Turbomachines can operate under various conditions with respect to its rotation speed and flow rate. Although this adaptability, the performance of the machine degrades as the operating condition leaves away from the design point. Turbo compressors especially, have operation limit in lower flow rate region due to the occurrence of flow oscillation like rotating stall or surge. Present report introduces efforts to visualize such phenomena in centrifugal compressor by utilizing PIV (Particle image velocimetry) technology.
The world’s first test facility that allows the visualization of cavitation on a rotating inducer in both cryogen and water was used for comparing the cavitation features in liquid nitrogen and water. The test inducer was a triple-threaded helical one with a diameter of 65.3 mm and a rotational speed range of 3500–6000 rpm. The inducer was made of Inconel superalloy, and the transparent casing was made of quartz glass. In addition, a mechanism to preserve the alignment of the casing with the inducer axis during thermal expansion or compression was used, so the tip clearance remained almost uniform and constant. The appearance of the cavitation was “foggy” in nitrogen and “foamy” in water. This difference can be explained by the maximum stable size of cavitation bubbles derived from the critical Weber number theory. Based on this theory, each cavitation bubble in nitrogen is four times smaller than that in water. The difference of each cavitation bubble size seems to be a cause of the thermodynamic effect.
Improving the theoretical prediction of propeller cavitation requires an investigation of unsteady propeller blade cavitation. We recently developed a new combination-line charge-coupled device camera-based method for measuring three-dimensional shapes that is faster and more accurate than conventional methods. To verify the system’s effectiveness, model experiments were conducted in the National Maritime Research Institute’s large cavitation tunnel using the highly skewed propeller of a training ship “Seiun-Maru-I”. In this paper, we introduce the measurement system and experimental results obtained for a model propeller blade and cavity shapes and show the effectiveness of the developed measurement system.
Compact centrifugal blowers with vaned diffusers are often used in home electric vacuum cleaners. To achieve high efficiency and low noise, it is important to improve the performance of impellers and diffusers. In this study, authors used a large solidity diffuser whose flow duct is low in height. Authors studied the influence of internal flow on the diffuser performance by using PIV and CFD. From our measurement results, low velocity was observed in the middle span section and the pressure side in the middle of the overlapped region. Area of the low velocity region became larger with the decrease of flow rate. From our vizualization results, the flow vortexes near the throat were observed along the pressure surface side of the vane. The inlet flow angle of the diffuser was decreased by the flow vortexes.
The combination of experimental and numerical analyses is essential to the elucidation of unsteady flow phenomena in turbomachinery. From this point of view, an instantaneous pressure field measurement technique and a smart visualization technique for CFD results are described in the present article. In the instantaneous pressure field measurement technique where 30 high response pressure transducers and a spatial-temporal interpolation of the pressure transducer signals are utilized, time-series data of the instantaneous pressure distribution on the casing wall of one pitch blade passage are obtained to capture transient phenomena in unsteady flow fields of rotating cascades. In the smart visualization technique for CFD results, the identification of vortex cores and topology analysis of limiting streamlines, which are based on the critical-point theory, are performed to extract fluid dynamic knowledge from CFD results. An application example of the instantaneous pressure field measurement technique and the smart visualization technique is presented, in which those are applied to the elucidation of the spike-type stall inception in an axial flow compressor rotor.