A novel thermal dispersion model to improve prediction of nanofluid convective heat transfer

This study numerically investigates the hydrothermal characteristics of the nanofluid in a laminar flow inside a straight tube. A new model is proposed for dispersion thermal conductivity while a theoretical approach is adopted to predict the particle distribution at the tube cross section considering the effects of non-uniform shear rate, Brownian diffusion and viscosity gradient on particle migration. It is observed that nanoparticles are not distributed uniformly at the tube cross section such that the values of concentration are higher at central regions of the tube and this non-uniformity intensifies at higher mean concentrations and Reynolds numbers. The particle distribution is applied in the proposed dispersion model. The findings show that this dispersion model presents more accurate results than traditional homogenous one. For dispersion model, the velocity profile is flatter than that obtained from the homogenous model. In addition, in the vicinity of the wall, the value of temperature and its gradient obtained from the dispersion model are respectively lower and higher than those from the homogenous model. Increasing the mean concentration results in convective heat transfer enhancement, while causing not much penalty in pressure drop. This indicates that application of nanofluids can result in energy efficiency improvement.     

A study on the effects of temperature and volume fraction on thermal conductivity of copper oxide nanofluid

By using copper oxide nanofluid fabricated by the self-made Submerged Arc Nanofluid Synthesis System (SANSS), this paper measures the thermal conductivity under different volume fractions and different temperatures by thermal properties analyzer, and analyzes the correlation among the thermal conductivity, volume fraction, and temperature of nanofluid. The CuO nanoparticles used in the experiment are needle-like, with a mean particle size of about 30 nm. They can be stably suspended in deionized water for a long time. The experimental results show that under the condition that the temperature is 40 degrees C, when the volume fraction of nanofluid increases from 0.2% to 0.8%, the thermal conductivity increment of the prepared nanofluid towards deionized water can be increased from 14.7% to 38.2%. Under the condition that the volume fraction is 0.8%, as the temperature of nanofluid rises from 5 degrees C to 40 degrees C, the thermal conductivity increment of the prepared nanofluid towards deionized water increases from 5.9% to 38.2%. Besides, the effects of temperature change are greater than the effects of volume fraction on the thermal conductivity of nanofluid. Therefore, when the self-made copper oxide nanofluid is applied to the heat exchange device under medium and high temperature, an optimal radiation effect can be acquired.     

Influence of dispersion on sensitivity of highly birefringent fibers to temperature and hydrostatic pressure

The influence of dispersion on the sensitivity of highly birefringent fibers to temperature and hydrostatic pressure was experimentally investigated. In fibers with geometric birefringence that shows high dispersion, great differences were observed between group and phase sensitivities to temperature and hydrostatic pressure. This difference may reach 400% in the case of temperature response. In contrast, in weakly dispersive fibers with stress-induced birefringence these differences were of the order of 8% and 14%, respectively, for temperature and pressure. The influence of the dispersion effect on the temperature compensation of white-light interferometric sensors based on highly birefringent fibers was also discussed.     

Driving simulator sickness: Impact on driving performance,influence of blood alcohol concentration,and effect of repeated simulator exposures

Simulator sickness is a major obstacle to the use of driving simulators for research, training and driver assessment purposes. The purpose of the present study was to investigate the possible influence of simulator sickness on driving performance measures such as standard deviation of lateral position (SDLP), and the effect of alcohol or repeated simulator exposure on the degree of simulator sickness. Twenty healthy male volunteers underwent three simulated driving trials of 1 h’s duration with a curvy rural road scenario, and rated their degree of simulator sickness after each trial. Subjects drove sober and with blood alcohol concentrations (BAC) of approx. 0.5 g/L and 0.9 g/L in a randomized order. Simulator sickness score (SSS) did not influence the primary outcome measure SDLP. Higher SSS significantly predicted lower average speed and frequency of steering wheel reversals. These effects seemed to be mitigated by alcohol. Higher BAC significantly predicted lower SSS, suggesting that alcohol inebriation alleviates simulator sickness. The negative relation between the number of previous exposures to the simulator and SSS was not statistically significant, but is consistent with habituation to the sickness-inducing effects, as shown in other studies. Overall, the results suggest no influence of simulator sickness on SDLP or several other driving performance measures. However, simulator sickness seems to cause test subjects to drive more carefully, with lower average speed and fewer steering wheel reversals, hampering the interpretation of these outcomes as measures of driving impairment and safety. BAC and repeated simulator exposures may act as confounding variables by influencing the degree of simulator sickness in experimental studies.     

高浓度石墨烯水分散液的制备与表征

利用高压均质液相剥离法,以鳞片石墨为原料,水为介质,制备高浓度石墨烯水分散液。采用紫外可见光谱研究表明活性剂浓度、高压均质压力和循环次数对石墨烯水分散液浓度C_G的影响。通过拉曼光谱、扫描电镜、透射电镜、激光粒度仪分析水分散液中石墨烯的结构和形貌。结果表明:通过调节各工艺参数,获得了浓度为324.3mg·L^-1的石墨烯水分散液,所得浓度是超声液相剥离法的10倍;石墨烯水分散液中石墨烯缺陷少、厚度薄、片径大,具有良好的品质;将所得石墨烯分散液制备石墨烯自支撑膜,其电导率可达3.2×10~4S·m^-1。     

温度和颗粒浓度对纳米流体粘度的影响

采用"两步法"制备了含有不同质量分数的TiO2-水纳米流体,并观察了其稳定性。测量了不同质量分数的纳米流体在15～40℃时的粘度,结果表明,纳米流体的粘度随颗粒浓度的增加而增大,随温度的升高而以指数形式降低,并且各种纳米流体的粘度随温度的变化趋势相似。结合实验数据,对已有粘度计算公式进行修正,提出了涉及温度和颗粒浓度的纳米流体粘度计算公式。     

表面活性剂对高浓度石墨烯水分散液制备的影响

分别选取阴离子型、阳离子型和非离子型3种类型表面活性剂,通过紫外可见光谱研究表面活性剂结构和浓度对高压均质-液相剥离法制备的石墨烯水分散液浓度的影响。通过高分辨透射电镜和激光粒度仪对所制备的石墨烯的品质进行分析。结果表明:长的疏水链段、双键和苯环官能团是促进表面活性剂作用发挥的关键结构,表面活性剂最优浓度略高于其临界胶束浓度。在测试范围内,Tween80效果最佳,其最佳作用浓度为0.012mmol·L^-1,所得石墨烯水分散液浓度为564.3mg·L^-1。表面活性剂的结构和浓度对石墨烯的品质无明显影响。     

A device for measuring the concentration and dispersion quality ofmagnetic particle suspensions

A technique, called rheomagnetic measurement, for studying the concentration and orientation of magnetic particles through inductance measurement is presented. The particles are oriented in a predominantly extensional flow field, and, because they are magnetic, their orientation can be detected with a weak magnetic sensing field. Because flocs of magnetic particles orient differently in a flow field than primary particles do, this method can be useful in obtaining information about the particle flocculation aspect of dispersion quality. A magnetic sensing field can also be used to detect the particle concentration in a quiescent flow. Experimental data on the effects of particle concentration and milling for rod-like γ-Fe₂O₃ and plate-like Ba-ferrite suspensions are discussed. The results for Ba-ferrite magnetic markedly contrast with those for the rod-like magnetic particles but showed similarity with those for rod-like γ-Fe₂O₃     

纳米Al2O3粒子浓度对弥散强化铜合金退火行为的影响

通过力学性能、金相和TEM观察对低、中浓度Cu-Al2O3弥散强化铜合金的退火行为进行了研究.结果表明:低浓度弥散强化铜合金具有一定的抗高温软化性能,500℃退火后发生再结晶,900℃退火后已基本完全再结晶,屈强比约为56%.中浓度合金抗高温软化性能较好,900℃退火后,合金仍然以回复过程为主,金相尺度下不能看到再结晶晶粒,屈强比可达70%.弥散强化铜合金优越的抗高温软化性能归功于铜基体内均匀弥散分布的纳米Al2O3粒子.Al2O3粒子尺寸在10～20nm内时,粒子间距＞200nm时,阻碍晶界迁移能力较差,粒子间距＜90nm时,可显著阻碍晶界的迁移,合金抗高温软化性能优越.     

Supersensitivity of transition-metal nanoparticle formation to initial precursor concentration and reaction temperature: understanding its origins

A supersensitivity of the formation and stabilization of transition-metal nanoclusters to the initial nanocluster concentration and temperature synthesis conditions is reported, then probed, herein for the specific case of prototype Ir(O)n nanoclusters prepared from the organometallic precursor [Bu4N]5Na3[(1,5-COD)Ir x P2W15Nb3O62] by reduction with H2 in propylene carbonate solvent. Fully isolable, redissolvable, near-monodisperse (i.e., < or = +/- 15% size distribution) and thus excellent Ir(O)n nanoclusters are formed using low temperature (22 degrees C) and moderate precursor concentration (1.2 mM) in propylene carbonate solvent. However, inferior, polydisperse (+/- 40% size distribution), non-redispersable nanoclusters are formed at the seemingly only moderately different conditions of 38 degrees C higher temperature (i.e., 60 degrees C) and 5-fold lower precursor concentration (0.24 mM). Investigation of this supersensitivity to the nanocluster synthesis conditions reveals that it derives from the dissociation of (1,5-COD)Ir(solvent)2+ from the P2W15Nb3O62(9-) polyanionic ligand/ stabilizer, subsequently resulting in a too fast, kinetically uncontrolled reduction of the quickly reduced (1,5-COD)Ir(solvent)2+ as the cause of the inferior synthesis of polydisperse, non-isolable, non-redissolvable nanoclusters. The results are significant in that they illustrate that understanding the mechanism of nanocluster formation, and then performing the nanocluster synthesis under kinetically carefully controlled, understood conditions, is necessary for the formation of superior nanoclusters in this, and by implication probably many other, cases.     

温度对大鼠肝脏中剪切波与频散特性影响研究

剪切波相速度和衰减的频散特性的准确测量具有重要的意义。温度变化可能会对生物软组织的力学特性产生影响,为了研究温度的变化对剪切波相速度和衰减频散特性测量的影响,以大鼠肝脏为研究对象,基于声辐射力的剪切波成像的方法,通过离体实验来获取大鼠肝脏内部不同温度下的剪切波相速度和衰减频散特性曲线。实验结果发现,随着温度的升高,剪切波相速度逐渐减少,衰减频散逐渐增大,特别是在较高的频段(270~380 Hz)的变化更为显著。结果表明温度是测量大鼠肝脏中剪切波相速度和衰减频散特性中不可忽略的因素,这对今后的动物实验研究以及疾病评估提供了实验参考,具有重要的意义。     

液态生物制品低温浓缩工艺综合节能降耗分析

把冷冻浓缩、膜分离浓缩与车间空调相结合,形成一种节能低温浓缩工艺,并对该工艺进行了理论分析。通过比较在120 m2GMP车间内,每小时将500 kg茶叶水提取液从初始质量浓度5 g/L浓缩到75 g/L各种浓缩工艺的能耗,结果表明:在中国广州地区,新低温浓缩工艺的能耗比真空蒸发浓缩节能56.7%,比膜分离浓缩节能16.6%,设备寿命期内可减少CO2排放量53.5 t;在中国上海地区,新低温浓缩工艺能耗比真空蒸发浓缩节能51%,比膜分离浓缩节能9.1%。     

Comment on the homogeneous nanofluid models applied to convective heat transfer problems

In several recent papers, the heat transfer characteristics of nanofluids have been investigated by simply replacing the transport coefficients of the base fluid by the effective transport coefficients of the nanofluids. The present note emphasizes, however, that the governing equations of these homogeneous nanofluid models (in which the velocity-slip effects of the nanoparticles are neglected) can be reduced with the aid of elementary scaling transformations to the respective equations of the regular fluids. Thus, the corresponding nanofluid results can be recovered from the solutions of already solved regular problems by simple arithmetic operations, without any additional research effort. This feature is illustrated here by the specific examples of the classical Blasius and Sakiadis forced convection heat transfer problems.     

Surfactant assisted exfoliation of high purity graphene in aqueous solution as a nanofluid using kitchen blender: Influence on dispersion,thermal conductivity and rheological properties

In this work, graphene is exfoliated from graphite flakes in distilled water solution containing Tween 80 as surfactant and by using the mechanical exfoliation method through a kitchen blender. The blending time is an important parameter for the size reduction and concentration of graphene exfoliation in a nanofluid. Graphene obtained using a kitchen blender is of high purity having no basal plane defects, as depicted from Raman spectroscopic investigation. The viscosity of graphene nanofluid increases with blending time up to 7 h and thereafter the viscosity decreases. All graphene nanofluid samples prepared using different blending time shows shear thinning behavior by fitting the power law of non-linear viscoelastic measurement data. The linear viscoelastic region measured was in the strain range of 0.01–0.8%. Further, the frequency sweep measurement using a rheometer has indicated the gel-like behavior of graphene nanofluid in a certain frequency range and depends on the concentration of graphene. The use of two-dimensional nanosheets-based nanofluid has attracted the attention of researchers for improving pool boiling properties. Herein, the pool boiling of graphene nanofluid at 7 h of blending shows the 77.4% enhancement in critical heat flux (CHF). This work has established mechanical exfoliation as a process to synthesize the graphene nanofluid and can be used for heat transfer applications.     

Environmental dispersion in a two-layer wetland: Analytical solution by method of concentration moments

For the most typical two-layer wetland with free-water-surface-effect, analytically explored in this paper is the environmental dispersion in terms of the longitudinal evolution of the depth-averaged concentration. Taylor’s classical analysis for soluble matter dispersion in a single phase fluid flow is rigorously generalized for the two-layer case of a wetland flow to develop the dispersion model. The velocity profile of the fully developed flow through the wetland is derived, with that for the single-layer wetland flow included as a special case. Aris’s method of concentration moments is applied to determine the dispersivity with an asymptotic time variation. For typical contaminant constituents of chemical oxygen demand, biochemical oxygen demand, total phosphorus, total nitrogen and heavy metal, the process of dispersion is shown characteristic of a hierarchical structure in the critical length and duration of the contaminant cloud.     

Effect of body acceleration on dispersion of solutes in blood flow

The unsteady dispersion of a solute in blood flow modeling blood as a Newtonian fluid under the influence of a body acceleration is studied using the generalized dispersion model proposed by Gill and Sankarasubramanian [13]. As a result, the total process of dispersion can be described in terms of a simple diffusion process with the effective diffusion coefficient as a function of time. The model brings out mainly the effect of body acceleration and radius of the artery on the overall dispersion process. In the absence of body acceleration, the dispersion coefficient is found to increase rapidly and maintains a steady value in aorta while in other arteries it oscillates about a mean value after reaching it. Body acceleration is observed to enhance the value of the dispersion coefficient in all arteries. The effective diffusivity is found to depend on body acceleration. In the presence of body acceleration, it is noticed that there is a decrease in the effective diffusivity in aorta, femoral, and carotid while in coronary it is increased.     

Effect of temperature,duration and speed of pre-mixing on the dispersion of clay/epoxy nanocomposites

The effect of temperature, duration and speed of pre-mixing on the dispersion of organoclay in epoxy was studied. The technique of high-speed mixing process was used. Epoxy and Cloisite 30B were pre-mixed at different temperatures, different speeds and for different durations of time. The quality of dispersion and intercalation/exfoliation of organoclay in epoxy after pre-mixing (before adding hardener) was analyzed by means of X-ray diffraction (XRD) and viscosity measurement. The dispersion and intercalation/exfoliation of organoclay in the epoxy nanocomposites (ENCs) after curing were characterized by XRD, field emission gun scanning electron microscopy (FEGSEM) and TEM. The results illustrate that the clay particles were broken down to smaller and smaller sizes with increase of pre-mixing temperature and especially with increase in pre-mixing speed. These parameters do not significantly affect the intercalation/exfoliation of organoclay in epoxy at the pre-mixing step, but they have indirect effect on intercalation/exfoliation at the curing step.