Numerical study on gravity-driven granular flow around tube out-wall: Effect of tube inclination on the heat transfer

Abstract

Heat recovery from granular flow in moving bed heat exchangers has wide industrial applications. However, more knowledge is still necessary for the heat transfer enhancement. For the slow descending of random particles, the flow structure has crucial roles on the heat transfer by conduction and radiation. In this paper, the effect of tube inclination was studied by discrete element method. Influences of particle update, contact and radial migration were carefully discussed. Moreover, heat resistance analysis was conducted to explore the clear law. It was found that, the heat is obviously improved by tube inclination due to the increasing area, but there are uncertainties for the local heat transfer coefficients. At the tube bottom, particles become denser with tube inclination, which is beneficial to increase the heat transfer coefficient. But for the top or middle area, there are two opposite effects. Tube inclination enhances radial migration around tube wall, while simultaneously weakens local particle update. Finally, contact resistance firstly increases and then decreases with inclination angle, but there is a reversal tendency for the sensitivity of penetration resistance to the flow rate. In actual applications, penetration resistance is the major resistance, and the inclination angle is therefore suggested between 15° and 37.5°.