Optimization of fan characteristics under various flow-rate

CFD SUPPORT is an engineering company dealing primarily with the CFD and FEA simulations, and also other activities related to numerical simulations of physical phenomena. Its own-developed TCAE software is a comprehensive environment of standalone modules for engineering simulations.

Challenge

Large centrifugal fans may be used for various industrial applications, such as in ventilation units for manufacturing facilities. Although the fan presented in this study is not a part of a used solution, it was derived from an existing fan, for which the comparison of the CFD results with measurement has been made.
The study shows a comprehensive analysis of main fan characteristics. These were observed for the varying volumetric flow rate, which is a typical flow parameter for analysing the fan performance. Additionally, the impeller geometry has changed during the analysis to find the most promising fan configuration for a given range of volumetric flow rate and a fixed volute geometry.

Fig. 1. Centrifugal Fan in the volute

Six parameters (volumetric flow rate, blade radius, blade thickness, number of blades, blade angle, b1,2 – blade depth), and six outputs (efficiency, total pressure difference, power, flow number, compression number and axial force) had to be evaluated in the study. During the analysis, the Uptimai tool successfully dealt with discontinuities in results, localizing them properly while suppressing oscillations in results. The created surrogate model can be used for the prediction of centrifugal fan characteristics without additional CFD computations.

Solution

At the beginning of the project, the Uptimai tool was very easily connected to the automated CFD analysis process used by the CFD Support company in their TCAE simulation environment. This loop received values of input parameters from the Uptimai software and was able to automatically generate the geometry of the fan, create the computational mesh, and run and post-process the CFD simulation. Values of input parameters were ordered by Uptimai’s smart algorithm and then it used CFD results to build surrogate models of the fan’s characteristics.

Fig. 2. Simulated flow through the fan

The Uptimai Sensitivity Analysis (Fig. 3.) confirmed the importance of the volumetric flow rate through the fan as the main flow parameter, which has to be considered in the fan design. From the impeller parameters point of view, the width of the impeller (b12) and the number of its blades have a main effect on the overall performance. Partially, it is due to the fact these inputs are interacting with the volumetric flow rate – the best setting of geometric variables depends on the flow rate. Thus, these interactions need to be considered when designing the fan to be working well in a wider range of the flow rate.

Fig. 3. Sensitivity Analysis of the Total Pressure Difference

Then, in the Uptimai Increment plot (Fig. 4.) is shown exactly how the efficiency of the fan is dependent on the flow rate and the impeller width. As indicated by the zero efficiency, it can be noted that the impeller of very low width is incapable of handling high flow rates. It suggests the width b12 should be higher than 0.2m to ensure there will be a reasonable efficiency of the fan for the whole range of possible flow rates. Also, it is the way to avoid excessive axial forces which might lead to an eventual fan malfunction.

Fig. 4. Fan efficiency (left) and axial force (right) based on impeller width and volumetric flow rate

The Uptimai Histogram Plot (Fig. 5.) shows the actual potential of the design under various operational conditions. There is a high probability to reach an efficiency of about 80% with a maximum of 86%. On the other hand, the histogram confirms the possibility of designs not matching specific flow rate conditions. As demonstrated above, these can be avoided by restricting ranges of geometry parameters.

Fig. 5. Probability distribution of efficiency of all theoretically possible designs

New ranges of inputs were identified using the Uptimai Preliminary Optimization tool which proposes input distributions leading to the statistical improvement in results. Once the volumetric flow rate was one of the input parameters, the optimization tool could be also used to visualize the flow rates best fitting for the current design concept. As seen in Fig. 6., it was generated for all outputs and these were compared against each other. It showed the best efficiency for flow rates about 25m/s and was checked if there is a conflict with e.g. axial forces.

Fig. 6. A trade-off can be easily done by comparing of Regions of Preference/Avoidance generated for different outputs

Benefits

  • Definition of influential parameters. There is only a minor effect of selected impeller parameters other than its width and also the number of blades.
  • Comprehensive design review. Taking multi-objective considerations into account for optimization of different outputs with specific constraints.
  • Identification of fan usability. Investigation of the design limits of all the variables to assure a correct fan operability.