The Influence of the blade aspect ratio on the pressure rise, the efficiency and the flow in axial turbomachinery is still an area of research that is not yet investigated in detail. For compressors this subject is treated more often in literature but most of these investigations are done for compressor cascades. The design parameters of axial fans are in some points different from compressors. Due to these differences smaller blade aspect ratios result for axial fans. Especially for small aspect ratios examinations are missing.

Challenging for any experimental investigation and validation is the view into a rotating turbomachinery. Often probe measurement technology in the form of five-hole or hot-wire probes is used for the investigation of the flow. The results of such measurements are influenced by the size of the probe, especially if details like turbulence quantities are to be measured. The choice of the measuring range and measuring position is limited by the accessibility to the flow area. The installation space is small so that problems arise to measure the entire area of interest. An alternative is the use of Laser Doppler Anemometry (LDA) measurement technology. With this method the components of the velocity vector can be recorded contactless with a high temporal and spatial resolution.

In this contribution an axial flow fan is designed with four different blade aspect ratios. These four blade aspect ratios are obtained by using four different number of blades. The blade aspect ratios are varied between 0.2 and 0.6. For the investigation the thickness ratio distribution and the spacing ratio are kept constant in the design process for all four blade aspect ratios. This means the blade thickness, the spacing and the blade length are adjusted for every blade aspect ratio.

Analysing small aspect ratios is challenging. From literature it is known that secondary losses and profile losses can only be examined separately up to an aspect ratio of 0.5. Using smaller blade aspect ratios the secondary losses of the shroud and the hub meet in the middle of the blade height so that the secondary losses and the profile losses cannot be separated anymore. To make an approach to quantify profile losses at the mid span of the blades for blade aspect ratios smaller than 0.5, 2D simulations at the mid span are carried out for supplementary examination. The Reynolds number of all investigations is related to the chord length of each blade. It is varied from 2.38x10e5 to 7.13x10e5. During the survey, the same stator with a constant blade aspect ratio as well as a constant spacing ratio is used.

In this contribution LDA is applied as the experimental method. Transient numerical simulations are performed for a full 3D simulation model to obtain a detailed view into the machine. Because of the low Reynolds numbers the k-w-SST transition model with the intermittency function is applied. With both methods, the flow is investigated on different planes inside the fan which are perpendicular to the machine axis. The velocity distribution on the planes are analysed and compared with the transient simulation results. In the boundary layer the velocity is dissolved as well with the laser technique.  


Analysis of the Losses in an Axial Fan With Small Blade Aspect Ratios Using CFD-Technique and Laser Doppler Anemometry

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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