Transient Thermal Process Research Articles

Investigation of two collinear cracks in fiber reinforced composites under thermal loading

The objective of this paper is to explore the transient behaviors of fiber reinforced composites with two embedded collinear cracks subject to a sudden thermal shock. The orthotropic composites with both the vertically oriented and horizontally aligned fibers are examined to present a systematic comparison. By means of the integral transform methodology and singular integral equations, the thermal and elastic governing equations are solved numerically to determine the transient thermal process and stress intensifications around the crack tips. Parametric investigations are carried out for fiber volume fractions and crack distances. The findings would be beneficial to a more thorough knowledge of fibrous composites’ fracture characteristics under thermal shocks, which helps material design and structural optimization of the composites with fiber reinforcements.

Thermal network model of rotary eddy current braking system for transient thermal process analysis

Thermal network model of rotary eddy current braking system for transient thermal process analysis

Transient heat transfer analysis of a sandwich panel with a cracked honeycomb core

Sandwich structures with ceramic honeycomb cores are extensively employed in thermal protection systems owing to their exceptional ability to resist high temperatures. This work aims at exploring the effect of cracking on the transient thermal process of the sandwich panel subject to impulsive and cyclic thermal loadings. Both the conventional and re-entrant hexagonal alumina honeycombs are considered for the core material. By the integral transform method, combined with singular integral equations, the transient temperatures of the whole sandwich panel are determined from the semi-analytical solution. The straightforward temperature difference of the crack face’s midpoints is exploited to characterize the heat intensification near the crack. Parametric investigations are carried out for the internal cell angle, the relative density, crack length, crack position, and thickness of face sheets, which provides a better understanding of the honeycomb materials working in thermal protection systems.

Ultrafast In-Situ synthesis of flexible MoO3 anode in five seconds for High-Performance aqueous zinc ion hybrid capacitor

The efficient synthesis of high-performance α-MoO3 on flexible conductive substrates is crucial for enhancing its value in the field of energy storage. However, traditional synthesis methods currently employed often suffer from slow heating rates, intricate reaction processes involving multiple steps, sluggish reaction kinetics, high energy consumption, and prolonged preparation times, thereby hindering efficient production. Presented herein is a facile, ultrafast, and versatile approach utilizing microwave carbon thermal shock, a one-step reaction synthesis of α-MoO3 on carbon substrates within 5 s. During microwave carbon thermal shock, the precursor salt experiences an extremely rapid heating rate, swiftly decomposing to form small-sized α-MoO3 crystals. Simultaneously, this process promotes the oxidation of adjacent carbon sites, thereby imparting multi-scale defects and oxygen-containing functional groups to the resulting carbon cloth (CC). The exceptionally low reaction energy barriers and superior Gibbs free energy (ΔG) further substantiate the advantages of the microwave carbon thermal shock strategy over traditional synthesis methods in terms of both kinetics and thermodynamics. The air-assisted transient microwave carbonthermal shock (AMCTS) process circumvents high energy requirements, multistep reactions, and extended preparation times, endowing AMCTS-CC@MoO3 with remarkable flexibility and processability. In demonstrating its practical utility within zinc-ion hybrid micro capacitors, it exhibits outstanding specific capacitance (up to 2300mF cm−2) and mechanical stability. This air-assisted transient microwave thermal shock process provides an efficient route for ultrafast and low-cost synthesis of flexible zinc-ion anode materials.

Modeling and Experimental Study of the Inrush Current of a High-Temperature Superconducting Transformer

Inrush current in high-temperature superconducting (HTS) transformers is a little-studied phenomenon. After connecting to the power grid, a current flow through the windings which exceeding the critical current value of the superconducting tape. It may cause significant overheating and thermal damage of winding. The purpose of the study is to develop a mathematical model for calculating inrush current pulses in a HTS transformer and its verification by physical experiments. To achieve the goal of the study, a mathematical model has been developed that accurately represents the electromagnetic and thermal transient processes after HTS transformer is turned on at idling or under load. The model considers the critical parameters of the HTS tapes, the process of heating and cooling of the windings, quench characteristics, and the electrical and magnetic parameters of the transformer. Good compliance of the experimental results and mathematical modeling with a deviation of 1.99 % allowed us to verify the model. The most important result is the creation of a mathematical model of the HTS transformer at the moment of connecting to power grid. This model represents the temperature changing of the windings during the loss of superconductivity. The developed model can be used in the analysis and modeling of inrush current in designed and operating HTS transformers for any power. The obtained results are significant for determine the optimal starting characteristics, geometric and electrical parameters of HTS transformers. The proposed methods for reducing the inrush current ensure safe and reliable operation of the HTS transformer when switched on at idling or under load.

Pressure-dependent thermal conductivity transient measurement of graphene

The thermal properties of graphene determine the thermal damage evaluation of graphene diaphragm and the lifetime of graphene-based devices. The conventional transient measurement method tends to result in the low-precision thermal conductivity (κ) for suspended monolayer graphene and complex interferometry setup. Hence, in order to improve κ measurement of graphene, a simple, miniaturized and portable Fabry-Perot (F–P) resonator with 10-layer graphene is proposed to perform the actuation and vibration detection of the suspended graphene. Then, the κ value can be calculated by means of laser-heated graphene's thermomechanical response, along with the established model for thermal time constant (τ) of a uniformly heated circular film. A quadratic fit is applied to the measured κ at different air pressures and an average fitting curve is obtained, thus determining that the measured κ relatively decreases by 13.32% with the increasing air pressure outside the F–P cavity from ∼22 kPa to ∼101 kPa. Subsequently, the COMSOL heat transfer simulation confirms that convective heat transfer is the primary reason for the external air pressure affecting transient measurements of κ of graphene. Hence, after taking the intercept of the average fitting curve, the predicted κ at 0 Pa of 10-layer suspended graphene is calculated to be 194.48 W/(m·K), which is close to the fitting curve calculated from previously reported κ of graphene with different layers. This study contributes to accurate transient measurement of κ in graphene, and is also instructive for investigating transient thermal process in other two-dimensional materials.

Development and application of a semi-detailed model for lithium-Ion battery thermal runaway chemistry

To overcome the disadvantages of detailed methodologies and existing simple methodologies for modeling lithium-ion battery thermal runaway chemistry, a semi-detailed model has been developed in the present work. This is important for mitigating battery thermal runaway. The semi-detailed model contains two parts: a ‘global part’ and an ‘elementary reaction mechanism part’. The thermal kinetics parameters of a recently published model are improved so that it can be selected for the ‘global part’ to model NCM811 (LiNi0.8Co0.1Mn0.1O2), NCM622 (LiNi0.6Co0.2Mn0.2O2), and NCM532 (LiNi0.5Co0.3Mn0.2O2). Temperature and heat release evolutions during battery thermal runaway can be obtained from the simulation of the ‘global part’. In the ‘elementary reaction mechanism part’, 53 reactions with frequency factors and activation energies have been proposed and adopted for describing flammable gas formation, lithium-ion consumption, gas phase interaction, and lithium-ion gas interaction during battery thermal runway. Detailed processes for coupling the ‘global part’ and the ‘elementary reaction mechanism part’ as well as solving the 53 reactions are developed, which is the major novelty of this work. Through the coupling of the two parts, chemical venting gas species generation and lithium-ion consumption during the transient thermal runaway process of an NCM811PC (LiNi0.8Co0.1Mn0.1O2 polycrystalline) pouch cell with nail penetration were calculated and compared with available experimental data in the literature. The comparison shows that the semi-detailed model can give correct species trends, indicating the two parts and their coupling in the semi-detailed model of the present work are reliable. The simulation results with transient venting gas chemical species provide useful insights for stopping or delaying battery thermal runaway.

Comprehensive analysis of the contribution from the heat capacity in thermal measurement on nanoscale thin films

Nanofilms (NFs) have been widely used in many emerging applications, such as microelectronic devices, spintronics, and optical instruments. Characterizing the thermal conductivity (k) of NFs, kNFs, is nontrivial for both fundamental science and industrial applications. Time-domain thermoreflectance (TDTR) is a powerful technique for thermal characterizations under nano-to-micro-scales. However, both the kNFs and the interface thermal resistance between layers are generally unknown parameters when analyzing TDTR signals. So that an effective thermal resistance model (ETRM) is often utilized, where the impact of heat capacity of NFs, cNFs, has been always ignored. Previous studies have proposed viewpoints on how to validate this assumption, however, which still needs to be verified further by considering the parameters' sensitivities for TDTR signals. In this work, we have highlighted the significance of sensitivity analysis for the investigation of the impact of cNFs with the example experiments on Ni NFs. The error of ETRM has been quantitatively and systematically studied. We found that the application requirements of ETRM in TDTR data analysis rely on the sensitivity relations between kNFs and interface thermal resistance. Finally, suggestions for future characterization of the kNFs have been discussed for reference. Our results and conclusions deepen the understanding of both the transient thermal transport process of multilayer NFs and the ability of TDTR on characterizing kNFs.

Input reduction for nonlinear thermal surface loads

A multiplicity of simulations is required to optimize systems with thermal transient processes in the presence of uncertain parameters. That is why model order reduction is applied to minimize the numerical effort. The consideration of heat radiation and convection with parameter-dependent heat transfer coefficients results in a nonlinear system with many inputs as these loads are distributed over the whole surface limiting the attainable reduced dimension. Therefore, a new input reduction method is presented approximating the input matrix based on load vector snapshots using singular value decomposition. Afterward, standard reduction methods like the Krylov subspace method or balanced truncation can be applied. Compared to proper orthogonal decomposition, the number of training simulations decreases significantly and the reduced-order model provides a high accuracy within a broad parameter range. In a second step, the discrete empirical interpolation method is used to limit the evaluation of the nonlinearity to a few degrees of freedom and proper orthogonal decomposition allows the fast adaptation of the emissivity. As a result, the reduced system becomes independent of the original dimensions and the computation time is reduced drastically. This approach enables an optimal method combination depending on the number of simulations performed with the reduced model.

Transient heat transfer analysis of a cracked strip irradiated by ultrafast Gaussian laser beam using dual-phase-lag theory

With the rapid development of ultrafast laser technology, the effect of non-Fourier heat conduction on the transient process has received considerable attention. In the present article, the non-Fourier dual-phase-lag heat conduction theory is adopted to analyze the transient thermal process of a cracked strip subjected to ultrafast laser heating. The originality of this work lies in considerations of Gaussian type distribution of the laser beam and the insulated crack's disturbance to the heat flow. Fourier transform and Laplace transform are utilized to reduce the complex boundary-value problems to the Cauchy-type singular integral equation, which is then solved numerically by the Lobatto-Chebyshev technique. With the aid of the numerical inversion of Laplace transform, the transient temperature characteristics of the cracked strip are displayed graphically to illustrate the effects of the two non-Fourier thermal lags, laser incident position, and the Gaussian type heating source.

An Online Identification Method of Thermal Dissipation State for Forced Air-Cooled System of Power Converters

Thermal stress is the primary cause of malfunction and failure in power modules. As the main heat dissipation component, the heatsink plays a significant role in improving the reliability of power converters. However, due to the continuous accumulation of dust and impurities on the heatsink’s air inlet side, the thermal dissipation performance of the cooling system constantly declines, which influences the reliable operation of power modules. In this article, a computational fluid dynamics (CFD)-based simulation analyzes the declining mechanism of the heatsinks’ thermal transfer performance. Moreover, a novel online method is proposed for the cooling system’s thermal dissipation state identification. This method regards the steady-state thermal resistance as the feature parameter of the thermal dissipation state evaluation. And a parameter identification method is employed to identify the steady-state thermal resistance due to the difficulty of obtaining it in the transient thermal process. In addition, an ac/dc/ac pulsewidth modulation (PWM) converter is built for verification. The experimental results demonstrate that the proposed method enables accurate and fast identification of the cooling systems’ thermal dissipation state. It can be one of a solution for replacing the existing off-line manual periodic detection method, which helps improve detection efficiency and decrease maintenance costs.

Experimental and numerical study on the transient heat transfer process for fluoride salt-to-air cooler

In order to better characterize the transient thermal behavior of Fluoride salt-to-Air Cooler (FAC) and then to forecast the risk of freezing under variable load, the numerical and experimental studies of the cooling of fluoride salts have been performed. Results show that the simulated thermal parameter is in agreements with the experiment data with a maximum deviation below 20%, which validate the reliability of the CFD model. Then, a detailed 3D CFD transient thermal process related flow distributions are conducted. Results show that the air-outlet velocity has a sudden rise at the immediate blower operating condition, but the transient thermal process will be worked well. Finally, the air-temperature response and fluoride-salt minimum temperature are evaluated considering the transient progression. Results show that gentle changes to the blower flowrate can effectively mitigate potential to overcool the salt and avoid solidification. Which offer helpful guidance for FAC transient regulation and operation.

Molecular dynamics study of instantaneous interfacial thermal resistance of droplets on flat crystalline surface during cooling and ice formation

Condensation and frost formation degrade the heat transfer performance of air-conditioners and refrigerators. Yet, the frost formation mechanism has not been fully understood. In the present study, we numerically investigate nanoscale H2O droplets during cooling and ice formation utilizing classical molecular dynamics simulations. The mW potential is employed for the H2O molecules. A nanoscale H2O droplet is placed on a flat solid wall consisting of Pt or Pb atoms. We examine where ice nucleation is formed and how the ice formation proceeds inside the droplet, and then evaluate the time change in instantaneous interfacial thermal resistance between the H2O molecules and the solid wall. In common with Pt and Pb surfaces, the time changes in the instantaneous interfacial thermal resistance and the density depletion length are qualitatively consistent in the present transient thermal energy transfer process together with the phase change. In addition, the relation of the time-averaged instantaneous interfacial thermal resistance and the density depletion length are qualitatively consistent with the relation of the solid-liquid interfacial thermal resistance in the steady-state.

Calculation of the transient thermal process for a linear heating device of oil pipelines

The economic and technological advantages of using induction heating of pipelines are considered. Comparison this method with other heating methods is carried out. A new design of an induction converter facility based on a linear heating device (LHD) is considered. The calculation of the transient thermal process for LHD as the most severe mode and taking into account the rapid transition to the steady state during LHD operation is carried out. Keywords induction heating, heating of pipelines, energy-saving technologies, transient mode, steady state

Numerical analysis of the non-stationary thermal state of the tool in the combined casting and extrusion of aluminum alloy

The results of a numerical analysis of unsteady heat transfer in the “metal-mold-environment” system during continuous combined casting and extrusion of an aluminum alloy in an installation with a horizontal carousel mold are presented. The heat engineering zones characterized by different intensity of heat transfer between the melt and the surface of the mold have been determined. A quantitative assessment of the influence of the rate of heating of the crystallizer on the temperature-time characteristics during the period of the transient thermal process is given. It is shown that an increase in the productivity of the installation reduces the duration of the transient thermal process when starting the installation from a cold state until it reaches a stationary thermal regime. The dependence of the time at which the installation reaches the stationary thermal regime on the rotation speed of the crystallizer wheel has been obtained.

Transient thermal analysis for circular laser hole-cutting in stable stainless steel sheets using a novel rotational pulsed-laser heat source model

Using the finite element method (FEM) and the element birth and death technique, a novel rotational pulsed-laser heat source model with consideration of angular velocity and laser pulse activation is reported for simulating the transient thermal process induced by circular laser hole-cutting (CLHC) of through holes in stable stainless steel sheets. The predicted through-hole geometry characteristics were validated with their measured results. It was shown that the simulated results agreed well with the experimental measurements with an acceptable prediction accuracy, and the error percentage for the simulation reported in this work ranged from 2.84% to 6.87%, demonstrating the feasibility and reliability of the rotational pulsed-laser heat source model developed. The temperature field on the workpiece transiently altered with the rotation of the laser beam. Under the combined effect of the uniform circular motion of the laser beam and the periodic irradiation induced by the successive laser pulses, the local temperature on the thermal cycle curve for a nodal location rose obviously with transient multi-peak temperature fluctuations when the rotational laser beam moved near to this calculation node. With the increase of the distance from the hole-entrance border to a certain location, the thermal effect of the rotational pulsed-laser heat source decreased, and the thermal fluctuation on the thermal cycle curve also weakened. The pulse repetition rate and angular cutting velocity slightly influenced the through-hole diameters predicted, while the beam spot size and pulse duration affected the calculated through-hole diameter more noticeably.

ДОСЛІДЖЕННЯ МЕТРОЛОГІЧНИХ ХАРАКТЕРИСТИК ТЕРМОАНЕМОМЕТРА НА ОСНОВІ ВИМІРЮВАЛЬНОГО КОНТРОЛЮ ПАРАМЕТРІВ ТЕПЛОВОГО ПОТОКУ МИКРОХВИЛЬОВОЇ КАМЕРИ

The purpose of the article is to study the rational operation of the hot-wire anemometer in accordance with the dynamic characteristics and measurement errors, when implementing the process of drying raw nuts in an industrial microwave chamber. The technique of studying the informative parameters of the heat flow was used to determine the rational mode of operation of the hot-wire anemometer and to reduce the time of heat treatment of nut raw materials. The characteristics of the thermal transient process of drying raw nut materials in a microwave chamber are investigated. Theoretical substantiation and an algorithm for determining the dynamic characteristics of the hot-wire anemometer under the external sinusoidal effect of the heat flow temperature are given, algorithms for determining the instantaneous values of the temperature increments of the hot-wire anemometer sensitive element in the process of drying nut raw materials are given. On the basis of the general theory of errors of indirect measurements in the investigated range of variation of the rate V of the heat flow, the values of the relative systematic errors of indirect measurements of the rate of heat flow in the range from 10 to 40 m / s were determined. It was found that in accordance with the dynamic characteristics and measurement errors when blowing a moving heat flow of nut raw materials, the most rational is the mode of constant resistance of the hot-wire anemometer. Solutions of the differential equation describing the dynamic heat process when the temperature of the moving heat flow changes according to the periodic law, i.e. stationary, non-stationary and complete. The results of calculating the constant times of the hot-wire anemometer thread τп and τтп of the moving heat flow, respectively, were τп = 0.59 s and τтп = 0.6 s, the time constant of the hot-wire anemometer is practically commensurate with the thermal time constant of the moving stream blowing over the nut raw material. It consists in the fact that the theoretical position of the hot-wire anemometer has been further developed in the direction of researching the characteristics of the thermal transient process of processing nut raw materials, all this makes it possible to determine the ranges of changes in the signals of the thermal device, which correspond to the ranges of change in the informative characteristics of the moving heat flow, and also allows the selection measuring equipment, establish rational operating modes of hot-wire anemometers and automated devices that use them in accordance with errors and sensitivities. The proposed algorithms for determining the informative parameters of the heat flow in the study of the transient process of heat transfer from the moving air flow to the sensitive element of the transducer, the results of the errors in measuring the velocity V of the moving heat flow, make it possible to calculate, design and create measuring automated devices that use hot-wire anemometers in food and processing industry, instrument making and chemical engineering.

Infrared visualization of lumped and non-lumped thermal transient processes in an introductory laboratory

The time dependence of the temperature distribution in cooling/heating bodies is studied with a simple, direct visual measurement of infrared radiation. The adoption of a smartphone thermal camera allows one to verify some essential features of theoretical models which describe the temperature field in a cylindrical symmetric configuration for both lumped (concentrated) and non-lumped (distributed) behaviors. Theoretical results and experimental temperature fields acquired via infrared thermal analysis of cooling solid cylinders compare well. The approach can be adopted to support an introductory route to the study of these classes of processes at very low cost and with a fast and simple laboratory procedure. This scheme is appropriate for undergraduate engineering and physics students.

Transient thermal analysis in nanofluid suspensions

An analytic approach is utilized to investigate the heat transfer mechanism in nanofluids containing spherical nanoelements. The transient thermal conduction process in a cylindrical geometry is assessed according to the Dual-Phase-Lag (DuPhlag) model and then compared with the classic Fourier solution. For the first several seconds, the wire temperature values calculated by the two models as well as their corresponding slopes are different; the curves merge at longer times depending on the nanofluid properties. At the initial stages, the realistic temperature magnitude of the suspension is also smaller than the Fourier solution, since in reality, the transient heat transfer rate is comparatively smaller owing to the certain time lags in the heat flux/temperature gradient relationship. The key parameters that govern in the initial stages of the heat transport inside the domain are the thermophysical properties of the components of a nanofluid, their relative velocity, the size/shape and concentration of the nanoparticles, and the system geometry. Two typical parameters, the conductivity and heat flux indexes, are also introduced to evaluate the deviation of the Fourier theory from the DuPhlag model. The conductivity index has a minimum value at the initial stages, and then, it is beginning to recover. A fluctuating distribution of the heat flux index is observed at the beginning of the process which is smoothed out after a lapse of time. A decrease in the nanoparticle diameter and/or an increase in the concentration may result in accelerating the transient response of the medium to the input thermal excitation.

Cylindrical body temperature field simulation which made in the transitional thermal process conditions out of polymer material

The advancing requirements for strength, relaxation, thermophysical, electrical, and other structural elements characteristics actualizes the polymer composite material use for the soft part and node point manufacture, which improves performance index. This paper reported the need to take into account relaxation phenomena in predicting the body’s thermal field development that is made of polymeric materials, and the thermal relaxation time and the thermal damping time proportional to the duration of transient thermal process certain periods. In this article three-period thermal process in a cylindrical body mathematical model is presented. cylindrical body made of a low-heat-conducting material by using a heat conduction hyperbolic equation that is reflecting the heat flow relaxation and thermal damping phenomenon. A numerical solution to the problem of unsteady heat conduction in a circular disk for a two-phase delay equation is presented, which is based on the grid method implementation by using a three-layer implicit difference scheme and the finite difference method use. Calculation formulas for the run-through coefficients as well as the temperature values at the outer boundaries are concluded using the boundary conditions approximation for the intermediate and upper time layers, taking into account the multi-period of the process. The implementation of the modified run-through method when solving the non-stationary heat conduction problem in a cylindrical body, taking into account the finite heat propagation speed and thermal damping is described. The calculation results for the cylindrical body temperature field are obtained by using the polymethyl methacrylate example upon sudden heating based on a model with a two-phase delay. The results presented in this paper aid in an increase in predicting temperature field accuracy in polymer composite materials in the transient thermal processes study.