Characterization of the continuous-flow mixing of non-Newtonian fluids using the ratio of residence time to batch mixing time

Continuous-flow mixing of pseudoplastic fluids possessing yield stress is a complex phenomenon exhibiting non-ideal flows within the stirred vessels. Electrical resistance tomography (ERT), a non-intrusive technique, was employed to measure the mixing time in the batch mode while dynamic tests were performed to study the mixing system in the continuous mode. This study attempts to explore the effects of the operating conditions and design parameters on the ratio of the residence time (τ) to the mixing time (θ) for the continuous-flow mixing of non-Newtonian fluids. To achieve these objectives, the effects of impeller types (four axial-flow impellers: A310, A315, 3AH, and 3AM; and three radial-flow impellers: RSB, RT, and Scaba), impeller speed (290–754 rpm), fluid rheology (0.5–1.5%, w/v), impeller off-bottom clearance (H/2.7–H/2.1, where H is the fluid height in the vessel), locations of inlet and outlet (configurations: top inlet-bottom outlet and bottom inlet-top outlet), pumping directions of an axial-flow impeller (up-pumping and down-pumping), fluid height in the vessel (T/1.06–T/0.83, where T is the tank diameter), residence time (257–328 s), and jet velocity (0.317–1.66 ms⁻¹) on the ratio of τ to θ were investigated. The results showed that the extent of the non-ideal flows (channeling and dead volume) in the continuous-flow mixing approached zero when the value of τ/θ varied from 8.2 to 24.5 depending on the operating conditions and design parameters. Thus, to design an efficient continuous-flow mixing system for non-Newtonian fluids, the ratio of the residence time to the mixing time should be at least 8.2 or higher.     

新型热塑性淀粉制备与性能研究

以甘油为增塑剂,制备了玉米和小麦两种热塑性淀粉。通过扫描电镜、拉伸强度以及含水率的表征等方法研究了分散时间、糊化温度及糊化时间对淀粉性能的影响。结果表明:在最佳制备条件下,玉米和小麦两种热塑性淀粉形态为均一的连续相,并且拉伸强度均可达到15MPa。     

基于FLUENT的有杆泵抽油流场数值研究

针对抽油泵上冲程流体进泵的全过程,在得出柱塞实际运动规律的情况下,根据国家标准建立抽油泵的几何模型,并进行网格划分和边界条件指定,结合动网格技术和 UDF 编程,应用计算流体动力学软件FLUENT对流体进泵的过程进行仿真和数值模拟,实现了柱塞运动和固定阀球运动与流场计算域的同步求解。得到不同时刻泵内流场的分布状况、泵内压力随时间变化规律和固定阀球的运动特性曲线。数值模拟的结果为抽油泵流道设计和工况研究提供了相关参考依据。     

Analysis of Optimal Heat Transfer in a PEM Fuel Cell Cooling Plate

An analysis of three‐dimensional computational fluid dynamics (CFD) is conducted to investigate the coupled cooling process involved in fluid flow and heat transfer between the solid plate and the coolant flow for optimization of the cooling design of a fuel cell stack. A conception of IUT (Index of Uniform Temperature) across the entire area is presented to evaluate the degree of uniform temperature profile across the cooling plates. Six cooling modes, including three serpentine‐type modes and another three parallel‐type modes, are presented and analyzed for optimization of the cooling mode of fuel cells. The prediction finds that the cooling effect of serpentine‐type cooling modes could be better than that of parallel‐type cooling modes.     

Modeling of an oxygen-staged membrane wall gasifier: effects of secondary oxygen

A new type of entrained flow gasifier with membrane wall and two-stage oxygen supply is being developed in China. The fraction of the secondary oxygen in total oxygen (FSO) is an important parameter for this kind of gasifier. A dynamic reduced order model (ROM) based on a reactor network model (RNM) is developed for this gasifier, which is used to investigate the effects of FSO on the slag layer thickness profile on the wall and explore the potential of FSO in dynamic slag control. The ROM adopts a flexible RNM blocking method, which varies with FSO to account for the influence of FSO on the flow pattern in the gasifier. Available industrial data was used to validate the model and a detailed sensitivity analysis for the calculation of slag layer thickness was performed. Static analyses show that FSO has a marked effect on the slag thickness distribution and higher FSO leads to lower heat loss through the wall. Finally, a slag control system, which introduced FSO as an auxiliary regulator, is proposed. Dynamic simulation shows that the new control system offers an improved performance in slag control and can broaden the regulating range of operating temperature.     

A mathematical programming formulation for temporal flexibility analysis

Realistic chemical processes are often operated in the presence of complex and uncertain dynamics. While an ill designed system may become inoperable due to variations in some process parameters at certain instances, the cumulative effects of temporary disturbances in finite time intervals can also result in serious consequences. The latter issue is studied in the present study on the basis of a novel concept – temporal flexibility. Specifically, the mathematical program used for evaluating the corresponding performance measure is built with a dynamic system model, which usually consists of a set of differential-algebraic equations (DAEs). The numerical technique of differential quadrature (DQ) is adopted to approximate these DAEs with equality constraints. As a result, any solution strategy for the conventional steady-state flexibility analysis is applicable. Two examples, a simple liquid storage tank and a solar thermal driven membrane distillation desalination process, are adopted to demonstrate the usefulness of temporal flexibility analysis. All results obtained in case studies show that the proposed approach is convenient and effective for assessing realistic issues in operating complex dynamic chemical processes.     

Influence of carbon black and plasticisers on dynamic properties of isotropic magnetosensitive natural rubber

Abstract

The dynamic shear modulus of magnetosensitive (MS) natural rubber composites is experimentally studied, where influences of carbon black, plasticiser and iron particle concentrations are investigated at various dynamic shear strain amplitudes and external magnetic fields within the lower structure borne frequency range. The iron particles embedded in natural rubber are irregularly shaped and randomly distributed; the plasticisers simplify the iron particle blending process, while carbon black reduces the production costs and improves the mechanical properties. The results show that the relative MS effect on the shear modulus magnitude increases with increased plasticiser and iron particle concentration and decreases with increased carbon black concentration. Furthermore, their relative contributions are quantified. Consequently, the study provides a basis for optimising the composition of MS natural rubber to meet a variety of requirements, including those of vibration isolation, a promising application area for MS materials.     

Fatigue peeling at rubber interfaces

Abstract

A fatigue peeling test has been developed to evaluate the failure of rubber to rubber interfaces under cyclic loading. Results obtained through this method have been compared to those of a typical fatigue crack growth experiment. The results show that the trends between these two failure modes are similar with the peeling necessary to drive the crack being slightly higher than the strain energy release rate at the same crack growth rate. Cyclic and time dependent contributions to the fatigue crack growth behaviour have been calculated using this test for an styrene–butadiene rubber compound and the results appear to be consistent with previous work although the origin of the cyclic contribution remains uncertain. The influence of pressure at the interface during vulcanisation has also been investigated and it has been observed that the fatigue peel behaviour is proportional to the surface area of contact developed during the curing cycle.     

Physico-mechanical and dynamic mechanical studies of nanographite reinforced chlorobutyl nanocomposites

Abstract

Nanographite reinforced chlorobutyl elastomer (CIIR) nanocomposites were prepared. The dispersion of the nanographite in the CIIR matrix has been investigated by scanning electron microscopy. The effect of increasing nanographite loadings (2, 4, 6 and 8 phr) on mechanical properties like tensile strength, hardness, elongation at break and modulus (100, 200 and 300%) has been studied. The study shows increase in tensile strength, hardness and modulus and decrease in elongation at break with nanographite loading, which can be attributed towards better CIIR–nanographite interaction. The above explanation was again verified from bound rubber measurements. It shows increase in bound rubber contents with nanographite loading. Dynamic mechanical analysis was used to study their relaxation behaviour as a function of temperature (?100 to 100°C) at frequency 1 Hz and 1% strain. The effect of increasing nanographite loadings on glass transition temperature was marginal in all the composites, and Tg value was in the range of ?10 to 10°C, which has been explained on the basis of relaxation dynamics of polymer chains in the vicinity of fillers. The effects of variation in nanographite loading and temperature on dynamic mechanical properties like loss tangent, storage and loss modulus have been reported. The effect of solvent (chloroform, benzene and tri-chloroethylene) on swelling properties at different periods of time (15, 30, 45 and 60 min) shows that the degree of swelling increases with time and decreases with concentration of nanographite loading.     

Dynamic mechanical properties of nanoclay filled TPU/PP blends with compatibiliser

Abstract

Blends of thermoplastic polyurethane (TPU) and polypropylene (PP) are highly incompatible because of large differences in polarities and high interfacial tensions. On one hand, PP is added to TPU to improve TPU's thermal stability, chemical properties, mechanical properties (modulus, strength and hardness) and processing performance and to reduce TPU's cost. On the other hand, TPU is blended with PP to improve PP's properties (e.g. abrasion, flexibility, tear strength, shock absorbing capabilities, impact strength, adhesion and paintability/printability). Earlier works in polyurethane/organoclay nanocomposites, PP/organoclay nanocomposites and TPU/PP blends were studied. In our experimental work, both ester and ether based TPU nanocomposites were prepared by melt blending using 3?wt-% Cloisite 10A (organically modified montmorillonite clay) as the nanoscale reinforcement and blended with PP with/without PP-graft-maleic anhydride as the compatibiliser. Blends of nanoclay filled TPU/PP were evaluated for dynamic mechanical properties such as storage modulus E′, loss modulus E″ and dissipation factor tanδ.