Questions and Answers of Fundamentals Thermal Fluid

What is the role of the baffles in a shell-and-tube heat exchanger? How does the presence of baffles affect the heat transfer and the pumping power requirements? Explain.
What is a regenerative heat exchanger? How does a static type of regenerative heat exchanger differ from a dynamic type?
When is a heat exchanger classified as being compact? Do you think a double-pipe heat exchanger can be classified as a compact heat exchanger?
Classify heat exchangers according to flow type and explain the characteristics of each type.
What is an electromagnetic wave? How does it differ from a sound wave?
By what properties is an electromagnetic wave characterized? How are these properties related to each other?
What is thermal radiation? How does it differ from the other forms of electromagnetic radiation?
How does microwave cooking differ from conventional cooking?
What is visible light? How does it differ from the other forms of electromagnetic radiation?
What is the cause of color? Why do some objects appear blue to the eye while others appear red? Is the color of a surface at room temperature related to the radiation it emits?
How do ultraviolet and infrared radiation differ? Do you think your body emits any radiation in the ultraviolet range? Explain.
Why is radiation usually treated as a surface phenomenon?
Electricity is generated and transmitted in power lines at a frequency of 60 Hz (1 Hz = 1 cycle per second). Determine the wavelength of the electromagnetic waves generated by the passage of
The speed of light in vacuum is given to be 3.0 × 108 m/s. Determine the speed of light in air (n = 1), in water (n = 1.33), and in glass (n = 1.5).
A radio station is broadcasting radio waves at a wavelength of 200 m. Determine the frequency of these waves.
A microwave oven is designed to operate at a frequency of 2.2 × 109 Hz. Determine the wavelength of these microwaves and the energy of each microwave.
An electromagnetic wave with a wavelength of 0.5 μm is being propagated in different mediums: air, water, and glass. The refractive index of air, water, and glass are 1, 1.33, and 1.5, respectively.
The electromagnetic spectrum that lies between 0.40 and 0.76 mm is what we call visible light. Within this spectrum, the color violet has the shortest wavelength while the color red has the longest
What is a blackbody? Does a blackbody actually exist?
Define the total and spectral blackbody emissive powers. How are they related to each other? How do they differ?
Why did we define the blackbody radiation function? What does it represent? For what is it used?
Consider a surface at a uniform temperature of 800 K. Determine the maximum rate of thermal radiation that can be emitted by this surface, in W/m2.
A flame from a match may be approximated as a blackbody at the effective surface temperature of 1700 K, while moonlight may be approximated as a blackbody at the effective surface temperature of 4000
At a wavelength of 0.7 mm, the black body emissive power is equal to 108 W/m3. Determine(a) The temperature of the blackbody(b) The total emissive power at this temperature.
The sun can be treated as a blackbody at 5780 K. Using an appropriate software, calculate and plot the spectral blackbody emissive power Ebλ of the sun versus wavelength in the range of 0.01 to 1000
A small body is placed inside of a spherical stainless steel chamber with a diameter of 3 m. The chamber is evacuated and has a constant surface temperature of 500 K. Determine the radiation incident
Consider a black spherical ball, with a diameter of 25 cm, is being suspended in air. Determine the surface temperature of the ball that should be maintained in order to heat 10 kg of air from 20 to
A thin vertical copper plate is subjected to a uniform heat flux of 1000 W/m2on one side, while the other side is exposed to ambient surrounding at 5°C. The surface of the plate is oxidized black
A circular ceramic plate that can be modeled as a blackbody is being heated by an electrical heater. The plate is 30 cm in diameter and is situated in a surrounding ambient temperature of 15°C
An incandescent lightbulb is desired to emit at least 15 percent of its energy at wavelengths shorter than 0.8 μm. Determine the minimum temperature to which the filament of the lightbulb must be
The temperature of the filament of an incandescent lightbulb is 2500 K. Assuming the filament to be a blackbody, determine the fraction of the radiant energy emitted by the filament that falls in the
The temperature of the filament of an incandescent lightbulb is 3000 K. Treating the filament as a blackbody, determine the fraction of the radiant energy emitted by the filament that falls in the
Reconsider Prob. 21–28. Using an appropriate software, investigate the effect of temperature on the fraction of radiation emitted in the visible range. Let the surface temperature vary from 1000 K
It is desired that the radiation energy emitted by a light source reach a maximum in the blue range (λ = 0.47 μm). Determine the temperature of this light source and the fraction of radiation it
The sun can be treated as a blackbody at an effective surface temperature of 10,400 R. Determine the rate at which infrared radiation energy (λ = 0.762100 μm) is emitted by the sun, in Btu/h·ft2.
A 3-mm-thick glass window transmits 90 percent of the radiation between λ = 0.3 and 3.0 mm and is essentially opaque for radiation at other wavelengths. Determine the rate of radiation transmitted
Consider the sun, which is considered to be a blackbody with a surface temperature of roughly 5800 K. Determine the percentage of solar energy(a) In the visible range,(b) At wavelengths shorter than
What is a graybody? How does it differ from a blackbody? What is a diffuse gray surface?
Define the properties emissivity and absorptivity. When are these two properties equal to each other?
Define the properties reflectivity and transmissivity and discus the different forms of reflection.
What is the greenhouse effect? Why is it a matter of great concern among atmospheric scientists?
A 5-in-diameter spherical ball is known to emit radiation at a rate of 550 Btu/h when its surface temperature is 950 R. Determine the average emissivity of the ball at this temperature.
A furnace that has a 40-cm × 40-cm glass window can be considered to be a blackbody at 1200 K. If the transmissivity of the glass is 0.7 for radiation at wavelengths less than 3 μm and zero for
The spectral emissivity function of an opaque surface at 1000 K is approximated asDetermine the average emissivity of the surface and the rate of radiation emission from the surface, in W/m2. 'ε
The emissivity of a tungsten filament can be approximated to be 0.5 for radiation at wavelengths less than 1 μm and 0.15 for radiation at greater than 1 μm. Determine the average emissivity of the
The variations of the spectral emissivity of two surfaces are as given in Fig. P21€“42. Determine the average emissivity of each surface at T = 3000 K. Also, determine the average
The emissivity of a surface coated with aluminum oxide can be approximated to be 0.15 for radiation at wavelengths less than 5 μm and 0.9 for radiation at wavelengths greater than 5 μm. Determine
The variation of the spectral absorptivity of a surface is as given in Fig. P21€“44. Determine the average absorptivity and reflectivity of the surface for radiation that originates from a
The reflectivity of aluminum coated with lead sulfate is 0.35 for radiation at wavelengths less than 3 μm and 0.95 for radiation greater than 3 μm. Determine the average reflectivity of this
The variation of the spectral transmissivity of a 0.6-cm-thick glass window is as given in Fig. P21€“46. Determine the average transmissivity of this window for solar radiation (T
An opaque horizontal plate is well insulated on the edges and the lower surface. The irradiation on the plate is 3000 W/m2, of which 500 W/m2is reflected. The plate has a uniform temperature of 700 K
Irradiation on a semitransparent medium is at a rate of 520 W/m2. If 160 W/m2 of the irradiation is reflected from the medium and 130 W/m2 is transmitted through the medium, determine the medium’s
Consider an opaque plate that is well insulated on the edges and it is heated at the bottom with an electric heater. The plate has an emissivity of 0.67, and is situated in an ambient surrounding
A horizontal plate is experiencing uniform irradiation on the both upper and lower surfaces. The ambient air temperature surrounding the plate is 290 K with a convection heat transfer coefficient of
What does the view factor represent? When is the view factor from a surface to itself not zero?
How can you determine the view factor F12 when the view factor F21 and the surface areas are available?
What are the summation rule and the superposition rule for view factors?
What is the crossed-strings method? For what kind of geometries is the crossed-strings method applicable?
Consider two coaxial parallel circular disks of equal diameter D that are spaced apart by a distance L. If the view factor is F12= 0.1, without altering the diameter of the disks, determine a
A room is to be heated by a coal-burning stove, which is a cylindrical cavity with an outer diameter of 32 cm and a height of 70 cm. The rate of heat loss from the room is estimated to be 1.5 kW when
A manufacturer makes absorber plates that are 1.2 m Ã— 0.8 m in size for use in solar collectors. The back side of the plate is heavily insulated, while its front surface is coated with
Consider two coaxial parallel circular disks of equal diameter D = 1 m spaced apart by 1 m, and two aligned parallel square plates (1 m Ã— 1 m) are also spaced apart by 1 m. Determine the
Consider an enclosure consisting of five surfaces. How many view factors does this geometry involve? How many of these view factors can be determined by the application of the reciprocity and
Consider a hemispherical furnace with a flat circular base of diameter D. Determine the view factor from the dome of this furnace to its base.
Cylindrical heaters are spaced equally at 5 cm apart in a row, and the heaters are positioned between two large parallel plates. If the diameter of the cylinders is 35 mm, determine the view factors
Consider a conical enclosure of height h and base diameter D. Determine the view factor from the conical side surface to a hole of diameter d located at the center of the base. -D- OF
Determine the four view factors associated with an enclosure formed by two very long concentric cylinders of radii r1 and r2. Neglect the end effects.
Determine the view factors from the very long grooves shown in Fig. P21€“62 to the surroundings without using any view factor tables or charts. Neglect end effects. (a) Semicylindrical (b)
Consider a cylindrical enclosure with A1, A2, and A3representing the internal base, top, and side surfaces, respectively. Using the length to diameter ratio, K = L/D, determine(a) The expression for
A circular cone of diameter D is positioned on a common axis with a circular disk, also of diameter, D, at a distance L, as shown in the figure. With a hypothetical area (A3) corresponding to the
A circular cone of diameter D with a length L is positioned on a common axis with a circular disk, also of diameter D, at a distance L (Fig. P21€“65). A cylindrical surface of diameter D
Determine the view factors from the base of a cube to each of the other five surfaces.
Determine the view factors F13and F23between the rectangular surfaces shown in Fig. P21€“67. 3 m A2 m A1 m Аз
Consider a cylindrical enclosure whose height is twice the diameter of its base. Determine the view factor from the side surface of this cylindrical enclosure to its base surface.
For the internal surfaces of the right circular cylinder shown in Fig. P21€“69, determine F13and F33. D2 = 8 cm (2) 10 cm (3) (1) D%3D 8 ст
Two infinitely long parallel plates of width ware located at w distance apart, as shown in the figure. Using the Hottel€™s crossed-strings method, determine the view factor F12. A2 L3 A,
Two infinitely long parallel cylinders of diameter D are located a distance s apart from each other. Determine the view factor F12 between these two cylinders.
Two infinitely long parallel plates of width w are located at a distance of w apart, as shown in Fig. P21€“72. Determine the view factors F12and F21. L2= w 2) ----- L3 4= w w/2
Determine the view factors F12and F21for the very long ducts shown in Fig. P21€“73 without using any view factor tables or charts. Neglect end effects. Oso 50 (0) (b) -------- (c)
Why is the radiation analysis of enclosures that consist of black surfaces relatively easy? How is the rate of radiation heat transfer between two surfaces expressed in this case?
How does radiosity for a surface differ from the emitted energy? For what kind of surfaces are these two quantities identical?
What are the radiation surface and space resistances? How are they expressed? For what kind of surfaces is the radiation surface resistance zero?
What is a reradiating surface? What simplifications does a reradiating surface offer in the radiation analysis?
What are the two methods used in radiation analysis? How do these two methods differ?
Consider a person whose exposed surface area is 1.9 m2, emissivity is 0.85, and surface temperature is 30°C. Determine the rate of heat loss from that person by radiation in a large room whose walls
Consider two black coaxial parallel circular disks of equal diameter D that are spaced apart by a distance L. The top and bottom disks have uniform temperatures of 500°C and 520°C,
Reconsider Prob. 21€“80. Using an appropriate software, evaluate the effect of the distance L between the black coaxial parallel disks (D = 1 m) on the radiation heat transfer coefficient.
Two coaxial parallel disks of equal diameter 1 m are originally placed at a distance of 1 m apart. If both disks behave as black surfaces, determine the new distance between the disks such that there
Two black parallel rectangles with dimensions 3 ft Ã— 5 ft are spaced apart by a distance of 1 ft. The two parallel rectangles are experiencing radiation heat transfer as black surfaces,
The room shown in Fig. P21€“84E is 20 ft by 20 ft wide and 9 ft high. The floor is at 100°F, the walls are at 60°F, and the ceiling is at 40°F. All surfaces are assumed black.
Two aligned parallel rectangles with dimensions 6 mx 8 m are spaced apart by a distance of 2 m. If the two parallel rectangles are experiencing radiation heat transfer as black surfaces, determine
A dryer is shaped like a long semicylindrical duct of diameter 1.5 m. The base of the dryer is occupied with water-soaked materials to be dried. The base is maintained at a temperature of 370 K,
A furnace is of cylindrical shape with R = H = 2 m. The base, top, and side surfaces of the furnace are all black and are maintained at uniform temperatures of 500, 700, and 1400 K, respectively.
Two parallel disks of diameter D = 0.6 m separated by L = 0.4 m are located directly on top of each other. Both disks are black and are maintained at a temperature of 450 K. The back sides of the
Consider a hemispherical furnace of diameter D = 5 m with a flat base, as shown in Fig. P21€“89. The dome of the furnace is black, and the base has an emissivity of 0.7. The base and the
A dryer is shaped like a long semicylindrical duct of diameter 2 m. The base of the dryer is occupied with watersoaked materials that are to be dried. The dome of the dryer has a constant temperature
Two parallel black disks are positioned coaxially with a distance of 0.25 m apart in a surrounding with a constant temperature of 300 K. The lower disk is 0.2 m in diameter and the upper disk is 0.4
Two parallel disks of diameter D = 3 ft separated by L = 2 ft are located directly on top of each other. The disks are separated by a radiation shield whose emissivity is 0.15. Both disks are black
Two infinitely long parallel plates of width w are located at w distance apart, as shown in the figure. The two plates behave as black surfaces, where surface A1has a temperature of 700 K and surface
Two long parallel 20-cm-diameter cylinders are located 30 cm apart from each other. Both cylinders are black, and are maintained at temperatures 425 K and 275 K. The surroundings can be treated as a

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