HEAT TRANSFER ENHANCEMENT AND FRICTION FACTOR BEHAVIOR INSIDE A RECTANGULAR DUCT EQUIPPED WITH TRANSVERSE PERFORATED BAFFLES FIXED ON TWO OPPOSITE WALLS

Abstract

The turbulent heat transfer and friction inside a rectangular channel with solid or perforated baffles have been studied experimentally. The baffles are attached on top and bottom walls inside the channel. The effects of baffle alignment (in-line and staggered), perforated baffle open-area ratio (), baffle pitch ratio (Pi/H), baffle height ratio (h/H) and flow Reynolds number are examined. The open-area ratios were 0, 15%, 25%, 35%, and 45%, the pitch ratios were 3.0, 6.0, and 9.0, the baffle height ratios were 0.3, 0.5, and 0.7 and the Reynolds number was varied from 9757 to 49257.

A test rig was designed and constructed for the purpose of blowing air through a test section of rectangular shape having a cross-sectional area of 300 x 50 mm and made from a top and a bottom Aluminum plates and the two other faces are from plexiglass sheets. The two Aluminum plates were heated by using two symmetrical electric heaters, placed adjacently to these plates, whereas the two other sides were kept adiabatic. The total heat liberated from the electric heaters was directed to the air stream, while the different losses were eliminated. Measurements were taken for the wall temperatures, air flow temperature and the pressure drop over the test section. Temperature measurements were carried out using Copper-Constatan thermocouples connected to a data logger, while pressure drop measurements were carried out using two pressure taps connected to a U tube micromanometer.

For the periodic module (region between two successive baffles), the distribution of the wall temperatures and local Nusselt number for both upper and lower walls were illustrated. Also, the distribution of the walls temperatures along the duct length on the middle of each module are also measured, to obtain the distribution of the local heat transfer coefficients along the baffled surface. Experimental results have been obtained for the local and average Nusselt numbers and friction factor. The results indicate that the perforated baffled geometry provides a higher thermal performance than the solid-type baffled one. The present work showed that the highest thermal performance factor under the same pumping power obtained from the experiments, was about 2.38 times more than that of smooth channel. The thermal performance of the perforated baffles is higher than the solid baffles by 2.6- 43.4%, depending on the baffle geometry. Furthermore, compact heat transfer and friction correlations are developed in terms of baffle geometry and flow parameters.