Transient Thermal Response Research Articles

Nonlinear transient thermal analysis of a convective–radiative fin: A comparative study of two approximate analytical methods

Developments of analytical solutions to nonlinear equations provide very good physical insights into the nonlinear phenomena. The nonlinear transient thermal responses of extended surfaces have been widely studied analytically with the aid of different methods based on power series solutions. Such solutions come with large numbers of terms that are not convenient for use in practice. In this work, a non-power series solution is presented for the nonlinear transient thermal analysis of the passive device. Also, a comparative study of two analytical methods, Laplace transform-Galerkin weighted residual and differential transformation methods is presented to analyze transient nonlinear thermal model of radiative–convective fin considering varying thermal conductivity. The obtained analytical solutions are verified analytically and numerically, and very good agreements are established. The symbolic results are employed to check the influences of model parameters on passive device’s thermal performance. It is found that as the conductive–convective, and conductive–radiative parameters increase, temperature distribution decreases since heat transfer becomes augmented and hence, the fin thermal efficiency is improved. The temperature distribution increases through the fin as thermal conductivity increases. At the different positions in the fin, the temperature increases with an increase in time. The time histories of the solution shows that transient solutions converge to a steady state and the fin tip temperature increases as time progresses. The developed approximate analytical solutions provide very good platforms for the nonlinear thermal analyses of fins and proper design of the extended surfaces in thermal systems.

Transient thermal and physiological responses from air-conditioned room to semi-outdoor space in the tropics

This study explored the feasibility of working in a semi-outdoor space in the tropics. Humans can work in the semi-outdoor space for short-term without compromising cognitive performance. However, transient thermal and physiological responses are unclear when people move from an air-conditioned (AC) room to semi-outdoor space. Experiments were carried out to understand transient thermal and physiological responses in Singapore. Eighty-nine tropically acclimatized participants voted on thermal responses in the first 30 min in an AC room of 24 °C and 40 min in a semi-outdoor space from 25.0 to 31.8 °C. Heart rate, skin temperature and skin conductance were measured. After moving to the semi-outdoor space, thermal sensation, thermal comfort and thermal acceptability remained similar without overestimating thermal responses in the first few minutes (p > 0.05). The stabilized times of thermal sensation, thermal comfort and thermal acceptability rapidly were within 1 min due to past thermal experiences and expectations. Mean skin temperature and mean heart rate increased by 0.7 °C (p < 0.001) and 2 bpm (p = 0.027) respectively, but mean skin conductance was stable (p > 0.05) after moving to the semi-outdoor space. The stabilized time of 7 min in mean facial temperature was shorter than the 30 min in the whole body mean skin temperature due to high density of blood vessels in the face. Mean facial temperature was more thermally sensitive than that in the whole body. There was little change in transient thermal and physiological responses after moving to semi-outdoor space.

Experimental Thermal Response Study of Multilayered, Encapsulated, PCM-Integrated Building Construction Materials

Thermal energy storage integration using phase change materials (PCMs) in buildings has great potential for energy conservation and greenhouse gas (GHG) emission reduction. Cutting-edge research and innovative ideas are required when using multilayered PCMs within typical construction materials to take advantage of their heat storage capability over a wide temperature range within buildings. This current study was carried out to experimentally test the efficacy of using dual PCMs RT28HC and RT21HC with different melting temperature ranges (28 °C and 21 °C) under variable thermal loading. The transient thermal response of various PCM-based configurations of concrete and cement blocks at different temperature inputs was obtained to determine the effectiveness of dual PCMs and their optimized configuration under experimental laboratory conditions. The range of the temperature input was varied from 22 °C to 50 °C, suitable for hot climatic conditions such as those in Pakistan. Laboratory ambient temperatures remained at ~17 °C for all experimental tests. Moreover, the results were compared using two parameters, i.e., decrement factor (DF) and time lag (TL). With DF and TL values of 0.10 and 5.72, respectively, in the high-temperature heating (HTH) regime and a low DF value of 0.08 and high TL of 5.17 in the moderate-temperature heating (MTH) regime, the RT28HC–RT21HC combination proved to be the most effective. The application of the RT28HC–RT21HC combination provided up to a 54.3% reduction in indoor temperatures in the HTH regime. This research contributes through experimental validation that these novel configurations are capable of providing substantial improvement in indoor thermal comfort.

Transition of interfacial friction regime and its influence on thermal responses in rotary friction welding of SUS304 stainless steel: A fully coupled transient thermomechanical analysis

Rotary friction welding (RFW) has proved to be a successful joining technology in advanced materials manufacturing. The highly transient thermal responses during friction heating, including heat generation and temperature distribution, are essential factors in the design and control of the RFW process. In this study, a 2D axisymmetric finite element (FE) model for the RFW process of SUS304 stainless steel is presented in order to investigate the transition of the interfacial friction regime and its influence on the transient thermal responses. By using the FE simulation, the in-process state variables, i.e., frictional stress and heat flux, and their spatial distribution at the welding interface are obtained. The FE simulation is validated by published experimental results. A new analytical model for the transient evolution of the friction regime is concluded from the FE simulation, which accurately predicts the shrinkage of the Coulomb friction zone while the Shear friction zone builds up from the workpiece periphery. Comprehensive analysis is conducted on the variation of interfacial heat flux, frictional stress and temperature resulted from the transition of friction regime. It is revealed that the transition of the interfacial friction regime from the Coulomb friction regime to the Shear friction regime is preferable for temperature homogenization at the welding interface.

Monte Carlo Sensitivities and Performance of Woven Deployable Entry Systems

A material response model is developed for the determination of erosion rates and thermal response of woven materials to enable accurate thermal protection material sizing and future missions to Venus and giant planets. The stagnation-point thermal response is evaluated using the model developed in Icarus, a material response finite volume solver. A Monte Carlo global sensitivity analysis is further performed using a stand-alone Python wrapper script developed to determine the interactions between key parameters in the material surface energy balance of the one-dimensional simulation, such as the convective blowing correction parameter and weave material properties. The time-accurate material response for the dual-weave erosion and surface temperatures show good agreement with stagnation flat face arcjet tests involving time-varying aeroheating–cooling cycles. The high-fidelity Monte Carlo sensitivity analysis technique was used to address challenges in thermal protection sizing for the Adaptive Deployable Entry and Placement Technology (ADEPT) system, a low ballistic coefficient hypersonic decelerator. The results provide new correlations for such weaves involving carbon–air equilibrium oxidation and wall enthalpy imposition through a B-prime formalism. Through the multivariate regression analyses and uncertainty rankings performed, only two properties were found to largely contribute to the transient thermal response of the gore, with emissivity contributing up to of the total output uncertainty at the front face and at the back face.

Transient Thermal Response of Blood Vessels during Laser Irradiation Monitored by Laser Speckle Contrast Imaging

Real-time monitoring of blood flow and thrombosis formation induced by laser irradiation is critical to reveal the thermal-damage mechanism and successfully implement vascular-dermatology laser surgery. Laser speckle contrast imaging (LSCI) is a non-invasive technique to visualize perfusion in various tissues. However, the ability of the LSCI to monitor the transient thermal response of blood vessels, especially thrombus formation during laser irradiation, requires further research. In this paper, an LSCI system was constructed and a 632 nm He-Ne laser was employed to illuminate a Sprague Dawley rat dorsal skin chamber model irradiated by a 1064 nm Nd: YAG therapy laser. The anisotropic diffusion filtering (ADF) technique is implemented after temporal LSCI (tLSCI) processing to improve the SNR and temporal resolution. The speckle flow index is used to characterize the blood-flow velocity to reduce the computational cost. The combination of the tLSCI and ADF increases the temporal resolution by five times and the SNR by 17.2 times and 16.14 times, without and with laser therapy, respectively. The laser-induced thrombus formation and vascular damage during laser surgery can be visualized without any exogenous labels, which provides a powerful tool for thrombus monitoring during laser surgery.

Transient thermal performance of multilayer thermal protection systems doped with phase change materials

In this article, the transient heat transfer process of the multilayer thermal protection system (TPS) doped with the phase change materials (PCM) is studied. The multilayer TPS consists of the C/SiC composite material, alumina aerogel, silica aerogel, and silica aerogel doped with the PCM. A thermal lattice Boltzmann model is adopted to consider the effect of interlayer thermal contact resistance on the transient thermal response of the TPS. An enthalpy-based lattice Boltzmann model is adopted to solve the solid–liquid phase change problem. The transient heat transfer characteristics of two kinds of TPSs in different layouts are analyzed in detail. The results show that whether the doped PCM enhances the thermal insulation performance of the TPS depends on the layout design of the multiple layers and service time. There exists an optimal layer thickness of the PCM that makes a TPS have the best thermal insulation performance, and the optimal value increases with the service time. The interlayer thermal contact resistance can have a significant effect on the transient thermal response of the TPS, and the optimal value of the PCM thickness will reduce.

Finite element thermal modelling of power MOSFET packages

This paper proposes a methodology for complete thermal description of power MOSFETs (LFPak56 and LFPak56D) to be used in electronic assembly level simulations. By describing all relevant heat paths through the packages, various types of cooling solutions can be assessed from early development phases. The described modelling approach comprises of measurement and calibration of the three paths through the package: via the slug, pins, and mold, by tuning each material property, to fit the thermal transient response (Zth). The automated calibration of all three heat paths of the MOSFETs described in the paper, greatly simplifies, and streamlines the buildup of the models lowering both development times and costs. The novelty of the paper is the introduction of concurrent calibration of all three paths with the help of optimization software and the measurement and model setups for an improved workflow. Using the described methods, standardized models can be built for quick assessments of different hardware configurations and thermal designs of electronic control units. The high accuracy of these models makes them appropriate for describing complex high-power transient loads and fast switching times.

Research on transient thermal behavior of semiconductor lasers under pulse current excitation by thermoreflection technique

We measured the response of the facet temperature of the semiconductor laser to a single pulse current with a magnitude of microseconds by using transient thermoreflection method, and then calculated the thermal time constants of the laser. The thermoreflection technique measures the reflectivity of facet which is affected by temperature in real time and thus only measures the thermal parameter. In addition, we select the falling edge of the pulse current where current and light dissipate immediately during the heat decline as the starting point for the study of the thermal response. Therefore, the electro-optical-thermal characteristics of the laser are decoupled in the measurement process. The transient thermal response is formulated as a second-order exponential function of time. The two time constants, i.e τ1 and τ2, represent thermal conductivity performance of laser chip and device package, respectively. We studied the transient thermal behavior of the laser under unsteady state based on the evolution trends of the thermal time constants with amplitude and width of the pulse current. When the pulse is narrow, the heat of the device is concentrated in the active region, even if the current amplitude is relatively large; With the increase of pulse width, the heat generated in the active region diffuses outward to other layers of the chip and heat sink. We also observed the mechanical deformation of the facet in the active region which results from heat accumulation, and the heat generation is related to light intensity.

Low Heat Capacity 3D Hollow Microarchitected Reactors for Thermal and Fluid Applications

Lightweight reactor materials that simultaneously possess low heat capacity and large surface area are desirable for various applications such as catalytic supports, heat exchangers, and biological scaffolds. However, they are challenging to satisfy this criterion originating from their structural property in most porous cellular solids. Microlattices have great potential to resolve this issue in directing transport phenomena because of their hierarchically ordered design and controllable geometrical features such as porosity, specific surface, and tortuosity. In this study, we report hollow ceramic microlattices comprising a 10 μm thick hollow nickel oxide beam in an octet-truss architecture with low heat capacity and high specific surface area. Our microarchitected reactors exhibited a low heat capacity for a rapid thermal response with a small Biot number (Bi &lt;&lt; 1) and large intertwined surface area for homogeneous flow mixing and chemical reactions, which made them ideal candidates for various energy applications. The hollow ceramic microlattice was fabricated by digital light three-dimensional (3D) printing, composite electroless plating, polymer removal, and subsequent thermal annealing. The transient thermal response and fluidic properties of the 3D-printed microstructures were experimentally investigated using a small-scale thermal and fluid test system, and analytically interpreted using simplified models. Our findings indicate that hollow microarchitected reactors provide a promising platform for developing multifunctional materials for thermal and fluid applications.

Heated wind-tunnel experiments and numerical investigations onhypersonic blunt cone aerodynamic heating

Aero-thermal analysis of hypersonic vehicles is of fundamental interest for structural design and material selection. Therefore, we investigated the aerodynamic heating characteristics of a blunt cone made from stainless steel in a combustion-heated hypersonic wind tunnel with a free stream of Mach 6. During ground testing, the temperature rise of the model from room temperature to thermal equilibrium under the influence of hypersonic flow was measured with thermocouples, and the equilibrium temperature of the blunt section was 1130–1150 K. In addition, the transient thermal response of the model to the aerothermodynamic load was simulated by fluid-thermal coupled analysis and compared with the measurement data. The obtained results correspond well to typical thermocouple data, and the temperature predicts the measurement with an accuracy on the order of 8.8%. On this foundation, some factors affecting the radiation equilibrium temperature of the blunt cone are also discussed. The analysis indicates that emissivity has an important influence, while turbulence intensity at the nozzle inlet has minimal impact.

Plasma-induced surface cooling

Plasmas are an indispensable materials engineering tool due to their unique ability to deliver a flux of species and energy to a surface. This energy flux serves to heat the surface out of thermal equilibrium with bulk material, thus enabling local physicochemical processes that can be harnessed for material manipulation. However, to-date, there have been no reports on the direct measurement of the localized, transient thermal response of a material surface exposed to a plasma. Here, we use time-resolved optical thermometry in-situ to show that the energy flux from a pulsed plasma serves to both heat and transiently cool the material surface. To identify potential mechanisms for this ‘plasma cooling,’ we employ time-resolved plasma diagnostics to correlate the photon and charged particle flux with the thermal response of the material. The results indicate photon-stimulated desorption of adsorbates from the surface is the most likely mechanism responsible for this plasma cooling.

Correlation between the sum of the flux conduction transfer function coefficients and a dimensionless function of the thermal properties of building walls

The estimation of the conduction transfer function (CTF) coefficients is the main step to obtain the transient heat flows on the inside and outside surfaces of a multilayer wall, and thus the heating and cooling loads in a building. Taking a wide variety of construction types, 76 walls from the ASHRAE Handbooks of Fundamentals, it is demonstrated that the aspects of wall transient thermal behaviour can be summarised by a non-dimensionalised function V of the thermal capacity and resistance of the layers of the wall. The sum of the flux CTFs, Σdk, is highly correlated with this measure and the correlation coefficient increases when the sampling time interval decreases. The current interest for nearly zero energy buildings demand management techniques requires the calculation of the CTFs for highly insulated walls and short sampling intervals. The study is extended to the possibility of the CTFs with shorter time steps but longer time steps are needed for thick walls. The theory of the plain slab runs as a thread through the study: the scatter of wall Σdk values cluster around the Σdk:V relation for a slab; the response factor function (exit flux following imposition of a temperature change at the other surface) follows readily from a classical expression in transient thermal response (and from which the slab CTF’s readily follow), and an appendix presents the slab’s recently discovered exact Σdk:V relation.

Transient thermal pattern separation and detection of conductive defects in composite insulators using eddy current pulsed thermography

Composite insulators are prone to internal conductive defects owing to their long-term high-voltage operation and outdoor aging. In this study, a method based on eddy current pulsed thermography (ECPT) combined with feature extraction transform algorithms is proposed for transient thermal pattern separation and defect detection in composite insulators with internal conductive defects. The principles of this method, including electromagnetic heating, thermal diffusion, and transient thermal response extraction, are investigated. Conductive defects in insulators have unique thermal patterns that differ from those of non-conductive defects in conductive materials. In this study, the original transient thermal responses of conductive defects are analyzed and compared with those of non-conductive defects. The effectiveness of the transient thermal pattern separation of this method is verified through experimental studies. Different feature extraction transform methods are utilized and compared to separate different thermal transient patterns and perform quantitative evaluations. It is found that ECPT combined with feature extraction transform algorithms can provide a visual and effective method to detect internal conductive defects in composite insulators.

Development of Electrothermal Models for Electrical Traction

In this paper, improved electrothermal models of the power diode and IGBT have been developed. The main local physical effects have been considered. The proposed models are able to deal with electrical and thermal effects. The models were confirmed by comparison with other models having similar characteristics for different circuits and different temperatures. The developed models are implemented in a traction unit to study the electrothermal performance in an electric vehicle system. The models were implemented in the Pspice circuit simulation platform using standard Pspice components and analog behavior modeling (ABM) blocks. The switching performance of the diode and the IGBT have been studied under the influence of different circuit elements in order to study and estimate the on-state and switching losses pre-required for the design of various topologies of converters and inverters. The comparison shows that these models are simple, configurable with the electrical circuit simulator software. They are better able to predict the main circuit parameters needed for power electronics design. Transient thermal responses have been demonstrated for single pulse and repetition modes. The obtained results show that our model is suitable for a fully electrothermal use of power electronic circuit simulations.

Detection of corrosion under insulation on aerospace structures via pulsed eddy current thermography

Corrosion degradation mechanics play a significant role in the safe operability and availability of aircraft components. To defend against corrosion, coating systems and insulation are used on aircraft structures to protect them from corrosion promoters. Due to this layer of protection, corrosion under insulation (CUI) and early onset corrosion remain difficult to characterize on exterior aircraft structures in a timely manner. Pulsed eddy current thermography (PECT) shows promise in enabling the detection of corrosion on aircraft while rapidly scanning large areas. However, the limitations of this technique and appropriate measurement parameters have yet to be explored on painted aluminum aircraft structures. In this work, pulsed phase thermography using electromagnetic coil excitation was performed on painted corroded aluminum samples to capture thermal transient localized responses for defect characterization. Defects ranging from 0.4 mm to 5 mm in a painted corroded aluminum sample were identified using PECT through the capture and analysis of changes in phase and amplitude. The results show that saturation of thermal response from larger defects overshadows smaller outlying defects, suggesting the need for an alternative approach to characterizing defects smaller than 1 mm in diameter. Despite inherently longer measurement times, phase measurements during the cooling period result in higher sensitivity thus enabling detection of smaller defects. This study helps to define a range of operation and measurement parameters for PECT to be effective as a means for corrosion detection on aluminum painted panels. The outcomes pave the way to further extend PECT as a non-destructive, non-contact method for detection of CUI.

Health Monitoring of RTV Silicone Rubber Coating on Insulators Based on Multimode Features of Active Infrared Thermography

Poor quality of room-temperature vulcanized silicone rubber (RTV-SIR) coating on insulators, such as damages, inclusions, debonding, uneven or substandard thickness, and so on, seriously deteriorates the external insulation performance and brings potential threats to the safety of power grids. This article proposed a convenient, quantitative, and nondestructive inspection method for room-temperature vulcanized (RTV) coating using multimode features of active infrared thermography (AIT). First, the thermal wave model of RTV coating was established to explain the theoretical basis for defect detection and thickness measurement of RTV coating based on the surface transient temperature field. Typical defects of RTV coating were detected, and the defect identification was enhanced by the statistical reconstructed thermal images. The frequency-domain features and the reciprocal square root of the minimum phase frequency (RSRMPF), which was proved to have a linear relationship with the coating thickness, were extracted from the phase spectrum of transient thermal response to quantitatively characterize the micrometer thickness of RTV coating. Particularly, the frequency aliasing phenomenon in small thickness measurement when the sampling frequency is not high enough was reported. And the genetic algorithm was adopted to implement the transient thermal response fitting to overcome the distortion of the phase spectrum induced by the frequency aliasing to obtain the accurate fitted linear calibration line. It has been demonstrated that AIT would have promising application prospects in industrial-quality inspection of RTV coating.

Transient thermal response with nonlocal radiation of a blast furnace main trough

A mathematical model for the transient thermal behaviour of the main trough of a blast furnace (BF) is presented. The proposed model consists of the transient heat equation with mixed radiation-convection boundary conditions to model the cooling process. The heat equation is coupled with an integral equation posed on the inner boundary, which models the radiative heat exchange on the internal cavity formed by the trough and the refractory cover placed over the trough. The main scope of this work is to address the evolution of the temperature field during a full BF tapping. A reliable algorithm, capable of simulating entire trough campaigns, is presented.The open-source computing platform FEniCS is used to numerically solve the model using a finite element method. A manufactured solution test for the heat diffusion coupled with 2D nonlocal radiation is defined with the purpose of verifying the implementation, comparing the performance of different time discretization schemes and the adaptive time stepping algorithm. Concerning the BF tapping problem, the results show that during the time interval corresponding to a single tapping, the temperature in the radiation enclosure swiftly reaches the steady state value. Nevertheless, to obtain a steady state in the bulk of the solids, much longer time scales are needed due to the large thermal inertia of the structure.

Thermal and hydraulic analysis of ultra-thin vapor chamber with copper columns considering Marangoni effect

The ultra-thin vapor chamber (UTVC) technology offers an attractive approach for thermal management of micro devices due to tiny volume, flat contact area and high temperature uniformity. In this paper, a 3D transient numerical model of UTVC is established to study the performance of the UTVC. The numerical method takes into account the variation of saturation pressure with saturation temperature. The Marangoni effect on the evaporation process is also discussed in wick microstructure. The modified boundary conditions are proposed to couple Marangoni effect on the liquid flow and pressure drop. According to the modified numerical method, the transient characteristics of UTVC are analyzed. The numerical results have excellent agreement with the experimental results and the maximum relative error is less than 1%. The numerical analysis result shows that the UTVC has rapid transient thermal response time and the time for UTVC to reach the steady state was less than 60 s at three heat loads. The pressure drop of UTVC is mainly in the vapor region, but the pressure drop of liquid increases by 12% after considering Marangoni effect. Comparing different copper column models, the increase of rows can increase the overall pressure drop of UTVC; but it can also improve the temperature uniformity of heating area and slightly reduce the overall thermal resistance before capillary limit is reached. The shape of copper column has little effect on the thermal performance of UTVC, while the influence of pressure drop is circular < diamond < square. These results provide valuable suggestions in design of copper columns to decrease pressure drop and improve thermal performance of UTVC.

Legitimacy of the Local Thermal Equilibrium Hypothesis in Porous Media: A Comprehensive Review

Local thermal equilibrium (LTE) is a frequently-employed hypothesis when analysing convection heat transfer in porous media. However, investigation of the non-equilibrium phenomenon exhibits that such hypothesis is typically not true for many circumstances such as rapid cooling or heating, and in industrial applications involving immediate transient thermal response, leading to a lack of local thermal equilibrium (LTE). Therefore, for the sake of appropriately conduct the technological process, it has become necessary to examine the validity of the LTE assumption before deciding which energy model should be used. Indeed, the legitimacy of the LTE hypothesis has been widely investigated in different applications and different modes of heat transfer, and many criteria have been developed. This paper summarises the studies that investigated this hypothesis in forced, free, and mixed convection, and presents the appropriate circumstances that can make the LTE hypothesis to be valid. For example, in forced convection, the literature shows that this hypothesis is valid for lower Darcy number, lower Reynolds number, lower Prandtl number, and/or lower solid phase thermal conductivity; however, it becomes invalid for higher effective fluid thermal conductivity and/or lower interstitial heat transfer coefficient.