Experimental Investigation Of Heat And Momentum Transfer Enhancement Of Nanofluids

Nanofluids, prepared by adding a certain proportion of metal or nonmetal nanoparticles into conventional fluids, is a new type of heat transfer or cooling medium. Nanofluids has the following advantages over conventional fluids. Firstly, Convective heat transfer coefficient does not increase at the expense of the flow resistance. Secondly, the size of particles in nanofluids is the nanometer level and its behavior is close to liquid molecules to avoid wearing away or blocking the tube. Finally, the effect of heat transfer of nanofluids depends on the type, proportion and shape of nanoparticles and thus heat transfer process of nanofluids could be properly designed according to the requirement of heat transfer quantity. Consequently, nanofluids is a developing heat transfer or cooling medium with abroad application future.There are two main reasons causing heat transfer enhancement of nanofluids, the increase of thermal conductivity and the movement of nanoparticles which could lead to momentum transfer enhancement of base fluid.The research of thermal conductivity of nanofluids has been carried out by the theoretical and experimental method. At the same time, the physical mechanism of the increase of thermal conductivity is confirmed. However, the effect of nanoparticles on the momentum transfer characteristic of base fluid is rarely reported. To this end, this work is carried out by us.Experimental method is adopted to study the heat and momentum transfer characteristic of nanofluids in this paper. The main contents are as following:Two-step method is used to prepare the nanofluids. It is found that the nanofluids appears well dispersion and suspension by the experiment of transmission electron microscopy and suspension stability.To master the basic characteristic of nanofluids, thermal physical parameters of nanofluids is measured. It is found that the thermal conductivity, viscosity and specific heat capacity of nanofluids varied with the volume concentration, diameter and shapes of nanoparticles.Experiment table is built to measure the fluid flow and heat transfer characteristic. The results show that enhancements of convective heat transfer coefficient of nanofluids achieve without at the cost of flow resistance coefficient. Thermal conductivity has little effect on the convective heat transfer coefficient; Volume concentration, diameter and shapes of nanoparticles have higher impact on convective heat transfer coefficient; Single-phase flow convective heat transfer correlation is not available for calculating heat transfer characteristic of nanofluids, thus creating a convective heat transfer correlation for nanofluids. Through the research above, it plays a decisive role in enhancement the convective heat transfer coefficient by the way of disturbance base fluid with nanoparticles, which could lead to momentum transfer enhancement.The flow visualization test rig is established for observational flow field. In order to compare the flow difference between the nanofluids and the base fluid more easily, the wavy-walled tubes are made by plexiglass. The electrochemical method by means of gain and losing electrons is adopted to measure the mass transfer coefficients of nanofluids, by which the momentum transfer characteristic of nanofluids could be studied quantity. The study found that the disorder degree of nanofluids is higher than base fluid at the same Re. Meanwhile, the mass transfer coefficient is also significantly higher than the base fluid at the same Re. Based on above experiment results and combined with the existing molecular dynamics and CFD simulation results, the theoretical analysis of the effect of nanoparticles on the momentum transfer characteristic of base fluid is presented, which further validate that disturbance base fluid with nanoparticles leads to momentum transfer enhancement, plays a decisive role in enhancement the convective heat transfer coefficient.The heat transfer enhancement is limited by the way of optimizing thermal physical parameters.In order to further enhance the momentum transfer, the way of imposition disturbance on the nanofluids is employed. Changing tube type, pulsating flow field and vibration could continue to increase the heat and mass transfer coefficient. The study found that the wave amplitude of wavy-walled tube has a significant impact on the performance of mass transfer and fluid flow; The effect of operating parameters, Reynolds number, proportion of pulsating flow and Strouhal number, on the mass transfer enhancement are discussed at pulsating flow field; Vibration frequency, vibration angle and filling ratio have important influence on heat transfer characteristics of nanofluids; The flow pattern in the tube becomes more chaos after imposition disturbance on the nanofluids. The results of mass transfer and visualization indicate that the imposition disturbance on the nanofluids further enhance the momentum transfer, which benefit heat and mass transfer.