Analysis of FDS 6 Simulation Results for Planar Air Curtain Related Flows from Straight Rectangular Ducts

Abstract

CFD results are discussed for jet flows through a straight square duct, which is an interesting configuration in the context of air curtain flows for smoke and heat control in buildings in case of fire. The CFD package Fire Dynamics Simulator, Version 6.0.1, is used. Special focus is given to the impact of the inlet boundary condition on the flow field in the near-field region. Investigation of different orifice configurations (W = 2 cm width, variable span-wise length), including calculations inside a straight square duct (2 cm × 2 cm, with variable length) ahead of the air orifice, reveals a small vena contracta effect when the orifice is flush with a solid boundary, leading to an acceleration of the flow in the symmetry plane in the near-field region. The vena contracta effect disappears if the co-flow at the nozzle exit is aligned with the jet. More important is the effect of the duct length (precursor domain length, serving as method to generate inflow turbulent conditions for the main computation): imposing a top hat velocity profile, a sufficiently long duct (i.e., L = 20W) is required for the flow to become fully developed at the orifice. The CFD results confirm an analytical correlation for the ratio of the entrance length to the hydraulic diameter of the duct as function of the Reynolds number, provided the duct width is used as characteristic length scale. Using a sufficiently fine mesh, i.e., 10 cells across the characteristic dimension of the nozzle, the evolution of the mean and RMS stream-wise velocity along the centerline, as well as their profiles across the nozzle width, are shown to be captured accurately in the CFD results.