Equivalent Thermal Resistance Research Articles

Thermal characterization of vertical interface by scanning photothermal radiometry.

In this work, scanning photothermal radiometry is used for imaging and to characterize a submicron crack. From the thermal images, the evolution of the crack is mapped in the space with micrometer resolution. A vertical contact interface at the steel-steel junction is used to represent a micro-crack with a thickness less than 0.5μm. The thermal quadrupole approach is used to model the heat transfer within the semi-infinite vertical crack. Then, using the phase mapping and that calculated from the model, the estimation of both the equivalent thermal boundary resistance of the interface and the average interface thickness was done.

Internal Temperature Estimation of Lithium Batteries Based on a Three-Directional Anisotropic Thermal Circuit Model

In order to improve the accuracy of internal temperature estimation in batteries, a 10-parameter time-varying multi-surface heat transfer model including internal heat production, heat transfer and external heat transfer is established based on the structure of a lithium iron phosphate pouch battery and its three directional anisotropic heat conduction characteristics. The entropy heat coefficient, internal equivalent heat capacity and internal equivalent thermal resistance related to the SOC and temperature state of the battery were identified using experimental tests and the least square fitting method, and were then used for online calculation of internal heat production and heat transfer in the battery. According to the time-varying and nonlinear characteristics of the heat transfer between the surface and the environment of the battery, an internal temperature estimation algorithm based on the square root cubature Kalman filter was designed and developed. By iteratively calculating the estimated surface temperature and the measured value, dynamic tracking and online correction of the internal temperature of the battery can be achieved. The verification results using FUDS and US06 dynamic working condition data show that the proposed method can quickly eliminate the influence of initial temperature deviations and accumulated process errors and has the characteristics of a high estimation accuracy and good robustness. Compared with the estimation results of the adaptive Kalman filter, the proposed method improves the estimation accuracy of FUDS and US06 working conditions by 67% and 54%, respectively, with a similar computational efficiency.

Virtual wall: Numerical model on interaction between trees and building façade to quantify heat exchange and microclimate in urban context

When simulating a building's energy demand, the tree surroundings should be accounted in their shade and reshaping microclimate, just representing an envelope layer as a virtual wall. This study delves into the effects of such tree-induced virtual walls on building energy dynamics, with a dataset derived from 24 distinct samples of building façades and adjacent trees within a campus environment. An experiment was designed to validate microclimate simulation that captures variations in sheltered radiation and temperature reductions. The study introduces and calculates the dynamic and static equivalent heat transfer coefficients and thermal resistances for the virtual wall. The findings reveal significant correlations between the calculated coefficients and resistances, and various factors such as seasonality, weather conditions, and façade orientation. The equivalent thermal resistance value for south façade is highest on sunny days, reaching up to 3.7 m2·K/W, and on cloudy days, east façade has the highest equivalent heat transfer coefficient and thermal resistance values, at around 8.5 W/m2·K and 10.3 m2·K/W, respectively. Moreover, the study reveals that the tree's impact is primarily determined by its height and canopy area, with the equivalent heat transfer coefficient negatively correlated with tree height and canopy area, and positively correlated with building height.

Techno-Economic Analysis of Electric Heating Load Participation in Power System Power Regulation

People are beginning to develop and use new energy sources, but it leads to a lack of power system regulation, so it is crucial to analyse the change of load power. In this paper, the electric heating load is taken as an example, and firstly, the properties of the steady state solution of the differential equation are analysed by changing the parameters of the indoor and outdoor equivalent heat capacity and equivalent thermal resistance, and it is found that the indoor equivalent heat capacity and thermal resistance have a significant effect on the results. Then, after differentiating the differential equations, the indoor temperature changes and the electric heating equipment switching state curves for one day were plotted, and it was found that the daily electricity cost increased as the temperature difference between indoor and outdoor increased. Then, the remaining switchable time values are analysed based on the load's current switch-on state and temperature-limited intervals, and the power upward and downward adjustments of the sustainable time are plotted schematically. In order to enhance the model universality so that it is no longer limited by the current switch on/off state, the switching value of each state is set to change dynamically to obtain a new power regulation diagram, and finally in this paper, we extend the model to multi-family, plot their one-day indoor temperature changes and equipment switching state diagrams and analyse the economic benefits.

Monitoring of some minor human skin lesions using a skin calorimeter

The growing interest of human skin thermal properties is motivating the development of new instruments, either by contact or by remote sensing. In this work, we show the ability of a skin calorimeter to monitor the temporal evolution of the heat capacity and the equivalent thermal resistance of the skin, in two small skin lesions. The first one consists of a forehead wound of 10 × 2 mm2. The other injury consists of a second degree burn on the volar wrist area, of 10 × 20 mm2. We studied the temporal evolution of the thermal properties of both injuries. The variation of heat capacity was significant. For first injury, heat capacity decreased by 21% and full recovery was achieved after ten days. For the second case, the heat capacity decrease was 55% and recovery was achieved after 3 weeks. These skin recoveries are monitored by the measured heat capacity value. The returning of the heat capacity to its normal value coincides with the recovery from the injury.

The integration of double-layer triangular micro-channel in three-dimensional integrated circuits for enhancing heat transfer performance

This paper proposed a new heat dissipation structure called double-layer triangular micro-channel (DLTMC) to solve the increasingly serious thermal problem of three-dimensional integrated circuits (3D-ICs). The matrix equation of heat transmission is established to derive steady-state temperature of 3D-ICs with embedded DLTMC, which is based on equivalent thermal resistance network. According to proposed computation thermal model, the heat transfer performances of 3D-ICs with embedded DLTMC concerning different cooling liquids are investigated, where the impacts of different bottom angle of DLTMC, volume concentration of nanofluid and flow velocity of cooling liquid are considered. The results show that the proposed DLTMC structure can significantly reduce steady-state temperature compared with conventional structure of single layer rectangular micro-channel (SLRMC), the corresponding steady-state temperature for them can be reduced over 32.098 %. Besides, it is demonstrated that the steady-state temperature for MWCNT-water and SWCNT-water nanofluids as cooling liquids can be reduced by 22.490 % and 19.801 % than conventional water case, respectively. Moreover, it is manifested that the increase of bottom angle of DLTMC, volume concentration of nanofluid and flow velocity of cooling liquid can evidently improve heat transfer performance of 3D-ICs. Therefore, the presented results in this paper are beneficial for solving complex thermal problems of 3D-ICs.

Evaluating the impact of virtual energy storage under air conditioning and building coupling on the performance of a grid-connected distributed energy system

Energy storage technologies are vital in improving the operation performance of grid-connected distributed energy systems. The adjustability of indoor temperature and the thermal inertia of buildings can form an excellent virtual energy storage. However, there are few studies on the impact of this virtual energy storage on the operation performance of grid-connected distributed energy systems. Therefore, this study first calculates the equivalent thermal resistance and thermal capacitance of a building by using EnergyPlus and establishes the first-order thermodynamic load calculation model. Subsequently, the system dynamic optimal scheduling model, considering the virtual energy storage, was developed based on the first-order thermodynamic load calculation model. The Gurobi nonconvex solver is used to optimize the model, and the results are compared with the optimal scheduling results without considering the virtual energy storage. The results indicate that, guided by time-of-use electricity pricing, the virtual energy storage effectively reduces the air conditioning load during high and peak tariff periods while increasing it during valley tariff periods. This change in air conditioning load leads to an increase in grid power consumption during valley tariff periods. However, it also improves the system's operation efficiency and ultimately reduces the total grid power and gas consumption. The lower energy consumption makes the primary energy saving rate and carbon dioxide emission reduction rate of Scenario 2 greater than zero compared to Scenario 1 and increases with the increase of renewable energy penetration of the integrated power system. Furthermore, affected by the difference in the proportion of grid power consumption under time-of-use electricity pricing and the total energy consumption, the system's operating cost in Scenario 2 is less than in Scenario 1. The reduction rates in summer and winter typical days are 1.95 % and 6.48 %, respectively. Therefore, fully utilizing the virtual energy storage under air conditioning and building coupling can reduce the operating cost, primary energy consumption, and carbon dioxide emissions of grid-connected distributed energy systems.

A new winding homogenization method based on thermal resistance concept

PurposeThe aim of this paper is to provide a simple yet accurate and efficient geometric method for thermal homogenization of impregnated and non-impregnated coil winding technologies based on the concept of thermal resistance.Design/methodology/approachFor regular windings, the periodic microscopic cell in the winding space is identified. Also, for irregular windings, the average microscopic cell of the winding is determined. An approximation is used to calculate the thermal resistance of the winding cell. Based on this approximation, the winding insulation is considered as a circular ring around the wire. Mathematical equations are obtained to calculate the equivalent thermal resistance of the cell. The equivalent thermal conductivity of the winding is calculated using equivalent thermal resistance of the cell. Winding thermal homogenization is completed by determining the equivalent thermal properties of the cell.FindingsThe thermal pattern of different windings is simulated and compared with the results of different homogenization methods. The results show that the proposed method is applicable for a wide range of windings in terms of winding scheme, packing factor and winding insulation. Also, the results show that the proposed method is more accurate than other winding homogenization methods in calculating the equivalent thermal conductivity of the winding.Research limitations/implicationsIn this paper, the change of electrical resistance of the winding with temperature and thermal contact between the sub-components are ignored. Also, liquid insulators, such as oils, and rectangular wires were not investigated. Research in these topics is considered as future work.Originality/valueUnlike other homogenization methods, the proposed method can be applied to non-impregnated and irregular windings. Also, compared to other homogenization methods, the proposed method has a simpler formulation that makes it easier to program and implement. All of these indicate the efficiency of the proposed method in the thermal analysis of the winding.

Numerical simulation analysis of high-temperature bent sodium heat pipes

Abstract High-temperature bent heat pipes are safety-level heat transfer components used in heat pipe cooled reactor, and their applications are becoming increasingly widespread. In this study, a comprehensive three-dimensional numerical model was established to evaluate the effects of different bending angles and radii on the performance of high-temperature heat pipes. The model solved for the temperature, pressure, and velocity fields in the wall, wick, and evaporator, considers the compressibility effect of the vapor. The study investigated parameters such as vapor pressure, vapor velocity, radial secondary flow field, wall temperature, wick temperature, and equivalent thermal resistance of powder sintered core sodium heat pipes with bending angles of 45°, 90°, 135°, 180°, and a bending radius under 90°. The results showed that when sodium vapor enters the bent pipe, the vapor flow deviates towards the outer side, resulting in higher vapor pressure at the outer side than the vapor pressure at the inner side, and the generation of radial secondary vortex. This leads to higher temperature in the evaporator and adiabatic sections of the bent heat pipe, while lower temperature in the condenser. Fractal patterns are observed in the outer and inner wall temperature curve in the adiabatic section, with lower temperatures on the inner side and higher temperatures on the outer side. The bent heat pipe exhibits higher equivalent thermal resistance than that of a straight heat pipe, and the equivalent thermal resistance increases with increasing bending angle, and the equivalent thermal resistance decreases with increasing bending radius.

On-Chip Micro Temperature Controllers Based on Freestanding Thermoelectric Nano Films for Low-Power Electronics

HighlightsDense and flat freestanding Bi2Te3-based thermoelectric nano films were successfully fabricated by sputtering technology using a newly developed nano graphene oxide membrane as a substrate.On-chip micro temperature controllers were integrated using conventional micro-electromechanical system technology, to achieve energy-efficient temperature control for low-power electronics.The tunable equivalent thermal resistance enables an ultrahigh temperature control capability of 100 K mW−1 and an ultra-fast cooling rate exceeding 2000 K s−1, as well as excellent reliability of up to 1 million cycles.

Waste heat recovery from the biomass engine for effective power generation using a new array-based system

Thermoelectric generator (TEG)-thermosyphon-based heat recovery system (HRS) for harvesting the heat from the high temperature sources is a well-known technology for power generation. However, various thermal resistances (a total of nine) are involved in the thermosyphon-based HRS between the heat sources and TEGs which generate irreversibility that ultimately reduces its performance. In this work, a new design of HRS i.e. TEG-array-based is proposed for effective power generation even at low temperatures of sustainable heat source by minimizing the thermal resistances. This TEG-array-based HRS is driven by a salt gradient thermal storage device (SGTSD) which is used for efficiently recovering and storing of sustainable waste heat generated from a biomass(renewable energy)-based engine-generator. The proposed system is experimentally studied to analyse the temperature variations in SGTSD, characteristics of sustainable waste heat temperature, and performance features of TEGs in terms of obtained electrical parameters, output power and conversion efficiency. The experimental results showed that the maximum open circuit voltage, output power and conversion efficiency of TEG are obtained as 81.62V, 7.438W and 4.63\\% respectively. With this proposed system, seven thermal resistances (out of total nine) are totally eliminated while the others two are lowered that leads to effectively transfer the energy from sustainable heat source to TEGs via minor equivalent thermal resistance of 0.01198oC/W. The proposed system is found to be capable of charging a 12 V, 80 Ah heavy-duty battery for real life applications.

Numerical modelling of an electric motor cooling jacket

The automotive industry is amid a sweeping change of propulsion technology to meet the increasingly stringent limits on emissions and fuel consumption. Traditional combustion engines are gradually being replaced by electric motors or hybrid power trains. The efficiency and power density levels achievable by electric motors are entirely dependent on the effectiveness of the employed cooling solution. Therefore, extensive analyses are needed to determine and optimise the thermal performance of these systems. In this work a numerical model is developed to determine the friction losses and the heat transfer properties of an electric motor cooling jacket. The cooling channels, which coil along the circumferential direction within the motor casing, are studied by means of CFD analysis of a basic periodic module. Flow and temperature fields are determined by applying a 3D Finite Volume approach. Numerical solutions are obtained by means of a validated conjugate heat transfer solver. Integral flow field results are employed to derive the equivalent Darcy friction factor and side-wall specific Nusselt numbers for several flow regimes. A lumped parameter thermal model, based on the graph theory and aforementioned CFD results is also developed to determine the overall system performance. The equivalent thermal resistances are computed from geometric parameters and CFD results. Finally preliminary numerical results on friction losses and heat transfer are compared with available experimental data.

Simulated and experimental study on effect of thermal insulation layer on temperature field and heat dissipation of roadway surrounding rock

As mining activities extend deeper, intense heat dissipation from surrounding rock has become the most significant factor leading to heat hazards in deep mines. Laying the thermal insulation layer (TIL) along the roadway walls is an effective measure to reduce heat dissipation. This study is to investigate the temperature field and heat dissipation evolution of roadway surrounding rock after applying thermal insulation layer (TIL) by numerical simulation and similar experiment. The study shows that (i) The simulation curves agree well with the experimental data, which confirms the reliability of the mathematical model and numerical simulator; (ii) The application of TIL slows down the cooling of surrounding rock temperature field, the cooling ring radius decreased by 8.13 % after 10 years of ventilation; (iii) The effect of TIL on the equivalent thermal resistance (ETR) is most significant in early ventilation, and the thermal insulation effect decreases with time; (iv) The heat insulation rate is positively correlated with the TIL thickness and the convective heat transfer coefficient, and negatively with the thermal conductivity of TIL. This study provides the theoretical basis for the use of thermal insulation materials in underground mines.

Innovative experimental approach for the dynamic Multi-Variable investigation of Pulsating heat Pipes

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the startup and the thermal crisis. An innovative experimental apparatus is designed to systematically investigate the above-mentioned phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 evaporator/condenser sections PHPs with a 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. First a steady-state characterization is performed. The condenser temperature is varied from 10 °C to 40 °C; the input power goes from 10 W to 40 W. Results show that both the increase of number of evaporator sections and condenser temperature enhance the PHP performance in terms of equivalent thermal resistance. Then, the effect of the condenser temperature on the initial liquid phase distribution is analyzed. It is observed that at low condenser temperatures (10 °C, 20 °C), the liquid phase tends to gather in the condenser sections rather than the evaporator sections and it is arranged in short liquid plugs; conversely, for high condenser temperatures (30 °C, 40 °C), the amount of liquid phase in evaporators is higher and it is arranged in long liquid plugs. Finally, the transient behavior is studied in terms of startup time. It is observed that the parameter influencing most the startup time is the initial fluid distribution; the startup time increases with the increase of evaporator sections volume occupied by liquid phase and with the length of liquid plugs; on the other hand, the startup time decreases with the increase of number of menisci in the evaporator sections.

An improved effectiveness-NTU method for streamlining the design and optimization of packed bed latent thermal energy storage

The packed bed thermal energy storage system (PBLTES) is one of the promising and effective ways to solve the contradiction between the supply and demand of renewable energy. As a simplified mathematical representation, the effectiveness-number of transfer units (NTU) method can characterize the PBLTES faster than numerical simulation, which is convenient for PBLTES design, analysis, and optimization. The p factor, which is a key parameter for calculating the equivalent thermal resistance in the effectiveness-NTU method, is used to define the path of charging and discharging of the PBLTES, that is, the synchronization of phase change processes of phase change materials in different layers, which determines the accuracy of the model. In this paper, the calculation method of average effectiveness is further refined based on existing studies and mathematical derivations. The phase change duration characteristic of the PBLTES is incorporated into the method. Then, a numerical simulation model of PBLTES with spherical capsules was established to investigate the influence of relevant parameters on the p-factor and the impact mechanism. Finally, the p-factor was fitted based on numerical simulation. The results show that the effectiveness solved by the fitting p-factor equation has a maximum error of less than 10 % and an average error of 5.11 % compared to the experimental results. The establishment of the p-factor calculation equation allows the effectiveness-NTU method to be used more accurately and quickly for the design and optimization of the structural and operating parameters of the PBLTES, which can advance the engineering applications of PBLTES.

Simulation Study of Heat Flow Coupling of Thermal Barrier Coatings Based on Virtual Coating Technology

To improve the simulation efficiencies and accuracy of heat flow coupling of thermal barrier coatings (TBCs), a nozzle guide vane model with virtual TBCs was established, by equivalent series thermal resistance theory and virtual TBCs modeling method. The difference in heat flow and temperature on the nozzle guide vane surface with real and virtual TBCs was discussed and analyzed. The results show that if the thickness of TBCs is less than 0.5mm, the temperature distribution of the virtual TBCs model is basically the same as the real one, the maximum relative deviation is less than 5%. The thermal resistance of TBCs is affected by the thickness and installation position of TBCs, when the thickness is 0.5mm, the maximum and minimum heat resistance of the TBCs is 40? and 5?, respectively. The efforts of this study provide a framework to predict the heat flow coupling of TBCs.

Experimental study on heat transfer performance of high temperature heat pipe under axial non-uniform heat flux

Non-uniform high-temperature large heat flux is commonly encountered in engineering applications, posing challenges to heat transfer processes and affecting system performance. High temperature heat pipes offer an effective solution to mitigate the adverse effects of non-uniform heat flux by leveraging the unique characteristics of their inner alkali metal (or alloy) working fluid, such as high latent heat of vaporization, strong heat flux density, and ability to operate at elevated temperatures. These features enable high temperature heat pipes to achieve excellent isothermal and uniform heat transfer performance, thereby improving overall thermal management in heat transfer systems. Building upon the investigation of the original high temperature heat pipe, this study designed and fabricated a specially shaped high temperature heat pipe with a spherical crown surface as the evaporator. Experimental research was conducted to assess its frozen start-up performance and heat transfer characteristics under the influence of axial non-uniform heat flux. The results were also analyzed theoretically, providing valuable insights into the frozen start-up performance and heat transfer performance of the high temperature heat pipe in the presence of non-uniform heat flux conditions. The analysis results reveal several important findings: (1)The high temperature heat pipe demonstrates its capability to convert axial non-uniform high-temperature large heat flux into a uniform heat flux, thus achieving excellent uniform heat transfer performance. (2)The theoretical solution obtained through the lumped heat capacity analysis method shows satisfactory agreement with the actual frozen start-up process of the high temperature heat pipe. This theoretical reference can aid in the transient analysis of the frozen start-up process under the influence of axial non-uniform high-temperature large heat flux. (3)The experiment investigated the influence of heating power on heat transfer performance and temperature uniformity. The results indicate that the temperature uniformity of the high temperature heat pipe exhibits a trend of initially increasing and then decreasing. Specifically, the best temperature uniformity is achieved when the heating power is 1084.7 W. Moreover, when the heating power is 1197.9 W, the high temperature heat pipe exhibits the lowest equivalent thermal resistance of 0.0265 K/W, indicating the best heat transfer performance. (4) The variable power heating approach demonstrates that increasing the heating power leads to an extension of the effective length of the heat pipe. The most favorable heat transfer performance and temperature uniformity are observed when the heating power is set to 1133.7 W, allowing the full length of the condenser of the high temperature heat pipe to participate actively in the heat transfer process. These results can provide quantitative experimental data and theoretical references for improving the thermal shock of non-uniform high-temperature large heat flux.

Analysis of the behavioural capabilities of electric heating loads to participate in power system power regulation

The high proportion of new energy access leads to the scarcity of power system regulation capacity, and it is important to provide regulation capacity for the power system by reasonably regulating the electricity consumption of electric heating temperature-controlled loads. In this paper, in order to analyse the characteristics of the electric heating behaviour and the electric heating load's ability to participate in grid regulation, the differential equations of a typical room temperature change process are constructed first to establish the equation of state model of a two-input system, and then the state equations are discretised into difference equations according to the bilinear discretisation method for iterative solving. This paper analyses that the equivalent heat capacity and thermal resistance parameters of indoor air and wall will affect the temperature response time and the size of the required heating power, with the lowering of the outdoor temperature of a single household room, the average indoor warming time increases, the average cooling time decreases, the average daily power consumption increases, and the total power consumption of the whole community power also rises.

Solid chamber for satellite electronic modules and evaluation of its heat conduction behavior

A novel solid chamber configuration for cooling satellite electronic modules enduring high heat is proposed. Solid chamber modules with different inner structures and solid materials with high thermal conductivity are designed, and their thermal performances were experimentally studied. The equivalent heat conduction coefficient and contact thermal resistance of a solid chamber module are defined and the estimation methods for the thermal behavior are proposed. It is found that the thermal performances of the two solid chamber modules with the inner structures respectively composed of Graphite/Al 630/30 composites and graphite plate are obviously better than those of the other modules. Based on the assessment of the thermal resistance and cracks generated in the modules after thermal tests, it is found that the powder metallurgy technology makes the interlayers bonded more tightly than diffusion welding in the manufacture of solid chamber, and the tightness of the interlayer interface plays a key role in the contact thermal resistance.

Application of moderate-temperature heat pipe instead of Ljungstrom in steam power plant: Parametric and Experimental study

The heat pipes are found as one of the promising technologies, which provide massive improvement of the energy performance in the energy systems. However, there are still some challenges in the employment of this technology in various applications, such as the ones with moderate-temperature operating conditions. The purpose of this study is the development of a moderate-temperature heat pipe on heat recovery, especially on replacing Ljungstrom in steam power plants with heat pipe heat exchangers. The temperature of the smoke passing through the Ljungstrom in the steam power plant is around 320 °C. For this purpose, 304 stainless steel vertical wickless heat pipe filled with special oil as working fluid is analyzed theoretically and experimentally. An experimental setup is fabricated, and the effects of aspect ratio, filling ratio, and exhaust temperature on the heat transfer rate and temperature distribution along the moderate-temperature wickless heat pipe are studied. Moreover, the operating limitations of the wickless heat pipe are investigated. A simple but accurate mathematical model is developed based on the equivalent thermal resistances and various physical limitations. By increasing the filling ratio of the heat pipe, the thermal performance of the system in all aspect ratios is reduced. The amount of heat transfer increased by increment of exhaust flow temperature up to 375 °C for aspect ratios of 11 and 25; however, for aspect ratio of 18, it is changed minimally. Based on the obtained results, the maximum heat transfer to cooling water, which is equal to 262.13 W, is achieved when FR and AR equal 30% and 18, consecutively. Furthermore, the average error for the equivalent thermal model is approximately 21% for a moderate-temperature wickless heat pipe, based on the assessment performed in this study.