Questions and Answers of Fundamentals Thermal Fluid

Consider a short cylinder whose top and bottom surfaces are insulated. The cylinder is initially at a uniform temperature Ti and is subjected to convection from its side surface to a medium at
Reconsider Prob. 17–68. Using an appropriate software, investigate the effect of the thickness of the insulation on the rate of heat loss from the steam and the temperature drop across the
Superheated steam at an average temperature 200°C is transported through a steel pipe (k = 50 W/m·K, Do = 8.0 cm, Di = 6.0 cm, and L = 20.0 m). The pipe is insulated with a 4-cm thick layer of
Steam exiting the turbine of a steam power plant at 100°F is to be condensed in a large condenser by cooling water flowing through copper pipes (k = 223 Btu/h·ft·°F) of inner
Repeat Prob. 17€“72E, assuming that a 0.01-in-thick layer of mineral deposit (k = 0.5 Btu/h·ft·°F) has formed on the inner surface of the pipe.Data from 17-72ESteam
Reconsider Prob. 17€“72E. Using an appropriate software, investigate the effects of the thermal conductivity of the pipe material and the outer diameter of the pipe on the length of the
A 2.2-mm-diameter and 10-m-long electric wire is tightly wrapped with a 1-mm-thick plastic cover whose thermal conductivity is k = 0.15 W/m·K. Electrical measurements indicate that a current
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
Chilled water enters a thin-shelled 5-cm-diameter, 150-m-long pipe at 7°C at a rate of 0.98 kg/s and leaves at 8°C. The pipe is exposed to ambient air at 30°C with a heat transfer coefficient of 9
Steam at 450°F is flowing through a steel pipe (k = 8.7 Btu/h·ft·°F) whose inner and outer diameters are 3.5 in and 4.0 in, respectively, in an environment at 55°F. The pipe is insulated with
Hot water at an average temperature of 70°C is flowing through a 15-m section of a cast iron pipe (k = 52 W/m·K) whose inner and outer diameters are 4 cm and 4.6 cm, respectively. The outer surface
In a pharmaceutical plant, a copper pipe (kc= 400 W/m·K) with inner diameter of 20 mm and wall thickness of 2.5 mm is used for carrying liquid oxygen to a storage tank. The liquid oxygen
Liquid hydrogen is flowing through an insulated pipe (k = 23 W/m∙K, Di = 3 cm, Do = 4 cm, and L = 20 m). The pipe is situated in a chemical plant, where the average air temperature is 40°C. The
An 8-m-internal-diameter spherical tank made of 1.5-cm-thick stainless steel (k = 15 W/m·K) is used to store iced water at 0°C. The tank is located in a room whose temperature is 25°C.
What is the critical radius of insulation? How is it defined for a cylindrical layer?
Consider an insulated pipe exposed to the atmosphere. Will the critical radius of insulation be greater on calm days or on windy days? Why?
A pipe is insulated to reduce the heat loss from it. However, measurements indicate that the rate of heat loss has increased instead of decreasing. Can the measurements be right?
Consider a pipe at a constant temperature whose radius is greater than the critical radius of insulation. Someone claims that the rate of heat loss from the pipe has increased when some insulation is
A pipe is insulated such that the outer radius of the insulation is less than the critical radius. Now the insulation is taken off. Will the rate of heat transfer from the pipe increase or decrease
A 0.083-in-diameter electrical wire at 90°F is covered by 0.02-in-thick plastic insulation (k = 0.075 Btu/h·ft·°F). The wire is exposed to a medium at 50°F, with a combined convection and
Repeat Prob. 17–88E, assuming a thermal contact resistance of 0.001 h·ft2·°F/Btu at the interface of the wire and the insulation.Data from 17-88E.A 0.083-in-diameter electrical wire at 90°F is
A 5-mm-diameter spherical ball at 50°C is covered by a 1-mm-thick plastic insulation (k = 0.13 W/m·K). The ball is exposed to a medium at 15°C, with a combined convection and radiation
Reconsider Prob. 17€“90. Using an appropriate software, plot the rate of heat transfer from the ball as a function of the plastic insulation thickness in the range of 0.5 mm to 20 mm.
Hot air is to be cooled as it is forced to flow through the tubes exposed to atmospheric air. Fins are to be added in order to enhance heat transfer. Would you recommend attaching the fins inside or
What is the reason for the widespread use of fins on surfaces?
What is the difference between the fin effectiveness and the fin efficiency?
The fins attached to a surface are determined to have an effectiveness of 0.9. Do you think the rate of heat transfer from the surface has increased or decreased as a result of the addition of these
Explain how the fins enhance heat transfer from a surface. Also, explain how the addition of fins may actually decrease heat transfer from a surface.
How does the overall effectiveness of a finned surface differ from the effectiveness of a single fin?
A 4-mm-diameter and 10-cm-long aluminum fin (k = 237 W/m·K) is attached to a surface. If the heat transfer coefficient is 12 W/m2·K, determine the percent error in the rate of heat
Consider a stainless steel spoon (k = 8.7 Btu/h·ft·°F) partially immersed in boiling water at 200°F in a kitchen at 75°F. The handle of the spoon has a cross section of 0.08
Reconsider Prob. 18€“15. Using an appropriate software, investigate the effects of the heat transfer coefficient and the final plate temperature on the time it will take for the plate to
Hot water is to be cooled as it flows through the tubes exposed to atmospheric air. Fins are to be attached in order to enhance heat transfer. Would you recommend attaching the fins inside or outside
Consider two finned surfaces that are identical except that the fins on the first surface are formed by casting or extrusion, whereas they are attached to the second surface afterwards by welding or
The heat transfer surface area of a fin is equal to the sum of all surfaces of the fin exposed to the surrounding medium, including the surface area of the fin tip. Under what conditions can we
Does the (a) efficiency (b) effectiveness of a fin increase or decrease as the fin length is increased?
Two pin fins are identical, except that the diameter of one of them is twice the diameter of the other. For which fin is the (a) fin effectiveness (b) fin efficiency higher? Explain.
Two plate fins of constant rectangular cross section are identical, except that the thickness of one of them is twice the thickness of the other. For which fin is the(a) Fin effectiveness(b) Fin
Two finned surfaces are identical, except that the convection heat transfer coefficient of one of them is twice that of the other. For which finned surface is the (a) fin effectiveness (b) fin
Consider a very long rectangular fin attached to a flat surface such that the temperature at the end of the fin is essentially that of the surrounding air, i.e., 20°C. Its width is 5.0 cm; thickness
Reconsider Prob. 17€“107E. Using an appropriate software, investigate the effects of the thermal conductivity of the spoon material and the length of its extension in the air on the
A DC motor delivers mechanical power to a rotating stainless steel shaft (k = 15.1 W/m·K) with a length of 25 cm and a diameter of 25 mm. In a surrounding with ambient air temperature of
A plane wall with surface temperature of 350°C is attached with straight rectangular fins (k = 235 W/m·K). The fins are exposed to an ambient air condition of 25°C and the convection
Two 3-m-long and 0.4-cm-thick cast iron (k = 52 W/m·K) steam pipes of outer diameter 10 cm are connected to each other through two 1-cm-thick flanges of outer diameter 20 cm. The steam flows
Steam in a heating system flows through tubes whose outer diameter is 5 cm and whose walls are maintained at a temperature of 180°C. Circular aluminum alloy 2024-T6 fins (k = 186 W/m·K) of
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 0.04 W. The board is impregnated with copper fillings and has an effective
A hot surface at 100°C is to be cooled by attaching 3-cm-long, 0.25-cm-diameter aluminum pin fins (k = 237 W/m·K) to it, with a center-to-center distance of 0.6 cm. The temperature of the
Reconsider Prob. 17€“114. Using an appropriate software, investigate the effect of the center-to-center distance of the fins on the rate of heat transfer from the surface and the overall
Circular cooling fins of diameter D = 1 mm and length L = 25.4 mm, made of copper (k = 400 W/m·K), are used to enhance heat transfer from a surface that is maintained at temperature Ts1=
A 40-W power transistor is to be cooled by attaching it to one of the commercially available heat sinks shown in Table 17€“6. Select a heat sink that will allow the case temperature of the
A 25-W power transistor is to be cooled by attaching it to one of the commercially available heat sinks shown in Table 17–6. Select a heat sink that will allow the case temperature of the
Consider two identical people each generating 60 W of metabolic heat steadily while doing sedentary work and dissipating it by convection and perspiration. The first person is wearing clothes made of
Cold conditioned air at 12°C is flowing inside a 1.5-cm-thick square aluminum (k = 237 W/m·K) duct of inner cross section 22 cm × 22 cm at a mass flow rate of 0.8 kg/s. The duct is exposed to air
Hot water is flowing at an average velocity of 1.5 m/s through a cast iron pipe (k 5 52 W/m·K) whose inner and outer diameters are 3 cm and 3.5 cm, respectively. The pipe passes through a 15-m-long
Steam at 235°C is flowing inside a steel pipe (k = 61 W/m·K) whose inner and outer diameters are 10 cm and 12 cm, respectively, in an environment at 20°C. The heat transfer coefficients inside and
A spherical vessel, 3.0 m in diameter (and negligible wall thickness), is used for storing a fluid at a temperature of 0°C. The vessel is covered with a 5.0-cm-thick layer of an insulation (k = 0.20
One wall of a refrigerated warehouse is 10.0-m-high and 5.0-m-wide. The wall is made of three layers: 1.0-cm-thick aluminum (k = 200 W/m·K), 8.0-cm-thick fibreglass (k = 0.038 W/m·K), and 3.0-cm
A 4-m-high and 6-m-long wall is constructed of two large 2-cm-thick steel plates (k = 15 W/m·K) separated by 1-cm-thick and 20-cm wide steel bars placed 99 cm apart. The remaining space
A typical section of a building wall is shown in Fig. P17€“126. This section extends in and out of the page and is repeated in the vertical direction. The wall support members are made of
Circular fins of uniform cross section, with diameter of 10 mm and length of 50 mm, are attached to a wall with surface temperature of 350°C. The fins are made of material with thermal
A total of 10 rectangular aluminum fins (k = 203 W/m·K) are placed on the outside flat surface of an electronic device. Each fin is 100 mm wide, 20 mm high and 4 mm thick. The fins are located
A plane wall surface at 200°C is to be cooled with aluminum pin fins of parabolic profile with blunt tips. Each fin has a length of 25 mm and a base diameter of 4 mm. The fins are exposed to an
Steam in a heating system flows through tubes whose outer diameter is 3 cm and whose walls are maintained at a temperature of 120°C. Circular aluminum alloy fins (k = 180 W/m·K) of outer
A 0.2-cm-thick, 10-cm-high, and 15-cm-long circuit board houses electronic components on one side that dissipate a total of 15 W of heat uniformly. The board is impregnated with conducting metal
What is the physical significance of the Biot number? Is the Biot number more likely to be larger for highly conducting solids or poorly conducting ones?
What is lumped system analysis? When is it applicable?
In what medium is the lumped system analysis more likely to be applicable: in water or in air? Why?
For which solid is the lumped system analysis more likely to be applicable: an actual apple or a golden apple of the same size? Why?
For which kind of bodies made of the same material is the lumped system analysis more likely to be applicable: slender ones or well-rounded ones of the same volume? Why?
Consider heat transfer between two identical hot solid bodies and the air surrounding them. The first solid is being cooled by a fan while the second one is allowed to cool naturally. For which solid
Consider heat transfer between two identical hot solid bodies and their environments. The first solid is dropped in a large container filled with water, while the second one is allowed to cool
Consider a hot baked potato on a plate. The temperature of the potato is observed to drop by 4°C during the first minute. Will the temperature drop during the second minute be less than, equal
Consider a potato being baked in an oven that is maintained at a constant temperature. The temperature of the potato is observed to rise by 5°C during the first minute. Will the temperature rise
Consider two identical 4-kg pieces of roast beef. The first piece is baked as a whole, while the second is baked after being cut into two equal pieces in the same oven. Will there be any difference
Consider a sphere and a cylinder of equal volume made of copper. Both the sphere and the cylinder are initially at the same temperature and are exposed to convection in the same environment. Which do
Obtain relations for the characteristic lengths of a large plane wall of thickness 2L, a very long cylinder of radius ro, and a sphere of radius ro.
Obtain a relation for the time required for a lumped system to reach the average temperature 1/2 (Ti + T∞), where Ti is the initial temperature and T∞ is the temperature of the environment.
A brick of 203 × 102 × 57 mm in dimension is being burned in a kiln to 1100°C, and then allowed to cool in a room with ambient air temperature of 30°C and convection heat transfer coefficient of
Metal plates (k = 180 W/m·K, ρ = 2800 kg/m3, and cp = 880 J/kg?K) with a thickness of 1 cm are being heated in an oven for 2 min. Air in the oven is maintained at 800°C with a convection heat
A 5-mm-thick stainless steel strip (k = 21 W/m·K, Ï = 8000 kg/m3, and cp= 570 J/kg·K) is being heat treated as it moves through a furnace at a speed of 1 cm/s. The air
A batch of 2-cm-thick stainless steel plates (k = 21 W/m·K, Ï = 8000 kg/m3, and cp= 570 J/kg·K) are conveyed through a furnace to be heat treated. The plates enter the
A long copper rod of diameter 2.0 cm is initially at a uniform temperature of 100°C. It is now exposed to an air stream at 20°C with a heat transfer coefficient of 200 W/m2·K. How long would it
Springs in suspension system of automobiles are made of steel rods heated and wound into coils while ductile. Consider steel rods (ρ = 7832 kg/m3, cp = 434 J/kg·K, and k = 63.9 W/m·K) with
Steel rods (ρ = 7832 kg/m3, cp = 434 J/kg·K, and k = 63.9 W/m·K) are heated in a furnace to 850°C and then quenched in a water bath at 50°C for a period of 40 seconds as part of a hardening
Reconsider Prob. 18€“29. Using an appropriate software, investigate the effect of the initial temperature of the balls on the annealing time and the total rate of heat transfer. Let the
In a manufacturing facility, 2-in-diameter brass balls (k = 64.1 Btu/h·ft·°F, Ï = 532 lbm/ft3, and cp= 0.092 Btu/lbm·°F) initially at 250°F are quenched in
How can we use the transient temperature charts when the surface temperature of the geometry is specified instead of the temperature of the surrounding medium and the convection heat transfer
A father and son conducted the following simple experiment on a hot dog which measured 12.5 cm in length and 2.2 cm in diameter. They inserted one food thermometer into the midpoint of the hot dog
An experiment is to be conducted to determine heat transfer coefficient on the surfaces of tomatoes that are placed in cold water at 7°C. The tomatoes (k = 0.59 W/m·K, α = 0.141 × 10-6 m2/s, ρ =
White potatoes (k = 0.50 W/m·K and a = 0.13 Ã— 10-6m2/s) that are initially at a uniform temperature of 25°C and have an average diameter of 6 cm are to be cooled by
A bare-footed person whose feet are at 32°C steps on a large aluminum block at 20°C. Treating both the feet and the aluminum block as semi-infinite solids, determine the contact surface
Reconsider Prob. 18€“79. Using an appropriate software, plot the soil temperature as a function of the distance from the earth€™s surface as the distance varies from 0 to 1 m, and
Reconsider Prob. 18€“91. Using an appropriate software, investigate the effect of the cooling time on the center temperature of the cylinder, the center temperature of the top surface of
How is the product solution used to determine the variation of temperature with time and position in three dimensional systems?
An egg is to be cooked to a certain level of doneness by being dropped into boiling water. Can the cooking time be shortened by turning up the heat and bringing water to a more rapid boiling?
To warm up some milk for a baby, a mother pours milk into a thin-walled cylindrical container whose diameter is 6 cm. The height of the milk in the container is 7 cm. She then places the container
The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a 1.2-mm-diameter sphere. The properties of the junction are k = 35 W/m·K, ρ = 8500 kg/m3,
In an experiment, the temperature of a hot gas stream is to be measured by a thermocouple with a spherical junction. Due to the nature of this experiment, the response time of the thermocouple to
A thermocouple, with a spherical junction diameter of 0.5 mm, is used for measuring the temperature of hot air flow in a circular duct. The convection heat transfer coefficient of the air flow can be
Pulverized coal particles are used in oxy-fuel combustion power plants for electricity generation. Consider a situation where coal particles are suspended in hot air flowing through a heated tube,

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