This paper presents effects of Reynolds number on the flow field and performance in a transonic centrifugal compressor impeller. Steady-state single passage RANS simulations with the low-Reynolds number k-omega turbulence model have been conducted to investigate the effects. The machine Reynolds number was changed in the range of 10⁴ to 10⁶ under the similarity rule in turbomahines. The substantial changes in vortical flow structures and separated flow patterns are described by taking advantage of the data mining techniques: vortex identification and limiting streamline topology analysis with the line integral convolution (LIC) method. The simulations also show the performance deterioration with Reynolds number decrease, but its trend is somewhat different from the expectation from the previous research papers.

Recently, fans have been used for cooling electric devices. A sirocco fan has been used in narrow places especially because it has the characteristics of high pressure and high flow rate. But the noise of a sirocco fan becomes loud, so it's very important to reduce the noise of a sirocco fan. So far, there were a lot of attempts to reduce the noise of the sirocco fan with large number of blades such as 40∼80 blades by performing experiments and numerical analysis. However, there is little knowledge for reducing the noise regarding the sirocco fan with small number of blades. There is the advantage that can reduce the production cost of the sirocco fan if it has small number of blades. The purpose of this study is to clarify the flow field around the blades of the small sirocco fan with small number of blades (Z=22, 11) by using LES compared with general number of blades (Z=40). As a result, it was clarified that the pressure fluctuation on the blade surface in the case of Z=22 was high compared with Z=40 and Z=11 at the maximum efficiency point. Because the spacing between the blades in the sirocco fan with Z=40 was smaller than Z=22, which restricted the flow separation on the suction side surface due to pressure fluctuation.

Recently noise reduction of a small axial flow fan for cooling electronic devices is required. When obstacles such as motor stays are located in back of a fan, noise of an axial flow fan becomes higher. In this study, numerical simulations were conducted by using LES (Large Eddy Simulation) with DSM (Dynamic Smagorinsky Model) about an axial flow small fan in order to clarify the noise reduction mechanism at high flow rate. The flow pattern around the fan blades and pressure fluctuation on the fan blade surfaces were compared under the conditions of different flow rates. The fan noise at high flow rate (Q=0.030 m³/s) is lower 1.9dB(A)than the noise at design point flow rate (Q=0.025 m³/s). The pressure fluctuation at pressure side surface of blades at high flow rate becomes low compared with the pressure fluctuation at design point flow rate. The axial flow velocity at the blade tip increases at high flow rate, which decreased pressure fluctuation on pressure side blade surface due to the interference between tip vortex and blade surface, and decreased pressure fluctuation on suction side surface due to the boundary layer separation.