Drag Reduction in Turbulent Flow of Gas-Solid Mixtures

Abstract

Abstract Well-developed one-dimensional steady turbulent flow of a dilute gas-solid mixture in a vertical pipe is considered on the basis of a closure model that accounts for the Reynolds stresses in both mixture phases and specific pseudoturbulent stresses that arise in the dispersed phase. The model is proven to be in an excellent agreement with experimental data as regards usually observed cross-sectional profiles of mixture concentration and mean velocities of the phases. This model is applied to evaluate a systematic variation in total gas flow rate caused by the insertion of fine particles into pure gas flow under the condition of the overall pressure drop being kept invariable. For this purpose, the ratio of gas flow rate in the pure gas flow to that in a mixture flow was studied as a function of the mixture concentration and other relevant parameters. All other things being equal, this ratio first decreases below unity as the amount of the inserted particles increases, reaches a minimum, and then begins to grow with concentration, eventually assuming values well above unity. This means that a drag reduction effect occurs if the concentration varies within a certain interval of relatively low concentrations, but if gives way to a drag enhancement effect as concentration further increases beyond the said interval.