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PhD Defence Jie Wang | Design for high efficiency of low-pressure axial fans: Use of blade sweep and vortex distribution

Design for high efficiency of low-pressure axial fans: Use of blade sweep and vortex distribution

Due to the COVID-19 crisis the PhD defence Jie Wang will take place (partly) online.

The PhD defence can be followed by a live stream.

Jie Wang is a PhD student in the research group Engineering Fluid Dynamics (EFD). His supervisors are prof.dr.ir. C.H. Venner and dr.ir. N.P. Kruyt from the Faculty of Engineering Technology (ET).

Rotor-only low-pressure axial fans with small hub-to-tip diameter ratio (HTR in short) are widely used in many branches of industry, especially for cooling and ventilation purposes. As energy- related products that account for a significant proportion of the consumption of natural resources and energy, optimization of their aerodynamic performance for high efficiency is important to reduce environmental impact. Differently from low-pressure axial fans with medium to high HTR, extensive regions of backflow near the hub are often present downstream of fans with small HTR.

In studies of low-pressure axial fans, Computational Fluid Dynamics (CFD) simulations have been frequently employed to analyze in detail the aerodynamic performance and the flow fields inside these machines. For optimization, according to investigations on axial fans with medium to high HTR, sweep, dihedral and skew of the blades stacking line as well as different vortex distribution designs form important methods. However, for fans with small HTR, only few studies have been reported in the scientific literature on appropriate CFD simulation strategy, three-dimensional stacking and vortex distribution design method.

The objective of this thesis is to investigate the effects of sweep, dihedral and skew on the aero- dynamic performance of low-pressure axial fans with small HTR and develop an optimal vortex distribution design method for high efficiency of such fans. CFD simulations are extensively used in these investigations.

Computational Fluid Dynamics Simulation Strategy

Firstly, in order to develop guidelines for obtaining accurate CFD predictions for such fans, validation simulations of a baseline axial fan with small HTR have been performed. The experimental and computed aerodynamic performance characteristics have been compared.

These CFD guidelines pay special attention to the trailing edge shape, presence of non-aerodynamically shaped blade sections, tip gap, and employed turbulence model. The results for the fan studied here show that the actual (rounded) trailing edge is necessary; the main blade (without non- aerodynamically shaped blade sections) well represents the aerodynamic performance of the whole fan blade; it is recommended not to take the tip gap into consideration (in an industrial context) due to the inadequate predictions of its influence on the aerodynamic performance. The use of the  Spalart-Allmaras turbulence model is advised for giving better agreement with measurements.

Effects of Sweep, Dihedral and Skew on Aerodynamic Performance

Secondly, investigations on axial fans with medium to high HTR have shown that forward sweep  of blades can give improved aerodynamic performance, especially for the total-to-total efficiency. Effects of sweep, dihedral and skew in axial and circumferential directions (in forward and backward  direction) on the aerodynamic performance of small HTR fans are investigated, with a linear stacking line.

The CFD results show that forward sweep and circumferential skew are beneficial for higher total-to-total efficiency and that higher total-to-static efficiency can be obtained by forward dihedral and axial skew. The backward shape variety generally gives negative aerodynamic effects. Forward sweep and circumferential skew shorten the radial migration path, but more flow separation is present near the hub. With forward dihedral and axial skew, the backflow region is reduced in radial size and axial extent, but a more significant hub corner stall region is found. The pressure reduction due to sweep and dihedral is more limited than what could be expected from wing aerodynamics.

Optimal Vortex Distribution Design Method

Finally, the vortex distribution (polynomial in spanwise coordinate) and the HTR have been determined by maximizing the total-to-static efficiency of a baseline axial fan with small HTR. For free vortex designs, analytical expressions for the maximum total-to-static efficiency and the optimal  HTR have been formulated. By combining the vortex distribution with a suitable choice for the spanwise lift coefficient distribution, fan blade designs have been established.

The CFD results for these designs show that the free and the polynomial vortex distribution designs satisfy the desired pressure rise, with significantly improved total-to-static and total-to-total efficiency (maximum improvement by 3.9% and 4.6%, respectively).

Flow field analyses show that no flow separation is present in the blade-to-blade plane, except near the hub region. For designs with small HTR, some backflow is present downstream of the rotor which affects the flow separation near the hub blade section.

Overall, the investigations in this thesis contribute to better understanding of small HTR axial fan aerodynamics. The results can be applied to the design of low-pressure axial fans with high efficiency.