泡沫铝-聚氨酯复合材料力学及吸能性能分析

目的 研究泡沫铝相对密度、孔径对泡沫铝-聚氨酯复合材料准静态压缩力学性能、吸能性能、吸能效率和理想吸能效率的影响。方法 将制备的泡沫铝-聚氨酯复合材料试样在万能材料试验机上进行准静态压缩试验,得出对应的应力-应变曲线,由应力-应变曲线分析材料的吸能性能、吸能效率、理想吸能效率。结果 当泡沫铝孔径一定,泡沫铝相对密度由0.350提升至0.384时,泡沫铝-聚氨酯复合材料屈服强度提升了4.38 MPa,而最大吸能效率由0.29下降至0.27,准静态压缩性能有所提高。当泡沫铝相对密度一定,泡沫铝孔径由5 mm增加至9 mm时,泡沫铝-聚氨酯复合材料屈服强度提升了6.16 MPa,而最大吸能效率由0.25升高到0.27,准静态压缩性能有所提高。结论 当进行准静态压缩时,泡沫铝-聚氨酯复合材料压缩性能随相对密度的增大而增大,随孔径的增大而增大;泡沫铝-聚氨酯复合材料的吸能性能随相对密度的增大而增大,随孔径的增大而增大;泡沫铝-聚氨酯复合材料的最大吸能效率随相对密度的增大而减小,随孔径的增大变化微小。     

低流量下阵列式微通道对流沸腾特性实验研究

设计了阵列式微通道热沉结构,进行了并R134a的沸腾流动换热实验。结果证明,在低干度区域由泡状流/弹状流/半环状流主导,主导换热机理为对流沸腾和蒸发,热交换系数随热流密度显著增加,随质量流量增大而略有增加。在高干度区域搅拌流/束状流主导沸腾流动,对流蒸发为主导换热机理,换热系数随流量增大而增大。该结构可以在低流量下提前紊流转捩;有效抑制压力波动,减小进出口压力差。实验观察发现搅拌流/束状流型,气液界面波失稳导致液膜破碎和卷携。液滴沉积会润湿局部蒸干壁面。当热流持续增大,液膜破碎并大量被卷携入气核后,壁面附着气膜且无法被润湿,形成反束状流型时,触发CHF。     

真空渗流法制备的通孔泡沫铝的吸声性能

研究了真空渗流法制备的通孔泡沫铝的吸声性能、通孔泡沫铝的孔结构参数及厚度对通孔泡沫铝吸声性能的影响。试验结果表明 ,真空渗流法制备的通孔泡沫铝具有较大的吸声系数 ,而且随孔径减小或孔隙率、厚度增加 ,所制备通孔泡沫铝的吸声性能提高     

多孔金属比表面积的计算方法

提出了一种根据泡沫金属的孔率和孔径这两个基本参量计算其比表面积的方法.利用泡沫金属比表面积与孔率和孔径的对应数理关系,结合有关实验数据,成功地计算出了电沉积法和高压渗流铸造法制备的泡沫金属的比表面积.     

聚甲醛/泡沫铝合金互穿复合材料导热性能的研究

使用硅烷偶联剂KH570对孔隙率不同的泡沫铝合金进行表面改性后,通过注塑成型法制备了聚甲醛/泡沫铝合金互穿复合材料。采用傅立叶变换红外光谱仪(FT-IR)、扫描电子显微镜(SEM)和导热系数测试仪分别研究了改性后的泡沫铝合金的表面的红外结构、复合材料的界面结合问题以及所制备复合材料的导热性能。结果表明,硅烷偶联剂KH570成功接枝到了泡沫铝合金的表面;泡沫铝合金与聚甲醛两相界面结合良好;泡沫铝合金的充填提高了复合材料的导热性能。当泡沫铝合金孔隙率为60.48%~75.41%时,聚甲醛/泡沫铝合金互穿复合材料的导热系数随孔隙率(λ)的增大而降低,其导热系数在10.2945~17.6471 W/(m·K)之间。     

电沉积铜的电阻率和硬度的时间效应

为了研究钢基体上电沉积铜的电阻率和硬度随阴极电流密度及时间的变化规律,通过制备不同电流密度的电沉积铜样品,分别采用直流电桥式电阻仪和显微硬度计测定样品的电阻率和硬度.结果表明,电沉积铜的硬度和电阻率随阴极电流密度增大而提高,然而经时效时理后会下降.阴极电流密度增加,电沉积铜层的晶粒变小,而时效处理能够提高晶粒度和降低缺陷密度.     

基于N-S方程的泡沫陶瓷渗流微观数值模拟

用喉道和孔隙体构造孔径均匀，高度有序的泡沫陶瓷微观结构，孔隙之间的连通性用配位数描述。用N-S方程描述孔隙中的流体运动，建立数学模型，将流体力学理论引入到渗流力学中，通过对两种具有不同配位数的泡沫陶瓷渗流过程进行微观数值模拟，获得陶瓷微孔道中的流速场和压力场分布状况。计算结果表明，由于孔隙结构对渗流过程的影响，在相同压差的条件下，配位数为3的Ⅰ型泡沫陶瓷渗透流量比配位数为4的Ⅱ型泡沫陶瓷渗透流量高21．29％，Ⅰ型泡沫陶瓷渗透性能明显优于Ⅱ型泡沫陶瓷。孔隙拓扑结构是影响泡沫陶瓷宏观渗透性能的重要因素，具有相同宏观统计参数（如孔隙率）的泡沫陶瓷，也会由于孔隙的分布方式不同而在性能上存在差异。研究结果对于多孔陶瓷制备与性能研究具有重要意义。     

泡沫Al孔结构的影响因素

揭示了泡沫化过程中Ａｌ熔体泡沫孔隙率、导热系数的变化规律,研究了泡沫Ａｌ的孔隙率、孔径以及导热系数的变化规律及其对应关系。结果表明：Ａｌ熔体泡沫的孔结构及导热系数随泡沫化时间而变化。Ａｌ熔体泡沫的孔结构直接影响其凝固后的孔结构,而导热系数则间接影响孔结构。     

基于孔尺度的泡沫金属强化相变储热材料传热性能数值模拟

导热系数低是影响相变储热材料应用的主要难题之一,而泡沫金属具有高热导率、高孔隙率以及高比表面积等特性,在相变材料中添加泡沫金属可实现强化传热。该文基于泡沫金属基3D微观结构W-P模型,重点分析了泡沫金属基复合相变材料有效导热系数与泡沫金属孔隙率以及孔径的关系,采用数值模拟方法利用该模型预测并验证了泡沫铝6101添加空气与水的有效导热系数,研究结果表明该模型能够精确预测泡沫金属材料有效导热系数,在此基础上预测了石蜡中添加泡沫铜的有效导热系数,结果表明,泡沫金属可以显著提高相变材料的导热系数,当泡沫铜的孔隙率为97.57%时,复合相变材料的导热系数与纯石蜡相比提高了13倍。研究结果对于相变储热材料的热物性强化研究具有一定参考价值。     

双氧水对镁系无机泡沫材料性能和孔结构的影响

以镁系胶凝材料为基体,双氧水为发泡剂,采用化学发泡工艺制备了内部含有大量密闭气孔的镁系无机泡沫材料。研究了双氧水添加量对泡沫材料发泡倍率、体积密度、压缩强度、弯曲强度、孔结构参数以及导热系数的影响。研究结果表明:双氧水添加量增加,导致胶凝体中的气泡核增加以及气泡生长的内动力增大,气孔孔径变大,体积密度和力学强度减小;随着双氧水添加量增加,材料的导热系数不断减小,在双氧水添加量大于16‰时增大。镁系无机泡沫材料的压缩强度和弯曲强度与体积密度的回归方程分别为y=-6.06+24.19x,y=-0.64+3.82x,均为密切线性相关关系。且在双氧水添加量为8‰时泡沫材料取得最大力学强度,此时气孔孔径最小,孔结构参数较优,导热系数为0.071 W/(m·K),体积密度仅为0.54g/cm3。     

Permeability and high temperature strength of porous mullite-alumina ceramics for hot gas filtration

The gas permeability and mechanical properties of mullite-alumina ceramics for potential use as filters in hot gas separation environments are examined. The mullite-alumina ceramics with different levels of induced porosity and pores sizes were fabricated by slip casting and characterised in terms of microstructure and strength properties at ambient and elevated temperatures. Permeability to nitrogen gas flow of the porous structures at ambient temperature was investigated over a range of flow velocities to quantify and assess the permeability. The strength at high temperatures is equivalent to ambient data signifying no discernible degradation. Nitrogen gas permeability tests reveal dramatic reductions in the pressure drop–gas velocity curves with increasing porosity. It is shown that the gas permeability increases with the level of porosity and pore size, with maximum Darcian permeability constant of k = 2.5×10⁻¹⁴ m² for a porosity of 71%.     

承压破碎岩石非Darcy渗流的渗透特性试验研究

地下工程中的破碎岩石在渗流过程中其结构发生两方面变化,一方面由于水压的作用引起孔隙结构发生变化,另一方面固体材料之间的接触应力造成棱角破损。因此,承压破碎岩石的渗流不同于自然堆放状态下的堆石渗流,其渗透特性随着位移或载荷的变化而产生显著变化。基于MTS815.02型岩石力学试验系统及破碎岩石渗透仪,利用稳态渗透法及轴向位移控制法测定了粒径分别为5mm―10mm、10mm―15mm、15mm―20mm、20mm―25mm以及混合粒径的破碎砂岩、煤矸石及灰岩在承压过程中非Darcy流的渗透特性,给出了岩样在不同轴向位移水平下的孔压梯度与渗流速度关系曲线,得到了渗透特性随孔隙率的变化规律,揭示了Darcy流偏离因子可能为负的现象。研究表明:1)随着孔隙率的减少,渗透率呈现整体下降的趋势,而Darcy流偏离因子的绝对值呈现整体上升的趋势;2)破碎岩石的渗透性不仅和外载水平及当前孔隙度有关,还与碎石的粒径、颗粒的排列方式以及初始孔隙结构(如初始孔隙通道)等有关;3)三种破碎岩石渗透率的量级为10-16m2―10-12m2,Darcy流偏离因子的量级为1012kg/m4―1015kg/m4;4)当颗粒的破碎细化现象严重时,Darcy流偏离因子出现负值的可能性较大。     

Influence of pore size on tensile strength, permeability and porosity of hyaluronan-collagen scaffolds

Recent investigations have shown the importance of scaffold pore size on the realisation of tissue engineered cartilage which promotes cell adhesion, proliferation and differentiation. The objective of this study was to investigate the influence of pore size on the mechanical properties, the permeability and the porosity of hyaluronan-collagen scaffolds. Hyaluronan-collagen scaffolds with three different mean pore sizes (302.5, 402.5 and 525 microm) have been produced according to a standardised protocol. The maximum stress at rupture, the Young's Moduli, permeability and porosity of the scaffolds were investigated. The permeability was determined both empirically and mathematically. Increased pore sizes indicated a larger stress at rupture as well as increased Young's Moduli. Porosity and permeability were raised by increasing pore sizes. The mathematically calculated permeability showed the same trend. The results indicate a higher mechanical stability for scaffolds with larger pores. The experimental and mathematical experiments both show increased permeability and fluid mobility for larger pores in scaffolds. Morphological changes resulting from the alteration of pore size led to non-correlation between the calculated and the experimental permeability.     

泡沫型多孔介质中非达西流动特性的研究

本文通过将泡沫材料复杂结构进行几何简化，形成立方框架结构，利用简化的等效单元流道，提出一种数理模型，它既考虑固体网架表面对流体的摩擦阻力作用，也考虑网架迎着流向的迎风阻力（形状阻力），得到了在多孔介质中非达西流的压力降与流速的关系式。并用简化的框架结构导出了预估泡沫材料渗透率ｋ和Ｆ－Ｗ关系式中流速平方项的系数Ｆ。另外，还通过专用的实验装置测定了四种不同孔径泡沫陶瓷的ｋ和Ｆ。结果表明，根据所给模型理论预测的结果与实验值一致性较好，在泡沫型多孔材料的应用中能够提供流动特性的相关数据。     

Experimental validation of the tetrahedral skeleton model pressure drop correlation for silica monoliths and the influence of column heterogeneity

This paper describes the use of computational fluid dynamics for the calculation of the flow resistance through computer-generated models resembling silica monoliths. This study was undertaken to determine the effect of skeleton heterogeneity on the flow resistance and, more precisely, to test the hypothesis that increased skeleton heterogeneity decreases the flow resistance. To evaluate the proposed model, 24 real silica monoliths have been prepared using the same method, covering a wide range of skeleton sizes (2.2 microm < d(s) < 8 microm) and porosities (0.47 < epsilon < 0.66). The permeability of these monoliths was determined by pressure drop measurements, and structural information was obtained by image analysis of laser scanning confocal microscopy-generated 3D images of the skeleton structure. The results indicate that the presence of preferential flow paths due to an increased heterogeneity of the flow through pore space reduces the flow resistance of monolithic media. It is also shown that the pore size is hence a much better suited scaling dimension than the skeleton size to reduce the permeability of monolithic columns.     

Pressure-drops control strategy in a fixed-bed reactor

This paper presents a strategy to control pressure-drops (head loss) in a biofilter designed according to the "Mist-Foam" concept. This concept is based on the mixing of the gaseous substrate and a liquid nutrient solution with an atomization nozzle to generate a mist passing subsequently through a synthetic polyurethane foam. In this type of bioreactor, the microbial growth reduces progressively the empty bed volume of the biofilter and causes an increase in the pressure-drops. This phenomenon can result in a complete clogging of the biofilter. The strategy of pressure-drops control presented here consists of successive interruption of the liquid flow, automatically controlled, resulting in a drying effect of the biomass. Tested during a 160 days experiment, this system has permitted to reduce and stabilize the pressure-drops in a biofilter in which the carrier exhibited a high likelihood of clogging.     

Characterizing pore volume,sizes, and connectivity in pervious concretes for permeability prediction

Methods of characterizing the pore structure features in a cement-based material with open pore structure, called pervious concrete, and the use of these pore structure features in permeability prediction is the focus of this paper. Porosity of several pervious concrete mixtures is determined using volumetric and area fraction methods whereas stereology and mathematical morphology based methods are used to extract the characteristic pore sizes. The characteristic pore sizes determined using several methods relate well to each other. A Weibull probability distribution function is found to adequately model the pore size distribution in pervious concretes. The values of porosity and the morphologically determined pore sizes, along with the pore phase connectivity represented using an electrical conductivity ratio, are used in a Katz–Thompson type relationship to predict the permeability of pervious concretes. It is shown in this paper that maximization of water transport behavior of pervious concretes is best achieved by increasing the pore connectivity factor.     

Effects of pore flow on the separation efficiency in capillary electrochromatography with porous particles

The effect of pore flow on the separation efficiency of capillary electrochromatography (CEC) has been studied using columns packed with particles with different pore sizes. A previously developed model was used to predict the (relative) pore flow velocity in these columns under various experimental conditions. Equations are derived describing the effect of pore flow on peak broadening in CEC. The theory has been compared with practice in the reversed-phase CEC separation of various polyaromatic hydrocarbons. It is shown, by theory and experimentally, that the mass-transfer resistance contribution to peak dispersion can be effectively eliminated when using porous particles with a high (> or =50 nm) average pore diameter. Moreover, at high pore-to-interstitial flow ratios the flow inhomogeneity contribution (the A term in the plate height equation) is also shown to decrease. Under optimal conditions, a reduced plate height of 0.3 for the nonretained compound could be obtained. It is argued that fully perfusive porous particles can be a more efficient separation medium in CEC than nonporous particles.     

The effect of microstructure on the permeability of metallic foams

Pressure drop was measured across complex and simple structure metallic foams at different velocity ranges using air as working fluid. Darcian and non-Darcian permeability parameters, K and C, were determined by fitting experimental data with widely accepted quadratic model of Hazen-Dupuit-Darcy. Generally, the experimental results are in good agreement with the model. The differences in K and C values between the two types of metallic foams are due to the different microstructure. For the simple structure specimens, permeability K increased whereas non-Darcian permeability C decreased with increasing pore diameter. The effect of pore size on the permeability of complex structure metallic foams seems to be opposite to that observed with the simple structure specimens and to results reported by other researchers on other porous medium. This discrepancy mainly stems from the differences in window concentration in addition to some heterogeneity in the foam that impeded the gas flow on one side of the specimens. The difference in pressure drop observed in the different metallic foams is due to combined effect of K and C. However, for simple structure foams, K and C could be predicted by Ergun-like model using appropriate values for the empirical constants. The permeability K is significantly affected by pore size and porosity. The quadratic term of Hazen-Dupuit-Darcy equation is mainly due to the inertia of the flow and partially to the drag exerted by the microstructure of the metallic foam. For both foams, as the porosity increases, pressure drop decreases and permeability, K, increases. The introduction of the open cross sectional area term enabled better understanding of the permeability of metallic foams with intricate morphologies.