Study Of Pressure Loss Characteristics Of Flexible Duct In Bending State

This paper aims to study on characteristics of pressure loss of aluminum foil flexible duct at bending state for engineering use. The research results will provide a technical support to project design. The simulation method is used to analyze the characteristics of part flexible duct flow field, and identify affected resistance loss factors.Based on numerical simulation and experimental investigation, reach the following conclusion:1) It has a dense metal thin-film inside and dispersive outside in the inflection of the flexible duct. The surface roughness is big inside and small outside in the inflection of the flexible duct. Pressure and velocity distribution of the bending flexible duct is more complex than rigid duct. Its secondary recirculation is a strength greater than rigid duct's with the same parameters;2) Flow distribution of the bending flexible duct show obvious feature. Pressure gradient in the bending flexible duct slants to the outlet. Strong secondary recirculation appears inside the flexible duct. Backflow region occupies most of the end of curved region. Turbulent center presents in a crescent-shaped which is localized in duct turning outside. The bending duct affects both upstream and downstream, One or two times the diameter distance in upstream and more than fifteen times the diameter distance in downstream;3) The change of diameter has little effect on the resistance loss of flexible duct, and a slight change in the secondary recirculation region. The entire flow and the range of backflow basically remained level. Because the absolute roughness of the flexible ducts which is made by the same materials is regarded as a constant. As the diameter decreases, the equivalent roughness increases. For the small diameter, its equivalent roughness is greater than big diameter's in the same drape height. Small diameter makes stronger secondary recirculation. It dissipates more energy in turbulent center. Therefore the loss of local resistance in small diameter flexible duct is more than big diameter. There is a little increase of the loss of local resistance range, equivalent roughness and the influence on drape in wall while the diameter increases to an extent;4) The inlet velocity has great effect on the pressure loss of flexible duct at bending state. As the increasing of inlet velocity, the resistance loss of flexible duct will be increased a lot. The essence of the pressure loss is that as the increasing of inlet velocity, the Reynolds number will be increased, the secondary recirculation will be increased , and the pressure loss will be increased too ,when the diameter unchanged;5) The bending angle has great effect on the pressure loss of flexible duct. As the increasing of the bending angle, the mainstream velocity of turbulence will be reduced, and the fold of the flexible duct at bending state will be lessened, resulting that the roughness will turn to lower, the recirculation zone of secondary vortex will become smaller, and the consumption will be bated by the secondary vortex, so the loss of partial resistance will be lessened;6) After calculating and measuring in experiment, the absolute roughness of the flexible duct in experiment is 0.00568m;7) According to the research achievement of this subject, it makes out a partial resistance correction factor tables to provide technical support for design, when the absolute roughness is 0.00568m, R/D=1.5, and the bending angle are separately 90°,110°,135°,150°.