This paper presents an experimental investigation of laminar gas flow through microchannels. The independent variables: relative surface roughness, Knudsen number and Mach number were systematically varied to determine their influence on the friction factor. The microchannels were etched into silicon wafers, capped with glass, and have hydraulic diameters between 5 and 96 μm. The pressure was measured at seven locations along the channel length to determine local values of Knudsen number, Mach number and friction factor. All measurements were made in the laminar flow regime with Reynolds numbers ranging from 0.1 to 1000. The results show close agreement for the friction factor in the limiting case of low Ma and low Kn with the incompressible continuum flow theory. The effect of compressibility is observed to have a mild (8 percent) increase in the friction factor as the Mach number approaches 0.35. A 50 percent decrease in the friction factor was seen as the Knudsen number was increased to 0.15. Finally, the influence of surface roughness on the friction factor was shown to be insignificant for both continuum and slip flow regimes.

No abstract

The basic physics of the underwater implosion of metal tubes is studied using small scale experiments and finite element simulations. A series of under-water implosion experimentshave been conducted with thin-wail aluminum alloy 6061-T6 tubes. The nominal tube dimensions are 2.54 cm outside diameter and 30.48 cm length. Two cylinders collapsed at their natural buckling pressure of 6895 kPa gauge pressure (1000 psig). Two additional cylinders were caused to implode at 6205 kPa gauge pressure (900 psig) using an initiator mechanism. Each of the four cylinders faiied with a mode 2 shape (collapsed shape is fiat with two lobes). The near field pressure time-history in the water is measured at a radial distance of 10.16 cm (4in.)from the centerline at three points along the cylinder's length. The pressure time-histories show very similar behavior between the cylinders which buckled naturally and those which were mechanically initiated at 90% of the buckling pressure. To aid in understanding the physical implosion phenomena, a computational model is developed with a fiuid-structure-interaction finite element code (DYSMAS). This model is validated against the experimental data, and it is used to explain the features of the implosion pressure pulse and how it is physically created.

Poiseuille number, the product of friction factor and Reynolds number (fRe) for quasi-fully-developed gas microchannel flow in the slip flow regime, was obtained numerically based on the arbitrary-Lagrangian-Eulerian method. Two-dimensional compressible momentum and energy equations were solved for a wide range of Reynolds and Mach numbers for constant wall temperatures that are lower or higher than the inlet temperature. The channel height ranges from 2 μm to 10 μm and the channel aspect ratio is 200. The stagnation pressure pstg is chosen such that the exit Mach number ranges from 0.1 to 1.0. The outlet pressure is fixed at atmospheric conditon. Mach and Knudsen numbers are systematically varied to determine their effects on fRe. The correlation for fRe for the slip flow is obtained from that of fRe of no-slip flow and incompressible theory as a function of Mach and Knudsen numbers. The results are in excellent agreement with the available experimental measurements. It was found that fRe is a function of Mach and Knudsen numbers and is different from the values by 96/(1+12Kn) obtained from the incompressible flow theory.

Underwater implosion experiments were conducted with thin-wall glass spheres to determine the influence that structural failure has on the pressure pulse. Four experiments were conducted with glass spheres having an outside diameter of 7.62 cm, thickness of 0.762 mm, and an estimated buckling pressure of 7.57 MPa. The experiments were performed in a pressure vessel at a hydrostatic pressure of 6.996 MPa. The average peak pressure of the implosion pressure pulse was 26.1 MPa, measured at a radial distance of 10.16 cm from the sphere center. A computational fluid structure interaction model was developed to assess how the failure rate of the glass structure influences the pressure time history. The model employed a specified glass failure sequence that is uniform in time and space. It was found that for the conditions of the test, a glass failure rate of 275 m/s provided a reasonable representation of the test data. The test data and the model results show that the failure time history of the structure has a significant influence on an implosion pressure pulse. Computational prediction of an implosion pressure pulse needs to include the failure time history of the structure; otherwise it will overpredict the pressure time history.