Study Of Microscale Nonlinear Flow Characteristics And Flow Resistance Reducing Methods

Microscale flow mechanism was studied in this dissertation based on the theory of intermolecular force between solid and liquid interface. From microflow experiment results and theoretical analysis, influence of boundary layer on microflow characteristics were carried out. The low velocity nonlinear flow mechanism of low permeability reservoir was clarified and the intrinsic reason of threshold pressure gradient phenomena can be revealed by this research.Nonlinear flow characteristic was found under low velocity microscale flow. Deionized water flow through microtubes with radius from 2.5μm to 10μm was investigated. Nonlinearity characteristic was appeared under low pressure and velocity; the smaller the tube's radius, the more obvious of the nonlinearity. Quadratic function fit the nonlinear flow well which showed that velocity was composed of three parts, boundary layer impact part(second order), fluid viscosity part and the threshold pressure gradient part. Generally, experimental flux is smaller than theoretical flux and is getting greater with the decreasing of the tube radius. The thickness of the boundary layer which intensively influenced by solid and liquid molecular force is altered as pressure changes. Thus, the changeable flow cross section induces microflow shows nonlinear characteristics. Function between thickness of effective boundary layer and pressure gradient was brought out. The thickness is related to the microtube radius and the pressure gradient.Reynolds number (Re) is the criterion of nonlinear flow. The lowest Re achieved from microflow experiments is near 10-5, and it is found that the departure of the resistance coefficient (f) from experimental value to theoretical value is significantly increased with the decreasing of Re and the radius. Analyze on relationship between f and Re reveal that Re can be the criterion of nonlinear flow. Critical Re is 1*10-3, and nonlinear characteristic began when Re is smaller than critical value.Microflow experiments obtained the flow equation and found the relation of flow parameters and the flow radius. Afterwards, regression method was used to establish the relationship between threshold pressure gradient and the microtube radius. It was testified that the results have high accuracy. Numerical solutions are very close to analytical solutions. The relationship between threshold pressure gradient and the radius can easily predict the threshold pressure gradient under microflow with a certain radius.For reducing microflow resistance force, methods to decrease solid and liquid interface molecular force were considered. Experiments results show that the resistance force was decreased as the temperature increases and the flux was increased at the same time. The greater the fluid viscosity is, the more significant the nonlinear characteristics show. Moreover, the threshold pressure gradient exponentially increased with the viscosity increasing. Wettability alteration of the solid wall can change the flow characteristics as well. Therefore, increasing temperature, altering wall feature, decreasing fluid viscosity and adding some surfactant may reduce the thickness of the fluid boundary layer to some extent. As a result, the flow resistance force can be decreased.