The design of an industrial axial flow fan can take great advantage from the knowledge of performance limits and favourable design choices determined from its specific fluid-dynamic characteristics.

As for other turbomachinery types, this fact is generally experienced through the entire design flowpath, from the preliminary design phase to the final optimization.

Correlations, data and charts available from literature as well as proprietary database, exploited with many techniques (including most advanced machine learning) are resources in widespread use for this purpose.

Despite the fluid-dynamics of axial flow fans can be considered a well-known topic, nevertheless some specific points (e.g. the maximum achievable total-to-static efficiency) can be the subject of discussions, misunderstanding or bad design choices.

The present work addresses this problem in two parts.

In the first part a simple 1D model is built, for fans with and without stator, from classical theory of axial fans for pressure rise coefficient, head coefficient, flow angles and diffusion efficiency.

The most relevant quantities (e.g. total-to-total efficiency, total-to-static efficiency), obtained with the 1D model are represented on hill charts as a function of the non dimensional pressure rise and flow coefficient.

The above charts can be used as a preliminary design tool (or for educational purposes) to set the main design variables according to the required performance.

The 1D results are also reported in terms of specific speed and diameter, allowing a comparison with traditional charts (e.g. Cordier diagram) and with similar recent studies.

In the second part of the work the main characteristics of a 3D phenomenon, observed both in CFD simulations and in experimental performance tests, that limits the performance of axial fans for industrial applications at high pressure rise and low-to-medium flow coefficients (e.g. automotive cooling fans) are discussed and investigated in some detail. Moving along a non dimensional performance curve toward low flow coefficients, at a certain critical point the outlet flow shows a sudden topology change characterized by a strong divergence of the streamlines in the radial direction.

A first study of this phenomenon has been done by 3D RANS simulations carried out on different single rotor geometries (in free-discharge configuration, with either ducted and free inlet conditions), designed to avoid flow separations due to excessive blade load at the working conditions of interest. The 1D flow maps are used as a reference to explore the critical design region.

The analysis of CFD results highlighted two main aspects in the flow behaviour.

At first a local radial divergence of the outlet streamlines starts when the ratio between the circumferentially averaged tangential and axial velocity components (swirl ratio) reaches a critical value close to one; typically at the outlet hub region (i.e. where common vortex laws used for fan design produce the highest swirl ratio). This local divergence gradually covers an increasing blade span percentage as the flow coefficient is lowered.

An abrupt change of flow topology is obtained when a major portion of the blades span is interested by the above flow structure. The outlet vortex equilibrium is suddenly broken, leading to a completely radial flow.

The starting flow divergence has characteristics (flow topology and swirl ratio value) typical to the vortex breakdown issue. A similar flow behaviour has been observed in other turbomachinery types (e.g. axial pump inducers, horizontal axis wind turbines).

The final topology change has similarities to the flow features found in some studies on swirling jets at high swirl ratio.

The authors did not find in-depth studies on both phenomena in the context of axial flow fans.

A 1D analytical model has been derived and added to the efficiency map to detect the bounds that avoid the above flow structure, in order to support the fan design process.


Analysis of the Design Bounds in Performance Limits for Industrial Axial Flow Fans

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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