片状纳米Al2O3/环氧树脂复合材料的制备及性能

采用水热法制备片状纳米Al₂O₃,经过偶联剂改性后与环氧树脂复合,通过溶液混合法制备了不同填充量的片状纳米Al₂O₃/环氧树脂复合材料,研究了片状纳米Al₂O₃用量对片状纳米Al₂O₃/环氧树脂复合材料介电性能和热性能的影响,利用SEM对复合材料的断口形貌进行了表征。结果表明: 片状纳米Al₂O₃在环氧树脂基体中分散良好;随着片状纳米Al₂O₃填充量的增加,复合材料的起始热分解温度升高、介电强度增大,当片状纳米Al₂O₃的填充量为7wt%时,复合材料的介电强度为 29.58 kV/mm,比纯环氧树脂的介电强度提高了30%;复合材料的介电常数(3.8~4.5)和介电损耗(0.015)比纯环氧树脂稍有增大,但仍维持在较好的介电性能范围内。     

Al2O3基多孔隔热材料表面Al2O3/MoSi2涂层的制备及其性能

采用刷涂法在Al₂O₃基多孔隔热材料表面制备Al₂O₃/MoSi₂涂层,涂层以硅溶胶作为粘结剂,纳米Al₂O₃与Al₂O₃纤维作为耐高温组分,MoSi₂为高发射率组分。通过SEM、XRD对Al₂O₃/MoSi₂涂层微观表面结构、物相组成进行分析。研究纳米Al₂O₃与Al₂O₃纤维的质量比和MoSi₂含量对Al₂O₃/MoSi₂涂层耐温性能的影响,并对Al₂O₃/MoSi₂涂层的抗热震性能、发射率进行表征。结果表明,当纳米Al₂O₃与Al₂O₃纤维的质量比小于1∶1时,热考核后Al₂O₃/MoSi₂涂层表面无裂纹产生;当纳米Al₂O₃与Al₂O₃纤维的质量比在1∶2~1∶4之间时,Al₂O₃/MoSi₂涂层中的纤维网络较完整。MoSi₂的含量为20%时,Al₂O₃/MoSi₂涂层抗热震实验循环25次后表面保持完好,热考核后在2.5~25 μm波段的平均发射率在0.85左右,具有较高的发射率。      

纳米Al2O3及酚酞聚芳醚酮改性环氧复合材料性能

通过机械分散技术制备了纳米Al₂O₃ /环氧、酚酞聚芳醚酮/环氧和纳米Al₂O₃/ 酚酞聚芳醚酮/环氧复合材料,并对比研究了其拉伸模量、拉伸强度、断裂性能和热性能。结果表明：纳米Al₂O₃及酚酞聚芳醚酮在环氧树脂中呈均匀的分散状态;纳米Al₂O₃使环氧树脂拉伸模量增加,使拉伸强度先增后降;酚酞聚芳醚酮使环氧树脂拉伸模量略微下降,对拉伸强度影响不明显;纳米Al₂O₃/酚酞聚芳醚酮/环氧三元复配体系的拉伸模量和拉伸强度呈非单调变化的趋势;纳米Al₂O₃和酚酞聚芳醚酮对环氧树脂均有增韧作用,三元复配体系增韧效果更明显,表现出协同增韧效果;高含量纳米Al₂O₃降低了环氧树脂的初始分解温度,而其余填料对环氧树脂热稳定性具有改善作用,填料均使环氧树脂玻璃化转变温度有所降低。     

Al2O3纤维对SiO2基陶瓷型芯的烧缩阻滞

向SiO₂基体粉料中添加Al₂O₃纤维,采用热压注法制备Al₂O₃/SiO₂陶瓷型芯。分析Al₂O₃纤维含量对陶瓷型芯性能的影响。研究结果表明：Al₂O₃纤维含量对Al₂O₃/SiO₂陶瓷型芯的线收缩率、体积密度和抗弯强度均有较大的影响。当Al₂O₃纤维含量大于1wt%时,Al₂O₃/SiO₂陶瓷型芯的线收缩率大幅度降低,稳定在0.335%左右,体积密度随之降低,稳定在1.790 g · cm^-3左右;当Al₂O₃纤维含量为1wt%时,陶瓷型芯抗弯强度达最大值20.48 MPa。分析了Al₂O₃纤维对Al₂O₃/SiO₂陶瓷型芯烧结收缩的阻滞作用机制。     

原位自生纳米Al2O3/Al-Zn-Cu复合材料的力学性能

将纳米ZnO粉末和Al粉球磨后冷压成Al-ZnO预制块,然后将其加到Al-Zn-Cu熔体中进行Al-ZnO原位反应,制备出纳米Al₂O₃颗粒增强Al-Zn-Cu基复合材料。能谱面扫描分析和透射电镜观察结果表明,复合材料由纳米Al₂O₃颗粒和Al₂Cu析出相两种颗粒/析出相组成。纳米Al₂O₃颗粒通过异质形核和晶界钉扎,细化了Al-Zn-Cu合金晶粒组织和Al₂Cu析出相。原位纳米Al₂O₃颗粒的生成提高了基体合金的拉伸性能,轧制+热处理使Al₂O₃/Al-Zn-Cu复合材料的拉伸强度比相同处理的基体合金提高约100%,总伸长率提高约98%。     

Al2O3掺杂BaTi0.85Sn0.15O3陶瓷的制备及性能表征

通过传统固相二次烧结法来制备x wt% Al₂O₃（x=0、1.0、1.5）/BaTi_0.85Sn_0.15O₃（BTS）陶瓷。研究了掺杂不同含量Al₂O₃对BTS陶瓷的微观结构、介电性能及挠曲电性能的影响。结果表明,掺杂Al₂O₃的BTS陶瓷不改变陶瓷的晶体结构,仍为标准钙钛矿结构晶型;Al₂O₃的掺入能够有效降低晶粒尺寸,具有明显的细晶作用。随着Al₂O₃含量的增大,Al₂O₃/BTS陶瓷的介电常数减小,介电损耗得到明显改善,居里峰逐渐宽化且向温度高的方向偏移。Al₂O₃/BTS陶瓷的挠曲电系数随着Al₂O₃含量的增加和测试环境温度的升高均减小。此外,Al₂O₃/BTS陶瓷的挠曲电系数和介电常数之间存在一种近线性关系,但当温度非常接近于居里温度时,这种线性关系减弱。     

Al2O3/6-6-3青铜复合材料的制备及性能

采用粉末冶金法制备出Al₂O₃/青铜复合材料, 研究了烧结温度、Al₂O₃颗粒尺寸、含量及表面状态对复合材料性能的影响。结果表明, 采用二次压制与烧结工艺制备的复合材料的组织致密,Al₂O₃颗粒分布均匀, 综合性能优于6-6-3青铜材料。Al₂O₃颗粒的化学包覆处理可以使复合材料的性能进一步提高。      

低毒单体DMAA用于ZrO2/Al2O3坯体注凝成型

采用低毒的单体N, N-二甲基丙烯酰胺(DMAA)制备了氧化锆增韧氧化铝(ZrO₂/Al₂O₃)坯体。讨论了分散剂的用量、 ZrO₂/Al₂O₃浆料的pH值、 粉体中ZrO₂含量、 粉体所占浆料的固相体积分数、 球磨时间、 预混液中DMAA的浓度(质量分数)对ZrO₂/Al₂O₃浆料黏度的影响。并研究了注凝成型ZrO₂/Al₂O₃坯体的性能和显微结构。结果表明, 当浆料pH值为9, 分散剂的添加量为ZrO₂/Al₂O₃粉体质量的0.6%, 球磨时间为6 h, ZrO₂/Al₂O₃浆料具有最小的黏度。固相体积分数的提高和DMAA加入量的增大都会提高ZrO₂/Al₂O₃浆料的黏度, ZrO₂的加入会降低浆料的黏度。用DMAA制备得到的ZrO₂/Al₂O₃坯体结构均匀, 抗弯强度达到25 MPa。      

球磨-转喷微注法制备纳米Al2O3颗粒/Al(7075)复合材料的组织及磨损特性

采用球磨-转喷微注相结合的新工艺制备纳米Al₂O₃颗粒(Al₂O_3p)/Al(7075)复合材料,设计一种转喷微注装置,该装置能将连续、微量的纳米Al₂O_3p注入到Al熔体中。观察纳米Al₂O₃增强相对Al(7075)基体合金材料微观组织的影响,并测试Al(7075)基体和纳米Al₂O_3p/Al(7075)复合材料的磨损特性。对纳米Al₂O_3p/Al(7075)复合材料和Al(7075)基体在不同载荷(15 N、25 N和35 N)下的磨损特性进行对比研究。结果表明:球磨-转喷微注法制备的纳米Al₂O_3p/Al(7075)复合材料晶粒较小,且增强相在基体中分布均匀且结合良好;随着载荷增大,纳米Al₂O_3p/Al(7075)复合材料磨损量的上升趋势慢于Al(7075)基体。载荷为35 N时,纳米Al₂O_3p/Al(7075)复合材料的磨损量较Al(7075)基体少,磨屑尺寸较小,其耐磨性能明显改善,这主要得益于纳米Al₂O_3p的支撑作用和材料的细晶强化作用。      

Al2O3包覆Li1.2Mn0.54Ni0.13Co0.13O2富锂正极材料的电化学性能

用溶胶凝胶法制备了Li_1.2Mn_0.54Ni_0.13Co_0.13O₂富锂锰基正极材料,用均匀沉淀法对其进行不同比例Al₂O₃的表面包覆改性,并对其进行XRD、TEM表征和电化学性能分析。结果表明,包覆后的材料保持了原来的层状结构,Al₂O₃均匀地包覆在材料颗粒表面形成纳米级包覆层。在0.1C、2.0~4.8 V条件下Al₂O₃包覆量(质量分数)为0.7%的正极材料首次放电容量为251.3 mAh/g,首次库仑效率达到76.1%,100次循环后容量保持率达92.9%。包覆Al₂O₃抑制了循环过程中的电压衰减,适量的Al₂O₃包覆使正极材料的电化学性能提高。     

不同空气侧风速下微通道蒸发器换热特性实验研究

搭建微通道蒸发器性能实验台,采用控制变量法研究不同空气侧风速下微通道蒸发器表面温度分布、制冷剂进出口压力的变化规律,计算换热量和换热系数,从而分析空气侧风速对微通道蒸发器的流量分配特性和换热效果的影响。结果表明,随着风速增大,微通道蒸发器制冷剂流量分配不均匀性增大,进出口压力波动振幅和周期增加,压降增大,风速2 m/s时微通道蒸发器换热效果最佳。     

Thermohydraulic performance and effectiveness of a mini shell and tube heat exchanger working with a nanofluid regarding effects of fins and nanoparticle shape

The influence of varied nanoparticle shapes on thermal–hydraulic efficacy of a boehmite nanofluid (BNF) in a mini shell and tube heat exchanger (MSTHX) in the cases of with and without fin is investigated. The five nanoparticle shapes with 90 °C inlet temperature at Reynolds number of 500 for the warm fluid side, and four Reynolds numbers of 500, 1000, 1500, and 2000 with 20 °C inlet temperature for the cold fluid side are considered. The warm fluid is the nanofluid that moves in the tube side, whilst the cool fluid is common water inside the shell side. With elevating Reynolds number, the heat transfer rate (q), overall heat transfer coefficient (U), pressure drop, effectiveness, and number of transfer unit (NTU) increase, while the performance index reduces. By increasing the Reynolds number from 500 to 2000 in the nanofluid having the oblate spheroid nanoparticles, the effectiveness rises 20%, and the performance index reduces 21.7%. The BNF with the platelet additives results in the largest q, whilst the smallest pressure loss is achieved for the Os-shaped additives. Also, the heat transfer rate, U, effectiveness, NTU, performance index, and pressure loss for the MSTHX with fin are larger than those for the MSTHX without fin.     

Numerical study on heat transfer and entropy generation of developing laminar nanofluid flow in helical tube using two-phase mixture model

Nanofluids and helical tubes are among the best methods for heat transfer enhancement. In the present study, laminar, developing nanofluid flow in helical tube at constant wall temperature is investigated. The numerical simulation of Al₂O₃-water nanofluid with temperature dependent properties is performed using the two-phase mixture model by control volume method in order to study convective heat transfer and entropy generation. The numerical results is compared with three test cases including nanofluid forced convection in straight tube, velocity profile in curved tube and Nusselt number in helical tubes that good agreement for all cases is observed. Heat transfer coefficient in developing region inside a straight tube using mixture model shows a better prediction compared to the homogenous model. The effect of Reynolds number and nanoparticle volume fraction on flow and temperature fields, local and overall heat transfer coefficient, local entropy generation due to viscous dissipation and heat transfer, and the Bejan number is discussed in detail and compared with the base fluid. The results show that the nanofluid and the base fluid have almost the same axial velocity profile, but their temperature profile has significant difference in developing and fully developed region. Entropy generation ratio by nanofluid to the base fluid in each axial location along the coil length showed that the entropy generation is reduced by using nanofluid in at most length of the helical tube. Also, better heat transfer enhancement and entropy generation reduction can be achieved at low Reynolds number.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Experimental hydrothermal characteristics of minichannel heat sink using various types of hybrid nanofluids

The hydrothermal characteristics of minichannel heat sink are analyzed experimentally by using deionized (DI) water based different nanoparticles mixture dispersed hybrid nanofluids. Al₂O₃, MgO, SiC, AlN, MWCNT and Cu nanoparticles are considered for this study. Different nanoparticles combinations (oxide-oxide, oxide-carbide, oxide-nitride, oxide-carbon nanotube and oxide-metal) in 50/50 vol ratio with base fluid (DI water) have been taken as coolants for volume concentration of 0.01%. Effects of volume flow rate (0.1–0.5LPM), fluid inlet temperature (20–40 °C) and Reynolds number (50–500) are studied for heat flux of 50 W/cm². Convective heat transfer coefficient and pressure drop are increased by about 42.24% and 22% for Al₂O₃ + MWCNT hybrid nanofluid. The maximum reduction of 21.36% in thermal resistance is obtained for Al₂O₃ + MWCNT hybrid nanofluid in comparison to DI water. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids which conclude that hybrid nanofluid is better option for electronics cooling over DI water. Al₂O₃ + MWCNT hybrid nanofluid is better in terms of heat transfer effectiveness; whereas, Al₂O₃ + AlN hybrid nanofluid (oxide-nitrite mixture) has maximum heat transfer coefficient to pressure drop ratio and coefficient of performance.     

A numerical study on heat transfer of the nanofluid flow inside helical tube through the two-phase method

In this study, a new numerical investigation was carried out to study the heat transfer characteristics of nanofluid flow inside a copper helical tube under constant heat flux. A nanofluid with different particle weight concentrations of 0.5%, 1.0%, and 2.0% was used. The effects of different parameters such as Reynolds number, nanofluid particle concentration, and constant heat fluxes (1500 and 3800?W/m²) on heat transfer coefficient were studied. For validation, Nusselt number and convection heat transfer coefficient obtained from the numerical model was compared with the experimental results. Also, to verify the accuracy of the method, grid independency was studied for each heat flux. The observations showed that the heat transfer coefficient increased by using nanofluid instead of base fluid. In addition, the convection heat transfer coefficient performance improved by increasing the nanoparticles’ concentration. The results from the numerical simulation compared with the experimental data showed that this new numerical method has high accuracy and could correctly predict the heat transfer behavior of nanofluids with different weight particle concentrations under constant heat flux.     

Effects of tube flattening on the fluid dynamic and heat transfer performance of nanofluids

In the present article, numerical simulation of Al₂O₃–water nanofluid flow in different flat tubes are performed to investigate the effects of tube flattening on the fluid dynamic and heat transfer performance of nanofluids. The numerical simulations of nanofluids are performed using two phase mixture model by FORTRAN programming language. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. The simulated results are compared with previously published data and good agreement is observed. The effects of tube flattening on different parameters such as heat transfer coefficient, wall shear stress, nanoparticles distribution, temperature distribution, secondary flow and velocity profiles are presented and discussed. The results show that with increasing the flattening, the heat transfer coefficient and wall shear stress increase. The rate of increasing is soft for all flat tubes except for the tube with the most flattening which has a severe increasing in heat transfer and wall shear stress values.     

CO_2微通道气冷器流量分配和换热特性的数值模拟及验证

本文建立了CO_2微通道气冷器集流管和微通道扁管两部分的物理模型并进行网格划分,模拟研究了扁管插入集流管深度f分别为4、5、6 mm和入口管在集流管1/6、1/2位置处对质量流量分配的影响,实验验证了CO_2微通道气冷器扁管壁面温度分布。结果表明:当f为4 mm、入口管位于集流管1/6处时,质量流量分配最均匀,此时不均匀度为0.4×10~(-3);模拟扁管内CO_2换热特性发现随着CO_2质量流量的增加,扁管换热量增加,流量由2.3 kg/h增至2.5 kg/h,换热量提高了21.4%;当质量流量一定时,CO_2的出口温度随着CO_2入口温度的升高而升高,在不同CO_2入口温度条件下,微通道扁管壁面温度实验值与模拟值误差在10%以内,验证了模拟的准确性。     

Ba(OH)2·8H2O/泡沫铜相变复合材料的制备及传热性能

采用SEM和X射线能谱仪分析方法研究了50次热循环后Ba(OH)₂·8H₂O与铝合金和紫铜的相容性,发现Ba(OH)₂·8H₂O对铝合金有一定的腐蚀性,与紫铜具有优良的相容性.以简单的真空吸附填充方法制备了Ba(OH)₂·8H₂O/泡沫铜相变复合材料.搭建了含和未含泡沫铜相变储能装置实验台,对Ba(OH)₂·8H₂O/泡沫铜相变复合材料进行室温下稳态和瞬态的传热实验.结果表明:Ba(OH)₂·8H₂O/泡沫铜相变复合材料比纯Ba(OH)₂·8H₂O传热速率快,导热性能好,有效地降低了Ba(OH)₂·8H₂O的过冷度.高温恒温箱的传热实验表明:Ba(OH)₂·8H₂O/泡沫铜相变复合材料的蓄热能力随外界环境温度的升高而降低,当环境温度高于材料相变点温度时,应考虑对相变复合材料采取一些保温措施.     

微重力环境低温流体无排气加注过程数值研究

针对加注系统受注贮箱,采用CFD方法就液氮贮箱无排气加注过程开展数值仿真,对比了不同重力水平下的无排气加注性能,分析了加注口结构、壁面初始温度、加注流体温度和加注流量等因素对微重力无排气加注性能的影响规律。所构建的二维轴对称模型将流体区与固壁区一起作为求解区域并划分网格,并通过植入用户自定义程序(UDF)计算加注口液体闪蒸过程及气液之间的热质交换。经过实验数据验证,该模型能够合理展示箱内温度场分布和相分布情况,并获得贮箱压力等参数变化信息。数值计算结果表明:(1)加注条件相同时,微重力工况较常重力工况体现出更好的无排气加注性能。(2)微重力条件下,无排气加注性能几乎不受加注口结构的影响,壁面初始温度和加注流体温度越高,贮箱压力越高,加注流量仅对加注时间有显著影响。