Heat transfer coefficients on the downstream face of an abrupt enlargement or inlet constriction in a pipe

Measurements were made of the local and average heat transfer coefficients on the downstream face of an enlargement step in a pipe. Two flow configurations were investigated: (a) an abrupt enlargement from a smaller diameter pipe to a larger diameter pipe and (b) partial constriction of a pipe inlet by a large baffle plate. Air was the working fluid. The transfer coefficients were determined by means of the naphthalene sublimation technique; axial pressure distributions were also measured. The highest values of the local transfer coefficient were found to occur on the portion of the enlargement face adjacent to the aperture through which the flow enters the enlarged space. On the other hand, the lowest coefficients occur in the corner where the enlargement face meets the wall of the enlarged pipe. The radial distributions of the transfer coefficient on the enlargement face vary with the Reynolds number. With regard to average transfer coefficients, higher values (by at least 50 percent) are attained for the constricted inlet than for the abrupt enlargement. The average coefficients for the enlargement face are much higher (by a factor of two or three) than those on the wall of the enlarged pipe for fully developed flow conditions.