90. For a glass, thermal conductivity, density and specific heat are 0.78 W/mK, 2700 kg/m³ and 0.84 kJ/kg K, respectively. The thermal diffusivity of glass in m²/s is (a) 2.91 x 106 (b) 3.44 x 10-7 (c) 2907.7 (d) 3.44 x 104

Analyze the following diagram. What is the heat rate to the left side of the heater? What is the heat rate to the right side of the heater? Tin = 16°C W h = 40- T = -20°C u = 188 m2K 60mm Air 60mm 10mm 6mm 10mm 14mm 6mm 2mm 2mm Blue: Epoxy Grey: Aluminum Green: Fiberglass Yellow: Silicone Orange: Heater Blanket, 2000w Note: Treat the outer left wall as flat plates in external convection. Note: Ignore the thickness of the heater blanket.

3 An electrical current of 700 A flows through a stainless steel cable having a diameter of 5 mm and an electrical resistance of 6 x 104 /m (i.e., per meter of cable length). The cable is in an environment having a tem- perature of 30°C, and the total coefficient associated with convection and radiation between the cable and the environment is approximately 25 W/m².K. (a) If the cable is bare, what is its surface temperature? (b) If a very thin coating of electrical insulation is applied to the cable, with a contact resistance of 0.02 m² K/W, what are the insulation and cable surface temperatures? (c) There is some concern about the ability of the insula- tion to withstand elevated temperatures. What thick- ness of this insulation (k = 0.5 W/m .K) will yield the lowest value of the maximum insulation temper- ature? What is the value of the maximum tempera- ture when this thickness is used? Note: The 25 W/m²K includes not only convection but also radiation.

Estimate the thermal conductivity of benzene at 30C.