Achievable Total-to-Static Efficiencies of Low-pressure Axial Fans

G3 - Fan Efficiency

Limits regarding total-to-static efficiency of axial fans are investigated as a function of design point. The design points examined are limited to the typical realm of axial fans according to the Cordier diagram. Initially, a purely theoretical approach is taken in which the optimal hub-to-tip ratio as well as the optimal swirl distribution are found by a local optimization algorithm. Friction losses are neglected. This approach yields a first estimate of the effect of design target in terms of flow coefficient and pressure coefficient on unavoidable exit losses due to the kinetic energy in the meridional and circumferential flow velocity downstream of the fan. However, realistic estimation of exit losses and hydraulic losses requires more advanced methodologies. Here, the realistically achievable total-to-static efficiency is estimated using an evolutionary optimization algorithm in which the target function is evaluated by multi-layer perceptrons (MLPs). The MLPs were trained based on the CFD simulation of 13,000 characteristic fan curves in an earlier study. Assuming a tip clearance ratio S/D = 0.001 and Reynolds numbers around 200,000, the maximum achievable total-to-static efficiency amounts to 68% and can be realized with medium specific fan speeds and comparatively large specific fan diameters. At most design points, the difference between theoretically optimized and MLP-optimized efficiency is mainly due to hydraulic losses. However, at some design points the MLP-optimized fans also feature significantly higher exit losses. Finally, the MLP-optimization is repeated with distinct geometrical constraints which are (i) imposing fixed sweep angles, (ii) restricting the allowable axial depth and (iii) avoiding undercuts. The impact of all three constraints changes with design point.