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Question

Difficulty: MediumHeat Transfer: Radiation

A heated metal component is suspended by thin, insulated wires inside a sealed test chamber. Engineers need to maintain the component's high temperature for as long as possible after its internal heat source is turned off. Which of the following chamber configurations will best minimize the rate of heat loss from the component?

  1. Polishing the component to a mirror finish and evacuating all air to create a vacuumAnswer
  2. B
    Painting the component flat black and evacuating all air to create a vacuum
  3. C
    Polishing the component to a mirror finish and filling the chamber with an inert gas
  4. D
    Painting the component flat black and filling the chamber with an inert gas

Answer

Polishing the component to a mirror finish and evacuating all air to create a vacuum
To minimize total heat loss, all transfer methods must be addressed. Creating a vacuum completely eliminates conduction and convection because these require a physical medium. To minimize the remaining transfer method—thermal radiation—the component must have a highly polished, reflective finish, as shiny surfaces are very poor emitters of radiant heat.

Step-by-Step Solution

1
Identify all potential modes of heat transfer from the component.
Heat can be lost through conduction, convection, and thermal radiation.
To minimize total heat loss, all active transfer mechanisms must be addressed.
2
Determine the best method to eliminate conduction and convection.
Evacuating the chamber to create a vacuum is the best method.
Both conduction and convection require a physical medium (like air or gas) to transfer thermal energy. A vacuum removes this medium.
3
Determine the best method to minimize thermal radiation.
Applying a highly polished, reflective mirror finish minimizes radiation.
Radiation travels via electromagnetic waves and can pass through a vacuum. Shiny, highly reflective surfaces are very poor emitters of thermal radiation compared to dull, dark surfaces.

Key Concept

Heat Transfer in a Vacuum and Surface Emissivity
Estimated Time:45s
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Topics

Heat Transfer: Radiation

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