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

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 \(50 < Re_n < 1000\) and oscillatory Reynolds numbers \(0 < Re_{osc}< 750\), for a dimensionless amplitude \(x_0/D = 0.5\) and \(Pr=65\). In absence of oscillatory flow, opposed baffles advance the transition to turbulence from \(Re_n = 100\) to \(50\), increasing the net friction factor (40 %) for \(Re_n > 50\) and the Nusselt number (maximum of 27 %) for \(Re_n < 150\). When an oscillatory flow is applied, augmentations caused by opposed baffles are only observed for \(Re_n < 150\) and \(Re_{osc} < 150\). Above \(Re_n\), \(Re_{osc}>150\), 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.