Approximate analytical methodology for calculating friction factors in flow through polygonal cross section ducts

Abstract

An approximate analytical methodology for calculating the friction factor within ducts of irregular cross-section is herein proposed. The approximations are developed by transforming the original governing PDEs into simpler ODEs, using approximation rules provided by the coupled integral equations approach. The transformed system is directly integrated and analytical solutions for the friction factor are readily obtained. Four different approximation cases are analyzed, which yield simple closed-form expressions for calculating the friction factor in terms of geometric parameters for rectangular, triangular, and trapezoidal ducts. The results of these expressions are compared with literature data, and very reasonable agreement is seen. After performing an error analysis of the results, regions for applicability of the methodology where accuracy requirements can be maintained are highlighted. Finally, enhanced approximation formulas yielding maximum errors as low as 3% are developed by using simple weighted averages of different approximation cases.