Thermal and material transfer in turbulent gas streams: Local transport from spheres

Abstract

Abstract Local convective thermal transfer is difficult to predict for nonuniform three‐dimensional boundary flows. Direct measurements of local transfer from objects of practical interest are therefore useful in the prediction of thermal transfer and in an understanding of multidimensional boundary flows. Measurements of the gross and local transfer were made upon a silver sphere 0.5 in. in diameter and a ceramic porous sphere of the same size from which n ‐heptane was permitted to evaporate. The air stream had a level of turbulence of approximately 5.4% and only small variation in velocity with position. Temperature distributions in the boundary flows around these spheres were determined, and from these distributions local transfer coefficients were established for the forward hemisphere. The gross transfers were established from the electrical energy added to the silver sphere and from the quantity of n ‐heptane evaporated from the porous sphere. The local thermal transfers were in reasonable agreement with some of the theoretical analyses based upon a three‐dimensional laminar‐boundary layer. Satisfactory agreement was obtained between spatial integration of the local transfer and the simultaneously measured over‐all values. These, in turn, were in fair agreement with correlated values of the gross thermal and material transfer from spheres.