Questions and Answers of Fundamentals Thermal Fluid

Can a medium involve(a) Conduction and convection,(b) Conduction and radiation,(c) Convection and radiation simultaneously? Give examples for the “yes” answers.
The deep human body temperature of a healthy person remains constant at 37°C while the temperature and the humidity of the environment change with time. Discuss the heat transfer mechanisms between
We often turn the fan on in summer to help us cool. Explain how a fan makes us feel cooler in the summer. Also explain why some people use ceiling fans also in winter.
Consider a 20 cm thick granite wall with a thermal conductivity of 2.79 W/m·K. The temperature of the left surface is held constant at 50°C, whereas the right face is exposed to a flow of
A solid plate, with a thickness of 15 cm and a thermal conductivity of 80 W/mˆ™K, is being cooled at the upper surface by air. The air temperature is 10°C, while the temperatures at the
Air at 20°C with a convection heat transfer coefficient of 25 W/m2·K blows over a horizontal steel hot plate (k = 43 W/m·K). The surface area of the plate is 0.38 m2with a thickness
An electronic package with a surface area of 1 m2 placed in an orbiting space station is exposed to space. The electronics in this package dissipate all 1 kW of its power to the space through its
Consider steady heat transfer between two large parallel plates at constant temperatures of T1 = 290 K and T2 = 150 K that are L = 2 cm apart. Assuming the surfaces to be black (emissivity ε = 1),
Consider a person standing in a room at 18°C. Determine the total rate of heat transfer from this person if the exposed surface area and the skin temperature of the person are 1.7 m2 and 32°C,
The inner and outer surfaces of a 25-cm-thick wall in summer are at 27°C and 44°C, respectively. The outer surface of the wall exchanges heat by radiation with surrounding surfaces at
A 2-in-diameter spherical ball whose surface is maintained at a temperature of 170°F is suspended in the middle of a room at 70°F. If the convection heat transfer coefficient is 15 Btu/h·ft2·°F
An 800-W iron is left on the iron board with its base exposed to the air at 20°C. The convection heat transfer coefficient between the base surface and the surrounding air is 35 W/m2·K. If
A 3-m-internal-diameter spherical tank made of 1-cm-thick stainless steel is used to store iced water at 0°C. The tank is located outdoors at 25°C. Assuming the entire steel tank to be at 0°C and
Solar radiation is incident on a = m2 solar absorber plate surface at a rate of 800 W/m2. Ninety-three percent of the solar radiation is absorbed by the absorber plate, while the remaining 7 percent
A flat-plate solar collector is used to heat water by having water flow through tubes attached at the back of the thin solar absorber plate. The absorber plate has a surface area of 2 m2with
The roof of a house consists of a 22-cm-thick concrete slab (k = 2 W/m·K) that is 15 m wide and 20 m long. The emissivity of the outer surface of the roof is 0.9, and the convection heat transfer
Consider a flat-plate solar collector placed horizontally on the flat roof of a house. The collector is 5 ft wide and 15 ft long, and the average temperature of the exposed surface of the collector
An AISI 304 stainless steel sheet is going through an annealing process inside an electrically heated oven. The ambient air inside the oven is 600°C, while the surrounding surfaces of the oven are
Engine valves (cp = 440 J/kg·K and ρ = 7840 kg/m3) are to be heated from 40°C to 800°C in 5 min in the heat treatment section of a valve manufacturing facility. The valves have a cylindrical stem
A cylindrical resistor element on a circuit board dissipates 1.2 W of power. The resistor is 2 cm long, and has a diameter of 0.4 cm. Assuming heat to be transferred uniformly from all surfaces,
The heat generated in the circuitry on the surface of a silicon chip (k = 130 W/m·K) is conducted to the ceramic substrate to which it is attached. The chip is 6 mm Ã— 6 mm in size
A 0.3-cm-thick, 12-cm-high, and 18-cm-long circuit board houses 80 closely spaced logic chips on one side, each dissipating 0.06 W. The board is impregnated with copper fillings and has an effective
A 40-cm-long, 800-W electric resistance heating element with diameter 0.5 cm and surface temperature 120°C is immersed in 75 kg of water initially at 20°C. Determine how long it will take for this
It is well known that wind makes the cold air feel much colder as a result of the wind chill effect that is due to the increase in the convection heat transfer coefficient with increasing air
An engine block with a surface area measured to be 0.95 m2 generates a power output of 50 kW with a net engine efficiency of 35%. The engine block operates inside a compartment at 157°C and the
Consider an electrical wire submerged in liquid water at atmospheric conditions. The wire has a diameter of 1 mm and a length of 15 cm. The current through the wire is increased until the water
A cylindrical fuel rod of 2 cm in diameter is encased in a concentric tube and cooled by water. The fuel generates heat uniformly at a rate of 150 MW/m3. The convection heat transfer coefficient on
Consider a person standing in a room maintained at 20°C at all times. The inner surfaces of the walls, floors, and ceiling of the house are observed to be at an average temperature of 12°C in
Reconsider Prob. 16–84. Using an appropriate software, plot the rate of radiation heat transfer in winter as a function of the temperature of the inner surface of the room in the range of 8°C to
Consider a 3-m × 3-m × 3-m cubical furnace whose top and side surfaces closely approximate black surfaces at a temperature of 1200 K. The base surface has an emissivity of ε = 0.4, and is
A soldering iron has a cylindrical tip of 2.5 mm in diameter and 20 mm in length. With age and usage, the tip has oxidized and has an emissivity of 0.80. Assuming that the average convection heat
A thin metal plate is insulated on the back and exposed to solar radiation on the front surface. The exposed surface of the plate has an absorptivity of 0.7 for solar radiation. If solar radiation is
Consider a flat-plate solar collector placed on the roof of a house. The temperatures at the inner and outer surfaces of the glass cover are measured to be 33°C and 31°C, respectively. The glass
An electric heater with the total surface area of 0.25 m2and emissivity 0.75 is in a room where the air has a temperature of 20°C and the walls are at 10°C. When the heater consumes 500 W of
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?
A 50-m-long section of a steam pipe whose outer diameter is 10 cm passes through an open space at 15°C. The average temperature of the outer surface of the pipe is measured to be 150°C. If
Consider heat conduction through a wall of thickness L and area A. Under what conditions will the temperature distributions in the wall be a straight line?
Consider heat conduction through a plane wall. Does the energy content of the wall change during steady heat conduction? How about during transient conduction? Explain.
What does the thermal resistance of a medium represent?
Can we define the convection resistance for a unit surface area as the inverse of the convection heat transfer coefficient?
Consider steady heat transfer through the wall of a room in winter. The convection heat transfer coefficient at the outer surface of the wall is three times that of the inner surface as a result of
How is the combined heat transfer coefficient defined? What convenience does it offer in heat transfer calculations?
Why are the convection and the radiation resistances at a surface in parallel instead of being in series?
Consider steady one-dimensional heat transfer through a plane wall exposed to convection from both sides to environments at known temperatures T∞1 and T∞2 with known heat transfer coefficients h1
Someone comments that a microwave oven can be viewed as a conventional oven with zero convection resistance at the surface of the food. Is this an accurate statement?
Consider two cold canned drinks, one wrapped in a blanket and the other placed on a table in the same room. Which drink will warm up faster?
Consider a surface of area A at which the convection and radiation heat transfer coefficients are hconv and hrad, respectively. Explain how you would determine (a) the single equivalent heat transfer
How does the thermal resistance network associated with a single-layer plane wall differ from the one associated with a five-layer composite wall?
Consider steady one-dimensional heat transfer through a multilayer medium. If the rate of heat transfer is known, explain how you would determine the temperature drop across each layer.
Consider a window glass consisting of two 4-mmthick glass sheets pressed tightly against each other. Compare the heat transfer rate through this window with that of one consisting of a single
Consider a 3-m-high, 6-m-wide, and 0.3-m-thick brick wall whose thermal conductivity is k = 0.8 W/m·K. On a certain day, the temperatures of the inner and the outer surfaces of the wall are measured
Consider a person standing in a room at 20°C with an exposed surface area of 1.7 m2. The deep body temperature of the human body is 37°C, and the thermal conductivity of the human tissue near the
Consider an electrically heated brick house (k = 0.40 Btu/h·ft·°F) whose walls are 9 ft high and 1 ft thick. Two of the walls of the house are 50 ft long and the others are 35 ft
A 12-cm × 18-cm circuit board houses on its surface 100 closely spaced logic chips, each dissipating 0.06 W in an environment at 40°C. The heat transfer from the back surface of the board is
A cylindrical resistor element on a circuit board dissipates 0.15 W of power in an environment at 40°C. The resistor is 1.2 cm long, and has a diameter of 0.3 cm. Assuming heat to be transferred
Consider a power transistor that dissipates 0.2 W of power in an environment at 30°C. The transistor is 0.4 cm long and has a diameter of 0.5 cm. Assuming heat to be transferred uniformly from
A 1.0 m × 1.5 m double-pane window consists of two 4-mm-thick layers of glass (k = 0.78 W/m·K) that are separated by a 5-mm air gap (kair = 0.025 W/m·K). The heat flow through the air gap is
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·K. Determine the steady rate of heat transfer through this glass window and the
Consider a 1.2-m-high and 2-m-wide doublepane window 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).
Repeat Prob. 17€“23, assuming the space between the two glass layers is evacuated.Data from 17-23Consider a 1.2-m-high and 2-m-wide doublepane window consisting of two 3-mm-thick layers of
Reconsider Prob. 17€“23. Using an appropriate software, plot the rate of heat transfer through the window as a function of the width of air space in the range of 2 mm to 20 mm, assuming
A wall is constructed of two layers of 0.7-in-thick sheetrock (k = 0.10 Btu/h·ft·°F), which is a plasterboard made of two layers of heavy paper separated by a layer of gypsum,
To defog the rear window of an automobile, a very thin transparent heating element is attached to the inner surface of the window. A uniform heat flux of 1300 W/m2is provided to the heating element
To defrost ice accumulated on the outer surface of an automobile windshield, warm air is blown over the inner surface of the windshield. Consider an automobile windshield with thickness of 5 mm and
An aluminum plate of 25 mm thick (k = 235 W/m·K) is attached on a copper plate with thickness of 10 mm. The copper plate is heated electrically to dissipate a uniform heat flux of 5300 W/m2.
The roof of a house consists of a 15-cm-thick concrete slab (k = 2 W/m·K) that is 15 m wide and 20 m long. The convection heat transfer coefficients on the inner and outer surfaces of the roof
A 2-m × 1.5-m section of wall of an industrial furnace burning natural gas is not insulated, and the temperature at the outer surface of this section is measured to be 80°C. The temperature of the
The wall of a refrigerator is constructed of fiberglass insulation (k = 0.035 W/m·K) sandwiched between two layers of 1-mm-thick sheet metal (k = 15.1 W/m·K). The refrigerated space is
Reconsider Prob. 17€“32. Using an appropriate software, investigate the effects of the thermal conductivities of the insulation material and the sheet metal on the thickness of the
Heat is to be conducted along a circuit board that has a copper layer on one side. The circuit board is 15 cm long and 15 cm wide, and the thicknesses of the copper and epoxy layers are 0.1 mm and
A 0.03-in-thick copper plate (k = 223 Btu/h·ft·°F) is sandwiched between two 0.15-in-thick epoxy boards (k = 0.15 Btu/h·ft·°F) that are 7 in Ã— 9 in in
Consider a house whose walls are 12 ft high and 40 ft long. Two of the walls of the house have no windows, while each of the other two walls has four windows made of 0.25-in-thick glass (k = 0.45
What is thermal contact resistance? How is it related to thermal contact conductance?
Will the thermal contact resistance be greater for smooth or rough plain surfaces?
Explain how the thermal contact resistance can be minimized.
A wall consists of two layers of insulation pressed against each other. Do we need to be concerned about the thermal contact resistance at the interface in a heat transfer analysis or can we just
A plate consists of two thin metal layers pressed against each other. Do we need to be concerned about the thermal contact resistance at the interface in a heat transfer analysis or can we just
The thermal contact conductance at the interface of two 1-cm-thick copper plates is measured to be 18,000 W/m2·K. Determine the thickness of the copper plate whose thermal resistance is equal to the
Two 5-cm-diameter, 15-cm-long aluminum bars (k = 176 W/m·K) with ground surfaces are pressed against each other with a pressure of 20 atm. The bars are enclosed in an insulation sleeve and, thus,
A 1-mm-thick copper plate (k = 386 W/m·K) is sandwiched between two 5-mm-thick epoxy boards (k = 0.26 W/m·K) that are 15 cm Ã— 20 cm in size. If the thermal contact
Two identical aluminum plates with thickness of 30 cm are pressed against each other at an average pressure of 1 atm. The interface, sandwiched between the two plates, is filled with glycerin. On the
A two-layer wall is made of two metal plates, with surface roughness of about 25 mm, pressed together at an average pressure of 10 MPa. The first layer is a stainless steel plate with a thickness of
An aluminum plate and a stainless steel plate are pressed against each other at an average pressure of 20 MPa. Both plates have a surface roughness of 2 μm. Determine the impact on the temperature
A thin electronic component with a surface area of 950 cm2 is cooled by having a heat sink attached on its top surface. The thermal contact conductance of the interface between the electronic
Consider an engine cover that is made with two layers of metal plates. The inner layer is stainless steel (k1 = 14 W/m∙K) with a thickness of 10 mm, and the outer layer is aluminum (k2 = 237
Inconel® refers to a class of nickel-chromium-based superalloys that are used in high-temperature applications, such as gas turbine blades. For further improvement in the performance of gas turbine
What are the two approaches used in the development of the thermal resistance network for two-dimensional problems?
The thermal resistance networks can also be used approximately for multidimensional problems. For what kind of multidimensional problems will the thermal resistance approach give adequate results?
When plotting the thermal resistance network associated with a heat transfer problem, explain when two resistances are in series and when they are in parallel.
A 10-cm-thick wall is to be constructed with 2.5-m-long wood studs (k = 0.11 W/m·K) that have a cross section of 10 cm × 10 cm. At some point the builder ran out of those studs and started using
Consider a 6-in Ã— 8-in epoxy glass laminate (k = 0.10 Btu/h·ft·°F) whose thickness is 0.05 in. In order to reduce the thermal resistance across its thickness,
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
A typical section of a building wall is shown in Fig. P17€“58. This section extends in and out of the page and is repeated in the vertical direction. The wall support members are made of
A 4-m-high and 6-m-wide wall consists of a long 18-cm Ã— 30-cm cross section of horizontal bricks (k = 0.72 W/m·K) separated by 3-cm-thick plaster layers (k = 0.22 W/m·K).
A 12-m-long and 5-m-high wall is constructed of two layers of 1-cm-thick sheetrock (k = 0.17 W/m·K) spaced 16 cm by wood studs (k = 0.11 W/m·K) whose cross section is 16 cm × 5 cm. The studs are
A 10-in-thick, 30-ft-long, and 10-ft-high wall is to be constructed using 9-in-long solid bricks (k = 0.40 Btu/h·ft·°F) of cross section 7 in Ã— 7 in, or identical size
Consider a 5-m-high, 8-m-long, and 0.22-m-thick wall whose representative cross section is as given in the figure. The thermal conductivities of various materials used, in W/m·K, are kA= kF=
In an experiment to measure convection heat transfer coefficients, a very thin metal foil of very low emissivity (e.g., highly polished copper) is attached on the surface of a slab of material with
What is an infinitely long cylinder? When is it proper to treat an actual cylinder as being infinitely long, and when is it not?
Can the thermal resistance concept be used for a solid cylinder or sphere in steady operation? Explain.

Showing 700 - 800 of 1421