Effects of wind tunnel orientation and mixed convection on heat transfer from a PQFP

The thermal resistance of a surface-mount, plastic quad flat package (PQFP) was measured over a range of heating powers, air velocities, and package orientations. The package was mounted in a low-speed wind tunnel which could be rotated in the vertical plane, thus changing the orientation of the package with respect to the gravity vector. Results show that the regime of mixed correction is limited, for a horizontal package, to air velocities less than 2.5 m/s at a heating power of 4 W, i.e., to Gr_L*/Re_L² greater than about unity. In the mixed convection regime, variation of the wind tunnel inclination angle from buoyancy-opposed to buoyancy-assisted orientations decreases the total thermal resistance by 5% or more when the air velocity is less than 0.5 m/s at a heating power of 4 W, i.e., for Gr_L*/Re_L² greater than about ten. For air velocities greater than 2.5 m/s, for all tunnel orientations, the total thermal resistance shows a steep decrease. Flow visualization revealed that the resistance decrease appears to be due to restarting of the surface boundary layer on the package