Abstract. Thermal and friction loss characteristics in a square-duct heat exchanger fitted with inclinedbaffles are experimentally examined. Air as the test fluid enters the test duct having a uniform surface heatflux. The baffles are placed repeatedly on both sides of a rectangular centre-cleared tape/frame before diagonally inserting the baffled frame into the test duct to produce longitudinal vortex flows through the test section. Effects of five different relative baffle height or flow blockage ratios (b/H=BR=0.1, 0.2, 0.3, 0.4 and 0.5) on heat transfer, pressure loss and thermal performance in the square duct are investigated for Reynolds number ranging from 4100 to 25,600. The relative baffle pitch or pitch ratio (P/H=PR) and baffle attack angle () are fixed at 3.0 and 30°, respectively. The experimental results reveal that the heat transfer and pressure drop in the form of respective Nusselt number (Nu) and friction factor (f) from using the baffle tend to increase with the rise of Reynolds number (Re) and BR. The maximum enhancement in Nu and f has been found to be 4.61 and 63.67 times above the smooth duct, respectively. The thermal enhancement factor () is maximum at BR=0.3.

An experimental work has been conducted to explore thermal and turbulent flow friction characteristics in a uniform heat-fluxed square-duct contained with inclined square-rings (SR). Experiments were performed for different geometric parameters of the SR elements, focused on their detailed effects on the optimal thermal performance. Influences of five different ring-to-duct height or blockage ratios (BR=b/H= 0.1, 0.15, 0.2, 0.25 and 0.3) on friction factor (f), Nusselt number (Nu), and thermal enhancement factor (TEF) in the square duct are investigated for Reynolds number (Re) from 4210 to 25,800. The inclination angle (a) and the ratio of axial pitch length of rings to the height of duct (called pitch ratio, PR=P/H) were kept constant at 60° and 3.0, respectively. The experimental result has shown that the SR insert gives greater heat transfer and friction loss than the plain duct acting alone. Increasing the blockage ratio (BR) results in the drastic enhancement in Nu and f values. The inserted duct with BR = 0.25 has the maximum TEF around 1.57. Comparing thermal performance at identical operating conditions, the SR insert at BR = 0.15 gives around 1.1 %, 2.3 %, 3.2 and 4.5 % higher than the one at BR = 0.2, 0.25, 0.3 and 0.1, respectively.

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The paper presents an experimental study on airflow friction and heat transfer behaviors in a heat exchanger square-duct fitted with winglet turbulators. The experiments are carried out by varying the airflow rate in terms of Reynolds number from 4000 to 25,000. The winglets were mounted in tandem with three attack angles (α=30o, 45o and 60o), two winglet-pitch to duct-height ratios, (called pitch ratio, PR=P/H=1.0 and 1.5) and a single winglet-to duct-height ratio, (called blockage ratio, BR=e/H=0.2). Effects of the winglet parameters on heat transfer and pressure loss in terms of Nusselt number and friction factor are investigated. The experimental result reveals that the application of the winglets provides considerably higher heat transfer and pressure loss values than the smooth duct alone. The winglet at α=60o and PR=1 gives the maximum heat transfer and pressure loss but the one at α = 30o and PR=1.5 yields the highest thermal enhancement factor of about 1.49 at the lowest Reynolds number.