Investigation On Enhanced Heat Transfer Of Nanofluids

With progresses of thermoscience and thermal engineering, many efforts have been devoted to heat transfer enhancement. Although a variety of techniques are available 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 carrier 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.Modem nanotechnology provides new challenges and opportuneities for thermo-science. Anew class of heat transfer fluids called "nanofluids" has been proposed. Nanofluids refer to a new kind of heat transport carriers 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.The purpose of this paper is to demonstrate experimentally and theoretically the feasibility of the concept of nanofluids. The research efforts for preparation and characterization the heat transfer behavior of nanofluids are indicated from the following aspects.1 Preparation and stability of nanofluidsThis paper presents a preparation method of nanofluids by directly mixing nanoscaled 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 ofnanoparticle, the properties of dispersants, are discussed.2 Experimental measurements on transport properties of nanofluidsNanofluids exhibit superior heat transfer performance to conventional heat transfer fluids. One of the 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. The thermal conductivities of some sample 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 mechanism analysis of the thermal conductivity of nanofluids is introduced to explain possible mechanisms contributing to enhancement of the thermal conductivity of the nanofluids.In order to estimate the enhancing heat transfer performance of nanofluids, investigation on the viscosity of nanofluids is carried out. NXE-l viscometer is applied to measure the apparent viscosity of the sample nanofluids in this paper. The viscosities of some sample nanofluids are given. Some factors of affecting the apparent viscosity of the nanofluids, such as the volume fraction, the shape, the dimension and the property of nanoparticle, are discussed.3 Aggregation structure and thermal conductivity of nanofluidsRandom motion of the suspended nanoparticles is a typical Brownian motion. Such irregular motion and aggregation process of the nanoparticles are simulated with the theory of Brownian motion and the DLCA model. The fractal dimension is a suitable parameter to indicate the aggregation structure of nanoparticles clusters. With considering physical properties of both the base liquid...