Experimental investigation on heat transfer enhancement by transversal ribs in channels

In this paper an experimental investigation is carried out on forced convection in channels with the lower ribbed wall heated with a uniform heat flux. The fluid is air. The experimental apparatus is made up of a channel with a transversal rectangular section, a test section, a convergent channel, to reduce the rectangular transversal section in a circular section and a fan between circular tubes. The unheated rectangular channel is long 2.50 m, the transversal rectangular section is 27.8 mm high and 250 mm wide. The test section is 300 mm long and it has the same rectangular transversal section of the unheated channel. In the test section the lower plate is in aluminum and is heated by two electrical resistances whereas the upper plate and the lateral walls are in glass. The experiments are carried out employing a smooth or a ribbed plate. The ribs are transversal and can be squared or triangular. In both cases the ribs are 5.0 mm wide and 5.0 mm high. The rib pitch is 40 mm and along the heated plate there are 7 ribs. Results are obtained in a Reynolds number range from 5000 to 15000 and wall heat flux of 3000 W/m2and 5000 W/m2. In the experiments two heated wall emissivity values are considered, 0.05 and 0.95. Results are given in terms of wall temperature profiles, air temperature, average Nusselt numbers and pressure drops.