In the present paper a comparison between the heat transfer enhancements obtained by small pulsating air jets, respectively aspirated or expired, is carried out. These air jets go in or out an aluminum vertical wall, heated by Joule effect, in the presence of air. Experimental measurements have been conducted with three temperature drops between wall and air (25, 40 and 70K respectively) with or without pulsating and continuous expired and aspirated jets. The thickness of the boundary layer near the wall is locally reduced due to the presence of the aspirated jets or the boundary layer is destabilized near the holes by means of expired jets: both of these phenomena improve the heat transfer coefficient. The experimental results have demonstrated that the pulsating expired jets are more efficient than the aspirated ones with lower differences wall-to-air temperature; the opposite occurs when that difference is greater then 40K. Also the time intervals of presence and absence, the air velocity and the jets array maximizing the heat transfer coefficient have been computed.
FREE CONVECTION HEAT TRANSFER ENHANCEMENT BETWEEN A VERTICAL WALL AND AIR IN PRESENCE OF PULSATING ASPIRATED AND EXPIRED JETS"
BARTOLI, CARLO
2007-01-01
Abstract
In the present paper a comparison between the heat transfer enhancements obtained by small pulsating air jets, respectively aspirated or expired, is carried out. These air jets go in or out an aluminum vertical wall, heated by Joule effect, in the presence of air. Experimental measurements have been conducted with three temperature drops between wall and air (25, 40 and 70K respectively) with or without pulsating and continuous expired and aspirated jets. The thickness of the boundary layer near the wall is locally reduced due to the presence of the aspirated jets or the boundary layer is destabilized near the holes by means of expired jets: both of these phenomena improve the heat transfer coefficient. The experimental results have demonstrated that the pulsating expired jets are more efficient than the aspirated ones with lower differences wall-to-air temperature; the opposite occurs when that difference is greater then 40K. Also the time intervals of presence and absence, the air velocity and the jets array maximizing the heat transfer coefficient have been computed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
