Experimental Study On The Micro Flow Characteristics In Porous Media

Abstract: Porous media widely exist in the nature and industry, and their structuresare very complex and changeable. Studying the internal characteristics of the flowfield and pressure difference are important for understanding the complexmomentum, energy and mass transfer in porous media. In this study, the MEMStechnology is used to process two different kinds of porous media models. One is thekind of micro-cylinder porous media and the other is packed micro-beads porousmedia. The characteristics of the flow field and pressure difference in the porousmedia models are studied by Micro-PIV technique and high precisionmicromanometer in this dissertation.Firstly, the Micro-PIV technique is employed to measure the flow fields in themicro-porous channels by using20×objective and1μm tracer particle under theReynolds number of0.1~1. Based on the measured velocities, the distributions ofshear stress are calculated. The results show that the velocity distributions have goodsymmetry, which are in accordance with the flow characteristics of low Reynoldsnumber. In addition, two symmetric streamlines exist between square uniformarranged micro cylinders, on which the velocity vectors remain constant. Twoasymmetric streamlines exist between square non-uniform arranged micro cylinders,and the velocity vectors on the streamlines remain constant. The two-dimensionalflow fields on multi-fluid planes are acquired by the high precision platform ofdisplacement to construct the three-dimensional flow fields. The constructedthree-dimensional flow fields reflect the full field flow characteristics in the porousmedia model. The RMS of shear stress is calculated, and the results show that theRMS of shear stress in different porous media models has a linear relationship withthe Reynolds number.Secondly, the flow fields near the lower wall are measured in the packedmicro-bead porous media. In order to obtain the high quality particle images, theimage processes are implemented at first. The measurements show that the magnitude of velocity is higher at wide pore locations. The flow fields show that theporous media flow fields have obviously characteristics of random distribution. Thetransverse and longitudinal velocity fields are studied using probability densityfunction (pdf). The statistical method shows that pdf of transverse velocitycomponents is in accordance with normal distribution. The statistical results ofdimensionless velocity components are not sensitive to the flow rate andmicro-bead’s diameter. The statistical results are compared with available results inliterature, and despite some differences at the peak section, the statistical curves arebasically the same.Finally, the high precision micromanometer is used to measure the pressuredifference of air flow through the porous media of two micro-cylinder arrays andpacked micro-bead. The results indicate that the pressure difference have linearrelationship with the flow no matter how the structure change. The pressuredifferences decrease as the porosity increases, which have the similar tendencies withthe shear stress. This indicates that the pressure differences of the porous mediamainly come from the energy loss of shear stress. The permeability of micro-cylinderporous media are calculated based on the pressure differences and average velocityof cross section, the results show that there are different functional relationshipbetween permeability and porosity in different porous media structures.