FORCED AND FREE CONVECTIVE HEAT-TRANSFER COEFFICIENTS FOR A MODEL PRINTED-CIRCUIT BOARD CHANNEL GEOMETRY

Experimental and theoretical analyses of the boundary layer developmen t and the heat transfer coefficients in various thermally simulated el ectronic components were carried out for both forced and free convecti on conditions in a channel. To determine the local heat transfer coeff icient and understand the physical phenomena involved, the effects of channel geometry, the component array height, the airflow rate, and th e heat flux were studied. The channel height was varied from 2 to 15 m m, 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/m(2), a nd the maximum velocity was varied up to 15 m/s. The apparatus used in the present study is geometrically, thermally, and hydrodynamically s imilar to a printed circuit board. The heat transfer coefficient of ea ch element lies between two limiting theoretical values. In forced con vection the upper analytical limit is evaluated from the expression fo r the average heat transfer coefficient on a plate where both the hydr odynamic and thermal boundary layers start at the leading edge of the heated element. The lower analytical limit is calculated from the expr ession for the average heat transfer coefficient where the hydrodynami c boundary layer starts at the beginning of the board while the therma l 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 cal culated from a composite relationship for the flow between two asymmet ric isothermal parallel plates.