Effects of 2-D and 3-D helical inserts on the turbulent flow in pipes

A constant search for more efficient heat exchangers motivates the use of innovative turbulators. Original flow-field measurements were performed with S-PIV to gain a better understanding of the role of 3-D wall obstacles in pipe flows. Such effects are compared with those of 2-D helically corrugated tubes, which are already provided in the literature. This work further investigates the impact of both continuous (2-D corrugation) and discontinuous (3-D corrugation) obstructions on the surrounding velocity field. The analysis of the flow structures is made by comparing two types of helicoidal turbulators. The continuous geometry has a pitch-to-diameter-ratio of p/D = 11 and an inclination angle of 80 degrees. An obstacle height-to-diameter-ratio of e/D = 3.6% is used, which is constant throughout the helix-wise direction. The discontinuous geometry is equivalent to the continuous one except that it has a varying obstacle height throughout the helix. The reattachment and redevelopment of the flow are determined by time-averaged velocity analysis, at Re = 20,000. Notably, the aim of this study is to examine the effects of the discontinuity in the tube corrugation on the flow separation/reattachment, turbulence statistics, vorticity (omega) and strain-rate (S-xy) of the flow particles, and compare them with a previously studied continuous turbulator. As a result, the discontinuous rib has a lower impact on the pressure losses, on the azimuthal swirl, and on the turbulent kinetic energy generation. In the field close to the wall, the changing height of the discontinuous rib generates a turbulent kinetic energy and all azimuthal velocity, respectively, 50% and 35% lower than the continuous configuration. The irregularity of the discontinuous obstacles provides a reduced skin friction coefficient (C-f) which is 0.57 times the one given by the continuous 2-D helical turbulator. On the other hand, the observed less intense wall-bounded turbulence levels associated with the 3-D helical turbulator suggest a lower local heat transfer enhancement.