in a helium gas environment, they were performed at gauge pressures of 0.05 bar, 0.5 bar, and 1 bar. The measurements were conducted in two scenarios: (i) constant stress condition at 0, 3, and 6 MPa, and (ii) Cyclic loading-unloading of 0.037–3 MPa and 0.037–6 MPa. It was found that the pebble bed's thermal conductivity rose in tandem with increased compressive stress in both the helium and air environments. Pebble bed’s thermal conductivity was found to be higher in a helium gas environment than in air. The thermal conductivity of the pebble bed increased slightly with rising helium gas gauge pressure from 0.05 bar to 1 bar. This work provides a design to incorporate 4-wire temperature measurement in the hot-wire method for thermal conductivity measurement of compressed pebble beds. It also contributes to the thermal conductivity data of Li2TiO3 pebbles in cyclic stress conditions in helium and air environments.

Enhanced thermal conductivity of plasma generated ZnO–MgO based hybrid nanofluids: An experimental study

Hybrid nanofluids (HNFs) of metallic oxide-based nanoparticles (NPs) have been prepared in different basefluids (BFs) employing the thermal plasma technique. NPs of ZnO–MgO were directly dispersed into pristine coolant, engine oil, distilled water (DW), and coconut oil. Plasma was generated between two identical electrodes applying 8.0 kV at the ambient conditions and proved economically viable in preparing stable HNFs. X-ray Diffractometry (XRD) showed ZnO and MgO NPs possessed hexagonal and cubic crystal structures, respectively. The band gap is calculated through UV-visible spectroscopy. The thermal conductivity (TC) of the HNFs has been measured using a thermal conductivity analyzer based on the transient hot wire method. The band gaps of pristine coolant and its HNFs were obtained to be 3.35 eV and 3.33 eV, respectively. In engine oil and its HNFs, band gaps of 3.16 eV and 3.02 eV have been extracted. There appears to be a slight reduction in band gap for coolant and engine oil-based HNFs. The band gap value of coconut oil-based HNFs was 4.05 eV, which showed a higher value than the pristine coconut oil-based HNFs (3.95 eV). The band gap calculated in the case of DW-based HNFs was 3.79 eV. TC of HNFs with volume concentration of 0.019 % for DW, 0.020 % for coolant, 0.016 % for engine oil, and 0.017 % for coconut oil were tested between 20 and 60 °C. An increase in TC was observed with the rise in temperature of the HNFs. Maximum increment in TC was observed at 60 °C for coolant-based HNFs, which was 19 %, followed by DW (18%), coconut oil (18%), and engine oil (16%), respectively. DW-based HNFs can be used as a coolant and optical filter for optoelectronics devices like photovoltaic cells for better performance. The study underscores precise control of NPs size as pivotal for band gap influence. HNFs hold promise as the next-gen heat transfer fluids (HTFs), revolutionizing thermal conductivity across industries. This research lays a firm foundation for plasma-synthesized HNFs' application in enhanced heat transfer and optoelectronic devices. Coolant-based HNFs excel in thermal conductivity, addressing heat transfer challenges.

Breaking graphite through a ball-milling process: the thermal conductivity and mechanical properties of polyethylene composites.

Exfoliated graphite platelets (EGPs) have attracted extensive attention owing to their exceptional combinations of thermal conductivity and mechanical properties. Mechanical exfoliation is a facile and high-throughput approach to produce single-layer or few-layer graphite platelets. Herein, octadecylamine (ODA)-grafted EGP (ODA@EGP) and subsequent polyethylene/ODA@EGP (PE/ODA@EGP) composites with different contents of ODA@EGPs were successfully prepared via ball-milling and melt-mixing methods, respectively. The thermal conductivity, crystallinity, and mechanical properties of the composites were investigated using tensile tests, the hot-wire method, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and thermogravimetric analysis (TGA). The results demonstrated that the thermal conductivity, mechanical properties, and thermal stability of the composites can be improved by regulating the additive contents of ODA@EGPs. When the content of ODA@EGPs was 10 wt%, the thermal conductivity of the composite reached up to 1.276 W (m-1 K-1), which is 216% higher than that of bare PE, while the tensile strength of the composite was 38.4% higher than that of PE. Additionally, thermal decomposition temperature increased by 16.2 °C. Therefore, the PE/ODA@EGP nanocomposites have great application potential in thermal management.

An overview of the Transient Hot Wire and preliminary studies for its implementation

The opportunity to design a new portable system for measuring thermal conductivity in fluids arose from the need to measure the phase state (liquid and/or vapour). As is well known, thermal conductivity varies in relation to the phase state of the fluid. The Transient Hot Wire Method was used to measure thermal conductivity. First, the work includes innovations in the analytical model of heat transfer, progressing over time to contemporary numerical models used in standards. The applications in the bibliography span a wide range, including gases, liquids, and the latest advances in the field of nanofluids. Secondly, the study delves into the transient model of heat exchange, focusing on conduction. The authors used standardized correction equations for the prototype, ensuring accuracy and relevance. Finally, the work includes the design and construction of the transducer. This process begins with a Monte Carlo simulation, predicting the expected results. This simulation helps to define measurement system components based on their performance characteristics and in-depth analysis.

Thermal conductivity measurements for n-nonane and n-undecane at elevated temperature and pressure

Hydrocarbons have wide applications in various industrial processes, and the knowledge of thermophysical properties is the foundation for their utilization. As an important thermophysical property, the thermal conductivity of fluids is vital in the evaluation of energy conversion quality, like heat transfer enhancement, thermodynamic cycle efficiency evaluation, etc. Therefore, obtaining accurate experimental data is of great importance for the utilization of hydrocarbons. Here, the thermal conductivities of liquid n-nonane and n-undecane were investigated using the transient hot-wire method (combined expanded uncertainty is 2.0 %) with a wide temperature and pressure range from 293.15 to 523.15 K and 0.1 to 15.0 MPa. For the convenience of engineering applications, a polynomial as a function of temperature and pressure was proposed to correlate the experimental data, the average absolute deviations between experimental data and fitted data are 0.29 % for n-nonane and 0.35 % for n-undecane, which indicates that the polynomial describes the experimental data well in the measured region. Meanwhile, the available literary data were collected and compared with our results, and good agreement was found between literary data and our results. This work is not only helpful for the industrial applications of hydrocarbons but also helps to understand the internal mechanism of the thermal conductivity of hydrocarbons.

A new transient hot-wire thermal conductivity apparatus and measurements of helium at temperatures from 20 K to 300 K

In this study, an apparatus based on the transient hot-wire method (THW) was designed to measure the thermal conductivity of cryogenic fluid within the temperature range of 20–300 K and the pressure range of 0–20 MPa, with a relative expanded uncertainty Ur(λ) of 0.0284 (k = 2, 95 %). The experimental data of methane, ethane, and carbon dioxide showed good consistency with the ECS model, validating the reliability of the apparatus. In addition to the linear temperature dependence of hot-wire resistance above 70 K, the nonlinear temperature dependence below 70 K is calibrated, and the feasibility of the THW method for measurements in the temperature range of 20–70 K is validated. Subsequently, the thermal conductivity data for helium are presented from 20 to 300 K. The temperature rises of hot-wire during the measurement agreed well with the simulation calculation results. Furthermore, a thermal conductivity model for helium gas up to 600 K was developed. The presented data in this work, as well as previously published data, demonstrated good agreement with the model calculation results, with an average absolute relative deviation (AARD) of 1.46 %.

Thermoelectric Properties of Annealed Bilayer SnSe/PbTe and SnSe/PbSe Thin Films by Thermal Evaporation Method

Tin Selenide, Lead Selenide, and Lead Telluride are known best thermoelectric materials for mid and high-temperature electric generation applications. The bilayer of these materials could enhance the quality of a thermoelectric generation. The present work deals with bilayer deposition of SnSe/PbTe and SnSe/PbSe in glass substrates using physical vapor deposition followed by annealing at 323K, 423K, and 523K. The structure and morphology of the films have been investigated by XRD, SEM, and FESEM studies. The thermoelectric pursuance of both bilayer thin films was studied with the temperature as a function in the range of 300K to 623K. Both films exhibit the maximum Seebeck coefficient. The electrical Conductivity and Power factor increased gradually for SnSe/PbTe thin films and SnSe/PbSe thin films for the samples annealed up to 573K and then decreases. The electronic thermal conductivity of both films was very low compared to the total thermal conductivity. The absolute thermal conductivity at room temperature was calculated by Transient Hot Wire (THW) method. The maximum Figure of Merit (ZT) value obtained for SnSe/PbTe and SnSe/PbSe at room temperature was 0.81 and 1.3 for 573K annealed thin films respectively.

Empirical models of thermal conductivity of cis-1,3,3,3-tetrafluoropropene (R1234ze(Z)) with measurements using transient hot-wire method

This article represents the empirical modeling for thermal conductivity of cis-1,3,3,3-tetrafluoropropene (R1234ze(Z)) with new experimental measurements conducted in liquid and vapor phases using the well-established transient hot-wire method. These new experimental data, covering a temperature range of 313 to 414 K for liquid and 354 to 452 K for vapor phase at a pressure range of 0.20 to 4.0 MPa, have an expanded uncertainty of less than 2.17% for liquid and 2.41 % for vapor measurements at a 95 % confidence level (k = 2). The thermal conductivity model of this fluid is presented using the extended corresponding states (ECS) and modified fluid specific residual entropy scaling (RES) techniques with the help of recently published respective viscosity models. Using the adjustable parameters found during the modeling process, the extended corresponding states and the residual entropy scaling technique model equations can represent the experimental data within reported uncertainties. Furthermore, the average absolute deviations for thermal conductivity were found to be 0.96 % and 0.72 % using the ECS and the RES methods, respectively.

MECHANICAL AND THERMAL CHARACTERIZATION OF CLAY WITH A VIEW TO POPULARIZING ITS USE AS AN ECOLOGICAL MATERIAL IN CONSTRUCTION

This work is carried out with the aim of enhancing the soils of Djarmaye in order to popularize its use as an ecological material in construction. However, a study of thermo-physical parameters was made to see the influence of cow dung in the mixture. As a result, thermal characterization tests were carried out, using the hot wire method for the determination of thermal conductivity, supported by the hot plane option for the determination of thermal conductivity.supported with the hot plane option for the determination of thermal effusivity was used. The test tubes are made with an incorporation of 0%, 2% and 4% by mass of cow dung. The parameters studied are the thermal conductivity, the thermal effusivity, and the thermal capacity by volume of the specimens. The results showed that the thermal conductivity decreases with cow dung, which confirms its use as an additive to obtain a good thermal insulator.

Study on thermal conductivity of improved soil under different freezing temperatures.

Based on the influence of moisture content, dry density and temperature (≦ 0°C) on the thermal conductivity of lime-modified red clay, the thermal conductivity was measured by transient hot wire method. A total of 125 data were obtained and the evolution law of thermal conductivity with influencing factors was analyzed. The fitting formula of thermal conductivity of lime-modified red clay and a variety of intelligent prediction models were established and compared with previous empirical formulas. The results show that the thermal conductivity of lime-modified red clay increases linearly with water content and dry density. The change of thermal conductivity with temperature is divided into three stages. In the first stage, the thermal conductivity increases slowly with the decrease of temperature in the temperature range of-2°Cto 0°C. In the second stage, in the temperature range of-5°Cto (-2)°C, the thermal conductivity increases rapidly with the decrease of temperature. In the third stage, in the range of-10°Cto (-5)°C, the thermal conductivity changes little with the decrease of temperature, and the fitting curve tends to be stable. The fitting formula model and various intelligent prediction models can realize the accurate prediction of the thermal conductivity of lime-improved soil. Using RMSE (Root Mean Square Error) and MAPE (Mean Absolute Percentage Error) to evaluate the model, it is found that the GBDT decision tree model has the best prediction effect, the RMSE value of the predicted value is 0.084, and the MAPE value is 4.1%. The previous empirical models have poor prediction effect on the thermal conductivity of improved red clay. The intelligent prediction models such as GBDT decision tree with strong universality and high prediction accuracy are recommended to predict the thermal conductivity of soil.