Numerical simulation and structure improvement of gravity-assisted phase transition cooling system used in heavy duty extrusion pelleting line

Abstract

High efficiency cooling system is an essential part of heavy duty extrusion pelleting line, which plays an important role in 300 kt/a production engineering of polyolefin. In this paper, a gravity-assisted phase transition cooling system (GAPTCS) used in heavy duty extrusion pelleting line was presented. Based on bubble dynamics theory and phase field method, a multi-bubble nucleation boiling model in GAPTCS was proposed. The phase transition, two-phase flow and heat transfer characteristics during the operation of GAPTCS were simulated by COMSOL for the first time, and corresponding experimental studies were carried out. The predicted temperature profile showed a satisfying agreement with experimental data with the maximum relative error of 1.11%, and the average relative error of 0.57%. Therefore, it was concluded that the numerical simulation successfully reproduced the heat and mass transfer process of the system. Aiming at the phenomenon that the vapor film was attached to the bottom wall, the structure was improved. The results showed that the average heat flux of the barrel wall in improved GAPTCS reached 28,790 W/m2, which was 1.8 times that of original GAPTCS. In addition, it took the improved GAPTCS 14 s to reach the stable state, which was 5 s less than the original structure. What's more, the stable average temperature of the wall in improved GAPTCS was 5.7 °C lower than that in original GAPTCS.