Question

Assertion A: Fluid property do not impact the heat transfer coefficient of natural convection.
Reason R: Density difference occurring in fluid impact the buoyancy forces which governs natural convection.
In the light of the above statements, choose the most appropriate answer:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Natural convection is all about the Grashof Number ($Gr$), which represents the ratio of buoyancy forces to viscous forces. Since buoyancy depends on density and expansion, fluid properties are absolutely critical!

Answer 1

The Correct Option is D

Concept: Natural (free) convection is driven by buoyancy forces that arise due to density gradients, which are caused by temperature differences within a fluid.

Step 1: Evaluate Assertion A.
The heat transfer coefficient ($h$) for natural convection is heavily dependent on fluid properties such as thermal conductivity ($k$), viscosity ($\mu$), density ($\rho$), and the coefficient of thermal expansion ($\beta$). These are captured in dimensionless numbers like the Grashof ($Gr$) and Prandtl ($Pr$) numbers. Therefore, Assertion A is incorrect.

Step 2: Evaluate Reason R.
Natural convection occurs because a change in temperature causes a change in density. Lighter (warmer) fluid rises and denser (cooler) fluid sinks due to gravity; this movement is driven by buoyancy forces. Thus, Reason R is correct.

Step 3: Conclusion.
Since A is false and R is true, the answer is clearly option (4).