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Difficulty: MediumHeat Transfer: Convection

A stationary industrial generator is housed inside a protective metal enclosure. The enclosure is designed with intake vents near the floor and exhaust grilles at the highest point of the roof. When the generator is operating, a steady upward flow of air is maintained through the enclosure even without the use of motorized fans. Which of the following best explains the mechanical principle driving this continuous airflow?

  1. As air absorbs heat from the generator, its density decreases, causing it to rise and exit the top grilles while drawing cooler, denser air in through the floor vents.Answer
  2. B
    The thermal energy causes the surrounding air to contract and become heavier, creating a downward pressure differential that forces air out of the enclosure.
  3. C
    Heat is transferred by direct physical contact between the generator and the metal enclosure, which then mechanically pumps the internal air upward.
  4. D
    Electromagnetic waves radiated by the hot generator physically repel the surrounding air molecules, pushing them away and out through the exhaust grilles.

Answer

The continuous airflow is driven by the fact that as air absorbs heat, its density decreases, causing it to rise and exit the top grilles while drawing cooler, denser air in through the floor vents.
The correct answer accurately describes natural convection. When a fluid like air is heated, the molecules gain kinetic energy and spread further apart (thermal expansion). This makes the heated air less dense than the cooler surrounding air. Because it is less dense, buoyancy forces cause the hot air to rise. As it escapes through the top grilles, cooler, denser air is drawn in through the bottom vents to replace it, establishing a continuous cycle without the need for mechanical fans.

Step-by-Step Solution

1
Analyze the thermal change occurring inside the enclosure.
The generator produces heat, which is transferred to the surrounding air.
Understanding the source of thermal energy is the first step in identifying the heat transfer mechanism.
2
Determine how the physical properties of the air change when heated.
The air undergoes thermal expansion, meaning its volume increases and its density decreases.
Density changes in fluids (gases and liquids) dictate how they behave under the influence of gravity.
3
Evaluate the movement of the air based on density.
The less dense, heated air becomes buoyant and rises to the top exhaust grilles, creating a low-pressure area at the bottom that pulls in cooler, denser air.
This establishes a continuous, naturally occurring cycle known as natural convection.

Key Concept

Heat Transfer: Convection
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Topics

Heat Transfer: Convection

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