$ Ra=\frac{g\beta\Delta TL^3\rho}{\mu\alpha} $ where: - $g$ gravitational acceleration - $\beta$ thermal expansion coefficient - $T_\text{op}$ operating temperature - $\Delta T$ temperature difference - $L$ characteristic length - $\rho$ fluid density - $\mu$ fluid viscosity - $\alpha$ thermal diffusivity In natural convection: - $Ra < 10^8$ suggests the flow is laminar - $Ra > 10^8$ suggests the flow is turbulent