Spatial Temperature Gradients Research Articles

Temperature and strain gradient monitoring of the buoyancy material curing reaction with a submillimeter spatial resolution fs-FBG array

Millimeter-scale temperature and strain gradient measurements are crucial for understanding the mechanical properties of buoyancy materials. In this paper, we successfully fabricated ultrashort and high-density fiber Bragg grating (FBG) array based on femtosecond laser point-by-point writing technology. By optimizing the energy and number of femtosecond laser pulse, and introducing secondary tilt apodization technology, submillimeter-spatial-resolution FBG arrays were prepared. To the best of our knowledge, this is the first application of femtosecond FBG array for high spatial resolution temperature and strain gradient in situ monitoring of buoyancy materials. FBGs exhibit excellent anti-chirp performance and achieve submillimeter spatial resolution curing monitoring under temperature and strain monitoring gradients of 2.6 °C/mm and 231 με/mm, respectively.

Study of Temperature Gradients in Concrete Products in the Process of Their Heat Moisture Treatment

Statement of the problem. The work is devoted to the problem of improving the technological process of production of concrete products, in particular, improving the performance of the stage of their heat and moisture treatment. As such indicators, the spatial and temporal temperature gradients in the thickness of the concrete product are chosen, the excess of which exceeds the maximum allowable values for this type of concrete and adversely affects the strength of the product. On the other hand, steam-moisture treatment of products with obviously low values of these gradients leads to an increase in the time of the considered stage of the technological process, i. e. to reduce line performance. In this regard, the task was set to quantify the temperature gradients in the sections of concrete products in the process of their heat and moisture treatment by using the appropriate mathematical model. Results. As a result of studies carried out on the model of temperature fields in the longest mode — cooling mode, a significant uneven distribution of temperature at the boundaries of the «concrete-air» media was shown and critical zones of these fields in the sections of products and time intervals of their existence were identified. Conclusions. A conclusion was made and confirmed on the expediency of reducing the temperature in the chamber according to the exponential law and on the need to control its gradients in the critical sections of products.

Mean reef fish body size decreases towards warmer waters

AbstractAquatic ectotherms often attain smaller body sizes at higher temperatures. By analysing ~15,000 coastal‐reef fish surveys across a 15°C spatial sea surface temperature (SST) gradient, we found that the mean length of fish in communities decreased by ~5% for each 1°C temperature increase across space, or 50% decrease in mean length from 14 to 29°C mean annual SST. Community mean body size change was driven by differential temperature responses within trophic groups and temperature‐driven change in their relative abundance. Herbivores, invertivores and planktivores became smaller on average in warmer temperatures, but no trend was found in piscivores. Nearly 25% of the temperature‐related community mean size trend was attributable to trophic composition at the warmest sites, but at colder temperatures, this was <1% due to trophic groups being similarly sized. Our findings suggest that small changes in temperature are associated with large changes in fish community composition and body sizes, with important ecological implications.

Multi-dimensional temperature sensitivity of protected tropical mountain rain forests

IntroductionTropical mountain rain forests (TMRF, natural forests at > 300 m asl) are globally important for biodiversity and ecosystem services and are believed to be highly vulnerable to climate change. But there are no specific approaches for rigorous assessment of their vulnerability at the landscape and local scales necessary for management for adaptation. We address the challenge of evaluating the ecological sensitivity to temperature of TMRF, applying a multidimensional approach in protected areas over a 440–2,950 m asl altitudinal gradient in Costa Rica, synthesizing results of a long-term research programme (2012-present). We evaluate the sensitivity to the current spatial temperature gradient of eleven ecosystem properties in three categories: forest composition and diversity, thermal characteristics of forest stands and forest structure and dynamics.MethodsData are from 29 to 32 plots of 50 m x 50 m (0.25 ha) distributed over the gradient, in which all trees, palms and tree ferns ≥ 10 dbh are identified to species and measured for recruitment, growth and mortality. An experimental study of leaf litter decomposition rates was carried out in twelve plots. Current and future (SSP 585, 2070) values of mean annual temperatures MAT were obtained from online climate surfaces. Thermal characteristics of forest stands were determined using MATs of species occurrences in GBIF and include a new index, the Community Thermal Capital Index (CTCI), calculated as CTI-MAT.ResultsWe classified degrees of sensitivity to temperature as very weak, weak, moderate or substantial. All eleven ecosystem properties are substantially sensitive, so changes in their values are expected under rising temperatures. Species density, the community temperature index CTI, tree recruitment and mortality rates and leaf litter decomposition rates are positively related to temperature, while the community weighted mean thermal niche breadth, the CTCI, net basal area increments, stand basal area and carbon in aboveground biomass are negatively related. Results point to zones of vulnerability in the protected areas.DiscussionIn montane forests, positive values of the CTCI–climate credit– robust basal area growth and very low mortality and leaf litter decomposition rates suggest healthy ecosystems and no risk of mountaintop extinction. Lowland forests may be vulnerable to degradation and biotic attrition, showing current basal area loss, high mortality and climate debts. National and local actors are participating in a process of adoption of the sensitivity analysis and recommendations regarding zones of vulnerability.

Effect of defects on flux expulsion of pure niobium superconducting radio frequency cavities via spatial temperature gradient

Since flux vortices are easily pinned by defects in superconducting radio frequency (SRF) cavities, trapped magnetic flux increases the surface resistance of the SRF cavity. It has been experimentally demonstrated that trapped magnetic flux can be expelled by cooling down a superconducting cavity under a large spatial temperature gradient. In this paper, we use the time-dependent Ginzburg–Landau (TDGL) theory to theoretically investigate the effect of defects on the flux expulsion ability of pure niobium superconducting cavities with an applied spatial temperature gradient. It is shown that the residual flux vortex density displays a notable stepwise decreasing trend with increasing temperature gradient at smaller residual fields, while it shows a stepwise rising with increasing defect size. Both larger defect number and size make vortices difficult to be expelled. Moreover, long cooling time is beneficial for vortices to be expelled, and the cooling time has nearly no significant effect on flux expulsion under conditions of large cooling times. The results in this paper provide some theoretical guidelines for improving the performance of superconducting cavities in practical situations.

Understanding why constant energy or constant temperature may affect nucleation behavior in MD simulations: A study of gas hydrate nucleation.

Molecular dynamics simulations have been widely used in exploring the nucleation behavior of many systems, including gas hydrates. Gas hydrates are ice-like solids in which gas molecules are trapped in water cages. During hydrate formation, a considerable amount of heat is released, and previous work has reported that the choice of temperature control scheme may affect the behavior of hydrate formation. The origins of this effect have remained an open question. To address this question, extensive NVE simulations and thermostatted (NPT and NVT) simulations with different temperature coupling strengths have been performed and compared for systems where a water nanodroplet is immersed in a H2S liquid. Detailed analysis of the hydrate structures and their mechanisms of formation has been carried out. Slower nucleation rates in NVE simulations in comparison to NPT simulations have been observed in agreement with previous studies. Probability distributions for various temperature measures along with their spatial distributions have been examined. Interestingly, a comparison of these temperature distributions reveals a small yet noticeable difference in the widths of the distributions for water. The somewhat reduced fluctuations in the temperature for the water species in the NVE simulations appear to be responsible for reducing the hydrate nucleation rate. We further conjecture that the NVE-impeded nucleation rate may be the result of the finite size of the surroundings (here the liquid H2S portion of the system). Additionally, a local spatial temperature gradient arising from the heat released during hydrate formation could not be detected.

Achieving optimal magnetic flux expulsion of a Nb3Sn superconducting radio-frequency cavity via spatial temperature gradient

The trapped magnetic flux leads to residual resistance, thereby reducing the quality factor of the superconducting cavity and affecting its power consumption. It has been experimentally shown that the trapped magnetic flux of the superconducting radio-frequency cavity can be expelled during cooldown under a spatial temperature gradient. In the present paper, we simulate the magnetic flux expulsion behavior during the cooling process of a Nb3Sn superconducting cavity using time-dependent Ginzburg-Landau (TDGL) theory. In order to achieve optimal magnetic flux expulsion, the effect of grain size, grain boundary strength |ψ|GB, and cooling time on flux expulsion ratio ε for different temperature gradients are studied theoretically. The results show that ε first increases rapidly and then varies slightly to a steady value with increasing temperature gradient, which is in good agreement with the experimental results. In general, for larger |ψ|GB, ε decreases with increasing grain size. While for smaller |ψ|GB, ε first increases to the maximum then decreases with increasing grain size. Moreover, It is found that a larger cooling time is more beneficial for the expulsion of the flux vortices from the sample. The results of this paper The vortex dynamics behavior can give an insight into the mechanism of magnetic flux expulsion, which can provide a theoretical basis for further enhancing the performance of superconducting cavities.

Design and Experimental Investigation of Temperature Control for a 10 kW SOFC System Based on an Artificial Neuronal Network

For a future carbon-neutral energy economy, fuel cells play an important role due their high efficiency. Especially the Solid Oxide Fuel Cell (SOFC) with demonstrated efficiencies beyond 60 % 1 can contribute to reasonable roundtrip efficiencies for hydrogen and e-fuels 2.To match the fluctuating electricity demand in a future electricity grid, dominated by renewable energy sources like wind and photovoltaic, dynamic operation of fuel cells is required. The research has shown that the degradation und therefore the lifespan of Solid Oxide Cell (SOC) stacks shows a significant dependency on the operating conditions and dynamic load changes 3. However, some research suggests that the degradation is not caused by the load changes itself but spatial temperature gradients during load changes 4–6.Therefore, controlling the temperature gradients in the stack during load changes can have a significant impact on the lifespan of SOC stacks. This is typically more dramatically for stacks with larger cell sizes or multiple cells per layer. Furthermore, a tight temperature control allows for running the stack at maximum efficiency without the risk of stack damage due to exceeding temperature limits.In this work the authors designed and experimentally evaluated different controller topologies for fuel cell operation (SOFC) of a reversible solid oxide cell (rSOC) system described previously 7. The controller design incorporates an artificial neuronal network (ANN) for real time state predictions. The training data for the ANN was generated by a dynamic model of this system. This model is implemented in Matlab Simulink and was validated against experimental data. The generated training data consists of about 1,000 simulated days of dynamic system operation with a sample interval of 10 s. Additionally, data for 16,000 different steady state operating conditions of the system were generated.One focus of this work is the robustness of the controller under real world conditions despite inaccuracies of the underlying model and SOC degradation effects over time. To compensate the aging of the stack, the ANN is trained on variable degradation states. The degradation state is then tracked by the controller during operation to maintain an accurate prediction. First system experiments showed promising results in this respect (Fig. 1).Fig 1. Maximum temperature (red) and its setpoint (red, dashed) as well as 8 other temperatures distributed over the stack (black) in response to a given current density profile (green) and the air flow (blue) set by the controller AcknowledgmentsThe authors would like to thank their colleagues at the Forschungszentrum Jülich GmbH, who helped realize this work, and the Helmholtz Society for financing these activities as part of the Living Lab Energy Campus. References(1) Peters, Ro.; Frank, M.; Tiedemann, W.; Hoven, I.; Deja, R.; Kruse, N.; Fang, Q.; Blum, L.; Peters, R. Long-Term Experience with a 5/15kW-Class Reversible Solid Oxide Cell System. J. Electrochem. Soc. 2021, 168 (1), 014508. https://doi.org/10.1149/1945-7111/abdc79.(2) Heydarzadeh, Z.; McVay, D.; Flores, R.; Thai, C.; Brouwer, J. Dynamic Modeling of California Grid-Scale Hydrogen Energy Storage. ECS Trans. 2018, 86 (13), 245–258. https://doi.org/10.1149/08613.0245ecst.(3) Kim, Y.-D.; Lee, J.-I.; Saqib, M.; Park, K.-Y.; Hong, J.; Yoon, K. J.; Lee, I.; Park, J.-Y. Degradation of Anode-Supported Solid Oxide Fuel Cells under Load Trip and Cycle Conditions and Their Degradation Prevention Operating Logic. J. Electrochem. Soc. 2018, 165 (9), F728–F735. https://doi.org/10.1149/2.1391809jes.(4) Hagen, A.; Høgh, J. V. T.; Barfod, R. Accelerated Testing of Solid Oxide Fuel Cell Stacks for Micro Combined Heat and Power Application. Journal of Power Sources 2015, 300, 223–228. https://doi.org/10.1016/j.jpowsour.2015.09.054.(5) Nakajo, A.; Wuillemin, Z.; Van herle, J.; Favrat, D. Simulation of Thermal Stresses in Anode-Supported Solid Oxide Fuel Cell Stacks. Part I: Probability of Failure of the Cells. Journal of Power Sources 2009, 193 (1), 203–215. https://doi.org/10.1016/j.jpowsour.2008.12.050.(6) Jiang, W.; Luo, Y.; Zhang, W.; Woo, W.; Tu, S. T. Effect of Temperature Fluctuation on Creep and Failure Probability for Planar Solid Oxide Fuel Cell. Journal of Fuel Cell Science and Technology 2015, 12 (5), 051004. https://doi.org/10.1115/1.4031697.(7) Peters, R.; Tiedemann, W.; Hoven, I.; Deja, R.; Kruse, N.; Fang, Q.; Blum, L.; Peters, R. Development of a 10/40kW-Class Reversible Solid Oxide Cell System at Forschungszentrum Jülich. ECS Trans. 2021, 103 (1), 289–297. https://doi.org/10.1149/10301.0289ecst. Figure 1

Rapid, selective and homogeneous brain cooling with transnasal flow of ambient air for pediatric resuscitation

Neurologic outcome from out-of-hospital pediatric cardiac arrest remains poor. Although therapeutic hypothermia has been attempted in this patient population, a beneficial effect has yet to be demonstrated, possibly because of the delay in achieving target temperature. To minimize this delay, we developed a simple technique of transnasal cooling. Air at ambient temperature is passed through standard nasal cannula with an open mouth to produce evaporative cooling of the nasal passages. We evaluated efficacy of brain cooling with different airflows in different size piglets. Brain temperature decreased by 3°C within 25 minutes with nasal airflow rates of 16, 32, and 16 L/min in 1.8-, 4-, and 15-kg piglets, respectively, whereas rectal temperature lagged brain temperature. No substantial spatial temperature gradients were seen along the neuroaxis, suggesting that heat transfer is via blood convection. The evaporative cooling did not reduce nasal turbinate blood flow or sagittal sinus oxygenation. The rapid and selective brain cooling indicates a high humidifying capacity of the nasal turbinates is present early in life. Because of its simplicity, portability, and low cost, transnasal cooling potentially could be deployed in the field for early initiation of brain cooling prior to maintenance with standard surface cooling after pediatric cardiac arrest.

Proxy quality control of biomass particles using thermogravimetric analysis and Gaussian process regression models

AbstractThe temperature experienced by reactants during preparation in a reactor is a key component in determining the yield and homogeneity of usable chemical products such as biomass particles. Thermocouples with sensors can be used to monitor spatial temperature gradients within reactors but these sensors are often too expensive and/or invasive. The present work proposes a strategy to identify optimal machine learning models to infer the maximum effective temperature experienced by particles during oxidative biomass torrefaction using key thermochemical combustion parameters. The maximum rate of weight loss, the corresponding temperature, and fixed carbon content on a dry‐ash‐free basis are used as literature‐based predictor variables obtained from thermogravimetric analysis. The evaluation of 24 machine‐learning models using the standard tenfold cross‐validation method suggests that the exponential Gaussian process regression (GPR) model is the most effective, followed by other GPR models. These high‐performing GPR models were also utilized to predict the effective preparation temperature distribution of reactor‐produced biomass particles under eight conditions of varying residence time and air‐to‐biomass ratio. The effective preparation temperature and residence time of individual biomass particles were then encoded into the torrefaction severity factor and used to estimate the energy yield of the reactor output as a novel quality control method. © 2023 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.

Depolymerization of plastics by means of electrified spatiotemporal heating.

Depolymerization is a promising strategy for recycling waste plastic into constituent monomers for subsequent repolymerization1. However, many commodity plastics cannot be selectively depolymerized using conventional thermochemical approaches, as it is difficult to control the reaction progress and pathway. Although catalysts can improve the selectivity, they are susceptible to performance degradation2. Here we present a catalyst-free, far-from-equilibriumthermochemical depolymerization method that can generate monomers from commodity plastics (polypropylene (PP) and poly(ethylene terephthalate) (PET))by means ofpyrolysis. This selective depolymerization process is realized by two features: (1) a spatial temperature gradient and (2) a temporal heating profile. The spatial temperature gradient is achieved using a bilayer structure of porous carbon felt, in which the top electrically heated layer generates and conducts heat down to the underlying reactor layer and plastic. The resulting temperature gradient promotes continuous melting, wicking, vaporization and reaction of the plastic as it encounters theincreasing temperature traversing the bilayer, enabling a high degree of depolymerization. Meanwhile, pulsing the electrical current through the top heater layer generates a temporal heating profile that features periodic high peak temperatures (for example, about 600 °C) to enable depolymerization, yet the transient heating duration (for example, 0.11 s) can suppress unwanted side reactions. Using this approach, we depolymerized PP and PET to their monomers with yields of about 36% and about 43%, respectively. Overall, this electrified spatiotemporal heating (STH) approach potentially offers a solution to the global plastic waste problem.

Low-drift-rate external cavity diode laser

We present the design, construction, and simulation of a simple, low-cost external cavity diode laser with a measured free-running frequency drift rate of 1.4(1) MHz/h at 852 nm. This performance is achieved in a compact aluminum structure held inside an airtight, temperature-controlled enclosure. The high thermal conductivity of the laser cavity and the stable temperature environment inside the enclosure minimize the time-varying, spatial temperature gradients across the laser cavity. We present thermal finite element method simulations, which quantify the effects of temperature gradients, and suggest that the drift rate is likely limited by the laser-diode and piezo-aging.

In situ probing of atmospheric-pressure warm air glow discharge for nitrogen fixation by multiple laser spectroscopies

The fixation of atmospheric nitrogen into valuable compounds through reactive plasma processes has attracted intense interests due to its easy operation and compatibility with distributed renewable energy sources. However, practical implementation of plasma-assisted nitrogen fixation is hampered because of its relatively low throughput, which is dominantly limited by the unclear underlying mechanisms. In this study, effort was focused on the in situ production of key species in a DC-driven warm air glow discharge at atmospheric pressure with the help of advanced laser spectroscopic diagnostics. Laser Rayleigh scattering was applied to determine the gas temperature distribution in the discharge column. And mid-infrared quantum cascade laser absorption spectroscopy and one/two-photon absorption laser-induced fluorescence were performed on molecular nitric oxide (NO), atomic oxygen and nitrogen (O, N) for their absolute densities in the discharge. It is found that the spatial distributions of gas temperature, O and N atoms show peaks in the hot discharge center. In contrast, a hollow ‘doughnut’ shape characterized by the NO molecule was observed, particularly under conditions of high discharge current but low airflow rate. The steady-state simulation shows that the hollow pattern of NO is dominantly induced by the radial diffusion of species due to the steep spatial gradient of gas temperature in the discharge cross-section. Moreover, the reverse conversion by atomic N leads to a negative effect on the NO synthesis, especially at the discharge center where the N density and gas temperature are high. From the steady-state modeling, a similar hollow distribution of NO2 was depicted in the air glow discharge. These results demonstrate the strong dependence on atomic O for the major formation process of NO, and the importance of suppressing the reverse paths dominated by atomic N for higher NO production in the studied warm air plasma.

Marine protected areas' positive effect on fish biomass persists across the steep climatic gradient of the Mediterranean Sea

Abstract The positive effect of fully protected marine protected areas (MPAs) on marine biodiversity, and specifically on fishes, has been widely documented. In contrast, the potential of MPAs to mitigate the impact of adverse climatic conditions has seldom been investigated. Here, we assessed the effectiveness of MPAs, quantified as increasing fish biomass, across wide geographic and environmental gradients across the Mediterranean Sea. We performed underwater visual surveys within and outside MPAs to characterize fish assemblages in 52 rocky reef sites across an extent of over 3300 km. We used the steep spatial temperature gradient across the Mediterranean as a ‘space‐for‐time’ substitution to infer climate‐driven temporal changes. We found that, as expected, Mediterranean MPAs increased fish biomass. At the same time, higher seawater temperatures are associated with decreased fish biomass, changes in species composition, and shifts towards more thermophilic species. Importantly, we found that the rate of decrease in fish biomass with temperature was similar between protected and fished sites. Taken together, these results suggest that the capacity of MPAs to harbour higher fish biomass, compared to surrounding areas, is maintained across a broad temperature range. At the same time, MPAs will not be able to offset larger‐scale biotic alterations associated with climate change. Policy implications. Our results suggest that sustained warming will likely reduce fish biomass in the Mediterranean Sea and shift community structure, requiring more conservative targets for fishery regulations. At the same time, protection from fishing will remain an important management tool even with future high‐water temperatures, and MPAs are expected to continue to provide local‐scale benefits to conservation and fisheries.

X-ray diffraction performance of thermally distorted crystals

Abstract The development of high-brightness X-ray free electron lasers (XFELs), such as hard X-ray self-seeding free electron lasers and XFEL oscillators (XFELOs), brings a severe challenge to the crystal monochromator due to a strong non-uniform thermal load. The distortion caused by spatial temperature gradients can severely affect the optical performance of crystals. Therefore, this paper presents a model to estimate the performance of non-uniform thermally distorted crystals. The model not only takes into account thermal strain, slope error and incident angle deviation, but also considers temperature-dependent factors such as the Debye–Waller factor and electric susceptibility. Our investigation indicates that the Debye–Waller factor reduces the height and bandwidth of rocking curves, and the impact of the electric susceptibility is tiny. The proposed model can describe the distortion of the reflectivity and transmissivity curves of non-uniform thermally loaded crystals and can be applied in the design of crystal monochromators, crystal splitters, crystal compressors and XFELOs.

Emissivity Prediction for an IR Camera During Laser Welding of Aluminum

Laser processing is becoming increasingly important in industrial applications. The success of the process relies on two fundamental parameters: the surface temperature of the medium and the thickness of the hardened layer. One of the most important factors during a laser process is certainly the temperature, which presents high temperature gradients. The speed at which a material undergoes a phase transition, the chemical reactions that take place during processing and the properties of the material are all dependent on temperature changes. Consequently, the measure of temperature is a demanding undertaking. This study proposes to measure temperature for the duration of laser welding with the infrared camera (IR) Optris PI. To restore the real temperature based on the brightness temperature values measured by the IR camera is needed to evaluate the emissivity to be attributed to the IR camera. For this purpose, firstly, the isotherms consistent with the melting point of aluminum (785 K) were assessed and then compared with the temperature distribution gauged in the zone of irradiation of the laser. Such data were then compared with the thickness of the melted zone. The use of the melting point isotherm allowed the calculation of the value of emissivity and the restoration of the temperature. Thermography software data acquisition wrongly presupposes the emissivity value does not change. This generates incorrect thermographic data. The surface emissivity normally hinges on temperature. Therefore, the values on which the literature relies may not work for materials of interest in the conditions of the process. This is particularly the case, where welding is carried out in keyhole mode (Tmax = Tvap). However, the physical phenomena involved, including evaporation and plasma plume formation, high spatial and temporal temperature gradients, and non-equilibrium phase transformations, influence the optical conditions of the brightness of the emission of light from the molten pool, making, De Facto, the emissivity value not constant. Thus, what we propose here is a methodological procedure that allows the measurement of the effective emissivity of the surface, at the same time taking into consideration the consequence of physical phenomena and the conditions of the surface. Two procedures (Standard and Simplified) capable of providing the correct emissivity value in relation to the working parameters have been proposed. The results showed that the procedures are correct, fast, and easy to use.

Heteroepitaxial Growth of Vertically-Aligned GaN Single-Crystalline Microrod Arrays on Silicon Substrates.

The heteroepitaxial growth of vertically aligned gallium nitride (GaN) single-crystalline microrod arrays on silicon substrates was achieved with high reproducibility by using the plasma-enhanced chemical vapor deposition (PECVD) method in the furnace. By reducing the plasma power from 70 to 15 W, the crystal morphology of GaN varied from thin films to microrod arrays with the decreased V/III gas ratio. The growth of GaN crystals occurred in the vertical direction of the substrate and in the lateral direction of the growth axis via the self-catalytic vapor-liquid-solid mechanism (VLS mechanism) and the vapor-solid mechanism (VS mechanism), respectively, contributing to the formation of inverted hexagonal GaN cone microrods. Furthermore, the morphology of inverted hexagonal GaN cone microrods shows extremely small contact areas between the microrods and the substrate, suggesting the potential to solve the problems of stress accumulation and poor crystalline qualities of heteroepitaxy. With the raised growth temperature of GaN from 930 to 980 °C, the material quality was improved and the high crystalline qualities were obtained, owing to the successful surface migration of gallium atoms. However, the density of GaN microrods became lower with the increased growth temperature because the spatial temperature gradient was reduced and the evaporation of gallium was enhanced, leading to fewer gallium atoms precipitating and remaining on the substrate. The growth direction of vertically aligned GaN single-crystalline microrod arrays with the (002) crystal plane is along the [0001] orientation (c axis) and normal to the substrate surface, which may bring about many device applications in future studies.

A hybrid approach to predict vertical temperature gradient of ballastless track caused by solar radiation

• A hybrid approach combining the field test and FEM with ML technique is proposed. • Six ML techniques are used to predict spatial and temporal temperature gradient. • The key features of hybrid approach to predict temperature gradient are analyzed. • The proposed hybrid approach has excellent performance and transferability. The spatial and temporal temperature evolution of ballastless track is difficult to be determined, due to its complex thermal conditions. Accurate acquisition of vertical temperature gradient (VTG) of ballastless track is of great significance for its design, maintenance and damage control. However, few researchers have studied the predictive approach of spatial and temporal VTG evolution. In this study, a hybrid approach that combines experimental and finite element analysis with machine learning (ML) technique is proposed to automatically predict VTG of ballastless track induced by solar radiation. Six different ML techniques, i.e., multiple linear regression (MLR), polynomial regression (PR), support vector regression (SVR), random forest (RF), extreme gradient boosting (XGBOOST), and categorical boosting (CATBOOST) with fine-tuned hyperparameters are trained and validated by a dataset of 1000 cases retrieved from macroscale finite element analysis. The maximum and minimum air temperatures, latitude, average wind speed, etc., considering spatial and temporal environment actions are used as input variables to estimate the VTG of ballastless track. The results indicate that the CATBOOST-based hybrid approach demonstrates the most appropriate method for determining VTG of ballastless track among the above ML techniques. The proposed feasible and efficient hybrid approach is expected to be widely used for the identification of temperature evolution in structural engineering.

Local heat generation management for temperature gradient reduction in tubular solid oxide fuel cells

• Heat generation management can reduce the cell temperature gradient. • Effects of flow characteristics on heat generation are analyzed. • Comparisons between flanged and tubular separators are preformed. • Cell temperature gradient and power density can be reduced at different rates. • Flanged separator can reduce the cell temperature gradient by 62% Reducing the temperature gradient in solid oxide fuel cells could improve their working stability. In this study, we have constructed a 2D-axisymmetric model to investigate the effect of the heat generation management on the temperature gradient reduction in a micro-tubular solid oxide fuel cell. The local heat generation is controlled by placing separators in the fuel channel, which could affect the fuel concentration distribution and the associated exothermic electrochemical reactions in the porous anode. We compared the electrochemical and thermal performance of the solid oxide fuel cells with tubular separators and flanged separators. We demonstrated both separators could effectively reduce the cell temperature gradient, but the flanged separator caused a more uniform temperature profile. A 25 mm long flanged separator could reduce the highest cell temperature gradient from 50 ℃/cm to 18.7 ℃/cm. The spatial temperature gradient diagram was proposed to evaluate the influence of different geometric parameters of the separator on its overall performance. It showed that the radius and length of tubular separators both affected the cell temperature profile while the length of flanged separators was the primary factor affecting the cell performance. This study helps improve our understanding about the thermal management of solid oxide fuel cells through the local heat generation control and build a foundation for the flow channel design.

Temperature and indoor environments.

From the thermodynamic perspective, the term temperature is clearly defined for ideal physical systems: A unique temperature can be assigned to each black body via its radiation spectrum, and the temperature of an ideal gas is given by the velocity distribution of the molecules. While the indoor environment is not an ideal system, fundamental physical and chemical processes, such as diffusion, partitioning equilibria, and chemical reactions, are predictably temperature-dependent. For example, the logarithm of reaction rate and equilibria constants are proportional to the reciprocal of the absolute temperature. It is therefore possible to have non-linear, very steep changes in chemical phenomena over a relatively small temperature range. On the contrary, transport processes are more influenced by spatial temperature, momentum, and pressure gradients as well as by the density, porosity, and composition of indoor materials. Consequently, emergent phenomena, such as emission rates or dynamic air concentrations, can be the result of complex temperature-dependent relationships that require a more empirical approach. Indoor environmental conditions are further influenced by the thermal comfort needs of occupants. Not only do occupants have to create thermal conditions that serve to maintain their core body temperature, which is usually accomplished by wearing appropriate clothing, but also the surroundings must be adapted so that they feel comfortable. This includes the interaction of the living space with the ambient environment, which can vary greatly by region and season. Design of houses, apartments, commercial buildings, and schools is generally utility and comfort driven, requiring an appropriate energy balance, sometimes considering ventilation but rarely including the impact of temperature on indoor contaminant levels. In our article, we start with a review of fundamental thermodynamic variables and discuss their influence on typical indoor processes. Then, we describe the heat balance of people in their thermal environment. An extensive literature study is devoted to the thermal conditions in buildings, the temperature-dependent release of indoor pollutants from materials and their distribution in the various interior compartments as well as aspects of indoor chemistry. Finally, we assess the need to consider temperature holistically with regard to the changes to be expected as a result of global emergencies such as climate change.