CFD Analysis of Axial Flow Fans for CSP Air-Cooled Condensers

Application of Analytical, Computational and Experimental Methods

The MinWaterCSP project aims to reduce the water consumption of concentrating solar power (CSP) plants by 75 to 95% relative to wet cooling systems and to improve plant efficiency by 2 to 3 % relative to current dry-cooled systems by introducing novel dry/wet cooling technology. This hybrid cooling system will make use of large axial flow fans to condense the process fluid.
As part of the project a new high efficiency, reduced noise axial flow fan was developed and manufactured for a 24 ft. full scale test facility at Stellenbosch University (constructed as part of the MinWaterCSP project). This fan is referred to as the M-fan. The M-fan was designed with an optimised hub-tip ratio and minimised exit kinetic energy velocity profile. At its design flowrate of 333 m³/s and a blade setting angle of 34°, full scale 3-D CFD simulations using ANSYS Fluent predicted a pressure rise of 114.7 Pa at a fan static efficiency of just below 60%. This full-scale simulation of the fan was also performed using the actuator disk method and indicated a fan static pressure rise of 111.4 Pa and a fan static efficiency of 63.1%.
The M-fan was also simulated at a diameter of 1.5 m diameter. At a design flowrate of 14.4 m3/s and a blade angle of 34° it indicated a fan static pressure rise of 116.5 Pa , and fan static efficiency of 57.9% . The fan is also simulated with a range of tip gaps which are found to reduce fan performance.
Concurrent RANS computations were also carried out using the simpleFoam solver of the OpenFOAM library for CFD computations with the non-linear closure of Lien et al. to further verify the performance of the fan and analyse the flow field in the blade-to-blade passage.
Two other fan configurations were also designed, namely the L3C2-fan and the T-fan. They where were designed for reduced noise signature and improved efficiency by adjusting the exit velocity profile, compared to the M-Fan. In particular, the L3C2 was obtained by optimizing the chord and pitch distributions of the M-Fan for reduced trailing edge noise, leading to a completely different shape of the rotor blade. With the T-Fan the design paradigm of the rotor was changed and a forced-vortex distribution of the work along the blade span was enforced to reduce the load at the tip of the blade. CFD computations were carried out also on these configurations, to verify performance of these new rotors.