隔热材料导热系数预测及其在发动机排气管隔热中的应用

由于排气管隔热在特种车辆中的重要性,在对排气管隔热方案分析的基础上,采用大型通用CFD软件STAR-CD,模拟氧化锆空心球隔热材料的导热系数,并对排气管真实的流固耦合模型进行大量的数值模拟试验。数值模拟可以分析隔热结构中隔热材料厚度、表面发射率、隔热方式等因素对排气管隔热效果的影响;通过隔热材料与真空层的匹配,得到既满足紧凑性要求,又能高效隔热的排气管隔热方案。     

电炉文摘WJ84417—84457

WJ84417特殊隔热粒料——《耐火物》,1983,Vol34,No 11,29～32,〈日文〉 近几年来鉴于节能的需要,发展高级耐火隔热材料,提高工业炉的热效率的愿望十分强烈。 本文着重介绍了特殊隔热粒料——氧化铝空心球的制作方法、性质及其应用。氧化铝空心球是人工制造的一种多孔质粒料,可用来制作导热系数小并具有一定强度的耐火隔热材料。其制造方法是将原料放入电弧炉内熔化,熔融后用压缩空气或高压蒸气喷吹成粗粒,然     

隔热涂层降低建筑空调负荷效果的参数分析

从降低建筑空调负荷的角度出发,对外壁面有隔热涂层的房间建立了稳态传热模型,用数值方法逐一分析了涂层的各种参数,包括导热系数、厚度、表面太阳吸收率、发射率以及地区因素等对空调负荷的影响。研究发现：隔热涂层能显著降低空调负荷及能耗；仅当涂层热阻与墙体热阻之比大于0．2时,涂层导热系数的降低才会对空调负荷产生显著影响；涂层表面太阳吸收率和发射率对空调负荷的影响非常大,是起主导作用的因素；隔热涂层用在太阳辐照密度越大、日最高气温越低、日较差越大的地区节能率越高,越有利于发挥其降低空调负荷及日累计能耗的功能；决定着空调运转时间长短的当地室外气温分布状况对于评价隔热涂层节能效果也有重要影响。     

陶瓷基点阵夹芯结构等效导热系数的数值模拟分析

为了评价陶瓷基点阵夹芯结构的热防护效能，选取陶瓷点阵夹芯结构的胞元作为研究对象，给出了表征其传热效能的等效导热系数计算方法。建立了点阵夹芯结构胞元的导热-辐射稳态耦合传热的有限元模型，通过对模型施加不同的边界条件，利用数值模拟的方法求出了点阵夹芯结构稳态温度场分布和等效导热系数，并对其换热机理和影响因素进行了分析。研究表明：点阵夹芯结构的辐射换热是结构热传导的主要方式，且随温度的升高其作用效应就越强；相对较小的材料发射率和较大的芯杆高度能够降低结构的传热效能；减小芯杆直径会降低结构热防护能力；芯杆倾角对结构的热防护性能影响较小。     

高温电加热下电阻率与导热系数随温度变化的传热过程模拟

高温、电加热条件下电阻率与导热系数随温度变化明显,因此研究在此条件下的热传导具有十分重要的意义.运用MATLAB中的PDE工具箱,对高温条件下,几种不同材料的发热导体电加热过程中导热系数、电导率变化所引起的导热等问题进行了数值求解.结果表明:当发热导体半径R＞5 cm时,温度的变化对发热导体的导热系数和电导率影响显著.     

点加热稳态导热系数测量方法研究

针对稳态导热系数测量方法测量过程时间较长、测量装置复杂、以及样品制备和加工工艺复杂等现状,提出了一种新型的点加热稳态导热系数测量方法,构建相应的三维稳态传热物理模型,使加热面温升只与热流密度、样品导热系数和测温点位置相关。通过聚焦连续激光加热样品,缩短样品达稳态时长至分钟量级;建立对照光路消除表面发射率和激光稳定性对温度测量的影响;红外热像仪测量加热表面稳态温度分布,结合物理模型实现导热系数测量。采用多种已知导热系数的标准材料和线性法对测量方法进行验证,并应用该方法测量硅藻土导热系数为0.49～0.60 W/(m·K),误差为6.06%。该方法的测量迅速及非接触特性使其可应用于工程实地测量。     

双面受热太阳能平板接收器热性能模拟研究

为提高太阳能光热转换效率,建立同轴非完整型平移抛物面聚光系统。分析双面受热平板接收器的能量传递及转换过程,采用热阻网络图的分析方法建立平板接收器的理论计算模型。利用MATLAB 7.0软件编制程序实现了平板接收器的热性能计算。在结构参数、环境参数和进口参数确定的情况下,当吸热板导热系数、厚度和吸热板表面发射率变化时,分析温度、能量及热性能的变化趋势。研究表明：导热系数和吸热板厚度达到一定数值,继续增加对于提高接收器的热性能基本没有太大意义;吸热板表面发射率对热性能影响显著,采用发射率为0.1的选择性涂层可实现能量最大转化;环境温度在0～30 ℃变化时,双面受热比单面受热的热效率提高了8.18%～37.01%。     

聚氨酯泡沫塑料热分析

针对聚氨酯开展导热性能分析,对聚氨酯导热系数、密度、闭孔率进行了实验研究,考察了聚氨酯密度、闭孔率对其导热系数的影响,并根据发泡过程中出现的孔洞开展模拟计算,建立基于多孔介质分形结构的物理模型,分析了孔洞对聚氨酯导热性能的影响。研究结果表明:随着密度的增大,聚氨酯的导热系数随之升高,聚氨酯的导热系数主要受固体孔壁结构的影响;聚氨酯闭孔率越低,导热系数越高,且过低的闭孔率会降低其密度;建立分形结构物理模型,通过数值模拟分析,闭孔率相同时,具有较大内部孔洞的聚氨酯导热系数较低。     

蜂窝型透明隔热材料有效导热系数的实验研究

用一套简易有效的实验装置测定了三种透明隔热材料的有效导热系数，并与通用计算模型进行对比，两者符合较好，验证了实验方法和通用计算模型的有效性。     

物性参数对硅单晶Czochralski生长过程的影响

为了了解硅单晶C zochra lsk i(C z)法生长时物性参数对熔体流动和氧传输过程的影响,利用有限元法对炉内的传递过程进行了全局数值模拟,假定熔体和气相中的流动都为准稳态轴对称层流,熔体为不可压缩流体,C z炉外壁温度维持恒定。结果表明:熔体的导热系数及发射率对熔体流动、加热器功率、结晶界面形状、晶体内轴向温度梯度和氧浓度有重要影响,而熔体的密度、黏度系数及熔解热对硅单晶C z法生长过程影响较小。     

Classical and minimum entropy generation analyses for steady state conduction with temperature dependent thermal conductivity and asymmetric thermal boundary conditions: Regular and functionally graded materials

Minimum entropy generation (MEG) temperature profiles are derived for steady conduction in a plane wall, a hollow cylinder and a hollow sphere and compared with the classical results. Cases of constant thermal conductivity, temperature dependent and location dependent thermal conductivities for each geometry are analyzed. Results are presented for the classical and the minimum entropy temperature profiles illustrating the effect of thermal asymmetry and variable thermal conductivity in each geometry. These results show that the difference between the classical and the MEG temperature profiles is largest when there is a strong thermal asymmetry. For all three geometries, the effect of temperature dependent thermal conductivity on the classical temperatures is moderate but its effect on the MEG temperatures is only small. Both the classical and minimum entropy generation rates, for each geometry, are found to be strong functions of thermal asymmetry and thermal conductivity variation parameter. Comparison of results for a hollow cylinder and a hollow sphere reveals that rate of entropy generation in a hollow sphere is much higher than in a hollow cylinder for the same thermal asymmetry and radius ratio.     

Numerical study on melt fraction during melting of phase change material inside a sphere

This paper presents a numerical study on the constrained melting of phase change material (PCM) inside a sphere to investigate the effect of various factors on the melt fraction. A mathematical model of melting processes of the PCM inside a sphere is developed. And experiments are conducted to verify the numerical method. On the basis of the model, the effects of the sphere radius, the bath temperature, the PCM thermal conduction coefficient and the spherical shell material on the melt fraction of PCM inside a sphere are discussed. The results show that the PCM inside a sphere melts fast as the sphere radius is small, the bath temperature increases, and the PCM thermal conductivity is high. And the metal shell with high thermal conductivity should be adopted preferentially. The present study provides theoretical guidance for the design and operation of the phase change heat storage unit with sphere containers.     

Thermal performance of a single-layer packed metal pebble-bed exposed to high energy fluxes

It is difficult to accurately measure the temperature of the falling particle receiver since thermocouples may directly be exposed to the solar flux. This study analyzes the thermal performance of a packed bed receiver using large metal spheres to minimize the measurement error of particle temperature with the sphere temperature reaching more than 700°C in experiments in a solar furnace and a solar simulator. The numerical models of a single sphere and multiple spheres are verified by the experiments. The multiple spheres model includes calculations of the external incidence, view factors, and heat transfer. The effects of parameters on the temperature variations of the spheres, the transient thermal efficiency, and the temperature uniformity are investigated, such as the ambient temperature, particle thermal conductivity, energy flux, sphere diameter, and sphere emissivity. When the convection is not considered, the results show that the sphere emissivity has a significant influence on the transient thermal efficiency and that the temperature uniformity is strongly affected by the energy flux, sphere diameter, and sphere emissivity. As the emissivity increases from 0.5 to 0.9, the transient thermal efficiency and the average temperature variance increase from 53.5% to 75.7% and from 14.3% to 27.1% at 3.9 min, respectively. The average temperature variance decreases from 29.7% to 9.3% at 2.2 min with the sphere diameter increasing from 28.57 mm to 50 mm. As the dimensionless energy flux increases from 0.8 to 1.2, the average temperature variance increases from 13.4% to 26.6% at 3.4 min.     

State-of-the-art of phase-change thermal storage at middle-low temperature

相变蓄热技术由于蓄热密度大,温度恒定,在国内外得到广泛的研究与应用,尤其在能源供给不连续的情况下,应用的尤其广泛.相变储热系统作为解决能源供应时间与空间矛盾的有效手段,是提高能源利用率的重要途径之一.本文从相变材料的选取,相变过程数值模拟,相变蓄热装置3个方面对中低温相变蓄热的研究进行了综述.首先介绍了中低温相变材料的种类及其循环稳定性,导热能力强化,其次总结了适用于中低温相变蓄热的数值模拟方法和理论,然后介绍了不同的相变储热器,最后指出了中低温相变蓄热的研究目标和方向.     

Numerical simulation of the effective thermal conductivity of wet clothing materials

Clothing materials may be considered composite materials composed of fiber, air, and moisture. For this paper, effective thermal conductivities of wet clothing materials were analyzed numerically using a proposed heat transfer model. The following simplifications were introduced. The clothing material fiber is woven with a single yarn, there is no air movement between fibers, and mass transfer is neglected. Numerical calculations were made using finite difference equations for steady three-dimensional heat conduction for several composite materials representing wet clothing materials. The main results obtained were as follows. The effective thermal conductivity of wet clothing material increases as the thermal conductivity of the yarn and the moisture content increase. We found that our numerical results agree qualitatively with those previously measured. The effective thermal conductivity of a wet layered material depends on the distribution of moisture and attains a maximum in the wet layer. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 243–254, 1998     

Back‐analyses of in‐situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (CFD) numerical analyses was performed in order to evaluate the performance of six full‐scale closed‐loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The high‐density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite‐volume method program, for analyzing the heat transfer process of the system. Two user‐defined functions accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The numerical simulations provide verification for the in‐situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m?K yielded by the back‐analysis was in better agreement with the in‐situ TRT result than with the ground thermal conductivity of 3.0 W/m?K. From the results of CFD back‐analyses, the effective thermal conductivities estimated from both the in‐situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m?K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in‐situ TRT decreases when borehole resistance decreases with a new three pipe‐type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U‐loop type heat exchanger. Copyright © 2012 John Wiley & Sons, Ltd.     

Contact thermal conductivity of a powder bed in selective laser sintering

Estimation of the temperature field in the powder bed in selective laser sintering process is a key issue for understanding the sintering/binding mechanisms and for optimising the technique. Heat transfer may be strongly affected by formation and growth of necks between particles due to sintering when the contact conductivity becomes predominant in the powder bed effective thermal conductivity. The necks often remain small as compared to the particle size. To calculate the effective contact conductivity of such structures a model of independent small thermal contacts is proposed. The conductivity of the considered cubic-symmetry lattices and the random packing of equal spheres depends on the three structural parameters: the relative density, the coordination number, and the contact size. The present model agrees with the known numerical calculations in the range of contact radius to particle radius ratio below 0.3. The strong dependence on the contact size is qualitatively confirmed by experimental data.     

Lattice Boltzmann modeling of the effective thermal conductivity for fibrous materials

This paper presents a full set of numerical methods for predicting the effective thermal conductivity of natural fibrous materials accurately, which includes a random generation-growth method for generating micro morphology of natural fibrous materials based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the energy transport equations through the fibrous material with the multiphase conjugate heat transfer effect considered. Using the present method, the effective thermal conductivity of random fibrous materials is analyzed for different parameters. The simulation results indicate that the fiber orientation angle limit will cause the material effective thermal conductivity to be anisotropic and a smaller orientation angle leads to a stronger anisotropy. The effective thermal conductivity of fibrous material increases with the fiber length and approach a stable value when the fiber tends to be infinite long. The effective thermal conductivity increases with the porosity of material at a super-linear rate and differs for different fiber location distribution functions.     

竖直双U型埋管地热换热器支管间热短路分析

采用地热换热器准三维传热模型和数值计算的方法,对竖直双U型埋管地热换热器支管间的热短路进行了分析,讨论了管间距和回灌材料的导热系数对热短路的影响,并提出了减少热短路的措施。     

Axial effective thermal conductivities of packed beds

Experimental investigations have been carried out to measure axial effective thermal conductivities of packed beds for a number of particles and catalyst pellets. Measurements were made for three gases (air, nitrogen and carbon dioxide) in beds packed with ball bearings, copper chromite, chromia alumina, alumina hollow cylinders and alumina spheres. A glass vacuum vessel was employed for most measurements, but a thin wall stainless steel vessel was used in a few experiments.

Empirical correlations to predict the axial effective thermal conductivity of packed bed reactors have been derived from the experimental results.