(186f) Pressure Loss In a Long Radius 90° Bend at High Re
Curved pipes are ubiquitous in industrial and lab settings, making flow through them and their subsequent pressure losses of great significance. In comparison to a straight pipe, flow through a bend may experience a greater pressure drop over an equivalent length due to flow separation. The flow through a bend is a function of the Re and the radius of curvature, R/r. The bend is characterized by the pipe curvature ratio: R/r, where R is the radius of the bend and r is the pipe radius. Pressure effects upstream of a bend have not received much attention and have been thought to exist only a few diameters upstream of the bend. The first goal of the investigation is to offer experimental data regarding the effects of a long radius pipe bend on the upstream pressure gradient at Re > 3*10^5. The second intent is to show the influence of a sudden expansion at the outlet of the bend on the pressure in the bend.
Experimentally, the effect of Re on pressure drop was investigated in a 180° vertical bend which consisted of two 90° bends of relative radius of curvature 24 and a horizontal straight pipe connection of length 7 pipe diameters. Pressure measurements were taken before the bend, throughout the inside and outside of the first 90° bend, and the connection pipe. Experiments of water at Re between 3.68*105 to 8.50*10^5 in smooth 3 inch (78 mm) stainless steel pipe were conducted. Two pipe configurations downstream of the 180° bend were investigated: a continuation of the 78 mm pipe used throughout the loop and a sudden pipe expansion that allowed for an effective freefall from the bend exit.
The results showed that at the lowest Re, 3.68*10^5, the pressure gradient was constant throughout the upstream tangent. However, the pressure gradient deviated from its upstream slope as it approached the bend at higher Re. This deviation propagated upstream with increasing Re; 7 pipe diameters at the highest Re. This phenomena was independent of the downstream configuration. The pressure drop in the first 90° bend was as expected when the downstream tangent contained the 3 inch (78 mm) steel pipe: the pressure on the inside of the bend was consistently less than that on the outside. When the flow was allowed to fall freely from the bend exit, pressure fluctuations throughout the bend were found. These fluctuations are consistent over the range of Re and appear to be a function of the sudden expansion and the bend curvature.