The use of flat oval ducts is advantageous in those applications where space limitations do not allow the installation of round ducts. However, there is a tendency for flat oval ducts to exhibit excessive deflections when exposed to substantial positive/negative static gauge pressures, particularly when large flat spans are utilized. In those instances, it may be necessary to select a thicker duct wall, or to provide some means of either internal or external reinforcement. Such reinforcements limit duct deflection, and therefore reduce the possibility of permanent damage to the ducts caused by large deformations. In order to further promote the use of flat oval ducts in HVAC applications, designers must have access to knowledge of the most economical combination of duct gauge and reinforcement for a given duct cross section, based on the expected pressure in the system. This paper describes an experimental program designed to measure the deformation of unreinforced 24 gauge 12 in. X 23 in. (305 mm X 584 mm) galvanized steel flat oval ducts subjected to positive internal static gauge pressures. The experiments were performed on spiral seam non-corrugated ducts. The duct deflection measurements were made at four different locations on the ducts. The purpose of the experiments was to supply deflection data to corroborate a finite element deformation model. This investigation sought to determine whether a simplified finite element approach could be used to model the presence of the continuous spiral seam. Duct deformation predictions resulting from a discrete ring seam model and an average wall thickness model were compared in this study, at prescribed locations along the duct wall. The finite element models were developed in ANSYS (a computer-aided engineering software including finite element simulation for structural analysis). The finite element models incorporated structural components such as the duct wall and seams. The discrete ring seam model employed both shell and beam elements to represent the duct wall, whereas the average wall thickness model was developed using shell elements only. Predicted deflections from the finite element models were tabulated and compared to experimental data. It was demonstrated that predictions from non-linear finite element analysis were capable of predicting the deflections considerably more accurately than linear finite element modeling. When self-weight was disregarded the discrete ring seam model typically under-predicted the deflections relative to the average wall thickness approach. Satisfactory agreement between model predictions and test data were obtained from a non-linear finite element analysis that employed a simplified average wall thickness to characterize the spiral seam and duct wall. It is straightforward to implement the average wall thickness methodology in finite element modeling of duct deformation. The investigation concluded that for the spiral seam flat oval duct that was investigated in this study, a non-linear duct deformation model using an effective wall thickness approach to simulate the presence of the continuous spiral lock seam was most successful in predicting duct deformation for positive internal pressures. For this particular case, which involved a relatively short flat span, the effects of self-weight were apparently unimportant. However, for more dependable duct deflection predictions, it is generally advisable to include the effects of self-weight in finite element modeling of spiral flat oval ducts.


Unreinforced Flat Oval Duct Deformation Testing and Modeling

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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