Thermal Resistance Model Research Articles

Heat transfer and stress distribution in the central part of vacuum glazing

Vacuum glazing (VG) is a super-insulator which can effectively reduce energy consumption in buildings. It consists of two glass panes and the gap in between is evacuated. To withstand the atmospheric pressure, spacers are arrayed in the gap and the edge is sealed. This study conducts heat transfer and stress analysis in the central part of VG. Previous studies used simple one-dimensional thermal resistance model to analyze the heat transfer. This study applies multi-dimensional heat transfer analysis and the effects of various geometrical parameters on the thermal resistance are concisely expressed in a fitted formula from the computational results. In addition, the stress concentration caused by the glass-spacer contact is numerically found and a few methods to avoid it are suggested. Finally, the heat transfer is analyzed again for a modified spacer to confirm the applicability of the previously fitted formula.

Conjugate heat transfer studies on steam reforming of ethanol in micro-channel systems

The heat transfer aspects of Steam Reforming of Ethanol (SRE) to produce hydrogen in a micro-channel reactor heated by co-flowing flue gas, have been modeled. Conjugate heat transfer models coupled with convective diffusion of reactants and single step kinetics at the catalyst coated walls of the reformer, are considered. The effects of different geometrical configurations (i.e., square, wide and tall channels) and 2D/3D aspects have been addressed in detail. Irrespective of the channel geometry, reformer system attains isothermal condition at the catalyst–coated wall, at a short axial distance from the inlet. A thermal resistance model developed for conjugate heat transfer across adjacent channels shows that the convective resistances within the channels dominate over the conduction resistance across the separating walls. Hence, the thermal conductivity of wall material has negligible influence on the overall heat transfer coefficient of the system. For the endothermic steam reforming process, availability of heat is one of the most important factors governing hydrogen yield. In adiabatic micro-channel reforming process depending solely on preheating of the reactant mixture, the conversion rates are observed to be very low (less than 10%). When flue gas flowing in adjacent channels supplies the necessary heat, nearly 100% ethanol conversion is possible; however overall hydrogen yield further depends on reformer fluid temperature, due to water-gas shift and methanation reactions occurring in the reactant mixture. It is also observed that, at low reformer wall temperatures, both reaction kinetics and heat availability are important; at high wall temperatures, the system performance is primarily heat transfer controlled.

Thermal Resistance Modeling of Oscillating Heat Pipes for Nanofluids by Artificial Intelligence Approach

In this study, thermal resistance of a closed-loop oscillating heat pipe (OHP) is investigated using experimental tests and artificial intelligence methods. For this target, γFe2O3 and Fe3O4 nanoparticles are mixed with the base fluid. Also, intelligent models are developed to predict the thermal resistance of the OHP. These models are developed based on the heat input into evaporator section, the thermal conductivity of working fluids, and the ratio of the inner diameter to length of OHP. The intelligent methods are multilayer feed-forward neural network (MLFFNN), adaptive neuro-fuzzy inference system (ANFIS) and group method of data handling (GMDH) type neural network. Thermal resistance of the heat pipe (as a measure of thermal performance) is considered as the target. The results showed that using the nanofluids as working fluid in the OHP decreased the thermal resistance, where this decrease for Fe3O4/water nanofluid was more than that of γFe2O3/water. The intelligent models also predicted successfully the thermal resistance of OHP with a correlation coefficient close to 1. The root-mean-square error (RMSE) for MLFFNN, ANFIS, and GMDH models was obtained as 0.0508, 0.0556, and 0.0569 (°C/W) (for the test data), respectively.

Thermal Modeling of Inductor and Transformer Windings Including Litz Wire

Design optimization of magnetic components, which considers both electrical and thermal performance, can help in designing power converters with a high power density. This paper proposes an approximate analytical model for thermal resistance of inductor and transformer windings, including litz wire, which only requires knowledge of thermal properties of constituent materials and geometric dimensions. The model is based on regular square and hexagonal packing of insulated wires and is derived by integration of infinitesimal thermal resistances along a specified heat flow path. The model closely agrees with finite-element analysis (FEA) and can be used in place of time-consuming FEA as part of larger thermal network models for magnetic components. The model is also experimentally validated using a toroidal inductor with a litz-wire winding; the model has less than 12% error compared to the experimental measurement. Application of the model for practical windings, including litz-wire windings, is also discussed. The model can be used during the design process of magnetic components of a switching power converter to evaluate the temperature and potential hot spots. The model provides closed-form results, hence fast computation times, and so can be used in design optimization procedures.

Thermal and hydrodynamic performance of a microchannel heat sink with carbon nanotube nanofluids

Increasing heat fluxes in decreasing sizes of a microchannel heat sink have necessitated studies into better systems, in particular a more capable coolant for improved thermal management. Nanofluids have been at the forefront with their higher thermal conductivity as compared to the base fluid alone. However, few investigations have looked into the role played by surfactants, a component used for dispersion and stability of nanofluids, and their influence on thermophysical properties and thermal performance. Optimized performances of carbon nanotube nanofluids with different surfactants at different volume fractions under laminar flow are studied. Using the thermal resistance model and experimental data for thermal conductivity and viscosity of nanofluids, the thermal resistance and pumping power are simultaneously minimized using multi-objective genetic algorithm. Results showed that the nanotube nanofluid with lignin as the surfactant performed better thermally and hydrodynamically, due to lower viscosity at high carbon nanotube concentration compared to the nanotube nanofluid with sodium polycarboxylate surfactant. As an example, a 29% and 28% increase in pressure drop is found for sodium polycarboxylate-based nanofluid at a volume fraction of 0.1% for a circular and square MCHS, respectively. A similar pattern is observed at higher volume fraction as well, even higher pressure drop with increasing volume fraction. Also, it is shown that microchannel heat sink with circular cross section performed better than that with square section.

Modeling of yeast thermal resistance and optimization of the pasteurization treatment applied to soft drinks

Yeast are usually responsible for spoilage of soft drinks and fruit beverages, because of the particular characteristics of these products (low pH, high C/N ratio). The microbial stability is guaranteed by thermal treatments. However, excessive heat treatments can affect food sensorial quality. In this work the thermal resistance of different yeasts strains (seven belonging to the species Saccharomyces cerevisiae and six belonging to the species Kluyveromyces marxianus, Zygosaccharomyces bisporus, Z. mellis, Z. rouxii, Schizosaccharomyces pombe and Saccharomycodes ludwigii) was assessed in a model system. The results showed non-linear death curves and a high variability also within the same species. The most resistant strain, belonging to the species S. cerevisiae, was chosen for further experiments in orange juice based industrial beverages: first, death curves were performed; then, the probability of beverage spoilage in relation to process parameters (initial inoculum, temperature, treatment time) was evaluated using a logistic regression model. Finally, a cross-validation was performed to investigate the predictive capability of the fitted model. Pasteurization in the soft drink industry is commonly applied according to parameters defined several decades ago, which does not consider the successive findings concerning microbial physiology and stress response, the process improvement and the more recent tools provided by predictive microbiology. In this perspective, this study can fill a gap in the literature on this subject, going to be a basis for optimizing thermal processes. In fact, the data obtained indicated an interesting possibility for food industry to better modulated (and even reduce) thermal treatments, with the aim to guarantee microbial stability while reducing thermal damage and energy costs.

Roles of kink on the thermal transport in single polyethylene chains

The trans-gauche state transformation commonly exists in polymers. However, the fundamental understanding of the roles of kink (gauche state) on the thermal energy transport in polymer chains is rather limited. From atomic simulations, we show that kinks greatly scatter phonons in single polyethylene chains, and even a single kink can reflect more than half of the phonons. Further studies show that kinks not only add extra thermal resistance to the chain but also break the whole chain into small segments and each with reduced thermal conductivity. A simple series thermal resistance model is proposed to estimate the effective thermal conductivity of single polymer chains with multiple kinks.

Year-round performance analysis of a photovoltaic panel coupled with phase change material

Phase change material (PCM) is in employed in photovoltaic (PV) system for thermal regulation and efficiency improvement. The hybrid system is named as a PV-PCM system. While progress is achieved in understanding system performance in laboratory and daily simulation, few results are reported in a seasonal and yearly basis, which requires substantial computation time to solve Navier-Stokes equations. In this study, an enhanced conductivity method is applied to simulate the convective effect without solving N-S equations. The model is developed based on 1-D thermal resistance model and validated with experimental data, possessing a good balance between accuracy and simplicity. The model is, therefore, adopted to examine and optimize seasonal and yearly PV-PCM performance. Within five systems simulated, the result suggests that systems perform diversely under different weather conditions. For instance, PV-PCM system with high melting temperature usually performs well in summer while might prevent heat transfer in winter since the PCM cannot be melt in cold days. Finally, the highest year-round improvement in electricity production 2.46% compared to reference PV system, which is lower than the value reported in most studies conducted only on a sunny day or in laboratory in a short-time scale. Moreover, the economic analysis indicates that, without a significant improvement in PCM performance or exploiting electricity-and-heat cogeneration, PV-PCM system may not be viable for real application at this stage.

Air-cooled Design of 4 kW DC Bus Converter on Airplane

To meet the application requirements of 4 kW DC 270 to DC 28 V converter on airplane, an air-cooled design method is proposed. According to converter’s circuit structure, the losses of main components and the air-cooled design structure are determined. The thermal resistance model of the air-cooled is built, and the process of the air-cooled design is illustrated. Then the air pressure and air volume of fan are determined. The results demonstrate that the temperature rise of the heat sink is less than 35°C when the output is 4 kW and the converter can work reliably when ambient temperature is 70°C.

Efficient waste heat recovery system for high-temperature solid particles based on heat transfer enhancement

A series of efficient heat recovery methods including PCHEE-A, PCHEE-K and a double-medium system is proposed in this paper, which is to provide comprehensive solutions for high temperature particle in different situations. The PCHEE-A and PCHEE-K can increase the heat transfer area and heat transfer coefficient, through extending specific area to 1000–2000 m2/m3 and inducing turbulence in the heat transfer boundary, respectively. For the double-medium system which can achieve deep heat recovery and utilization, a multi-dimensional thermal resistance model is established to conduct heat analysis. Based on the model, Pareto frontier optimization of the double-medium system is carried out, and the optimal medium arrangement as well as the heat load distribution is derived. By combing the above methods, three typical systems (single-medium vertical plate-type with PCHEE-A, double-medium vertical tube-type and horizontal-type with PCHEE-K) are established and can get heat recovery efficiency at around 80%.

Creeping control strategy for Direct Shift Gearbox based on the investigation of temperature variation of the wet multi-plate clutch

Proposing an appropriate control strategy is an effective and practical way to address the overheat problems of the wet multi-plate clutches in Direct Shift Gearbox under the long-time creeping condition. To do so, the temperature variation of the wet multi-plate clutch is investigated first by establishing a thermal resistance model for the gearbox cooling system. To calculate the generated heat flux and predict the clutch temperature precisely, the friction torque model is optimized by introducing an improved friction coefficient, which is related to the pressure, the relative speed and the temperature, before being demonstrated experimentally using a full scale powertrain test. After that, the verified heat transfer model and the reasonable friction torque model are employed by the vehicle powertrain model to construct a comprehensive co-simulation model for the Direct Shift Gearbox vehicle, capable of simulating the dynamic responses and predicting the temperature variations of two clutches. A creeping control strategy is then proposed and, to evaluate the vehicle performance, the safety temperature (250°C) is particularly adopted as an important metric. From the variations in torque and speed obtained from the simulation results, the vehicle can track the desired speed (1.5 km/h) satisfactorily, with only 3% fluctuation, and have good longitudinal dynamic performance (8.5 m/s3). But above all, during the entire 174 s creeping process, the temperature of two clutches is always under the safety value (250°C), which demonstrates the effectiveness of the proposed control strategy in avoiding the thermal failures of clutches.

A Novel Thermal Network Model Used for Temperature Calculation and Analysis on Brushless Doubly-Fed Generator With Winding Encapsulating Structure

In recent years, magnetic-barrier rotor has been put forward for brushless doubly-fed generator (BDFG) application owing to its desirable performance, such as high power density and strong magnetic coupling ability. However, it also causes significant losses induced by harmonics and high temperature problem. Hence, it is a major challenge to evaluate Bref_DFG temperature rise fast and accurately. So far, aiming at magnetic-barrier rotor, there has been no accurate thermal network model established achieving this goal. In addition, wingding end encapsulating technology has been used for a few years and its heat dissipation potential is determined by the materials. Therefore, the application of novel material in winding encapsulating structure is desirable to improve the heat dissipation potential. In this paper, an equivalent thermal network model, thermal resistance models of magnetic-barrier rotor, and a winding encapsulating structure with novel materials are further established for thermal analysis. Subsequently, temperature distributions for BDFG components are also calculated by finite element method, while the results are compared with those by analytical method. The BDFG prototype is manufactured with experimental tests performed. The correctness of the equivalent thermal network models proposed in this paper is verified by the test results.

Modeling of the temperature field in a porous thermal barrier coating

Abstract Thermal barrier coatings (TBCs) are advanced materials systems with low thermal conductivity. One of the reasons for the low thermal conductivity in TBCs is that they contain porous structures created by a network of micro-voids. In the present investigation, experimental and analytical studies of heat transfer in TBCs having different levels of porosity were performed. The ceramic coatings were analyzed using scanning electron microscopy to calculate the level of porosity and micro-pore size distribution. A two-dimensional FE model was then developed, where a stochastic method was used to define randomly distributed porous structures equivalent to porosities of 1%, 3%, and 5%. The results showed that the heat flux and temperature gradient were affected by the interactions between neighboring micro-pores and micro-pores/ceramic coatings, and that the effect of the micro-pores was limited to a small area (2–2.5 times the micro-pore radius). Based on the obtained results, a set of effective thermal conductivity equations are proposed which more clearly describe the heat transfer process in a porous TBC structure. Two different equivalent thermal resistance models were used to study the heat transfer process under low porosity ( 3%) conditions.

Thermal transport in monocrystalline and polycrystalline lithium cobalt oxide.

Efficient heat dissipation in batteries is important for thermal management against thermal runaway and chemical instability at elevated temperatures. Nevertheless, thermal transport processes in battery materials have not been well understood especially considering their complicated microstructures. In this study, lattice thermal transport in lithium cobalt oxide (LiCoO2), a popular cathode material for lithium ion batteries, is investigated via molecular dynamics-based approaches and thermal resistance models. A LiCoO2 single-crystal is shown to have thermal conductivities in the order of 100 W m-1 K-1 with strong anisotropy, temperature dependence, and size effects. By comparison, polycrystalline LiCoO2 is more isotropic with much lower thermal conductivities. This difference is caused by random grain orientations, the thermal resistance of grain boundaries, and size-dependent intra-grain thermal conductivities that are unique to polycrystals. The grain boundary thermal conductance is calculated to be in the range of 7.16-25.21 GW m-2 K-1. The size effects of the intra-grain thermal conductivities are described by two empirical equations. Considering all of these effects, two thermal resistance models are developed to predict the thermal conductivity of polycrystalline LiCoO2. The two models predict a consistent thermal conductivity-grain size relationship that agrees well with molecular dynamics simulation results. The insights revealed by this study may facilitate future efforts on battery materials design for improved thermal management.

Modeling and application of thermal contact resistance of ball screws

Aiming at determining the thermal contact resistance of ball screws, a new analytical method combining the minimum excess principle with the MB fractal theory is proposed to estimate thermal contact resistance of ball screws considering microscopic fractal characteristics of contact surfaces. The minimum excess principle is employed for normal stress analysis. Moreover, the MB fractal theory is adopted for thermal contact resistance. The effectiveness of the proposed method is validated by self-designed experiment. The comparison between theoretical and experimental results demonstrates that thermal contact resistance of ball screws can be obtained by the proposed method. On this basis, effects of fractal parameters on thermal contact resistance of ball screws are discussed. Moreover, effects of the axial load on thermal contact resistance of ball screws are also analyzed. The conclusion can be drawn that the thermal contact resistance decreases along with the fractal dimension D increase and it increases along with the scale parameter G increase, and thermal contact resistance of ball screws is retained almost constant along with axial load increase before the preload of the right nut turns into zero in value. The application of the proposed method is also conducted and validated by the temperature measurement on a self-designed test bed.

Modeling of concrete thermal power resistance during the high-temperature heating

A model of concrete deformation under the high temperature heating in a loaded state is proposed. It reflects the physical nature of deformation has a rather compact form and requires a minimum number of normalized parameters. In order to describe the curve of concrete deformation and temperature parameters change exponential relationships were assumed and calibration of their coefficients was performed.

Avoiding Being Trapped in False Analogical Modeling of Composite Wall Thermal Resistance

Because Analogy is considered as a double-edged sword, thermal engineers should be cautious in analogical maneuvering between electrical and thermal domains in order not to be slipped into building misconceptions about thermal resistance concept. Composite wall thermal resistance (CWTR) modeling is one of the practical examples that illustrates the probability of misusing analogy. Heat transfer undergraduate textbooks coverage of CWTR suffers a lean towards “cookbook” coverage that reports concise statements that lack deep clarification and illustration. Transparent Thinking Approach (TTA) is employed to present a detailed calculation and illustration of a typical CWTR modeling based on isothermal and adiabatic assumptions. The calculation of a typical CWTR for different values of wall thermal conductivities shows that the difference in parallel walls thermal conductivity is creating a large discrepancy that may reach 80% between heat flows calculated based on isothermal and adiabatic assumptions. It is found that for a series-parallel arrangement of composite walls with high difference in parallel wall thermal conductivity values, the true value of heat flow is bracketed between the isothermal and adiabatic heat flow values. The transparent way of presenting CWTR modeling can be readily included in any standard heat transfer textbook and result in greatly enhancing CWTR modeling coverage.

Accurate and efficient analysis of the upward heat flow in InGaP/GaAs HBTs through an automated FEM‐based tool and Design of Experiments

AbstractThis paper presents an extensive analysis aimed at quantifying the impact of all the key technology parameters on the upward heat flow in state‐of‐the‐art InGaP/GaAs heterojunction bipolar transistors (HBTs) for various emitter areas and shapes. Extremely accurate thermal simulations are conducted in a relatively short time with a tool relying on a commercial 3‐D finite‐element method (FEM) solver and an in‐house routine for automated geometry construction, optimized mesh generation, sequential solution, and data storing/processing. Design of Experiments is used to define a thermal resistance model as a function of the aforementioned parameters on the basis of a few FEM data.

A Universal Scalable Thermal Resistance Model for Compact Large-Signal Model of AlGaN/GaN HEMTs

This paper presents a universal scalable thermal resistance model for AlGaN/GaN high electron-mobility transistors (HEMTs), which can be easily implemented into compact large-signal models and is very suitable for rapid device modeling. It contains all the key HEMT device geometric parameters such as gate length, gate width, gate pitch, and different layer thicknesses. The temperature-dependent conductivities of different materials are taken into consideration. The via-holes effects on the thermal resistance of a device are also included, which is considered for the first time in a compact largesignal model. Two uniform parameters are used to simplify the complicated analytical closed-form expression of thermal resistance, which is difficult to be implemented into the compact model. Another two uniform parameters are used to simplify the complicated transcendental function of the temperature rise with respect to the temperature-dependent thermal conductivity. Thus a simple, accurate, and scalable thermal resistance model can be obtained. The results are verified by comparing with experimental and finite-element numerical simulations. The dc I-V characteristics and large-signal performances of HEMTs devices with different gate lengths 0.15, 0.25, and 0.4 μm are used for further verifications. The proposed thermal resistance model can also be used to optimize the structural geometry of a device to obtain minimum thermal resistance.

Analysis on Self-Heating Effects in Three-Stacked Nanoplate FET

In this paper, self-heating effects (SHEs) in three-stacked nanoplate FETs were investigated through the TCAD simulation. In order to obtain high reliability, the evaluation of SHEs was performed after ${I}_{D}$ – ${V}_{G}$ curve fitting based on the experimental data. First, general analysis on SHEs was conducted to confirm the influence of SHEs to electrical characteristics. The optimized nanoplate width for great electrical properties was proposed by using the figure-of-merit factor under the consideration of SHEs. In addition, difference of heat flux between FinFET and stacked nanoplate FET was analyzed. Based on the analysis, the two-step thermal resistance ( ${R}_{\text {th}}$ ) model depending on drain voltage was proposed. The two-step ${R}_{\text {th}}$ model in the stacked nanoplate FET matched well with the Berkeley short-channel IGFET model—common multigate model compared the other ${R}_{\text {th}}$ models. A seven-stage ring oscillator with the proposed ${R}_{\text {th}}$ model was demonstrated, and SHEs in the circuit level were confirmed.