Examkin

Question

Difficulty: MediumHeat Transfer: Convection

In an older gravity-fed residential heating system, a boiler in the basement heats water, which then travels up to radiators on the second floor. This system operates entirely without the use of a mechanical circulation pump. Which of the following best describes the primary mechanism causing this upward flow of water?

  1. The heated water expands and decreases in density, allowing it to be displaced upward by cooler, denser water.Answer
  2. B
    Heat transfers continuously through the solid metal pipes via direct contact, drawing the surrounding water upward.
  3. C
    The heated water contracts and increases in density, creating a pressure differential that pulls fluid toward the upper floors.
  4. D
    The boiler increases the weight of the water molecules, generating a greater buoyant force that pushes them up the pipes.

Answer

The heated water expands and decreases in density, allowing it to be displaced upward by cooler, denser water.
The correct answer accurately describes natural convection. When fluid is heated, its molecules move apart (thermal expansion). This decreases the fluid's density relative to the surrounding cooler fluid. Gravity pulls the denser, cooler fluid downward more strongly, which mechanically displaces the warmer, lighter fluid, pushing it upward.

Step-by-Step Solution

1
Analyze the system described in the scenario.
A boiler heats water, which moves upward to radiators without a pump (natural flow).
Identifying the absence of mechanical force indicates the system relies on physical property changes in the fluid.
2
Identify the relevant physical principle acting on the fluid.
When a fluid is heated, it undergoes thermal expansion, increasing its volume and lowering its density.
Understanding thermal expansion is necessary to explain why the fluid's behavior changes.
3
Evaluate the resulting fluid dynamics.
The cooler, denser water from the radiators sinks due to gravity, displacing the warmer, less dense water and pushing it upward.
This describes the continuous convection loop that drives the gravity-fed system.

Key Concept

Natural Convection and Fluid Density
Estimated Time:1m 0s
Rate this question

Topics

Heat Transfer: Convection

More questions from this topic