CFB cyclones at high temperature: Operational results and design assessment

Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775 ℃.
The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones, generally for operation at ambient temperature and at low Cs [〈0.5]. None of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions. The cut size d50 can be estimated using direct empirical methods or using the Stokes number, Stk50. Both methods were used to compare measured and predicted values of d50. With the exception of Muschelknautz and Krambrock, none of the equations made accurate predictions.
Finally, an alternative method to determine the friction factor of the pressure drop equation （Euler number, Eu） and of the cut size is proposed. The Eu number is determined from the geometry of common cyclones, and the derived value of Stk50 defines more accurate cut sizes. The remaining discrepancy of less than 5%, when compared with the measured values, is tentatively explained in terms of a reduced cyclone diameter due to the solids layer formed near its wall. Further measurements, mostly using positron emission particle tracking, elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper.     

CFB cyclones at high temperature: Operational results and design assessment

Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775℃.The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones, generally for operation at ambient temperature and at low Cs[0.5]. None of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions. The cut size d50 can be estimated using direct empirical methods or using the Stokes number, Stk50. Both methods were used to compare measured and predicted values of d50. With the exception of Muschelknautz and Krambrock, none of the equations made accurate predictions.Finally, an alternative method to determine the friction factor of the pressure drop equation (Euler number, Eu) and of the cut size is proposed. The Eu number is determined from the geometry of common cyclones, and the derived value of Stk50 defines more accurate cut sizes. The remaining discrepancy of less than 5%, when compared with the measured values, is tentatively explained in terms of a reduced cyclone diameter due to the solids layer formed near its wall. Further measurements, mostly using positron emission particle tracking, elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper.     

Protecting pressure gauges with grease in high temperature facilities

The thermal protection of pressure gauges used in shock tubes has to avoid inducing losses in response time and accuracy. A study was undertaken to this purpose, and a procedure for mounting and covering the gauges with a protection layer of grease has been developed. Results up to date show satisfactory behaviour.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

钢-复合材料组合结构材料选型优化设计映射解法

建立了基于拓扑分布的钢-复合材料组合结构材料选型优化的数学模型,通过变量连续化,用映射函数对材料选型优化模型进行函数变换,使其能适应更多的优化算法进行求解.分别采用线性函数、指数函数、对数函数、阶跃函数、反三角函数和分段函数等对原始模型进行了变换,得到相应材料选型优化模型并求解.以组合浮筏材料选择动力学优化为例,对比研...     

A radiation and convection fluxmeter for high temperature applications

Heat flux is an essential parameter for the diagnostic of thermal systems. In high temperature industrial environment, there are difficulties in measuring incident radiation heat flux as well as in differentiating between the convective and radiative components of heat flux on the heat transfer surface. A new method for heat flux measurement is being developed using a porous sensing element. The gas stream flowing through the porous element is used to measure the heat received by the sensor surface exposed to the hot gas environment. A numerical model of sensor with appropriate boundary condition has been developed in order to perform analysis of possible options regarding its design. The analysis includes: geometry of element, physical parameters of gas and solid and gas flow rate through the porous element. For the optimal selection of parameters, an experimental set-up was designed, including the sensor element with respective cooling and monitoring systems and a high temperature radiation source. The experimental set-up was used to obtain calibration curves for a number of sensors. The linear dependency of the heat flux and respective temperature difference of the gas were verified. The accuracy analysis of the sensor reading has proved high linearity of the calibration curve and accuracy of ±5%.     

Residual fracture energy of cement paste, mortar and concrete subject to high temperature

Quantifying high temperature damage is an issue that can hardly be dealt with experimentally because of the complexity of the loading control, of temperature and of moisture. The experimental investigation was carried out. The measurement of the mechanical characteristics (fracture energy, tensile strength, elastic modulus and thermal damage parameter) of five cementitious materials, cement paste, mortar, ordinary concrete and two HPC concretes were performed by three-point bending tests after heating/cooling cycles at 120, 250 and 400 °C. The tests showed that the cementitious materials behave almost identical when the fracture energy G_f is considered as a function of maximum temperature. The thermal damage due to heating from 120 to 400 °C increases the fracture energy by 50% with the reference tests at room temperature. A more tortuous crack surface is one reasonable explanation for the significant increase in G_f. It is demonstrated that the temperature exposure makes all cementitious materials tested significantly more ductile and less resistant.     

Material forces and phase transitions in elasticity

A computational scheme for the determination of the interface in a strain-induced phase-transition problem for an elastic bar is proposed. The algorithm is based on the material force notion and more specifically on the simultaneous solution of equilibrium equations for the physical and material forces. The weak form of both equations is derived with the aid of a variational principle that accounts for the variations of the dependent and the independent variables. The whole scheme concludes in a nonlinear algebraic system which is numerically solved by the Newton method. The numerical results thus derived seem to be quite encouraging for further application of the concept of material forces in computations related to phase transition problems. The austenite–martensite transformation could be a possible application of the proposed model.     

CONTINUOUS SELECTION THEOREMS FOR FAN-BROWDER MAPPINGS IN TOPOLOGICAL SPACES AND THEIR APPLICATIONS

The concept of Fan-Browder mappings was first introduced in topological spaces without any convex structure. Then a new continuous selection theorem was obtained for the Fan-Browder mapping with range in a topological space without any convex structure and noncompact domain. As applications, some fixed point theorems, coincidence theorems and a nonempty intersection theorem were given. Both the new concepts and results unify and extend many known results in recent literature.     

Analysis of the fluid-bed stability of FCC catalysts at high temperature in terms of bed elasticity

The effect of temperature on the fluid-bed stability of three FCC catalysts has been analyzed through considerations on fluid-bed elasticity. Experimental findings on the effect of temperature on the elasticity modulus at minimum bubbling conditions, （E）mb, were analyzed using the hydrodynamic fluid-bed stability model developed by Foscolo and Gibilaro （1984） and adopting the interparticle-forces-based stability criterion developed by Mutsers and Rietema （1977）. For both models, the parameters which control changes in （E）mb with temperature are discussed, in order to establish the origin of the fluid-bed elasticity and analyze the ability of these models to discriminate between the relative importance of the hydrodynamic and interoarticle forces on the stability of the fluidized catalysts.     

High temperature fracture parameter for a weld interface crack

A fracture parameter is introduced to characterize crack growth at elevated temperature for a weld interface crack. Finite element analyses are made for a compact tension (CT) specimen under constant load where an interface is modeled along the crack plane to simulate the dividing line of the weld metal (WM) and base metal (BM). A constitutive relation for an elastic-creeping material is used. The proposed time dependent fracture parameter is obtained for different creep constants of WM and BM. A transition time and fracture parameter for the homogeneous and welded specimens are defined such that the results can be normalized and presented in a general form for assessing the crack growth life.     

最小概率DWO带宽选择及激光陀螺温度误差模型辨识

在原有研究的基础上,针对实验数据观测点疏密分布均匀或不均匀的工程实际情况,分别运用全局准则和局部准则,研究最小概率DWO非线性辨识方法中的带宽选择关键问题,提出了校正AIC准则和LCV准则两种不同的带宽选择方法,并将这些方法应用于四频差动激光陀螺的温度误差模型辨识中,比较和验证了这些方法的正确性和适应性。研究结果表明：①对于＂分布均匀＂的情况,宜采用校正AIC准则;②对于＂分布不均匀＂的情况,宜采用LCV准则;③形成了自动带宽选择算法。总之,这些方法为解决＂带宽选择＂问题提供了有效途径,从而进一步提高了最小概率DWO方法的工程应用价值。     

A new fabrication process for a flexible skin with temperature sensor array and its applications

This paper reports a novel technique for fabrication of a flexible skin with a temperature sensor array (40×1 sensors). A simplified MEMS technology using platinum resistors as sensing materials, which are sandwiched between two polyimide layers as flexible substrates is developed. The two polyimide layers are deposited on top of a thin aluminum layer, which serves as a sacrificial layer such that the flexible skin can be released by metal etching and peeled off easily. The flexible skin with a temperature sensor array has a high mechanical flexibility and can be handily attached on a highly curved surface to detect tiny temperature distribution inside a small area. The sensor array shows a linear output and has a sensitivity of 7.5mV/℃ (prior to amplifiers) at a drive current of 1 mA. To demonstrate its applications, two examples have been demonstrated, including measurement of temperature distribution around a micro heater of a micro PCR (polymerase chain reaction) chip for DNA amplification and detection of separation point for flow over a circular cylinder. The development of the flexible skin with a temperature sensor array may be crucial for measuring temperature distribution on any curved surface in the fields of aerodynamics, space exploration, auto making and biomedical applications etc.     

An application of a damage constitutive model to concrete at high temperature and prediction of spalling

A characteristic feature of concrete under uniaxial compression is the development of cracks parallel to the loading direction. A damage constitutive model proposed by Ortiz [Ortiz, M., 1985. A constitutive theory for the inelastic behaviour of concrete. Mech. Mater. 4, 67–93] can predict the transverse tensile stress responsible for these cracks by considering the interaction between the aggregate and the mortar and the development of damage in the latter. When concrete is exposed to high temperature, as is the case during fire, the failure mode is thermal spalling. In order to improve the prediction of the stresses involved in this failure Ortiz’s model is extended to account for the effects of high temperature. Published experimental results for uniaxial and biaxial compression at high temperatures are used to calibrate the temperature dependence of some of the material properties. The transient creep strain is accounted for by modifying the constrained thermal strain. The stress analysis is coupled with hygro-thermal analysis of heat, mass transfer and pore pressure build-up. The effect of pore pressure on the damage evolution is modeled by applying a body force in the stress analysis module proportional to the pressure gradient. A numerical example of concrete under fire is solved and the computed results are discussed. Spalling is predicted when the damage variable reaches its maximum value of unity. The predicted depth and time of spalling for a range of variation of permeability and initial liquid water content are presented. They are in good agreement with published experimental results.     

Effect of temperature on the effective thermal conductivity of n-tetradecane-based nanofluids containing copper nanoparticles

Nanofluids were prepared by dispersing Cu nanoparticles (∼20 nm) in n-tetradecane by a two-step method. The effective thermal conductivity was measured for various nanoparticle volume fractions (0.0001–0.02) and temperatures (306.22–452.66 K). The experimental data compares well with the Jang and Choi model. The thermal conductivity enhancement was lower above 391.06 K than for that between 306.22 and 360.77 K. The interfacial thermal resistance increased with increasing temperature. The effective thermal conductivity enhancement was greater than that obtained with a more viscous fluid as the base media at 452.66 K because of nanoconvection induced by nanoparticle Brownian motion at high temperature.     

高温下混凝土的本构模拟及破坏分析

针对已建立的高温下混凝土中化学-热-水力-力学耦合过程分析的分级数学模型,发展了混凝土的化学-热-水力-力学(CTHM)耦合本构模型。在已有的Willam-Warnke弹塑性屈服准则基础上发展了考虑脱水和脱盐引起的材料损伤及化学塑性软化、塑性应变硬化/软化和吸力硬化的广义Willam-Warnke本构模型,模拟高温下混凝土的材料非线性行为。为保证全局守恒方程的Newton迭代过程的二阶收敛率,导出了非线性化学-热-水力-力学(CTHM)耦合本构模型的一致性切线模量矩阵。数值结果显示了本文所发展的化学-热-水力-力学(CTHM)耦合本构模型在模拟高温下混凝土中复杂破坏过程的能力和有效性。     

A technique for measuring the dynamic behavior of materials at high temperatures

A new experimental technique has been developed for the performance of high temperature, high-strain-rate experiments in the compression Kolsky bar (split-Hopkinson pressure bar or SHPB). The new technique (referred to as the High-Temperature Compression Kolsky Bar or HTCKB) uses an infra-red spot-heater to rapidly heat the specimen to the desired temperature, a!nd an electropneumatic actuation system to minimize the development of temperature gradients in the sample. The technique is cheap and relatively easy to implement and yet provides accurate, repeatable results. As an illustration of the application of the technique, we have examined the high-temperature response of the BCC metal vanadium at high-strain rates. Stress–strain curves are obtained for the material at strain rates of 4 × 10³ s⁻¹ and at temperatures ranging from 300 to 1100 K (27–800°C). Quasistatic (10⁻³ s⁻¹) experiments have also been performed on vanadium over a slightly smaller range of temperatures, and the results are compared with the new high-temperature, high-strain-rate data. It is observed that the rate of thermal softening is a function of the strain rate. These results illustrate the importance of including the coupling between temperature and strain r!ate in thermoviscoplastic constitutive models.     

Numerical simulations of high pressure carbon dioxide fluid fluidized beds

Hydrodynamics of carbon dioxide fluid-particle mixtures are predicted using a low density ratio-based kinetic theory of granular flow in high pressure carbon dioxide fluid fluidized beds. A coexistence of particle waves and particle aggregates exists along bed height. The threshold to identify the occurrence of particle aggregates is suggested based on standard deviation of solid volume fractions in aggregative fluidization. The existence time fractions and frequencies of particle aggregates are predicted along axial direction. The effect of carbon dioxide fluid temperature and pressure on volume fraction and velocity distributions are analyzed at different inlet carbon dioxide velocities and particle densities in high pressure carbon dioxide fluidized beds. Simulated results indicate that the carbon dioxide-particles fluidization transits from particulate to aggregative states with the increase of inlet carbon dioxide velocities. The computed fluid volume fractions and heterogeneity indexes are close to the measurements in a high pressure carbon dioxide fluidized bed.     

干涉应变计法在焊缝材料力学性能测试中的应用

在焊接结构中,焊缝的不同区域(如热影响区和熔合区)的材料力学性能对整个结构的安全性、可靠性起着至关重要的影响。本文介绍了用于高精度应变测量的干涉应变计法(ISDG),并利用自行研制的微小试样拉伸试验机,对20Cr合金钢压力容器中部分焊缝材料,分热影响区和熔合区对其材料力学性能进行测试,在国内首次获得了焊缝不同区域的全历程应力-应变曲线,对热影响区材料的软化现象进行了定量的描述。     

Mircopolar Model of Fracture for Composite Material

Failure of a composite is a complex process accompanied by irreversible changes in the microstructure of the material. Microscopic mechanisms are known of the accumulation of damage and failure of the type of localized and multiple ruptures of the fibers delamination along interphase boundaries, and also mechanisms associated with fracture of fibers. In this work, we propose a mathematical model of the local mechanism of failure of a composite material randomly reinforced with a system of short fibers. We implement the Cosserat moment model of crack tip for filament material, reinforced with whiskers or in fiber- reinforced polycrystalline materials. It is assumed that the angular distribution of the fibers is isotropic and the elastic characteristics of the fibers are considerably higher than the elastic constants of the matrix. We implement the homogenization procedure for the effective Cosserat constants similarly to the effective elastic constants. The singular solution in the vicinity of the crack tip in the Cosserat moment model is found. Using this solution, we examine the bending stresses in the filaments due to effective moment stresses in the material. The constructed model describes the phenomenon of fracture of the fibers occurring during crack propagation in those composites. The following assumptions are used as the main hypotheses for the micromechanical model. The matrix contains a nucleation crack. When the load is increased the crack grows and its boundary comes into contact with the reinforcing fibers. A further increase of the stress causes bending of the fiber. When~the fiber curvature reaches a specific critical value, the fiber ruptures. If the stress at infinity is given, the fibers no longer delay the development of failure during crack propagation The degree of bending distortion of the fiber in the vicinity of the boundary of the crack is determined by the moment model of the material. The necessity to take into account the moment stresses in the failure theory of the reinforced material was stressed in [Muki and Sternberg (1965) Zeitschrift f angew Math und Phys 16:611–615; Garajeu and Soos (2003) Math Mech Solids 8(2):189–218; Ostoja-Starzewski et al (1999) Mech Res Commun 26:387–396]. The moment Cosserat stresses were accounted also for inhomogeneous biomechanical materials by Buechner and Lakes (2003) Bio Mech Model Mechanobiol 1: 295–301. We should also mention the important methodological studies [Sternberg and Muki (1967) J Solids Struct 1:69–95; Atkinson and Leppington (1977) Int J Solids Struct 13: 1103–1122] concerned with the moment stresses in homogeneous fracture mechanics.