Near-field characteristics of drift under crossflow conditions

초록

A drift from a cooling tower is an important research topic in both academic and practical perspectives due to a similarity to the jet in crossflow and possible relationship to Legionella transmission. The dynamic and thermal characteristics of a drift under crossflow are investigated numerically using an Eulerian-Lagrangian approach and droplet phase change model, focusing on the near-field development under varying Grashof numbers and velocity ratios. Large eddy simulations are performed for the base, low atmospheric temperature, and low velocity ratio (LVR) cases. The development of counter-rotating vortex pair (CVP) in a LVR range shows the opposite trend to that in a high velocity ratio range. Unlike the jet centerline, the droplet trajectory shows higher sensitivity to the Grashof number and distinctive double inflection points in the LVR case. A modified semi-empirical formula for the drift radius is presented with improved prediction accuracy. Time evolution of a two-dimensional probability density function (PDF) on the Lagrangian droplet phase presents two distinctive temperature regimes based on the droplet ejection location and explains the mechanisms driving a branch shape in the PDF. The histograms of the droplet mass show higher sensitivity of smaller droplets to the phase change and increased Grashof number. The LVR case shows distinct droplet evolution mechanism due to the compact and strong CVP collecting hot and small droplets inside. The higher intensity CVP increases the mass of smaller droplets via increased condensation, while stronger crossflow momentum decreases the mass of larger droplets via mixing.

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