THEORETICAL AND EXPERIMENTAL STUDY OP TURBULENT HEAT TRANSFER IN THE ENTRY TUBE REGION AT HIGH PRANDTL NUMBERS
The paper considers the process of stabilization: of heat transfer in the entry tube region under conditions of hydrodynamically stabilized turbulent flow and at Pr>>1 when the variation in turbulent diffusion coefficient Ε with the distance to the wall, y, can be described, to a good degree of accuracy, by the exponential function Ε/ν= b(yu*/ν)n. Using the integral method, a general solution of the problem is obtained, which can be used for any values of the problem parameters: x/d, Pr, b, and n. This solution is compared with the results of numerical calculations.
An experimental investigation of the problem was carried out by the electrochemical method of studying mass-transfer processes. The experiment covered the range of Reynolds numbers from 4 × 103 to 230 × 103, Prandtl numbers from 600 to 35,000, and l/d from 4 × 10−3 to 8. The experimental data obtained are compared with the theoretical results. The values of the empirical constants b and n appearing in the calculation equations have been determined. In particular, it has been established that the best agreement between theory and experiment is obtained by assuming the validity of the "third-power law" for the damping of the effective turbulent diffusion coefficient of the passive impurity near the wall.
Simple calculations are recommended for determining, under the conditions discussed, the intensity of the local and integral heat- and mass transfer in the entry region of straight channels of arbitrary cross section.