Pressure Loss Coefficients of Ductwork Fitting Combinations With and Without Vanes (Baffles)

Abstract

Ductwork components in ventilation systems are often installed in close proximity to each other due to space constraints. Published pressure loss coefficients are, however, only valid if there are sufficient distances between components. In this work, pressure losses of common combinations of two, three, and four bends and the addition of internal vanes were investigated with computational fluid dynamic simulations and validated with reference data. Pressure losses of combinations of bends without vanes were higher or lower than the sum of the single pressure losses of the components (combination effect), depending on the components and orientation. For bends with abrupt deflections, strong combination effects occurred; in some cases, pressure losses doubled. The spacing between the components was also a relevant factor. Combination effects were most pronounced for spacing lengths of the same order as the length of the flow separation region of the upstream bends. Consequently, certain spacer lengths were particularly unfavorable. Combination effects were found to be complex to predict and fluid simulations proved to be useful for analyzing the interactions. Adding vanes was found to be very useful. For bends and combinations with sharp deflections, the pressure loss coefficients were reduced by a factor of ten with vanes. Vanes also effectively reduced detrimental combination effects. The combination pressure losses with vanes were in all cases lower than the sum of the single components pressure losses. Pressure loss coefficients for combinations with vanes had a strong dependency on the Reynolds number. Furthermore, the downstream flow distributions were more homogeneous with vanes.