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Chapter 9: Problem 114

Consider a double-pane window whose air space width is \(20 \mathrm{~mm}\). Now a thin polyester film is used to divide the air space into two 10-mm-wide layers. How will the film affect \((a)\) convection and (b) radiation heat transfer through the window?

Short Answer

Expert verified
Answer: (a) Convection heat transfer is likely to be decreased, as the film divides the air space into smaller layers, limiting the air particle movement and current formation. (b) Radiation heat transfer might be slightly affected, as the film introduces additional radiation exchange surfaces. The overall impact on heat transfer depends on the properties of the film, such as emissivity, and the balance between the convective and radiative mechanisms.

Step by step solution

01

Understand Convection Heat Transfer

Convection heat transfer is the process of heat transfer through the movement of fluid particles (in this case, air) subjected to temperature differences. It occurs due to the energy transfer by the circulating air particles in the space between the two panes. In the double-pane window, the air space width is essential for the convection heat transfer process.
02

Analyze the Impact of the Film on Convection Heat Transfer

When a thin polyester film is introduced, the air space width is divided into two layers, each with a width of 10 mm. With the smaller air space width, the air particles have less freedom to move around, which generally limits the convection currents. As a result, the convection heat transfer will be decreased when the air space is divided by the film.
03

Understand Radiation Heat Transfer

Radiation heat transfer is the process of heat transfer that occurs due to the emission of electromagnetic energy, which travels as waves that carry energy from one surface to another without any medium (air or fluid). In the case of a double-pane window, the primary radiation heat exchange happens between two glass surfaces facing each other. The film will affect the radiation when it is placed between them.
04

Analyze the Impact of the Film on Radiation Heat Transfer

The polyester film is a thin, transparent material. Although it will block some of the radiation heat exchange, the majority of the radiation transfer might still occur. However, the film will also absorb and emit heat, depending on its emissivity and temperature. This introduces an additional layer for the radiation exchange process. So, although the radiation heat transfer may not be significantly reduced, it might be slightly affected due to the additional radiation exchange occurring at the film surfaces.
05

Conclusion

In conclusion, the addition of a thin polyester film in the double-pane window can affect both convection and radiation heat transfer. \((a)\) The convection heat transfer is likely to be decreased, as the film divides the air space into smaller layers, limiting the air particle movement and current formation. \((b)\) The radiation heat transfer might be slightly affected, as the film introduces additional radiation exchange surfaces. The overall impact on heat transfer depends on the properties of the film, such as emissivity, and the balance between the convective and radiative mechanisms.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Convection Heat Transfer
Convection heat transfer involves the movement of fluid particles, often air or a liquid, transporting heat from one place to another. This occurs when a temperature difference exists, leading to the circulation of these particles.

In the context of a double-pane window, the air trapped between the two panes plays a significant role in this type of heat transfer. The width of the space impacts how freely the air particles can circulate.

When a thin polyester film divides the space, each section becomes narrower, reducing the ability of the air to move. This decreased movement lessens the convection currents that typically transfer heat. With limited air particle movement in the smaller divided layers, convection heat transfer through the window is reduced.
Radiation Heat Transfer
Radiation heat transfer is the process by which heat is transferred between objects or surfaces via electromagnetic waves. Unlike convection, radiation does not require any medium for the heat to travel, so it can occur in a vacuum.

In a double-pane window, the main radiation transfer takes place as electromagnetic waves move between the inner surfaces of the window panes. When a polyester film is placed between the panes, it introduces an additional surface for the radiation exchange.

This film, being a thin and somewhat transparent layer, interacts with the radiation heat in two ways: it absorbs part of the heat and can emit it, depending on its temperature and emissivity. Although this additional layer may not drastically change the radiation heat transfer, it does add complexity by slightly modifying how radiation occurs between the surfaces.
Double-Pane Window
A double-pane window is a popular construction element designed for improved thermal insulation. It comprises two layers of glass with an air space in between, acting as a barrier to heat flow.

The purpose of this design is to reduce heat transfer, thereby enhancing energy efficiency and maintaining desirable indoor temperatures.

When a thin polyester film is added to the space between the two panes, it transforms this setup into a triple-layer system. This new configuration can further reduce heat transfer due to diminished convection currents and altered radiation paths.

The film's material properties, like its emissivity and thermal conductivity, are critical in determining how effectively this new setup can control the overall heat transfer through the window.

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Most popular questions from this chapter

A room is to be heated by a coal-burning stove, which is a cylindrical cavity with an outer diameter of \(32 \mathrm{~cm}\) and a height of \(70 \mathrm{~cm}\). The rate of heat loss from the room is estimated to be \(1.5 \mathrm{~kW}\) when the air temperature in the room is maintained constant at \(24^{\circ} \mathrm{C}\). The emissivity of the stove surface is \(0.85\), and the average temperature of the surrounding wall surfaces is \(14^{\circ} \mathrm{C}\). Determine the surface temperature of the stove. Neglect the heat transfer from the bottom surface and take the heat transfer coefficient at the top surface to be the same as that on the side surface. The heating value of the coal is \(30,000 \mathrm{~kJ} / \mathrm{kg}\), and the combustion efficiency is 65 percent. Determine the amount of coal burned a day if the stove operates \(14 \mathrm{~h}\) a day. Evaluate air properties at a film temperature of \(77^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\) pressure. Is this a good assumption?

A \(12-\mathrm{cm}\)-high and 20-cm-wide circuit board houses 100 closely spaced logic chips on its surface, each dissipating \(0.05 \mathrm{~W}\). The board is cooled by a fan that blows air over the hot surface of the board at \(35^{\circ} \mathrm{C}\) at a velocity of \(0.5 \mathrm{~m} / \mathrm{s}\). The heat transfer from the back surface of the board is negligible. Determine the average temperature on the surface of the circuit board assuming the air flows vertically upward along the 12 -cm-long side by (a) ignoring natural convection and ( \(b\) ) considering the contribution of natural convection. Disregard any heat transfer by radiation. Evaluate air properties at a film temperature of \(47.5^{\circ} \mathrm{C}\) and 1 atm pressure. Is this a good assumption?

Consider a \(0.3\)-m-diameter and \(1.8-\mathrm{m}\)-long horizontal cylinder in a room at \(20^{\circ} \mathrm{C}\). If the outer surface temperature of the cylinder is \(40^{\circ} \mathrm{C}\), the natural convection heat transfer coefficient is (a) \(3.0 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (b) \(3.5 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(3.9 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(4.6 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(5.7 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)

Determine the \(U\)-factor for the center-of-glass section of a double-pane window with a 13-mm air space for winter design conditions. The glazings are made of clear glass having an emissivity of \(0.84\). Take the average air space temperature at design conditions to be \(10^{\circ} \mathrm{C}\) and the temperature difference across the air space to be \(15^{\circ} \mathrm{C}\).

Two concentric spheres of diameters \(15 \mathrm{~cm}\) and \(25 \mathrm{~cm}\) are separated by air at \(1 \mathrm{~atm}\) pressure. The surface temperatures of the two spheres enclosing the air are \(T_{1}=350 \mathrm{~K}\) and \(T_{2}=\) \(275 \mathrm{~K}\), respectively. Determine the rate of heat transfer from the inner sphere to the outer sphere by natural convection.
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