Study on Chimney Effect in Natural Air Cooled Electronic Equipment Casings under Inclination (The Proposal of a Thermal Design Correlation with Inclination)

Natural air-cooling needs neither a fan nor a blower. However, cooling capability of natural air-cooling is low, so the improvement of its capability is indispensable. In this paper are presented the results of the experimental casings on the chimney effect in natural air-cooled electronic equipment. The system box is inclined for the purpose to raise the capability of natural air-cooling. Experiments have been carried out using three models of thin laptop PC that have two kinds of heat sources. It has inlet and outlet vents on the front and rear walls of the casing. The box was inclined 0 to 90 degrees up from the horizontal plane. The influence by the box inclination, the box dimensions, and the heating element size was investigated. The result shows that the temperature inside casing and heater surface temperature are gently diminished by the effect of inclination, and the increase in natural circulation flux in the box is quantitatively proven by experiments. But, a substantial benefit of casing inclination is only at angles lower than about 40 degree. In the other hand, the experimental data were reduced to a Nusselt number-Rayleigh number correlation by using modified the reference length we supposed that does not include explicitly the box dimensions, the heat source size, and the inclination angle. The experimental data almost fall on a straight line which gives a correlation Nu* =0.0038 Ra*0.6, and are collapsed in a narrow band.