Sharp entry and transition effects for laminar combined convection of water in vertical tubes

In previous work the authors showed a sharp entry to have a definite effect on laminar forced convection to water in a vertical circular tube. The study has been extended to situations of strong aiding natural convection, and new experimental data and numerical predictions are reported. The expected entry effect is confirmed, but it is found to be less marked in a strong combined convection field than in the previous forced convection study. The experimental data also include evidence of transition from laminar flow, and a possible criterion of transition is investigated, based on the axial location of minimum local Nusselt number. This is experimental and predictive data for this criterion are compared with the experimental correlation of Lawrence and Chato based on a criterion of temperature fluctuations