Temperature-dependent Thermal Properties Research Articles

An inverse method for the estimation of a long-duration surface heat flux on a finite solid

The nonlinear inverse heat conduction problem (IHCP) is resolved for the estimation of surface heat flux from two temperature measurements inside a finite domain. An improvement is achieved in this study by optimizing the rescaled time used to linearize the heat conduction equation with temperature-dependent thermal properties. A closed-form solution of the transient temperature is derived, considering the inhomogeneous boundary conditions. The direct solution is further incorporated into a sequential technique for inverse analysis, and the uncertainty of inverse solution with respect to input parameters is obtained. The results from a representative example show that the method is computationally efficient and the inverse solution can be stabilized by increasing the rescaled time step size. Thus, the effect of the real time step on the ill-posedness can be reduced and the increase of sampling rate in experiment is possible. The present method is useful for the prediction of long-duration heat flux in thermal engineering.

Challenges in Radiofrequency Pasteurization of Shell Eggs: Coagulation Rings.

A total of 50 different configurations of simple radiofrequency (RF) heating at 27.12 MHz of a shell egg were simulated using a finite element model with the purpose of pasteurizing the egg. Temperature-dependent thermal and dielectric properties of the yolk, albumen, and shell were measured, fitted, and introduced into the model. A regression equation that relates the top electrode voltage to the gap between the electrodes and vertical position of the egg was developed. Simulation and experimental results had good agreement in terms of temperature deviation (root mean squared error ranged from 0.35 °C to 0.48 °C) and both results demonstrated the development of a "coagulation ring" around the air cell. The focused heating near the air cell of the egg prevented pasteurization of the egg due to its impact on quality (coagulation). Analysis of the electric field patterns offered a perspective on how nonuniform RF heating could occur in heterogeneous food products. The results can be used to guide development of RF heating for heterogeneous food products and further development of RF pasteurization of eggs.

Mechanical and thermophysical properties of rare-earth monopnictides

The present paper addresses the temperature dependent elastic, mechanical and thermal properties of NaCl structure (B1 type) holmium monopnictides, HoX (X = N, P, As, Sb, Bi) computed using Coulomb and Born repulsive potentials extended up to second nearest neighbors. The second-order elastic constants (SOECs) of single crystals HoX are calculated as a function of temperature in the range 0–500[Formula: see text]K. The compounds under study are found to be brittle in nature. Beside these calculations, the theoretical hardness has been obtained for various rare-earth monopnictides using the elastic properties in the polycrystalline approach. The obtained hardness values indicate HoN to be hard, but cannot be considered super hard. The anisotropic nature of the chosen single crystal is an important physical quantity in studying the directional dependent thermal properties such as Debye temperature and thermal conductivity computed using ultrasonic velocities along different crystallographic directions. The obtained results are discussed in correlation with mechanical and thermophysical properties of similar materials.

An approximate analytical method for nonlinear transient heat transfer through a metallic thermal protection system

Metallic thermal protection system (MTPS) is being investigated more popular in present. In preliminary design, the MTPS can be simplified as a multilayer plate, and a one-dimensional transient heat transfer analysis is commonly used to size the thickness of MTPS. A novel approximate analytical method based on separation of variables and orthogonal expansion technique is presented for temperature prediction through MTPS subjected to convection and radiation boundary conditions, which considers the effects of temperature-dependent thermal material properties. The proposed method applies a set of linear functions with space-variable to describe the distribution of thermal diffusivity through the thickness direction, and it adopts a numerical solution dividing the whole process of heat transfer into a chosen number of time steps to solve the problem of boundary conditions variation with time. An advanced linearized approximation for radiation boundary condition is developed for modified use with the analytical model. Implementation of the proposed method is demonstrated by applying it to determine the problem of nonlinear transient heat transfer through a MTPS made of Titanium 6-4 and Saffil, and a good agreement is presented by comparing with the finite element method (FEM) results.

Application of an improved firework algorithm for simultaneous estimation of temperature-dependent thermal and optical properties of molten salt

Accurate measurement of the thermal and optical properties of molten salt is important in harvesting of solar energy. In this study, a secondary optimization technique that can be used to simultaneously retrieve the temperature-dependent absorption coefficient and the thermal conductivity of molten salt was proposed. The forward coupled radiation–conduction problem in a participating slab exposed to a pulse laser was solved by the finite volume method. An improved firework algorithm (FWA) was introduced to solve inverse problems, which was proven to be more efficient and robust than the original FWA and particle swarm optimization. All research outcomes indicated that the thermal and optical properties of high-temperature molten salt could be retrieved accurately by the proposed secondary optimization technique from the optical and thermal measurement signals.

A novel instrument for rapid measurement of temperature-dependent thermal properties of conduction-heated food up to 140 °C

Estimating thermal properties for thick or solid foods at temperatures greater than 100 °C is challenging for two reasons: the long time needed to reach a constant temperature, and the pressure needed to be maintained in the sealed container. An instrument (“Thermal Properties Cell”, or “TPCell”) was developed based on a rapid non-isothermal method to estimate the temperature-dependent thermal properties within a range of commercial food processes (20–140 °C). The instrument design consists of a custom sample holder and special fittings to accommodate the heater within a pressurized environment. The total time of the experiment is less than 1min., compared to existing isothermal instruments requiring 5–6 h to cover a similar temperature range. Thermal conductivities of different food materials were estimated for the temperature range of commercial food processes. The novelty of the instrument lies in its ability to analyze transient temperature data using a nonlinear form of the two-dimensional heat conduction equations.

Three-Dimensional Thermomechanical Simulation and Experimental Validation on Failure of Dissimilar Material Welds

Dissimilar material weld joints, consisting of low-alloy steel and 304LN austenitic stainless steel (SS), have critical application in boiling water reactors in the nuclear industry. It was predicted that phase transformation adjacent to the fusion boundary and stress distribution across the transition joint play a key role in the structural degeneration of these welds. Quantitatively, to evaluate their contribution, two different joints were considered. One was fabricated with buttering material 309L SS (M/S Mishra Dhatu Nigam Limited, Hyderabad, India), and the other was produced with buttering material IN182 (M/S Mishra Dhatu Nigam Limited, Hyderabad, India). Base materials remained the same for both. Thermomechanical simulation on dissimilar material welds was performed using finite-element modeling to predict the thermal effect and stress prone area. Temperature-dependent thermal and structural properties were considered for simulation. Simulation results were compared with microstructural characteristics, and data were obtained from the in-situ tensile test. Simulation results exhibited that stress was at maximum in the buttering material and made the zone weaker with respect to adjacent areas. During the validation of results, it was observed that failure occurred through buttering material and endorsed the inference. The variation in mechanical properties of the two welds was explained considering the effect of thermal state and stress distribution.

Thermal Property Database for Investment Casting Shells

Reliable and realistic thermal properties data for investment casting shell molds are required to correctly simulate the solidification and predict the shrinkage. Investment casting shells exhibit several phase transformations during firing and pouring which affect their transient thermal properties. These properties are dependent upon time, temperature and process history. This study presents the thermal properties (thermal conductivity and specific heat capacity) of seven industrially produced ceramic molds using an inverse method in which pure Ni was poured into ceramic molds equipped with two thermocouples (inside the mold cavity and in the shell). MAGMASOFT® software (hereafter known as Software A) was used to simulate virtual cooling curves which were fitted to experimental curves by adjusting the temperature dependent thermal properties of the ceramic mold. The thermal properties data obtained from the inverse method were compared with measurement results from laser flash and the differences were discussed. The dataset thus developed will serve to improve the accuracy of investment casting simulation.

Mitigation of residual stresses and distortions in thin aluminium alloy GMAW plates using different heat sink models

This study aims to minimise the residual stresses and distortions in thin butt-welded aluminium alloy 5052-H32 plates during gas metal arc welding process by using three different heat sink models employing thick backing plate and cooling fluids. Experiments are conducted to investigate the efficacy of these models on the temperatures, longitudinal and transverse residual stresses and distortions produced in the welded plates. The results are compared against numerical simulation results obtained using a commercial finite element analysis software ANSYS. Temperature-dependent thermal and mechanical properties of plates are used and coupled transient thermal–structural analysis based on thermo-elastic–plastic approach is conducted. Residual stresses are measured using X-ray diffraction and distortions using a coordinate measuring machine. Temperatures were measured using thermocouples. Results show good conformity between experimental and numerical results and that the heat sink model using a mixture of argon gas and water is the most suitable scheme to achieve the mitigation.

Steady- and Transient-State Analyses of Fully Ceramic Microencapsulated Fuel with Randomly Dispersed Tristructural Isotropic Particles via Two-Temperature Homogenized Model—I: Theory and Method

As a type of accident-tolerant fuel, fully ceramic microencapsulated (FCM) fuel was proposed after the Fukushima accident in Japan. The FCM fuel consists of tristructural isotropic particles randomly dispersed in a silicon carbide (SiC) matrix. For a fuel element with such high heterogeneity, we have proposed a two-temperature homogenized model using the particle transport Monte Carlo method for the heat conduction problem. This model distinguishes between fuel-kernel and SiC matrix temperatures. Moreover, the obtained temperature profiles are more realistic than those of other models. In Part I of the paper, homogenized parameters for theFCM fuel in which tristructural isotropic particles are randomly dispersed in the fine lattice stochastic structure are obtained by (1) matching steady-state analytic solutions of the model with the results of particle transport Monte Carlo method for heat conduction problems, and (2) preserving total enthalpies in fuel kernels and SiC matrix. The homogenized parameters have two desirable properties: (1) they are insensitive to boundary conditions such as coolant bulk temperatures and thickness of cladding, and (2) they are independent of operating power density. By performing the Monte Carlo calculations with the temperature-dependent thermal properties of the constituent materials of the FCM fuel, temperature-dependent homogenized parameters are obtained.

Thermo-mechanical response FEM simulation of ceramic refractories undergoing severe temperature variations

The development of thermal stresses inside refractory ceramics experiencing severe thermal shock is studied. The present analysis departs from the linear theory of thermal stresses as it accounts for temperature dependent thermal and mechanical material properties. Radiation surface heat exchange between the refractory component and its surroundings is included. A Finite Element Procedure is presented and a MATLAB code is developed. The nonlinear heat equation is solved for the temperature field and this solution is subsequently used in order to determine the stress field evolution. The finite element code is validated and used for the thermal shock estimation of a refractory brick. Material properties corresponding to Al2O3 are selected. A thermal cycle consisting of a heating stage followed by cooling down is simulated. The results are compared with those predicted by the linear thermal stresses theory and significant deviations are observed for the examined values of the Biot number.

Finite element analysis of thermal behavior of metal powder during selective laser melting

The thermal behavior during the selective laser melting of metal powders is a key issue to the product quality in 3D printing, so it is of benefit to the development of 3D printing industries to explore the heat transfer characteristics during the selective laser melting process. Based on the TiAl6V4 powder system, the energy conservation equation with a moving Gaussian energy source is developed, in which the temperature-dependent thermal physical properties of materials are taken into account. By means of the finite element methods, the temperature distribution and molten pool dimensions are presented, and the related modeling and numerical methods are validated by previous experimental and numerical works. The effects of the linear energy density, volume shrinkage, scanning track length, hatch spacing and time interval between two neighboring tracks on the temperature distribution and molten pool dimensions are obtained and analyzed. The results show that the increased laser power is superior to the reduced scan speed in thermal performance, which can improve the temperature distribution and molten pool dimensions. Shorter track, shorter hatch spacing and less time interval can lower the temperature gradient and increase the temperature of each track. The average temperature and dimensions of the molten pool are influenced by the volume shrinkage, which should be considered during the numerical simulation.

Temperature-dependent thermophysical properties of Ganurgarh shales from Bhander group, India

Thermal conductivity is a very important property of rocks which determines the rate of heat transfer between two adjacent layers of rock. Temperature-dependent thermal properties of Ganurgarh shale are analyzed and presented over a temperature range from air dried to 900 °C. Physico-mechanical properties, porosity, density and P-wave velocity are determined as per ISRM standards. In addition, changes in internal microstructure are studied using scanning electron microscopy (SEM). Thermal conductivity is predicted using P-wave velocity and porosity of Ganurgarh shale. The effect of temperature on the microstructure, porosity, and mineralogical composition of shale and dependency of thermal conductivity on porosity of shale is discussed. It has also been found that as temperature increases, porosity of Ganurgarh shale increases up to 2.3 times of its initial porosity because of thermal cracking which also acts as barrier of heat flow and hence thermal conductivity decreases. Uncertainties of measured quantities are calculated and the total combined expanded uncertainty for thermal conductivity value of Ganurgarh shale at 95 % confidence level is less than 6 %. All the results obtained using a state-of-the-art single hot guarded parallel plate apparatus.

Temperature-Dependent Short-Circuit Capability of Silicon Carbide Power MOSFETs

This paper presents a comprehensive short-circuit ruggedness evaluation and numerical investigation of up-to-date commercial silicon carbide (SiC) MOSFETs. The short-circuit capability of three types of commercial 1200-V SiC MOSFETs is tested under various conditions, with case temperatures from 25 to 200 °C and dc bus voltages from 400 to 750 V. It is found that the commercial SiC MOSFETs can withstand short-circuit current for only several microseconds with a dc bus voltage of 750 V and case temperature of 200 °C. The experimental short-circuit behaviors are compared, and analyzed through numerical thermal dynamic simulation. Specifically, an electrothermal model is built to estimate the device internal temperature distribution, considering the temperature-dependent thermal properties of SiC material. Based on the temperature information, a leakage current model is derived to calculate the main leakage current components (i.e., thermal, diffusion, and avalanche generation currents). Numerical results show that the short-circuit failure mechanisms of SiC MOSFETs can be thermal generation current induced thermal runaway or high-temperature-related gate oxide damage.

Temperature dependent elastic and thermal expansion properties of W3Co3C, W6Co6C and W4Co2C ternary carbides

W3Co3C, W6Co6C, and W4Co2C are known as η-phases of WC–Co materials that form at the expense of the Co phase, because Co allows C diffusion. Their intrinsic properties are generally poorly understood. We report here first-principles calculations of the temperature-dependent elastic and thermal expansion properties of these three phases. We calculated the elastic constants by using the strain–stress relations. The effects of temperature on the elastic properties were examined in the framework of Debye–Grüneisen theory with the quasistatic approximation. Using the results of the elastic properties, we examined the rigidness, brittleness, and ductility of the ternary carbides. In addition, we confirmed the reliability of our Debye calculations by comparing the results of previous experiments and phonon density of states with quasi-harmonic approximation. The results expand our understanding of the ternary carbides in the WC–Co system.

The explosion characteristics of methane, hydrogen and their mixtures: A computational study

The maximum pressure rise rate during gas explosions in enclosures and the deflagration index are important explosion characteristics of premixture. They can be used to quantify the potential severity of an explosion. However, there are large discrepancies in the deflagration index measured by different researchers for the same methane/air or hydrogen/air mixture. In this study, outwardly propagating spherical flames in a closed vessel are simulated by considering detailed chemistry as well as temperature-dependent thermal and transport properties. From simulation, the maximum pressure rise rate and deflagration index of methane, hydrogen and their mixtures are obtained. The influence of equivalence ratio, initial temperature and initial pressure on the maximum pressure rise rate and deflagration index is examined. It is found that the deflagration index has not been accurately measured in previous experiments, and that experiments conducted in cylindrical vessels have under-predicted greatly the deflagration index. For hydrogen/methane mixtures with hydrogen blending level above 70%, the deflagration index is observed to increase exponentially with hydrogen blending level. Moreover, the deflagration index is found to be greatly affected by initial pressure; while the initial temperature has little influence on deflagration index. Finally, based on theoretical analysis we propose a correlation to calculate the maximum pressure rise rate and deflagration index of methane at a broad range of initial pressure. The performance of this correlation is examined and it is demonstrated to provide accurate prediction.

Study of temperature effect on thermal conductivity of Jhiri shale from Upper Vindhyan, India

Shale is a very important sedimentary rock that is used as building construction material and found predominantly underground. Shales have been found to contain both oil and gas, which can be extracted only when it is fractured. In this paper, temperature dependent thermal properties of Jhiri shale are presented over a temperature range from air-dried to 900 °C. Porosity, density, and compressional wave velocity measurement have been done as per ISRM standard. Furthermore, internal microstructure is studied using scanning electron microscopy (SEM). In this study, thermal conductivity is predicted using P-wave velocity and porosity of Jhiri shale. The effect of temperature on microstructure, porosity, and mineralogical composition and dependency of thermal conductivity on porosity is discussed. It was found that as temperature increases, porosity of Jhiri shale also increases because of thermal cracking, which acts as a barrier of heat flow, reducing thermal conductivity. Uncertainties of measured quantities are calculated and the total combined expanded uncertainty for thermal conductivity value at 95 % confidence level have been found to be less than 6 %.

Thermal analysis of longitudinal fin with temperature-dependent properties and internal heat generation using Galerkin's method of weighted residual

In this study, heat transfer in a longitudinal rectangular fin with temperature-dependent thermal properties and internal heat generation is analysed using Galerkin's method of weighted residual. The simple, but highly accurate solution was validated by the exact solution for the linear problem, and also by numerical method and differential transformation method in the case of the non-linear problem. The developed heat transfer models were used to investigate the effects of thermo-geometric parameters, coefficient of heat transfer and thermal conductivity (non-linear) parameters on the temperature distribution, heat transfer and thermal performance of the longitudinal rectangular fin. From the results, it shows that the fin temperature distribution, the total heat transfer, the fin effectiveness, and the fin efficiency are significantly affected by the thermo-geometric and thermal parameters of the fin. The analysis revealed that the operational parameters must be carefully chosen to ensure that the fin retains its primary purpose of removing heat from the primary surface. Therefore, the results obtained in this analysis serve as basis for comparison of any other method of analysis of the problem, and they also provide a platform for improvement in fin design of fin in heat transfer equipment.

Multiphysics Modeling of Microwave Heating of a Frozen Heterogeneous Meal Rotating on a Turntable.

A 3-dimensional (3-D) multiphysics model was developed to understand the microwave heating process of a real heterogeneous food, multilayered frozen lasagna. Near-perfect 3-D geometries of food package and microwave oven were used. A multiphase porous media model combining the electromagnetic heat source with heat and mass transfer, and incorporating phase change of melting and evaporation was included in finite element model. Discrete rotation of food on the turntable was incorporated. The model simulated for 6 min of microwave cooking of a 450 g frozen lasagna kept at the center of the rotating turntable in a 1200 W domestic oven. Temperature-dependent dielectric and thermal properties of lasagna ingredients were measured and provided as inputs to the model. Simulated temperature profiles were compared with experimental temperature profiles obtained using a thermal imaging camera and fiber-optic sensors. The total moisture loss in lasagna was predicted and compared with the experimental moisture loss during cooking. The simulated spatial temperature patterns predicted at the top layer was in good agreement with the corresponding patterns observed in thermal images. Predicted point temperature profiles at 6 different locations within the meal were compared with experimental temperature profiles and root mean square error (RMSE) values ranged from 6.6 to 20.0 °C. The predicted total moisture loss matched well with an RMSE value of 0.54 g. Different layers of food components showed considerably different heating performance. Food product developers can use this model for designing food products by understanding the effect of thickness and order of each layer, and material properties of each layer, and packaging shape on cooking performance.

Thermal Behaviour of a Gypsum Fibre Board Associated with Rigid Polyurethane Foam Under Standard Fire Conditions

Due to its low thermal conductivity (λ ≈ 20 mW/m.K), rigid polyurethane (PUR) foam has the potential to improve the thermal performance of buildings without increasing the thickness of construction elements. Nevertheless, PUR foam has the drawback of having a low resistance to fire: non-flaming thermal degradation of PUR starts at temperatures about 150-180°C, and flash- ignition around 300°C. Taking advantage of the stable behaviour of gypsum fibre at high temperature, a new panel composed of gypsum fibre and PUR has been developed, with the objective of resisting to fire for 30minutes. At first, the thermal properties of the different materials at high temperature have been established. Then, a heat transfer model using temperature-dependent thermal properties and apparent specific heat capacity has been developed. Finally, the model has been compared to full-scale test results performed according to EN 1365-1.