Effect of Flow Structure on Air-Water Mixing Process at Different Bend Angles

Abstract

—The secondary flow generated within curved ducts accompanies a substantial change in flow structure due to the imbalance between centrifugal forces and transverse pressure gradient. The current study investigates the effect of changing bend angle on the evaporative cooling of an airstream via spray of water in fogging mode. The experimental Particle image velocimetry (PIV) technique has been implemented for visualizing the secondary flow layout when an air-water mixture travels through the bend duct. The bend angles considered are (45 o , 90 o and 135 o ) at velocities of (5, 3.75 and 2.5 m/s) for air flow through 50 cm square duct. PIV results manifested that a single pair of counter rotating vortices appears at 45 o bend angle, while the other two bend angles evinced two pairs of vortices (four-cell pattern) across the duct cross-section. The water spray was set to tilt at three orientations, namely (45 o , 0 o and – 45 o ) to the axial flow direction to suit the change in bend angle. ANSYS FLUENT 19.R1is used in numerical simulation with RNG-k-ε turbulent model to study the spray characteristics which impacts the cooling process. The best cooling of the airstream is obtained when the water is sprayed axially, i.e. at 0 o , prior to the 135 o bent duct ranking 30% reduction in temperature corresponding to 6.5% improvement in heat transfer between the two phases.