Effect of three-orifice baffles orientation on the flow and thermal–hydraulic performance: Experimental analysis for net and oscillatory flows

Abstract

Three-orifice baffles equally spaced along a circular tube are investigated as a means for heat transfer enhancement under net, oscillatory and compound flows. An unprecedented, systematic analysis of the relative orientation of consecutive baffles – aligned or opposed – is accomplished to assess the changes induced on the flow structure and their impact on the thermal–hydraulic performance. The results cover the Nusselt number, the net and oscillatory friction factors and the instantaneous velocity fields using PIV in an experimental campaign with a 32 mm tube diameter. The study is conducted in the range of net Reynolds numbers 50<Ren<1000 and oscillatory Reynolds numbers 0<Reosc<750, for a dimensionless amplitude x0/D=0.5 and Pr=65. In absence of oscillatory flow, opposed baffles advance the transition to turbulence from Ren=100 to 50, increasing the net friction factor (40%) for Ren>50 and the Nusselt number (maximum of 27%) for Ren<150. When an oscillatory flow is applied, augmentations caused by opposed baffles are only observed for Ren<150 and Reosc<150. Above Ren, Reosc>150, opposed baffles are not recommended for the promotion of heat transfer, owing to friction penalties. However, the chaotic mixing and lack of short-circuiting between baffles observed with flow velocimetry over a wide range of operational conditions point out the interest of this configuration to achieve plug flow.