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

TL;DR

This experimental study investigates how the orientation of three-orifice baffles in a circular tube affects flow behavior and heat transfer efficiency under various flow conditions, revealing optimal configurations for enhanced thermal-hydraulic performance.

Contribution

It provides a systematic analysis of baffle orientation effects on flow and heat transfer, including new experimental data on turbulence transition and flow structure in oscillatory and net flows.

Findings

Opposed baffles promote earlier turbulence transition at lower Reynolds numbers.

Opposed baffles increase heat transfer (up to 27%) at low Reynolds numbers.

Flow visualization shows chaotic mixing and plug flow potential with certain baffle orientations.

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 $50 < Re_n < 1000$ 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.