Thermal Conductivity Of Porous Materials Research Articles

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.

Air-Permeable Building Envelopes for Building Ventilation and Heat Recovery: Research Progress and Future Perspectives

Air-permeable building envelopes (APBEs) utilize the infiltrated or exfiltrated airflow within porous materials to directly change their temperature distribution to reduce heat loss/gain. APBEs effectively integrate building ventilation and heat recovery to achieve excellent thermal insulation while improving indoor air quality. This paper presents a comprehensive review of the fundamentals and classifications, historical evolution over time, opportunities and benefits, and future views on APBEs. It can be treated as a responsive building envelope that enables building envelopes to dynamically change the U-values by varying the infiltrated or exfiltrated airflow rate within a porous material. Previous studies have indicated that the U-value of 0.1 W/(m2·K) can be realized by employing APBEs. Moreover, some research demonstrates that APBEs could act as high-performance air filters that reduce over 90% of particulate matter within fresh, ventilated air. Some factors, such as airflow rate, thickness, and thermal conductivity of porous materials, have a significant influence on the effectiveness of APBEs. For practical applications, integrating the APBE with passive building ventilation can help reduce the initial cost and facilitate decarbonization in buildings. Moreover, advanced control strategies could collaboratively optimize the operation of ABPEs and build energy systems to maximize their energy-saving potential.

An interpretable deep learning strategy for effective thermal conductivity prediction of porous materials

The effective thermal conductivity of porous materials plays a pivotal role in advancing novel thermal insulation materials. Existing theoretical prediction methods are computationally demanding with restricted applicability, and deep learning methods lack interpretability. More importantly, current approaches fail to explore the material structures themselves and leverage those insights to improve the accuracy of thermal conductivity prediction. Herein, we for the first time propose an interpretable deep learning approach with feedback for thermal conductivity prediction. A 3D convolutional neural network is trained on material structures-thermal conductivity data, and heat maps are generated from weighted feature maps to elucidate the prediction processes. Through image processing, incongruous regions are eliminated, resulting in porous structures with improved reasonability. Consequently, more accurate effective thermal conductivities can be obtained. This work may offer a novel predictive approach that refines both the material structure and the predictive model, thereby achieving more precise thermal conductivity predictions for porous materials.

Mullite fibers/whiskers hierarchical porous materials: Experimental characterization and fractal model to predict thermal conductivity

It is an effective method to predict thermal conductivity of porous materials by using fractal models. In this work, mullite fibers/whiskers hierarchical porous materials (MFWs) were prepared via in-situ growth method. The fractal dimension was obtained by calculating the microstructure photo after binarization according to the box-dimension method. Based on fractal theory and fractal dimension, improved Sierpinski carpet models were established. The models were then converted into electrical fractal patterns to solve the iterative formulas of effective thermal conductivity (Keff) and porosity. The results show that Keff decreases gradually with the increase of porosity. The thermal conductivity of the solid phase has a significant effect on Keff at low porosity. Pore size and its spatial distribution, especially the spatial distribution of the larger pore, have a substantial impact on Keff, and the smaller and uniformly distributed pore can reduce Keff, effectively. The calculated results are close to the experimental results, suggesting that the fractal model is suitable for predicting the thermal conductivity of MFWs. More crucially, the calculated results have guiding significance for the experimental preparation, and it is possible to reduce the thermal conductivity of MFWs by increasing whiskers length and growing efficiency.

Machine learning boosting the discovery of porous metamaterials with an abnormal thermal transport property

Normally, the introduction of porous structures into materials can tune their thermal conductivity, showing great applications in thermal management and thermoelectric energy harvesting. However, the ability of disorder changing the thermal conductivity of porous materials has seldom been explored. In this work, we show that an introduction of disorder into the macroscopic porous materials with a certain porosity can lead to a desired effective thermal conductivity over a large range, where an abnormal enhancement of ∼7.9% and a normal reduction of ∼44% at room temperature are predicted by the machine-learning-optimized algorithm. All of these theoretical calculation results are further verified by our experiments performed in the current work by using the steady-state thermal flux method. Moreover, when these periodic units are artificially connected, a structural anisotropy up to 40 is achieved, which can be further used to adjust the direction of the thermal flux in a well-controlled way. Our work provides an efficient and convenient approach for designing high-performance porous materials with specific thermal conductivity and high structural anisotropy for various applications in thermal management.

The Effects of Effective Thermal Conductivity of Porous Materials Under Vapor Flow in Sudden Enlargement-Contraction Channel on Local Heat Transfer

This study aims to obtain the local heat penetration and local heat convection characteristics in a system where a flow of heated vapour cedes thermal energy to a horizontal heat-sink plate. The heat penetrates a bed of porous materials with varia

A NOVEL ANALYTICAL MODEL OF THE EFFECTIVE THERMAL CONDUCTIVITY OF POROUS MATERIALS UNDER STRESS

With the increasing demand for energy, heat and mass transfer through porous media has been widely studied. To achieve accuracy in studying the behavior of heat transfer, a good knowledge of the effective thermal conductivity (ETC) of porous materials is needed. Because pore structure dominates the ETC of porous materials and effective stress leads to a change in pore structure, effective stress is one of the key influencing factors affecting ETC. In this study, considering the structure of surface roughness and pore size, based on fractal theory, a novel analytical solution at the pore scale for ETC of porous materials under stress conditions is proposed. Furthermore, in this model, capillaries in porous materials saturated with multiple phases have sinusoidal periodically constricted boundaries. The derived ETC model is validated against available experimental data. Moreover, the influences of the effective stress, initial effective porosity, roughness structure characterization, and wetting phase saturation on the ETC are analyzed. Compared with previous models, the rough surfaces of porous materials and the coupling of heat conduction and mechanics are taken into consideration to make the model more reasonable. As a result, this ETC model can better reveal the mechanism of heat conduction in porous media under stress conditions.

A Fractal Model of Effective Thermal Conductivity of Porous Materials Considering Tortuosity

Accurate estimation of the thermal conductivity of porous materials is crucial for the modeling of heat transfer and energy consumption calculation in energy, aerospace, biomedicine and chemical engineering, etc. The series-parallel model is a simple and direct method and is usually used in the prediction of the effective thermal conductivity (ETC) of porous materials. In this work, the weighted coefficients of the series and parallel section were obtained based on the tortuosity of the porous materials. Then, the physical model of the ETC of the porous materials was established. Furthermore, the ETC of the porous materials was developed using the fractal model to calculate the pore cross-sectional area of the porous materials. Finally, quantitative analysis of the characteristic parameters, e.g., porosity, tortuosity, tortuous fractal dimension and pore diameter distribution, of the ETC of the porous materials was conducted. The results show that the proposed model can provide an accurate prediction of the ETC of porous materials.

Tailoring thermal resistance of porous materials with void filling for improved hydrogen adsorption

Porous materials such as 5A molecular sieve (5A) display huge thermal resistance due to high porosity and lots of voids between grains that is negative to hydrogen isotope separation engineering. Generally, introducing thermal conductive fillers contributes to reducing thermal resistance while results in decreasing volume ratio of porous materials and then certainly causes declined hydrogen adsorption capacity. Here, a liquid polydimethylsiloxane (PDMS) is pressed into the voids between 5A grains, which is further developed into a silicon, oxygen and carbon (SiOC) structure suffering from 350 °C sintering. 5A/SiOC composite at 10 wt% polydimethylsiloxane (5A/SiOC10) displays 0.74 W/mK of thermal conductivity, which is about 300% higher than that of neat 5A. More importantly, enhanced rather than reduced hydrogen adsorption capacities at a fixed volume of the composite are determined. 5A/SiOC10 shows adsorption capacities of H2 (175.4 mL/cm3) and D2 (188.4 mL/cm3) while neat 5A shows that of H2 (171.6 mL/cm3) and D2 (165.8 mmol/cm3) at 77 K with 1 bar. Besides, enhanced thermal conductivity of porous materials shortens the cycle time of hydrogen isotope separation that contributes to reducing energy consumption. This work proposes a novel strategy on void filling to tailor thermal resistance of porous materials, which open a window to improve hydrogen isotope separation with thermal management materials.

Effective Thermal Conductivity of Structured Porous Medium: Numerical Study

The paper presents a numerical study of thermal conductivity of porous structures using the Ansys software package. Unlike the well-known porous materials used in construction and engineering, it is proposed to use porous materials with an ordered law of cavity placement. The porous material proposed is formed by dividing the volume into cubes of equal size with a spherical cavity placed in the center of each cube. The numerical calculation of an effective thermal conductivity coefficient of a porous medium is performed using the Ansys Mechanical computer modeling tool. The values obtained are compared with solutions based on classical methods for determining the effective thermal conductivity of porous materials. A dependency graph of effective thermal conductivity in a porous material based on pores geometric parameters (distance between cavities, diameter of cavities), as well as an analytical dependence to obtain the effective thermal conductivity value is presented. Additive technologies available today provide producing the proposed porous material with an ordered law of cavity placement with any accuracy and any pore geometric parameters. Such materials open up wide opportunities for engineers, especially in the field of thermal power engineering, because it has predictable thermophysical and mechanical properties.

Thermal conductivity analysis of ceramic membranes for recovering water from flue gas

Condensation is the common mechanism of water intake from gases. Transport membrane condenser is a newly proposed technology to recover water from flue gas based on the mechanism of condensation. Most of the existing heat transfer research on transport membrane condenser focuses on water vapor phase change or fluid convection heat transfer, and there is a lack of research on the thermal conductivity of ceramic membranes. The significance of studying thermal conductivity is to optimize the overall heat transfer performance. The thermal conductivity of alumina ceramic solid materials is not good enough, which can affect the industrial application performance to a certain extent. There is currently a lack of experimental measurements or theoretical calculations to quantitatively characterize the thermal conductivity of ceramic membranes. This affects the optimization of overall heat transfer performance of transport membrane condenser. In this paper, coal fly ash is used as the raw materials to prepare macroporous ceramic membranes. Coal fly ash is a very cheap material. It can greatly reduce the manufacturing cost of transport membrane condenser using coal fly ash to prepare ceramic membranes. The classical theoretical model of thermal conductivity of porous materials is introduced, the calculation and experimental results are compared, and the validity and limitation of the classical models in analyzing the heat transfer problem of transport membrane condenser are discussed. Finally, based on the composite effective medium theory, combining with the experimental results, a modified model is proposed. The modified model reduces the average theoretical error to 7.20%, and enriches the research on the heat transfer theory of transport membrane condenser.

Thermal conductivity analysis of porous NiAl materials manufactured by spark plasma sintering: Experimental studies and modelling

This work presents a comprehensive analysis of heat transfer and thermal conductivity of porous materials manufactured by spark plasma sintering. Intermetallic nickel aluminide (NiAl) has been selected as the representative material. Due to the complexity of the studied material, the following investigation consists of experimental, theoretical and numerical sections. The samples were manufactured in different combinations of process parameters, namely sintering temperature, time and external pressure, and next tested using the laser flash method to determine the effective thermal conductivity. Microstructural characterisation was extensively examined by use of scanning electron microscopy and micro-computed tomography (micro-CT) with a special focus on the structure of cohesive bonds (necks) formed during the sintering process. The experimental results of thermal conductivity were compared with theoretical and numerical ones. Here, a finite element framework based on micro-CT imaging was employed to analyse the macroscopic (effective thermal conductivity, geometrical and thermal tortuosity) and microscopic parameters (magnitude and deviation angle of heat fluxes, local tortuosity). The comparison of different approaches toward effective thermal conductivity evaluation revealed the necessity of consideration of additional thermal resistance related to sintered necks. As micro-CT analysis cannot determine the particle contact boundaries, a special algorithm was implemented to identify the corresponding spots in the volume of finite element samples; these are treated as the resistance phase, marked by lower thermal conductivity. Multiple simulations with varying content of the resistance phase and different values of thermal conductivity of the resistance phase have been performed, to achieve consistency with experimental data. Finally, the Landauer relation has been modified to take into account the thermal resistance of necks and their thermal conductivity, depending on sample densification. Modified theoretical and finite element models have provided updated results covering a wide range of effective thermal conductivities; thus, it was possible to reconstruct experimental results with satisfactory accuracy.

Characteristics of effective thermal conductivity of porous materials considering thermal radiation: A pore-level analysis

Porous materials have attracted considerable attention in the fields of heat transfer and solar energy harvesting. Due to the complicated pore structures and multiple reflections of radiation beams at interior interfaces, the analysis of coupled radiative and conductive heat transfer in porous materials is complex and challenging. In this paper, heat transfer characteristics in porous materials considering the coupled modes of conduction and radiation are studied at pore level. A discrete ordinate ray-tracing (DORT) method is proposed to solve the radiative heat transfer in porous materials, which is then combined with the finite volume method to solve the energy balance equation. The geometry of porous structure is represented by the implicit function, which ensures the calculation accuracy of interface normal vectors during the solution of radiative heat transfer. A dimensional analysis is conducted and a governing dimensionless parameter (NSRC) is introduced, which well characterizes the scaling characteristics of the effective thermal conductivity (ETC). The proposed approach is demonstrated to be an effective way to simulate the coupled radiative-conductive heat transfer in porous materials at pore level. Due to the thermal radiation, the ETC for open-cell porous materials increases more dramatically with NSRC than that for closed-cell porous materials. A simple thermal network model is developed for both open- and closed-cell porous materials, which predicts the trends of the ETC with NSRC very well.

NOx emission reduction in ammonia-powered micro-combustors by partially inserting porous medium under fuel-rich condition

Based on the fact that NOx generation will be inhibited under the condition of high equivalence ratio, the micro-combustors with double-rib structure partially inserted with porous medium (PM) are selected to examine the NOx emission performances in fuel-rich combustion of ammonia. The effects of: 1) the inlet velocity, 2) the porous material and 3) the porosity are evaluated on NOx emission reduction. Besides, the kinetic rate of key reactions contributing NOx production are analyzed. It is found that, the PM can effectively reduce NO generation and emission under a larger inlet velocity. With the increased thermal conductivity of porous material, the emission of NO is shown to be decreased by 99.98% from 2.70 × 104 ppm to 5.17 ppm, while N2O emission is increased by 10000% from 24.8 ppm to 2.52 × 103 ppm. In addition, the porous material with a higher thermal conductivity is beneficial to improving the uniformity of the combustor outer wall temperature. When the PM’s porosity is increased from 0.5 to 0.9, NO emission is increased by 4120% from 3.53 × 102 ppm to 1.49 × 104 ppm, while N2O emission is decreased by 71.5% from 3.29 × 103 ppm to 9.38 × 102 ppm. The present work reveals that the NOx emission can be effectively reduced, while increasing the thermal performance of micro-combustors by adopting appropriate porous medium.

Thermo-Mass Coupling Fractal Study of Wet Phase-Change Rough Porous Materials

The complex internal structures and moisture states of porous materials are of great significance to heat and mass transfer, and their coupling heat and mass transfer processes widely exist in energy development and engineering heat insulation. Beyond the unilateral analysis of heat and mass transfer characteristics of porous materials under ideal conditions, the distribution parameters of porous channels, rough surface, wet states and phase-change were considered, and the fractal theory was used to deduce the expressions of the seepage coefficient and the coupling equivalent thermal conductivity of porous materials with wet phase-change rough surface. The results show that, the seepage coefficient is positively correlated with the area fractal dimension and the moisture saturation, and negatively correlated with the relative roughness and the tortuous fractal dimension. The coupling equivalent thermal conductivity is positively correlated with the seepage coefficient and the phase variable, but negatively correlated with the relative roughness. In addition, the phase variable and the gas expansion pressure difference caused by phase-change also have important effects on the coupling heat and mass transfer.

Каркасна теплопровідность пористих металевих матеріалів

The influence of a number of physical characteristics and parameters of metallic fiber materials on their thermal conductivity is studied in this work. Such porous materials are intended, among other things, for their effective use in two-phase heat transfer devices (heat pipes). The use of heat pipes in aircraft and space vehicles provides a number of thermophysical advantages. In particular, heat pipes significantly expand the possibilities of air cooling of heat-loaded technical devices. The thermal conductivity of capillary-porous materials-structures, which are important elements of heat pipes, significantly affects the intensity of two-phase heat transfer inside heat pipes. Frame thermal conductivity is equivalent to the thermal conductivity of materials that are conditionally continuous medium. Studies of the influence of structural characteristics of porous materials, such as porosity and parameters (dimensions) of discrete particles-fibers (fractions of the studied materials), were performed using special experimental equipment created at the I.M. Frantsevich Institute for Problems of Materials Science of the National Academy of Sciences of Ukraine (Kyiv). Porous metal structures (coatings) made of copper, nickel, and steel fibers (MPM) were investigated under conditions similar to those for space heat pipes. The porosity values ​​of the prototypes of materials were in the range of 40 to 93%. The research results showed that the following physical characteristics of capillary structures, such as values ​​of thermal conductivity of metallic materials (fiber fractions), the porosity of capillary-porous metal materials (structures), significantly affect the value of thermal conductivity of porous materials. The dimensions of discrete particles-fibers also affect in a certain way the value of the MBM thermal conductivity but to a lesser degree. The results obtained in this work are summarized in the form of empirical dependencies – formulas, providing engineering calculations of the thermal conductivity values ​​of metal fiber materials. The research results are intended for practical application in aviation and spacecraft apparatus engineering. In particular, the presented results are necessary for the development and creation of effective heat pipes with metal fiber capillary structures.

Effects of pore structure characteristics on performance of sintered bi-porous Ti3AlC2 wicks

Loop heat pipe, carried out by liquid-gas phase transition, has been attracted as an efficient heat management device in high-heat-flux, long-distance and anti-gravity situations. The capillary pressure provided by the porous wick in the evaporator drives circulation for working liquid, affecting the heat exchange efficiency for loop heat pipes directly. In this work, the bi-porous Ti3AlC2 wick was produced by reaction sintering and pore formers dissolution and porosity and pore size were regulated by varying pore formers content and cold pressure. The pore size distribution was characterized by mercury intrusion and effects on capillary performance and thermal conductivity were also analyzed. The porosity and proportion of large pores increased with the increased pore formers content, causing the enhancement of capillary performance and the reduction of thermal conductivity. As the cold pressure increased, pore size distribution concentrated, the proportion of fine pores increased and the porosity fluctuated slightly, increasing the capillary pressure and improving capillary performance and thermal conductivity. Based on experimental results, the parameter of pore size proportion was proposed to modify the Alexander model to predict the thermal conductivity of porous materials accurately, guide the design for pore structure and promote the transfer capacity.

Thermophysical properties of porous materials

The study of the porosity of thermal insulation made of refractory materials is an important task for the power industry, since the thermal conductivity of porous materials depends on the shape and especially the location of the pores. An analytical review of existing technologies shows that research in this area is not enough to simulate the process of heat and mass transfer in porous alumina material. Experimental determination of the characteristics of heat and mass transfer in porous materials during the formation of a porous structure is a pressing scientific problem. This article analyzes the influence of the composition of materials on the formation of pores, as well as the effect of various impurities and temperature on the thermal conductivity of the material.

Measurement and comparison of thermal conductivity of porous materials using box, dual-needle, and single-needle probe methods-A case study

Thermal conductivity controls heat transfer through materials, so accurate assessment of thermal conductivity is necessary when modeling the thermal behavior of a material. In this study, the thermal conductivities of a silty clay specimen, a sand specimen, and six types of mortar specimens with various mixing ratios of sand as an additive, were measured using box, dual-needle, and single-needle probes, with and without thermal grease, under saturated and oven-dried conditions. The results obtained with the needle probes were compared to those measured using the box probe, given the high accuracy of the box probe method. After applying thermal grease to single- and dual-needle probes, the thermal contact resistance decreased, resulting in increased thermal-conductivity values for soil and mortar specimens under saturated (wet) and oven-dried conditions. The thermal-conductivity values measured via the single-needle probe were as accurate as those measured by the box probe, whereas those measured using the dual-needle probe underestimated the material's thermal conductivity. Further research is required regarding thermal-conductivity measurements and the design of the tests and equipment used. The results of this study provide useful information for future studies of thermal-property measurements and probe development.

Effects of Pore Complex Shape, Distribution and Overlap on the Thermal Conductivity of Porous Insulation Materials

The effective thermal conductivity of porous materials is determined by many factors. Previous works were mainly focused on the pore content and the thermal conductivity of two components. However, few researches pay attention to the effects of pore complex shape, overlap and distribution. In this study, the effects of pore complex shape, distribution and overlap are investigated at the same time. It is found that the best pore should have relatively larger contact areas in the direction vertical to heat flux. I-shaped and T-shaped pores have the larger contact area in any type of the pore distributions. They have the most impact on the thermal conductivity of the porous material. I-shape pore with directional distribution is better than random distribution. The rhombic, elliptical and rectangular pores have larger thermal insulation in random distribution than in directional distribution. The efficiency of square, triangular and T-shaped pores are similar in directional and random distributions. The pore overlap only has the apparent effect on the I-shaped pore. New methods to control the distribution and overlapped direction of pores should be investigated in the future.