Question 1

What does the natural convection heat transfer calculator compute?

Accepted Answer

This tool calculates the natural convection heat transfer coefficient h, Rayleigh number Ra, Nusselt number Nu, and heat flux q″ = h·ΔT in real time for four geometries: vertical plate (Churchill-Chu), horizontal plate heated from above, horizontal plate heated from below, and horizontal cylinder. The calculation uses Ra = gβΔTL³/(να), Nu = f(Ra, Pr) via the appropriate correlation, and h = Nu·k/L. Fluid options include air at 20°C and 100°C, water at 20°C and 60°C, and a custom fluid mode.

Question 2

How do I use the natural convection calculator?

Accepted Answer

Select the geometry (vertical plate, horizontal plate top/bottom surface, or horizontal cylinder) in the left panel, then set the characteristic length L and temperature difference ΔT = T_surface − T_fluid using the sliders. Choose a fluid preset or select Custom to enter density, dynamic viscosity, thermal conductivity, Prandtl number, and thermal expansion coefficient manually. The right panel instantly updates Ra, Nu, h, and q″. The h vs ΔT and Nu vs Ra charts show how these quantities vary across a range of conditions.

Question 3

What is the Rayleigh number and how does it affect natural convection?

Accepted Answer

The Rayleigh number Ra = gβΔTL³/(να) is the product of the Grashof number and Prandtl number. It represents the ratio of buoyancy-driven convective forces to viscous and thermal diffusive forces (α = thermal diffusivity, ν = kinematic viscosity). For a vertical plate, Ra < 10⁹ indicates laminar flow and Ra > 10⁹ indicates turbulent flow. Larger Ra values produce stronger convection and higher Nu and h values. Because Ra scales with L³ and ΔT, these parameters have a strong nonlinear effect on the heat transfer coefficient.

Question 4

How can this tool be used for natural air-cooling design of electronics?

Accepted Answer

Use the vertical plate model with the characteristic length set to the heat sink height and ΔT set to the allowable surface-to-air temperature difference. The resulting h (typically 5–25 W/m²K for air natural convection) and heat flux q″ allow you to estimate the required surface area for a given power dissipation. Since natural convection coefficients are much lower than those for forced convection, finned heat sinks with large surface areas are typically required. Increasing ΔT improves h but means higher component temperatures, so reliability must be considered.

Natural Convection Heat Transfer Calculator Heat Transfer

Geometry & Fluid Conditions

h vs ΔT

Nu vs Ra (Log-Log)

Boundary Layer Temperature Profile (Schematic)