Blade Design and Optimization of Low Pressure Axial Fans using a Cascade Approach

Application of Analytical, Computational and Experimental Methods

Background: Low to intermediate solidity axial fans (0.4 ≤ sigma ≤ 1.2) can often be found in heating, ventilation and air conditioning (HVAC) systems. In many applications (e.g. automotive cooling fans) the aerodynamic-design aims for high flow rates at low pressure rises. Therefore those fans usually have no stator, a low hub-to-tip ratio (≤ 0.4) and few blades.
While cascade based design strategies are by definition used for high solidities (sigma ≥ 1), the isolated airfoil approach is valid for low solidities only (sigma ≤ 0.7). For medium solidity fans the literature recommends mostly the isolated airfoil approach with additional empirical corrections to take interference-effects into account. With the aim of using a cascade based fan-design for low to medium solidities, a new design methodology with suitable cascade-data was developed.

Used Methods: For the purpose of low to medium solidity fan-blade-design a MATLAB based program was developed. This tool consists of three main classes and generates the three-dimensional fan geometry in the following manner:
1. Prediction of the velocity triangles at in- and outlet according to the required operating point using a general fixed vortex approach
2. Determination of best efficiency airfoil data (e.g. deviation and incidence angles) based on cascade simulations
3. Generation and stacking of two-dimensional blade-sections allowing for a flexible incorporation of blade sweep
4. Aerodynamic load corrections to take blade-sweep into account
The efficiency of this initial design is further improved by doing 3D-CFD based optimizations using OpenFOAM. Therefore an optimization procedure with nine geometric parameters is used.

Results: Comparing the predicted pressure rise and the velocity triangles with 3D-CFD data, a good agreement can be found. Together with the optimization procedure a fast and reliable design process for low-to-medium solidity fans was developed. The paper shows the design principles using the example of an automotive cooling fan to get reliable initial designs in minutes. Furthermore the optimization strategy is described and results are discussed.