Wide angle diffusers with passive boundary-layer control

The effect of surface material and roughness on the conductance of a channel at molecular flow was measured for N2, Ar, He, and H2. The experimental results were compared with the results of Monte Carlo calculations. The channel consisted of two parallel disks with different surfaces including polished stainless steel (SS), roughly polished SS, quartz, Ti, Cu, Al, alumina with smooth surface, alumina with rough surface, Au coating, Pt coating, and diamondlike carbon (DLC) coating. The conductances of the experimental channel depended on the slope of the surface roughness, rather than the arithmetic surface roughness (Ra) or the material. In the case of smooth surfaces, such as polished SS, quartz, and DLC coating, the experimental results for N2, Ar, and H2 show good agreement with the calculation, assuming diffuse reflection. The momentum accommodation coefficients on these surfaces for N2, Ar, and H2 were estimated to be in the range from 0.98 to 1.00, based on the assumption that specular reflection wa...

Effect of surface material and roughness on conductance of channel between parallel disks at molecular flow

The conical diffuser is one of the oldest and most versatile hydraulic devices, however its performance is often less than optimal. This is because boundary layer separation and instabilities in th...

Effects of boundary layer suction control on flow through an axisymmetric diverging channel

Usage of a wide-angle diffuser may result in unfavorable separated flow and a significant diffuser loss. To improve the performance of the diffusers, inserting short splitter vanes is known as a useful method that has been demonstrated experimentally. Regarding the role of the vane in the diffuser flow, Senoo & Nishi (1977) qualitatively explained that the lift force acting on the vane should be a key factor. However, its quantitative verification remains since then. To challenge this issue, numerical simulations of incompressible flow in a wide angle of 28° two-dimensional diffuser with and without a short splitter vane were conducted in the present study. An improvement of pressure-recovery by the vane and oscillatory flows in the diffuser are reasonably reproduced from comparison with the experimental results made by Cochran & Kline (1958). It is also found that the lift force acting on the vane varies periodically in an opposite phase with the detachment point moved back and forth on a diverging wall, since one vane is not sufficient to fully suppress the flow separation that occurred on the wall and the incoming main-flow shifts toward the other diverging wall in the diffuser. Thus, as a role of splitter vane in the diffuser, “the lift force of the vane is a key factor” may be quantitatively verified from the present numerical simulation. Further, it is confirmed by the local loss analysis that the turbulent kinetic energy production observed in mixing layers contributes most of the loss in the diffuser. Consequently, the present numerical technique may be usable to investigate the flow character in a diffuser with splitter vanes at a design stage.

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A Numerical Study on the Flow Mechanism of Performance Improvement of a Wide-Angle Diffuser by Inserting a Short Splitter Vane

In this study, a new computational modeling of the gas-surface interaction is proposed to explain the results of the scattering experiments of molecular beams from solid surfaces, especially from industrial surfaces. The characteristic feature of the model is to settle the adsorbed gas layer and the collision layer which involve adsorbed molecules and surface molecules, respectively. Incident molecules experience force due to the gas-solid potential gradient, changing their trajectories which is computed by Molecular Dynamics method. The gas molecules are scattered from the surfaces after collisions with adsorbed or surface molecules. The simulated results are compared with the experimental ones: i.e., flux distributions, TOF spectra and the average energy of scattered molecules.

Computational modeling of gas-surface interaction

Experimental studies were carried out on the caracteristics of diffusers of rectangular cross section which have an angle of expansion 2θ=30°, 60°, 90°, and 180° and an area ratio of 4. Remarkabel improvement of the pressure coefficient and energy loss coefficient is achieved by means of suction at the entrance. It is found that the most effective position of suction is at the point right down stream of the corner. The diffuser efficiency increases with increase of aspect ratio. The efficiency, however, tends to be constant as the aspect ratio exceeds four. When the angle of expansion is 20°, 30°, 60°, and 90°, the rate of optimum suction is 3 to 4 per cent of the total flow rate. With this optimum suction, the efficiency of these wide angle diffusers approaches that of the diffuser of 2θ=10°.

Performance of the Two-Dimensional Diffusers with Suction at the Entrance

A high-intensity molecular beam apparatus has been constructed and the spatial distributions of argon and nitrogen beam scattered on polycrystalline platinum surfaces were measured. A skimmer-orifice having a new configuration, and related measuring devices were constructed in order to carry out an accurate measurement of the flux distribution. In relation to the flux distribution of reflected molecules, diffuse scattering patterns are obtained for surface temperatures lower than 100°C, while the flux distributions obtained at surface temperatures higher than 100°C can be explained by the simple superposition of diffuse and lobular scattering patterns. The pattern of the lobular reflection qualitatively agrees with the hard-cube theory.

Tetsuo Fujimoto

Papers

Performance of the Two-Dimensional Diffusers with Suction at the Entrance

Experiments on Scattering of Molecular Beams on Polycrystalline Platinum Surfaces