Question

The ........... number may usually be ignored in baffled mixing vessels.

• A. Reynold’s
• B. Froude’s
• C. Nusselt’s
• D. Prandtl’s

Hint:

Remember: Baffles = No Vortex = No Froude Number. Gravity only matters for power consumption calculations when a free surface (vortex) is present.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question relates to fluid mechanics and heat transfer in mixing operations, specifically focusing on dimensionless numbers and their relevance in agitated vessels equipped with baffles.

Step 2: Detailed Explanation:

• Mixing and agitation are fundamental unit operations in food processing used to ensure uniformity, enhance heat transfer, and facilitate mass transfer.

• Dimensionless numbers are used to characterize the flow patterns. The Reynold's number (\(Re\)) represents the ratio of inertial forces to viscous forces. The Froude number (\(Fr\)) represents the ratio of inertial forces to gravitational forces.

• In unbaffled vessels, the centrifugal force generated by the impeller often causes the fluid to swirl and form a central vortex. The depth of this vortex is influenced by gravity, making the Froude number a critical parameter in such systems.

• To prevent vortexing and improve mixing efficiency, baffles are installed along the vessel walls. Baffles transform the circular flow into axial and radial flow patterns, thereby suppressing the formation of a vortex.

• Since the gravitational effect (vortex formation) is neutralized by the presence of baffles, the Froude number becomes irrelevant for describing the power consumption or flow characteristics in a fully baffled system.

• In contrast, the Reynold's number remains essential as it determines whether the flow is laminar or turbulent, which directly impacts energy consumption and mixing time.

Step 3: Final Answer:
In baffled mixing vessels, the vortex is suppressed, and therefore the Froude number can be ignored.