The emphasis in the study is for the flow of air in a window cavity. Detailed plots of the stream patterns and isotherms in a multicellular flow are shown and vertically averaged Nusselt numbers as well as typical local values are given. The calculations compare favorably with the experiments of ElSherbiny, Raithby, and Hollands. A working formula for window spacings is suggested.
Sign In or Create an Account. Sign In. Advanced Search. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume , Issue 3. Previous Article Next Article. Article Navigation. Research Papers. Korpela , S. This Site. These serve to keep harmful UV light and heat out, which is especially beneficial in warm climates.
Benefits of Double Pane Windows Improve efficiency and save money: Double pane windows insulate better than single pane versions, making your home more efficient. A home that retains heat in winter and keeps heat out in summer is more affordable to heat and cool. In this way, double pane windows deliver savings to help pay for themselves. The air space between double pane windows insulates not only against heat transfer, but noise as well.
Reduce condensation: Single pane windows tend to condensate when a big temperature difference exists between the interior and exterior, which most often occurs in the winter.
If enough condensation forms, beads of water run down the glass and damage the sill. Mold can start to grow on or behind the sill when this happens. Fortunately, this occurs less often with double pane windows. Other Considerations Benefits only come with a complete upgrade: Because all the windows in your home contribute to drafts and energy waste, you must replace all of them if you expect to enjoy the benefits of double pane windows.
While this results in a higher purchase price, energy savings can help you recoup your investment. As a result, the heat transfer coefficient remains nearly constant, as shown in Fig. Therefore, it makes no sense to use an air space thicker than 13 mm in a double-pane window unless a thin polyester film is used to divide the air space into two halves to suppress convection currents.
The film provides added insulation without adding much to the weight or cost of the double-pane window. The thermal resistance of the window can be increased further by using triple- or quadruple-pane windows whenever it is economical to do so.
Note that using a triple-pane window instead of a double-pane reduces the rate of heat transfer through the center section of the window by about one-third. Another way of reducing conduction heat transfer through a double-pane window is to use a less-conducting fluid such as argon or krypton to fill the gap between the glasses instead of air. The gap in this case needs to be well sealed to prevent the gas from leaking outside.
Of course, another alternative is to evacuate the gap between the glasses completely, but it is not practical to do so. The glasses in double- and triple-pane windows are kept apart from each other at a uniform distance by spacers made of metals or insulators like aluminum, fiberglass, wood, and butyl. Continuous spacer strips are placed around the glass perimeter to provide edge seal as well as uniform spacing.
Heat transfer in the edge region of a window is two-dimensional, and lab measurements indicate that the edge effects are limited to a 6.
The U-factor for the edge region of a window is given in Fig. The curve would be a straight diagonal line if the two U-values were equal to each other. Note that this is almost the case for insulating spacers such as wood and fiberglass. But the U-factor for the edge region can be twice that of the center region for conducting spacers such as those made of aluminum.
Values for steel spacers fall between the two curves for metallic and insulating spacers. The edge effect is not applicable to single-pane windows. The framing of a window consists of the entire window except the glazing. Heat transfer through the framing is difficult to determine because of the different window configurations, different sizes, different constructions, and different combination of materials used in the frame construction. The type of glazing such as single pane, double pane, and triple pane affects the thickness of the framing and thus heat transfer through the frame.
Most frames are made of wood, aluminum, vinyl, or fiberglass. However, using a combination of these materials such as aluminum-clad wood and vinyl-clad aluminum is also common to improve appearance and durability. Aluminum is a popular framing material because it is inexpensive, durable, and easy to manufacture, and does not rot or absorb water like wood. However, from a heat transfer point of view, it is the least desirable framing material because of its high thermal conductivity.
It will come as no surprise that the U-factor of solid aluminum frames is the highest, and thus a window with aluminum framing will lose much more heat than a comparable window with wood or vinyl framing. Heat transfer through the aluminum framing members can be reduced by using plastic inserts between components to serve as thermal barriers.
The thickness of these inserts greatly affects heat transfer through the frame. For aluminum frames without the plastic strips, the primary resistance to heat transfer is due to the interior surface heat transfer coefficient. The U-factors for various frames are listed in Table 17 as a function of spacer materials and the glazing unit thicknesses. Note that the U-factor of metal framing and thus the rate of heat transfer through a metal window frame is more than three times that of a wood or vinyl window frame.
Heat transfer through a window is also affected by the convection and radiation heat transfer coefficients between the glass surfaces and surroundings. The effects of convection and radiation on the inner and outer surfaces of glazings are usually combined into the combined convection and radiation heat transfer coefficients hi and ho, respectively.
Under still air conditions, the combined heat transfer coefficient at the inner surface of a vertical window can be determined from. Here the temperature of the interior surfaces facing the window is assumed to be equal to the indoor air temperature. This assumption is reasonable when the window faces mostly interior walls, but it becomes questionable when the window is exposed to heated or cooled surfaces or to other windows. The commonly used value of h i for peak load calculation is.
The values of h i for various temperatures and glass emissivities are given in Table
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