Experimentally Determined Equivalent Roughness Values For Two Types of Longitudinal Straight Duct Produced by a Small Company

This study evaluates the static pressure losses due to friction in longitudinal straight ducts. These losses are related to the flow properties (velocity, density, etc.) by means of the Darcy friction factor, f. In the case of hydraulically smooth wall conditions the f value is related to the Reynold's number (Re), while for rough conditions it is dominated by the relative roughness (ϵ/Dh). The Colebrook equation is often used to determine the f value as it also provides a reasonable value when the flow is in the transient region between hydraulically smooth and rough wall conditions.;Experimental data was collected for these galvanized steel ducts over a range of 1500-5000 fpm with nominal diameters of 4-8 inches. It was found that hydraulically smooth conditions could be applied for all the tested cases. However, the hydraulically smooth predictive equations resulted in underestimating the pressure losses, while applying an effective roughness value of ϵ = 0.0001 feet (determined from a recursive back calculation from the experimental data) yielded better predictions of the pressure losses. A regression analysis indicated that both the hydraulic diameter (Dh) and Re were statistically significant factors in determining the f-value. The regression model for the f-value based on Dh and Re resulted in a higher coefficient of determination (R2) than a model based on Re alone. The regression model was applied to the dataset and resulted in less error than applying the Colebrook equation with an effective roughness value of ϵ = 0.0001 feet.