高超声速飞行器与发动机的热环境非常恶劣，随着飞行马赫数的不断提高以及飞行器结构的复杂化，热防护成为了亟待解决的核心关键问题。目前以燃料为冷却介质的主动冷却技术是国内外公认的有效热防护方式之一。而主动冷却技术的关键在于如何提高燃料的对流换热效率。从上世纪90年代纳米流体这一概念提出以来，很多学者针对不同类型的纳米流体开展了关于分散稳定性、热物性参数以及流动特性的研究。纳米流体就是指在基础液体介质中添加纳米颗粒所形成的能够达到分散稳定的胶体介质。作为一种新型的冷却介质，纳米流体具有改善冷却介质导热性能以及提高主动冷却换热效率的应用潜力。目前针对非极性液体煤油的纳米流体的研究很少，煤油基纳米流体的热物性以及对流传热特性都有待更深入系统的研究。

本文首先针对纳米流体的悬浮稳定性及其制备方法展开了研究。分别以去离子水和航空煤油为基础液体，以铝、氧化铝、二氧化钛为添加物，研究了纳米流体的分散稳定性。纳米流体的制备主要分为一步法和两步法。其中一步法所制备的纳米流体稳定性较好但制备效率非常低。因此，为了满足工程应用的需求，本文研究了两步法配制纳米流体的方法以及影响纳米流体分散稳定性的因素，并通过沉降观测法、测量吸光度及Zeta电势等方法对纳米流体的分散稳定性进行了定性与定量的分析。本文最终确定以十二烷基苯磺酸钠作为水基纳米流体的分散剂，以油酸作为煤油基纳米流体的分散剂。所制备水基纳米流体能够达到15天左右的分散稳定，所制备煤油基纳米流体能够达到7天左右的分散稳定。以上稳定时间达到了目前两步法制备水基和油基纳米流体稳定时间的国际先进指标。

本文针对纳米流体的导热系数与粘性系数展开了实验和理论研究。首先采用基于一维非稳态传热理论的瞬态双热线法测量了不同种类纳米流体导热系数，发现添加纳米颗粒能够显著增加基础液体的导热性能。常温状态下，颗粒质量浓度3g/L的铝-去离子水纳米流体的导热系数增加了8.7%；当温度升高到50℃时，导热系数增加了20.4%。类似地，常温下颗粒质量浓度3g/L的铝-航空煤油纳米流体导热系数增加了13.3%，当温度升高到100℃摄氏度时导热系数增加了27.7%。本文从纳米颗粒微观运动角度分析了影响纳米流体导热系数的因素，包括颗粒种类、粒径、颗粒浓度、液体温度等，并建立了具有较好普适性的纳米流体导热系数理论预测模型。此外，本文还发展了一种基于纳米流体宏观物理参数（导热系数、温度、质量浓度）确定纳米颗粒吸附层厚度的间接测量方法，并得到了吸附层厚度随温度、质量浓度的变化结果。本文还研究了纳米流体粘性系数。采用旋转法进行粘性系数的测量，并对影响纳米流体粘性系数的主要因素进行了分析。与基础液体相比，水基纳米流体的粘性系数没有非常明显的提高，而煤油基纳米流体的粘性系数相较基础液体有比较明显的增大，部分原因是由于作为分散剂的油酸其粘性系数较大所导致的。

最后，本文针对煤油基纳米流体的圆管流动与传热特性开展了实验研究。实验结果表明，煤油基纳米流体的摩阻系数、努塞尔数和对流传热系数相较基液煤油都有明显的提高。本文所用三类纳米流体（铝-煤油、氧化铝-煤油、二氧化钛-煤油）在质量浓度为1g/L时，摩阻系数较煤油增大了约11%，努塞尔数提高了约12%，对流换热系数提高了约18%。本文分析了三类煤油基纳米流体的热工因子，定量评估添加纳米颗粒能否对介质的流动与传热综合性能带来增益。三类纳米流体在三种质量浓度条件下热工因子均大于1，且热工因子的数值随颗粒质量浓度的增大而增大。这意味着添加颗粒能够给煤油的流动与传热综合性能带来增益，且颗粒浓度越高增益越大。本文还提出了考虑纳米颗粒浓度效应以及粘性系数增大效应的煤油基纳米流体传热努塞尔数理论预测公式。通过与实验数据的比对，证明该公式能够较准确地预测煤油基纳米流体的对流传热性能。

本文针对水基和煤油基纳米流体分别开展了分散稳定性、制备方法、热物性参数、流动与传热性能的研究，系统研究了煤油基纳米流体的导热和对流传热性能，同时提出了具有较好适用性的纳米流体导热系数理论模型和煤油基纳米流体努塞尔数预测公式。以上工作为深入了解纳米流体的物理机制以及促进纳米流体技术的工程应用提供了有益的参考

As known, thermal environment of supersonic combustor is very severe due to significant aerodynamic heating and combustion heat releasing. When the flight Mach number is 6, the total temperature in engine combustor could reach 3000K, and the maximum wall heat flux could reach 5MW/m². Therefore, thermal protection of combustor is one of the key technologies for scramjet applications. And the active cooling with fuel as coolant is one of the efficient thermal protection methods, then, the way to improve the cooling efficiency of fuel is becoming a key issue. In 1995, the concept of nanofliuds, a kind of colloid made by adding nanoparticles in base fluid, was proposed by Choi. Since then, the studies about dispersion stability, thermophysical characteristics and flowing properties of nanofluids have been researched extensively. As a new type of heat transfer medium, it is potential for nanofluids to improve the cooling efficiency of fuel, and of active cooling. However, the current research on kerosene-based nanofluids is not comprehensive, and the thermophysical and heat transfer characteristics of kerosene-based nanofluids need to be further studied.

Firstly, the preparation and dispersion stability of nanofluids are studied in this paper. Deionized water and RP3 aviation kerosene were chosen as base fluid and aluminum, alumina and titanium dioxide were chosen as dispersion phase. There are two methods to prepare nanofluids, one-step method and two-step method. The stability of nanofluids prepared by one-step method is much better, but the preparation efficiency is pretty low. Thus, in order to meet the needs of engineering application, the two-step method and the factors influencing the dispersion stability of nanofluids is studied in this paper. The dispersion stability of nanofluids was analyzed qualitatively and quantitatively by means of sedimentation observation, absorbance measurement and Zeta potential measurement. In this paper, sodium dodecyl benzenesulfonic acid was used as dispersant of water-based nanofluids and oleic acid as dispersant of kerosene-based nanofluids. The water-based nanofluids and the kerosene-based nanofluids could respectively reach the dispersion stability of about 15 days and 7 days.

Then the thermophysical properties of nanofluids are studied in this paper. Firstly, the thermal conductivity of nanofluids is focused on. Transient hot wire method was used to measure the thermal conductivity of different kinds of nanofluids. The thermal conductivity of aluminum-deionized water nanofluids with a mass concentration of 3g/L increased by 8.7% at room temperature and 20.4% when the temperature rose to 50℃. The thermal conductivity of aluminium-kerosene nanofluids with a mass fraction of 3g/L increased by 13.3% at room temperature and 27.7% when the temperature rose to 100℃. Then the factors affecting the thermal conductivity of nanofluids were analyzed, and a new theoretical prediction model for the thermal conductivity of nanofluids was established. Besides, the viscosity of nanofluids is also studied in this paper. Rotational method was applied to measure the viscosity of nanofluids. Compared with deionized water, the viscosity of water-based nanofluids was not significantly increased, and the addition of nanoparticles did not influence the change rule of viscosity of base fluid. However, kerosene-based nanofluids is a little different. Compared with aviation kerosene, the viscosity of the kerosene-based nanofluids is obviously higher, which is caused by the high viscosity of oleic acid as dispersant.

Finally, the flow and heat transfer characteristics of kerosene-based nanofluids are studied experimentally. Experimental results show that the Nusselt number, friction coefficient and convective heat transfer coefficient of nanofluids are all higher than those of kerosene. The friction coefficient was increased by 11% of aluminium/alumina/titanium dioxide-kerosene nanofluids with mass fraction of 1g/L, the Nusselt number was increased by 12% and the convective heat transfer coefficient was increased by 18% or so. In order to get whether the addition of nanoparticles could improve the comprehensive performance of flow and heat transfer, the thermal performance parameter was calculated. The thermal performance parameters of the three kinds of nanofluids are all greater than 1 under three different mass fractions, and the value of the thermal performance parameter increases with the increase of particle mass fraction. It means that the addition of particles can improve the comprehensive performance of flow and heat transfer of kerosene, and the higher the mass fraction, the greater the improvement. The prediction formula of Nusselt number of kerosene-based nanofluids is also proposed in this paper. By comparing with experimental data, it is believed that this formula can accurately predict Nusselt number of kerosene-based nanofluids.

The preparation method, characteristics of thermophysical properties and flow and heat transfer performance were studied in this paper. In this paper, the thermal conducting and heat transfer properties of nanofluids are systematically studied for the first time. At the meantime, new theoretical model of thermal conductivity and prediction formula of Nusselt number were proposed in this paper. The research in this paper can provide the reference for engineering application of nanofluids.