In analyzing the performance of a thermal system, the

engineer must be able to identify the relevant heat transfer processes. Only then can the system behavior be properly quantified. For the following systems, iden- tify the pertinent processes, designating them by appro- priately labeled arrows on a sketch of the system. Answer additional questions that appear in the problem statement.

(a) Identify the heat transfer processes that determine the temperature of an asphalt pavement on a sum- mer day. Write an energy balance for the surface of the pavement. GS Cover glass Air space Absorber plate Water tubing Insulation

(b) Microwave radiation is known to be transmitted by plastics, glass, and ceramics but to be absorbed by materials having polar molecules such as water. Water molecules exposed to microwave radiation align and reverse alignment with the microwave

radiation at frequencies up to 109_s⫺1_{, causing heat}

to be generated. Contrast cooking in a microwave oven with cooking in a conventional radiant or convection oven. In each case, what is the physical mechanism responsible for heating the food? Which oven has the greater energy utilization efficiency? Why? Microwave heating is being con- sidered for drying clothes. How would the opera- tion of a microwave clothes dryer differ from a conventional dryer? Which is likely to have the greater energy utilization efficiency? Why? (c) To prevent freezing of the liquid water inside the

fuel cell of an automobile, the water is drained to an onboard storage tank when the automobile is not in use. (The water is transferred from the tank back to the fuel cell when the automobile is turned on.) Con- sider a fuel cell–powered automobile that is parked

outside on a very cold evening with T_앝⫽ ⫺20⬚C.

The storage tank is initially empty at Ti,t⫽ ⫺20⬚C,

when liquid water, at atmospheric pressure and tem-

perature Ti,w⫽ 50⬚C, is introduced into the tank. The

tank has a wall thickness tt and is blanketed with

insulation of thickness tins. Identify the heat transfer

processes that will promote freezing of the water. Will the likelihood of freezing change as the insula- tion thickness is modified? Will the likelihood of freezing depend on the tank wall’s thickness and material? Would freezing of the water be more likely if plastic (low thermal conductivity) or stainless steel (moderate thermal conductivity) tubing is used to transfer the water to and from the tank? Is there an optimal tank shape that would minimize the probabil- ity of the water freezing? Would freezing be more likely or less likely to occur if a thin sheet of alu- minum foil (high thermal conductivity, low emissiv- ity) is applied to the outside of the insulation?

Water Transfer tubing To fuel cell tt t_ins h, T_∞ Tsur

(d) Your grandmother is concerned about reducing her winter heating bills. Her strategy is to loosely fit rigid polystyrene sheets of insulation over her double-pane windows right after the first freezing weather arrives in the autumn. Identify the relevant heat transfer processes on a cold winter night when the foamed insulation sheet is placed (i) on the inner surface and (ii) on the outer surface of her window. To avoid condensation damage, which configuration is preferred? Condensation on the window pane does not occur when the foamed insulation is not in place.

(e) There is considerable interest in developing building materials with improved insulating qualities. The development of such materials would do much to enhance energy conservation by reducing space heat- ing requirements. It has been suggested that superior structural and insulating qualities could be obtained by using the composite shown. The material consists of a honeycomb, with cells of square cross section, sandwiched between solid slabs. The cells are filled with air, and the slabs, as well as the honeycomb matrix, are fabricated from plastics of low thermal conductivity. For heat transfer normal to the slabs, identify all heat transfer processes pertinent to the performance of the composite. Suggest ways in which this performance could be enhanced.

Insulation on outer surface Cold, dry night air Exterior pane Air gap Interior pane Insulation Warm, moist room air Insulation on inner surface Cold, dry night air Exterior pane Air gap Interior pane Insulation Warm, moist room air

(f) A thermocouple junction (bead) is used to measure the temperature of a hot gas stream flowing through a channel by inserting the junction into the mainstream of the gas. The surface of the channel is cooled such that its temperature is well below that of the gas. Identify the heat transfer processes associated with the junction surface. Will the junction sense a tem- perature that is less than, equal to, or greater than the gas temperature? A radiation shield is a small, open- ended tube that encloses the thermocouple junction, yet allows for passage of the gas through the tube. How does use of such a shield improve the accuracy of the temperature measurement?

Surface slabs

Cellular air spaces

(h) A thermocouple junction is used to measure the tem- perature of a solid material. The junction is inserted into a small circular hole and is held in place by epoxy. Identify the heat transfer processes associated with the junction. Will the junction sense a tempera- ture less than, equal to, or greater than the solid tem- perature? How will the thermal conductivity of the epoxy affect the junction temperature?