A tube of diameter D = 1.25 cm is placed at the focus of a parabolic solar collector. The solar energy received by the tube is q” = 12 kW/m2 (Fig. 6P-5). Water is heated while it flows with velocity V = 1 m/s through the tube. The ambient air temperature and the inlet water temperature are the same, Ti = T∞ = 25 °C. Determine the length of the tube (and the collector) at which the water temperature reaches Te = 50 °C.