Investigation On Preparation And Transport Properties Of Nanofluids

With progresses of thermoscience and thermal engineering, many efforts have been devoted to heat transfer enhancement. Although a variety of techniques, such as addition of surface area, swirl flow devices, vibration of heated surface, vibration of fluid, are applied to enhance heat transfer, major improvements in heat transfer capabilities have been held back because of the low thermal conductivity of conventional heat transfer fluids. Improvement of the thermal properties of energy transmission fluids may become a promising trick of augmenting heat transfer.An effective way of improving the thermal conductivity of fluids is to suspend small solid particles in the fluids in order to change transport properties, flow and heat transfer features of the liquids. Traditionally, solid particles of micrometer or millimeter magnitudes were mixed in the base liquid. Although such solid additives may improve heat transfer coefficient, practical application are limited because of the fact that the micrometer and/or millimeter-sized particles settle rapidly, clog flow channels, erode pipelines and cause severe pressure drops.Modern nanotechnology provides new challenges and chance for thermo-science. A new class of heat transfer fluids called "nanofiuids" has been proposed. Nanofluids refer to a new kind of heat transport fluids by suspending nanoscaled metallic or nonmetallic particles in base fluids. Nanofluids are expected to exhibit heat transfer properties superior to those of conventional heat transfer fluids. Compared with the existing techniques for enhancing heat transfer by adding millimeter and/or micrometer-sized particles in fluids, nanofluids appear to be ideally suited for practical application with incurring little or no penalty in pressure drop because the nanoparticles are so small that the nanofiuid behaves like a pure fluid.The purpose of this paper is to demonstrate experimentally the feasibility of the concept of nanofluids. The research efforts to produce and characterize the heat transfer behavior of nanofluids, include such two main aspects as preparation ofnanofluids and experimental determination of the transport properties of nanofluids.Preparation of nanofluids is the first key step in applying nanophase particles to heat transfer enhancement. This paper presents a preparation method of nanofluids by directly mixing nanophase powders and base fluids. Some auxiliary dispersants are necessary to obtain the even distributed and stabilized suspensions. With this method, several sampled nanofluids with different volume fraction have been prepared. The TEM photographs are given to illustrate the stability and evenness of suspensions. Some factors of affecting the stability and evenness of nanofluids, such as the property of fluid, the dimension and the property of nanoparticle, the properties of dispersants, are discussed.Nanofluids exhibit superior heat transfer performance to conventional heat transfer fluids. One of reasons is that the suspended nanoparticles remarkably increase the thermal conductivity of the nanofluids. In this paper, the transient hot-wire instrument for measuring the thermal conductivity of sample nanofluids is developed. Detailed discussions on the measurement principle and error analysis are presented. The thermal conductivities of the oil-nanoparticles nanofluids and the water- nanoparticles nanofluids are measured by using this instrument. The effects of the volume fraction, dimensions, properties of the nanoparticles and temperature on the thermal conductivity of nanofluids are discussed. The theoretical study of the thermal conductivity of nanofluids is introduced to explain possible mechanisms contributing to enhancement of the thermal conductivity of the nanofluids. In addition, by analyzing the experimental results, a correlation of the thermal conductivity of the nanofluids is presented.In order to estimate the enhancing heat transfer performance of nanofluids, investigation on the viscosity of nanofluids is carried out. NXE-1 viscometer is ap...