Question

Forced convection in a liquid bath is caused by

• A. Density difference brought about by temperature gradients
• B. Molecular energy interaction
• C. Flow of electrons in a random fashion
• D. Intense stirring by an external agency

Hint:

Convection Types:
FORCED CONVECTION = Driven by EXTERNAL MECHANICAL AGITATION (Stirrer Pump).
Free Natural Convection = Driven by buoyant density differences ($Gr$ number).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Convective heat transfer fluid dynamics: Free/Natural convection is driven by buoyancy forces due to temperature-induced density gradients, whereas Forced Convection is driven by mechanical energy input from an external agency (pump, propeller agitator, stirrer).
Key Formula or Approach:
\[ \text{Convection Regimes: } \begin{cases} \text{Natural / Free Convection} \& \longrightarrow \mathbf{\text{Buoyancy Forces (Density Gradient } \Delta \rho)} \mathbf{Forced \text{ } Convection} \& \longrightarrow \mathbf{\text{External Mechanical Energy (Stirrer, Pump, Impeller)}} \end{cases} \]

Step 2: Detailed Explanation:
In fluid flow and convective heat transfer mechanisms in dairy processing:
1. Natural (Free) Convection (A): Fluid motion is induced purely by internal buoyancy forces caused by density differences arising from temperature gradients within the liquid.
2. Forced Convection (D): Fluid motion and macroscopic bulk mixing are forced and driven by an external mechanical agency (such as an electric motor stirrer, propeller agitator, circulation pump, or blower).
- In a jacketed dairy vat or heating bath, intense mechanical stirring forces high-velocity turbulent fluid currents across heat transfer surfaces, thinning the boundary layer and vastly increasing the convective heat transfer coefficient ($h$).

Step 3: Final Answer:
Therefore, forced convection is caused by Intense stirring by an external agency, corresponding to option (D).