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