单螺杆挤出过程固体粒子输送的离散单元法模拟

用离散单元法对单螺杆挤出固体粒子输送运动进行模拟,所得结果与实验和塞流模型对比,进而验证了离散单元法的有效性。速度计算结果显示了螺槽内各粒子的运动情况,此外还研究了物料参数如颗粒大小、摩擦系数以及螺杆转速对固体输送量的影响。     

固体推进剂螺旋压伸挤出过程流变模型建立

在对固体推进剂的螺旋压伸过程进行研究的基础上,探究了一种新型的固体推进剂流变模型的建立方法,即采用最小二乘法拟合基于毛细管流变仪测试所得数据,得到初步固体推进剂流变本构模型,利用Polyflow有限元仿真软件模拟毛细管流变仪测试过程,对流变本构模型参数进行修正,确定最终的推进剂流变本构模型。通过实验数据拟合与有限元模拟相结合的方法建立推进剂的流变本构模型,为用有限元法仿真模拟固体推进剂的螺旋挤出过程奠定基础。结果表明,采用数据拟合和数值模拟相结合得到的推进剂流变模型进行模拟实验得到的数据与真实数据误差均在10 %左右,符合数值模拟预期结果。     

固体推进剂固相组分的混合研究

研究了含固体组分的两相体系固体推进剂混合过程的特点。用固体组分的浓度、粒径及其分布等参量研究分析了改性双基推进剂代料的混合特点。根据实验结果，认为改性双基推进剂的混合过程属分散混合。固体推进剂的性能将取决于分散混合后固体颗粒的尺寸。     

流化床密相区内固体颗粒对流换热模型研究进展

为深入理解流化床密相区内活性颗粒表面换热过程,论述了乳化相模型、单颗粒和双颗粒模型、变物性模型以及接触热阻模型等流化床密相区内固体颗粒对流换热模型的研究进展,提出了模型发展方向。当流化风速较低时,固体颗粒对流换热系数可采用乳化相模型进行预测。针对流化风速较大的工况,有学者提出了单颗粒和双颗粒模型、变物性模型以及接触热阻模型,并采用适当假设简化模型,然而模型的可靠性仍需进一步验证。最后提出现应加强固体颗粒对流换热过程的模型研究,提高模型在较宽风速范围内对固体颗粒对流换热系数预测的适用性;重点研究活性颗粒在密相区内的运动形式及活性颗粒表面固体颗粒对流换热关键参数测量方法。     

颗粒移动床内不稳定运动的计算颗粒动力学模拟

采用考虑滚动摩擦的三方程离散单元法（DEM）模型对侧开孔的移动床中的颗粒流动进行了数值研究。结果表明，计算颗粒动力学(CGD)方法可对复杂颗粒系统内颗粒的运动行为进行准确的预测，包括时均速度场和脉动速度场。讨论了模型中颗粒摩擦参数的重要影响，并对颗粒流动表现出的间歇现象进行了分析。颗粒流动与流体流动有相似之处，都存在随机的脉动，但颗粒流的随机脉动机理与流体中的湍流机理有很大不同，颗粒流动会表现出很强的不连续性。     

磁粒研磨中磁性磨料的动力学行为仿真研究

为了探究磁粒研磨中磁性磨料的最佳动力学参数,利用有限元软件ANSYS Maxwell结合离散元软件EDEM,对磁粒研磨加工过程进行动态仿真。首先进行微观参数标定,得到了磁性磨粒离散元仿真所需的参数,最终确定滑动摩擦系数为0.9、滚动摩擦系数为0.109为最优的参数组合。在此参数基础上,对磁粒研磨加工过程进行动态仿真。结果表明：在其他条件一定的情况下,磁场转速越高,单位时间内磁性磨粒与管件表面之间的作用次数越高,加工效率越高。通过加工实验,得到了在不同加工参数下表面粗糙度数据。模拟仿真结果与实验结果的变化趋势具有一致性,验证了利用数值分析的手段对磁粒研磨加工过程进行理论分析的可行性。     

不规则多孔性颗粒物料形状系数和密度的确定

在化工生产过程中,诸如:流化床和固定床反应,过滤、干燥、结晶、气流输送以及固体浸出等重要的单元操作,颗粒状物料都起着很重要的作用,对设备进行设计计算时,形状系数和密度是两个常用的物性参数,对一般普遍物料在手册中不难查出该参数值,但对某些特殊的,不规则的或是多孔性颗粒物料,有时确很难查到,必须通过实验确定,如显微镜分析法、沉降法、淘析法、光散射法等。这些方法过程复杂,精确度也不高,特别对多孔性物料,由于流体渗入孔内,测定值往往高于实际值,最近国外报导了一个新的方法,实验、计算和图解法相结合,通过测定压力降,利用已知的流体粘度,层床间隙率等数据,可以精确、简单地得到形状系数和密度这两个重要参数。     

固体热载体法褐煤热解过程中的传质传热特性

为揭示在固定床反应器中固体热载体法快速热解褐煤工艺过程中的热、质传递机理,建立了固体热载体法褐煤热解过程中的传质传热模型。模型包括球型颗粒的一维非稳态导热方程和基于分布活化能模型的动力学模块,分别采用有限容积法与Matlab软件中遗传算法工具箱对二者进行数值计算。通过呼伦贝尔褐煤热重实验数据与温度测定实验数据分别验证了预测的动力学参数及颗粒传热模型结果。研究发现,热、质变化在固体热载体法褐煤热解工艺中呈现复杂的耦合特性。此外,考察了在不同初始温度、热载体进料比与煤颗粒半径条件下,褐煤在热解过程中颗粒内部温度场在径向上随时间的变化规律,并分析了产物释放速率与温度场的关联性,结果表明热历程改变是工艺条件对热解产物分布造成影响的根本原因。     

基于液桥效应圆筒制粒机的铁矿粉制粒机理研究

针对圆筒制粒机中存在有不同量级粒径颗粒的制粒问题，本工作对现有的液桥力计算式进行了修正，建立了相对应的离散元模型，通过实验测试与仿真相结合，探讨了修正系数对颗粒运动的影响，得到了修正系数以0.6适宜；以铁矿粉混合料的堆积角为参考，通过仿真和实验测试得到了混合料的物性参数，以此为基础，探讨了铁矿粉混合料颗粒的运动规律与团聚机理及其团聚体的分布，结果表明，团聚体颗粒的自旋速度越大、所受剪切力越小，则越有利于制粒；团聚体粒径沿圆筒的径向呈现先增大后减少的变化，且在靠近混合料表层下存在一个“高效制粒区”，在该区域团聚体的粒径最大、剪切力最小、自旋速度较大，同时建议可采用颗粒碰撞频率或能量损失作为判据来终止颗粒的团聚，以达到节能降耗的目的，所得结论可为铁矿粉圆筒制粒机的研发提供设计依据。     

三级活塞推料离心机数值模拟与操作参数优化

针对新型三级活塞推料离心机的研发,在转鼓尺寸和物料增大情况下,操作参数对分离过程中转鼓内侵蚀现象和分离效率有重要影响。采用样机实验结合CFD数值模拟,对三级活塞推料离心机进行三维建模和仿真,并利用密集离散粒子模型(DDPM)模拟侵蚀过程。基于粒度分析可知,氯化钠颗粒粒径dm为0.070~0.200 mm时,随颗粒粒径增加,固相对离心机转鼓内部的侵蚀越大。结合响应面分析法可知,响应模型各因素的交互影响明显,以更高分离率与更低侵蚀率作为评价指标,得出最优值为推料频率40次/min、转鼓转速1431 r/min、进料浓度60%。与实验数据进行对比,模拟结果的误差在可接受范围内。     

Effect of ultrafine aluminum on the combustion of composite solid propellants at subatmospheric pressures

This paper presents the results of an experimental study of the combustion of composite solid propellants with a double oxidizer (ammonium perchlorate/HMX) at pressures of 0.03–0.1 MPa. Systems containing a micron-sized aluminum powder (ASD-4) and the Alex ultrafine aluminum powder were investigated. It was shown that the replacement of ASD-4 by Alex in propellant systems led to an increase in the burning rate. The aluminum particle size and the oxidizer excess coefficient were found to affect the exponent in the power-law burning-rate dependence. The range of the oxidizer excess coefficient was determined that corresponded to the effective replacement of micron-sized aluminum by ultrafine aluminum for which the exponent in the power-law burning rate dependence decreases. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 47–55, January–February, 2009.     

PROBABILISTIC THREE-DIMENSIONAL MODEL FOR SLOW SHEARING PARTICULATE FLOW: DRY FRICTION

A probabilistic model for the slow shearing motion of spherical particles dominated by dry inter-particle frictional forces is proposed. The particles can move by sliding, rolling, solid body rotation and jumping following the criterion of minimum energy dissipation. The continuous deformation of particle assembly is regarded as a Markov process in discrete time and continuous space. The components of the contact angle between two neighboring particles are taken as the primary random variables. The coefficients of the probabilistic governing equation are determined from the rate of energy dissipation and micromechanical analysis of particle motion. The inter-particle sliding and rolling friction laws are input data. The constitutive relationships, apparent viscosity and internal friction angle of the particulate material as a whole are obtained by integration. The validity of the proposed model is verified by comparing the model predictions to analytical solutions and available experimental results.     

热固性酚醛树脂再生活性细料物性参数标定及应用

为准确评价再生热固性酚醛树脂再生活性细料的机械混合程度,提出了通过休止角合理推测其他参数的思路,并提出了一种粉体休止角测定方法。以休止角作为参照,基于Hertz-Mindlin接触模型与前馈神经网络的预测模型,采用离散元参数标定的方法,对细料离散元物性参数进行标定。采用K-means聚类分析法确定细料离散元参数范围,选择一组最优离散元参数应用于细料机械混合再生制备试验。结果表明,再生活性细料颗粒随机混合浓度的相对标准偏差(RSD)约为0.15,接近均匀混合状态,获得综合性能较优的再生复合保温板,验证了细料离散元参数标定方法的有效性与可行性。     

Nonreinforcing filler–elastomer systems. I. Experiments based on model systems

Five methods were modified to test for interactions between binder and filler of composite solid propellants. Methods based on uncured binder or model compounds were rate of solution of binder from composite mixtures; centrifugal separation of binder from the filler; of composite mixtures; measurement of the contact angle between binder and filler; and adsorption of binder or model compounds by filler from solution. Stress–strain–birefringence was measured on cured binders containing small amounts of filler. In addition to the polymers used as binders and the model compounds, three physical forms of aluminum, and the additives tris [1-methylaziridinyl] phosphine oxide (MAPO) and a polysebacate of methyl-N-diethanolamine were included in the study. The filler was ammonium perchlorate in all experiments. The polysebacate was an effective adhesion improver in the polyurethane–ammonium perchlorate composite. MAPO was not as effective in the system polybutadiene–ammonium perchlorate. Fibrous forms of aluminum result in a weaker propellant then does powdered aluminum. Nonfunctionally terminated polymers were poorer in adhesion, contact angel, and adsorption tests compared with carboxyl- and hydroxyl-terminated types. The nonpolymeric model compounds yielded inconclusive data.     

Laser-induced ignition of solid propellants for gas generators

An experimental study was conducted to evaluate the ignitibility of a family of potential gas-generator solid propellants under different CO₂ laser heat fluxes and chamber operating conditions. Six types of solid propellant were tested: one baseline and five variations. The ingredients of the baseline propellant were ammonium nitrate, guanidine nitrate, potassium nitrate, and polyvinyl alcohol. The other five propellant variations were obtained by adding different amounts of carbon black, ammonium perchlorate, RDX, and triamino guanidine nitrate to the baseline formulation. The propellants were tested in a high-pressure windowed strand burner, which was pressurized with either air or argon purge gases. Propellant ignition was accomplished by a 50-W CO₂ laser. The ignition delay times of the propellants were measured at pressures of 1 and 69 atm using high-speed video cameras. In this paper, the test results and comparison of the ignition and combustion characteristics of these propellants were presented.     

Discrete element method models of elastic and elastoplastic fiber assemblies

Geometry-dependency and plasticity are considered in the contact force models for the discrete element method simulations of elastic and elastoplastic fiber assemblies subject to uniaxial compression. It is observed that the contact force models have a significant impact on compressive loads. A simplified normal contact force model leads to smaller loads at large solid volume fractions compared to the experimental results, while the present geometry-dependent models give better predictions. Simulations with a simple Coulombic tangential contact force model (considering sliding friction only) significantly underestimate the loads, while a Mindlin tangential force model that considers static friction improves the predictions. For the modeling of the fibers that undergo large plastic deformations, plasticity should be considered for both the fiber bending and fiber–fiber normal contact in order to obtain correct simulation results. Elastic models for fiber–fiber contact and fiber bending deformation remarkably overpredict the loads for the plastic fibers.     

A comparison of discrete element simulations and experiments for ‘sandpiles’ composed of spherical particles

Discrete element simulations, with the particle–particle interaction model based on classical contact mechanics theory between two non-adhesive spheres, were carried out and compared with ‘sandpile’ experiments using spherical particles in order to assess the validation of the simulation data. The contact interaction model is a combination of Hertzian theory for the normal interaction and Mindlin–Deresiewicz theory for the tangential interaction. To ensure the consistency of the simulations with the experiments, the measurement of sliding friction, a key parameter in DEM simulations, was highlighted. A simple experimental method for establishing the value of the friction coefficient was proposed and used in measuring the friction of the rough glass beads and steel balls to be modelled in the simulations. The simulations were carried out for two cases according to particle arrangements: the first is quasi-two-dimensional (Q2D), with a smaller flat cuboidal box containing the spherical particles inside another box for discharge, and the second case is axisymmetric (3D). For both cases, simulations and experiments were carried out for assemblies of polydisperse rough glass beads under the same conditions. Comparisons were made that showed that the profiles and hence the measured angles of repose, in each case, were in good agreement, thus supporting the validity of the discrete element model used. Further numerical–experimental comparisons were carried out for 3D conical piles using smooth monodisperse steel balls and the same conclusions were obtained.     

Diffusion rates and the role of diffusion in solid propellant rocket motor adhesion

Diffusion rates can give important information for the adhesion process across the bondline between insulation and propellant in solid propellant rocket motors. Diffusion coefficients of low molecular weight species such as crosslinkers and plasticizers have been measured by the weight of uptake method in polymer materials that are candidates for propellant contact. The materials were EPDM insulation sheets and “liners,” based on HTPB, HTPE, or GAP, and with different degrees of particle filling. Plots of relative mass gain as a function of the square root of time showed good linearity up to 20–50% weight increase and the diffusion coefficients could thus be determined with good accuracy. The diffusion coefficients for the low molecular weight isocyanates and plasticizers in these materials vary between 10^?11 and 10^?17 m² s^?1, dependent on material types and particle filling. In most cases, the results can be explained by the solubility parameters of the organic liquids and polymers. For the particle filled samples, the diffusion coefficients decrease with increasing degree of particle filling, and the decrease is faster than predicted by the Maxwell–Fricke or the Keller models for arrays of smooth spheres. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1529–1538, 2007     

Agglomeration Characteristics of Aluminum Particles in AP/AN Composite Propellants

Most solid rockets are powered by ammonium perchlorate (AP) composite propellant including aluminum particles. As aluminized composite propellant burns, aluminum particles agglomerate as large as above 100 μm diameter on the burning surface, which in turn affects propellant combustion characteristics. The development of composite propellants has a long history. Many studies of aluminum particle combustion have been conducted. Optical observations indicate that aluminum particles form agglomerates on the burning surface of aluminized composite propellant. They ignite on leaving the burning surface. Because the temperature gradient in the reaction zone near a burning surface influences the burning rate of a composite propellant, details of aluminum particle agglomeration, agglomerate ignition, and their effects on the temperature gradient must be investigated. In our previous studies, we measured the aluminum particle agglomerate diameter by optical observation and collecting particles. We observed particles on the burning surface, the reaction zone, and the luminous flame zone of an ammonium perchlorate (AP)/ammonium nitrate (AN) composite propellant. We confirmed that agglomeration occurred in the reaction zone and that the agglomerate diameter decreased with increasing the burning rate. In this study, observing aluminum particles in the reaction zone near the burning surface, we investigated the relation between the agglomerates and the burning rate. A decreased burning rate and increased added amount of aluminum particles caused a larger agglomerate diameter. Defining the extent of the distributed aluminum particles before they agglomerate as an agglomerate range, we found that the agglomerate range was constant irrespective of the added amount of aluminum particles. Furthermore, the agglomerate diameter was ascertained from the density of the added amount of aluminum particles in the agglomerate range. We concluded from the heat balance around the burning surface that the product of the agglomerate range and the burning rate was nearly constant irrespective of the added amount of aluminum particles. Moreover, the reduced burning rate increased the agglomerate range.     

水泥粉体流动行为及其与不同材质库壁粘附性研究

水泥粉体流动性与其储存、输送、给料、混合等操作密切相关。通过休止角、内摩擦角、粘聚力和无侧限抗压强度测试方法,研究了含水率对水泥粉体流动性的影响,对比了水泥粉体与砂浆、钢、涂料、防腐卷材四种壁面材料的壁摩擦角和附着力。结果表明:随着水泥粉体含水率增加,休止角、粘聚力和无侧限抗压强度增加,水泥粉体流动性变差。松散堆积水泥粉体随着放置时间增加,由于水泥水化引起颗粒尺寸增加造成流动性变好。固结应力增加,无侧限抗压强度明显增大,导致水泥粉体流动性变差。水泥粉体在不同壁面材料的粘聚力均随着含水率的增加而增大,在水泥库内表面涂覆涂料和防腐卷材均可以起到降低水泥粉体与壁面的摩擦力和附着力的作用。