Understanding pressure loss mechanisms in rectangular air duct elbows: The role of guide vanes

Abstract

This study numerically investigates flow characteristics and pressure losses within 90° rectangular duct elbows, with a particular focus on flow separation, secondary flows, Dean vortices, and frictional losses. Three-dimensional simulations were conducted to evaluate the impact of guide vanes on loss reduction across a range of curvature radii from 1 to 6 inches. The steady Reynolds-Averaged Navier-Stokes (RANS) equations, associated with the standard k-ε turbulence model, were solved to obtain the flow fields. Key flow parameters, including side surface area, secondary kinetic energy, and curvature ratio, were utilized to quantify secondary flow and Dean vortex behavior. Results demonstrate that flow separation constitutes the primary loss mechanism at smaller radii, whereas secondary flows and Dean vortices become increasingly influential as the radius increases. The analysis identified three distinct performance regions: a high gain region where guide vanes effectively suppress flow separation, resulting in significant loss reductions; a marginal gain region characterized by moderate loss reductions where the benefits of vanes are less pronounced; and a negative gain region beyond a critical curvature ratio of 5, where increased frictional losses from the vanes outweigh their advantages, leading to higher overall pressure losses. This critical curvature ratio marks the transition point where the interplay between diminished secondary flow intensity and an expanding wetted area determines the vane's effectiveness. These findings offer valuable insights for optimizing guide vane placement and curvature radius to achieve efficient elbow designs, tailored to specific operational and economic requirements.