Models of the sun indicate that only about 10% of the total hydrogen in the sun will participate in nuclear reactions, since it is only the hydrogen in the central regions that is at a high enough temperature. use the total energy radiated per second by the sun, 3.8 × 1026 watts, alongside the exercises and information given here to estimate the lifetime of the sun. (hint: make sure you keep track of the units: if the luminosity is the energy radiated per second, your answer will also be in seconds. you should convert the answer to something more meaningful, such as years.)

Asteel tube (k=50 w/mk) of inner and outer diameter di = 20mm and do=26mm, respectively, is used to transfer heat from hot gases flowing over the tube (h-hot = 200w/m2k) to cold water flowing through the tube (h-cold = 8000 w/m2k). what is the cold-side overall heat transfer coefficient uc? to enhance heat transfer 16 straight fins of rectangular profile are installed longitudinally along the outer surface of the tube. the fins are equally spaced around the circumference of the tube, each having a thickness of 2mm and a length of 15mm. what is the corresponding overall heat transfer coefficient uc? *solve for uc in both cases*