Experimental Investigation of Turbulent Heat Transfer in the Entrance Region of a Circular Conduit

Abstract

The local heat transfer characteristics for air flowing turbulently in the entrance region of a circular duct have been determined experimentally over a flow Reynolds number range of 10 000 to 110 000. A wall-boundary heating condition of uniform heat flux was imposed. The entrance configurations investigated included a long calming section, bellmouth, orifice plate arrangements and various entries of practical significance—elbow, T-piece, etc. A correlation of heat transfer under fully developed conditions is included together with a discussion of the effects of fluid property variation in an axial direction. Entrance-region heat transfer data are presented in the form of local heat transfer film coefficients and as the dimensionless ratio , this quantity being defined in such a manner as to eliminate the effects of fluid property variation in an axial direction. Data for average heat transfer over tubes of lengths from 1/4 diameter to 320 diameters are presented in the form of the ratio . In the case of the long calming section the results are compared with the results of previous investigations, both experimental and theoretical, the agreement being satisfactory. The use of thermal entrance lengths is discussed and the values obtained are compared with previously accepted values. The heat transfer characteristics obtained with the bellmouth yielded interesting information pertaining to the position and nature of the transition from a laminar to a turbulent boundary layer.