Heat transfer enhancement in natural convection between vertical and downward inclined wall and air by pulsating jets

In this paper are reported the results of an experimental investigation about the heat transfer enhancement from vertical and downward inclined wall to air, in natural convection, by means of expired jets. The air was maintained at room temperature and the wall is heated by Joule effect. The temperature difference between the wall and the air was prefixed at 25 K and the wall placement had been varied from the vertical to different downward inclined positions (5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees and 30 degrees). The experimental results had shown an increase in the heat transfer coefficient, at the different wall inclinations, due to the jets; actually they had induced turbulence in the dynamic field, by interrupting the laminar flow. Our aim was to optimize the variables involved in the phenomenon, in order to find out the highest enhancement of the heat transfer coefficient values, caused by the jets. The variables were the number of active horizontal lines of jets, their activity and the inactivity time and their velocity. We had also investigated the conditions without jets and we found out that the heat transfer coefficient trend versus the downward inclination angles, could not be described by correlations present in literature. In fact, in the investigated range of the downward wall inclinations, the heat transfer coefficient did not may be simply correlated with the cosine of the angle, but it increased up to a maximum value, reached at 15 degrees, and then decreased. The visualization by smoke had confirmed these results: the boundary layer thickness had followed this trend. In correspondence to the wall inclination equal to 15 degrees, it reached its minimum value. Finally, to the purpose to link the experimental data, we had proposed a numerical correlation. The values calculated thanks to this one reported differences much lower than the computed experimental error. (C) 2010 Elsevier Inc. All rights reserved.