Research On The Transport Mechanism Of Turbulent Fluid Across Porous Media And The Law Of Interface Slip

The porous medium is a substance composed of a solid skeleton and pores divided by a skeleton.Natural cracks and artificial cracks in oil and gas production,heat pipes,building envelopes and insulation materials can all be regarded as porous media.Heat and mass transfer between porous media and fluids is a complex coupling process that is widely used in various fields of production and engineering applications.For example,in oil and gas mining;the coupling flow of oil and gas between the oil and gas layer and the crack,the coupling flow and heat exchange between the working medium and the porous capillary in the heat pipe.The transmission mechanism between the fluid and the porous medium is very complicated,and the turbulent flow is more difficult to solve.In particular,the slip effect at the interface when the turbulent fluid traverses the porous medium has a great influence on the fluid flow in the mixed region and the transport mechanism between the fluid and the porous medium.Therefore,the influence of the treatment of the mixed area interface and the slip effect is the focus of many studies.Based on the previous studies,this thesis studies the transmission mechanism and slip law of turbulent fluid across porous media by combining theoretical analysis,numerical simulation and experimental verification.In this thesis,the mixing region of turbulent fluid across the porous medium is simplified into a two-dimensional physical model.The same mathematical approach as Mohais was chosen to describe the flow of turbulence in the fluid region.The pure fluid region is regarded as a channel,the channel wall is equivalent to a porous medium wall,and the perturbation method is used to solve the nonlinear equation with the Reynolds number as the perturbation.Unlike previous studies,the shear jump conditions proposed by Ochoa-Tapia and Whitaker were used at the interface between the fluid and the porous medium.Analytical solutions related to flow and heat transfer are obtained by using shear force jump conditions as the main boundary conditions.Through the analysis of the analytical solution,the effects of permeability coefficient,channel width,shear jump and effective dynamic viscosity on the flow and heat transfer in the channel were studied The variation of the axial velocity in the channel with the characteristic parameters and the heat transfer sensitivity are obtained A two-dimensional turbulent mathematical model characterized by the characteristic parameters(h,K,?,?]of the mixed region is establishedThe finite volume method is used to simulate the mixing region of fluid and porous media at macro scale.In this thesis,the single-zone method is used to simulate the fluid flow in the mixed region,considering the free flow and porous media as a whole.The single-zone method is used to solve the mixed-zone problem,which can satisfy the continuity of the boundary conditions of interface temperature,shear force,velocity,pressure and related flow without processing the interface.The user defined function(UDF)in fluent is used to define the entrance velocity function of fluid and the source function of transport equation.The governing equations of fluids and porous media are different.By controlling the source term defined by UDF in fluent,we achieve the goal of controlling the whole mixed area with a set of control equations,thus realizing the single-domain method The accuracy and feasibility of the method are verified by comparing with the numerical simulation of the two-domain method Then,the influence of different physical parameters on the interface slip characteristics between fluid and porous media is discussed,and the mechanism of velocity slip effect is summarized,which lays a foundation for the study of heat and mass transfer.A visualized fluid and heat transfer test bench was set up to experimentally study the heat and mass transfer of fluid across the porous medium,especially the slip effect at the interface.This laboratory can directly obtain the flow condition inside the fluid porous medium by using Ultrasound Doppler Velocimetry(UDV).The flow distribution of the fluid and the porous medium can be obtained by measuring the instantaneous velocity of the multi-point.The influence of the characteristic Parameters of the porous medium such as porosity and permeability on the slip and the velocity distribution of the entire flow field can be studied by changing the bottom bed of different porous media.It is found that the Reynolds number and the relative water level have a great Influence on the slip velocity.The slip velocity at the interface increases with the increase of the Reynolds number and decreases with the increase of the relative water depth.The effect of porosity on slip speed is small.The free flow is multiphase flow and the size of the porous medium is small enough that the capillary force will not be negligible.The phase change heat of the two-phase flow slip under capillary action needs to be further studied Based on the unidirectional flow simulation,the VOF model and UDF are used to simulate the two-phase flow model of the porous medium and the fluid mixing region by adding the capillary force equal volume source term.The experimental results show that the maximum pumping height of the porous medium is determined by the capillary force provided by the internal microchannel under the premise that the volume fraction gradient remains basically unchanged.