Numerical investigation into the effect of cross-flow on the performance of axial flow fans in forced draught air-cooled heat exchangers

Abstract

Air-cooled heat exchangers (ACHEs) which utilise large arrays of axial fans, commonly suffer from inlet flow losses related to off-axis flow into the fans. This investigation aims to extend current knowledge on the effect of off-axis inflow on the performance of axial fans in this type of installation. An actuator disk fan-model was developed for the Computational Fluid Dynamics (CFD) code, FLUENT TM, and validated against experimental data for off-axis inflow angles up to 45°. Agreement between numerical and experimental pressure rise was good, although fan power consumption and fan static efficiency were under and over-predicted respectively. Experimentally observed trends were confirmed numerically: fan static pressure rise and efficiency were adversely affected, while fan power consumption was not significantly affected by the presence of cross-flow into the fan. The investigation revealed that while the torque characteristics over the outer portion of the fan blades are fundamental in determining the global fan power requirements, the net effect of cross-flow in this region is very small. Local variations of blade torque at diametrically opposed orientations more or less cancel each other out, explaining the independence of fan power consumption to cross-flow conditions. The adverse effect of off-axis inflow on fan static pressure rise was attributed to two factors: increased kinetic energy per unit volume at the fan exit, and greater dissipation through the fan itself. Off-axis inflow was found to affect fan-blade loading characteristics, with implications for blade fatigue.