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Difficulty: Very hardHeat Transfer: Convection

An industrial liquid cooling system normally relies on a mechanical pump for forced convection. An engineer is modifying the system to include a fail-safe thermosiphon loop that relies entirely on natural convection if the pump breaks down. Based on the principles of fluid dynamics and thermal energy transfer, which of the following design configurations is required for this fail-safe to work, and what mechanism drives it?

  1. The radiator must be mounted physically higher than the heat source, allowing the heated fluid to expand, decrease in density, and rise into the cooling fins.Answer
  2. B
    The radiator must be mounted physically lower than the heat source, allowing the heated fluid to thermally contract, increase in density, and sink downward.
  3. C
    The radiator must be bolted flush against the heat source to maximize the direct transfer of thermal energy through the static fluid and metal casing.
  4. D
    The system must use a fluid with a minimal specific heat capacity so that it absorbs and releases thermal energy at a rapid rate without requiring circulation.

Answer

The radiator must be mounted physically higher than the heat source, allowing the heated fluid to expand, decrease in density, and rise into the cooling fins.
The correct answer accurately identifies that a natural convection loop (thermosiphon) requires the heat sink to be elevated above the heat source. When the fluid at the heat source warms up, thermal expansion decreases its density, making it buoyant. This hotter, lighter fluid naturally rises into the elevated radiator. As it cools in the radiator, its density increases, causing it to sink back down to the heat source, establishing a continuous, self-sustaining flow without a pump.

Step-by-Step Solution

1
Analyze the difference between forced and natural convection.
Forced convection uses a pump; natural convection relies solely on buoyancy forces caused by density differences in the fluid.
Understanding the driving force of the fail-safe system is required to determine the necessary physical layout.
2
Determine how heat affects the density of the cooling fluid.
As the fluid absorbs heat from the engine block, its molecules spread out (thermal expansion), which decreases its density and causes it to become buoyant.
Identifying the density change dictates the direction the hot fluid will naturally move (upwards).
3
Evaluate the placement of the radiator (heat sink) based on fluid movement.
Because hot fluid naturally rises and cold fluid naturally sinks, the radiator must be placed above the heat source to allow the rising hot fluid to enter it, cool down, become denser, and sink back down to complete the loop.
A continuous circulation loop cannot form if the radiator is placed below the heat source, as the hot fluid would remain trapped at the top.

Key Concept

Natural convection is driven by buoyancy forces resulting from the thermal expansion and decreased density of heated fluids.
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Topics

Heat Transfer: Convection

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