Question

In natural convection, the driving force for fluid motion is:

• A. Pressure gradient imposed externally
• B. Buoyancy due to density differences caused by temperature
• C. Viscous shear
• D. Mechanical stirring

Hint:

To identify natural convection problems, look for terms like "buoyancy," "density difference," "temperature gradient," and "Grashof number."
If there is no pump, fan, or stirrer mentioned, the heat transfer mode is natural convection.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the physical driving force that initiates and sustains fluid motion during natural (free) convection.
This is a fundamental concept in convective heat transfer.

Step 2: Key Formula or Approach:
In natural convection, fluid motion is not generated by an external source like a pump or fan.
Instead, it is driven by density variations within the fluid under the influence of gravity.
These density differences are typically caused by temperature gradients.

Step 3: Detailed Explanation:

• Mechanism of Natural Convection: When a fluid is in contact with a hot surface, the fluid layer closest to the surface absorbs heat and expands.
Thermal expansion reduces the density of this hot layer of fluid.
The cooler fluid further away remains denser.

• Buoyancy Generation: Under the influence of gravity, the denser, cooler fluid sinks, which forces the lighter, warmer fluid to rise.
This buoyancy-driven circulation forms convection currents.
The volumetric thermal expansion coefficient \( \beta \) relates the temperature change to density variation:
\[ \beta = -\frac{1}{\rho} \cdot \left(\frac{\partial \rho}{\partial T}\right)_P \]

• Forced Convection Comparison: Forced convection is driven by external means, such as pressure gradients from pumps or fans, or mechanical stirring.


Step 4: Final Answer:
The driving force for fluid motion in natural convection is buoyancy arising from density differences caused by temperature variations.