Abstract
To help engineers predict the pressure drop of a fluid passing along ducts of non-circular cross-section, the concept of `equivalent diameter' of a circular duct has been extensively used. This paper re-analyses two earlier sets of research data for non-circular ducts, such an analysis being easier now than at the time of the original research, 1948 and 1975. This new work reveals that equations for equivalent diameter of non-rectangular ducts lifted from the two prime sources are totally inapplicable; in short they are myths which should no longer be reiterated. In analysing the experimental results as constructively as possible to extract a more appropriate and correct relationship, the author finds that the experimental results are not sufficiently consistent to draw any reliable quantitative conclusions. Thus the experimental work needs to be repeated to verify whether hydraulic diameter may be used as an `equivalent diameter' or whether an alternative relationship can be found.
Practical application: In showing that there are no reliable practical results to guide engineers in calculating pressure drop along non-circular ducts, the paper demonstrates that further practical research is necessary. In showing that the relationships used in some texts and reputedly emanating from practical research, are both invalid, the author hopes that their future use will cease. Until a proven relationship for `equivalent diameter' is forthcoming, there is no other choice than to use the hydraulic diameter.
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