Streamlining Performance: Drag Reducing Riblets for Boosting Industrial Axial-Fan Efficiency

Streamlining Performance: Drag Reducing Riblets for Boosting Industrial Axial-Fan Efficiency


Investigating the effects of Riblets on the efficiency of high performance axial fan blades.


In-depth CFD modeling, followed by extensive tests on specific testing facilities.


Demonstration of the significant efficiency improvements for axial fans by applying Riblet surfaces.

Figure 1: High-performance axial fan in a wind tunnel
Project details
Industrial fans play a pivotal role in various sectors, serving as essential components for processes ranging from ventilation and cooling to material handling, combustion and creating air flow in a wind tunnel. As industries continue to strive for enhanced productivity, reduced energy consumption, and enforced environmental sustainability, the significance of increasing efficiency and performance of industrial fan equipment becomes a major goal. Improved fan efficiency directly translates to reduced operational costs, minimized energy consumption, and decreased environmental impact. The achievable efficiency increases through continuous improvements of existing technology approach a limit and are minor if a technology is already highly mature. In such situations, only disruptive innovation and new technologies enable significant improvements.
The potential of Riblet surfaces to reduce fluid-dynamic drag in turbulent flows and increase the efficiency of facilities is significant and proven. Regarding industrial fans, the application of Riblet surfaces enables performance increases to an extent which is hardly possible through other measures. Over this project’s course, we examined the application of Riblets to the blades of a high performance axial fan stage together with TLT Turbo. Starting with extensive CFD simulations, an optimal Riblet design was developed. After applying the Riblet structures to the facility, the effects of Riblets on the fan stage were investigated for a performance map with variations in blade angles, rotational speeds and different Riblet film mappings.
For the best possible configuration of Riblets, the design process starts with a baseline simulation, which means the numerical analysis of the untreated axial fan. This numerical investigation started with a steady state RANS (Reynolds Averaged Navier Stokes) simulation of a single passage domain with a mixing plane model interface between rotor and stator and a 10 million hex-cell mesh. For the steady state simulation, a transition model was used to determine the transition line on the rotor and the stator. To further improve the accuracy of the results, a second setup with a Large Eddy Simulation was performed. Three different operating points were calculated for the blade angle, the design point, a low flow-rate and a high flow-rate operating point. To accurately calculate the ideal Riblet size the boundary layer was resolved with y+ below 1. The resulting flow field enables the calculation of the ideal Riblet sizes for the given geometry and flow by the use of our in-house developed Riblet algorithms. From this ideal configuration, a feasible real Riblet distribution is derived. The real Riblet distribution aims to achieve the highest possible impacts by the use of single size Riblets on plastic film.
Figure 2: Unsteady baseline-simulation showing the static pressure on the rotor, stator and hub
The testing was carried out on a model fan test bench designed according to ISO 5801. Besides measuring the temperature and pressure at six planes along the flow path, the test bench was also capable of measuring acoustic conditions. On the downstream side, an additional auxiliary fan was used to extend the maximum flow rate and enable the measurement of a wider range of the fan performance map.
Figure 3: TLT Turbo testing facility


The results of the measurements show a significant efficiency increase with Riblets at the design point of the facility. Also the operating points apart from the design points were affected significantly by the applied Riblet surfaces. The overall best efficiency was achieved with smooth film on the leading edge and Riblets on the pressure and suction side of rotor and stator.
The findings contribute to the understanding of optimizing rotating machines with Riblets. This project successfully demonstrated the power of Riblet surface structures for improving the performance of industrial fans and the technology’s readiness for extensive industrial applications. Regarding the maturity of fan technology, the measured efficiency increase is significant and constitutes a considerable progress. Furthermore, this project serves as a successful use case for following applications of Riblets in industrial fans.