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