Colloquia

Samenvatting

Low-pressure axial fans are widely used in industrial applications for ventilation and cooling purposes, mainly to displace high volumes of flow with a low pressure difference over the fan. Due to EU's regulations, the efficiency of fans driven by motors with an electric power input is required to be improved. It has been shown that for axial fans with low hub-to-tip ratio, regions of backflow or secondary flows may occur. Backflow causes secondary flow losses and decrease in efficiency. The objective of this research is to improve the efficiency and investigate the influence of vortex distribution on the backflow area of a baseline Howden DLM fan. The vortex distribution describes the energy transfer from the fan blade to the flow and it determines the specific work and pressure rise over the fan. 

In order to improve the efficiency of the Howden DLM fan, new fan blades have been designed according to optimizations of the vortex distribution and hub-to-tip ratio. In the design method, the fan performance is optimized using a classical approach, which relies on empirical methods and simplified analyses. It involves the dimensionless performance parameters, two dimensional inviscid flow theory and cascade analysis, with a prescribed vortex distribution along spanwise direction. A sequential quadratic programming (SQP) approach that finds a minimum of a constrained nonlinear multi-variable function, is used to find the optimized vortex distribution and hub-to-tip ratio that minimizes the exit kinetic losses of the fan and hence maximizes the total-to-static efficiency.

The newly designed fan blades are analyzed by flow visualization using the CFD software package Numeca Fine Turbo. The pressure and velocity field are described by the continuity equation and the Reynolds-averaged Navier-Stokes momentum equation. Effects of turbulence are captured by the Spalart-Allmaras turbulence model. Although the efficiency has not been improved, the extent of the resulted backflow area of the newly designed fan blades are decreased from approximately 40% to 30% in radial direction, compared to the Howden DLM fan blades. No good agreement was found between the flow velocity and pressure field of the fan resulting from the design method study and CFD, due to 3D viscous flow effects. Further research is required to evaluate whether the approach in the design method study is suitable to predict the influence of the vortex distribution on the backflow area.