玻璃池窑中流动与传热的二维数学模型

在理解熔体流动和传热机理的基础上,确立了非电热玻璃池窑中流动和传热的二维数学模型.高温下玻璃熔体物理性质的计算程式如下:根据Boussinesq近似,除浮力项外,在力距公式中密度被视为常数;以有效导热系数表示真实热导和辐射热导的综合作用.根据生产实际情况,确定边界条件;应用SIMPLE法在IBM-AT计算机上计算了窑内玻璃液的温度场和速度场.在求解方程时,应用"Line-by-line"求解法并采用了分块修正法以加速度收敛速度.选择数个方案,研究了池窑冷却部池深、窑底保温及出口流量变化等对池窑内玻璃液的温度场和速度场的影响.结果表明:减少池深可提高降温效果;窑底保温可增加玻璃液的循环流速,特别是逆流速度,从而提高了玻璃液的化学均匀性和温度均匀性;使窑内自然对流增强,远远大于作业流;随着出口流量的增大,冷却部玻璃液温度将逐渐增高.数学模拟计算的结果定量地显示了窑炉结构设计与作业制度的相对应关系.     

U型管式换热器进口截面流场数值模拟

建立了U型管式换热器进口截面的三维稳态流动数学模型,求得了U型管式换热器内部的压力场、速度场和温度场分布;在此基础上对U型管式换热器内部的温度场、压力场和速度场进行了讨论。研究结果表明,一方面,增大U型管式换热器进口热流体速度,可以增加U型管式换热器的换热量,增大出口截面的速度,增大内部压强,提高内部温度;另一方面,运行时间越长,U型管式换热器内部温度越低、出口截面速度越大,总传热率越低,压力损失先减小后趋于稳定;同时,离U型管换热器越近的外导流筒冲刷腐蚀越严重。     

矩形电渗流道中焦耳热效应的数值模拟研究

对玻璃和聚二甲基硅氧烷(PDMS)制成的矩形微流道中电渗流(EOF)的温度场进行了数值模拟研究。焦耳热效应的数学模型包括控制电势场的Poisson-Boltzm ann方程,控制流场的修正Navier-Stokes方程和控制温度场的能量方程。电势场、流场和温度场通过与温度有关的流体属性耦合在一起,将耦合的控制方程简化之后,应用有限元方法完成了矩形电渗流道中温度场的仿真计算。数值模拟结果表明,相同条件下,PDMS制成的微流道中的溶液温度明显高于玻璃制成的微流道中的溶液温度,且尺寸较大的PDMS流道中(h=48μm,b=96μm)的溶液温度明显高于尺寸较小的PDMS流道(h=32μm,b=96μm)中的溶液温度。     

换热器负压阀的传热特性仿真与分析

为了确保换热器负压阀在较大的热载荷条件下仍保持良好的稳定性和可靠性,基于有限元方法对负压阀内的流体进行了仿真与分析。基于ANSYS软件建立了传热耦合模型,通过ICEM划分网格,在极限载荷条件下得出负压阀内流体的速度场、压力场和温度场。研究表明,负压阀的阀座底端和阀杆位置温度较高,可通过增压冷却回路的方式改善其传热特性。     

RESS法制备微细颗粒的喷嘴内流动特性的数值模拟

为了对RESS法制备微细颗粒过程中喷嘴内流体规律进行研究,通过对超临界流体快速膨胀法（rapid expansion of supercritical solution,RESS）流动过程的研究与分析,建立了喷嘴内超临界流体流动数学模型。对喷嘴内流场和温度场进行研究,考察了预膨胀压力、预膨胀温度、长径比等操作参数对RESS过程的影响,模拟结果表明,喷嘴内部的密度曲线在喷嘴入口段,几乎没有发生变化,而在直管段和出口膨胀段超临界流体密度发生急剧下降;随着长径比的增大,喷嘴内密度曲线变陡;随着长径比的增大,喷嘴出口处流体的温度都变小,过饱和度变大,结晶颗粒使得更为细小。该模型和模拟过程能够为实现制备均一微细颗粒的实际操作条件和优化过程参数奠定基础。     

密炼室三维温度场分析

本文利用有限元法对密炼机混炼过程中密炼室的三维温度场分布进行了分析。重点分析了密炼室的三维温度场及其热量传递的分布情况,以及强制冷却对炼胶温度的影响,同时通过实验验证了密炼室温度场有限元模拟的正确性。     

玻璃纤维窑炉玻璃液流场数学模拟研究

玻璃纤维窑炉作为玻璃纤维生产最重要的热工设备之一,被称为玻璃纤维生产的心脏.为了实现玻璃液熔制的高效节能和玻璃液高质量的输出,需掌握窑炉中高温玻璃液的运动轨迹,对生产过程实现实时控制.通过对窑炉内玻璃液的流动状态进行模拟,可以在提高产品质量、节约能源、降低废物排放、提高单位产出率等方面得到优化.以年产8万t玻璃纤维窑炉玻璃液为研究对象,通过使用ANSYS软件对窑炉生产状态进行数值模拟,分析了模型中温度场分布、速度场分布和电功率场分布情况,同时计算了不同电助熔参数设置对窑炉内玻璃液温度场分布以及速度场分布的影响,结合生产实际提出了窑炉内部实际的玻璃液流动场优化情况.     

塑料熔体在注塑模中的三维流动模拟

建立了非等温条件下黏性、不可压缩、非牛顿流体流动的控制方程.为了避免同时求解耦合的压力场、速度场,通过修改传统方法的变分方程导出了关于压力场的拟Poisson方程,用迭代法独立求解连续性方程、动量方程,并进行速度场-黏度迭代求出最终的压力场、温度场.这种方法可以减少内存,提高数值方法的稳定性,避免了Hele-Shaw模型中容易引起争议的“中面”概念,并能模拟中面方法不能模拟的一些物理现象.算例表明数值结果与实验结果吻合较好,这种方法可以成功地预测注射成型流动过程中的重要特征.     

换热管内插螺旋强化传热的数值模拟

换热管内插入钢丝螺旋改变了管内的流动状态。通过数值模拟,研究内插螺旋换热管内速度场与温度场的分布特性,对空管和内插螺旋换热管进行了比较。模拟结果表明,在相同条件下,内插螺旋能够有效地改善换热管内速度场和温度场,验证了管内插螺旋是提高换热性能的有效手段。     

挤塑过程熔体流动的数值模拟

通过对挤出过程不可压缩粘弹性熔体流动的数值模拟、求出速度场、温度场、压力场以及膨胀比,有利于确定挤塑过程的工艺条件和口模尺寸。     

Effect of die temperature on the flow of polymer melts. Part I: Flow inside the die

The effect of die wall temperature on the flow of polymer melts in circular capillary dies was studied. At constant flow rates, it was found that die wall temperature had a greater effect on the pressure drop than melt temperature. A capillary die with two circular channels with different diameters was designed to simulate the profile extrusion. Changes of wall temperature varied the flow rate ratio between the two channels. An implicit finite difference method was used to simulate the velocity and temperature profiles inside the die. Values predicted by this model matched well with experimental data for both dies.     

Heat transfer to molten polymers flowing through tubes

A theoretical and experimental study of heat transfer to polymer melts flowing through circular tubes is presented. The mathematical model provides for shear heating and expansion cooling effects, and also heat of reaction during flow for various wall boundary conditions. Experimental results, obtained using low density polyethylene, show reproducible temperature and velocity profiles. The measured inlet melt temperature profile and the axial wall temperature profile provide the boundary conditions for the calculations. The experimental data confirm the predictions of the magnitude of the shear heating and expansion cooling effects during tube flow.     

Viscous dissipation in capillary flow of rigid PVC and PVC degradation

The steady state, non-isothermal behavior of rigid polyvinyl chloride melt, flowing in capillaries of circular cross-section, was investigated by solving, with the aid of a digital computer, the momentum and energy balance equations. It was assumed that the polymer melt can be described by the “Power Law” constitutive equation. The shear rate, temperature and pressure dependent properties of the fluid were obtained experimentally. The effects of the thermal degradation of PVC on its viscosity, were also introduced in the equations of momentum and energy. The velocity, temperature and pressure profiles, obtained for both adiabatic flow and flow through a tube of constant wall temperature, indicate that considerable heating of the melt, due to viscous dissipation, can be achieved at moderate flow rates. Thermal degradation occurs in the capillary under certain conditions of temperature history and residence time of the fluid. The results of this work are in fair agreement with experimental results in this area.     

The injection molding of thermoplastics part I: Theoretical model

A mathematical model is proposed for the quantitative treatment of the injection molding of thermoplastics as it relates to the behavior of polymer in the cavity. The model is based on setting up the equations of continuity, motion, and energy for the system during each of the stages of the injection molding cycle (filling, packing and cooling) and the coupling of these equations with practical boundary conditions. The treatment takes into consideration the non-Newtonian behavior of the melt, the effect of temperature on density and viscosity, the latent heat of solidification, and the differences in thermal properties between the solid and the melt. In employing the model, it is necessary to know the pressure-time variation at the cavity entrance. Numerical solutions have been obtained for the case of spreading radial flow in a semi-circular cavity. The numerical results yield significant data on the progression of the melt front, the flow rate, and the velocity profiles at different times and positions in the cavity. They also yield temperature and pressure profiles throughout the packing and cooling stages.     

水平管油气两相段塞流及其传热特性

海底油气管道的冷却传热过程是结蜡、水合物等海洋石油工业流动保障问题的关键控制因素。采用电容探针与热电偶、热电阻等流动及温度测量手段对不同冷却条件下空气-油段塞流的流动参数和传热参数进行实验测量,分析了空气-油段塞流流动参数对传热特性的影响,并与空气-水对流换热进行对比。结果表明,空气-油段塞流对流传热系数主要受液相折算速度的影响,且冷却液温度越低,管底热流体黏度越大,导致热边界层越厚,传热系数降低;受黏性力及边界层影响,对流传热系数远小于空气-水;沿管壁周向,从管顶到管底的对流传热系数不断增大。提出了适用于冷却条件下的油气段塞流传热关联式和传热模型。     

THERMAL DISPERSION AND HEAT TRANSFER IN NONISOTHERMAL BUBBLE COLUMNS†

Local axial and radial temperatures were measured at steady-state conditions in a 0.078-m-I.D. bubble column heat exchanger. Nitrogen and water superficial velocity ranges were 0-0.6 m/s and 0-0.02 m/s, respectively. Average column pressures were 3.0, 5.1, and 7.1 atm. The axial temperature profile varied significantly with all conditions encountered. Radial temperature profiles were found to be nearly constant, indicating very good radial mixing.

An axial thermal dispersion heat transfer model, capable of representing nonisothermal systems, was employed to characterize the measured bubble column temperature profiles. Thermal dispersion was apparent from large temperature changes in the entrance of the bubble column. Heat transfer coefficients depended on the gas and liquid flow rates. However, the thermal dispersion coefficients depended on linear gas velocity and were a weak function of liquid flow rates. The thermal dispersion coefficients obtained in this study were found to be consistent with other investigations. In addition, they were compared to the mass dispersion coefficients obtained by other studies and found to be in good agreement     

Wall slip of concentrated suspension melts in capillary flows

The effects of wall slip of concentrated suspension melts in capillary flows were investigated at elevated temperature. The modeled material is a mixture of polymer EVA (Ethylene Vinyl Acetate) and non-colloidal spherical powder (glass microspheres) with mean particle size within 53∼63 μm. The effect of particle concentration on wall slip was studied experimentally in a capillary rheometer. For suspensions with different particle loadings (35%, 40%, and 45% by volume), the slip velocity V_s increased with an increase of particle concentration at the same testing temperature. A master slip curve can be obtained by plotting slip velocity versus the product of wall shear stress and square root of particle concentration. As such, a new particle concentration-dependent slip model is proposed. A theoretical approach coupled with the new slip model and flow equation is employed to characterize the flow behavior of concentrated suspension in a capillary rheometer, with reasonable agreement obtained with experimental observations.     

Theoretical analysis on the flow of melts in a curved circular tube

A theoretical study is made of molten polymer flow in a curved circular tube. To obtain the velocity profile formed in the tube, a variational approach is used. A variational equation for viscous dissipation has been so determined that its Euler equation satisfies the equation of motion. An approximate velocity profile which satisfies the prescribed boundary conditions is assumed as a function involving unknown coefficients and is substituted into the variational equation to determine the unknown coefficients. Various velocity profiles of polymer melts for different flow behavior indices have been obtained under the assumption that polymer melts obey a power law.     

Numerical investigation of the error on flow measurements due to exhaust gas heating and cooling in the SBI‐configuration

The possible flow measurement error due to heating or cooling of exhaust gases in the Single‐Burning‐Item (SBI) test is estimated from numerical experiments. It is illustrated that there is no one‐to‐one correspondence between the velocity profile shape and the instantaneous Reynolds number, due to the time‐dependent temperature and density profile evolution in the exhaust gas pipe. A non‐ambiguous relation is found between the velocity profile shape and an ‘effective’ Reynolds number, based on the turbulent viscosity. Maximum variations of the velocity correction factors, relating the mean velocity to the velocity on the pipe axis, are found to be in the order of 2% for limiting circumstances for the SBI test. The primary effect is caused by instantaneous Reynolds number variations. The effect of heating or cooling of the flow by the hot or cold pipe is noticeable, too. The statements are proved to be valid independent of the computational grid, the turbulence model and the time steps taken to obtain the numerical solutions. Copyright © 2006 John Wiley & Sons, Ltd.     

Fluorescence monitoring of polymer injection molding: Model development

We have developed models to describe the behavior of an optical fiber sensor which was used to detect fluorescence from a polymer resin during the cooling phase of injection molding. The optical fiber sensor was positioned at the wall of the mold cavity by using the ejector pin channel as access to the cavity. The sources of fluorescence were dyes, which were chosen because of their sensitivity to temperature and which were mixed with the resin at dopant concentrations (parts per million by weight). The behaviors of a molecular rotor dye, dimethylamino diphenyl hexatriene, doped into polyethylene, and an excimer producing dye, bis-(pyrene) propane, doped into polystyrene were the subjects of the modeling calculations. The models consist of two modules: (a) a solution to the thermal diffusion equation for the resin cooling in the mold and (b) using temperature/time profiles and, in the case of polyethylene, crystallinity/time profiles obtained from the thermal diffusion equation, fluorescence intensity as a function of time was computed. Factors incorporated in the models are: adiabatic heating and cooling, light scattering due to microcrystals of polyethylene, crystallization kinetics, temperature and pressure shift factors for viscoelastic relaxation near the glass transition temperature of polystyrene, and the thermal resistance at the resin/mold interface.