Combined Free and Forced Convection in a Constant-Temperature Vertical Tube

An experimental study of superposed free and forced convection for air in a vertical tube is examined. The Nusselt modulus, based on the log-mean-temperature difference, ranged from 15.0 to 57.1; the Graetz modulus, based on the bulk or average temperature of the air, ranged from 40.2 to 1710; and the Grashof-Prandtl D/L modulus, based on properties of air at the wall temperature, ranged from 1.05 × 105 to 1.30 × 106. The Martinelli equation for heating fluids flowing vertically upward does not correlate the data satisfactorily. An analysis of the system from a macroscopic viewpoint leads to the derivation of an equation which fits the experimental data.