Forced and free convective heat transfer coefficients for a model printed circuit board channel geometry

Experimental and theoretical analyses of the boundary layer development and the heat transfer coefficients in various thermally simulated electronic components were carried out for both forced and free convection conditions in a channel. To determine the local heat transfer coefficient and understand the physical phenomena involved, the effects of channel geometry, the component array height, the airflow rate, and the heat flux were studied. The channel height was varied from 2 to 15 mm, the component heights were varied in three arrays (smooth channel, 2 mm, and 4 mm), the maximum heat flux was varied up to 5000 W/m2, and the maximum velocity was varied up to 15 m/s. The apparatus used in the present study is geometrically, thermally, and hydrodynamically similar to a printed circuit board. The heat transfer coefficient of each element lies between two limiting theoretical values. In forced convection the upper analytical limit is evaluated from the expression for the average heat transfer coefficient on a plate where both the hydrodynamic and thermal boundary layers start at the leading edge of the heated element. The lower analytical limit is calculated from the expression for the average heat transfer coefficient where the hydrodynamic boundary layer starts at the beginning of the board while the thermal boundary layer starts at the leading edge of the heated element. In free convection, the upper analytical limit is calculated as the heat transfer from an isolated vertical plate, while the lower limit is calculated from a composite relationship for the flow between two asymmetric isothermal parallel plates.