Ni/MH电池过充电过程热效应分析

论文采用二维热模型分析了圆柱型Ni/MH电池在过充电过程中的热效应.实验提供了更为精准的数据以建立精确的热模型.利用石英频率微量热仪对电池的热容量以及电池在不同电流过充电时的发热量和散热速率进行了测量,继而将散热速率曲线拟合成线性函数和三段不同的指数函数.线性阶段之后的散热过程符合数学微分表达式,这些表达式有助于理解过渡阶段和过充电阶段散热速率的变化规律.热传导方程中产热速率采用理论计算值.最后使用FEM模拟了电池在1C,3C,5C充电过程每一阶段末时刻的电池内部温度场分布,结果相对准确.     

Computational study of a latent heat thermal energy storage system enhanced by highly conductive metal foams and heat pipes

Numerical simulations are performed to analyze the thermal characteristics of a latent heat thermal energy storage system with phase change material embedded in highly conductive porous media. A network of finned heat pipes is also employed to enhance the heat transfer within the system. ANSYS-FLUENT 19.0 is used to create a transient multiphase computational model to simulate the thermal behavior of the storage unit. Copper foam is the porous medium used to enhance the heat transfer and is impregnated with the phase change material, potassium nitrate (KNO₃). The effects of the porosity of the metal foam and the quantity of heat pipes on the thermal characteristics of storage unit have been investigated. The results indicated that increasing the quantity of the embedded heat pipes leads to drastic acceleration of both charging and discharging process. Impregnating the copper foam with potassium nitrate phase change material significantly affects the total charging and discharging times of the storage unit. It was shown that the porosity of the metal foam plays a key role in the thermal behavior of the system during the charging and discharging processes.

     

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au−SiO2 Nanoparticles

Relaxation dynamics of plasmons in Au−SiO₂ core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.     

Characterization of irregular open-cell cellular structure with silicone pore filler

The paper presents the results of studying the influence of silicone polymer pore filler on the macroscopic quasi-static and dynamic compressive behaviour of aluminium foam with irregular open-cell structure. The study is based on a mechanical experimental testing programme, where the deformation mechanism and mechanical energy absorption capacity of aluminium foam with silicone pore filler have been observed for the first time. As plastic yielding is accompanied by significant heat energy dissipation, this study was additionally supported by thermal imaging, which enables visualization of plastification to better understand the deformation process of observed specimens. The influence of specimen size on the behaviour of aluminium foam specimens has also been investigated. The results show that introduction of silicone pore filler considerably increases the energy absorption capacity at almost unchanged densification strain under both quasi-static and dynamic loading conditions. The silicone pore filler also significantly influences the deformation behaviour of aluminium foam specimens, which is manifested in a different stress distribution and a significant transverse deformation with conical plastification front. However, only a minor difference in response of different size specimens has been observed.     

Numerical simulation and parametric analysis of latent heat thermal energy storage system

This paper presents the numerical analysis of the transient performance of the latent heat thermal energy storage unit established on finite difference method. The storage unit consists of a shell and tube arrangement with phase change material (PCM) filled in the shell space and the heat transfer fluid (HTF) flowing in the inner tube. The heat exchange between the HTF, wall and PCM has been investigated by developing a 2-D fully implicit numerical model for the storage module and solving the complete module as a conjugate problem using enthalpy transforming method. A comparative investigation of the total melting time of the PCM has been performed based on natural convection in liquid PCM during the charging process. The novelty of this paper lies in the fact it includes convection in PCM and this investigation includes a detailed parametric study which can be used as a reference to design latent heat storage. The results indicate that natural convection accelerates the melting process by a significant amount of time. In order to optimize the design of the thermal storage unit, parametric study has been accompanied to analyze the influence of various HTF working conditions and geometric dimensions on the total melting time of the PCM. Another important feature considered in this work is the influence of the inner wall of the tube carrying the HTF on the entire melting time of the PCM. An error of around 7.2% is reported when inner wall of the tube is ignored in the analysis.

     

Thermal Energy Storage and Heat Transfer of Nano-Enhanced Phase Change Material (NePCM) in a Shell and Tube Thermal Energy Storage (TES) Unit with a Partial Layer of Eccentric Copper Foam

Thermal energy storage units conventionally have the drawback of slow charging response. Thus, heat transfer enhancement techniques are required to reduce charging time. Using nanoadditives is a promising approach to enhance the heat transfer and energy storage response time of materials that store heat by undergoing a reversible phase change, so-called phase change materials. In the present study, a combination of such materials enhanced with the addition of nanometer-scale graphene oxide particles (called nano-enhanced phase change materials) and a layer of a copper foam is proposed to improve the thermal performance of a shell-and-tube latent heat thermal energy storage (LHTES) unit filled with capric acid. Both graphene oxide and copper nanoparticles were tested as the nanometer-scale additives. A geometrically nonuniform layer of copper foam was placed over the hot tube inside the unit. The metal foam layer can improve heat transfer with an increase of the composite thermal conductivity. However, it suppressed the natural convection flows and could reduce heat transfer in the molten regions. Thus, a metal foam layer with a nonuniform shape can maximize thermal conductivity in conduction-dominant regions and minimize its adverse impacts on natural convection flows. The heat transfer was modeled using partial differential equations for conservations of momentum and heat. The finite element method was used to solve the partial differential equations. A backward differential formula was used to control the accuracy and convergence of the solution automatically. Mesh adaptation was applied to increase the mesh resolution at the interface between phases and improve the quality and stability of the solution. The impact of the eccentricity and porosity of the metal foam layer and the volume fraction of nanoparticles on the energy storage and the thermal performance of the LHTES unit was addressed. The layer of the metal foam notably improves the response time of the LHTES unit, and a 10% eccentricity of the porous layer toward the bottom improved the response time of the LHTES unit by 50%. The presence of nanoadditives could reduce the response time (melting time) of the LHTES unit by 12%, and copper nanoparticles were slightly better than graphene oxide particles in terms of heat transfer enhancement. The design parameters of the eccentricity, porosity, and volume fraction of nanoparticles had minimal impact on the thermal energy storage capacity of the LHTES unit, while their impact on the melting time (response time) was significant. Thus, a combination of the enhancement method could practically reduce the thermal charging time of an LHTES unit without a significant increase in its size.     

The Thermal Charging Performance of Finned Conical Thermal Storage System Filled with Nano-Enhanced Phase Change Material

A latent heat thermal energy storage (LHTES) unit can store a notable amount of heat in a compact volume. However, the charging time could be tediously long due to weak heat transfer. Thus, an improvement of heat transfer and a reduction in charging time is an essential task. The present research aims to improve the thermal charging of a conical shell-tube LHTES unit by optimizing the shell-shape and fin-inclination angle in the presence of nanoadditives. The governing equations for the natural convection heat transfer and phase change heat transfer are written as partial differential equations. The finite element method is applied to solve the equations numerically. The Taguchi optimization approach is then invoked to optimize the fin-inclination angle, shell aspect ratio, and the type and volume fraction of nanoparticles. The results showed that the shell-aspect ratio and fin inclination angle are the most important design parameters influencing the charging time. The charging time could be changed by 40% by variation of design parameters. Interestingly a conical shell with a small radius at the bottom and a large radius at the top (small aspect ratio) is the best shell design. However, a too-small aspect ratio could entrap the liquid-PCM between fins and increase the charging time. An optimum volume fraction of 4% is found for nanoparticle concentration.     

Heat Transfer in Gas Phase Olefin Polymerisation

A fixed bed microreactor has been used to study heat transfer during the initial transient state of gas phase olefin polymerization on a supported catalyst. It has been shown that heat transfer during this stage of the polymerisation is critical, and under conditions found commercially problems can arise with hot spots and polymer melting. It is proven how the thermal properties of the gas mixture flowing on the catalytic bed exert great influence on heat dissipation reducing the sudden increase in temperature by as much as a factor of 5. Flow rate and especially the process gas composition are the key factors in controlling the bed temperature.     

Thermal behavior of annealed organic glasses

The influence of annealing processes on the thermal behavior of organic glasses in the glass-transition interval has been investigated and analyzed quantitatively. In detailed annealing studies of atactic polystyrene and Aroclor 5460, the absorption of thermal energy superposed on the increase in the specific heat at the glass transition, observed with suitably chosen heating rates, was followed by the differential thermal method. It is concluded that the absorption of thermal energy observed under these conditions parallels the extent of molecular relaxation that has taken place during the annealing period. It is not necessary to postulate a first-order process to account for the energy absorption. Moreover, such a postulate leads to severe conceptual difficulties regarding the development of crystallinity in crystallizable materials. The areas and the shapes of the endotherms are considered in terms of the original physical properties of the quenched glasses and the anticipated equilibrium properties. Relationships between the extent of energy absorption and time-dependent processes such as volume relaxation are discussed.     

Variable chemical species and thermo-diffusion Darcy–Forchheimer squeezed flow of Jeffrey nanofluid in horizontal channel with viscous dissipation effects

This research communicates the applications of thermos-diffusion effect associated to the squeezing flow of Jeffrey nanofluid due to horizontal channel. The problem presents the applications of inertial effects by following the Darcy–Forchheimer flow. Moreover, the effects of viscous dissipation and activation energy phenomenon has been discussed. The dimensionless attention of problem is retained. The shooting technique is implemented to present the numerical computations. The numerical validation of results is reported. The essential assessment of physical flow parameters is studied. The numerical outcomes are presented for heat and mass transfer phenomenon. It is observed that presence of inertial forces control to velocity flow in the regime. The enhancing contribution of thermal and concentration rate is noted for inertial constant.     

Influence of additives on the thermal behavior of nickel/metal hydride battery

This study discusses the thermal behavior of the 6.5 Ah cylinder Ni/MH hydride battery with 0.5 wt% ytterbium oxide (Yb₂O₃) in nickel electrode and 1.0 wt% super absorbent polymer (SAP) in hydrogen-storage alloy (MH) electrode during charging to 150% of its rating capacity. Quantity of heat and heat generation rate of the battery during charging are studied by quartz frequency microcalorimeter. Heat generation curve is fitted into a function, and heat transport equation is solved. Using measured data, the internal temperature profiles at the terminal moment of charging at 1C, 3C, and 5C are simulated by FEM. Influence of Yb₂O₃ and SAP on the thermal behavior of Ni/MH battery is examined by the two-dimensional thermal model. Results show that addition of Yb₂O₃ and SAP can achieve substantial improvement for thermal behavior of Ni/MH battery at 1C,3C, and 5C charging.     

Hyperthermal energy distribution functions of the adspecies in recombinative adsorption at catalytic surfaces and their relevance to ‘hot atom’ reactions

We report on the modeling of the energy distribution functions of the adspecies in diatom formation at catalytic surfaces under steady state conditions. To this end master equations are employed, in the case of either continuous or discrete set of adatom energy levels in the adsorption potential well, and the impact of the distribution function on reaction rate investigated. The transition from thermal to hyperthermal reaction rates has been studied as a function of rate coefficients for recombination and energy dissipation processes. Experimental data available from the literature, have been analysed in the framework of the theoretical model. It is shown that hyperthermal energy distribution functions entail a “hot atom” reaction mechanism.     

Experimental studies on performance augmentation of single hole cored brick sensible heat storage system using turbulence inducers

This article presents experimental analysis on performance augmentation of a single hole cored brick regenerator using turbulence inducers. Experiments were carried out for different velocities with air as the working fluid for both charging and discharging processes of a 455 mm long aluminum regenerator with inner and outer diameters of 26 mm and 40 mm, respectively. Two numbers of turbulence inducers of 1.5 mm diameter and 13 mm long were placed in ten different combinations and the results were compared with the trials wherein no inducers were used. The mean temperature of the cored brick, exit temperature during discharge, ratio of heat transfer rate to pressure drop, and exergetic efficiencies are the characteristics that were used to study the performance of the regenerator. Placement of inducers increased the mean temperature of the regenerator and the ratio of heat transfer rate to pressure drop by about 15% and a maximum of 40%, respectively, during charging. The exit air temperature during discharge exhibited maximum improvement of 18%. Increased exergetic efficiencies of more than 10% and 5% were estimated for charging and discharging, respectively. It was also observed that the addition of inducers does not necessarily result in an increased performance, and some of the combinations in fact deteriorated the performance of the regenerator.

     

低能离子散射谱分析中的荷电效应及中和作用

用低能离子散射谱（ISS）对比分析了与样品托有良好电接触的和与样品托绝比的金属银片，观察到荷电效应对ISS分析有严重影响。实验还表明在ISS分析时使用低能电子中和枪可以有效地消除荷电效应，还发现电子中和枪的使用对于惰性气体离子的中和作用无明显影响。     

Thermal Charging Optimization of a Wavy-Shaped Nano-Enhanced Thermal Storage Unit

A wavy shape was used to enhance the thermal heat transfer in a shell-tube latent heat thermal energy storage (LHTES) unit. The thermal storage unit was filled with CuO–coconut oil nano-enhanced phase change material (NePCM). The enthalpy-porosity approach was employed to model the phase change heat transfer in the presence of natural convection effects in the molten NePCM. The finite element method was applied to integrate the governing equations for fluid motion and phase change heat transfer. The impact of wave amplitude and wave number of the heated tube, as well as the volume concertation of nanoparticles on the full-charging time of the LHTES unit, was addressed. The Taguchi optimization method was used to find an optimum design of the LHTES unit. The results showed that an increase in the volume fraction of nanoparticles reduces the charging time. Moreover, the waviness of the tube resists the natural convection flow circulation in the phase change domain and could increase the charging time.     

Superlyophilic Interfaces Assisted Thermal Management

Thermal management has become a critical issue owing to the increasing need for various devices including heat dissipation and adsorption. Recently, the rapid growth of scientific reports is seen to improve thermal management efficiency by developing materials with high transfer coefficient and surface improvement to enhance heat transfer rate. Inspired by nature, constructing superlyophilic interfaces has been proved to be an effective way for thermal management and applied in industry and daily life. Herein, state-of-the-art developments of superlyophilic interfaces assisted thermal management are reported mainly from four perspectives around boiling, evaporation, radiation, and condensation. In particular, we discussed the unique role of superlyophilic interfaces during the heat transfer process, such as increasing bubble detachment rate, superspreading assisted efficient evaporation, directional liquid transfer in textiles during radiative cooling, and so forth. Finally, challenges of thermal management assisted by superlyophilic interfaces toward future applications are presented.     

Fire behaviors study on 18650 batteries pack using a cone-calorimeter

To investigate the effects of different state of charges (SOCs), external heating powers and charging/discharging treatment on the fire behaviors of 18650 batteries pack, three groups of abuse experiments were conducted with the help of a cone-calorimeter. The fire hazards of batteries pack were characterized by measuring the flame photographs, battery surface temperature, ignition time, thermal runaway time, heat release rate and radiative heat flux. According to the results, it is found that the fire behaviors of batteries pack will appear in advance and behave more violent with the increase in SOC. Additionally, the higher heating power will exacerbate the fire hazards of batteries pack by increasing the surface temperature rise rate, the total heat released and the total heat flux of pack leading to an earlier thermal runaway and more rigorous consequence. Finally, the pack with discharging/charging treatment has a much lower heat released compared to the pack without any treatment due to the incomplete burning and incomplete release of energy. Besides, their fire behaviors also exhibit earlier and severer.

     

Phase-Transition Thermal Charging of a Channel-Shape Thermal Energy Storage Unit: Taguchi Optimization Approach and Copper Foam Inserts

Thermal energy storage is a technique that has the potential to contribute to future energy grids to reduce fluctuations in supply from renewable energy sources. The principle of energy storage is to drive an endothermic phase change when excess energy is available and to allow the phase change to reverse and release heat when energy demand exceeds supply. Unwanted charge leakage and low heat transfer rates can limit the effectiveness of the units, but both of these problems can be mitigated by incorporating a metal foam into the design of the storage unit. This study demonstrates the benefits of adding copper foam into a thermal energy storage unit based on capric acid enhanced by copper nanoparticles. The volume fraction of nanoparticles and the location and porosity of the foam were optimized using the Taguchi approach to minimize the charge leakage expected from simulations. Placing the foam layer at the bottom of the unit with the maximum possible height and minimum porosity led to the lowest charge time. The optimum concentration of nanoparticles was found to be 4 vol.%, while the maximu possible concentration was 6 vol.%. The use of an optimized design of the enclosure and the optimum fraction of nanoparticles led to a predicted charging time for the unit that was approximately 58% shorter than that of the worst design. A sensitivity analysis shows that the height of the foam layer and its porosity are the dominant variables, and the location of the porous layer and volume fraction of nanoparticles are of secondary importance. Therefore, a well-designed location and size of a metal foam layer could be used to improve the charging speed of thermal energy storage units significantly. In such designs, the porosity and the placement-location of the foam should be considered more strongly than other factors.     

A Fast and Room-temperature Self-healing Thermal Conductive Polymer Composite

Thermal conducting materials may be damaged during long-term use, resulting in the increase of thermal resistance and therefore inefficient heat dissipation. The introduction of self-healing ability may solve this problem, but the realization of fast and room-temperature selfhealing in thermal conducting composites is quite challenging. Herein, we choose a flexible poly(dimethylsiloxane) polymer material(PDMSCOOH) as the matrix and graphene nanosheets as the thermal conductive filler to prepare ...     

石墨烯导热研究进展

石墨烯具有目前已知材料中最高的热导率,在电子器件、信息技术、国防军工等领域具有良好的应用前景。石墨烯导热的理论和实验研究具有重要意义,在最近十年间取得了长足的发展。本文综述了石墨烯本征热导率的研究进展及应用现状。首先介绍应用于石墨烯热导率测量的微纳尺度传热技术,包括拉曼光谱法、悬空热桥法和时域热反射法。然后展示了石墨烯热导率的理论研究成果,并总结了石墨烯本征热导率的影响因素。随后介绍石墨烯在导热材料中的应用,包括高导热石墨烯膜、石墨烯纤维及石墨烯在热界面材料中的应用。最后对石墨烯导热研究的成果进行总结,提出目前石墨烯热传导研究中存在的机遇与挑战,并展望未来可能的发展方向。