General engineering analysis

Case Study

Design optimisation of an axial impeller hub

Air Radiators Pty Ltd of Geelong, Australia - now part of the Austrim group of companies, used LUSAS finite element analysis to minimise weight, increase strength and minimise the cost of their high grade aluminium alloy mid diameter industrial axial impeller. The impeller varies from 700mm to 1100mm diameter and operates at speeds ranging from 720 to 2880 rpm.

The fans are used extensively in commercial and industrial installations where large air volumes have to be moved. Operating conditions vary from extremes of temperature in applications such as blast freezers and smoke spill fans to the highly corrosive environments encountered in brick drying kilns. Other installations range from general air-conditioning, air blast heat exchangers and air extraction systems. The new hub design was required to satisfy all current operating conditions and in addition, cut manufacturing costs and improve product quality. To achieve this, the number of parts in the assembly was reduced from 27 to 2, rationalising tooling and reducing assembly times.

LUSAS was used to analyse the existing hub and determine the stresses and displacements in both this design and a new proposed design. Model data was imported using the LUSAS CAD Toolkit. Due to the complexity of the geometry, a full 3 dimensional analysis was needed using 3D parabolic brick elements. A comprehensive linear stress analysis was carried out for a range of loads such as radial blade loading, bolt clamping forces, bending moments induced by impeller air pressure development and centrifugal forces. These load cases were combined using the Load Combination facility in LUSAS.

The results focused design efforts in the areas of the hub that showed the highest stress levels and also enabled the designer to meet the stringent dimensional and cross-sectional restraints of the pressure die casting process. The analysis also provided the designers with a comparison of the stresses between the new and existing fan designs, giving a level of confidence in the final design. Using LUSAS for this analysis has saved Air Radiators Pty Ltd time and money building and testing physical prototypes.

Stress on a 30 degree section of blade seat

Stresses on mid-part of blade seat

Air Radiators are now considering automating the whole analysis process for their entire range of fans by using the parametric command facility in LUSAS. Using this facility, engineers can generate prompting menus for use during the input of model and design data, and automate the geometry generation, meshing and loadcase assignment for components of similar shape. The parametric command language can also generate results plots and graphs for reports that are consistent and comprehensive. As the parametric approach builds-in the expert knowledge of the FE analyst, Air Radiators Pty Ltd can use this feature as a way of retaining accumulated expertise whilst permitting junior engineers to run many "what if" trials to a consistent QA standard.



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