TURBULENT HEAT-TRANSFER IN A SQUARE DUCT

The characteristics of the flow and temperature fields in turbulent fl ow through a straight duct with a square cross section are considerabl y complex owing to the momentum and heat transport by the secondary fl ow of the second kind. For a basic understanding of the turbulent heat transport process in such a complex turbulent flow, the detailed char acteristics of the turbulent temperature field must be made clear. Thi s paper presents experimental results concerning the fluctuating tempe rature intensity, cross-correlation coefficients between the fluctuati ng velocity and temperature, and the turbulent heat fluxes obtained fo r forced-convection heat transfer in a square duct. The contours of th e turbulent heat flux in the streamwise direction are distorted greatl y toward the duct corner, similar to those of the fluctuating temperat ure intensity. Similarity has been also found between the distribution s of the turbulent heat flux in the transverse direction and the turbu lent shear stress. Based on the results of the measurements, the eddy- viscosity, eddy thermal diffusivity, and turbulent Prandtl number have been also obtained. The distributions of the eddy-diffusivities and t urbulent Prandtl numbers on the symmetric axis of a square duct are si milar to those in a circular pipe. As the duct corner is approached, h owever, the eddy-diffusivities become smaller, and the turbulent Prand tl number becomes larger, and thus the assumption of the constant turb ulent Prandtl number is invalid for turbulent heat transfer in a squar e duct. (C) 1997 by Elsevier Science Inc.