An experimental study : using honeycomb to reduce secondary flows in curved ducts

This experimental study evaluated the effectiveness of using honeycomb to improve flow quality by reducing secondary flow in curved ducts. An experimental setup that consisted of two 90° elbows joined by a 6-inch straight connector duct was used. Traditional solid honeycomb and honeycombs of varying porosity (20%, 30%, and 40%) were installed at either of two locations in the flow: after the 1 st elbow or after the 2nd elbow. Surface oil flow visualization, pressure measurements, and Particle Imaging Velocimetry (PIV) were used to determine the improvement in flow quality with the honeycombs installed and to determine if the secondary flows in the exit duct diminish within a shorter distance downstream compared to the baseline flow. Surface oil flow visualization provided a qualitative representation of flow behavior in the exit duct; analysis of pressure measurements provided insight on how the flow traveling through the elbows and into the exit duct was affected by different free-stream velocities; and PIV described the vorticity contours in the exit duct. The experimental results revealed that the honeycombs significantly improve the flow quality in the exit duct by reducing secondary flow generated by the elbows. It was also revealed that the porous-cell honeycombs were able to further eliminate more of the nonuniformities in the flow than the traditional solid honeycomb. The solid and porous-cell honeycombs helped to generate a more uniform flow within a shorter distance downstream in the exit duct than the baseline flow.