Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. There were some studies on the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully developed to allow performing thermal-mechanical fatigue (TMF) test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.     

基于实验的数值反演的滚动轮胎稳态温度场的有限元分析

根据轮胎温度场的单向解耦分析思想,形成了一个基于ABAQUS程序的轮胎稳态温度场的分析方法,单向解耦过程分为变形、损耗、热传导三个分析过程。变形分析中,采用了平衡态的超弹性材料模型;损耗分析中,依据变形分析获得的应力应变场,结合材料粘性损耗特性来获得损耗能量;热传导分析中,依据实测的轮胎胎侧温度场,提出了一种基于实验的数值反演方法来确定胎侧的对流热边界条件。由于轮胎胎侧的形状和结构细节,其对流热边界不同于旋转平圆盘的对流热边界,本文的数值反演方法避免了实测胎侧对流热交换系数的困难。     

回转筒内沙石物料混合均匀性的数值模拟

采用离散单元法对回转筒内沙石物料的混合均匀性进行数值模拟研究,并选取回转筒转速、直径、提升条个数和石子填充率作为影响因素,分别设立3个水平,进行回转筒内物料混合的正交模拟,旨在对各因素的敏感性影响程度进行分析。基于试验结果选择物料混合时间和颗粒接触数率作为试验指标,对四个因素进行极差分析和方差分析。结果表明,回转筒转速、直径和石子填充率对物料混合时间的影响显著,提升条个数对混合时间的影响不显著;石子填充率、回转筒转速和提升条个数对物料混合后颗粒接触数率的影响显著,回转筒直径对物料混合后颗粒接触数率的影响不显著。     

Effect of drum structure on particle mixing behavior based on DEM method

The optimization of the drum structure is beneficial to improve the particle motion and mixing in rotary drums. In this work, two kinds of drum structures, Lacy cylinder drum (LC) and Lacy-lifters cylinder drum (LLC), are developed on the basic of cylinder drum to enhance the heat transfer area. The particle motion and mixing process are simulated by DEM method. Based on the grid independence and model validation, the contact number between particles and wall, particle velocity profile, thickness of active layer, particle exchange coefficient, particle concentration profile and mixing index are demonstrated. The influences of the drum structure and the operation parameters are further evaluated. The results show that the contact number between particles and wall is improved in LC and LLC compared to cylinder drum. The particle velocity in LC is higher than that in cylinder drum at high rotating speed, and the particle velocity of the particle falling region is significantly improved in LLC. Compared to cylinder drum and LC, the thickness of active layer in LLC is smaller, while the local particle mixing quality is proved to be the best in the active region. In addition, the particle exchange coefficients between static region and active region in the three drums are compared and LLC is found tending to weaken the particle flow. Besides, the fluctuations of particle concentration in the active region, static region, and boundary region are weakened in LLC, and the equilibrium state is reached earlier. In addition, the overall particle mixing performance in cylinder drum, LC and LLC is analyzed. The particle mixing performance in cylinder drum is the worst, while the difference in mixing quality of LC and LLC depends on the operation conditions.     

混凝土搅拌车搅拌筒磨损数值分析

混凝土运输过程中搅拌筒的磨损一直是一种常见潜在危害,磨损严重时会导致叶片失效,对搅拌的质量和出料匀质性产生影响。通过实验获取搅拌筒内部的磨损费时费力,因此有必要采取一种数值分析方法对搅拌筒的磨损进行预测并提出改进。本文采用摩擦磨损实验的方法来标定颗粒与搅拌筒之间的Archard磨损常数,采用JKR接触模型表征混凝土的流动性能,采用离散元方法(DEM)对搅拌筒筒体及叶片磨损进行预测分析。通过法向接触能量与切向接触能量的对比,证明搅拌筒中的磨损主要为伴有冲击作用的磨粒磨损,搅拌车搅拌筒中搅拌叶片顶部的磨损较为严重。针对磨损比较严重的叶片顶部进行改进,采用T型耐磨结构等改进叶片结构,叶片顶部结构改进后搅拌筒使用寿命能得到明显提升。     

Transient thermal behavior of a cylindrical brake system

A mathematical model is presented to describe the thermal behavior of a brake system which consists of shoe and drum. The model is solved analytically using Green's function method for any type of the stopping braking action. In terms of the obtained solutions, the transient temperature distribution of the brake is described. The thermal behavior is investigated for three specified braking actions which are the impulse, unit step and trigonometric stopping actions. Received on 14 April 1999     

Measuring dynamic-strain profile and history with a drum camera

A low-speed drum camera has been used to continuously record the strain profile in several different types of impact tests. The drum camera was selected because it is inexpensive and simple to use, and it overcomes many of the inherent disadvantages of other dynamic-recording techniques. The use of the drum camera in recording the strain profile in tensile and torsional impact is illustrated, as well as the determination of impact velocity in collinear plastic impact of two bars. A detailed discussion of determination of the strain profile from the tensile-test film record is presented, including an analysis of the accuracy achieved. It was found that the drum camera is capable of measuring strains to an accuracy of approximately 1 percent for a gage length of 0.05 in. Strains varying from 1 to 100 percent were measured in the tensile test, with a nominal strain rate of 20 sec^?1. Strain rates of up to 2000 sec^?1 were successfully recorded in the torsional impact tests.     

Discrete element method study of shear-driven granular segregation in a slowly rotating horizontal drum

Segregation and mixing of granular materials are complex processes and are not fully understood. Motivated by industrial need, we performed a simulation using the discrete element method to study size segregation of a binary mixture of granular particles in a horizontal rotating drum. Particles of two different sizes were poured into the drum until it was 50% full. Shear-driven segregation was induced by rotating the side-plates of the drum in the opposite direction to that of the cylindrical wall. We found that radial segregation diminished in these systems but did not completely vanish. In an ordinary rotating drum, a radial core of smaller particles is formed in the center of the drum, surrounded by larger revolving particles. In our system, however, the smaller particles were found to migrate toward the side-plates. The shear from anti-spinning side-plates reduces the voidage and increases the bulk density. As such, smaller particles in the mixer tend to move to denser regions. We varied the shear by changing the coefficient of friction on the side-plates to study the influence of shear rate on this migration. We also compared the extent of radial segregation with stationary side-plates and with side-plates moving in different angular directions.     

Effect of non-condensable gas on condensation in a highly-rotating drum with a scraper

The effect of non-condensable gas on condensation in a highly rotating drum with a scraper is explored analytically. The formulation is first carried out generally, and then the application of the general formulation to the steam-air system gives heat transfer results for a wide range of operating conditions. It is found that a very small amount of non-condensable gas can induce a significant reduction in heat transfer. The reduction is accentuated at larger temperature differences, thinner condensate thickness and, to one's surprise, at higher operating pressures. By comparison, it is indicated that the present situation is more sensitive to non-condensable gas than forced convection condensation along a horizontal plate. Received on 14 August 1997     

Computational fluid dynamics simulations of aerosol behavior in a high-speed (Heubach) rotating drum dustiness tester

Potential exposure from hazardous dust may be assessed by evaluating the dustiness of the powders being handled. Dustiness is the tendency of a powder to aerosolize with a given input of energy. Previously we used computational fluid dynamics (CFD) to numerically investigate the flow inside the European Standard (EN15051) rotating drum dustiness tester during its operation. The present work extends those CFD studies to the widely used Heubach rotating drum. Air flow characteristics are investigated within the Abe-Kondoh-Nagano k-epsilon turbulence model; the aerosol is incorporated via a Euler-Lagrangian multiphase approach. The air flow inside these drums consists of a well-defined axial jet penetrating relatively quiescent air. The spreading of the Heubach jet results in a fraction of the jet recirculating as back-flow along the drum walls; at high rotation rates, the axial jet becomes unstable. This flow behavior qualitatively differs from the stable EN15051 flow pattern. The aerodynamic instability promotes efficient mixing within the Heubach drum, resulting in higher particle capture efficiencies for particle sizes d < 80 μm.     

Suppression of harmonic perturbations and bifurcation control in tracking objectives of a boiler–turbine unit in power grid

In the presence of harmonic disturbances, boiler–turbine units may demonstrate quasi-periodic behaviour due to the occurrence of various types of bifurcation. In this article, a nonlinear model of boiler–turbine unit is considered in which drum pressure, electric output and drum water level are controlled via manipulation of valve positions for fuel, steam and feed-water flow rates. For bifurcation control in tracking problem, two controllers are designed based on gain scheduling and feedback linearization (FBL). To investigate the efficiency of control strategies, three cases are considered for desired tracking objectives (a sequence of steps, ramps/steps, and a combination of them). According to the results, FBL controller works successfully in suppression of harmonic perturbations and consequently bifurcation control. As it is implemented, quasi-periodic solutions (caused by Hopf bifurcation) are vanished; leading to the appearance of periodic solutions with low amplitudes. Consequently, appropriate tracking performance with less oscillatory behaviour is observed for the drum pressure, electric output, and drum water level (desirable for the power grid). In addition, when FBL controller is used, less control efforts are predicted for the bifurcation control.     

Heat transfer in intumescent heat- and fire-insulating coatings

The physicochemical and thermophysical processes that jointly occur in intumescent heat- and fire-insulating coatings are studied numerically with the use of a model based on the hypothesis that the gaseous products of thermal expansion of an initial material are transparent. It is found that with the radiative mechanism of heat transfer in a layer of intumescent-coating coke taken into account, the temperature at the surface of a structure to be protected is almost equal to a temperature reached at this surface if a nonintumescent coating is used. Institute of Applied Mathematics and Mechanics, Tomsk State University, Tomsk 634050. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 3, pp. 143–149, May–June, 1999.     

ENERGY AND MASS TRANSPORT PROCESSES IN THE GRANULAR BED OF AN INDIRECTLY HEATED ROTARY KILN

The transport mechanisms of momentum,mass,species,and energy are invertigated in detail for the ro-tary kiln process.The residence time prediction of the granular bed is well improved by considerng differet flow patterns in the drum.Introducing a mixed flow pattern of the basic slipping and slumping behaviour has the most important effect on the improvement of the residence time prediction.The granular bed is assumed to hehave as a Bingham fluid in the active layer of the bed.The transport mechanisms of momentum,species,and energy are modelled on the basis of this assumption and using the kinetic gas theory.Additionally,a mathematical transformation is presented to save comput a-tional time.The model results of the temperature field are in very good agreement with experimental data.     

Numerical simulation of tetrahedral particle mixing and motion in rotating drums

A regular tetrahedron is the simplest three-dimensional structure and has the largest non-sphericity. Mixing of tetrahedral particles in a thin drum mixer was studied by the soft-sphere-imbedded pseudo-hard particle model and compared with that of spherical particles. The two particle types were simulated with different rotation speeds and drum filling levels. The Lacey mixing index and Shannon information entropy were used to explore the effects of sphericity on the mixing and motion of particles. Moreover, the probability density functions and mean values and variances of motion velocities, including translational and rotational, were computed to quantify the differences between the motion features of tetrahedra and spheres. We found that the flow regime depended on the particle shape in addition to the rotation speed and filling level of the drum. The mixing of tetrahedral particles was better than that of spherical particles in the rolling and cascading regimes at a high filling level, whereas it may be poorer when the filling level was low. The Shannon information entropy is better than the Lacey mixing index to evaluate mixing because it can reflect the real change of flow regime from the cataracting to the centrifugal regime, whereas the mixing index cannot.     

Cataracting-centrifuging transition investigation using nonspherical and spherical particles in a rotary drum through CFD simulations

This paper aims to systematically investigate the cataracting-centrifuging transition in a rotary drum involving spherical and nonspherical particles by using the Multiphase Granular Eulerian Model (MGEM). The effects of drum length and particle shape on the cataracting-centrifuging transition behavior were analyzed. The results showed that drum length plays an important role in the cataracting-centrifuging transition, although most related works in the literature do not consider this. The particle shape also significantly affects the cataracting-centrifuging transition behavior. Nonspherical particles required lower rotation speeds than spherical particles to reach the centrifuging condition. The particle shape was shown to be related to the critical solid fraction (α_sc) from Schaeffer’s model, although further investigations are required to completely correlate particle spherecity with critical solid volume fractions.     

Application of a screw conveyor with axial tilt blades on a shearer drum and investigation of conveying performance based on DEM

Screw conveyors are widely employed in industrial fields for conveying bulk materials. The shearer drum which uses the screw conveying principle is responsible for excavation and conveying coal particles onto the chain conveyor. Screw conveyor performance is affected by potential factors, such as the blade axial tilt angle and style, core shaft form and diameter. The effect of blade axial tilt angle on the conveying performance was investigated with the help of DEM. In the case of the screw conveyor, the mass flow rate, and particle axial velocity increased with increasing positive axial tilt angle, and declined with increasing negative axial tilt angle. In the case of the drum, the mass flow rate, particle axial velocity, and loading rate first increased and then decreased with increasing positive axial tilt angle, and decreased with increasing negative axial tilt angle. These results can be considered as a benchmark for screw conveyor and drum structural designs with axial tilt screw blades.     

综合管廊早龄期混凝土温度应力仿真模拟

以汉中市综合管廊为工程背景,通过有限元软件,首先模拟分析综合管廊早龄期温度场,随后将温度场结果附加到管廊结构中求解温度应力,从而分析综合管廊早龄期混凝土温度应力与浇筑长度的关系。模拟分析发现:综合管廊在混凝土浇筑早期,温度场沿长度方向变化不显著,且随着浇筑长度的增加,温度场不变;在墙体温度上升阶段,墙体内部沿长度方向的应力主要为压应力,当墙体混凝土内部温度达到最高,混凝土开始降温,结构内变形由膨胀逐渐变为收缩,内力由压应力逐渐变为拉应力,随着时间的变化,墙体内外温差趋于稳定,墙体温度应力也逐渐平稳,此时温度应力主要受外界环境变化影响;当综合管廊浇筑长度在60m范围内,浇筑长度越大,温度应力越大,但当浇筑长度超过60m,温度应力不再增加。     

采用混合边界温度的滑动轴承热动力润滑计算

根据流量连续和能量守恒原理，在滑动轴承热动力润滑计算中采用了一种确定混合边界温度的简单方法。与通常的平均边界温度方法相比，采用该混合边界温度使得轴承油膜温度提高，同时对轴承静特性系数、动特性系数，以及系统稳定性均构成不同程度的影响。最后的实验验证了该方法的可行性。     

Anomalous elongational flows and change of type

Anomalous effects on elongational flows at high rates of elongation reported by Ferguson et al. [1] are here treated as a change of type. Analysis predicts that the vorticity near the drum is hyperbolic, elliptic away from the drum, under the supercritical conditions in the experiment when the shearwave speeds have the values which we measured with our wave-speed meter.     

2D DEM simulation of particle mixing in rotating drum:A parametric study

Mixing behaviors of equal-sized glass beads in a rotating drum were investigated by both DEM simulations and experiments. The experiments indicated that higher rotation speed can significantly enhance mixing. The particle profiles predicted by 2D DEM simulation were compared with the experimental results from a quasi-2D drum, showing inconsistency due to reduction of contacts in the single-layer 2D simulation which makes the driving friction weaker than that in the quasi-2D test, better results could be rea...