Transient Thermal Conditions Research Articles

On the significance of short-duration regional metamorphism

Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in metamorphic geology. In this review, it is defined as metamorphism on time scales that limit length scales (of the associated thermal anomaly) to significantly less than the thickness of the orogenic crust (<10 myr) or subducted oceanic lithosphere (<5 myr). Without appealing to exceptional heat sources, thermal models have been unable to account for peak metamorphic temperature during collisional orogenesis and subduction. This observation, combined with restricted time scales for regional metamorphism, suggests that metamorphic facies series can record atypical and transient thermal conditions (related to punctuated and localized heat advection and/or production), rather than normal, ambient conditions for the tectonic setting to which they are allied. High-precision geochronology can resolve short-duration metamorphic estimates of 1 – 10 myr. However, diffusion geospeedometry typically yields extremely short metamorphic durations (<1 myr); tools in metamorphic geology may have matured to the point that the discipline is beginning to recognize episodicity and criticality in deep processes. New, very high-precision petrochronology techniques offer great potential to probe the veracity of extremely short metamorphic durations being obtained from diffusion geospeedometry. Benchmarking of these new very high-precision petrochronology techniques must become a priority for metamorphic geology.

Thermal Stresses in Adhesively Bonded Joints/Patches and Their Modeling: A Critical Review

The adhesive bonding technique is widely used for joining similar or dissimilar materials in various engineering areas. Mismatches in the thermal expansion coefficients of adhesive and adherend materials in the vicinity of bi-material interfaces result in incompatible thermal strains and discontinuous thermal stress distributions along the bi-material interfaces. During the adhesive curing process, thermal residual stresses are induced due to the chemical and physical changes in adhesive material. The expansion of the adhesive with changes in moisture and temperature levels is the reason for hygroscopic residual stresses. Non-uniform temperature or material composition distributions also result in non-uniform thermal strain and stress distributions. Loading with a relatively high frequency cyclic component may result in a temperature rise due to internal heat generation and consideration of viscoelastic time-temperature effects may become necessary in the stress analysis even at moderately low temperatures. The crack nucleation, initiation and propagation around bi-material notches, and the fatigue crack growth in adhesively bonded integrated structures are important design considerations under cycling thermal loads. Therefore, the thermal strain and stress states of the adhesive joints should be analysed in order to predict the joint strength accurately and to improve joint service life. This article reviews from simple to more complicated mathematical models to analyse the thermal stress and deformation states of various adhesive joints under both steady state and transient thermal conditions.

Improvement of the Stolwijk model withregard to clothing, thermal sensation and skin temperature.

The original Stolwijk model is not equipped with clothing, thermal sensation, comfort indices, individual characteristics and performance loss models. This study attempts to modify the model to include clothing, thermal sensation as well as the calculation of the percentage of dissatisfied as a result of general discomfort. The model is useful for the evaluation of thermal comfort in the built environment by professionals. Methods described in literature with regard of clothing, the research of Fiala as well as some in the literature recommended and validated adjustments, to improve the simulation of the skin temperature per body segment, are implemented in the here assembled Stolwijk computer model. Finally, for verification of the above adjustments, the model was compared with experiments conducted in the field of thermal sensation at various levels of temperature change. By improving the simulation of the skin temperature per body segment and by adding clothing and thermal sensation, suitable for the assessment of steady state and transient thermal conditions, and fixed with this the percentage of dissatisfied, the scope of the Stolwijk model has become larger than it was before. On the basis of the calculations and the experimental results, it was concluded that the adjusted Stolwijk model was suitable for the simulation of the thermal sensation under steady state and transient thermal conditions.

Thermoregulation in premature infants: A mathematical model

PurposeIn 2010, approximately 14.9 million babies (11.1%) were born preterm. Because preterm infants suffer from an immature thermoregulatory system they have difficulty maintaining their core body temperature at a constant level. Therefore, it is essential to maintain their temperature at, ideally, around 37°C. For this, mathematical models can provide detailed insight into heat transfer processes and body-environment interactions for clinical applications. MethodsA new multi-node mathematical model of the thermoregulatory system of newborn infants is presented. It comprises seven compartments, one spherical and six cylindrical, which represent the head, thorax, abdomen, arms and legs, respectively. The model is customizable, i.e. it meets individual characteristics of the neonate (e.g. gestational age, postnatal age, weight and length) which play an important role in heat transfer mechanisms. The model was validated during thermal neutrality and in a transient thermal environment. ResultsDuring thermal neutrality the model accurately predicted skin and core temperatures. The difference in mean core temperature between measurements and simulations averaged 0.25±0.21°C and that of skin temperature averaged 0.36±0.36°C. During transient thermal conditions, our approach simulated the thermoregulatory dynamics/responses. Here, for all infants, the mean absolute error between core temperatures averaged 0.12±0.11°C and that of skin temperatures hovered around 0.30°C. ConclusionsThe mathematical model appears able to predict core and skin temperatures during thermal neutrality and in case of a transient thermal conditions.

Thermal performance of two heat exchangers for thermoelectric generators

The thermal performance of a heat exchanger is important for the potential application in an integrated solar cell/module and thermoelectric generator (TEG) system. Usually, the thermal performance of a heat exchanger for TEGs is analysed by using 1D heat conduction theory which ignores the detailed phenomena associated with thermo-hydraulics. In this paper, thermal and momentum transports in two different heat exchangers are simulated by means of a steady-state, 3D turbulent flow k-ε model with a heat conduction module under various flow rates. In order to simulate the actual working conditions of the heat exchangers, a hot block with an electric heater is included in the model. The TEG module is simplified by using a 1D heat conduction theory, so its thermal performance is equivalent to a real TEG. Natural convection effects on the outside surfaces of the computational domains are considered. Computational models and methods used are validated under transient thermal and electrical experimental conditions of a TEG. The two heat exchangers designed in this paper have better thermal performance than an existing heat exchanger for TEGs. More importantly, the fin heat exchanger is more compact and efficient than the tube heat exchanger.

Development of thermal discernment among visitors: Results from a field study in the Hermitage Amsterdam

Building energy and occupant health concerns have increased the desire for variable, dynamic indoors and hence the interest in comfort of non-uniform and/or transient thermal conditions. An extended thermal comfort field study in the Hermitage Amsterdam museum afforded a unique opportunity to analyse evolving subjective perception of occupants, upon their moving indoors, over the time they spent in the museum. Visitors’ responses were grouped depending on how long they had been inside when they filled up the survey. The mean thermal sensation vote of each time group bore a strong correlation with their average time duration. For visitors who had been inside for 20 min or less, the thermal sensation vote had a significant relation with the outdoor temperature but not the indoor temperature. As visitors spent longer indoors, percentage of them feeling warm decreased and percentage of neutral or cool feeling increased. In tandem, the percentage of visitors preferring to be warmer also increased with time. Gender based differences in thermal sensation and preference also had a gradual and logical evolution with time. In an evidence of alliesthesial response, all the visitors inside for 20 min or less, accepted their thermal environment. The overall evidence suggests that visitor’s subjective perception of the thermal environment undergoes a distinct evolution during their first hour indoors.

Synchrophasor based thermal overhead line monitoring considering line spans and thermal transients

A synchrophasor-based thermal line monitoring scheme enabling the measurement of line spans and therewith temperature profiles is presented. Further, the measurement of thermal transients with synchrophasors has become feasible by incorporating non-steady state heat equations and local weather data. The proof of concept is executed with an electro-thermal line modelling approach in combination with sensitivity analyses of various power flow scenarios, data processing time steps and weather updating rates. It has been shown, that the relative measurement error of this method is ±0.2% at ideal thermal static and ±2.0% at ideal thermal transient conditions for various load currents with reference to the original temperature. Finally, the accuracy at highly volatile weather conditions as well as the suitability for practical implementation is discussed.

A two-dimensional model for calculating heat transfer in the human body in a transient and non-uniform thermal environment

A thermal model for the human body is used to assess the temperature of the body and a person’s thermal comfort level. Most models available in the literature were developed for a uniform thermal environment and have limited applications. This study developed a 12-segment model for transient and non-uniform surrounding conditions by considering two-dimensional heat transfer in each segment of a human body. This heat transfer included convection, radiation, and evaporation on bare skin and skin covered with clothing. The model allows non-uniform clothing insulation across different body segments. The heat transfer between two body segments was estimated from blood circulation through counter-current heat exchange. This study evaluated the model’s performance for subjects with and without clothing under a wide range of transient and non-uniform thermal environmental conditions. Good agreement was observed between the measured and calculated skin and rectal temperatures, although there were small discrepancies. The two-dimensional model developed in this study is a step forward in predicting thermal comfort under transient and non-uniform environmental conditions.

Thermal sensations and comfort investigations in transient conditions in tropical office

The study was done to identify affective and sensory responses observed as a result of hysteresis effects in transient thermal conditions consisting of warm-neutral and neutral - warm performed in a quasi-experiment setting. Air-conditioned building interiors in hot-humid areas have resulted in thermal discomfort and health risks for people moving into and out of buildings. Reports have shown that the instantaneous change in air temperature can cause abrupt thermoregulation responses. Thermal sensation vote (TSV) and thermal comfort vote (TCV) assessments as a consequence of moving through spaces with distinct thermal conditions were conducted in an existing single-story office in a hot-humid microclimate, maintained at an air temperature 24 °C (±0.5), relative humidity 51% (±7), air velocity 0.5 m/s (±0.5), and mean radiant temperature (MRT) 26.6 °C (±1.2). The measured office is connected to a veranda that showed the following semi-outdoor temperatures: air temperature 35 °C (±2.1), relative humidity 43% (±7), air velocity 0.4 m/s (±0.4), and MRT 36.4 °C (±2.9). Subjective assessments from 36 college-aged participants consisting of thermal sensations, preferences and comfort votes were correlated against a steady state predicted mean vote (PMV) model. Local skin temperatures on the forehead and dorsal left hand were included to observe physiological responses due to thermal transition. TSV for veranda–office transition showed that no significant means difference with TSV office-veranda transition were found. However, TCV collected from warm-neutral (−0.24, ±1.2) and neutral-warm (−0.72, ±1.3) conditions revealed statistically significant mean differences (p < 0.05). Sensory and affective responses as a consequence of thermal transition after travel from warm-neutral-warm conditions did not replicate the hysteresis effects of brief, slightly cool, thermal sensations found in previous laboratory experiments. These findings also indicate that PMV is an acceptable alternative to predict thermal sensation immediately after a down-step thermal transition (≤1 min exposure duration) for people living in a hot-humid climate country.

Design of welded joint based on transient thermal condition

Design of welded joint based on transient thermal condition

Modelling transient ground surface temperatures of past rockfall events: towards a better understanding of failure mechanisms in changing periglacial environments

Despite the rising interest in mountain permafrost due to climatic changes and a noticed increase of registered rockfall events in the European Alps and other mountain ranges, little is known about transient thermal conditions in the detachment areas of rockfalls. Temperature conditions prior to the rockfall events of 144 past events in the European Alps were modelled with a physically based ground temperature model. To minimise the impact that uncertainty has on interpretations, only relative values were used, that is, percentiles obtained from cumulative distribution functions of the modelled ground surface temperatures from the beginning of the meteorological measurement series up to the event dates. Our results suggest that small and mid‐sized rockfalls (volumes up to 100 000 m3) from high elevation occurred mainly during short‐term periods of unusually high temperatures. This was neither found to be a result of the seasonal distribution (most analysed events in higher elevations occurred from July to September) nor of the longer‐term temporal distribution (most analysed events occurred after 2000) only. Plausible explanations are either a destabilisation related to advective thaw or failure due to stress redistribution caused by large temperature variations. Large deep‐seated rock slope failures (≥100 000 m3) in high elevation occurred all year round.

Performance of straight tungsten mono-block test mock-ups using new high heat flux test facility at IPR

Paper describes the newly established high heat flux test facility (HHFTF) at IPR in India. The facility is being used to simulate typical steady state and transient thermal load conditions for testing Divertor plasma facing components using a 200kW Electron Beam as heat source. After successful installation and commissioning of the HHFTF, two straight tungsten mono-block type test mock-ups are tested for several hundred thermal cycles with absorbed heat flux up to 11MW/m2 (corresponding incident heat flux 20MW/m2). Maximum temperature on tungsten surface observed using a two-color pyrometer is found to be 1150°C for absorbed heat flux up to 11MW/m2 (corresponding incident heat flux 20MW/m2). Present paper gives a brief introduction to the HHFTF followed by experimental details and some results of high heat flux tests were performed on two straight tungsten mono-blocks.

Transient Thermoelectroelastic Response of a Functionally Graded Piezoelectric Material Strip with Two Parallel Cracks in Arbitrary Positions

In this article, the problem of two parallel cracks in arbitrary positions of a functionally graded piezoelectric material (FGPM) strip is analyzed under transient thermal loading conditions. It is assumed that the thermoelectroelastic properties of the strip vary continuously along the thickness of the strip, and that the crack faces are supposed to be insulated thermally and electrically. By using both the Laplace transform and the Fourier transform, the thermal and electromechanical problems are reduced to two systems of singular integral equations. The singular integral equations are solved numerically, and a numerical method is then employed to obtain the time dependent solutions by way of a Laplace inversion technique. The intensity factors versus time for various geometric and material parameters are calculated and presented in graphical forms. Temperature change, the stress and electric displacement distributions in a transient state are also included.

Semi-Analytical Solution of Functionally Graded Circular Short Hollow Cylinder Subject to Transient Thermal Loading

Abstract In this paper, by using a semi-analytical solution based on multi-layered approach, the authors present the solutions of temperature, displacements, and transient thermal stresses in functionally graded circular hollow cylinders subjected to transient thermal boundary conditions. The cylinder has finite length and is subjected to axisymmetric thermal loads. It is assumed that the functionally graded circular hollow cylinder is composed of N fictitious layers and the properties of each layer are assumed to be homogeneous and isotropic. Time variations of the temperature, displacements, and stresses are obtained by employing series solving method for ordinary differential equation, Laplace transform techniques and a numerical Laplace inversion.

Modeling of morphological evolution of columnar dendritic grains in the molten pool of gas tungsten arc welding

A macro–micro coupled model for epitaxial nucleation and the subsequent competitive dendrite growth was developed to study the morphological evolution of both dendrite and grain structures in molten pool of the gas tungsten arc welding (GTAW) for Fe–C alloy. The simulation of heat and mass transfer in molten pool was conducted by the three-dimensional finite element (FE) model to obtain the transient solidification conditions. The process of epitaxial nucleation and the competitive dendrite growth was simulated by a two-dimensional cellular automata (CA) model. The size and random preferential orientations of substrate grains were considered in this model. The transient thermal conditions used in the CA model were obtained from the results of FE model through the interpolation method. In addition, the effects of the substrate grain size and the welding speed on the morphologies of both dendrite and grain structures were investigated. The simulated results indicate that dendrites with the preferential orientations parallel to the direction of the highest temperature gradient are more competitive during the competitive dendrite growth, and the morphology of resulting columnar grains is determined by the competition between different dendritic arrays. Under the same welding conditions, with the increase of substrate grain size, the average width of resulting columnar grains becomes larger, and the characteristics of dendrite structure within the columnar grains do not change obviously. Without considering the new nucleation in the melt, with the increase of welding speed, the dendrite structure in weld seam becomes much finer, and the average columnar grain width within the calculation domain of the CA model does not change obviously. The trend of the simulated results of dendrite arm spacing under various welding conditions are consistent with the analytical and experimental data.

A Note on the Normalized Approach to Simulating Moisture Diffusion in a Multimaterial System Under Transient Thermal Conditions Using ansys 14 and 14.5

Moisture can have significant effects on the performance and reliability of electronic components. Accurately simulating moisture diffusion is important for designers and manufacturers to obtain a realistic reliability evaluation. Beginning with version 14, ansys is capable of simulating diffusion and related behaviors, such as hygroscopic swelling, with newly developed elements. However, a normalized approach is still required to deal with the discontinuity of concentrations at the material boundaries, and normalization of the moisture concentration in transient thermal conditions is tricky. Case studies have shown that normalizing the moisture concentration with respect to a time- or temperature-dependent material property will lead to erroneous results. This paper re-addresses the issues of performing diffusion simulations under transient thermal conditions and more general anisothermal conditions (temporally and spatially), and suggests an easy-to-use approach to cope with the limitations of the current version for users in the electronic packaging industry.

Dynamic Thermal Receptor Response and Comfort in Cold and Warm Environments

IntroductionAthletes are permanently exposed to or in the state of transient thermal conditions, because of their own varying metabolic output or environmental factors such as temperature, humidity, wind velocity and radiation. This causes thermal stress, discomfort and even inhibits performance. The human body constantly regulates its internal temperature receiving neuronal signals from thermal receptors in the skin. Both warm and cold receptors have static and dynamic responses to thermal transitions in the environment. The latter occurs at sudden thermal changes that lead to peak the receptors firing rate. It is well known how humans thermally perceive transient thermal conditions due to starting temperature, temperature gradient and the size of the skin surface area. Not known is their comfort response to quick environmental changes. Therefore this study investigates local comfort response of sudden thermal changes, whether they can improve thermal comfort and imply useful applications for hot or cold environments in sports. MethodologyEight healthy, male subjects were exposed to three warm and cold environments in standardized clothing. After 20minutes of acclimatization nine body parts were locally treated with cool and warm convection to trigger peak receptor responses (“overshoot”). Local skin temperatures were measured and subjects were asked about their thermal comfort and perception. ResultsResults show highly significant global comfort improvement by local overshoot in both environments (p=.00, α= .01) at every body part. Furthermore results show tendencies for major impact on comfort of core body parts. ConclusionTo sum up this research found the possibility for temporal local treatment as highly efficient for significant comfort improvement. For instance local heating via technical textiles can improve comfort in harsh skiing conditions or local cooling can do the same in extreme heat. Nonetheless further research is needed to quantify physiological impacts of temporal local treatments for further implications e.g. on sports performance as well as possibilities of technical realizations.

FEM Study of Fatigue Crack Growth in a Power Semiconductor Chip Subjected to Transient Thermal Loading

A detailed finite element analysis of fatigue crack growth between metallisation layers of a power semiconductor device subjected to active cycling conditions is carried out. The active cycling and the resulting transient thermal loading is the source of the thermally-induced cyclic stresses in the microelectronic device, which may cause the fatigue failure. To model the fatigue crack formation and propagation under the transient thermal loading conditions, a coupled thermomechanical cyclic cohesive zone model based on an irreversible damage evolution equation is utilised. The contact formulation of the implemented cohesive zone model allows to consider evolving tractions and heat flux across the cohesive zone both in open and closed crack states. To accelerate simulations, a cycle jump technique without the need of damage extrapolation is utilised.

Multiscale Transient Thermal, Hydraulic, and Mechanical Analysis Methodology of a Printed Circuit Heat Exchanger Using an Effective Porous Media Approach

Printed circuit heat exchangers (PCHE) and the similar formed plate heat exchangers (FPHE) offer highly attractive economics due to their higher power densities when compared to more conventional shell-and-tube designs. However, their complex geometry makes them more vulnerable to damage from thermal stresses during transient thermal hydraulic conditions. Transient stresses far exceed those predicted from steady state analyses. Therefore, a transient, hydraulic, thermal, and structural analysis is needed to accurately simulate and design high performing PCHE. The overall length of the heat exchanger can be thousands of times larger than the characteristic length for the heat transfer and fluid flow. Furthermore, simulating the thermal hydraulics of the entire heat exchanger plate is very time consuming and computationally expensive. The proposed methodology mitigates this by using a multiscale analysis with local volume averaged (LVA) properties and a novel effective porous media (EPM) approach. This method is implemented in a new computer code named the compact heat exchanger explicit thermal and hydraulics (CHEETAH) code which solves the time-dependent, mass, momentum, and energy equations for the entire PCHE plate as well as hot and cold fluid streams using finite volume analysis (FVA). The potential of the method and code is illustrated with an example problem for a Heatric-type helium gas-to-liquid salt PCHE with offset strip fins (OSF). Given initial and boundary conditions, CHEETAH computes and plots transient temperature and flow data. A specially developed grid mapping code transfers temperature arrays onto adapted structural meshes generated with commercial FEA software. For the conditions studied, a multiscale stress analysis reveals mechanical vulnerabilities in the HX design. This integrated methodology using an EPM approach enables multiscale PCHE simulation. The results provide the basis for design improvements which can minimize flow losses while enhancing flow uniformity, thermal effectiveness, and mechanical strength.

Transient Thermoelectroelastic Response of a Functionally Graded Piezoelectric Strip with Two Parallel Axisymmetric Cracks

In this article, the problem of two parallel axisymmetric cracks in a plate of a functionally graded piezoelectric material (FGPM) strip is analyzed under transient thermal loading conditions. It is assumed that the thermoelectroelastic properties of the strip vary continuously along the thickness of the strip, and that the crack faces are supposed to be insulated thermally and electrically. By using both the Laplace and Hankel transforms, the thermal and electromechanical problems are reduced to two systems of singular integral equations. The singular integral equations are solved numerically, and a numerical method is then employed to obtain the time dependent solutions by way of a Laplace inversion technique. Systematic numerical calculations are carried out, and the field intensity factors versus time are presented for various values of dimensionless parameters representing the crack geometry and the material non-homogeneity.