CuO纳米颗粒悬浮液中各组分对悬浮液稳定性及黏度的影响

由CuO纳米颗粒、分散剂和水组成的悬浮液黏度是研究其流动与换热的重要基础数据。对纳米氧化铜颗粒悬浮液来说，十二烷基苯磺酸钠(SDBS)和阿拉伯树胶是两种分散性能优异的分散剂，特别是在一定的pH值时分散性可达到最佳。通过实验给出使悬浮液分散性能达到最佳时有机小分子分散剂SDBS与分散相CuO质量分数的相对比值和SDBS在纳米CuO悬浮液中的最大值，同时得出分散相CuO质量分数为0．04时悬浮液体系稳定性最好的结论；作为对比，研究了高分子分散剂阿拉伯树胶在可溶性范围内质量分数对CuO纳米颗粒悬浮稳定性的影响。基于此，得出纳米颗粒悬浮液的黏度在一定分散相质量分数范围内取决于分散剂的黏度，而在低质量分数和高质量分数时出现变异的结论。     

十五烷微胶囊潜热型功能流体的制备及其性能北大核心CSCD

为了防止相变过程中材料的泄漏,采用原位聚合法制备了以十五烷(Pen)为芯材,脲醛树脂(UF)为壁材的低温相变微胶囊。研究了升温速率、聚合pH值和聚合转速对微胶囊制备的影响,采用SEM、FT-IR、DSC和马尔文激光粒度仪测试了微胶囊的形貌、化学构成、热力学性质和粒径分布。升温速率为1.0℃/min、聚合pH值为3.5和聚合转速500 r/min为十五烷微胶囊的最佳制备条件,在此条件下制备的微胶囊球形形貌明显,表面光滑,仅有少数UF颗粒粘附,粒径分布均匀,相变温度和相变潜热分别为8.20℃和115.3 J/g,平均粒径为50.0μm,包裹率达到77.3%。实验结果表明,芯材和壁材仅为简单的物理嵌合,具有良好的储热性能和热稳定性。以不同质量分数的乙醇溶液为基液分散十五烷微胶囊,采用24 h静置实验得到了稳定的潜热型功能流体(LHFF),LHFF在乙醇含量为70%的基液中最为稳定。采用导热系数测定仪和旋转黏度计对LHFF的导热率和黏度进行测试分析表明,LHFF的导热率随着温度的升高而增加,随着微胶囊的添加量的增加而逐步降低。LHFF的黏度随着温度的升高而逐步减小,随着微胶囊的添加量增加而逐步升高。潜热型功能流体作为空调系统的载冷剂,提高了制冷机组的性能,降低泵的输送能耗,提高了蓄冷空调系统的经济性。     

圆管内相变微胶囊悬浮液换热特性实验研究

实验研究了质量浓度为2%和5%的相变微胶囊悬浮液在等热流加热圆管内的对流换热特性,结果表明:相变微胶囊颗粒的加入明显增强了溶液的冷却效果,有效降低了管道壁面温度,并且悬浮液浓度越高,管道壁面温度越低;相变微胶囊悬浮液使管道入口段的局部努塞尔数明显增大,但随着流程进行,对流换热增强的幅度减弱;和水相比,浓度2%的相变微胶囊悬浮液的平均努塞尔数提高11%,悬浮液具有较好的强化换热效果;但浓度5%的悬浮液的平均努塞尔数低于水,这是由于5%的悬浮液粘度增加明显,悬浮液的高粘度和低导热率减缓了圆管中心处微胶囊颗粒的吸热相变,降低了5%悬浮液的强化换热效果。     

等热流圆管内相变微胶囊悬浮液层流换热实验研究

用基液代替水来配置微胶囊相变悬浮液,并对实验数据的准确性进行了检验。在等热流密度环境下对管道内的该悬浮液进行加热实验,对相变微胶囊悬浮液的质量分数、St、入口过冷度、粒径和Re等因素影响强化换热的效果进行了分析。结果显示影响微胶囊相变悬浮液管内层流换热最主要的因素是微胶囊的质量分数和St。     

纳米流体导热系数的影响因素分析

采用"两步法"制备质量分数为0.5%、1.0%、2.0%和4.0%的Cu-乙二醇纳米流体,添加了聚乙烯吡咯烷酮(PVP)和十二烷基苯磺酸钠(SDS)作为表明活性剂。利用Hot Disk 2500s热常数分析仪测试Cu-乙二醇纳米流体导热系数,在分析温度和颗粒浓度对其导热系数的影响基础上,重点研究了两种不同表面活性剂及添加量对纳米流体导热系数的影响。结果表明:Cu-乙二醇纳米流体的导热系数随质量分数的增大而增大,随温度的升高而增大。添加了PVP的纳米流体导热系数随PVP的添加量增加呈先增加后减小的趋势,但添加了SDS的纳米流体的导热系数小于无表明活性剂时纳米流体导热系数。适当的纳米颗粒浓度与PVP表面活性剂浓度比例,对提高Cu-乙二醇纳米流体的导热系数有帮助。     

纳米Fe2O3/硬脂酸/十八醇纳米复合相变蓄热材料的性能研究

针对有机相变材料热导率低的问题,将高热导率的纳米Fe₂O₃添加到硬脂酸/十八醇二元有机复合蓄热相变材料中,制备纳米复合蓄热相变材料。从分散剂的种类、分散剂与纳米材料的添加量以及超声时间4个方面研究其对纳米复合相变蓄热材料的稳定性及热物性的影响。结果表明,阴离子表面活性剂的分散效果优于阳离子和非离子表面活性剂。复合相变材料中添加质量分数为0.8%,十二烷基苯磺酸钠(SDBS)和质量分数为0.4%Fe₂O₃的体系,超声时间为80 min时,纳米Fe₂O₃在相变材料中的分散效果最好。添加纳米Fe₂O₃后复合蓄热相变材料的相变潜热及相变温度有所下降,热导率提高34.9%。300次热循环复合相变材料的相变温度波动区间不超过0.41℃,相变潜热波动区间不超过4.0%,热稳定性良好。     

基于均匀设计法的Al_2O_3纳米流体稳定性影响因素的实验研究

采用均匀设计法对利用两步法制备的氧化铝纳米流体进行了稳定分析。将悬浮液的zeta电位作为氧化铝纳米流体稳定性的衡量指标。在实验中,选择了四个因素,分别为纳米流体的浓度、分散剂的浓度、pH值、超声震荡时间,并对这四个因素都取了12个水平,这样的实验设计可以系统地反映出各个因素如何影响氧化铝纳米流体的稳定性。随后通过对实验结果进行回归分析,得到了关于这四个因素与纳米流体zeta电位的方程式,并分析得出悬浮液的pH值和超声震荡时间对纳米流体稳定性起着非常重要的作用。最后找到了在实验范围中的最佳纳米流体溶液的配方,并按此配方配制纳米流体,测得实际zeta电位值与预测值吻合较好。     

相变蓄热材料微胶囊制备工艺的正交试验分析

以常低温相变蓄热材料作为芯材,脲醛树脂作为壳材,通过原位聚合法合成了常低温相变蓄热材料微胶囊。采用正交试验优化,确定了微胶囊的最佳制备工艺。利用SEM分析了乳化剂的种类对微胶囊微观形貌的影响,并结合FTTR和DSC等测试仪器考察了通过最佳制备工艺得到的微胶囊化学结构及热性能。实验结果表明:在乳化转速为3000r/min下使用由吐温60和司盘60组成的混合型乳化剂,用量为芯材质量5%,所制备的微胶囊具有良好的热稳定性、球体表面光滑、粒径均一。在吸热过程中,相变温度为30.10℃,相变潜热为63.85J/g。     

纳米铜/微胶囊复合体系相变过程及传热特性

针对相变材料的微胶囊化过低的导热性能限制了其大规模应用,通过将具有优良导热性能的纳米铜粉末(CNPs)填充在微米级相变微胶囊颗粒间,构建CNPs质量分数1.0%～8.0%的复合相变体系,研究复合体系相变过程和相变传热特性。研究结果表明,相比于微胶囊相变材料,复合相变体系有效导热系数随CNPs含量增加而提高,有效热扩散系数、相变速率与相变焓值随CNPs含量增加而先减小后增大,且有效热扩散系数与相变速率变化趋势基本保持一致,均在CNPs质量含量1.0%时出现增长的转变,且在CNPs质量含量低于3.0%时两者的数值均低于微胶囊相变材料,而复合体系相变温度则不受CNPs影响。     

相变微胶囊悬浮液层流强迫对流换热实验研究

对以溴代十六烷(C16H33Br)为相变材料的相变微胶囊悬浮液(MPCMS)作了管内层流恒定热流密度加热条件下的强化传热性能测试和分析.悬浮液质量浓度为5.0%、10.0%和15.8%(文中百分数若无特别指出均为质量分数),整体雷诺数和斯蒂芬数分别在400～1900和1.1～8.8范围内.实验结果表明:实验工况下与水相比,使用MPCMS可降低壁面温度3.9℃,Nux数可提高27%～42%,传热实验过程中MPCMS样品压降增加不大,压降可降低26.7%(um=1.3m/s),在其输传热综合效果较好.     

Experimental investigation on pool boiling heat transfer of ZnO,and CuO water-based nanofluids and effect of surfactant on heat transfer coefficient

Comparison of boiling performance of nanofluids and mixtures of nanofluids with surfactant is an objective of this research. Experimental investigation has been performed with different heat flux and concentrations of nanoparticles and surfactant. CuO and ZnO water-based nanofluids are used and sodium dodecyl sulfate (SDS) is used as surfactant. The size of nanoparticles is measured from Field Emission Scanning Electron Microscopy pictures. Roughness of rod heater is calculated by using Atomic Force Microscopy picture. The pure water is tested after each run with nanofluid and mixture of nanofluid with SDS and the results are presented. Result from experiments demonstrates that the addition of SDS to nanofluids solution resulted in improving boiling performance. Experimental results also show an inefficient process by excluding surfactant. An optimum value for heat transfer coefficient is found by increasing of surfactant concentration within CuO nanofluid (0.01wt%CuO). Pictures of coated and clean surface in boiling pure water are employed for understanding the dynamics of bubbles. Gorenflo constant (h₀) is utilized to show the effect of addition of nanoparticles and SDS in boiling performance of base fluid.     

Enhanced Fe–Cr dispersion on mesoporous silica support using surfactant-assisted melt-infiltration for the water-gas shift reaction in waste-to-hydrogen processes

Iron-chromium/SBA-15 (Fe–Cr/SBA-15) catalysts for high-temperature water-gas shift reactions (HTS) were prepared using the surfactant-assisted melt-infiltration (SAMI) method. The SAMI method was used to synthesize an efficient catalyst that could simultaneously achieve high dispersion and loading (Fe: 40 wt% and Cr/Fe molar ratio: 0.25) without a liquid solution. According to X-ray diffraction (XRD) and transmission electron microscopy analyses, it was observed that the Span60 surfactant enhanced Fe₃O₄ particle dispersion. N₂ adsorption-desorption analysis indicated that the Fe₃O₄ and chromium oxide particles were thoroughly infiltrated inside the mesopores of the SBA-15 support. XRD and temperature-programmed-reduction analyses revealed that the well-dispersed Fe₃O₄ acted as active sites for the HTS reaction when Span60 was added to the Fe–Cr catalyst. In other words, the Fe–Cr catalyst synthesized via SAMI exhibited better performance and stability for the HTS reaction than that prepared without the surfactant.     

Effects of surface modification on the dispersion and thermal conductivity of CNT/water nanofluids

In the present work, effects of different surface modification methods (surfactant, acid, base, amide, sulfate) on multi walled carbon nanotubes (CNTs) are studied. The dispersion stability of CNTs in aqueous media was confirmed and the effects of the type of treatment on the thermal conductivity of CNT/water nanofluids were investigated. The surface of the CNTs was modified with acid mixtures (H₂SO₄–HNO₃), potassium persulfate (KPS), tetrahydrofuran (THF), octadecylamine and sodium dodecyl sulfate (SDS). UV–visible spectral data indicate that the CNTs treated first with the acid mixture and then with KPS show the best dispersion stability. The basic treatment and SDS treated CNT/water nanofluids (SDS-KCNT/water) showed the highest conductivity of 0.765 W/mK which increases 24.9% of water as a base fluid conductor.     

Preparation of a stable nanocomposite phase change material (NCPCM) using sodium stearoyl lactylate (SSL) as the surfactant and evaluation of its stability using image analysis

In this study, a new method was proposed for the preparation of a stable Al₂O₃-paraffin nanocomposite phase change material (NCPCM). Sodium stearoyl lactylate (SSL) was used as the surfactant to improve the dispersion of Al₂O₃ nanoparticles (2.5, 5, 7.5, and 10 wt.%) in paraffin with a SSL/Al₂O₃ mass ratio of 1:3.5. After preparation, one group of the samples was placed in an incubator at a temperature of 60 °C for one week to evaluate its stability during the time elapsed. The other was left alternately in two incubators, one at 60 ^°C and the other at 25 ^°C over time intervals of 1 h in order to evaluate the stability of the sample after given numbers of melting/freezing cycles. This latter treatment has been seldom ever reported elsewhere. For NCPCM stability evaluation, the samples were broken into two parts equal in volume and the change ratios of surface layer Al³⁺ concentration are determined. Also, image analysis is used for evaluating the stability of nanofluids. The results obtained from the two methods are found to be in good agreement. Image analysis is, therefore, proposed as a nondestructive method with good accuracy, especially for evaluating the stability of high concentration nanofluids.     

Thermal performance of multi-walled carbon nanotubes (MWCNTs) in aqueous suspensions with surfactants SDBS and SDS

The surfactants of sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) are used in multi-walled carbon nanotubes (MWCNT) aqueous solution respectively due to the hydrophobic nature of MWCNT. Thermal conductivities of nanofluid solutions are measured via the LAMBDA measuring system by transient hot wire method and compared as function of dispersing two different surfactants. MWCNT (hereinafter sometime referred to as CNTs) nanofluid gets a good dispersion and long time stability with both surfactants within 3/1 relative ratio mixture. However, the thermal conductivity of nanofluid decreases with increasing the concentration of both surfactants, and CNT nanofluid with SDBS exhibits better thermal conductivity than that with SDS dispersant. Finally the proper mixture ratio of CNT nanofluid with SDBS and pH value is examined and results show that 0.5 wt.% CNT nanofluids with 0.25 wt.% SDBS, at pH ≈ 9.0 condition display the best thermal performance which increases by 2.8% totally on thermal conductivity compared with that of base fluid distilled water (DW).     

Enhanced specific heat and thermal conductivity of ternary carbonate nanofluids with carbon nanotubes for solar power applications

Specific heat and thermal conductivity are important thermal properties of high-temperature heat transfer fluids and thermal storage materials for supercritical solar power plants. In the present work, nanofluids composed of ternary carbonate Li₂CO₃-K₂CO₃-Na₂CO₃ (4:4:2, mass ratio) and 1.0 wt.% carbon nanotubes (CNT) were prepared to obtain high-temperature heat transfer and storage media with enhanced specific heat and thermal conductivity. The dispersion of CNTs in the nanofluids was tuned by changing the evaporation temperature (100, 140, 180 and 220 °C) and adding surfactants such as sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), or gum Arabic (GA). The results showed that GA and SDS facilitate good dispersion of CNT in nanofluids at the evaporation temperatures of 140 °C and 180 °C, resulting in the formation of more needle-like nanostructures. The higher increase in the specific heat and thermal conductivity of the nanofluids with SDS at 500 °C was 78.3% and 149.2%, respectively. Additionally, the specific heat of as-prepared ternary carbonate nanofluids exhibits a good thermal stability after 30 cycles of thermal shock experiments.     

Surfactant assisted synthesis of Pt-Pd/MWCNT and evaluation as cathode catalyst for proton exchange membrane fuel cell

This study evaluates the role of two types of surfactants, the anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) on the synthesis and properties of multi-walled carbon nanotube (MWCNT) supported bi-metallic Pt-Pd (atomic ratio 1:1) nanoparticles (NPs) and assess their performance as cathode catalyst in proton exchange membrane fuel cell (PEMFC). Pt-Pd/CNT catalysts are prepared through microwave assisted, polyol route in presence of SDS (Pt-Pd/CNT-S) or CTAB (Pt-Pd/CNT-C) and their performance is compared with Pt-Pd/CNT prepared in an identical way without any surfactant. The study shows the prominent influence of the type of surfactant in disaggregation of MWCNT bundles and the dispersion of Pt-Pd NPs on MWCNT support, which in turn controls their catalytic activity for oxygen reduction reaction. In-situ electrochemical characterization studies show improvement in the catalytic activity of Pt-Pd/CNT-S compared to Pt-Pd/CNT-C and Pt-Pd/CNT, demonstrating SDS as an efficient disperser for MWCNT. High fuel cell performance achieved with Pt-Pd/CNT-S having less Pt content (～12 wt %) than the commonly employed pure Pt or Pt/C (20 wt %) catalysts demonstrates that it is a potential cathode catalyst for PEMFC that can help its wider commercialization.     

Experimental research on titanium dioxide nanofluid in vacuum flash evaporation

This study is purposed to conduct experimental investigation into the characteristics of vacuum flash evaporation with nanofluid under adsorption conditions, with the effects of nanoparticle concentration, surfactant concentration, and nanofluid stability taken into consideration. By measuring absorbency, solute concentration, and sedimentation graph, a new nanofluid with excellent dispersion was developed for the vacuum ice-making system. It has been demonstrated that the low concentration of additive is conductive to not only the maintenance of vacuum but also the generation of binary ice. For nanofluid, there are two distinct freezing stages in the process of vacuum flash evaporation. The supercooling stage of nanofluid is delayed with the increase of surfactant concentration, whereas the TiO₂ nanofluid applied to the vacuum ice-making system under adsorption shows stability. Moreover, it is indicated that, for the vacuum ice-making system with TiO₂ nanofluid, the pH level is around 8.5, the surfactant is sodium dodecyl benzene sulfonate-span 60 (SDBS-SPAN), the ratio of SDBS to SPAN is about 10:7, the concentration of TiO₂ nanoparticle is around 0.20 wt%, and the ratio of TiO₂ nanoparticle to SDBS-SPAN is about 10:5. Compared with pure water, undercooling is reduced by 77.92%; the ice packing factor and thermal conductivity are increased by 40.61% and 67.27%, respectively. The results of test and analysis can be applied to conduct further research on vacuum flash evaporation.     

An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers

In the present study, the effects of surfactants on the thermal performance of the hybrid nanofluid (Alumina–Silver) at constant wall temperature and laminar flow have been experimentally studied in a helical coil heat exchanger. Different surfactants such as anionic Sodium Dodecyl Sulfate (SDS) and nonionic Poly Vinyl Pyrrolidone (PVP) in the concentration of range of 0.1–0.4 wt.% are employed. It is found that the thermal performance can be maximized by using the 0.2 vol.% hybrid nanofluid and 0.1 wt.% SDS anionic surfactant in the helical coil. The maximum thermal performance in the presence of hybrid Alumina–Silver nanofluid and SDS anionic surfactant is 16% higher than that of the pure distilled water. The presented results can have potential application in process intensification and optimum design of heat exchangers.     

Efficient hydrogen production from glycerol photoreforming over Ag2OTiO2 synthesized by a sol–gel method

Photoreforming hydrogen production from glycerol aqueous solutions has been investigated over Ag₂OTiO₂ catalysts with variable compositions (0.72–6.75 wt.% of Ag₂O) synthesized by a sol–gel method. The structural and morphologic characteristics were examined by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET) method etc. By comparing the results of FESEM and DLS, aggregations of all samples were found to occur in both dry powder and water suspension states. The TiO₂ based catalyst with 2.82 wt.% Ag₂O calcined under 500 °C showed less average particle size differences between in air and in aqueous state, suggesting better dispersion stability which could lead to a more efficient of using light. It was found that mixing certain amounts (0.72–2.82 wt.%) of Ag₂O to TiO₂ could reduce anatase grain size and increased the specific surface area of the catalysts. The hydrogen production results indicated that the photocatalytic activity and the reaction rate of TiO₂ were significantly improved by mixing Ag₂O. The optimal Ag₂O mixing amount and calcination temperature for H₂ production was found. With an appropriate catalyst composition, the heterostructures could be well generated to enhance the separation of electrons and holes and it was also found that the desirable heterostructure was likely to disappear after the calcination temperature of 700 °C.