Numerical Study On Dynamic Behavior And Heat And Mass Enhancement Of Freely Rotatable Square Cylinder In Microchannel

In recent years,with the rapid development of micro-nano systems represented by micro heat sinks and micro mixers,the demand for heat and mass transfer in micro scale keeps increasing.The vortex-induced vibration(VIV)of cylinder in flow field is a common and typical kind of fluid-structure interaction problem.It is of great scientific significance to study the interaction between solid and liquid,the mechanism of cylinder motion and its influence on heat and mass transfer in this phenomenon.However,the researches on vortex-induced vibration pay more attention to the dynamic interaction between solid and liquid in large space under open conditions at present.There are few reports on kinetic patterns of solid structures under the action of fluid in microchannel.And the relevant applied research is also very rare.Therefore,the numerical investigation on the dynamic behavior and heat and mass transfer enhancement of a freely rotatable square cylinder in microchannel is conducted in this thesis.The detailed research contents and main results are summarized as follows:1.Construction of moving mesh numerical model.Based on the finite volume method and moving mesh technology,a numerical model for solid-liquid two-phase flow is established,which can precisely reproduce the momentum exchange between the liquid and square cylinder.The model continuously reconstructs the mesh during the motion of the square cylinder to adapt to the position of it.At the same time,the boundary layer mesh which has no relative motion with the square cylinder is constructed to ensure the accuracy of the force calculation at the wall of the square cylinder.Thus,the simulation of the motion state of the freely rotatable square cylinder in the open space was successfully realized,which laid the foundation for the subsequent research.2.Dynamic behavior of freely rotatable cylinder in microchannel.By simulating the motion state of the freely rotatable square cylinder in microchannel with the range of Reynolds number from 40 to 600 at different blockage ratios(Br = 0.25,0.4,0.55).The pressure,shear stress,torque,flow field around the cylinder and so on are measured.As a result,the motion state of the cylinder will change from static to periodic oscillation and finally turn to rotation with the increase of Re.In the cases of periodic oscillation,the equilibrium position tends to change from 0 to π/4 as Br increases.3.Heat and mass transfer enhancement in microchannel with freely rotatable cylinder.By applying uniform heat flux density to the upper and lower walls of the microchannel,the effect of freely rotatable square cylinder on the heat transfer in the microchannel is investigated.It is discovered that compared to a static square cylinder at the same Re,the freely rotatable square cylinder can disturb the original Poiseuille flow in the microchannel more significantly and destroy the thermal boundary layers on the heated walls,thus improve the heat transfer efficiency.The effect of the freely rotatable square cylinder on the mass transfer in the microchannel was studied by introducing two liquids with different concentrations at the entrance of the channel.The horizontal flow of the vortices can promote the mixing of solute in the microchannel.The mixing efficiency will be increased by 70.5% and 65500% compared to the no cylinder case and static cylinder case at Br = 0.25,Re = 400.