Heat transfer fundamentals: Natural (free) convection depends on fluid properties and body force. Which of the following quantities influence free convection flow?

Introduction / Context:
Free (natural) convection is fluid motion driven by buoyancy forces that arise from density variations in a gravitational field. Understanding the parameters that control this motion is key to predicting heat transfer in enclosures, around hot surfaces, and in the atmosphere.

Given Data / Assumptions:

• No external pumping; motion arises solely due to buoyancy.
• Properties considered: density, viscosity; field variable: gravity.
• Temperature differences create density gradients through thermal expansion.

Concept / Approach:
The intensity of free convection is captured by dimensionless groups such as the Grashof number Gr and the Rayleigh number Ra. In simplified form, Ra = Gr * Pr, with Gr proportional to g * beta * ΔT * L^3 / ν^2. Here g (gravitational acceleration), β (thermal expansion), ΔT (temperature difference), L (length scale), and ν (kinematic viscosity) directly influence buoyant flow. Density appears in the definition of buoyancy (via beta and fluid weight), and viscosity appears in ν and in damping of motion.

Step-by-Step Solution:

Verification / Alternative check:
Using Ra = g * beta * ΔT * L^3 / (α * ν) shows explicit dependence on gravity g and viscous/thermal diffusivities (which depend on viscosity and density).

Why Other Options Are Wrong:
Picking only one factor neglects the essential interplay of buoyancy (g, density) and viscous damping (viscosity) that defines the flow regime.

Common Pitfalls:
Assuming only gravity matters. Without viscosity and density (thus buoyancy contrast), flow intensity and boundary layer development cannot be predicted.

Final Answer:
All of these