The traditional method of calculating pressure drop in a pipe that conveys a fluid involves use of the Moody Diagram. This diagram is a correlation of data with friction factor plotted as a function of Reynolds number and relative roughness. A new graphical representation of these data has been formulated, and makes use of what is known as the constricted flow diameter. The background for this new correlation is based on using a pipe diameter less twice the average roughness height. A new “modified” Moody Diagram has been produced based on the constricted flow diameter. The presence of roughness features on the inside pipe wall has an effect on the flow along the pipe surface which is not accounted for in the traditional Moody diagram. The new diagram accounts for this effect. To demonstrate the use of the new diagram, several example problems have been formulated and solved using the traditional and the modified diagrams. Calculations indicate that at the smaller pipe sizes, the use of the constricted flow diameter yields significantly different results from those obtained in the traditional way. These results have a major influence on modeling flows in mini- and in micro-channels. Laminar and turbulent flows are both affected.

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