Heat Transfer Theory Research Articles

A Review of Silo Temperature Field Studies

Silo is a large thin-walled structure, due to the operation period will be affected by different temperatures, such as sunshine temperature difference, seasonal temperature difference and the temperature difference between inside and outside the silo wall, of which the most influential is the sunshine temperature difference, silo due to the unevenness of the temperature distribution is prone to stress concentration and more likely to occur damage. This paper briefly introduces the three-dimensional heat transfer theory and ASHRAE solar radiation model, and discusses in detail the research progress of different experts and scholars at China and abroad on the temperature effect of silos. Finally, it is concluded that there are fewer temperature regulations in China codes, and there is a need to develop a perfect temperature theory to prevent silos from damage caused by temperature inhomogeneity.

Measurement method of hot forging dimension based on green laser depth information.

The application field of large forgings is extensive. Accurate dimensional measurement is an important factor to ensure the quality of the finished forging product when it is forged at high temperature. Therefore, this paper proposes a green laser scanning measurement method based on depth information. First, a geometric measurement model based on depth information is established by studying the relationship between green laser depth information and forging dimension. Then, based on the heat transfer theory, distribution of the temperature field around hot forgings is studied, and an error function caused by light refraction is established using a ray tracing algorithm. After that, the error function is used to modify the measurement model to obtain the accurate distance between the forging and background plane and then obtain the dimensional information of the forging. Finally, this measurement method was experimentally verified in the laboratory, and the experimental results show that the measurement error of this method meets the dimensional measurement requirements of large hot forgings.

Modeling and analysis of ice condensation on bridge deck pavement surface based on heat transfer theory and finite element method

Bridge surfaces usually freeze more easily than road surfaces in wet and cold conditions, which is unfavorable to traffic safety. This study aims to develop an ice prediction model for bridge deck pavement structures by integrating external climatic factors and internal structure parameters, and to analyze the mechanism of ice condensation. This model adopted finite element analysis to deal with the heat transfer problem and the latent heat during the phase change process. A series of experiments were designed to validate the model, and the relative difference was around 10 %. Results shown that: among external climatic factors, the impact of changes in water accumulation on ice formation is not significant. Meanwhile, the air temperature has the greatest impact, and there is a non-linear relationship between wind speed and the initial freezing time. In the study on the effect of aging, it was found that aged asphalt concrete pavement would freeze earlier and melt later compared to new. When comparing different structures, it was observed that the initial freezing time of the bridge deck pavement structure was 0.8 h earlier than the road structure, and the total freezing time was 2.8 h shorter than the road structure. It was also found that the lower thermal conductivity would delay the occurrence of icing when the bridge deck pavement structure was in a cooling stage. Finally, it was observed that the vehicle speed played a critical role in determining the direction of the heat flow with a specific value, and the value would increase as the temperature difference decreased. This prediction model is useful for transportation agencies to understand the mechanism of pavement ice formation and ensure traffic safety in winter.

Conjugate Multimode Heat Transfer Analysis of Cryogenic EXLO Manipulation.

In this study, a conjugate radiation/conduction multimode heat transfer analysis of cryogenic focused ion beam (FIB) milling steps necessary for producing ex situ lift out specimens under cryogenic conditions (cryo-EXLO) is performed. Using finite volume for transient heat conduction and enclosure theory for radiation heat transfer, the analysis shows that as long as the specimen is attached or touching the FIB side wall trenches, the specimen will remain vitreous indefinitely, while actively cooled at liquid nitrogen (LN2) temperatures. To simulate the time needed to perform a transfer step to move the bulk sample containing the FIB-thinned specimen from the cryo-FIB to the cryo-EXLO cryostat, the LN2 temperature active cooling is turned off after steady-state conditions are reached and the specimen is monitored over time until the critical devitrification temperature is reached. Under these conditions, the sample will remain vitreous for >3 min, which is more than enough time needed to perform the cryo-transfer step from the FIB to the cryostat, which takes only ∼10 s. Cryo-transmission electron microscopy images of a manipulated cryo-EXLO yeast specimen prepared with cryo-FIB corroborates the heat transfer analysis.

Determination of Pipe Diameter Using Optimization Method

During the design phase of engineering networks, a critical issue remains the selection of pipe diameters that minimize capital investments for transporting the heat carrier from the heat source to the consumers. Object of study: a pressurized pipeline of circular cross-section with a moving heat carrier. Subject of study: total monetary costs for transporting the heat carrier as a function of the chosen pipeline diameter. Objective of study: to determine the pipe diameter that achieves maximum cost savings for transporting the heat carrier under given design conditions. Research methods: theory of hydraulic calculation for circular cross-section pipelines and theory of heat transfer through a single-layer cylindrical wall under steady-state conditions. Research results: using an example from a centralized heating system pipeline section, it was established that for a mass flow rate of the heat carrier equal to 32.9 t/h, the optimal pipe size would be 76×3 mm (nominal diameter 70 mm). Under current tariffs for thermal energy (24.82 USD/Gcal) and electrical energy (6.65 USD/(MW⋅h)), the total monetary costs for transporting the heat carrier over a heating season would be 56.28 USD per 1 running meter of pipeline. In comparison, with a nominal diameter of 50 mm, the total costs amounted to 90.37 USD; with a diameter of 80 mm, the costs were 63.29 USD. The developed method for hydraulic design calculations is universal and can be applied in the design of engineering networks where the working medium is a moving heat carrier (steam or hot water).

An expansion effect based pseudo-boiling critical point model for supercritical CO<sub>2</sub>

Heat transfer deterioration (HTD) is one of the important issues in the study of supercritical fluid (SCF) heat transfer. However, when the SCF crosses the pseudo-critical point, the none-quilibrium process occurs in liquid, so SCF is very complicated. Recently, the existence of SCF pseudo-boiling on a macro scale has sparked controversy. There is still no unified understanding of the mechanism of gas-like and liquid-like transition affecting heat transfer. In this work, it is assumed that SCF has a macroscopic phenomenon similar to subcritical flow boiling. By analogy with subcritical boiling heat transfer, a boiling critical point model is proposed to describe the HTD in supercritical CO<sub>2</sub>. Our study reveals that the HTD caused by pseudo-boiling only occurs under large temperature gradient, which makes the superheated liquid-like layer cover the wall, and the gas-like and liquid-like may present different distribution forms, thus changing the heat transfer characteristics. When the wall temperature is higher than the pseudo-critical temperature and the enthalpy of the fluid layer covering the wall exceeds a certain value, the HTD may occur. The proposed theoretical model can explain the experimental results well, and the prediction accuracy of heat transfer correlation considering pseudo-boiling is greatly improved. In this work, the connection between supercritical heat transfer and subcritical heat transfer is established theoretically, which provides a new idea for studying the deterioration of SCF heat transfer, thus enriching the theory of supercritical heat transfer.

Development of complex methods for exergetic losses calculating in heat transfer through wall in air-heater heat utilizers

Ensuring high economic indicators of power plants of various purposes, which include waste gas heat utilization systems, is possible with the use of effective heat utilization equipment. One of the criteria for evaluating the thermodynamic perfection of power plants is the amount of exergy losses in the structural elements of the plant. A decrease in exergy losses corresponds to an increase in exergy efficiency. Determination of the structural elements in the heat utilization system, in which the maximum exergy losses are localized, will allow to influence them in a targeted manner in order to increase the exergy efficiency of the installation. The paper presents the results of the development of complex methods for calculating local exergy losses in the processes of heat transfer through flat and cylindrical walls of air-heated heat exchangers, which are included in the heat utilization systems of power plants of various purposes. Complex methods are based on integral equations for calculating exergy losses, as well as differential equations of heat transfer theory when applied to heat utilization systems. The classification of exergy losses for heat utilization systems of power plants has been carried out, and the location and type of local exergy losses have been determined. Three types of local exergy losses are considered: losses during heat transfer from the heat carrier to the wall, heat transfer from the wall to the heat carrier, and exergy losses in heat conduction processes. A system of integrodifferential equations was developed and analytical expressions were obtained for the calculation of local exergy losses in heat transfer processes through the flat and cylindrical walls of air-heating heat exchangers. The introduction of new complex research methods expands the possibilities of using exergy analysis methods to increase the exergy efficiency of heat utilization systems. Research using the specified methods will allow to develop heat utilization systems with high indicators of energy and exergetic efficiency.

Flow and heat transfer characteristics of high-pressure natural gas in the gaps of high-speed motors with a high radius ratio

A motor stator-gap-rotor model is established based on the numerical heat transfer theory by using the finite volume method. The flow evolution of high-pressure natural gas in the gap with a radius ratio of 0.971 is investigated. The results demonstrate that the flow patterns of high-pressure natural gas in the motor gap can be categorized into turbulent, spiral Taylor-Couette, and turbulent Taylor-Couette flow; the flow ranges are determined based on the Ta/Re2. Then, the flow and heat transfer characteristics of the cooling medium in the gap under different flow regimes as well as the mechanism of locally enhanced heat transfer in the gap by the Taylor-Couette flow are explored. Finally, the mathematical expressions for the Nusselt number of motor gap are determined in terms of the Reynolds number, Taylor number, and Prandtl number by fitting using the Levenberg-Marquardt and global optimization methods. Using these expressions, the flow and heat transfer characteristics in the motor gap can be predicted. Overall, this study provides useful and novel insights on the design of cooling systems for high-speed motors.

Flow and heat transfer characteristics of high-pressure natural gas in the air gap of high-speed motor

High speed motors are widely used in industrial applications owing to their unique features, such as compact framework, high performance, and high reliability. Based on the finite volume method and numerical heat transfer theory, this study establishes a stator-air gap-rotor model, and the flow and heat transfer in the air gap between the stator and rotor in a high speed motor are investigated with high pressure natural gas as the cooling medium. Meanwhile, the ?radial tri-vortex partition, alternating axial distribution? feature of the turbulent Taylor-Couette vortex in the air gap of the motor is determined. Then, the optimal structural parameters which can realize the heat transfer enhancement of motor air gap are obtained. Finally, an optical fiber grating temperature measurement system based on the wavelength division multiplexing technology is utilized to attain the temperature distributions on the stator and rotor surfaces. The simulation results are compared with the experimental data to evaluate the simulation method?s precision.

Analysis on the laying method and thermal insulation effect of tunnel insulation layer in high-altitude cold regions

To mitigate freeze-thaw damage in tunnels located in high-altitude cold regions, insulation layers are implemented to prevent the freezing of surrounding rocks. Currently, the selection of laying methods lacks a solid scientific basis, with the merits and demerits of various techniques remaining insufficiently evaluated. This study seeks to establish a scientifically grounded equilibrium between the anti-freezing efficacy and the construction impact of tunnel insulation in cold regions through the optimization of insulation design via numerical calculations. First, the advantages and disadvantages of the four insulation layer laying methods were summarized. Then, a multilayer media heat transfer model that accounts for the latent heat of phase change was developed, grounded in solid heat transfer and porous media heat transfer theories, and corroborated by typical case studies. Finally, taking Duolong tunnel as a case study, the insulation effect of various laying methods at different positions of the tunnel was analyzed based on the finite element method. The results show that the unfavorable position of the four laying methods is at the inverted arch of the tunnel, and the unfavorable time point occurs when the temperature rises from below 0℃ to above 0℃. Among the four laying methods, off-wall laying exhibits the superior insulation performance at tunnel vault, while sandwich laying has best insulation effect at the arch foot and inverted arch. The research results can provide reference and basis for the thermal insulation and anti-freezing design of tunnels in high-altitude cold regions.

Determination of the thermal state of a block gravel filter during its transportation along the borehole

Purpose. Development of a methodology for determining the thermal state of block inverse gravel filters manufactured using low-temperature technology during their transportation in a well based on computer and mathematical modeling. Methods. The study of hydrodynamic processes occurring during the transportation of the filter in the borehole, as well as the calculation of thermal fields in the filter body, was performed using the Ansys Fluent software package. To determine the effective thermophysical characteristics of the filter, the approaches of the heat transfer theory in porous media were applied. To investigate the thermal conditions of the filter at heat exchange with well fluid, the use of analytical solution of the heat conduction problem in a cylindrical shell, taking into account the properties of porous dispersed water-saturated medium, is proposed. Findings. The methodology for calculating the thermal state of a gravel filter during its operation in a well has been developed. For estimation of the filter surface temperature, the expression obtained on the basis of the analytical solution of the heat conduction equation, taking into account the characteristics of the porous filter medium, is proposed. Hydrodynamic and thermal fields in the borehole and the filter body during the filter transportation process in the borehole have been obtained. Originality. For the first time, the problem of heat exchange of a gravel inversion filter, manufactured by low-temperature technology in a well is considered. The influence of hydrothermal conditions in the well on the process of filter heating during its transportation in the well is shown. The hydrodynamic fields during the flow of the drilling mud around the inverse gravel filter are determined. Practical implications. The proposed approaches and results of the study allow to determine and can be used in the development of technological regulations for the use of block gravel filters produced by low-temperature technology for the equipment of hydrogeological wells. The methodology for modelling the process of two-phase inversion transition of aggregate state of the binding agent during transportation of inverted block-type gravel filter during well construction has been developed.

Modeling large-scale bioreactors with diffusion equations. Part I: Predicting axial dispersion coefficient and mixing times.

Bioreactor scale-up is complicated by dynamic interactions between mixing, reaction, mass transfer, and biological phenomena, the effects of which are usually predicted with simple correlations or case-specific simulations. This two-part study investigated whether axial diffusion equations could be used to calculate mixing times and to model and characterize large-scale stirred bioreactors in a general and predictive manner without fitting the dispersion coefficient. In this first part, a resistances-in-series model analogous to basic heat transfer theory was developed to estimate the dispersion coefficient such that only available hydrodynamic numbers and literature data were needed in calculations. For model validation, over 800 previously published experimentally determined mixing times were predicted with the transient axial diffusion equation. The collected data covered reactor sizes up to 160 m3 , single- and multi-impeller configurations with diverse impeller types, aerated and non-aerated operation in turbulent and transition flow regimes, and various mixing time quantification methods. The model performed excellently for typical multi-impeller configurations as long as flooding conditions were avoided. Mixing times for single-impeller and few nonstandard bioreactors were not predicted equally well. The transient diffusion equation together with the developed transfer resistance analogy proved to be a convenient and predictive model of mixing in typical large-scale bioreactors.

High-effective fog collection on metal surfaces by temperature gradients

The acquisition of global freshwater resources has always been the direction of people's efforts. At present, metal materials based on special wettability and special structure have become an effective way to collect fog. However, it is still a bottleneck to further improve the water collection efficiency by carefully designing the wettability and special structure of materials. Therefore, we propose to use temperature gradient to enhance fog collection in metal surface. The mechanism of temperature gradient enhancement improves fog collection efficiency by 54 % and surface refresh rate by about 541 %. Further scattering measurements show that the capture time of fog particles at low cooling surface is shortened by about 40 %. Based on the theory of heat transfer and thermophoresis, the temperature gradient around the metal and the behavior of fog particles are analyzed, and it is concluded that the enhancement of fog collection performance on the metal surface can be caused by the temperature gradient caused by thermophoresis force. This work provides ideas for designing better fog collection equipment and materials.

A transient transformation theory for the design of convective thermal metamaterials: Cloaks, concentrators, and rotators

Convective thermal metamaterials have attracted particular attention since they can simultaneously manipulate the fluid flow and heat transfer. However, the current transformation theory for designing convective thermal metamaterials is only applied to steady state heat-transfer systems or porous media systems. Therefore, we prove the weak forms of convective heat-transfer governing equations with time-dependent terms for non-porous media flows possess form invariant under coordinate transformation and develop the transient transformation theory for creeping viscous potential flows. Based on this theory, three convective thermal metamaterials are designed under transient heat-transfer systems, including cloaks, concentrators, and rotators. The validity of the transient transformation theory is verified through quantitative numerical simulation, which validate the functionalities of the metamaterials in uniform, non-uniform, steady-state, and transient heat-transfer systems. These cloaks, concentrators, and rotators in the transient convective heat-transfer system ensure that the heat fluxes and flows are transported around the object, amplified at specified times in the center region, and are rotated to a specified angle, respectively, without disturbing the background. Especially, the convective thermal cloak is characterized by approximately drag-free characteristics in transient velocity fields. Furthermore, comparative results reveal that transient convective thermal metamaterials constitute an extension of their steady-state counterparts. The established transient theory of transformation heat transfer not only guides the design of more richly functional thermal metamaterials but also broadens their application scenarios.

Numerical simulation of fracture temperature field distribution during oil and gas reservoir hydraulic fracturing based on unsteady wellbore temperature field model

In the hydraulic fracturing oil and gas reservoir, the temperature variation of the fracturing fluid has a great impact on its flow and rheology, affecting the sand-carrying capacity and friction resistance of the fracturing fluid, and also affecting the settling speed of proppant in the fracture, thus changing the sand setting profile and sand laying concentration, and finally affecting the geometry of the fracture. Based on the energy balance and continuity equations, a numerical model for the distribution of wellbore temperature field in an extended-reach well is developed, and a variation law for the wellbore and reservoir temperature fields during fracturing also is formulated. A 3D mathematical model of the temperature distribution in hydraulic fracture and near-fracture formations has been developed based on heat transfer theory and finite-difference methods, taking into account the temperature gradients of fracture length and height according to the energy balance principle and fracture fluid continuity equations. The sensitivity factors of the temperature field are clarified, the wellbore temperature field and the fracture matrix temperature field model are coupled, and the influence rule of the temperature field change of the construction layer on the fracture shape while fracturing is revealed, which provides a theoretical basis for the hydraulic fracturing design optimization.

Energy Conversion in Systems-Contained Laser-Irradiated Metallic Nanoparticles – Comparison of Results from Analytical Solutions and Numerical Methods

Abstract This work introduces the theoretical method of metallic nanoparticles’ (NPs’) heat and mass transfer where the particles are coated on a surface (base), together with considering the case wherein nanoparticles move freely in a pipe. In order to simulate the heat transfer, energy and radiative transfer equations are adjusted to the considered issue. NPs’ properties are determined following the nanofluidic theories, whereas absorption and scattering coefficients are described using either Mie-Lorenz theory or Rayleigh-Drude approximation. Thermal boundary conditions are implemented based on the microscale heat transfer and Smoluchowski theory. Results are compared with the classical Fourier transport differential solutions that have been adjusted to laser irradiation.

Safety assessment of wireless chargers for electric vehicles considering thermal characteristics.

This study employs the transient finite element method and electromagnetic heat transfer theory to assess the heating generated by high-power wireless chargers during electric vehicle charging. The analysis includes simulating and analyzing the temperature distribution of two different types of shielding plates of the wireless charger and the specific absorption rate (SAR) and head temperature rise of both adults and children in close proximity to the charger. Simulation results show that the maximum temperature rise of the copper shielding plate is 16°C lower than that of the aluminum shielding plate after charging for 1h. This temperature increase does not affect the chassis' s equipment. Regarding human safety, the induced electric field strength and SAR values in the child's head tissue are higher than those in the adult, meeting the International Commission on Non-Ionizing Radiation Protection (ICNIRP) limits. When the initial temperature is set to 37°C, the temperature rise in the heads of both adults and children is approximately equal after 1h of charging, reaching a maximum temperature rise of 0.21 and 0.23°C, respectively. These values remain below the thermal limit of ICNIRP (2°C for Type 2 tissues). The findings indicate that the copper shielding plate can provide both electromagnetic shielding and heat dissipation functions, and the electromagnetic exposure absorbed by the human body and head temperature rise within safe ranges.

All electromagnetic scattering bodies are matrix-valued oscillators

Scattering theory is the basis of all linear optical and photonic devices, whose spectral response underpins wide-ranging applications from sensing to energy conversion. Unlike the Shannon theory for communication channels, or the Fano theory for electric circuits, understanding the limits of spectral wave scattering remains a notoriously challenging open problem. We introduce a mathematical scattering representation that inherently embeds fundamental principles of causality and passivity into its elemental degrees of freedom. We use this representation to reveal strong constraints in the mathematical structure of scattered fields, and to develop a general theory of the maximum radiative heat transfer in the near field, resolving a long-standing open question. Our approach can be seamlessly applied to high-interest applications across nanophotonics, and appears extensible to general classical and quantum scattering theory.

Study on ambient temperature and humidity in the construction tunnel under the influence of ventilation and hot water gushing

The influx of hot water will intensify the detrimental effects of heat and humidity during the excavation of high geothermal tunnels. The spatiotemporal evolution of the ambient temperature-humidity field in the construction tunnel under the coupling action of ventilation and hot water is critical. Based on the theory of heat and mass transfer, a numerical model was developed to simulate the temperature and humidity conditions within a construction tunnel. The model considers the coupling effects of heat and moisture between airflow, hot water, and the surrounding rock. The distribution of temperature and humidity in the tunnel was analyzed, and the study investigated the influence mechanisms of ventilation volume (Va) and water gushing conditions on the ambient temperature and humidity in the construction tunnel. The main conclusions include: (1) During tunnel construction, the airflow temperature (Ta) and relative humidity (RH) varied as follows: heating and humidifying, constant humidity heating, and constant humidity cooling. (2) Altering the water temperature (Tw) will affect the distribution pattern of RH from the working face to the tunnel portal. Similarly, modifying the length of the ditch without the cover plate (Lc) will impact the distribution trend of Ta and RH. (3) When the Va affects the RH mainly through the airflow velocity rather than the evaporation coefficient, the linearity between the Va and RH is weak. Water volume and Tw both impact the humidity distribution by altering the amount of heat needed for evaporation. (4) Given the significance of the impact of individual factors on the temperature and humidity, it is advised that the Ta and RH be regulated by increasing the Va and decreasing the Lc, respectively. The research can give direction for temperature and humidity control in a tunnel where hot water gushes during construction.

Derivation of generalized thermoelectric energy equations and the study of thermoelectric irreversible processes based on energy, exergy, and entransy analysis

AbstractThermoelectric (TE) generation is becoming a valuable and promising research direction. Many researchers have carried out system analysis and performance optimization of thermoelectric technologies based on the generalized thermoelectric energy balance equations. However, it is assumed that TE legs have no heat exchange with the ambient except at the junctions of the hot and cold ends where heat flows in and out. Based on basic thermoelectric effects and fundamental theories of heat transfer, a detailed derivation of the revised generalized thermoelectric energy equations considering convective heat transfer between TE legs and the ambient has been carried out. Irreversible heat transfer processes have been analyzed by employing energy analysis based on the first law of thermodynamics and exergy analysis based on the second law of thermodynamics. The results show that convective heat transfer leads to a decrease in both energy and exergy efficiencies: the rate and magnitude of the decrease in exergy efficiency are greater than those of the decrease in energy efficiency. The exergy efficiency is relatively high despite the low energy efficiency in operation, revealing the features and advantages of thermoelectric generators (TEGs) in low‐grade energy utilization. For TEG efficient operation, the load resistance value should match the system's internal resistance, or at least be greater than that, to avoid a sharp drop in power output and efficiencies. In an attempt at theoretical analysis, the concept of entransy was first introduced into thermoelectric analysis, yielding two concise relational equations which reflect the intrinsic link between Carnot cycle efficiency, energy efficiency, exergy efficiency, and entransy flow transfer efficiency. The entransy analysis based on the index of entransy flow transfer efficiency, together with energy analysis and exergy analysis, may be a novel and valuable guideline for the operation and optimization of TEGs, which needs to be further investigated.