Thermal Generation Rate Research Articles

Analysis of Thermal Mixing and Entropy Generation during Natural Convection Flows in Arbitrary Eccentric Annulus

The present study presents a computational investigation into the thermal mixing along with entropy generation throughout the natural convection flow within an arbitrarily eccentric annulus. Salt water is filled inside the eccentric annulus, in which the outer and inner cylinders have Tc and Th constant temperatures. The Boussinesq approximation is used to develop the governing equations for the natural convection flow, which are then solved on a structured quadrilateral mesh using the OpenFOAM software package (FOAM-Extend 4.0). The computational simulations are performed for Rayleigh numbers (Ra=103–105), eccentricity (ϵ=0,0.4,0.8), angular positions (φ=0∘,45∘,90∘), and Prandtl number (Pr=10, salt water). The computational results are visualized in terms of streamlines, isotherms, and entropy generation caused by fluid friction and heat transfer. Additionally, a thorough examination of the variations in the average and local Nusselt numbers, circulation intensity with eccentricities, and angular positions is provided. The optimal state of heat transfer is shown to be influenced by the eccentricity, angular positions, uniform temperature sources, and Boussinesq state. Moreover, the rate of thermal mixing and the production of total entropy increase as Ra increases. It is found that, compared to a concentric annulus, an eccentric annulus has a higher rate of thermal mixing and entropy generation. The findings show which configurations and types of eccentric annulus are ideal and could be used in any thermal processing activity where a salt fluid (Pr=10) is involved.

Non-Markovian Hole Excess Noise in Avalanche Amorphous Selenium Thin Films.

Enhancing the signal-to-noise ratio in avalanche photodiodes by utilizing impact ionization gain requires materials exhibiting low excess noise factors. Amorphous selenium (a-Se) as a wide bandgap at ∼2.1 eV, a solid-state avalanche layer, demonstrates single-carrier hole impact ionization gain and manifests ultralow thermal generation rates. A comprehensive study of the history dependent and non-Markovian nature of hot hole transport in a-Se was modeled using a Monte Carlo (MC) random walk of single hole free flights, interrupted by instantaneous phonon, disorder, hole-dipole, and impact-ionization scattering interactions. The hole excess noise factors were simulated for 0.1-15 μm a-Se thin-films as a function of mean avalanche gain. The hole excess noise factors in a-Se decreases with an increase in electric field, impact ionization gain, and device thickness. The history dependent nature of branching of holes is explained using a Gaussian avalanche threshold distance distribution and the dead space distance, which increases determinism in the stochastic impact ionization process. An ultralow non-Markovian excess noise factor of ∼1 was simulated for 100 nm a-Se thin films corresponding to avalanche gains of 1000. Future detector designs can utilize the nonlocal/non-Markovian nature of the hole avalanche in a-Se, to enable a true solid-state photomultiplier with noiseless gain.

Impact assessment of vehicle electrification pathways on emissions of CO2 and air pollution in Xi'an, China

To assess the environmental impact of promoting the use of electric vehicles in road traffic on emissions of CO2 and air pollution in Xi'an, China, both the proportion of electric vehicles and the power generation mix should be considered. Here, vehicle ownership in 2021 served as the baseline scenario, and the vehicle development trend through 2035 was projected. Using emission factor models for fuel vehicles and the electricity generation required for running electric vehicles, this study estimated the related pollutants' emission inventories at 81 corresponding scenarios, in which differing vehicle electrification paths were coupled with power generation mix. Further, the degree to which different vehicle electrification paths impacted the CO2 and air pollutant emissions was also evaluated. The results show that, to achieve the goal of peak carbon emission in the road transport sector in Xi'an by 2030, the penetration rate of electric vehicles must reach at least 40 % in 2035, and the thermal power generation rate should satisfy the necessary coupling conditions. Although reducing the thermal power generation rate could mitigate the environmental problems, we find that electric vehicle development in Xi'an during 2021–2035 would still exacerbate SO2 emissions despite reducing the thermal power generation rate to 10 %. Finally, to avoid exacerbating the adverse effects on public health from vehicle-related pollutants, the penetration rate of electric vehicles should be at least 40 % in 2035, at which time for the 40 %, 50 %, 60 %, and 70 % scenarios, the corresponding thermal power generation rate should not exceed 10 %, 30 %, 50 %, and 60 %. This study systematically analyzed plausible development paths of electric vehicles from the perspectives of peak carbon emissions, air pollution control, and human health, whose findings can serve as a timely and valuable reference for reducing pollution and carbon in the field of road transport.

Soft Wearable Thermal Devices Integrated with Machine Learning

AbstractCore body temperature (CBT) is a vital parameter that provides insight into individuals' overall health. However, existing methods to monitor CBT are mainly invasive and limited to applications in operating rooms. This work reports a soft wearable thermal device with low power operation to accurately monitor the core temperature and overcome these limitations. The thermal device comprises multiple temperature sensors separated with insulating materials of different thermal conductivities. The design provides a well‐defined thermal gradient to characterize the heat flux across the device. Thermal simulation of the devices with finite element analysis provides guidelines on the device design. Experimental studies involving tissue phantom and human subjects characterize and validate the device performance. A machine learning approach can account for heterogeneous, hard‐to‐measure parameters among individuals such as tissue thermal conductivity and heat generation rate. The machine learning algorithms can be trained to accurately quantify the core temperature in human subjects using the zero‐heat‐flux device measured temperature as a reference. The results show that the mean core temperature difference between the zero‐heat‐flux and the devices is 0.01 °C with 95% limits of agreement in the range of −0.08 °C and 0.1 °C.

Electric Field-Enhanced Generation Current in Proton Irradiated InGaAs Photodiodes

Dark current degradation due to 49.7 MeV proton irradiation is studied on lattice-matched InGaAs PIN photodiodes. This degradation is described in terms of electric field enhanced Shockley-Read-Hall generation current in the depletion region. In this paper we present a model of the radiation-induced generation current which includes the role of the electric field through the generation rate field enhancement factor (GRFEF). Using a combination of dark current-voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_d – V</i> ) and capacitance-voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C – V</i> ) measurements, an experimental GRFEF is extracted for the thermal generation rate. This GRFEF has been used to define a new damage factor for InGaAs at low electric field that has been compared to previous literature studies. As a final result, the low-field generation lifetime degradation as a function of the fluence has been extracted.

High spatial resolution direct conversion amorphous selenium X-ray detectors with monolithically integrated CMOS readout

Recent progress in the field of micron-scale spatial resolution direct conversion X-ray detectors for high-energy synchrotron light sources serve applications ranging from nondestructive and noninvasive microscopy techniques which provide insight into the structure and morphology of crystals, to medical diagnostic measurement devices. Amorphous selenium (a-Se) as a wide-bandgap thermally evaporated photoconductor exhibits ultra-low thermal generation rates for dark carriers and has been extensively used in X-ray medical imaging. Being an amorphous material, it can further be deposited over large areas at room temperatures and at substantially lower costs as compared to crystalline semiconductors. To address the demands for a high-energy and high spatial resolution X-ray detector for synchrotron light source applications, we have thermally evaporated a-Se on a Mixed-Mode Pixel Array Detector (MM-PAD) Application Specific Integrated Circuit (ASIC). The ASIC format consists of 128 × 128 square pixels each 150 μm on a side. A 200 μm a-Se layer was directly deposited on the ASIC followed by a metal top electrode. The completed detector assembly was tested with 45 kV Ag and 23 kV Cu X-ray tube sources. The detector fabrication, performances, Modulation Transfer Function (MTF) measurements, and simulations are reported.

The formation mechanism of the typical hot dry rocks in Hunan Province, China

Abstract Calculated by using the pipeline model, the burial depths of the 150 and 180°C hot dry rocks (HDRs) are 3.20–3.56 and 3.92–4.36 km in the Reshuiwei geothermal field, respectively, while those in the Huitang geothermal field are 3.13–3.50 and 3.84–4.28 km, respectively. Based on the analysis of the geophysical and terrestrial heat flow data, rock thermal generation rates, and the hydrochemical thermometric scale data, we summarized the formation mechanisms of the HDRs in Hunan Province: (i) the boundary deep faults in the welding area have made the intersection part of the deep faults an open tectonic environment, which is beneficial to the upwelling of deep-mantle-sourced thermal matter. (ii) The southeastern Hunan Province is the front region of the West Pacific subduction and retreat. Lithospheric thinning in this area is significant since the Mesozoic era, especially in the Dongting Lake basin, which was formed during extension and depression. This kind of regional lithospheric thinning is favorable for the lateral and upward migration and the intrusion of the deep-mantle-sourced hot substances. (iii) Rocks with high radioactive heat production rates are important crustal thermal sources for the formation of HDRs. Besides that, plutons with similar geological conditions are favorable for the formation of HDR resources. These plutons include the Baimashan pluton and its surrounding areas, and the eastern Dongting Lake plain. Moreover, the intermediate and acid granites distributed in the Chenzhou-Linwu deep fault are also part of the plutons, and the area where the Chenzhou-Linwu deep fault convergent with the Changde-Anren deep fault.

Conversion of Sewage Sludge to combined heat and power: Modeling and optimization

Combined Heat and Power (CHP) generation from Sewage Sludge (SS) offers two simultaneous advantages: greenhouse gas emission reduction and increase of renewable energy generation as promoted by the European Union Green Deal 2021. In this work, a numerical model has been developed via Aspen Plus for the evaluation of CHP generation potentiality from SS through gasification integrated with an internal combustion engine system. The model is applied to the case of Italy and eight other European countries for the first time. The gasification model has been developed based on the experimental data on syngas generation from SS in a bench-scale rotary kiln reactor under laboratory conditions available in the literature. Sensitivity analysis revealed optimal operating temperature and equivalence ratios for gasification were 900 °C and 0.2 respectively. The CHP generation potentiality of SS resulted to be 2.73 kWh per kg SS as dry solid.According to the statistical analysis used in the present study, SS generation will reach 680 kt per year as dry solid by 2030 based on the current sludge generation rate as well as improvement in the wastewater collection and treatment expected for the future in Italy. Within this time, the projected electrical and thermal energy generation rate per year can reach 714 GWh and 1142 GWh respectively. Electrical and thermal energy generation rates from sewage sludge have been estimated for eight EU countries in 2015 and compared with the Italian scenario, founding the highest one in Spain and the lowest in Luxembourg.

Thermal runaway front in failure propagation of long-shape lithium-ion battery

Long, large-format lithium-ion batteries have become prominent in recent years in high-power application scenarios, such as in electrochemical energy storage stations, electric vehicles, and electric ships. In these batteries, failure is always initiated from a local point and then propagates to the full cell, requiring countermeasures to quench the in-cell thermal runaway propagation. This study investigates the thermal runaway propagation behaviors of long, large-format lithium-ion batteries. A thermal runaway front exists during the propagation of thermal runaway; it separates the failure zone and normal zone and carries significant information regarding the thermal runaway reactions. The characteristics of the thermal runaway front are investigated through experiments and simulations. The thermal runaway front moves forward with an average velocity of approximately 24.14 mm·s−1, driven by the large temperature gradient between the failure and intact zones. The velocity of the thermal runaway front is correlated with the thermophysical properties of the battery. A modeling analysis indicates that the velocity of the thermal runaway front in propagation has a square root correlation with the thermal conductivity and heat generation rate. This square root correlation links the failure process and the thermophysical properties of the battery and can contribute to the future safety design of large-format batteries.

Combined Rankine Cycle and dew point cooler for energy efficient power generation of the power plants - A review and perspective study

The Rankine Cycle is a thermodynamic process widely applied to power plants such as coal-fired power plants or nuclear reactors. The thermal efficiency of a power plant is largely dependent upon the temperature difference between a heat source and a cold source; an increased temperature difference leads to the significant increase in the thermal efficiency and power generation rate of the power plant. Integrating the dew point cooler, i.e., Maistosenko Cycle system, into the traditional Rankine Cycle is an effective approach to reduce the temperature of the cold source (i.e. condenser), and thus, increase the thermal efficiency and power generation rate of the power plant. The paper presents a close overview of the combined Rankine/Maisotsenko Cycle, concerning with: (1) current development of Rankine Cycle power systems and Maisotsenko Cycle evaporative cooling technologies; (2) existing pre-cooling technologies for the cooling tower; and (3) thermal efficiency of the combined Rankine/Maisotsenko Cycle and its relevant impact factors. It is concluded that the combined Rankine/Maisotsenko Cycle can overcome the limit of the existing similar technologies, has great energy-saving potential and is economically viable. The further research should focus on a number of critical issues which require effective solutions to enable the energy efficient power generation. These include: (1) Optimisation of the combined Rankine/Maistsenko cycle using advanced multi-objective evolutionary optimisation algorithms; (2) Prediction of the future operational performance of the combined Rankine/Maistsenko cycle based power plant using future weather forecast scenarios and associated machine learning algorithms; (3) Analysis of thermal economics performance of the combined cycle using entropy and exergy analytic approach; and (4) Analysis of energy costs of the combined cycle using the exergo-economic and emergoeconomic analytical methods. This benefit also thanks to the recent development of the dew point cooling technology with highly enhanced COP, especially in hot and dry climates.

Indirect calorimetry of metabolic rate in college-age Japanese subjects during various office activities

Indoor climate regulations are based on an empirical thermal comfort model developed in the 1960s. The standard values for one of its fundamental parameters, which is the metabolic rate, is based on the data pertaining to European and North American men measured at that time and may overestimate the metabolic rate of modern Japanese, especially women. Therefore, applying standard values to modern Japanese may lead to inaccurate thermal comfort evaluation. The purpose of this research is to investigate the metabolic rates of Japanese individuals performing different office activities using accurate body surface area data in order to determine whether there exist differences between the measured and standard values. Three-dimensional measurements of the body surface area and indirect calorimetry were applied to determine their metabolic rates accurately. This study measured the mean metabolic rates of 45 college-age Japanese subjects while they were sitting quietly, sitting and typing, standing quietly, and standing and typing at 0.9 and 0.8 met, respectively, and 2.6 met each while they were walking at a speed of 4.0 km/h. These values are significantly lower than the standard values specified in the current international standards for all activity levels. Moreover, females exhibited lower metabolic rates than males, and significant differences between both biological sexes were found for all activity levels excluding walking. It was found that applying the metabolic rate values specified in the existing thermal comfort standards to college-age Japanese, particularly females, can lead to inaccurate thermal comfort evaluation and CO2 generation rate estimation for building occupants.

Numerical Study of NOx Generation in a Trapped Vortex Combustor Fuelled by Kerosene Blended with Ethanol

The study of alternative fuels is of great importance, and the study of ethanol blended with kerosene is necessary. The trapped vortex combustor is a new type of combustor and NOx emissions are important in the study of trapped vortex combustors. In this article, the NOx generation rate of trapped vortex combustor was studied using kerosene blended with ethanol in different mass fractions as fuel. The results show that the thermal NOx is the main source of NOx in the combustor, which is mainly generated at the back of the combustor, and the thermal NOx generation rate decreases with the increase of ethanol mass fraction. The prompt NOx generation rate decreases with the increase of ethanol mass fraction. NOx is also easily produced near the sidewall of the combustor.

Defect and Impurity-Complex Depassivation During Electron-Beam Irradiation of GaAs

Significant increases in thermal generation rates in molecular-beam epitaxy (MBE)-grown p-i-n-i-p GaAs structures are attributed to depassivation of hydrogen-defect/impurity complexes during high-intensity, low-energy electron-beam irradiation. Quantum calculations of relevant defect energy levels and reaction pathways suggest that depassivation of oxygen impurities is the likely cause of the observed degradation. Electric-field and excess-carrier-induced dehydrogenation of O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">As</sub> -H complexes is identified as a likely rate-limiting process, leading to the observed enhanced thermal carrier generation in these structures.

Heating Detection and Temperature Control Method of Power Lithium Battery in Pure Electric Vehicle

For the power battery of electric vehicles, especially pure electric vehicles, there is no perfect and comprehensive detection system and management system in the production and use links. By analyzing the working principle, structure and electrochemical characteristics of lithium-ion battery, the heat generation mechanism of lithium-ion battery was studied. In this paper, the factors that affect the temperature characteristics of Li ion battery are described, and the corresponding relationship between the temperature rise of the battery and the ambient temperature is established. At the same time, the optimal temperature range of the battery pack discharge efficiency is determined. In this paper, the thermal effect model and heat generation rate model of lithium-ion battery are established, and then the thermal conductivity, specific heat capacity, density and other parameters of the thermal model are calculated. Finally, the initial and boundary conditions of the thermal model are determined, the simulation of heat generation temperature field is realized, and the temperature distribution of the battery after heat generation is obtained. In this paper, the flow mode of air is analyzed, and the fluid structure coupling model of battery air is established. Finally, the thermal field of the battery pack is simulated by setting the solver mode and boundary conditions, which makes a theoretical analysis for the preliminary design of the temperature control battery box. The test results show that the method proposed in this paper can meet the technical requirements of power lithium battery heating management of pure electric vehicles..

Local thermal non-equilibrium (LTNE) model for developed flow in porous media with spatially-varying Biot number

The local thermal non-equilibrium (LTNE) model has been extensively used to model convective heat transfer in porous media. Most of the past LTNE-based models account for thermal interactions between fluid and solid phases in the form of a constant Biot number (Bi). This work presents a local thermal non-equilibrium model for fully developed flow in a channel filled with a porous medium where Bi itself varies across the channel. A set of differential equations for fluid and solid phase temperature fields are derived under this condition, which are shown to be generalizations of previously presented results for constant Bi. Results from the present analysis are shown to reduce to and agree with past work for the special case of constant Bi. The variable Bi model is used to investigate the effect of thermal properties such as thermal conductivity on the fluid and solid temperature profiles. The nature of temperature distributions are correlated with the spatial variation in Bi, including for parabolic and sinusoidal variation. Specifically, for periodic Bi, the locations of maxima and minima in temperature fields are found to be well correlated with corresponding maxima and minima in Bi. Nusselt number (Nu) for different values of thermal conductivities and heat generation rates are determined for variable Bi. This work accounts for an important physical consideration in porous media and generalizes previously-presented LTNE models for porous media in a channel.

Core temperature estimation based on electro‐thermal model of lithium‐ion batteries

The performance of battery management systems relies on the core temperature estimation, which is one of the major technical bottlenecks for electric vehicles. Aiming to tackle this problem, a lumped-parameter two-state thermal model for lithium-ion batteries is established in this paper. Then, this thermal model is coupled with a temperature-dependent second-order RC equivalent circuit model to form an electro-thermal model for lithium-ion batteries. Using the proposed electro-thermal model, an adaptive estimation algorithm based on joint Kalman filtering is proposed for battery core temperature estimation, considering the heat transfer condition variations between the battery surface and ambient media. The verification results show that the proposed algorithm enhances the temperature estimation accuracy, compared to the results obtained directly from the electro-thermal model. Besides, the verification results demonstrate the high adaptability of the proposed algorithm, that is, it is robust to variations in ambient temperature, as well as variations in thermal resistance between the battery surface and ambient media. Considering the influences of temperature and SOC on the thermal generation rate of the battery, an electro-thermal model for lithium-ion batteries is established. This model is composed of a lumped-parameter two-state thermal model and a temperature-dependent second-order RC equivalent circuit model. An adaptive core temperature estimation algorithm based on joint Kalman filtering is proposed, considering the heat transfer condition variations between the battery surface and the ambient media.

Study of process of avalanche switching of silicon thyristors without bias voltage

Operation of the high-power thyristor without bias voltage switching in the impact-ionization mode was studied by numerical simulation methods. In calculations, the rate of voltage build-up on the dV/dt structure varied from 0.5 to 10 kV/ns, the temperature of the T structure was from 25 to 200 °C. It is shown that the increase in temperature affects the process of switching thyristors both due to an increase in the rate of thermal generation of carriers, and due to a decrease in the intensity of impact ionization processes. During switching processes of impact ionization occur at the same time in two regions of n-base: in the part of a base filled with the majority carriers and in the space charge region (SCR) near the n-p junction. At T>180 °C, owing to increase in concentration of thermo-generated carriers in the base, ionization processes occur only in SCR. It leads to increase in duration of switching process and increase in residual voltage. However, despite it if dV/dt>9 kV/ns, the effect of fast switching of the thyristor exists up to 200 °C.

Minority carrier lifetime in mid-wavelength interband cascade infrared photodetectors

A simple and effective electrical method is developed to extract the thermal generation rate and minority carrier lifetime in type-II (T2) InAs/GaSb/Al(In)Sb superlattices (SLs) by taking advantage of the features of interband cascade infrared photodetectors (ICIPs). This method is more generally applicable, considers the parasitic shunt and series resistances found in practical devices, and can account for various transport mechanisms including Auger and Shockley-Read-Hall processes. The investigated ICIPs have cutoff wavelengths in the close vicinity of 4.3 μm at 300 K. The carrier lifetimes in these ICIPs at high temperatures (200–340 K) were extracted and found in the range of 8.5–167 ns, with values correlated with the material quality. The temperature dependence of the extracted carrier lifetime exhibited different characteristics from previously reported behavior obtained using other methods for T2SL detectors, especially at high temperatures.

Performance prediction of p-i-n HgCdTe long-wavelength infrared HOT photodiodes.

Recently, an enhanced computer program was applied to explain in detail the influence of different recombination mechanisms (Auger, radiative, and Shockley-Read-Hall) on the performance of high-operation-temperature, long-wavelength, infrared p-i-n HgCdTe heterojunction photodiodes. It is shown that the photon recycling effect drastically limits the influence of radiative recombination on the performance of small pixel HgCdTe photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. Both the distribution of thermal carrier generation and recombination rates, and spatial photon density distribution in photodiode structures have been obtained. In comparison with two previously published papers in the Journal of Electronics Materials [J. Electron. Mater.45, 4587 (2016)JECMA50361-523510.1007/s11664-016-4566-6 and J. Electron. Mater.46, 6295 (2017)JECMA50361-523510.1007/s11664-017-5736-x], our paper provides an additional insight on the ultimate performance of long-wavelength infrared, high-operation-temperature HgCdTe arrays with pixel densities that are fully consistent with background- and diffraction-limited performance due to system optics.

Gamma non-ionizing energy loss: Comparison with the damage factor in silicon devices

The concept of non-ionizing energy loss (NIEL) has been demonstrated to be a successful approach to describe the displacement damage effects in silicon materials and devices. However, some discrepancies exist in the literature between experimental damage factors and theoretical NIELs. 60Co gamma rays having a low NIEL are an interesting particle source that can be used to validate the NIEL scaling approach. This paper presents different 60Co gamma ray NIEL values for silicon targets. They are compared with the radiation-induced increase in the thermal generation rate of carriers per unit fluence. The differences between the different models, including one using molecular dynamics, are discussed.