Dynamic Thermal Model and Temperature Control of Proton Exchange Membrane Fuel Cell Stack

A dynamic thermal transfer model of a proton exchange membrane fuel cell (PEMFC) stack is developed based on energy conservation in order to reach better temperature control of PEMFC stack. Considering its uncertain parameters and disturbance, we propose a robust adaptive controller based on backstepping algorithm of Lyaponov function. Numerical simulations indicate the validity of the proposed controller.

Extended Generalized Riccati Equation Mapping for Thermal Traveling-Wave Distribution in Biological Tissues through a Bio-Heat Transfer Model with Linear/Quadratic Temperature-Dependent Blood Perfusion

Analytical thermal traveling-wave distribution in biological tissues through a bio-heat transfer (BHT) model with linear/quadratic temperature-dependent blood perfusion is discussed in this paper. Using the extended generalized Riccati equation mapping method, we find analytical traveling wave solutions of the considered BHT equation. All the travelling wave solutions obtained have been used to explicitly investigate the effect of linear and quadratic coefficients of temperature dependence on temperature distribution in tissues. We found that the parameter of the nonlinear superposition formula for Riccati can be used to control the temperature of living tissues. Our results prove that the extended generalized Riccati equation mapping method is a powerful tool for investigating thermal traveling-wave distribution in biological tissues.

Thermal Behavior of Externally Driven Spindle: Experimental Study and Modelling

This paper focuses on model development for computer analysis of the thermal behavior of an externally driven spindle. The aim of the developed model is to enable efficient quantitative estimation of the thermal characteristics of the main spindle unit in an early stage of the development process. The presented work includes an experimental validation of the simulation model using a custom-built test rig. Specifically, the effects of the heat generated in the bearings and the heat flux from the bearing to the adjacent spindle system elements are investigated. Simulation and experimental results are compared and demonstrate good accordance. The proposed model is a useful, efficient and validated tool for quantitative simulation of thermal behavior of a main spindle system.

Unexpected Relationships between Thermal and Radiative Energy Transfer

A simple experiment is described where the IR (infrared) radiation level is kept constant while the temperature of an IR absorbing and a non-absorbing solid object are changed. The two objects, made from black-painted and highly polished Al foil envelopes, respectively, are placed in a chamber where the temperature is controlled. When heated by the surrounding air the black object becomes about 40% colder than the non-IR absorbing object! However, when the two objects are cooled by the surrounding air, the black becomes ca. 40% warmer than the non-IR absorbing object (and the surrounding air). This effect was surprising to us, and it gave us an opportunity to quantify the relationship between IR radiation flow and thermal energy flow. The unexpected large value of the (Fourier) thermal conductivity coefficient was found to be the reason for the reduced warming/cooling of the black object. The interaction between radiative and thermal energy transfer, when an IR absorbing object (like the surface of the Earth) is warmed, should be included in the climate models used by the Intergovernmental Panel on Climate Change (IPCC), since the global land temperature is measured in the air above Earth’s surface. This leads to ca. 15% of the temperature increase predicted by the climate models.

Thermal Modeling of Cylindrical LiFePO₄Batteries

Thermal management of Li-ion batteries is important because of the high energy content and the risk of rapid temperature development in the high current range. Reliable and safe operation of these batteries is seriously endangered by high temperatures. It is important to have a simple but accurate model to evaluate the thermal behavior of batteries under a variety of operating conditions and be able to predict the internal temperature as well. To achieve this goal, a radial-axial model is developed to investigate the evolution of the temperature distribution in cylindrical Li-ion cells. Experimental data on LiFePO4 cylindrical Li-ion batteries are used to determine the overpotentials and to estimate the State-of-Charge-dependent entropies from the previously developed adaptive thermal model [1]. The heat evolution is assumed to be uniform inside the battery. Heat exchange from the battery surfaces with the ambient is non-uniform, i.e. depends on the temperature of a particular point at the surface of the cell. Furthermore, the model was adapted for implementation in battery management systems. It is shown that the model can accurately predict the temperature distribution inside the cell in a wide range of operating conditions. Good agreement with the measured temperature development has been achieved. Decreasing the heat conductivity coefficient during cell manufacturing and increasing the heat transfer coefficient during battery operation suppresses the temperature evolution. This modified model can be used for the scale-up of large size batteries and battery packs.

油浸式变压器的全域热网络模型

热点温度是衡量变压器绝缘性能和安全裕度的重要指标,对其进行快速准确计算具有重要的意义。对油浸式变压器内部的产热和传热机理进行了分析,得到自然对流边界层中速度和温度分布与流体普朗特数Pr的关系,考虑到局部自然对流换热系数的沿程变化以及不同区域传热特性的差异,对油浸式变压器进行了分区分层处理。基于热电类比法定义了多种热阻类型,构建了油浸式变压器的全域热网络模型;通过有限元仿真对比了热网络模型的计算结果。二者所得绕组温度变化趋势相同,热点温度相差约1.9%,热点位置相差约7.7%。相比于有限元仿真,热网络模型在求解时间和效率上更有优势,能够实现油浸式变压器运行状态的在线监测、实时分析和动态评估。

基于不同接箍形式的隔热油管传热特性模拟分析

热力采油方法是稠油开采的主要方式之一,在注蒸汽热采过程中,隔热油管可以有效减少井筒径向传热损失;常规隔热油管使用一段时间后,热损失明显增加,如何改进隔热油管结构,降低注汽过程热损失,需要进一步研究。基于常规隔热油管,提出了2种改进隔热油管结构:接箍处含隔热衬套油管和直连式隔热油管;在对3种隔热油管物理结构分析基础上,建立了热损分析模型,根据所建模型,利用COMSOL软件进行了模拟计算分析,给出了3种隔热油管结构的温度分布和热损失变化特性,并进行了对比分析。结果表明:接箍处含隔热衬套油管热损失最小,直连式隔热油管热损失适中,建议对普通隔热油管接箍采取保温措施。本研究为注蒸汽热采过程减少热损失、提高稠油采收效果提供一定支撑。

回转式空气预热器积灰模型构建与积灰工况传热性能研究

为了量化研究积灰状态下回转式空气预热器(空预器)对运行条件的敏感性,构建基于局部热非平衡模型的三方程回转式空预器热力计算模型,探究了积灰分布与运行条件对空预器传热性能和硫酸氢铵(ABS)沉积的综合影响,对比分析了积灰热阻与热容对空预器的影响规律。计算结果表明:积灰热阻对传热性能的削弱效果远大于积灰热容对传热的强化效果;冷段积灰比例的增加使得ABS沉积区比例增大;当蓄热元件表面积灰厚度增加时,提高转速、增大流量或提升机组负荷对改善传热性能及减小ABS沉积区比例的效果尤为显著;改变流体温度对空预器性能的影响较小;当机组负荷由50%增加至100%时,ABS沉积区比例在清洁状态下的减小量为5%,在冷段积灰厚度为2 mm时提高至7%。研究结果可为针对性地调整空预器运行参数,从而优化传热效果及减轻积灰腐蚀提供参考。

饱和砂土地基中碳化硅能源桩的传热特性测试与分析

采用标准试块测试与模型试验,对碳化硅能源桩的传热特性进行研究。通过桩基混凝土试块导热系数和力学性能测试,确定碳化硅能源桩的桩身材料配合比;通过饱和砂土地基中能源桩高温释热工况模型试验,分析饱和砂土地基中碳化硅能源桩的传热性能以及轴向和径向温度分布及变化规律。结果表明:碳化硅代砂率达到16.0%时,桩基混凝土试块的导热系数、抗压强度以及抗折强度会分别提升64.1%、19.9%和11.4%,该代砂率下的碳化硅能源桩桩身材料配合比为最佳配合比。碳化硅能源桩的最高温度出现在桩体中部。相较于普通能源桩,碳化硅能源桩具有更高的桩体温度、温度增长速率和换热功率,表现出更优的传热性能。在碳化硅能源桩进行热循环时,桩土间的热量传递效率自桩身向外沿径向递减;热循环后,桩周土体呈现热量堆积现象,且这种现象随着接近桩身区域而逐渐增强。研究结果可为碳化硅能源桩的设计及其在饱和砂土地基中的应用提供试验依据。

热转印系统色带传动过程张力分析与建模

为保证热转印过程中薄膜色带传动稳定进而实现高质量转印,建立正确的热转印色带传动张力模型十分关键。首先,研究了热转印色带传动系统组成及转印原理,并根据色带传动路径分辊间段、放卷段和收卷段3阶段对色带传动系统张力进行了建模与分析。然后,结合热转印色带卷材的黏弹性分析张力形成机制,改进了卷绕系统经典张力公式,建立了辊间段薄膜色带张力模型。针对传动系统的放卷区域,分析了摩擦对薄膜色带传动张力的影响,提出张力下降系数以评估张力损失。考虑色带张力非线性时变的特点,利用步进电动机负载模型求解收卷区域色带张力。最后,通过对比仿真与实验所得的色带张力变化曲线,验证了模型的准确性,为后续张力控制方案设计奠定了基础。

多孔介质传热模型在多孔壁湍流中的适用性

为了考查不同的多孔介质传热模型在不同工况下的适用性,对带有高孔隙率多孔介质壁面槽道湍流及其传热进行了直接数值模拟研究。在多孔介质层外流体区域,通过有限差分方法求解不可压缩Navier-Stokes方程和温度对流扩散方程;在多孔介质层内,使用修正的Darcy-Brinkman-Forchheimer模型描述高孔隙率多孔介质阻力,以及分别采用局部热平衡(local thermal equilibrium,LTE)模型、局部非热平衡(local thermal non-equilibrium,LTNE)模型、理想金属(ideal metal foams,IMF)模型计算温度分布。通过对所得热场的统计特性的分析比较,探究了不同Biot数下水和空气两类流体介质的多孔介质传热模型的有效性。研究表明:LTE模型不足以准确预测金属泡沫多孔介质内传热问题,其等效导热系数因仅考虑孔隙率因素而低估了多孔介质层的传热能力;IMF模型在小比热容流体介质的算例中表现良好,可以代替LTNE描述多孔介质层内的传热,而在大比热容流体介质的算例中表现不佳,需要考虑比热容以及流固两相间的传热能力对预估的固体相温度分布进行修正。

锂离子电池失控机理模拟

锂离子电池由于存在热失控风险,安全性不能得到保障,在实际应用中仍存在局限。因此,了解热失控触发后电池的温度场变化规律,分析影响温度变化的因素是很有必要的。使用ANSYS FLUENT对锂离子电池进行模拟,建立了单体锂离子电池三维瞬态生热模型,通过模拟发电端不稳定导致的不同充放电波动功率输入下电池的温度场变化来研究电池内部热扩散规律;模拟锂离子电池局部内短路触发热失控后的温度场变化来分析热扩散规律。结果表明,锂离子电池的温度和壁面热流密度随输入电流的波动而波动。锂离子电池局部内短路触发后,热失控沿着电极纵向扩展的速率大于沿径向向外扩散的速率,增加对流换热系数能有效降低电池温度升温速率和幅度,降低电池触发热失控的可能性。研究热扩散规律对于实际应用中预测热失控、提高安全性、改善电池冷却工艺等提供了研究依据。

板翅式换热器传递函数动态模型及参数确定方法

飞机综合环控/热管理系统迫切需要建立快捷高效的换热器动态仿真模型,以满足先进控制系统设计需求。基于此,针对板翅式换热器动态模型,提出一种包含2个延迟环节和4个一阶惯性环节的传递函数矩阵形式,并从换热器机理模型出发,利用拉普拉斯变换推导模型中4个时间常数的计算公式,提出基于换热器效率的传热热阻计算关系式的辨识方法,解决了主要模型参数设置难题。以板翅式换热器为研究对象,在Simulink仿真平台搭建其传递函数动态模型,并与AMESim仿真平台搭建的机理模型进行对比分析,结果显示:2种模型所得空气和冷却水出口温度分别随入口温度和质量流量阶跃变化的动态响应曲线吻合很好,其中,4种工况空气和冷却水出口温度最大稳态偏差分别为0.034℃和0.029℃,当冷却水入口质量流量阶跃变化时,空气出口温度动态响应相对偏差最大,为9.27%,当空气入口质量流量阶跃变化时,冷却水出口温度动态响应相对偏差最大,为7.03%。

基于热阻模型的城市轨道交通铝合金车辆围护结构传热系数计算方法

[目的]根据城市轨道交通铝合金车辆围护结构的特点,研究一种基于热阻模型的K值计算方法,以此提高计算准确度和效率。[方法]将围护结构分为三个层级,根据热阻串并联关系建立了围护结构的热阻模型。位于第三层级的串联热阻,除包括对流热阻、车体热阻和隔热材料热阻外,还包括将传热路径上所有其他热阻等效替代的“简化热阻”。通过理论计算和试验标定的方法确定上述四个热阻后,即可通过热阻串并联关系计算出围护结构的总热阻,进而计算出K值。[结果及结论]与试验结果对比后发现,该方法的计算结果相对偏差在7%以内。由于目前K值试验数据较少,因此需要持续积累数据并标定该热阻模型,以此达到不断提高K值计算准确度的目标。基于该方法还开发出了应用程序,提高了计算效率。

基于土壤分层当量物性的地埋管换热器换热模型研究

文章综合考虑了土壤分层与地下水渗流等情况,结合实测数据,通过地埋管外壁温度计算得出地埋管内部流体温度与不同分层土壤的热物性参数,根据土壤当量物性进行分层,据此建立地埋管换热模型。经过对比验证实验发现,分层换热模型比常规均质模型更接近实测值,误差更小;基于土壤分层当量物性的地埋管换热器换热模型模拟出水温度与实测值平均绝对误差为0.21℃,模拟进、出口换热温差与实测值平均绝对误差为0.14℃,具有较高的准确度,可为工程设计提供依据并作为后续地埋管换热优化研究的基础。

用于农宅供暖的三角形太阳能空气集热器集热性能研究

用于农宅供暖的太阳能空气集热器均为平板型,通常安装于农宅南墙外侧,但农宅南墙除了门窗外,可用于安装集热器的面积有限,所以集热量也有限。为此,设计一种可安装在南、东、西墙的新型三角形太阳能集热器,以提高对太阳能资源的利用率。基于有限体积-能量法,建立了集热器的光学模型;联合光学模型,构建集热器的动态传热模型,并通过实验验证了模型的准确性;分析了集热器安装在农宅南、东、西墙的集热性能,并与传统贴墙安装平板太阳能空气集热器以及倾斜安装平板太阳能空气集热器进行了对比。结果发现,集热器安装在农宅东墙和西墙的集热量约为安装在南墙的60%和80%;三角形太阳能空气集热器安装在南墙的供暖季集热量比贴墙安装平板太阳能空气集热器高33.5%,比倾斜安装平板太阳能空气集热器高15.0%。

110~500 kV氧化锌避雷器对流换热系数计算及数值拟合

针对氧化锌避雷器在长期运行中老化、潮湿或短路等导致的故障,结合110~500 kV避雷器参数,建立典型避雷器电热耦合模型。介绍避雷器多物理场仿真建模方法,提出基于外流场的氧化锌避雷器外壁面对流换热系数计算方法,获得瓷外套和复合外套氧化锌避雷器对流换热系数的变化规律及函数。通过电-热耦合计算与三维流-热耦合温度场计算对比,验证对流换热系数计算结果的有效性,可为避雷器多物理场仿真提供数值参考。

基于多场耦合的无线电侦扰一体主机散热特性分析

针对无线电侦扰一体主机因高度集成引起的高热流密度导致散热难的问题,通过分析整机的散热路径,构建传热网络图;应用传热学和流体力学理论,建立主机热−流−固耦合散热数学模型;基于ANSYS Icepak仿真平台,构建主机的三维全尺寸数值模型研究其散热特性。结果表明:当风扇风量约为56 m^(3)/h时,无线电侦扰一体主机的散热性能较好且噪声较小;当电源箱的热源同侧分布,侦扰箱和导航诱骗箱的热源异侧分布,且热源靠近进风口或出风口时,主机的散热性能好且效率高;当支架孔隙和设备间距分别为15 mm和10 mm时,主机的散热性能较好且结构强度高,同时主机质量和体积均较小;当散热器翅片的高度、厚度和间距分别为7.5 mm、1 mm和5 mm时,强化散热效果较好且主机质量小。

Estimation of temperature elevation generated by ultrasonic irradiation in biological tissues using the thermal wave method

In most previous models,simulation of the temperature generation in tissue is based on the Pennes bio-heat transfer equation,which implies an instantaneous thermal energy deposition in the medium.Due to the long thermal relaxation time τ（20 s-30 s） in biological tissues,the actual temperature elevation during clinical treatments could be different from the value predicted by the Pennes bioheat equation.The thermal wave model of bio-heat transfer（TWMBT） defines a thermal relaxation time to describe the tissue heating from ultrasound exposure.In this paper,COMSOL Multiphysics 3.5a,a finite element method software package,is used to simulate the temperature response in tissues based on Pennes and TWMBT equations.We further discuss different factors in the bio-heat transfer model on the influence of the temperature rising and it is found that the temperature response in tissue under ultrasound exposure is a rising process with a declining rate.The thermal relaxation time inhibits the temperature elevation at the beginning of ultrasonic heating.Besides,thermal relaxation in TWMBT leads to lower temperature estimation than that based on Pennes equation during the same period of time.The blood flow carrying heat dominates most to the decline of temperature rising rate and the influence increases with temperature rising.On the contrary,heat diffusion,which can be described by thermal conductivity,has little effect on the temperature rising.

Effects of calculation approaches for thermal conductivity on the simulation accuracy of billet continuous casting

An unsteady, two-dimensional, explicitly solved fmite difference heat transfer model of a billet caster was presented to clarify the influence of the thermal conductivity of steel on model accuracy. Different approaches were utilized for calculating the thermal conductivity of solid, mushy and liquid steels. Model results predicted by these approaches were compared, and the advantages of advocated approaches were discussed. It is found that the approach for calculating the thermal conductivity of solid steel notably influences model predictions. Convection effects of liquid steel should be considered properly while calculating the thermal conductivity of mushy steel. Different values of the effective thermal conductivity of liquid steel adopted could partly be explained by the fact that different models adopted dissimilar ap- proaches for calculating the thermal conductivity of solid and mushy steels.