Explicit vs Implicit Numerical Modelling of Air-cooled Condenser Fans
E1 Methods and Instruments for Transient Phenomena
The numerical analysis of the flow field about large-scale air-cooled condensers (ACCs), which typically find application in the thermoelectric power industry, represents an enduring challenge to engineering efforts. Numerical analyses of ACCs are complicated by the scale of the systems and the innate complex flow phenomena that transpire during their operation. By virtue of their scale, computational fluid dynamic (CFD) models of ACCs are typically constrained to utilizing some or other implicit formulation to represent the system’s large array of axial flow fans. However, customary implicit axial flow fan model formulations are only able to provide a limited approximation of actual fan performance within the ACC’s complex flow environment, especially under conditions characteristic of operation during windy periods. Accordingly, these implicit fan models limit our ability to derive quantitative information from the associated ACC simulations over all operating points of interest. Therefore, to contribute towards deriving new understandings that can facilitate the improvement of these implicit fan models (and ultimately ACC wind effect analyses), an explicit 3D fan model capable of delivering detailed insight into ACC fan behaviour mechanics has been developed.
This explicit model is analysed under low inlet flow rate conditions characteristic of those under which actuator-disk type implicit fan models suffer. The aerodynamic information determined by the explicit 3D model is compared to the airfoil characteristics used by a 2D-referenced and a 3D-corrected implicit actuator-disk fan model. The differences between the data sets are then highlighted to gauge the potential shortcomings of the implicit models, guide improvement efforts and advance current understanding of 3D effects in the context of low pressure-rise, low hub-to-tip ratio axial flow fan operation. The results of this paper show the important influence the velocity evaluation method has on the description of 3D effects and the results suggest that potential improvements to the 3D-corrected actuator-disk model may be found through a revised treatment of the near-hub augmentation and the scaling of the airfoil polar correction with flow rate.