Free convective heat transfer from a constant heat flux vertical circular tube with different entrance restrictions length

Free convection from the inside surface of vertical circular tube with a uniformly heated surface under constant wall heat flux was investigated experimentally for laminar air flow in the ranges of Ra_L from 6.9 × 10⁸ to 5 × 10⁹. The effect of restrictions length placed at upper side of the heated tube on the surface temperature distribution, the local and average heat transfer coefficients were studied. The experimental apparatus consists of an aluminum tube with 900 mm length and 30 mm inside diameter. The entry restrictions included a circular tube of the same diameter as the heated tube but with different lengths of 60 cm, 90 cm, 120 cm, 150 cm and 180 cm. It was found that the surface temperature along the tube surface had higher values for the restriction with a length of 180 cm and smaller values for the restriction with a length of 120 cm. The results show that the local Nu_x and average Nusselt number over(Nu, -) were higher for the restriction with a length of 120 cm and lower for the restriction with a length of 180 cm. The results obtained were correlated by dimensionless groups as Log over(Nu_L, -) against Log over(Ra_L, -) for each case, and a general correlation for all cases was proposed.