The experimental study on transition criteria of natural convection inside a vertical pipe

摘要

Natural convection experiments inside a vertical pipe were performed for Grashof number of 5.2 x 10~6 to 3×10~(10) to seek the proper transition criteria. In order to achieve high Grashof number easily, the analogy concept was employed and a cupric acid-copper sulfate electroplating system was adopted as the mass transfer system. Nusselt numbers measured for laminar natural convection for all Prandtl numbers showed good agreement with the existing heat transfer correlation. However, those for turbulent flow decreased as Prandtl number increased; Nu_H=0.65Ra_H~(0.25) for laminar and Nu_H=0.11Ra_H~(0.28)Pr~(0.13) for turbulent. It is concluded that the proper governing parameter for the description of transition should be Grashof number rather than Rayleigh number and that the transition occurs around Grashof number of 10~9.