圆管聚合物热流中黏性耗散分析的无网格模拟

以与温度相关的指数定律作为本构方程,应用无网格方法模拟了外表面为恒温时的圆管内具有黏性耗散的聚合物流动热传导问题,给出了离入口不同位置处的温度分布。计算结果表明：根据黏性耗散模型计算的温度比无黏性耗散模型高出64℃,从而说明了黏性耗散在聚合物流动热传导问题中具有举足轻重的作用。并且, 无论是无黏性耗散模型,还是黏性耗散模型,其极限温度与壁面温度有很大的关系,但与入口温度无关。     

气固两相流中颗粒运动强化器壁对流传热的机理

提出了颗粒碰撞壁面过程中颗粒打破边界层和颗粒与壁面导热的强化传热模型,以分析气固两相流中颗粒强化对流传热的机理,通过模型计算研究了风速、颗粒浓度和粒径等因素对颗粒在壁面的停留特性、颗粒在边界层的扰动及对导热和边界层破坏两种强化机制传热比重的影响．以气固两相流横掠圆管的传热过程为例进行计算,计算与实验结果一致,并获得了具有实用价值的关联计算式．     

垂直受热通道中气液两相流传热性能的研究

在垂直受热通道中气液两相流的传热是一种从单相流、泡状流、弹状流、搅拌流直至环状流的各种流型的气、液两相复杂传热。本文采用了特制的实验研究设备,以水及其蒸汽为介质,研究了各种流型的传热特性及其影响因素(进料质量流速、热通量以及干度等),探讨了成膜条件,并用J.C.Chen双机理传热模型计算、分析了在不同情况下各机理对传热所起的作用。     

直流蒸汽发生器蒸干及蒸干后传热数值分析

准确预测直流蒸汽发生器流动沸腾及蒸干对其设计、安全可靠运行极其重要。通过对BW公司直流蒸汽发生器进行合理简化,引入两流体三流场数学模型及壁面热通量分区模型,分别进行基于常热通量和耦合传热的蒸汽发生器流动沸腾数值模拟。结果表明:蒸干发生时传热性能急剧下降,常热通量边界下壁温升高的幅度相当大(约300K·m~(-1)),而耦合传热边界下壁温飞升幅度约为25K·m~(-1),与实际情形相一致;两种热边界中预热区会发生过冷沸腾,壁面处传热由液相对流换热、淬火换热和蒸发换热3部分构成,核态沸腾区蒸发换热为主要换热方式,同时伴随着液相对流换热和淬火换热,蒸干发生时淬火换热和蒸发换热全部降到0,在蒸干后传热区域换热方式为气相对流换热。     

直流蒸汽发生器蒸干及蒸干后传热数值分析

准确预测直流蒸汽发生器流动沸腾及蒸干对其设计、安全可靠运行极其重要。通过对B&W公司直流蒸汽发生器进行合理简化,引入两流体三流场数学模型及壁面热通量分区模型,分别进行基于常热通量和耦合传热的蒸汽发生器流动沸腾数值模拟。结果表明：蒸干发生时传热性能急剧下降,常热通量边界下壁温升高的幅度相当大（约300 K·m^-1）,而耦合传热边界下壁温飞升幅度约为25 K·m^-1,与实际情形相一致;两种热边界中预热区会发生过冷沸腾,壁面处传热由液相对流换热、淬火换热和蒸发换热3部分构成,核态沸腾区蒸发换热为主要换热方式,同时伴随着液相对流换热和淬火换热,蒸干发生时淬火换热和蒸发换热全部降到0,在蒸干后传热区域换热方式为气相对流换热。     

三相流沸腾传热

对三相流沸腾传热进行了理论分析和实验研究，研究了三相流沸腾传热特性，提出了三相流沸腾传热系数的计算方法，实验结果表明，由于固体粒子的存在，强化了沸腾传热，三相流沸腾传热系数是相同条件下汽－液两相流沸腾传热系数的两倍。固体粒子的存在，能有效地防止和清除传热壁面上的污垢。     

微通道内Taylor流的计算流体力学数值模拟研究进展

简要阐述了计算流体力学(CFD)中流体体积函数法(VOF)的概念和原理。重点综述了采用CFD方法研究微通道中两相Taylor流的最新进展,包括两相流速度场、气泡或微液滴的生成、发展等流动特性、壁面处液膜厚度的影响因素、微通道内的传质传热特性等。最后展望了微通道内多相流CFD数值模拟的研究重点和应用方向。     

微通道内液-液泰勒流传热的计算流体力学模拟

微通道内的液-液两相流型在低流速时以泰勒流为主,本文使用计算流体力学方法,对微通道内液-液泰勒流的传热特性进行了研究。首先考察了分散相流速、物系和管径对微通道壁面温度的影响。结果表明:分散相流速和物系热导率增大使得微通道壁面温度降低,管径的改变对微通道壁面温度影响较小。然后针对当量直径为0.5mm的微通道内工作介质为甲苯和水的两相泰勒流模型,考察了微通道壁面剪切力、界面涡度和努塞尔数对壁面和内部温度的影响,并与文献中气-液两相泰勒流的传热性质作了比较。结果表明:壁面剪切力和界面涡度对管壁和界面温度的波动性变化有一定影响,壁面剪切力和界面涡度的波峰往往出现在温度的波峰附近,并且有一定的时间滞后性。甲苯-水两相泰勒流动下的壁面努塞尔数比气-液两相流大得多,液弹单元的平均努塞尔数是相同条件下单相流体流动的1.3倍。     

分相式多孔壁微通道流动冷凝传热强化与减阻

微通道换热器应用广泛,强化传热和减阻是新型换热器设计的重要目标。为了同时实现这两相目标,本文提出了一种分相式多孔壁微通道冷凝器,利用微针肋阵列组成的多孔壁在冷凝传热过程中实现了汽液两相分离。采用实验研究方法对比了分相式多孔壁微通道与普通实心壁微通道的流动和传热特性,结果证明分相式微通道在冷凝传热中同时具备强化传热和减阻的作用。深入研究了通道内两相流动摩擦耗散原理并提出了相分离减阻理论,指出汽液两相流内部摩擦耗散的减小是分相流减阻的关键。另一方面,分相过程使针肋换热面侧壁直接与高温蒸汽接触,极大消减了蒸汽与换热壁面之间的传热液膜厚度。沿流动方向不断扩展的液通道截面与不断减缩的汽通道截面积适应了流动冷凝过程延工质流动方向"水渐多,汽渐少"的规律,保证沿程传热效果不会恶化。     

液滴碰撞倾斜热壁面数值模拟研究

为了研究液滴碰撞复杂热壁面过程中液滴的铺展特性和传热特性,文中基于FTM(Front-Tracking Method)方法对液滴碰撞倾斜固体热壁面过程进行了模拟研究,通过改变壁面倾斜角度、表面张力系数σ,从压力场、流场、惯性力、表面张力等角度对换热现象进行分析和研究。结果表明:液滴撞击倾斜壁面时,在液滴下滑方向一侧三相点处热流密度取得最大值;壁面倾斜角度越大,液滴铺展程度越低,传热减弱;σ影响液滴中后期的铺展特性,σ越大液滴最大铺展系数越小,且回缩越迅速;由于传热大部分发生在液滴碰撞壁面初期铺展阶段,所以σ对液滴与壁面间传热特性影响较小。     

矩形截面微模具通道中熔体的黏性耗散效应

基于对微注塑成型过程中聚合物熔体充模流动时黏性耗散效应的理论分析，以聚丙烯（PP）和高密度聚乙烯（HDPE）两种聚合物材料，在不同工艺参数作用下流经不同当量直径和长径比矩形截面微模具通道时，由黏性耗散效应引起的微通道中熔体温度变化进行了试验测量和数值模拟。结果显示，微通道出口熔体温度的试验测量和数值模拟值与理论计算值非常吻合，且其平均误差小于1℃。同时研究发现，增大微模具通道当量直径和长径比时，熔体流动时的黏性耗散热量增多，通道出口熔体温度升高；而当微通道几何尺寸一定时，其黏性耗散热量随注射速度和注射压力的升高而增加，随熔体温度和模具温度的升高而降低；但同样试验条件下，对剪切作用敏感性强的PP材料的黏性耗散热量明显高于对剪切敏感性弱的HDPE材料。     

Effect of viscous dissipation on the temperature of the polymer during injection molding filling

During injection molding, viscous dissipation changes the temperature distribution by playing the role of an energy source, which affects heat transfer rates. Understanding the effect of the viscous dissipation assists the designing of the cooling system in injection molding process. In this article, the effect of the viscous dissipation on the temperature distribution throughout a rectangular channel for different polymers at different inlet velocities and temperatures is studied. A cross type rheological model depending on the temperature and pressure is assumed for polymer materials polystyrene (PS) and polypropylene (PP). The evolution of the flow velocity inside the channel is presented. The quantity of heat added due to viscous dissipation to the polymer is also calculated up to different positions through the channel. A numerical finite volume code for the simulation of polymer melt flow in a channel is used and a validation of this numerical code is presented. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Haake密炼机三维流场的非等温数值模拟研究

本文利用流体力学计算软件FLUENT对69cm3哈克密炼机内的聚合物熔体进行了三维非等温非稳态数值模拟,得到了三维流场的瞬时温度分布,并对熔体与密炼室之间的热量传递过程进行了分析。当粘性耗散生热量等于向外传热量时,达到热平衡状态,熔体平均温度不再变化。由于聚合物熔体具有较高的粘性生热,仅靠自然对流不足以使密炼室壁保持初始的设定温度,壁温会有所增加。流场的混合指数分布说明混合流场中剪切流动占主导地位,还包括一小部分拉伸流动和收敛流动。     

Experimental investigation of the energy balance for the metering zone of a twin screw extruder

The object of this study is to investigate experimentally the mechanical power transported from the screws to the polymer melt, the heat dissipated in the polymer melt, and the heat transferred to the barrel of the metering zone of a co‐rotating twin screw extruder. For the experimental investigations, different screw elements, mixing elements, and kneading discs are used. The experimental results for the dissipation show good agreement with known calculation models. For the calculation of the heat transfer, a new equation is used to calculate the mean temperature difference under consideration of the dissipation. The Nusselt‐number can be calculated in good agreement with the experimental results as a function of the Brinkmann‐number.     

Fundamentals of melt rheology,heat generation,and heat transfer as applied to polymer processing

The flow of polymer melts is normally laminar, but pseudoplastic in character. That is, flow rate increases in proportion to a power higher then 1, usually between 1.5 and 4, of the applied pressure difference. Viscosities are extremely high—more then a mollion times that of water—so considerable heat is generated in the flowing melt by viscous dissipation of flow energy. This mode of heat generation is put to good practial use in the screw extruder, a device that converts plastic pellets into hot melt for shaping into pipe, sheets, coatings and molded products. Equations are given for computing flow rates and viscous dissipation rates. Heating (except by viscous working) and cooling of polymers are slow processes because all these materials are poor heat conducts nad their extreme viscosities make convection impracticable. The princliples of heat transfer are reviewed, and the solutions are given for some transient-conduction problems frequenlty encountered in processing. Equations are given for judging the operation of extruders.     

Conjugate thermal simulation for sheet extrusion die

The thermal effects on manifold temperature uniformity and output flow uniformity are important for polymer extrusion die design. Lin and Jaluria (Lin and Jaluria, Polym. Eng. Sci., 37, 1582 (1997)) has carried out a numerical study on conjugate heat transfer for extrusion polymer flow under the assumption that the die body surface is in uniform temperature or heat transfer coefficient. In this study, we have solved the non‐uniform body surface temperature as part of the simulation solutions based on heat flux boundary conditions (including radiation and convection heat transfer). The body temperature is computed in conjugated with the melt polymer flow with non‐linear viscous shear heating effect. The relative tough thermal conditions are set to test uniformity of the temperature distribution on the manifold wall. We also give the results of the heat transfer effect on the flow velocity distribution. POLYM. ENG. SCI., 54:682–694, 2014. © 2013 Society of Plastics Engineers     

蒸汽在过冷液体喷雾上直接接触相变换热过程实验研究

以水为工质,通过实验研究了饱和水蒸汽与过冷水喷雾逆流直接接触冷凝换热过程,考察了不同入口液相温度下液膜厚度及破碎长度变化、液膜轴向及径向的温度分布;基于实验数据计算出了液膜局部传热系数及总传热系数。实验研究的结果表明,直接接触冷凝换热过程中,低入口液相温度时的液膜厚度和破碎长度更大;液膜在径向方向上存在温度梯度变化,液膜表面的温度较高,中心存在1个最低温度;随着液膜运动轴向距离的增大,液膜温度逐渐升高,喷嘴出口处液膜的温升最快,在整个喷雾的冷凝换热过程中,液膜温升占喷雾换热总温升的80%～85%,因此相比液滴,液膜起主要换热作用;喷嘴出口处的局部传热系数最大,并随着轴向距离增大逐渐减小。实验得到总传热系数的值远大于传统的膜状冷凝传热系数,体现了蒸汽-过冷液体喷雾这类直接接触换热方式的优势。     

Heat transfer to molten polymer flow in tubes

This article presents the results of a numerical study (finite differences) of the heat transfer problem in flowing polymer melts. The tube wall is assumed to be at a constant temperature. The rheological behavior of the melt is described by a power law temperature-dependent model. A convective and a viscous dissipation term are included in the energy equation. Temperature profiles, bulk temperatures, and Nusselt numbers are presented for a variety of flow entry temperatures.     

Effects of Viscous Dissipation and Rarefaction on Parallel Plates with Constant Heat Flux Boundary Conditions

The effect of viscous dissipation and rarefaction on heat transfer characteristics of hydrodynamically and thermally fully developed flow between parallel plates with constant heat flux conditions is analyzed. Three different types of heat flux boundary conditions, i.e., both plates kept at different constant heat fluxes, both plates kept at equal constant heat fluxes, and one plate insulated, are applied. In all cases, closed form expressions are obtained for the temperature distribution and Nusselt number. Viscous dissipation, rarefaction, and heat flux ratio are found to influence the heat transfer substantially. The present predictions are verified for the cases which neglect the microscale and viscous heating effect. The obtained results are in good agreement with other analytical results.