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Difficulty: MediumHeat Transfer: Convection

A heavy-duty diesel engine utilizes a belt-driven mechanical water pump to circulate liquid coolant between the hot engine block and the radiator. If the water pump's drive belt breaks while the engine is operating, which of the following best describes the resulting change in the fluid's heat transfer mechanism?

  1. The primary mechanism shifts from forced convection to natural convection, significantly reducing the cooling efficiency.Answer
  2. B
    The mechanism changes entirely to direct conduction, as heat moves through the stationary liquid without any bulk fluid movement.
  3. C
    The system continues to utilize forced convection because rapid thermal expansion mechanically forces the fluid through the cooling lines.
  4. D
    The cooling rate remains constant because convective heat transfer depends solely on the coolant's specific heat capacity, not its velocity.

Answer

The primary mechanism shifts from forced convection to natural convection, significantly reducing the cooling efficiency.
The correct answer identifies that when the mechanical pump fails, forced convection ceases. However, because the engine continues to heat the coolant, the hotter fluid expands, becomes less dense, and rises, while cooler fluid from the radiator sinks. This buoyancy-driven movement is natural convection, which operates at a much lower flow rate than forced circulation, drastically reducing overall cooling efficiency.

Step-by-Step Solution

1
Identify the initial heat transfer mechanism.
The system uses a mechanical water pump to move the coolant, which indicates forced convection.
A pump uses mechanical energy to physically overcome fluid resistance and drive flow.
2
Determine the physical effect of the pump failing.
The forced mechanical circulation completely stops, but the fluid is still being heated by the engine block.
The mechanical driving force is removed, leaving only the thermal properties of the fluid to dictate movement.
3
Identify the resulting heat transfer mechanism.
The heated coolant expands, becomes less dense, and rises, while cooler coolant from the radiator sinks. This buoyancy-driven circulation is natural convection.
Density differences caused by temperature gradients create a natural fluid flow that operates at a much lower velocity than a mechanical pump, thus reducing the cooling rate.

Key Concept

Forced vs. Natural Convection
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Topics

Heat Transfer: Convection

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