Question

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

Short answer

Answer: Introducing a thin polyester film in a double-pane window effectively restricts air movement and reduces convective heat transfer. The film may also reduce the overall radiation heat transfer due to additional absorption and re-emission, which can decrease the effective temperature difference driving the heat transfer. However, the exact impact on radiation heat transfer depends on the film's properties.

Step-by-step solution

(a) Effect of film on convection heat transfer)

(With the thin polyester film dividing the airspace into two separate 10-mm-wide layers, the volume of the air under consideration is effectively halved. In a smaller volume, the air movement is restricted, and, as a result, the convective heat transfer is reduced. The rate of convective heat transfer is directly proportional to the temperature difference between the solid and fluid interface, surface area, and air movement. By adding the polyester film, air movement is restricted, leading to a decrease in convective heat transfer.)

(b) Effect of film on radiation heat transfer)

(Radiation heat transfer involves the transfer of heat between two surfaces by emitting and absorbing thermal radiation. The rate of radiation heat transfer depends on factors such as the emissivity of the surfaces, the temperature difference between the surfaces, and the intervening medium's absorption and reflection properties. The thin polyester film acts as an additional surface in the radiation heat transfer path. As the film has its own emissivity and absorptivity, it will affect the overall radiation heat transfer through the window. The film will absorb some of the thermal radiation from the inner pane and re-emit it to the outer pane. The overall radiation heat transfer depends on the film's properties and can either increase or decrease compared to the case without the film. In general, it is expected that the film would reduce the overall radiation heat transfer through the window due to additional absorption and re-emission, which would decrease the effective temperature difference driving the heat transfer.)