The characteristics of flow and wall-to-bed heat transfer for vertical dense-phase transport flow

The current work demonstrates the heat transfer capacity of dense-phase transport flow as a heat transfer fluid (HTF) in a vertical tube. Dense-phase transport flow is an ideal HTF that can be utilized in the industrial field, especially for concentrating solar power. Experiments were conducted with quartz particles (317 mu m, 184 mu m) and desert particles (164 mu m) in a self-established 49 mm I.D. x 4 m industrial pipe under the uniform heat flux. The thermal behaviors are investigated for different particle mass fluxes and solid loading ratios, whilst covering the different hydrodynamic flow regimes of plug flow, slug flow, turbulent flow, and dense-riser upflow. The wall-to-bed heat transfer coefficients are calculated from experimental data, ranging from 300 W/(m(2) K) to 1200 W/(m(2) K). It is sensitive to the particle mass flux and hence flow regimes. A semi-empirical correlation is developed relating the Nusselt number to Froude number, Reynolds number, and particle-tube diameter ratio. It provides a fair estimate of the heat transfer coefficient compared with the empirical predictions of two other published dimensionless correlations. The application of the new semi-empirical correlation to other published experimental data is also investigated for an extrapolation analysis. With knowledge of heat transfer characteristics, the dense-phase flow is expected to open new opportunities for high-efficiency thermodynamic cycles in concentrating solar power.