Question

Consider a double-pane window whose airspace is flashed and filled with argon gas. How will replacing the air in the gap with argon affect \((a)\) convection and \((b)\) radiation heat transfer through the window?

Short answer

Answer: Replacing air with argon gas in a double-pane window reduces convection heat transfer due to argon's lower thermal conductivity. However, argon gas does not significantly impact radiation heat transfer since it does not affect the emissivity of the surfaces. To decrease radiation heat transfer, low-emissivity coatings should be applied to the glass panes.

Step-by-step solution

(Step 1: Understand the properties of Argon gas)

Argon is a noble gas, and it is heavier and less thermally conductive than air. This means that heat is transferred through argon gas at a slower rate than through air. We will keep this property in mind as we analyze how argon gas affects convection and radiation heat transfer in the double-pane window.

(Step 2: Analyze the convection heat transfer mechanism in a double-pane window)

Convection heat transfer occurs when heat moves through a material due to circulating currents within the material. In a double-pane window, convection heat transfer between the panes of glass occurs due to the air (or gas) within the gap circulates as it gets heated, transferring heat from the warmer side to the colder side. The thermal conductivity of the gas trapped in the gap affects the rate of heat transfer through convection.

(Step 3: Determine the effect of argon gas on convection heat transfer)

Since argon gas has a lower thermal conductivity compared to air, its presence in the gap between the panes will reduce the movement of heat due to convection. As a result, by replacing the air in the gap with argon gas, the convection heat transfer through the window will decrease.

(Step 4: Analyze the radiation heat transfer mechanism in a double-pane window)

Radiation heat transfer occurs through the emission of electromagnetic waves (infrared radiation) from a warm surface. In a double-pane window, radiation heat transfer between the panes happens due to the emission of warm surfaces toward each side of the gap. The primary factor that affects radiation heat transfer in a double-pane window is the emissivity of the surfaces, which is a measure of how effective the surfaces are at emitting infrared radiation.

(Step 5: Determine the effect of argon gas on radiation heat transfer)

Argon gas, having lower thermal conductivity, forms a barrier that minimizes heat transfer through conduction and convection. However, it does not significantly impact radiation heat transfer, as it does not appreciably alter the emissivity of the surfaces. To decrease radiation heat transfer, the use of low-emissivity coatings on the panes plays a more crucial role than changing the type of gas in the gap.

(Conclusion)

Replacing the air in the gap of a double-pane window with argon gas has the following effects on heat transfer: (a) Convection: It reduces convection heat transfer since argon has lower thermal conductivity than air, slowing down the circulation of gas within the gap and thus reducing the movement of heat through convection. (b) Radiation: Argon gas does not significantly impact radiation heat transfer since it does not affect the emissivity of the surfaces. To decrease radiation heat transfer in double-pane windows, low-emissivity coatings should be applied to the glass panes.