An array of electronic chips is mounted within a sealed rectangular enclosure, and cooling is implemented by attaching an aluminum heat sink (k 180 W/m K). The base of the heat sink has dimensions of w1 w2 100 mm, while the 6 fins are of thickness t 10 mm and pitch S 18 mm. The fin length is Lf 50 mm, and the base of the heat sink has a thickness of Lb 10 mm. If cooling is implemented by water flow through the heat sink, with u 3 m/s and T 17C, what is the base temperature Tb of the heat sink when power dissipation by the chips is Pelec 1800 W