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

Abstract 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 numbers of blades. The blade aspect ratios are varied between 0.2 and 0.6. For this 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 chord are adjusted for every blade aspect ratio. Analysing low aspect ratios is challenging. Previous literature treating compressors suggests that at low aspect ratio the secondary losses and the freestream losses combine, making it difficult to separate the profile loss from the secondary flow losses. An approach to quantifying profile losses at the midspan of the blades for blade aspect ratios lower than 0.5, 2D simulations at the midspan 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 2.38 × 105 to 7.13 × 105. 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 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-ω-SST transition model with the intermittency function is applied. With both methods, the flow is investigated on different planes inside the fan. The velocity distribution on the planes is analysed and compared to the transient simulation results. In the boundary layer, the velocity is resolved as well using the laser technique.