Heat transfer problems
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Transcript of Heat transfer problems
Heat Transfer
Sheet #2 – Conduction_I
1‐ Consider a cold canned drink left on a dinner table. Would you model the heat transfer to the drink as one‐, two‐, or three‐dimensional? Would the heat transfer be steady or transient? Also, which coordinate system would you use to analyze this heat transfer problem, and where would you place the origin? Explain. 2‐ Consider a round potato being baked in an oven. Would you model the heat transfer to the potato as one‐, two‐, or three‐dimensional? Would the heat transfer be steady or transient? Also, which coordinate system would you use to solve this problem, and where would you place the origin? Explain. 3‐ Consider a medium in which the heat conduction equation is given in its simplest form as
(a) Is heat transfer steady or transient? (b) Is heat transfer one‐, two‐, or three‐dimensional? (c) Is there heat generation in the medium? 4‐ Consider a medium in which the heat conduction equation is given in its simplest form as
(a) Is heat transfer steady or transient? (b) Is heat transfer one‐, two‐, or three‐dimensional? (c) Is there heat generation in the medium? 5‐ Shown below is the 3‐D transient heat conduction equation in spherical coordinates.
22 2 2 2
1 1 1sin
sin sin
T T T Tkr k k g C
r r r r r t
a) Cross out the term(s) that go away if heat transfer is steady. b) Underline the term(s) that go away if there is no internal heat generation. c) Circle the term(s) that go away if temperature is a function only of r. 6‐ Consider one‐dimensional heat conduction through a large plane wall with no heat generation that is perfectly insulated on one side and is subjected to convection and radiation on the other side. It is claimed that under steady conditions, the temperature in a plane wall must be uniform (the same everywhere). Do you agree with this claim? Why?
7‐ It is stated that the temperature in a plane wall with constant thermal conductivity and no heat generation varies linearly during steady one‐dimensional heat conduction. Will this still be the case when the wall loses heat by radiation from its surfaces? 8‐Consider a 1.2‐m‐high and 2‐m‐wide glass window whose thickness is 6 mm and thermal conductivity is k = 0.78 W/m ∙ °C. Determine the steady rate of heat transfer through this glass 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 ‐5°C. Take the convection heat transfer coefficients on the inner and outer surfaces of the window to be h1= 10 W/m2∙°C and h2= 25 W/m2∙°C, and disregard any heat transfer by radiation. 9‐ Consider a 1.2‐m‐high and 2‐m‐wide double‐pane window consisting of two 3‐mm‐thick layers of glass (k = 0.78 W/m∙°C) separated by a 12‐mm‐wide stagnant air space (k= 0.026 W/m∙°C). 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 ‐5°C. Take the convection heat transfer coefficients on the inner and outer surfaces of the window to be h1= 10 W/m2∙°C and h2 = 25 W/m2∙°C, and disregard any heat transfer by radiation. Answers: 114 W, 19.2°C 10‐ Clothing made of several thin layers of fabric with trapped air in between, often called ski clothing, is commonly used in cold climates because it is light, fashionable, and a very effective thermal insulator. So it is no surprise that such clothing has largely replaced thick and heavy old‐fashioned coats. Consider a jacket made of five layers of 0.1‐mm‐thick synthetic fabric (k = 0.13 W/m∙°C) with 1.5‐mm‐thick air space (k = 0.026 W/m∙°C) between the layers. Assuming the inner surface temperature of the jacket to be 28 °C and the surface area to be 1.1 m2, determine the rate of heat loss through the jacket when the temperature of the outdoors is 5 °C and the heat transfer coefficient at the outer surface is 25 W/m2∙°C. What would your response be if the jacket is made of a single layer of 0.5‐mm‐thick synthetic fabric? What should be the thickness of a wool fabric (k = 0.035 W/m∙°C) if the person is to achieve the same level of thermal comfort wearing a thick wool coat instead of a five‐layer ski jacket? 11‐ A thin silicon chip and an 8‐mm thick aluminum substrate (k= 238 W/m.K) are separated by 0.02 mm thick epoxy joint. The chip and substrate are each 10 mm on a side, and their exposed surfaces are cooled by air, which is at a temperature of 25 oC and provides a convection coefficient of 100 W/m2.K. If the chip dissipates 104 W/m2 under normal conditions, and the maximum operating temperature of the epoxy is 85 oC, will this assembly operate safely? 12‐ Will the thermal contact resistance be greater for smooth or rough plain surfaces? 13‐ Consider two surfaces pressed against each other. Now the air at the interface is evacuated. Will the thermal contact resistance at the interface increase or decrease as a result? 14‐ A 1‐mm‐thick copper plate (k = 386 W/m∙°C) is sandwiched between two 5‐mm‐thick epoxy boards (k = 0.26 W/m∙°C) that are 15 cm by 20 cm in size. If the thermal contact conductance on both sides of the copper plate is estimated to be 6000 W/m∙°C, determine the error involved in the total thermal resistance of the plate if the thermal contact conductances are ignored.
15‐ Steam at 320°C flows in a stainless steel pipe (k = 15 W/m∙°C) whose inner and outer diameters are 5 cm and 5.5 cm, respectively. The pipe is covered with 3‐cm‐thick glass wool insulation (k = 0.038 W/m∙°C). Heat is lost to the surroundings at 5 °C by natural convection and radiation, with a combined natural convection and radiation heat transfer coefficient of 15 W/m2∙°C. Taking the heat transfer coefficient inside the pipe to be 80 W/m2∙°C, determine the rate of heat loss from the steam per unit length of the pipe. Also determine the temperature drops across the pipe shell and the insulation. 16‐ Consider a 2‐m‐high electric hot water heater that has a diameter of 40 cm and maintains the hot water at 55 °C. The tank is located in a small room whose average temperature is 27 °C, and the heat transfer coefficients on the inner and outer surfaces of the heater are 50 and 12 W/m2∙°C, respectively. The tank is placed in another 46‐cm‐diameter sheet metal tank of negligible thickness, and the space between the two tanks is filled with foam insulation (k = 0.03 W/m∙°C). The thermal resistances of the water tank and the outer thin sheet metal shell are very small and can be neglected. The price of electricity is $0.08/kWh, and the home owner pays $280 a year for water heating. Determine the fraction of the hot water energy cost of this household that is due to the heat loss from the tank. Hot water tank insulation kits consisting of 3‐cm‐thick fiberglass insulation (k = 0.035 W/m ∙ °C) large enough to wrap the entire tank are available in the market for about $30. If such insulation is installed on this water tank by the home owner himself, how long will it take for this additional insulation to pay for itself? Answers: 17.5 percent, 1.5 years 17‐ The boiling temperature of nitrogen at atmospheric pressure at sea level (1 atm pressure) is ‐196°C. Therefore, nitrogen is commonly used in low‐temperature scientific studies since the temperature of liquid nitrogen in a tank open to the atmosphere will remain constant at ‐196°C until it is depleted. Any heat transfer to the tank will result in the evaporation of some liquid nitrogen, which has a heat of vaporization of 198 kJ/kg and a density of 810 kg/m3 at 1 atm. Consider a 3‐m‐diameter spherical tank that is initially filled with liquid nitrogen at 1 atm and ‐196°C. The tank is exposed to ambient air at 15 °C, with a combined convection and radiation heat transfer coefficient of 35 W/m2∙°C. The temperature of the thin‐shelled spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Determine the rate of evaporation of the liquid nitrogen in the tank as a result of the heat transfer from the ambient air if the tank is (a) Not insulated, (b) Insulated with 5‐cm‐thick fiberglass insulation (k = 0.035 W/m∙°C), and (c) Insulated with 2‐cm‐thick super‐insulation which has an effective thermal conductivity of 0.00005 W/m∙°C.
FIG. P. 9 Fig. P. 10
FIG. P. 14
FIG. P. 16
FIG. P. 17