HEAT TRANSFER CHARACTERISTICS IN FORCED CONVECTION THROUGH A RECTANGULAR CHANNEL WITH RIBBED SURFACES

In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are also presented for comparison. The channel has a 120 mm wide, 12 mm high cross-section, and it is operated with the lower and upper walls at fixed temperature whereas the side ones are adiabatic. Ribs have a square cross section and are mounted transversely to air-flow direction; the tested configurations differ for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls, in order to attain developed velocity conditions at the test channel entry. Air flow rates have been varied in order to investigate Reynolds numbers, based on the duct hydraulic diameter, between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number for the tested ribbed channels ranging between 1.1 and 4.6 with respect to the flat channel.