Study on the resistance calculation method for ventilation duct systems under non-fully developed flow conditions

Abstract

This study addresses the challenge of accurately calculating duct resistance under non-fully developed flow conditions and proposes a novel generalized method for evaluating the resistance of local fittings. The accuracy and applicability of the method are validated through a combination of Computational Fluid Dynamics (CFD) simulations and full-scale experiments. For elbow fittings, five coupling systems were validated. The results demonstrate that, with different straight duct lengths between fittings (0D, 4D, and 7D, where D is the equivalent diameter), the error of the proposed method was reduced by 25.45 %, 21.35 %, and 11.87 %, respectively, compared to traditional calculation methods. Additionally, normalized parameters for the generalized resistance calculation method for elbows are provided. For tee fittings, whose resistance characteristics are significantly influenced by the flow rate ratio between the main and branch directions, a parameter table for different types of tees was established and validated through full-scale coupling experiments. The results show that, in the tee coupling system, the average error in the branch direction was reduced by 31.33 %, and in the main direction, the error decreased by 51.67 %. The findings indicate that the proposed method significantly improves the accuracy of local resistance calculation under non-fully developed flow conditions, effectively compensating for the shortcomings of existing design standards under complex flow conditions.