Problem Of Thermal Conductivity Research Articles

On the finite Radon transform for the computational homogenization of conductivity

AbstractThis paper investigates the computational homogenization of thermal conductivity problems using a finite Radon transform as proposed by Derraz and coworkers, implemented using the Fourier slice theorem to allow utilization of the fast Fourier transform‐based frameworks and methods. For the finite Radon transform both the original approach and the consistent approach proposed by Jabs and Schneider are used. The two discretizations are compared to the Moulinec–Suquet discretization using numerical examples. In doing so the convergence of the consistent approach is shown and the discussion of the original Radon approach is extended.

Numerical study of the influence of temperature data error on the determination of frictional heat generation

Subject of research: determination of the friction torque and functions of the specific intensity of heat generation in system of sliding bearings. The purpose of the study: To develop a method for determining the friction torque in sliding bearings using temperature data. Based on computational experiments, study the influence of errors in temperature data on the solution of the inverse problem. Methods and object of research: A system of sliding bearings made of a polymer composite material on which a rotating shaft rests is considered. A mathematical model of the thermal process in the considered system of friction units is presented, taking into account the spatial distribution of temperature and its change over time. The temperature is measured at several points in each bearing. Frictional heat generation is determined by solving the inverse heat conduction problem from the condition that the measured and calculated temperatures are close. To ensure continuous data processing and determine the friction torque during long-term tests, the inverse problem of determining the friction torque from temperature data is solved at successive short time intervals. Then the resulting solutions are «glued together». The main results of the study: With such a restoration, the discrepancy between the specified and restored functions of the specific intensity of heat generation is 10–15 % with an error level in temperature data of 10 %. The developed algorithm for solving the inverse problem of thermal conductivity can be used to determine the friction torques in real systems of self-lubricating sliding bearings.

Analysis of the thermomechanical behavior of a three-layer rectangular plate of the ceramic-metal-ceramic structure due to non-stationary convective heating

A rectangular isotropic layered plate is considered, which is convectively heated by the external environment. The thermal stress state of the plate is determined in two stages. At the first stage, the nonstationary temperature field is determined from the ratios of the non-stationary thermal conductivity problem. At the second stage, using the five-modal mathematical model of the shear theory of thermoelasticity, the parameters of the stress-strain state are determined.

Study of nonequilibrium regenerative heat exchange in positive displacement compressors

BACKGROUND: An important task in the design of thermal engines is the evaluation of power losses in different processes to determine the installation efficiency. One of the processes that generates power losses in the compressor is the process of nonequilibrium regenerative heat exchange (NRHE) of the compressed gas with the working cavity walls. This heat exchange occurs at a significant temperature difference, which can lead to noticeable power losses. Modern compressor design methods do not consider losses from nonequilibrium regenerative heat exchange and do not describe them separately from other types of losses. AIM: This study investigates the mechanism of losses during regenerative heat exchange between the gaseous working flow out and the working cavity walls and evaluates the scale of these losses. METHODS AND RESULTS: This work provides a qualitative description of the mechanism for the formation of losses from NRHE. As a result of solving the problem of unsteady thermal conductivity, an analytical expression for calculating such losses is derived based on the Gouy-Stodola theorem. CONCLUSIONS: The study revealed that power losses from NRHE account for a significant share in the overall balance of machine losses. Parameters influencing the magnitude of these losses were also determined, and recommendations were developed to reduce them.

Numerical search for the effective thermal conductivity of cracked media

Spacecraft parts accumulate damage during operation and defects that are invariably present even in new designs may grow. This leads to changes in the behavior of individual parts of the space vehicle and, consequently, to the risk of fracture. A more accurate assessment of spacecraft safety requires internal defects to be included in the material models under consideration. One of the main hazardous effects on space objects is multiple temperature heating and cooling due to periodic action of solar rays. This paper presents a study of thermal conduction of media containing cracks. It is carried out with the help of a technique developed by the authors to determine the effective thermal conductivity of materials and based on approximate numerical solution of the steady-state thermal conduction problem for a three-dimensional medium with cracks by the boundary element method. This technique allows to obtain the distribution of the temperature field and heat flux density at any point of the body under consideration, as well as to calculate the effective parameters of materials with high accuracy at relatively low calculation time using ordinary personal computers of average power. The basis of the numerical method presented in this paper is the decomposition of the desired solution into a series of some pre-calculated analytical solutions of the heat conduction equations. The dependence of the effective thermal conductivity on the density of thermally insulated cracks was considered. The formula of this dependence is proposed. Verification of the proposed methodology was carried out by comparing the numerical results of a number of problems with the results of other authors.

Аналіз розподілу температури у двошаровій композитній циліндричній оболонці зумовленої конвективним теплообміном на її зовнішній поверхні

A non-stationary thermal conductivity problem for a two-layer cylindrical shell is formulated. The materials of the constituent layers of the shell are assumed to be homogeneous and isotropic. This shell is convectively heated by the external environment. The system of linear two-dimensional equations for the integral temperature characteristics summed over the component layers of the shell was used as the initial system of equations of the problem under consideration. When obtaining this system, the hypothesis of a linear distribution of temperature along the thickness of the entire shell was applied. On the basis of the developed two-dimensional mathematical model of thermal conductivity for layered cylindrical shells, a general solution to the thermal conductivity problem for a two-layered cylindrical shell was obtained. A temperature distribution was found for such a shell, which is locally convectively heated by the external environment in a rectangular region on the outer surface. For the numerical analysis, a metal-ceramic cylindrical shell was considered, the inner layer of which is made of tungsten, and the outer layer is made of ceramics. Under such a structure and conditions of heat exchange with the external environment for the considered shell, the effect of its geometric parameters and thermophysical characteristics of the materials of its component layers on the temperature field on the outer surface of the shell was investigated. The revealed new qualitative and quantitative regularities can be used to estimate temperature fields in composite elements of layered structures and in structures with one-sided coatings. The dependences of temperature distributions on geometric parameters and thermophysical characteristics obtained in this work are the basis of the theoretical basis for the analysis of temperature regimes of metal-ceramic cylindrical shells. In particular, metal-ceramic dental crowns to predict their temperature regimes are modeled by the two-layer metal-ceramic cylindrical shells described above, which are locally convectively heated by the external environment

Exergo and enviro-economic analysis of thermal energy storage assisted finned solar still

The current investigation focuses on improving the performance of a single slope solar still integrated with Phase Change Material as energy storage media. The problem of low thermal conductivity of PCM which hinders the energy storage capability has been addressed by incorporating fins with different profiles into the energy storage unit. Initially, an experimental investigation was conducted and analyzed to assess the effective performance of a Conventional Single Slope Solar Still (CSS) using various water depth levels: 1–4 cm. The optimal basin water depth for testing solar still with energy storage material along with fins was identified as 2 cm. Subsequently, the CSS was subjected to performance enhancement investigation under three different conditions: Case 1 – Solar Still (SS) with PCM storage unit, Case 2 – SS with fins at the PCM storage unit, and Case 3 – SS with fins at the storage unit and a parallel plate attached to the other end of the fins. The experimental results demonstrated the production of fresh water and efficiency improvements are as follows 26.95%, 55.86%, 69.14% and 24.34%, 52.83%, 68.83% for cases I, II, and III, respectively. Further, it was observed that Case III exhibited a lower cost of water production and a shorter payback period compared to CSS. The enviro-economic analysis revealed that the net CO2 emissions (NCEM) and Carbon Credits Earned (CCE) for Case III were 15.60 tons and 312.07 USD respectively against 9.22 tons and 184.31 USD for CSS.

Polystyrene shell-based “coconut-like” and “pomegranate-like” microencapsulated phase change materials: Formation mechanism, thermal conductivity/stability enhancement and their application in thermal management

Encapsulation of phase change materials (PCMs) using polystyrene (PS) can be an effective solution to the leakage of PCMs, but the low thermal conductivity/stability of the prepared microencapsulated phase change materials (MEPCMs) remain unresolved. Here, we investigated the formation mechanism of PS shell-based MEPCMs via Pickering emulsion polymerization: coconut-like and pomegranate-like MEPCMs were obtained for the first time before and after the introduction of divinylbenzene (DVB), and the thermal conductivity and stability of the former were lower than that of the latter with the same graphene nanosheets (GNSs). Among them, MEPCM-4–7 wt% GNSs (St:DVB=1:1) has the highest thermal conductivity of 0.9358 W∙m−1∙k−1 (222.47 % higher than MEPCM without GNSs) and optimal anti-leakage performance. The thermal stability (thermogravimetric results) is slightly weaker than MEPCM-3–7 wt% GNSs (St:DVB=5:1), but has the highest thermal reliability (the melting enthalpy only decreases by 0.19 % after 100 cycles). The phase change composites (PCCs) was prepared by dispersing MEPCMs in PDMS matrix and applied to the simulated chip thermal management, which can achieve a 10.85 % decrease in the chip equilibrium temperature compared with PDMS. The method of improving the heat transfer properties and thermal stability of MEPCMs based on their structure provides a new idea to solve the problem of low thermal conductivity and stability of MEPCMs, and the prepared PCCs show great potential for practical applications in the field of thermal management.

NiCo@EG-based composite PCMs with boosted thermal conductivity and photothermal conversion efficiency for solar energy harvesting

The application of phase change materials (PCMs) in solar energy is hampered by challenges such as insufficient photothermal conversion efficiency, low thermal conductivity and leakage. In this study, a novel composite PCMs based on NiCo@EG were prepared to solve the problems of low thermal conductivity, high leakage rate and inefficient photothermal performance. The NiCo@EG is obtained through calcination of the product of Ni-Co-BTC in-situ on expanded graphite (EG). In the composite PCMs, the octadecanol (OD) acts as a latent heat storage unit, the EG retains its porous network structure as an encapsulating skeleton. The nickel and cobalt nanoparticles derived from Ni-Co-BTC bimetallic organic frameworks are uniformly fixed on the surface of the EG, which could not only construct multiple thermal conduction pathways, but also enhance the photon-trapping ability. Benefiting from the synergy effect of nickel, cobalt metal nanoparticles and EG, the prepared OD/NiCo@EG-550 composite PCMs display high latent heat (168.3 J/g), excellent photothermal conversion efficiency (98.71 %), and improved thermal conductivity of 12.873 W/(m·K). Furthermore, the composite PCMs demonstrate exceptional shape stability, thermal stability, and robust thermal reliability. In summary, the high-efficiency photothermal composite PCMs display significant potential in improving the utilization of solar energy.

REFINING THE EMPIRICAL MODEL OF THE HEAT TRANSFER BOUNDARY CONDITIONS OF THE PISTON OF A HIGH-SPEED DIESEL ENGINE

The level of temperature of ICE pistons is a factor that greatly affects their reliability. Therefore, the need to simulate the temperature state of pistons with sufficient accuracy and minimal allocation of resources and time exists in research and during design. Primary approaches for determining the boundary conditions of the problem of thermal conduction of the piston on the basis of criterion equations, high-level mathematical models, and empirical models are considered. The latter ones have become widespread by virtue of simplicity and ease of use in practice; their usability is limited to a certain set of operating modes of one engine or a few ones of a similar design. On the basis of data from four series of experimental tests for a well-studied diesel engine 4ChN12/14 the existing empirical models of boundary conditions, suitable for identifying the temperature state of the piston only within the respective separate series separately, are refined in the paper. Processing of the data set on the measured temperatures in the control zones of the piston was performed, and linear function approximations were found that satisfactorily describe the effect of diesel power on the specified temperatures. The model of boundary conditions for a set of eighteen separate regions on the surfaces of the piston was proposed, which takes into account the dependence of the piston temperature state on the brake power of the ICE, the crank angle of the start of fuel injection, and the conditions of cooling the piston with oil. The influence of the heat insulation layer on the piston crown surface on the heat transfer was accounted for by introducing an additional thermal resistance term into the boundary condition equations. The model made obtaining the temperature field of the piston possible in simulation that is consistent with the results of all series of experiments in the control zones of the piston periphery, the edge of the combustion chamber, the vicinity of the groove of the first piston ring, the cooled surface opposite to the combustion chamber, and the piston skirt. The importance of a gradual change in the temperature of the oil and coolant during the transition of the internal combustion engine from one mode of operation to another regarding the temperature state of the piston is demonstrated. Taking the temperatures of oil from previous modes of operation into account is recommended during the analysis of engine transient processes.

The influence of stochastic interface defects on the effective thermal conductivity of fiber-reinforced composites

In this paper, a novel microscopic modeling strategy is proposed to investigate the effective thermal conductivity of composites with consideration of stochastic interface defects. To this end, the subdomain boundary element method combined with asymptotic homogenization is proposed to effectively solve the thermal conduction problem. In order to accurately capture the heat flux on the boundary and the internal region in the representative volume element (RVE), a parameterized sub-cell is constructed to discretize the RVE. On this basis, the influence of stochastic interface defects on the thermal conductivity of composites is investigated by utilizing the Monte Carlo method. Specifically, the effect of the location, length, thickness, and area of the interface defects on the thermal conductivity is investigated. A proportional decrease in the transverse thermal conductivity coefficient is found for interface defect areas ranging from 1% to 10%.

Optimization of electrostatic seeding technique for wafer-scale diamond fabrication on β-Ga2O3

Integrating diamonds with β-Ga2O3 is a promising solution for addressing the problem of poor thermal conductivity and limited p-type dopability of β-Ga2O3. However, growing diamonds on β-Ga2O3 is challenging, especially without any interfacial layer, and achieving large-scale uniform diamond films is also tricky. This paper explores the vital role of an electrostatic seeding technique involving the use of Polydiallyldimethylammonium chloride (PDDAC) polymer with a positive zeta potential and diamond nano slurries with a negative zeta potential to facilitate the integration of diamond with β-Ga2O3. We conduct a detailed analysis of each step of this seeding technique, adjusting the spinning speed of the polymer coating, spinning period, baking period, and ultrasonication period to understand the underlying mechanisms preventing large-scale diamond growth and to identify strategies for achieving wafer-scale diamond fabrication on single crystal β-Ga2O3 film. This process facilitates the attainment of a uniform dispersion of diamond nanoparticles on β-Ga2O3 at a seeding density of ∼1.82 × 1011 cm−2. The optimized seeding technique has enabled the successful growth of wafer-scale diamond films on β-Ga2O3, as confirmed by scanning electron microscopy (SEM). Additionally, the high quality of the diamond film is affirmed by a quality factor of 96.75 % and a narrow full-width half maximum of 10.28 cm−1 in the T2g peak of the Raman spectra. The X-ray diffraction pattern reinforces the excellent quality of polycrystalline diamond films with minimum stress. In summary, this research provides valuable insights into the polymer-assisted electrostatic seeding technique, paving the way for the uniform growth of wafer-scale diamond films on β-Ga2O3, which is critical for realizing β-Ga2O3-based heterostructure devices.

Development and Generalization of the Method of Reflections in Problems of Electrostatics and Thermal Conductivity of Plane-Layered Media

Development and Generalization of the Method of Reflections in Problems of Electrostatics and Thermal Conductivity of Plane-Layered Media

Preparation and Properties of Stearic Acid/Nanocellulose Composite Phase Change Energy Storage Materials

To solve the problems of liquid phase leakage and poor thermal conductivity of organic phase change energy storage materials, a novel composite phase change energy storage material (SA/CNF) based on stearic acid (SA) and nanocellulose (CNF) was prepared in this study and added graphene to enhance its thermal conductivity. For the prepared composite phase change materials (SFs) with different ratios, shape stability, and testing experiments showed that the maximum adsorption rate of CNF on SA reached 80 %. The properties of SA/CNF were then characterized by various instruments. The results show that SA and CNF are compounded together by physical interaction. More than 93 % of SA in the composite phase change energy storage material SA/CNF is able to store and release heat through the phase change process, and the latent heat of phase change of pure SA is 206.3 J/g, while the latent heat of phase change of the composite phase change material SA/CNF with 20 % of CNF is 156.2 J/g, and they both have good cycling stability. The addition of graphene enhances the thermal conductivity of SA/CNF to a large extent, and the thermal conductivity at a graphene ratio of 5 % is 261 % higher than that of SA/CNF without graphene. SA/CNF is an environmentally friendly energy storage material with very high application prospects.

An inverse estimation of frozen sand mold-melt heat transfer coefficient in the solidification of A356 alloy

Casting aluminum alloys is widely used in aerospace, rail transportation, automotive, ships, and other fields. The frozen sand mold casting (FSMC) technology provides a greener and low-cost solution for the sand mold casting industry, which has a better operating environment than the resin sand mold casting and a higher degree of subcooling for solidification process. The interfacial heat transfer coefficient (IHTC) is a significant parameter that affects the structure and properties of the frozen casting. It is also an important boundary condition in numerical simulation. In this paper, a frozen sand mold casting (FSMC) process thermal conductivity problem (IHCP) inversion model is proposed based on the 1-D heat transfer assumption using a combination of finite difference method (FDM) and conjugate gradient method (CGM). The inversion model is validated for its rationality and accuracy in terms of experiments and numerical calculations, respectively. The IHTC is determined under various frozen sand mold casting conditions, including different initial frozen temperatures, water contents, and molding sand. The accuracy of results is verified by comparing the experimental and numerical data. A new method is provided for determining IHTC in frozen casting.

Semi-Analytical Solution for a Single-Phase Thermal Conduction Problem of a Hollow Cylinder with a Growing or Receding Inner Radius

Abstract A semi-analytical solution for the thermal conduction of a single phase, homogeneous, circular, hollow-cylinder with a growing or receding inner radius at a constant rate under unit-loading was derived using conformal mapping, Laplace transformation, and a Zakian series representation of the inverse Laplace transform. All solutions allow for convection on the fixed outer radius. Predictions were compared to finite-element simulations with excellent agreement observed. Given the changing thickness, thermal transients could not reach true steady-state equilibrium, especially for faster growth or recession rates. Indeed, the temperature states become somewhat linear with respect to time, reflecting the constant velocity of growth or recession. In practice, the resulting solutions can be used to determine temperatures during machining, wear, erosion, corrosion, and/or additive manufacturing, especially for lower temperature solid-state methods such as cold-spray.

Investigation of Thermal Performance of Optimized Tree-Shaped Fins in Latent Heat Storage Units

An innovative tree-shaped fin is proposed to address the problem of low thermal conductivity of phase change materials (PCMs) in latent heat energy storage units. Numerical methods are applied to investigate the thermal properties of the melting process of PCMs. This paper investigates the effects of fin arrangement, fin geometry parameters, and temperature of the heat source on melting characteristics of PCMs in the latent heat energy storage unit. Results show that the melting time of PCMs with the inwardly-tilted fins at the top increases by 21.3% compared to the outwardly-tilted fins at the bottom. The response surface methodology is adopted to obtain the fin parameter combination with lower quality and better performance. The optimal fin dimensions are a thickness of 1.06 mm, a length of 3.38 mm, and a spacing of 12.61 mm. The reduction of the ambient temperature from 35 °C to 30 °C increases the melting time of the latent heat energy storage unit by 85%, and from 35 °C to 40 °C decreases the melting time by 29%.

Cement-based external wall insulation material with thermal performance improvement by partial substitution of calcium silicate

External wall composite insulation materials have shown great potential for energy saving and emission reduction in the construction industry, but cement-based material as one of the external wall composite insulation materials has the problem of high thermal conductivity. Herein, a composite insulation material was prepared using the press method by expanded polystyrene (EPS) and cement of different calcium silicate content. The addition of porous calcium silicate provides nucleation sites for cement hydration to promote hydration reaction and increase the porosity of the inorganic cementitious phase from 75 % to 85 % and refine the average pore size from 2826.09 nm to 421.31 nm. The cement-based composite insulation material which the calcium silicate content is 20 % has 0.0423 W/(m·K) in the thermal conductivity, which is 23.1 % lower than the non-calcium silicate content of cement-based composite insulation material (0.055 W/(m·K)). The simulation analysis proves that the improved insulating properties are attributed to the increased interface between solid-phase and gas phase, thinner heat transfer walls, and longer heat transfer paths due to the pore structure changes in the inorganic cementitious phase. This proposed composite insulation material provides a new choice for external wall insulation.

Stability analysis of a multilayer diffusion-reaction heat transfer problem with a very large number of layers

The stability of thermal conduction problems is of much practical interest in the context of Li-ion batteries, power electronic devices, chemical reactors and other engineering systems. While past work in predicting the thermal stability of a multilayer diffusion-reaction problem has been carried out using pole analysis or by determining imaginary eigenvalues of the problem, such techniques are impractical when the number of layers in the problem is very large. This work presents an exact analysis of the stability of a multilayer diffusion-reaction problem, in which the number of layers may be very large. Using homogenization and Heaviside functions based representation of thermophysical properties and parameters, it is shown that the stability of such a system may be easily predicted by computing the determinant of a matrix containing contributions from each layer/interface. Results from the present work are shown to correctly reduce to results from past work for pure-diffusion and single-layer special cases, as well as agree well with numerical simulations. Threshold for thermal stability of a representative Li-ion battery pack containing 1000 Li-ion cells is determined. It is shown that stability of the system is governed by a balance between temperature-dependent heat generation, diffusion and heat removal from the boundary. A key result of practical interest is that even small improvement in thermal diffusivity of Li-ion cells may greatly help reduce the risk of thermal instability. Additionally, practical guidelines for the maximum permissible operating time of a battery pack to avoid thermal runaway are discussed. This work advances the theoretical understanding of multilayer diffusion-reaction problems, and helps understand the thermal stability of Li-ion battery packs containing a very large number of cells, which is very difficult to analyze using traditional techniques.

Integrated gypsum composite material for energy storage and thermal insulation: Assessment of mechanical performance, thermal properties and environmental impact

The development of gypsum-based construction materials with energy storage and thermal insulation functions is crucial for regulating indoor temperatures, reducing building energy consumption, and mitigating CO2 emissions. In this study, graphene and expanded vermiculite (EV) were used as paraffin carriers to prepare a novel dual-carrier composite energy storage material called P/G-EV, which was developed through ultrasound, a constant-temperature water bath, and vacuum adsorption. In addition, a novel energy storage–thermal insulation integrated–gypsum (ESTIIG) composite material was developed using P/G-EV as the energy storage layer (ESL) and EV as the thermal insulation layer (TIL) compounded with phosphorus building gypsum. The thermal properties, mechanics, apparent density, temperature control performance in a real environment and environmental impact of ESTIIG were evaluated, and the temperature control mechanism was analyzed. The results reveal superior thermal stability and thermal conductivity of the P/G-EV carrier.P/G-EV overcomes the low thermal conductivity and leakage problems associated with paraffin, exhibiting a phase change temperature and latent heat of 21.7 °C and 101 J/g, respectively, during the heat absorption stage. In ESTIIG, P/G-EV and EV are compounded with phosphorus building gypsum to form an ESL and a TIL. The overall mechanical performance of ESTIIG meets the requirements of construction applications. Among the variants of ESTIIG, ESTIIG-1, with an ESL/TIL thickness ratio of 3:1, exhibits the best real-world temperature control performance and environmental benefits, providing a more stable and comfortable indoor temperature. Furthermore, this study provides data and theoretical references for the design of ESTIIG through real-world temperature control monitoring and analysis of the temperature control mechanism.