Computing Research on Nanoparticle Thermophoresis Deposition in Supercritical Carbon Dioxide Fluid

The movement and deposition of nanoparticles in supercritical carbon dioxide is a current international frontier subject. In the development of new nuclear reactors, using supercritical carbon dioxide Brayton cycle instead of traditional steam Rankine cycle can greatly improve the cycle efficiency, reduce equipment size and improve safety. In this new cycle, the movement and deposition of nanoparticles play a significant role under normal and abnormal operating conditions in nuclear power plants. Nanoparticles, due to their small particle size and higher relative surface energy, are more penetrable. When it under the thermophoresis force will emerge the thermophoresis deposition, causing collision and erosion to the pipeline. And it may be more destructive to supercritical devices or systems. To study the movement and deposition characteristics of nanoparticles in supercritical media, then further explore the factors affecting nanoparticles deposition, and find their qualitative and even quantitative relationship. This paper studies this problem from the aspects of particle size, wall temperature difference, incoming flow velocity, and so on to try to clarify the motion mechanism of nanoparticle deposition. In specific studies, stainless steel nanoparticles and supercritical carbon dioxide were selected as the objects, a 1 meter long horizontal straight pipe was taken as the flow geometric channel, based on the control single variable method, the factors affecting the thermophoresis deposition of nanoparticles were calculated and analyzed by Fluent software. According to the calculation to obtain the curve picture, the results show that the higher the fluid temperature, the greater the temperature gradient, and the greater the thermophoresis force on nanoparticles, which increases the dimensionless thermophoresis deposition velocity and the thermophoresis deposition rate. In addition, the increase of fluid temperature leads to the decrease of fluid viscosity and viscous resistance of particle movement, which also promotes the movement of particles and increases their deposition rate to a certain extent. In terms of particle size, the change of thermophoresis deposition rate caused by the change of particle size on a small scale (1100 nm) is more obvious than that on a large scale. With the increase of particle size, the thermophoresis force increases, but the increasing speed slows down. At the same time, the viscous resistance and lift increase in geometric multiples with the particle size. Under the combined action of these forces, the thermophoresis deposition efficiency decreases. The pipe diameter and flow velocity do not directly affect the thermophoresis deposition rate. The pipe diameter increases the average distance and reduces the thermophoresis gradient, while the flow velocity affects the heat transfer coefficient. In addition, the increase of external flow velocity enhances the carrying capacity of fluid, and nanoparticles are easier to be coerced out of the channel, thus reducing their thermophoresis deposition. Totally, for thermophoresis deposition, the temperature difference between the fluid and the wall is the most important factor affecting the deposition rate, and there is a positive correlation. The flow velocity, particle size, and pipe diameter are negatively correlated with deposition rate.