SiO2气凝胶导热系数与密度的关系探讨

本文以双层玻璃窗平面热传导模型为基础,建立了SiO2气凝胶的热传导"立方体"模型,探讨了SiO2气凝胶导热系数与密度的关系。计算结果与实际测试值具有结果一致性。     

纳米SiO2填充硅橡胶的补强机理研究进展

简单介绍了纳米SiO2的表面化学性质及其粒子的聚集状态;归纳了物理吸附作用、氢键作用方式及共价键结合方式等基本的纳米SiO2与硅橡胶的作用方式;以纳米SiO2-纳米SiO2、纳米SiO2-硅橡胶的不同作用为基础列举了键合胶、复合团聚体及互穿网络模型等几种补强模型,并总结了其在补强过程中发挥的作用,从而对其补强机理研究进展进行更为详细的概括和介绍.     

球形SiO2颗粒的合成及反应机理

利用Stober方法合成了球形SiO2颗粒.在乙醇和水的混合溶剂中,以正硅酸乙酯(TEOS)为硅源、氨水作催化剂,在40℃水浴下制备SiO2球形颗粒.通过改变反应物TEOS以及催化剂氨水的加入量,制备了不同粒径的球形SiO2颗粒.利用扫描电镜(SEM)可以看出,随着TEOS和氨水加入量的增加,SiO2颗粒的粒径增大,并探讨了其变化机理.     

纳米低温保护剂导热系数研究

用瞬态热线法测量了SiO2纳米粉体加入到浓度为10%、20%、30%、40%、50%的丙三醇溶液中制备成的纳米低温保护剂在260K~290K下的导热系数,研究纳米低温保护剂导热系数与纳米颗粒含量、保护剂浓度以及温度的关系。结果表明：随着纳米颗粒的添加量增多（0.5g/L-1g/L),甘油溶液导热系数增大;甘油浓度从10%升高到50%时,纳米低温保护剂导热系数增加率也随之提高,从0.4%提高到6.8%;纳米低温保护剂导热系数随温度降低而减小,当温度低于260K后,纳米颗粒对保护剂导热系数几乎无影响。建议应用在解冻和复温过程中。     

木质素／纳米SiO2复合材料的制备及其应用

采用高沸醇（HBS）木质素制备得到了HBS木质素／纳米SiO2复合材料。对产物进行红外光谱、透射电镜分析，可以明显看出材料复合后的特征，表明木质素／无机纳米复合材料的制备是可行的。把木质素／纳米SiO2复合材料作为橡胶的补强剂应用于乙丙橡胶。实验发现，HBS木质素／纳米SiO2复合材料添加到橡胶中可提高胶料的断裂伸长率，从165％提高到229％，但拉伸强度略有降低。     

采用3ω法测量多壁纳米碳管悬浮液的导热系数

为了减小瞬态热线法测量纳米流体导热系数时易受电磁干扰和自然对流等因素的影响,更好地探究新型换热工质的强化传热机理,研制了3ω法实验台,采用锁相放大器测得交流加热铂丝的3倍频电压响应,拟合算出待测液体的导热系数.先通过对常规液体蒸馏水、乙二醇以及酒精溶液的测量,验证了实验台的精度和可靠性.然后采用两步法合成了稳定性较高的多壁纳米碳管悬浮液,测得其各体积分数和各温度下的导热系数.实验结果表明,3ω法具有较好的电磁兼容性,测量时温升不超过0.5K,可以有效地减小对流传热和辐射传热的影响,且可以通过1ω电压来判断纳米流体的稳定性;纳米碳管悬浮液的导热系数比基液和Hamilton-Crosser预测值明显提高,并且分别随纳米碳管含量的增加和温度升高而加大.     

PBS/SiO2纳米复合材料的制备及其结构与性能分析

以聚丁二醇丁二酸酯（PBS）为基材,经-甲基丙烯酰氧基丙基三甲氧基硅烷偶联剂（KH570）改性过的纳米SiO2和未改性的SiO2为填料,采用熔融共混法制备了PBS/SiO2纳米复合材料。研究了所得纳米复合材料的热稳定性能、力学性能和降解性能等。结果表明：当经KH570表面改性的纳米SiO2（KH570与纳米SiO2的质量之比为1：5）的添加质量分数为4%时,复合材料的维卡软化点约提高了10℃,拉伸强度约提高30%,同时复合材料的降解性能比PBS纯料的降解性能有一定的提高。     

低浓度Al2O3-水纳米悬浮液粘度的研究

分别对粒径为40nm和65nm的Al2O3-水纳米悬浮液的粘度在不同浓度、温度下进行了实验研究.结果表明,Al2O3-水纳米悬浮液的粘度随浓度的增加而增加,随温度的升高而降低;在相同体积分数下,随颗粒尺寸的减小而增加.当Al2O3纳米颗粒体积分数为0.1%时,粒径为40nm和65nm的Al2O3-水纳米悬浮液的粘度比水分别提高了11.03%和0.83%;当Al2O3纳米颗粒体积分数为0.5%时,粘度比水分别提高了28.28%和17.50%.与粘度理论值比较发现,测量值远大于理论值,且测量值与体积分数呈非线性关系,而理论值与体积分数呈线性关系.     

纳米Al2O3与SiO2的制备及在陶瓷釉料中的应用

以普通无机盐为原料采用沉淀法制备了纳米Al2O3和SiO2.XRD分析表明样品为无定形结构,SEM分析表明得到的纳米Al2O3和SiO2均为球形颗粒,直径分别为90m和300nm.将合成的纳米材料添加至陶瓷面釉进行烧结测试,结果表明,添加纳米材料釉料的烧结温度比普通釉料的烧结温度降低了30℃,釉层性能明显得到改善,釉料良好的性能源于纳米材料较大的表面积及高的烧结活性.     

包覆型纳米SiC—Al2O3水悬浮液流变特性研究

本文通过在纳米SiC颗粒表面包覆一层Al2O3,使包覆型SiC与Al2O3具有相似的表面化学性质,从而包覆型SiC-Al2O3混合粉悬浮液具有相似于单相Al2O3悬浮液的流变性质,研究了分散剂,颗粒匹配及温度对包覆型SiC-Al2O3混合粉悬浮液流变性质的影响,结果表明,对于亚微米粉,尤其纳米粉,高聚物电解质分解剂不能改善其流变性,合适的颗粒匹配及温度可以改善悬浮液的流动性。     

金属氧化物纳米流体的导热性能研究

采用瞬态热线法测量了4种不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米颗粒属性、体积分数、悬浮稳定性及温度等因素对纳米流体导热系数的影响.实验结果表明,在流体中加入纳米颗粒将显著提高流体的导热系数.     

方腔内Al2O3-H2O纳米流体热毛细对流的格子Boltzmann模拟

采用格子Boltzmann方法研究纳米颗粒形状影响下方腔内纳米流体热毛细对流的强化传热效果,主要分析了纳米粒子体积分数、颗粒形状以及Marangoni数Ma等相关参数对于纳米流体热毛细对流换热过程的影响。结果表明:长径比（长/半径）对纳米流体换热效果有影响,形状因子越大,平均Nu数Nu_ave越大。随着体积分数的增加,棒状、盘状和正方体状纳米颗粒均使热毛细对流的Nu_ave数减少,球状纳米颗粒条件下热毛细对流的Nu_ave数增加。Ma数越大,纳米流体热毛细对流的自由表面速度越大,对流换热效果也随之增强。      

Modeling Thermal Conductivity for Alumina-Water Nanofluids

This article presents results of an ongoing investigation into the modeling of thermal conductivity for Alumina-Water nanofluids. In spite of having the promise of being an improved heat transfer medium, fundamental understanding and modeling of important thermo-physical properties of nanofluids (such as thermal conductivity) have remained a difficult task due to the possible influence of several particle and base-fluid properties on the behavior of nanofluids. The existing theories to explain the phenomenon of thermal conductivity augmentation have provided different and sometimes contrasting mechanisms. In this study, seven existing theoretical models for thermal conductivity of nanofluids have been evaluated for their accuracy by comparing the predicted versus experimental data for a wide range of test conditions. The existing models were found to provide inaccuracies (over/underpredictions) in the range of 3 to 58%. A new model has been developed using dimensionless analysis, which includes Prandtl number and a new dimensionless number that is a ratio of Reynolds number to the square root of Brinkman number for particle and fluids. The new model has been found to generally predict the thermal conductivity ratio (nanofluids to base fluids) within 5% accuracy range.     

Thermal Conductivity of AlN–Ethanol Nanofluids

Aluminum nitride (AlN) particles of 20 nm diameter were dispersed into ethanol by a two-step process, first magnetic striation and then ultrasonic agitation. Castor oil was added as a dispersant to improve the stability of the AlN suspension. The thermal conductivities of AlN–ethanol nanofluids were measured by a hot-disk method from 0.5 vol% to 4.0 vol% at temperatures of 273.15 K and 297.15 K. Results show about 20% increase in the thermal conductivity of ethanol with the addition of 4.0 vol% at 273.15 K, and a strong temperature dependence of the thermal conductivity.     

Al2O3纳米粉体悬浮液热物性实验研究

分别采用瞬态热线法、比较量热法和旋转粘度计测试了不同温度、粒子浓度和粒径下的Al2O3-DW（蒸馏水）纳米流体的导热系数、比热容、粘度等热物性参数。试验结果表明,粒子浓度、粒径和温度都是影响Al2O3-DW纳米流体热物性参数的重要因素。与水相比,纳米流体导热系数和粘度增加,常温4％体积份额下增幅分别为21．5％和52．3％;纳米流体比热容随着粒子体积份额增加而降低,并推导出了常温下低浓度纳米流体比热容的预测公式。     

Thermal Conductivity of Nanofluids – Experimental and Theoretical

The thermal conductivity of nanofluids has been studied experimentally using the transient hot-wire method, and it is shown that a significant increase can be obtained. Existing methods for the prediction and correlation of the thermal conductivity are discussed. It is shown that a lot of work still needs to be done in this area.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

影响纳米流体导热系数实验测试的因素

导热系数是衡量纳米流体强化换热的最重要的参数,但在不同学者的研究中,对于同一种纳米流体所测得的导热系数却有很大差别。本文针对影响纳米流体导热系数实验测量的因素进行研究,在相同的实验条件下,分别运用Hotdisk导热仪和闪光法导热仪对水基二氧化钛纳米流体的导热系数进行了测量。实验结果表明,用Hotdisk法的测量结果比用闪光法测得的高21%～34%。通过计算分析发现:自然对流是引起纳米流体导热系数测量结果多变的重要原因之一。     

瞬态热丝法测量纳米流体的导热系数

推导了用双铂丝测量流体导热系数的瞬态热丝法的实验关联式,并据此设计实验装置,分析误差因素,给出了系统误差修正系数.用该装置测量了几种纳米流体的导热系数并作了相关分析.     

Al_2O_3-H_2O纳米蓄冷箱蓄冷效果的影响因素

本文采用低密度海绵吸附纳米流体(相变蓄冷剂)中的纳米颗粒,改变蓄冷剂(PCM)的热物理性质,以强化蓄冷箱的蓄冷效果。采用正交实验法,以胡萝卜为实验对象,箱内空气平均温度对时间的积分值为对比指标,分析海绵厚度(8 mm、16 mm、20 mm)、纳米流体质量分数(0.1%、0.15%、0.2%)、每千克蔬菜对应的蓄冷材料质量(0.429 kg、0.5 kg、0.571 kg)、蔬菜预冷温度(-1℃、4℃、12℃)对蓄冷效果的影响。研究结果表明:影响蓄冷效果的主次因素为蔬菜预冷温度、纳米流体质量分数、蓄冷材料质量、海绵厚度;蓄冷效果随着海绵厚度、PCM质量分数和PCM质量的增大而增强,预冷温度在4℃时较佳;由极差值可知,各因素的最优水平为海绵厚度20 mm、纳米流体质量分数0.2%、蓄冷材料质量0.571 kg、预冷温度4℃,但是从经济角度考虑,海绵厚度可以用16 mm代替20 mm,纳米流体质量分数可以用0.15%代替0.2%。该实验方法及结果具有指导借鉴作用,可以为后期的实验及分析提供理论基础。