Influence of heat transfer and curing on the quality of pultruded composites. I: Experimental

Blister formation, a major concern in pultrusion, has not been studied in detail. The objective of this study is to investigate the mechanism of blister formation and the effect of process variables such as die temperature, pulling speed, die length, and composite thickness on blister formation. Dies with different length and a stop-start method were used to investigate blister formation. The results show that for a given die and resin system, low temperature and a higher pulling speed lead to blister formation. Longer dies can prevent blister formation. Based on the experimental results, process windows for 4-mm-thick and 6-mm-thick composites are established for a vinyl ester resin. This study suggests that heater power input should be optimized in high-speed pultrusion.     

Modeling of heat transfer and crystallization kinetics in thermoplastic composites manufacturing: Pultrusion

Whereas pultrusion with thermoset resins has been widely analyzed, there is a scarcity of knowledge about pultrusion with thermoplastic resins. The aims of this work were to strive towards deeper knowledge of temperature distribution and degree of crystallinity in the composite during processing, to develop a tool for process simulations with a variety of processing parameters, and to provide input data to models for pressure and matrix flow. The heat transfer model presented herein describes the temperature distribution within the composite throughout the thermoplastic pultrusion process. The transient heat transfer situation is modeled one-dimensionally through a transverse cross section far from the edges of a high aspect ratio composite. Temperature-dependent thermal properties, partly non-infinite contact conductance, and heat contribution from crystallization were taken into consideration. The degree of crystallinity within the composite was determined as well. The analysis also includes an experimental verification of the heat transfer model. Results show good agreement with experimental data.     

橡胶硫化反应热对硫化温度场的影响

探讨长方体胶块和轮胎硫化反应热对硫化温度场的影响。用热电偶硫化测温仪测试胶块和轮胎硫化过程中的温度,用差示扫描量热法测试硫化反应热,用有限元分析法进行硫化热模拟。通过硫化模拟温度与实测温度比较得出,胶块和轮胎硫化反应热对硫化温度场有明显影响。     

橡胶硫化传热的模拟研究

采用有限元方法对橡胶制品硫化传热过程进行模拟,并与实测结果对比.结果表明,硫化过程中的放热及橡胶交联程度等对传热过程影响较小;由于模具-橡胶界面温度的加载较为复杂,应该通过实测的边界温度进行模拟;必须考虑不同胶种对传热的影响,但可忽略由于不同胶种的叠加产生的界面对传热的影响.     

Modeling of the heat transfer in cross-flow heat exchangers

The heat transfer in plate heat exchangers with cross flows of heat-transfer media is considered. At constant values of thermophysical parameters using a model of a one-dimensional flow in cach channel, analytical expressions for the temperature and heat flux distributions are obtained. The efficiency of heat exchangers at different values of dimensionless parameters is estimated. A model is proposed to take into account the two-dimensional flow of heat-transfer media in channels on the basis of empirical relations for the flow resistance and the heat transfer in flat channels.     

硫化体系对EPDM耐热老化性能的影响

研究了普通硫化体系 (CV)、半有效硫化体系 (SEV)和有效硫化体系 (EV)的硫化特性及其对EPDM力学性能和耐热老化性能的影响 ,以及防老剂对EPDM耐热老化性能的影响。试验结果表明 ,采用SEV和EV体系硫化的硫化胶有较好的耐热老化性能 ,采用CV体系的硫化胶有较好的力学性能 ;防老剂对EPDM的耐热老化性能无明显影响。     

板-泡式换热器传热及阻力特性数值模拟

开孔泡沫金属由于其复杂的三维网状结构,当流体通过时会发生非线性扰动,湍流程度增强,使得泡沫金属和流体之间发生强迫对流换热,基于开孔泡沫金属能强化传热的特性开发了高效紧凑的板-泡式换热器。利用FLUENT多孔介质模型对板-泡式换热器的传热及阻力特性进行数值模拟。研究结果表明:在导热隔板间填充铝泡沫金属,换热器的传热效率明显提高;在相同速度下,换热效率随孔隙率的增大而减小;努塞尔数随着流道高度的增加而增大,且随着雷诺数的增大,其影响越来越明显。同时以数值计算结果为基础,拟合得到400Re4 000范围内努塞尔数以及阻力系数准则关系式。研究结果可为板-泡式换热器的结构优化和设计制造紧凑换热设备提供参考。     

Studies on melt spinning. II. Analysis of the deformation and heat transfer processes

A generalized, empirical equation is proposed which takes into account the dependence of elongational viscosity on both elongation rate and temperature. From this, a mathematical model for simulating the melt spinning process has been developed. The model has been tested against experimentally observed velocity profiles in fibers of polystyrene and high-density polyethylene spun into an isothermal chamber. It has been found that predicted velocity profiles agree well with experimentally observed ones. The mathematical model has been used to predict velocity and temperature profiles in fibers spun into a cooling medium. The simultaneous solution of momentum and energy balance equations by means of a numerical integration scheme has generated important information such as distributions of force components involved in spinning and distributions of the total rate of heat transfer along the spinning way.     

Modeling and simulation of heat and mass transfer during drying of solids with hemispherical shell geometry

A heat and mass transfer model was proposed to describe the moisture and temperature evolution during drying of solid products with hemispherical shell geometry (HSG). The dimensionless form of the model was numerically solved for both several drying conditions and values of a geometrical factor related with the inner radius of the HSG to obtain their moisture and temperature profiles. In addition, average drying kinetics were calculated from the volume integration of local moisture values. A theoretical and numerical approach was used to develop a mass transfer analogy between the proposed HSG and a simpler flat slab-shaped product. These analogies provide simple mathematical expressions for drying process simulation and estimation of diffusion coefficients in solids with the proposed geometry, and may be applicable to other mass and heat transfer operations. Furthermore, the presented procedure may be used to develop similar expressions in other non-traditional or dissection geometries.     

板泡式换热器对流与导热耦合换热的数值模拟

由于在结构和强化传热方面的优越性,开孔泡沫金属在换热器行业中具有很大的应用潜力。为了探究板泡式换热器的强化传热机理,建立了泡沫金属微尺度几何模型,采用商业软件Fluent 12.1对板泡式换热器内流体流动进行了数值模拟,通过计算其速度、温度和压力分布,分析了换热器内对流与导热的耦合换热情况。研究结果表明:泡沫金属复杂的三维立体结构强化了流体的扰动,增强了二次流,起到了强化传热的作用;泡沫金属与流体分界面分别采用恒壁温和耦合换热2种边界条件时,其温度分布呈现不同特点,耦合换热边界条件更能真实反映实际温度分布情况;去除进口效应的影响,中间段流体的压力梯度随单胞模型呈周期性变化。这为板泡式换热器的设计提供了一定的依据和指导。     

蜂窝夹套传热性能的数值分析

利用 FLUENT 软件对蜂窝夹套内流体的传热进行了计算分析,将模拟所得水出口温度及夹套传热系数与实验值进行了比较,结果表明两者的变化趋势一致,误差较小.并得到蜂窝夹套传热面积设计所用的给热系数的估算式,可以应用于工程设计.     

涡结构对对流传热影响的实验和模拟研究

利用计算流体动力学(CFD)模拟和粒子图像测速技术(PIV)实验相结合的方法研究了一个类似于板式换热器的模型在内部转子旋转的条件下,产生涡结构,提高传热效果的过程。首先采用PIV技术测定了在一定流量不同转速的条件下,模型中流场的变化。然后,运用CFD方法计算流动传热过程。计算的流场结构和PIV测定流场结构较为一致,显示出采用计算方法的可行性和有效性。计算结果表明,与没有转子转动相比,转子转动会使流场中出现涡结构,并且发生明显的扰乱。这优化了模型中流场结构,强化了换热效果。流动换热过程中,在压力损失的增加不变的条件下,换热效果得到了极大地提高。而且转速越快,换热效果提高越明显。     

Modeling batch melting: Roles of heat transfer and reaction kinetics

Development of mathematical models of heat and mass transfer in glass-melting furnaces began in the 1970s and progressed rapidly with advances in sophisticated experimental/numerical techniques and increasing computational power. Today, practically all newly built or rebuilt furnaces are optimized with these models to meet stringent quality requirements, reduce the unit costs of manufacturing, or control emissions. One remaining hurdle is to model the batch-to-glass conversion accurately enough to reliably assess the glass production rate. This article summarizes two key aspects of the batch-conversion modeling—the heat transfer and the kinetics of conversion—and reviews the current state-of-the-art approaches to simulating them. We critically examine the advantages of the commonly used heat transfer approach, but also explain that its predictive capabilities are significantly restricted by the dependence of batch thermal properties on the time-temperature history. We argue that kinetic approaches to the batch-conversion modeling would offer a significant improvement when coupled with the heat transfer approach. Finally, we summarize key areas requiring further research on the way toward a realistic model of the batch blanket.     

Studies on blown film extrusion. II. Analysis of the deformation and heat transfer processes

Having investigated the elongational flow behavior of polymer melts (part I of this series), we have carried out both theoretical and experimental studies in order to better understand the deformation and heat transfer processes involved in blown film extrusion. For the experimental study, nonisothermal experiments were carried out, using high-density and low-density polyethylenes. Measurements were taken of the axial tension, bubble diameter, and film thickness at a series of extrusion conditions (i.e., flow rate, pressure difference across the film, and take-up speed). For the theoretical study, an analysis was carried out to simulate the blown-film extrusion process, by setting up the force- and energy-balance equations on the blown bubble moving upward. The approach taken in the theoretical study may be considered as an extension of the earlier work by Pearson and Petrie who considered the isothermal operation of Newtonian fluids. In the present study, However, we have considered the nonisothermal operation of power law fluids, whose rheological parameters were determined by an independent experimental study an described in part I of this series. Four highly nonlinear differential equations were solved numerically with the aid of the CDC 360 digital computer, using the fourth-order Runge-Kutta method. The mathematical model predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Comparison is made of the experimentally observed bubble shapes with the theoretically predicted ones, showing a reasonable agreement.     

Modeling of heat transfer in rotational molding

Rotational molding is a process for manufacturing hollow or open‐sided plastic products using a rotating mold subjected to heating and then cooling. The process is attractive for the production of stress‐free objects at a competitive cost. In this article, a modified model for heat transfer in rotational molding is proposed, which assumes that the heat transfer at the mold‐powder interface is because of convection, whereas the powder particles are heated up by conduction. Heat transfer through the mold–air contact is also included. A source‐based formulation is used for modeling the layer‐by‐layer nonisothermal deposition of plastic. The reduced heat transfer due to warpage is calculated by using a modified heat transfer coefficient. Good overall agreement is found between the cycle times as predicted by the model and the experimental data. The model is then used for calculating the cycle time for particulate composites, based on their effective properties. A reduction in the cycle time is observed in the case of reinforced composites. This is attributed to the increase in thermal conductivity of the particulate composites and the reduced mass fraction of the polymer. Numerical calculations of the cycle time for the glass‐bead reinforced composites are found to be in good agreement with the experimental results. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Influence of the processing on the completion of curing in epoxy‐based composites

Differential scanning calorimetry analysis was used to investigate interrelationships between several thermal parameters and the processing conditions of particle‐filled thermosetting resins. On the same piece, obtained by injection molding, epoxy‐based composites exhibited sensible differences within a set of measurements of the glass‐transition temperature, the width of the transition, the difference in the heat capacity, and the conversion degree. Statistical analysis showed a strong intercorrelation between these thermal parameters, but it could not provide any explanation for the disparities. The dispersion of the measured properties could, in a second step, be directly related to a sample's position with respect to the injection point in the mold. Moreover, even the postcuring stage could not erase this topological effect. As a result, a phenomenological model is proposed that fairly describes the experimental trends. This simple polynomial approach can subsequently be used either to determine the thermal parameters of any point of a molded piece or to shed some light on phenomena responsible for the large variations of the measured quantities. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1368–1376, 2005     

一种新型受热面传热和流动特性的数值模拟及实验研究

针对余热利用过程中低温热源的含尘量高、不连续及不稳定等特点, 提出了一种新型菱形受热面结构。在传热过程中, 该受热面表现出管束叉排布置的特征, 传热过能力较强, 流动阻力较大, 壳侧对流换热表面传热系数较高。在实施吹灰过程中, 该受热面呈现出管束顺排布置的特征, 易清洗, 吹灰效率高。采用数值模拟和实验方法研究了新型受热面结构的传热和流动特性, 给出了壳侧的Nusselt数和摩擦因子随Reynolds数的变化规律。实验结果和数值分析均表明, 该受热面能够适应现有余热利用过程的基本要求, 在便于清灰和除垢的同时实现高效传热。     

泡状管换热器强化传热与整体性能的数值模拟

针对泡状管管壳式换热器,采用FLUENT软件对其进行管程和壳程耦合传热的整体数值模拟,并将模拟结果与具有相同形式相同换热面积的光滑圆管换热器对比。通过结果对比分析泡状管强化传热的效果、压降及总传热系数的差异;评价泡状管壳式换热器的传热性能。结果表明:相比圆管,泡状管表面形状的变化对管壳两侧均起到强化传热的作用,同时管程流动阻力增大,但壳程阻力有所降低。总体传热系数较圆管有较大幅度提高。综合考虑总传热系数和压降,泡状管对壳程传热性能提升明显。     

梅花形孔板换热器壳程流动与换热特性模拟

换热器是化工生产的重要设备之一,为了解决传统的弓形折流板换热器存在压降过大和换热效率低的问题,设计了一种新型的梅花形孔板换热器。针对使用简化模型进行模拟研究的局限性,构建了梅花形孔板换热器壳程全三维模型。采用CFX模拟比较了梅花形孔板换热器与弓形折流板换热器壳程的流动换热特性,并进一步研究了孔板间距与换热器性能的相关性,得到了换热管外表面平均对流换热系数h,以及壳程压降Δp随雷诺数Re变化的规律。研究结果表明:梅花形孔板换热器可以有效消除折流板换热器中存在的流动死区并减少壳程压降,同时孔板处流体形成射流破坏流动边界层,强化换热。在模拟雷诺数范围内,孔板换热器的综合性能参数h/Δp相比折流板换热器平均高约12%;相同雷诺数下孔板间距越大换热器综合性能越高。     

Modeling and simulation approaches in the resin transfer molding process: A review

A review of current approaches in modeling and simulation of the resin transfer molding (RTM) process is presented. The processing technology of RTM is discussed and some available experimental techniques to monitor the process cycle are presented. A master model is proposed for the entire process cycle consisting of mold filling and curing stages. This master model contains the fundamental and constitutive sub‐models for both stages. The key elements of the master model discussed in this study are: flow, heat and mass balance equations for fundamental sub‐models, permeability, cure kinetics, resin viscosity and void formation for constitutive sub‐models. At the end, numerical methods widely used to simulate the filling process are presented and published simulation results of mold filling and process cycle are reviewed.