Investigation On Particulate Fouling Deposition Characteristics Of Nanofluids On Heat Transfer Surface

As a new kind of heat transfer medium with high heat transfer coefficient,nanofluids have very broad prospects and great potential economic value in the field of heat transfer enhancement.As a heat transfer medium,whether it can maintain excellent heat transfer performance for a long time and whether it may deposit on the heat transfer surface to reduce the heat transfer performance are the important factors which affect nanofluids whether to be widely used.Therefore,the research of nanofluids is imperative for the deposition characteristics of nanofluids and the effect of heat performance which affect by the fouling.For this purpose,nanoparticle suspension and nanofluids are chosen as the research objects.The deposition characteristics of nanoparticle suspension and nanofluids on the heat transfer surface have been investigated.Furthermore,the mechanism of particles agglomeration and deposition behavior has been revealed by molecular dynamics simulation to track the trajectories of microscopic particles and the force between the particles.Firstly,in order to study the effect of heat flux,inlet temperature,particulate concentration and mass flow rate on the characteristics of particulate agglomeration and deposition on the heat transfer surface,γ-Al₂O₃/water suspension is chosen as the working fluid in the experiment system of annular channel.Both single-phase flow state and subcooled-flow boiling state are considered.The results show that the effect of heat flux on the fouling resistance under the two flow states has an inverse relationship,and there exists a minimum value of fouling resistance between these two states.In the single-phase flow state,the asymptotic value of fouling resistance decreases with increasing the inlet temperature.In the subcooled-flow boiling state,the effect on fouling resistance by the inlet temperature is negligible.The effects of concentration and mass flow on the fouling characteristics under these two states are same.The asymptotic value of fouling resistance increases with increasing the concentration and decreases with increasing the mass flow rate.Secondly,in order to study the effect of heat flux,mass flow rate,inlet temperature,kinds of surfactant and nanoparticles on the characteristics of nanofluids deposition on the heat transfer surface,γ-Al₂O₃ nanofluids prepared by two-step method is chosen as the working fluid in the experiment system of annular channel.The heat transfer performance varies before and after fouling formation is considered.The results show thatγ-Al₂O₃ nanofluids can maintain good heat transfer performance in the first 20 hours under single-phase flow state,which is difficult to deposit on the heat transfer surface.The asymptotic value of fouling resistance increases with increasing the heat flux and the inlet temperature.The appropriate heat flux and inlet temperature are crucial to keep the heat transfer performance after fouling formation.The mass flow rate has the same effect on the fouling characteristics and the heat transfer characteristics.Increasing the mass flow rate can reduce the fouling formation and improve the heat transfer performance.The fouling characteristics of nanofluids prepared by different kinds of surfactants and different kinds of nanoparticles present obvious differences under certain condition.Therefore,the deposition on the heat transfer surface could reduce by choosing the appropriate surfactants and nanofluids under different heat transfer conditions.Finally,according to the factors which affect the agglomeration of nanoparticles and deposition characteristics by the experimental method,molecular dynamics simulation method is used to study the effect of temperature on the agglomeration of particles and the characteristic of particles deposit to the heat transfer surface.The factors such as particle size,wall temperature and the velocity were analyzed as well.The factors which affect the agglomeration and the deposition characteristics of the particles are explained from microscopic by tracking the trajectories of the microscopic particle,the variation of microcluster structure,counting the interparticle force and the energy transfer from interparticle.The results show that the collision and agglomeration of the particles are accelerated with increasing the fluid temperature.The time of particles with larger size for the first collision to the surface is longer,and the particles with smaller size are easier to adhere to the surface.The particles have a movement trend to move from high temperature surface to low temperature surface.The larger temperature difference is,the more obvious the trend.The particles deposit on the surface significantly affected by the velocity of the fluid.With the increase of velocity,the particles are more difficult to deposit on the surface and only agglomerate in the fluid.The simulation results are consistent with the characteristics obtained by the experiments.