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1–15 Consider a 150-W incandescent lamp. The filament of the lamp is 5-cm long and has a diameter of

0.5 mm. The diameter of the glass bulb of the lamp is 8 cm. Determine the heat flux, in W/m2, (a) on the surface of the filament and (b) on the surface of the glass bulb, and (c) calculate how much it will cost per year to keep that lamp on for eight hours a day every day if the unit cost of electricity is $0.08/kWh.

1–23 Water is heated in an insulated, constant diameter tube by a 5-kW electric resistance heater. If the water enters the heater steadily at 15°C and leaves at 60°C, determine the mass flow rate of water.

1–53 The inner and outer surfaces of a 4-m 3 7-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m·K are maintained at temperatures of 26°C and 8°C, respectively. Determine the rate of heat transfer through the wall, in W.

1–69 An electric current of 5 A passing through a resistor has a measured voltage of 6 V across the resistor. The resistor is cylindrical with a diameter of 2.5 cm and length of 15 cm. The resistor has a uniform temperature of 90°C and the room air temperature is 20°C. Assuming that heat transfer by radiation is negligible, determine the heat transfer coefficient by convection.

2–25 Consider a medium in which the heat conduction equation is given in its simplest form as 𝑑 2 𝑇 1 𝑑𝑇 = 𝑑𝑥 2 ∝ 𝑑𝑡 (a) Is heat transfer steady or transient? Ans: transient (b) Is heat transfer one-, two-, or three-dimensional? Ans one dimensional (c) Is there heat generation in the medium? Ans no heat generation (d ) Is the thermal conductivity of the medium constant or variable? Ans constant

2-26 Consider a medium in which the heat conduction equation is given in its simplest form as 𝑑 2 𝑇 𝑑 2 𝑇 1 𝑑𝑇 = + 𝑑𝑥 2 𝑑𝑦 2 ∝ 𝑑𝑡 (a) Is heat transfer steady or transient? Ans: transient (b) Is heat transfer one-, two-, or three-dimensional? Ans: two dimensional (c) Is there heat generation in the medium? Ans: no heat generation (d ) Is the thermal conductivity of the medium constant or variable? Ans: constant

2-28 Consider a medium in which the heat conduction equation is given in its simplest form as 1𝑑 𝑑𝑇 𝑑 𝑑𝑇 (𝑘𝑟 ) + (𝑘 𝑑𝑍) + 𝑒𝑔𝑒𝑛 = 0 𝑟 𝑑𝑟 𝑑𝑟 𝑑𝑧 (a) Is heat transfer steady or transient? Ans: steady (b) Is heat transfer one-, two-, or three-dimensional? Ans: two dimensional (c) Is there heat generation in the medium? Ans: heat generation (d ) Is the thermal conductivity of the medium constant or variable? Ans: variable 2–64 Electrically heated draw batch furnaces are commonly used in the heat treatment industry. Consider a draw batch furnace front made of a 20-mm thick steel plate with a thermal conductivity of 25 W/m·K. The furnace is situated in a room with surrounding air temperature of 20°C and an average convection heat transfer coefficient of 10 W/m2·K. If the inside surface of the furnace front is subjected to uniform heat flux of 5 kW/m2 and the outer surface has an emissivity of 0.30, determine the inside surface temperature of the furnace front.

2–59 Consider a large plane wall of thickness L 5 0.4 m, thermal conductivity k 5 1.8 W/m·K, and surface area A 5 30 m2. The left side of the wall is maintained at a constant temperature of T1 5 90°C while the right side loses heat by convection to the surrounding air at T` 5 25°C with a heat transfer coefficient of h 5 24 W/m2·K. Assuming constant thermal conductivity and no heat generation in the wall, (a) express the differential equation and the boundary conditions for steady one-dimensional heat conduction through the wall, (b) obtain a relation for the variation of temperature in the wall by solving the differential equation, and (c) evaluate the rate of heat transfer through the wall.

3–25 Consider a 1.2-m-high and 2-m-wide double-panewindow consisting of two 3-mm-thick layers of glass (k = 0.78 W/m·K) separated by a 12-mm-wide stagnant air space (k = 0.026 W/m·K). Determine the steady rate of heat transfer through this double-pane window and the temperature of its inner surface for a day during which the room is maintained at 24°C while the temperature of the outdoors is 25°C. Take the convection heat transfer coefficients on the inner and outer surfaces of the window to be h1 = 10 W/m2·K and h2 = 25 W/m2·K, and disregard any heat transfer by radiation....