Heating and Cooling Air and Carbon Dioxide in the Thermal Entrance Region of a Circular Duct With Large Gas to Wall Temperature Differences

The local heat-transfer characteristics for air and carbon dioxide in the thermal entrance region have been determined experimentally and are compared with predicted values computed from an extension of the theoretical analysis due to Deissler. The experiments were conducted in smooth round tubes having a fully developed turbulent velocity profile and a uniform temperature distribution at the entrance. The boundary conditions of uniform heat flux for heating the gas, and wall temperature constant and variable with axial distance for cooling the gas, were investigated with large temperature differences between the gas and the tube wall. For heating, the experimental results yielded one per cent thermal entrance lengths ranging from 11 to 27 diameters over the bulk Reynolds number range from 50,000 to 246,000. For cooling, one per cent thermal entrance lengths ranging from 12 to 26 diameters were obtained for bulk Reynolds numbers ranging from 17,000 to 218,000. The agreement between theory and experiment was favorable in most cases.