Time-dependent Temperature Distribution Research Articles

Experimental and numerical simulation studies of electrical resistance of cementitious materials under varying temperature history

Many in-service concrete structures are normally exposed to varying temperature condition. The time-dependent and non-uniform temperature distribution in concrete structures could affect the accurate assessment of their electrical properties. Therefore, the influences of temperature change over time and the spatial variation in temperature on the measured electrical resistance of cementitious materials are still in need of investigation. The coupled thermal-electrical responses of two types of cementitious materials under varying temperature history were investigated by a series of thermal and electrical tests in this study. Then a coupled thermal-electrical finite element (FE) model was developed to simulate the coupled thermal-electrical responses. The results show that the input direct current has a minor effect on the measured electrical resistance by embedded four-probe method and the specimen temperature when it is no greater than 3 mA. The relationship between the measured temperature at 10-mm depth of the specimen and the apparent resistance is well captured by a linear equation. The coupled thermal-electrical finite element model can predict well the developments of specimen temperature and apparent electrical resistance under varying temperature history. The FE results reveal that the non-uniform temperature variation results in the redistribution of electrical current within the tested specimen. However, electrical field between the two inner electrodes is always uniform when the embedded four-probe method is used, which is not affected by the non-uniform temperature variation.

Thermal boundary conditions for heat transfer analysis of bridges considering non-uniform distribution of internal air temperature by computational fluid dynamics

Heat transfer analysis has been used to calculate the temperature distribution in bridges. Thermal boundary conditions play a critical role in this analysis. However, existing studies on thermal boundary conditions simplify the air temperature inside the bridge deck as uniform, which is not realistic and thus causes inaccurate simulation results. This study proposes a new approach to thermal boundary conditions in the heat transfer analysis of bridges. For the first time, computational fluid dynamics is used to calculate non-uniform air temperatures inside the bridge deck. In addition, non-approximate heat exchange equations for long-wave radiation are also incorporated into the approach. The techniques are applied to the 1377-m main span Tsing Ma Suspension Bridge to calculate the internal air temperatures of a deck segment. Transient heat transfer analysis is then conducted to calculate the time-dependent temperature distribution of the segment. As compared with the field monitoring results, the proposed approach can simulate the temperature distribution of the bridge with an average discrepancy of 0.88 °C and is more accurately than other existing approaches.

Analysis of Earth-Moving Machinery Cabin Windshield Defrosting Performance with Computational Fluid Dynamics Method and Verification by Testing

Driver and passenger safety and thermal comfort are seen as the most important parameters in the development of earth-moving machinery cabin air conditioning systems. The formation of a layer of ice and fog on the windshield restricts the driver's field of vision and distracts driving attention. For this reason, the earth-moving machinery air conditioning system has an important place. The ability of the defrosting and defogging system is measured by how quickly and effectively the ice layer formed on the glass is dissolved and to what extent the mist formed inside the glass is cleared. In this study, three-dimensional air conditioning numerical analysis was carried out using the Computational Fluid Dynamics (CFD) method in order to remove ice formed on the windshield of the earth-moving machinery cabin. The earth-moving machinery cabin was used as a mathematical model. Boundary and initial conditions were chosen to be the same as the test data to ensure appropriate validation. By solving the numerical analysis energy equation, time-dependent temperature distribution and defrosting time in the windshield area were analysed. Validation was provided by comparing the results with test data. As a result of the analysis, it was understood that the windshield defrosting time, which is of vital importance for driving safety due to visibility restrictions, especially in regions with cold climates, can be optimized by analysing it with numerical analyses.

Experimental study of a latent heat thermal energy storage system using erythritol for medium temperature applications

Using solar energy and waste heat for medium-temperature thermal applications depends on efficient and economical heat storage development. Developing a compact thermal energy storage system is essential to use excess thermal energy from a source in a process or to shift the utilisation time of solar thermal energy or recovered waste heat. Phase change materials are commonly used for energy storage applications globally. In this work, a latent heat thermal energy storage (LHTES) system with 2.37 kg of Erythritol as the phase change material (PCM) has been studied. A shell and tube heat storage system is selected due to its effectiveness in heat transfer. Synthetic silicone oil Hi-Tech Therm-60 was used as heat transfer fluid. Thermal stability tests in a differential scanning calorimeter (DSC) for a small sample mass and a specifically designed experiment for a larger sample mass were performed to assess the stability of the PCM. The time-dependent temperature distribution inside the storage system and the effect of heat transfer fluid (HTF) inlet temperature on the charging behaviour were studied during the charging process. Charging time and melt fraction have been calculated. No significant change in melting temperature and a 10% reduction in latent heat of PCM was observed in the thermal stability test for a small sample mass in DSC. The charging time of the storage device was observed to be 91–97 min and 195 min for 140 °C and 130 °C HTF inlet temperatures, respectively. The total stored energy in the system was 922 kJ with a Stefan number of 0.59. Temperature variation during the charging process inside the storage also suggests a buoyancy effect, leading to increased natural convection during the melting process.

Ultrasonic measurement of temperature distributions in extreme environments: Electrical power plants testing in utility-scale steam generators

Thermal heterogeneities within energy conversion and storage, material processing, nuclear processes, aerospace, and military applications are often inaccessible to characterization by insertion sensors. When sensor deployment is possible, conventional pointwise temperature probes quickly degrade when inserted into harsh environments typical of such processes. We developed spatially-resolved ultrasonic thermometry to noninvasively measure the spatial distributions of thermal properties in such applications, even when sizable thermal gradients are present. Our method divides the path of ultrasonic propagation into segments bound by echogenic features, which create echoes in pulse-echo mode, encoding the information about interior temperature distributions. We use the acquired ultrasonic responses to estimate the internal temperature distributions by solving an inverse problem or concatenating segmental estimates. This work describes the implementation and industrial testing of the developed method at a coal-fired electrical power generation plant. We inserted an echogenically segmented Inconel 625 waveguide into the combustion zone of the utility-scale boiler and continuously acquired ultrasonic data while keeping sensitive components away from the damaging combustion environment. The accuracy of the time-dependent temperature distributions reconstructed from the ultrasonic measurements was comparable to that of thermocouples. The resiliency of ultrasonic thermometry to harsh combustion conditions was far superior to conventional insertion sensors. The measurements obtained during plant operation captured daily steam generation cycles in response to changing customer demand and intermittent contributions of renewable power sources to the power grid. These measurements have revealed new insights into the relationship between the dynamic power generation load and the conditions inside the steam generator. The successful industrial testing of spatially-resolved ultrasonic thermometry in solids indicates that the developed technology has matured to become an attractive alternative to conventional sensing in solving challenging problems of long-term thermal characterizations in extreme environments.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Parametric numerical study of dislocation density distribution in Czochralski-grown germanium crystals

Dislocations strongly affect the quality of Czochralski-grown germanium crystals used for various applications, however, the influence of different growth parameters on the dislocation distribution has not been extensively studied in the literature. In the present work, a simplified parametric crystal growth model is introduced which is suitable for large-scale numerical studies. The time-dependent temperature and dislocation density distributions in the crystal are calculated and analyzed. The Alexander–Haasen model is applied for dislocation modeling. Model parameters are taken from previously published discrete dislocation dynamics simulations, establishing a novel coupling between mesoscopic and macroscopic simulations. A W-shaped radial distribution of the dislocation density is obtained in the simulations. It is shown that the effective ambient temperature and crystallization front deflection have the strongest influence on the dislocation distribution. The results are compared to the experiment, showing a qualitative agreement, but also several deviations. Limitations of the considered dislocation modeling approach and the simplified thermal model are discussed.

Thermal arching and interfacial damage evolution of CRTS-II slab track under solar radiation in alpine and plateau regions

Arch deformation and interface failure of CRTS-II slab track have become crucial issues for the operation safety of high-speed railways. However, few researchers investigated the mechanics and deformation behaviour on CRTS-II slab track exposed to high-altitude plateau climatic environments. This study proposes an analysis approach for thermal arching and interface damage evolution of the slab track structure under solar radiation in alpine and plateau environments. The time-dependent solar temperature distribution is loaded on the refined finite element model (FEM) of the slab track structure’s thermal deformation in high-altitude plateau climatic environments. The analysis approach is validated by comparing the calculated and experimental data. The track slab's maximum and minimum vertical temperature gradient (VTG) are 94.49 °C/m and −45.02 °C/m, respectively. The narrow joint damage is an incentive for the inverted V-shaped thermal arching and interface separation of the slab track structure under solar radiation. The interfacial gap of slab track structure in alpine and plateau regions reaches more than 14 mm during summer. The proposed analysis approach is a valuable reference to the construction temperature, maintenance, and damage monitoring of slab track structure in alpine and plateau regions.

Effect of fin width ratio on thermal performance of fin based-heat sink with phase change materials

The rapid development of electronics, coupled with need of miniaturization in the design and manufacture, makes heat dissipation, which is critical for the performance, lifetime, and reliability of the components extremely challenging. To achieve higher heat dissipation rates, novel fin geometry with variable cross-section considering melting behaviour in medium is presented for PCM-based heat sink beyond previous efforts in literature. In this experimental study, effect of different variable cross-section, which refer different fin width ratio (s = 1, 0.5, 0.2 and 0) values are performed on the thermal performance of plate fin-based heat sink with PCM to enhance melting heat transfer. Decreasing fin width ratios point out increasing heat transfer surface area downwardly. N-eicosane is used. The constant input power is provided at the heat sink base. Thermal performance of novel fin design is evaluated by time-dependent temperature distributions, solid–liquid interface tracking photographs, reliable operating time, enhancement ratio, and thermal conductance. The results reveal that the proposed plate-fin geometry has a significant effect on the cooling performance of PCM-based heat sink. With the decrease of fin width ratio, higher temperature profile with more uniformity of 52% in the PCM medium, and so lower temperature distribution of 4.5% on the heat sink base are provided. Reliable operating time and thermal conductance values go up 15 % and 38%, respectively, for proposed fin geometry. The improvement on the thermal performance is achieved by the form of the plate-fin, which has increased surface area downward. This provides a great contribution to the conduction and convection heat transfer mechanisms at the lower half of the enclosure. Further, in this study, nearly the same reliable operating time is figured out with proposed fin geometry and fin number of s = 0 compared to higher fin number installation of s = 1 previously reported.

Decrypting the mechanisms of wicking and evaporation heat transfer on micro-pillars during the pool boiling of water using high-resolution infrared thermometry

Surfaces with micrometer-scale pillars have shown great potential in delaying the boiling crisis and enhancing the critical heat flux (CHF). However, physical mechanisms enabling this enhancement remain unclear. This knowledge gap is due to a lack of diagnostics that allow elucidating how micro-pillars affect thermal transport phenomena on the engineered surface. In this study, for the first time, we are able to measure time-dependent temperature and heat flux distributions on a boiling surface with engineered micro-pillars using infrared thermometry. Using these data, we reveal the presence of an intra-pillar liquid layer, created by the nucleation of bubbles and partially refilled by capillary effects. However, contrarily to conventional wisdom, the energy removed by the evaporation of this liquid cannot explain the observed CHF enhancement. Yet, predicting its dry out is the key to delaying the boiling crisis. We achieve this goal using simple analytic models and demonstrate that this process is driven by conduction effects in the boiling substrates and, importantly, in the intra-pillar liquid layer itself. Importantly, these effects also control the wicking flow rate and its penetration length. The boiling crisis occurs when, by coalescing, the size of the intra-pillar liquid layer becomes too large for the wicking flow to reach its innermost region. Our study reveals and quantifies unidentified physical aspects, key to the performance optimization of boiling surfaces for cooling applications.

Reconstruction of the Thermal Load of a Functionally Graded Hollow Sphere by Surface Displacements

We formulate and solve a problem of reconstruction of the unknown time-dependent temperature distribution on one boundary surface of a functionally graded hollow sphere according to the temperature and radial displacements on the other boundary surface. We suggest a procedure reducing the formulated problem to the inverse thermoelasticity problem. By applying the finite-difference method, we construct a numerical algorithm aimed at solving the obtained inverse problem. By using the solution of the direct thermoelasticity problem, we analyze the stability of the obtained solution of the inverse problem and the distributions of displacements and stresses obtained on the basis of this solution against the errors of the input data.

Air dehumidification performance study of a desiccant wheel by a three-dimensional mathematical model

Hitherto air-conditioners have severe shortcomings because of the inherent need to maintain the cooling coil temperature significantly below the air's dew point. Their energy efficiency can be improved by employing desiccant wheel dehumidifiers that offer independent humidity control. Although numerous experimental and theoretical studies have evaluated desiccant wheels’ performance, no existing investigation yields a detailed account of its localized temperature and humidity distributions. Therefore, a new three-dimensional mathematical model was developed to study the time-dependent and steady-state temperature, humidity, and reaction distributions. The model predicted the performance with the maximum discrepancy of ±10% in both temperature and humidity ratio. It also revealed that the countercurrent airflow configuration has a more uniform reaction distribution than the concurrent configuration. The parametric analysis using the model revealed that higher regeneration temperatures require smaller regeneration angles. With the regeneration temperatures ranging between 110 and 50 °C, the optimal regeneration angles span 120–180°. Additionally, the dehumidification performance could be easily improved by increasing the wheel's rotation speed at low desiccant mass since coating more desiccant on the wheel is costly and inconvenient.

Trefftz-Type Functions Applied for Subcooled Flow Boiling Heat Transfer Calculations in a Minichannel of Different Spatial Orientation

ABSTRACT The paper describes the results of subcooled flow boiling heat transfer in an asymmetrically heated group of five minichannels 1 mm deep. The heated element for the cooling fluid (FC-72) flowing in the minichannels was a foil made of Haynes-230 alloy. The time-dependent temperature distribution of the outer heated foil surface was recorded as a result of infrared thermography. The test section was oriented in several spatial orientations, that is, 30°, 45°, 60° and 75° inclinations to the horizontal plane. The objective of the numerical calculations, referring to the experimental results, was to determine the time–dependent heat transfer coefficient on the wall-fluid contact surface from the Robin boundary condition. Both the foil and the fluid temperatures were the result of solving the inverse and direct two-dimensional heat transfer problem in two neighboring domains: the heated foil and flowing fluid. The problem was solved using the Trefftz method and the FEM with Trefftz type basis functions. The heat transfer coefficient obtained by two numerical methods appears to be similar. During subcooled flow boiling, at higher minichannel inclination angles, higher values of the heated foil temperature were obtained, as well as higher values of the heat transfer coefficient.

Unmoderated molten salt reactors design optimisation for power stability

The critical region of unmoderated molten salt reactors consists in a cavity filled with a liquid fuel. The lack of internal structure implies a complex flow structure of the circulating fuel salt. A preliminary core shape optimization has been performed during the EVOL European project to limit recirculation and hotspots. This optimization was based on a Reynolds Averaged Navier Stokes (RANS) approach, but the latter only provides time-averaged values for velocity and temperature. However, the power stability is sensitive to thermal fluctuations induced by the flow turbulence itself, even at steady state without pump flow rate or heat extraction variation. This phenomenon is studied using a Detached Eddies Simulation approach to solve the turbulence in the reactor and get a time dependent temperature distribution and then the reactivity fluctuations. A new geometry is proposed to limit the total power fluctuations from 7.5% for the preconceptual EVOL geometry down to 1.2%.

Experimentally validated modeling of the temperature distribution and the distortion during the Fused Filament Fabrication process

This paper presents the numerical approach to compute process-induced temperature distribution and distortion in Polylactic Acid (PLA) parts manufactured using the Fused Filament Fabrication (FFF) process. Thermal simulations are performed first to compute the time-dependent temperature distribution which is then input as solution-independent boundary condition into a mechanical simulation to compute warpage of the manufactured part. The developed procedure used experimentally measured process parameters and was experimentally verified at each step of the simulation. The thermal step in the simulation workflow was experimentally validated using thermography yielding quantitative predictions for 25 out of 27 considered points with the highest accuracy of <1°C. To verify mechanical step, the computed process-induced distortions were compared against those measured with an optical 3D scanner. The developed procedure yielded qualitative and, in some cases, quantitative predictions of process-induced geometrical distortions.

The Impact of Conventional Power Block Startup Procedures on the Fatigue Behavior of Drum Materials

Varying demands for electricity within the energy sector require the modification of operating modes to ensure both an uninterrupted electricity supply and the upholding of safety standards. This paper presents a method for the prediction of drum fatigue life, via the local analysis of stress–strain fields and low-cycle fatigue tests of the drum material. The analysis compares the fatigue properties of an unused material and a material that has undergone many years of operation. In addition, this paper suggests drum operating conditions based on the results of load testing under industrial conditions. The drum model was developed on the basis of technical documentation. The analysis includes the calculations of time-dependent temperature distributions, stresses, and strains for various drum startup modes. The drum material fatigue properties are determined under low-cycle conditions. Using the modeling results and fatigue properties, predictions of drum life for different startup modes are presented. The paper summarizes the impact of a range of startup procedures and the drum material fatigue properties on the fatigue life of a working drum, under various mechanical and thermal loads. In addition, this paper proposes a methodological approach to fatigue life assessment, as a combination of multiple research methods.

Evaluation of residual stresses in additively produced thermoelastic cylinder. Part II. Residual stresses

The time-dependent evolution of self-stressed state in additively produced thermolastic cylinder during the additive process and after its completion is studied. The time-dependent temperature distribution is taken from the solution of heat evolutionary problem, derived in the first part of this work (Part I. Thermal fields). The representations for strains and stresses are obtained analytically in the framework of linear semi-coupled thermoelasticity (theory of thermal stresses). Based on the obtained solutions the numerical analysis for different scenarios of additive processes is carried out. It focuses on the influence of auxiliary high frequency inductive heating on the distributions for internal stresses and shape distortion during the processes and their residual counterparts. It is shown that the use of induction heating makes it possible to halve the maximum stress intensity. The calculations are carried out in the dimensionless form. The dimensionless parameters that are used correspond to Titanium.

Thermal Properties of Warm- versus Heated-Needle Acupuncture.

Background Warm-needle acupuncture (WA) and fire-needle acupuncture are treatment techniques that use the combination of acupuncture and thermal stimulation. In clinical practice, a new method of fire-needle acupuncture called “heated-needle acupuncture (HA)” has been proposed, wherein the needle is directly heated after insertion. WA and HA share similarities in their methods, and no previous study has sought to assess whether their thermal outcomes are also similar. Methods We controlled environmental variables and measured the maximum temperatures and temperature changes of a silicon phantom in which K-type thermocouples were embedded at depths of 0, 2, 5, 7, and 10 mm. WA and HA were also performed with acupuncture needles of various thicknesses (0.30 × 40 mm, 0.40 × 40 mm, and 0.50 × 40 mm). Results Different time-dependent temperature distributions were observed between the two acupuncture methods: HA yielded a higher maximum temperature and temperature change on the surface, whereas WA yielded higher temperatures at the other tested depths. The thermal patterns were similar among the needles of different thicknesses for each method, with the following exception: while the temperature change and maximum temperature did not differ significantly by needle thickness for WA, these parameters increased significantly with needle thickness for HA. Conclusion The two acupuncture procedures yielded different thermal patterns in a controlled environment. Further studies are necessary to reflect the effect of external environment variables occurring in reality.

The wall heat flux partitioning during the pool boiling of water on thin metallic foils

• We study nucleate boiling on substrates of small thermal capacity using IR camera. • Microlayer does not fully evaporate due to limited thermal capacity of the heater. • Microlayer evaporation and rewetting are less important compared to thick heaters. • Most energy is removed by convective effects created by bubble growth and departure. • We re-derived heat flux partitioning model to quantify each heat transfer mechanism. In this work, we studied the wall heat flux partitioning during the pool boiling of water on thin metallic surfaces. We conducted boiling experiments on surfaces where we engineered nucleation sites by nanosecond-fiber-laser texturing. These nucleation sites form triangular lattice patterns with different pitches. We measured the time-dependent temperature and heat flux distributions on the boiling surface using an infrared camera. We developed post-processing algorithms to measure, based on these distributions, all the fundamental boiling parameters used in heat flux partitioning models (e.g., nucleation site density, bubble wait and growth time, and bubble footprint radius) and the actual partitioning of the heat flux, i.e., how much heat is transferred by evaporation of the microlayer, rewetting of the surface, and convective effects. This work reveals that the mechanisms of heat transfer on substrates of small thermal capacity are very different compared to substrates of large thermal capacity. With water, the bubble microlayer typically does not dry out and the surface temperature at rewetting is practically the same as the rewetting fluid temperature. These effects limit the efficiency of microlayer evaporation and rewetting heat transfer. Instead, convective effects generated by the bubble growth process remove most of the energy from the heated surface. This behavior is captured by a heat flux partitioning model that we re-derived from first principles to describe the heat transfer mechanisms on substrate of small thermal capacity.

Numerical study on the effect of residual stress on mechanical properties of laser welds of aluminum alloy 2024

The fusion zone is the weak area of weld joint which will experience complex thermal mechanical process during laser welding aluminum alloy 2024-T4. The resulting residual stress in the weld joint may exceed the yield stress, which will significantly affect the hardness and tensile strength. The purpose of this study is to predict the residual stress, and analyze its relationship with hardness and yield stress of the aluminum alloy 2024-T4 weld joint. Herein, three sequential models are established: (i) a thermal finite element model using double cone combined heat source that allows the prediction of time-dependent temperature distribution during laser welding of aluminum alloy 2024; (ii) a mechanical model using sequential coupling method and loading temperature in each increment to predict strain and residual stress; (iii) a theoretical empirical model to link the residual stress, hardness and yield stress. The simulated temperature field from the thermal model is in good agreement with the experimental result. The predicted stress field shows that the average von Mises stress at the center of the Al 2024-T4 and Al 2024-O weld joints are 303.60 MPa and 62.85 MPa, respectively, which are close to their yield stress. Moreover, it is found that the hardness decreases by 6 HV0.1 under the influence of residual stress in Al 2024-T4 fusion zone. The results indicate that the yield stress of 2024-T4 weld joint decreases by 16.5 MPa as a result of the residual stress. Above results indicate that residual stress has non-negligible influences on the mechanical properties of laser weld joint.