- Bejan, A. and Lage, J.L., Prandtl number effect on the transition in natural convection along a vertical surface,
Journal of Heat Transfer, Transactions ASME, vol. 112 no. 3
pp. 787 - 790 .
(last updated on 2007/04/08)
The present study was motivated by certain observations and the apparent suggestion that in low-Pr fluids the laminar flow expires at unexpectedly low Rayleigh numbers. This idea is particularly interesting if we think of the natural convection boundary layer near a vertical wall, for which the textbook teaches us to associate the constant Ra approximately equals 109 with the height of transition to the turbulent flow, regardless of the Prandtl number. In order to test this idea, we re-examined the experimental record of observations on transition in vertical natural convection boundary layer flow. Indeed, the empirical data described show that the Prandtl number has a strong influence on the transition Rayleigh number. In low-Pr fluids the transition occurs at Rayleigh numbers much lower than the often-mentioned Ra approximately equals 109, while in high-Pr fluids the transition Rayleigh number is higher than 109. It appears that the constant Grashof number Gr approximately equals 109 (i.e., not Ra approximately equals 109) marks the transition in the wide Pr range 0.001-1000.
Flow of Fluids--Boundary Layer;