Numerical prediction of turbulent flow and heat transfer with phase change in crystallizer of inverse casting

horoductionInverse casting is a technique for Producing nearnet-shape cast strips. T'he main idea of this technique isdeveloped from a successful inveshgation['], in which ithas been Proved that the inverse cashng teclmiquewould be applicable to produce the steel strip with aabackness of 0.5-3 nun, with a good material prOPertyand with a lower energy consumption in contrast withconventional conhnuous cashng process. The POssibilitytO p~ce composite strips is also one of the mostattrachve pro…     

组合式高温相变蓄热器蓄热过程的数值模拟

通过对相同管径蓄热管组成的高温相变蓄热器蓄热过程进行模拟,得到了蓄热过程中相变材料(PCM)温度和液相率随时间的变化曲线以及不同时刻液相率的分布云图。针对温度曲线和云图所显示的问题,在保证蓄热量的前提下,提出了蓄热管组合式排布的设计方案,并对其蓄热性能进行了模拟。研究结果表明,采用所提出的组合式方案有效地减少了蓄热时间,降低了"死区"对蓄热器蓄热性能的影响,使PCM的液相率分布更加均匀,可为用于太阳能热发电的高温相变蓄热器的优化设计提供理论依据。     

Creep damage prediction of the steam pipelines with high temperature and high pressure

Creep is the significant factor that caused failure of steam pipelines with high temperature and high pressure in the period of long-term service. In this paper, the creep tests were performed at serviced temperature of 520 °C for 1.25Cr–0.5Mo pipe material, and the creep and fracture constants were obtained by fitting the creep test data. Based on the modified Karchanov–Rabotnov constitutive equation, compiled the user subroutine computing the damage of the pipe element or 3D solid element, the creep damage prediction was carried out by finite element methods using ABAQUS codes for the steam pipelines with high temperature and high pressure, which serviced in a petro-chemical plant, the damage distribution and maximum damage location of the pipelines were obtained, which is testified by metallographic examination result. Furthermore, the local creep damage analysis of a tapered pipe serviced for 100,000 h was also carried out because tapered pipes used in the main steam pipeline is one of weakness in the piping system. Damage distribution and evolution in the analyzed tapered pipe were obtained. The location with the maximum damage value was determined, which is coincident with cracking position of the actual tapered pipe.     

固-固相变蓄热材料在太阳能高温空气集热系统中的应用研究

为利用太阳能获得稳定持续的高温空气工质,除了有效集热外,还需要解决因太阳辐射强度变化导致输出工质温度波动的问题。在性能优良的太阳能集热系统中采用蓄热技术是解决此问题的有效途径。根据给定的设计目标,研究将固-固相变蓄热材料季戊四醇应用到太阳能集热蓄热一体化的实验装置中。实验结果表明:按集热蓄热一体化思路设计的实验装置,集热单元能够输出最高温度超过220℃的高温空气,蓄热单元能够将高温空气的温度稳定在蓄热材料的相变温度附近。并且随着蓄热管级数的增加,空气出口温度稳定的时间就越长,为利用太阳能获得稳定持续的高温热媒工质奠定了基础。     

A comparison of extended Kalman filter,particle filter,and least squares localization methods for a high heat flux concentrated source

State estimation procedures using the extended Kalman filter, particle filter, and least squares are investigated for a transient heat transfer problem in which a high heat flux concentrated source is applied on one side of a thin plate and ultrasonic pulse time of flight is measured between spatially separated transducers on the opposite side of the plate. This work is an integral part of an effort to develop a system capable of locating the boundary layer transition region on a hypersonic vehicle aeroshell. Results from thermal conduction experiments involving one-way ultrasonic pulse time of flight measurements are presented. Comparisons of heating source localization measurement models are conducted where ultrasonic pulse time of flight readings provide the measurement update to the extended Kalman filter, particle filter, and least squares. Two different measurement models are compared: (1) directly using the one-way ultrasonic pulse time of flight as the measurement vector and (2) indirectly obtaining distance from the one-way ultrasonic pulse time of flight and then using these obtained distances as the measurement vector. For the direct model, the Jacobian required by the extended Kalman filter and least squares is obtained numerically using finite differences and a finite element forward conduction solution. For the indirect model, the derivatives with respect to the state variables are obtained in closed form. Heating source localization results and convergence behavior are compared for the three inverse methods and the two measurement models. The extended Kalman filter, least squares, and particle filter methods using the one-way ultrasonic pulse time of flight measurement model (direct model) produced similar results when considering accuracy of converged solution, ability to converge to the correct solution, and smoothness of convergence behavior. The results provide quantified justification for moving forward with development of an extended Kalman filter-based localization solution.     

An analytical analysis for the prediction of nonlinear inverse temperature profiles in solid explosives

A novel analytical method developed by Mansour and Hussein (11) has been used for the evaluation of critical thermal conditions for a non-linear inverse temperature distribution model of an explosive. The analytical results presented in this work have been compared with previously published numerical results and shown to be accurate. The advantage of this new method is that it can be easily applied to other inverse non-linear models arising in boundary layer, kinetics, electronics, vibration, combustion … etc.     

Transient analysis of coupled high temperature nuclear reactor to a thermochemical hydrogen plant

A very high temperature reactor (VHTR) has been considered for generation of hydrogen by water-splitting through thermochemical processes using process heat. The development of this technology requires understanding of the coupled behavior of VHTR and the hydrogen generation plants and key safety issues during transients. A system level modeling of the coupled VHTR and sulfur-iodine hydrogen generation process is carried out where the thermal and neutronic models of the VHTR and the chemical models of the sulfur-iodine cycle were developed and benchmarked with available experimental data, models and simulation codes. Transient analysis of the possible accidents emanating either from VHTR or hydrogen plants was carried out. The results indicate that on VHTR side the response of the reactor is affected by the temperature feedback and xenon buildup feedback. On the hydrogen plant side the slowest reaction controlled the speed of the overall response of the chemical plant.     

侧壁面正弦加热条件下电场强化固液相变研究

本文采用格子Boltzmann方法对侧壁面正弦加热条件下的方腔潜热储热单元的蓄热过程进行模拟与分析,系统研究了电场对相变材料相变过程的强化效果以及加热壁面温度分布变化的振幅、角频率、初相对电场强化效果的影响.结果表明,与没有电场相比,施加电场以后,潜热储热单元的蓄热速率显著提升,且电场越强,蓄热速率提升越明显.此外,加...     

Thermal characteristics of shape-stabilized phase change material wallboard with periodical outside temperature waves

Thermal characteristics of shape-stabilized phase change material (SSPCM) wallboard with sinusoidal temperature wave on the outer surface were investigated numerically and compared with traditional building materials such as brick, foam concrete and expanded polystyrene (EPS). One-dimensional enthalpy equation under convective boundary conditions was solved using fully implicit finite-difference scheme. The simulation results showed that the SSPCM wallboard presents distinct characteristics from other ordinary building materials. Phase transition keeping time of inner surface and decrement factor were applied to analyze the effects of PCM thermophysical properties (melting temperature, heat of fusion, phase transition zone and thermal conductivity), inner surface convective heat transfer coefficient and thickness of SSPCM wallboard. It was found that melting temperature is one important factor which influences both the phase transition keeping time and the decrement factor; for a certain outside temperature wave, there exist critical values of latent heat of fusion and thickness of SSPCM above which the phase transition keeping time or the decrement factor are scarcely influenced; thermal conductivity of PCM and inner surface convective coefficient have little effect on the phase transition keeping time but significantly influence the decrement factor; and the phase transition zone leads to small fluctuations of the original flat segment of inner surface temperature line. The results aim to be useful for the selection of SSPCMs and their applications in passive solar buildings.     

Self regulation of photovoltaic module temperature in V-trough using a metal-wax composite phase change matrix

The present study was carried out to take advantage of the enhanced solar insolation in V-trough while limiting the temperature of the photovoltaic (PV) module at around the maximum (ca. 65 °C) observed for conventional usage without any concentration. Paraffin wax of 56-58 °C melting range was chosen as phase change material (PCM) and incorporated at the rear of the module to absorb the excess heat. The problem of low thermal conductivity of the wax was solved with the help of packed metal turnings wherein the wax resided. Two sets of experiments were performed indoor and outdoor. Employing a 0.06 m thick bed of the PCM matrix, the module temperature in the indoor experiment could be maintained at 65-68 °C for 3 h whereas in its absence the temperature rose beyond 90 °C within 15 min. In outdoor studies, the module temperature in V-trough could be reduced from 78 °C to 62 °C with the PCM assembly and operation could be sustained throughout the day. Using the V-trough PV-PCM system, the output power over the day could be enhanced 1.55 times with self regulation of temperature. The molten wax formed during operation re-solidified during the evening and night and could be re-used. A significant finding was the safe operation of the module even under low wind velocity conditions without sacrificing operational simplicity.     

Estimation of effective solar irradiation using an unscented Kalman filter in a parabolic-trough field

Measurement of direct solar radiation has been shown to be very useful to improve control performance and disturbance rejection in solar fields by anticipating the effect of sudden changes in solar radiation due to clouds. Since direct solar radiation is measured locally by pyrheliometers, important errors in the estimation of the overall effective solar radiation can be produced when the pyrheliometer is covered by clouds while the rest of the solar field is not or viceversa. Furthermore, estimation of the overall efficiency affected by the reflectivity and absortance of metal tubes is very difficult because only local measurements can be obtained. This work proposes an algorithm for estimating overall solar radiation and efficiency at the field. The algorithm uses an unscented Kalman filter and it is validated by data obtained at the Plataforma Solar of Almer?´a (Spain).     

Quasi-steady-state model of a counter-flow air-to-air heat-exchanger with phase change

Using mechanical ventilation with highly efficient heat-recovery in northern European or arctic climates is a very efficient way of reducing the energy use for heating in buildings. However, it also presents a series of problems concerning condensation and frost formation in the heat-exchanger. Developing highly efficient heat-exchangers and strategies to avoid/remove frost formation implies the use of detailed models to predict and evaluate different heat-exchanger designs and strategies. This paper presents a quasi-steady-state model of a counter-flow air-to-air heat-exchanger that takes into account the effects of condensation and frost formation. The model is developed as an Excel spreadsheet, and specific results are compared with laboratory measurements. As an example, the model is used to determine the most energy-efficient control strategy for a specific heat-exchanger under northern European and arctic climate conditions.     

相变材料与水套式液冷结构耦合的圆柱型锂离子电池组热管理仿真分析

针对圆柱型锂离子电池组散热问题,设计了一种新型的相变材料(PCM)-水套式液冷耦合散热结构模型.首先研究了电池组在PCM模型的散热下,不同电池间距对电池组表面温度的影响,并得出PCM模型的最佳电池布局.然后根据PCM模型的最佳电池布局,优化PCM-水套式液冷耦合散热结构模型,即找出PCM散热模型的最佳流道结构.通过仿真分析结果表明,在6流道结构模型下,电池之间的最佳间距为8 mm;PCM-水套式液冷耦合散热模型的效果最佳,在3 C和5 C高倍率放电时,电池组的表面最高温度分别为33.78、41.11℃,相比于同尺寸PCM散热模型的最高温度,分别降低了7.23、1.06℃.采用PCM-水套式液冷耦合散热模型,电池之间的最大温差均维持在5℃以内.结果表明:该新型的PCM-水套式液冷耦合散热结构能在一定程度上保证电池组的正常工作,并提高电池组的安全性和耐用性.     

A 3D computational model of heat transfer coupled to phase change in multilayer materials with random thermal contact resistance

An improved understanding of the heat transfer in materials consisting of two layers (splat and substrate) is essential for many industrial applications. We are interested in the deposition, rapid cooling and solidification of metal droplets (known as splats) brought into contact with a cold substrate. We therefore need to understand the temperature history in both the splat and the substrate, including phase change phenomena. A new model of the thermal contact resistance based on a random distribution of contact points, rather than the uniform distribution commonly used, is presented in this paper.Phase change has been also considered, using the enthalpy-porosity formulation. Simulations have been conducted with the commercial package CFX-4. The computational results for the cooling rate of the splat obtained using the random contact distribution model are in good agreement with available experimental results. In addition, results obtained from the random model provide information on the inhomogeneity affecting the temperature at the interface between the splat and the substrate.     

Analysis of a flat plate collector with fluid undergoing phase change

This paper presents a theoretical analysis of the performance of a flat plate solar collector with the heat removal fluid undergoing a phase change. The resultant efficiency expression is a modified Hottel-Whillier-Bliss equation. Numerical computations are made to investigate the effect of vaporisation and operational parameters on the collector's performance. The collector's efficiency increases with the increase in liquid length until a point is reached when the region of superheating the vapour disappears. The efficiency is higher when a heat removal fluid of high latent heat of vaporisation is used in the collector. An increase in the saturation temperature of the working fluid (with increase of pressure) in the collector reduces its efficiency.     

3D transient multiphysics modelling of a complete high temperature fuel cell system using coupled CFD and FEM

Full commercialisation of the solid oxide fuel cell (SOFC) technology faces many technological challenges that prevent the incorporation of the technology into the global energy sector. The effort to increase the transient thermomechanical reliability of the interacting fuel cell components and the associated fuel cell system requires a comprehensive understanding of the complex multiphysics, occurring within the system. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. The present study introduces a transient, coupled 3D computational fluid dynamics/computational solid mechanics model of a complete solid oxide fuel cell system and its experimental validation. The model includes all system components; namely the fuel cell stack, afterburner, pre-reformer, air pre-heater and the auxiliary components. All components are presented in their real geometrical resolution. The capabilities of the 3D system level model are demonstrated by simulating the heating-up process and the critical system locations susceptible to thermomechanically induced stress, over time.     

Active (air-cooled) vs. passive (phase change material) thermal management of high power lithium-ion packs: Limitation of temperature rise and uniformity of temperature distribution

The effectiveness of passive cooling by phase change materials (PCM) is compared with that of active (forced air) cooling. Numerical simulations were performed at different discharge rates, operating temperatures and ambient temperatures of a compact Li-ion battery pack suitable for plug-in hybrid electric vehicle (PHEV) propulsion. The results were also compared with experimental results. The PCM cooling mode uses a micro-composite graphite–PCM matrix surrounding the array of cells, while the active cooling mode uses air blown through the gaps between the cells in the same array. The results show that at stressful conditions, i.e. at high discharge rates and at high operating or ambient temperatures (for example 40–45 °C), air-cooling is not a proper thermal management system to keep the temperature of the cell in the desirable operating range without expending significant fan power. On the other hand, the passive cooling system is able to meet the operating range requirements under these same stressful conditions without the need for additional fan power.