Analysis of Effect of Heat-Transfer Variables on Thermal Stress Resistance of Brittle Ceramics Measured by Quenching Experiments

Heat-transfer theory was used to analyze the variables which control the thermal stress fracture of brittle ceramic specimens subjected to rapid cooling by quenching into fluid media. Expressions were derived for the maximum quenching temperature difference to which circular solid cylindrical specimens can be subjected in terms of the pertinent properties of the material and fluid media. Appropriate thermal stress resistance parameters were derived. The specimen density was introduced as an additional property which controls thermal stress resistance. Recommendations were made for procedures to be followed in conducting quenching experiments.     

Thermal Stress Fracture of Brittle Ceramics by Conductive Heat Transfer in a Liquid Metal Quenching Medium

Observed and calculated values of the critical quenching temperature difference for circular rod specimens of a soda-lime-silica glass and polycrystalline alumina quenched in a liquid Pb-Sn alloy indicate that thermal stress fracture was caused by conductive heat transfer, in support of the original theory of Hencke, Thomas, and Hasselman.     

Effect of Spatially Varying Thermal Conductivity on the Magnitude of Thermal Stress in Brittle Ceramics Subjected to Convective Heating

The effect of a spatial variation of the thermal conductivity on the magnitude of the maximum tensile thermal stress in a solid circular cylinder subjected to sudden convective heating was calculated by finite element methods. The general results show that by lowering the thermal conductivity in the surface region of the cylinder, the magnitude of the maximum tensile thermal stress at the center of the cylinder is reduced significantly. The negative temperature dependence of the thermal conductivity in dielectric materials, which indirectly creates a spatial variation in thermal conductivity, also causes a significant decrease in the magnitude of the tensile thermal stresses, as demonstrated by a numerical example for aluminum oxide. It is also shown that a spatial variation in thermal conductivity can significantly affect the time to maximum tensile stress as well.     

Effect of Drop Height on Critical Temperature Difference (ΔTc) for Brittle Ceramics Subjected to Thermal Shock by Quenching into Water

The critical quenching temperature differences required to initiate thermal stress failure of soda-lime-silica glass, polycrystalline Al₂O₃, mullite, and reaction-sintered Si₃N₄ quenched into water were found to be independent of the height from which the specimens were dropped. These results suggest that, within the range of usual laboratory practice, drop height need not be closely controlled. The implications of these observations for the mechanism of heat transfer responsible for thermal stress failure in water quenching are discussed .     

Approximate Theory of Thermal Stress Resistance of Brittle Ceramics Involving Creep

The effect of stress relaxation by creep on the thermal stress fracture of brittle ceramics at high temperature under conditions of quasi-static heat flow is discussed. It is shown that, to a good approximation, thermal stress relaxation rates can be calculated on the basis of creep rates which correspond to the minimum temperature of the ceramic workpiece. For materials exhibiting linear stress-creep rate dependence, expressions for the relaxation time and maximum temperature difference or heat flux to which ceramic bodies can be subjected are derived in terms of the material variables affecting thermal stress fracture and stress relaxation by creep. A numerical example shows that high-temperature creep can materially affect the thermal stress behavior of brittle ceramics. Appropriate thermal stress parameters are proposed to form the basis of proper material selection for high-temperature environments involving thermal stress and stress relaxation by creep. Conditions for which thermal stress calculations should be based on an elastic or viscoelastic analysis are outlined.     

Role of Physical Properties in the Resistance of Brittle Ceramics to Failure by Thermal Buckling

An analysis is presented of the role of the physical properties which affect the thermal buckling behavior of ceramic materials for various geometric and thermal conditions. For a column with slight initial curvature or a curvature as the result of a transverse temperature gradient, expressions are derived for the maximum increase in temperature, Δ T _max or transverse temperature gradient, or heat flux to which the column can be subjected to avoid failure under the influence of the bending stresses. From the solutions obtained several "thermal buckling resistance parameters" are defined for comparing the relative thermal buckling resistance of brittle materials. Numerical examples indicate that thermal buckling of brittle materials of the proper geometry can occur relatively easily.     

Role of Material Properties in the Thermal-Stress Fracture of Brittle Ceramics Subjected to Conductive Heat Transfer

An analysis is presented of the thermal stresses in brittle ceramics subjected to thermal shock by quenching into a liquid medium in which conduction is the primary mechanism of heat transfer. The magnitude of maximum stress was shown to be a function of the coefficient of thermal expansion, Young's modulus, and Poisson's ratio of the ceramic, as well as the values of the thermal conductivity, specific heat, and density of the ceramic and quenching medium. The magnitude of thermal stress was shown to be independent of specimen size and, on cooling, to occur at the instant of cooling ( t =0). Appropriate thermal-stress resistance parameters were derived.     

传热系数法评定超声波的抗垢作用

通过池动态模拟实验,利用自动数据采集系统,以CaCO3污垢溶液为研究对象,配置了不同浓度的模拟溶液,在不同的污垢溶液浓度及热通量等工艺条件下引进了超声波清洗,以减小污垢在金属表面的沉积. 跟踪系统不同点的温度,采用数学处理方法转换成传热系数,以此评价阻垢作用. 研究表明,超声波清洗不仅明显强化了传热效果,也有效阻止了污垢在加热器表面的沉积,具有良好的抗垢作用.     

晶化温度对锂铝硅微晶玻璃热膨胀系数和热稳定性的影响

通过制备锂铝硅基础微晶玻璃,对其进行差热分析,在不同温度下进行晶化处理,对微晶玻璃的热膨胀系数和热稳定性进行测试分析,并讨论了晶化温度对这两种性能的影响。     

民用建筑围护结构传热系数的检测技术研究

基于目前国内外检测建筑围护结构传热系数方法的优缺点,改进现有的检测技术,本文提出控温热流计检测方法,可解决现有围护结构传热系数检测技术的缺点。开发这种检测方法的检测设备,为建立河北省建筑节能评估体系提供技术支持。     

热处理对AlN陶瓷的热导率及介电性能的影响

采用放电等离子烧结技术制备Al N陶瓷,并利用真空无压烧结方法进行热处理。研究了不同烧结助剂在高温热处理过程中对Al N陶瓷的物相组成、显微结构、导热性能及介电性能的影响。结果表明,Al N陶瓷经过热处理后,Al N晶粒进一步发育长大,尽可能实现面接触,材料中的晶间相分布更加合理。与只引入单一稀土氧化物为烧结助剂的Al N陶瓷相比,引入复合烧结助剂稀土氧化物及氟化钙(Ca F2)的Al N陶瓷经过热处理后,在有效除氧及生成具有挥发性晶间相双重效果作用下,材料热导率及介电损耗改善效果明显。     

Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics

Dense sialon ceramics along the tie line between Si₃N₄ and Nd₂O₃·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T_cool) of ∼50°C/min) or quenching (T_cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd_xSi_{12–4S x} Al_{4.5 x} O_{1. 5 x} , N_{16–1.5 x} . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si_6-zAl_z)N_10-zO_z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K _lc= 4.4 MPa·m^1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.     

Calculation of the Heat Transfer in Coke Dry Quenching by Numerical Modeling

The air flow and heat transfer in a coke dry quenching (CDQ) system are analyzed by simulation using FloEFD software, within the framework of a modified k–ε model of turbulence. The calculated temperatures are compared with operational data. The accuracy of the simulation is acceptable. The proposed method may be used for engineering calculations in the design and refinement of CDQ systems or their components, with significant decrease in experiment costs.     

旋液流态化流体阻力与传热的试验研究

摘要对某公司换热器的运行情况。采用旋液流态化技术设计了模拟试验方案。首先在φ57mm×3．5mm×2000mmPVC管内进行旋液流态化清洗试验,然后在碳钢管上内置不同螺距的钢丝螺旋线进行流体阻力测试,并进行旋液流态化与空管的对比传热试验。结果表明：旋液流态化技术清洗管道中人工污垢只需时间145s,没有发生粒子沉积和堵塞的现象,安全可靠性好;旋液流态化流体阻力△p随着Re的增大而增大,摩擦系数随着Re的增大而减小,流体总阻力48．7kPa低于供水压力,总传热系数平均提高幅度13．3％,强化传热效果好。     

Experimental and Finite Element Study of the Thermal Conductivity of α-SiAlON Ceramics

The thermal conductivity of monolithic Y-Sm/α-SiAlON was evaluated using experimental data and finite element analysis. The thermal diffusivities of Y, Y-Dy, and Y-Ce/α-SiAlON ceramics were also investigated experimentally for comparison. The maximum achievable thermal conductivity of Y-Sm/α-SiAlON has been calculated by the linear extrapolation of the temperature-based experimental inverse diffusivity data and was used for the numerical calculations. Two-dimensional model microstructures were built on the base of real microstructure images and applied for calculations. Experimental data and numerical calculations were compared for Y-Sm/α-SiAlON, and it was revealed that both results are in good agreement.