5. In considering the following problems involving heat transfer in the natural

Question

5. In considering the following problems involving heat transfer in the natural environment (outdoors), recognize that solar radiation is comprised of long wavelength components. If this radiation is incident on a semi transparent medium, such as water or glass, two things will happen to the nonreflected portion of the radiation. The long wavelength component will be absorbed at the surface of the medium, whereas the short wavelength component will be transmitted by the surface. influence the heat loss a. The number of panes in a window can strongly Compare the single- from a heated room to the outside ambient air. and double- paned units shown by identifying relevant heat transfer processes for each case. b. In a typical flat-plate solar collector, energy is collected by a working fluid that is circulated through tubes that are in good contact with the back face of an absorber plate. The back face is insulated from the surroundings, and the absorber plate receives solar radiation on its front face, which is typically covered by one or more transparent Identify the relevant heat first for the absorber plate with no cover plate and then for the absorber plate with a single cover plate. c. energy collector design shown below has been used for agricultural applications. Air is blown through a long duct whose cross section is in the form of an equilateral triangle. One side of the triangle is comprised of a double-pane semitransparent cover, the other two sides are constructed from aluminum sheets painted

Explanation / Answer

5. a. A single pane window has only 1 layer of glass. These have little or no insulation. They transmit almost all of the non reflected heat into the room and outside the room. A doube pane window has two layers of glass, seperated by air, which is a non conductor of heat. These are very effective in conservation of heat inside the room, and trap the heat in the insulating layer of air. The long wavelength rays will be absorbed by the double pane window.

b. Without the cover plate, the absorber plate will lose much of its heat into the ambient air and will not be very efficient, since heat loss will take place by radiation. With the cover plate, these losses are minimized and heat is transfered to the tubes by conduction, mainly.

c. With the double paned transparent, heat is conducted by radiation and trapped inside. The black colour of the aluminium plates on the inside makes it a good absorber of heat, since black has a high emissivity. It also emits all of the heat absorbed. The outside layer of styrofoam acts as an insulation, and prevents loss of heat into the surroundings.

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