The Analysis And Experimental Researches Of Dynamic Stability, Physical Properties And Convective Heat Transfer Characteristic Of TiO₂-Ammonia Water Nanofluids

Nanofluid is more efficient than ordinary fluid in heat transfer, and its'stability is better than that of the micron-fluid and milli-fluid, so it has been concerned by so many researchers. At the same time, energy supply is really tight. And the ammonia water absorption refrigeration system can use the low-grade energy which can relatively save energy, but how to improve its'COP and miniaturize the equipment has been a problem. So the nanofluid used in the ammonia water absorption refrigeration system seems to be a subject which is worthy to be researched. But if the nanofluid is used in the whole system, the dynamic stability of the ammonia water nanofluid should be known. So this subject means to study the dynamic stability of the ammonia water nanofluid and its convective heat transfer characteristics. And the researches of which kind of nano particles is more stable in the ammonia water have found that:a-TiO2 is more suitable for the ammonia water. So a-TiO2-ammonia is choosed as the work fluid for all the researches. All the work can be divided into the following parts:(1) Theoretical study of thermal conductivity of TiO2 nanofluid:the new calculation model of thermal conductivity is proposed and the drag reduction of nanofluid in some special cases is analyzed.On the basis of the classical model, the new calculation model takes the influence of the dispersion agent and the axial thermal conductivity of cylindrical like nanoparticles into account. The calculated values are compared with the experimental date in the literature and the existing models'dates. It can be found that the new model's calculating date is more close to the experimental date.(2) The building of the experiment platform for the researches of the dynamic stability, physical properties and the convective heat transfer of TiO2-ammonia water nanofluid:the experiment platform can be divided into two parts, one is the dynamic stability and physical properties of TiO2- ammonia water nanofluid part, and the other is the convective heat transfer part. The dynamic stability test section is a closed pipe channel which uses the pump to provice the circulation power of the fluid. The convective heat part is the casing pipe, the fluid in and out of the pipe is the countercurrent flow.(3) The experimental of TiO2-ammonia water nanofluid's dynamic stability and physical properties: the TiO2-ammonia water nanofluids which particle concentration is 0.1%wt,0.2%wt,0.3%wt and 0.5%wt (each concentration will be divided into containing or not containing two kinds) are added into the experimental platform. And the fluids will be uninterrupted circulating flowed in the system for 72 hours. The values of the absorbance, density, and surface tension are measured for every 3 hours. All the dates will be analyzed, and it can be found that the absorbance of the TiO2-ammonia water nanofluild which is under dynamic condition is decreasing, but overall the dynamic stability is much better than the static ones. And the mechanism for dynamic stability is different form the mechanism of the static state. Viscosity and surface tension are shown to be reduced over the whole process, but the reduction is not large.(4) Researches for convective heat transfer characteristic:In this experiment, the Wilson plot method is accepted. At first, water is used to check weather the heat convection tube is proper. And it can be found that the convective heat transfer coefficient obtained by the Wilson plot method is much closer to the date calculated by the classical formula. And the error is about 5%, it can be deemed acceptable. Heat transfer experiments for different particle convective were carried out, and the results showed that the heat transfer performance of the TiO2-ammonia water nanofluid was significantly enhanced when it was compared with the base fluid. And the maximum enhancement is 15.1% when the particle concentration is 0.3%.