Numerical Simulation Of Fluid Flow Driven By Temperature/Chemical Concentration

The flow phenomena driven by temperature gradient or chemical concentration gradient are not only common in daily life,but also widely exist in industrial applications.Such flow phenomena mainly include Marangoni effect and natural convection.In this paper,a numerical method which can simulate this kind of problem is developed.Combined with numerical simulation and theoretical analysis,the movement of self-rewetting droplets on a plate with temperature gradient and the fluid behavior in the process of reactor heating are studied.The main work is as follows:(1)Based on the conservative Level-Set interface capture method,a new method is proposed to deal with the multiphase interfacial flow with Marangoni effect caused by chamical concentration gradient.The improved diffusion convection equation is used to make the miscible fluid meet the normal non-penetration condition at the interface.The intermediate step advancing volume fraction of miscible fluid is proposed to ensure the compatibility between the diffusion convection equation and the conservative Level-Set method.Therefore,the Marangoni effect caused by chemical concentration gradient can be simulated numerically.Meanwhile,we also discuss the Marangoni effect caused by temperature and the simulation method of natural convection.Through several examples,the algorithm is verified from many aspects,which shows the accuracy and robustness of the method.Finally,we apply the program to the flow problem of droplets in multi-fluid environment,and show the potential computing power and application value of the method.(2)The movement of self-rewetting droplets on a plate with temperature gradient is investigated numerically,and the influence of surface tension gradient on the deformation and displacement of the droplets is discussed.Self-rewetting fluids are different from normal fluids(e.g.oil,water,etc.).The surface tension coefficient of self-rewetting fluids varies nonlinearly with temperature,i.e.first decreasing and then increasing.This change is approximated as a quadratic function in our discussion.First,the center of mass of the droplet is placed at the minimum of the surface tension coefficient.We find that the droplet deforms symmetrically and there are two motion modes,i.e.elongation and deformation modes,to characterize whether the droplet can continue to spread.The critical condition of the two modes is quantitatively given by force analysis of the droplets.Further,we study a more general case,that is,the initial position of the droplet is near the minimum of the surface tension coefficient.It is found that the migration of the droplet obeys an exponential function with time at early stage.Also,we discuss the displacement and deformation relationship of the droplet qualitatively.(3)The fluid flow behavior during the heating process of the reactor was studied by numerical simulation.The experimental results show that the cross-section of the graphene oxide and phenol-formaldehyde resin(GO/PF)produced by the hydrothermal reaction.in the reactor has an annular structure.However,we found that this structure is closely related to the flow in the reactor.By simulating the movement of rod-shaped particles in the reactor,we revealed that the orientation of graphene oxide nanosheets and the flow line direction in the reactor are very consistent,which explained the formation reason of GO/PF materials.Based on the generation and decay of plume,we found three flow modes:no plume,plume persistence and plume decay.The critical conditions of plume generation and the behavior characteristics of plume decay are discussed.