The Effect of Coefficient of Restitution in Modeling of Sand Granular Flow for Core Making: Part I Free-Fall Experiment and Theory

摘要

Coefficient of restitution (COR) is known to be lower for liquid-coated wet particles than dry particles due to higher inelastic effect caused by cohesion force of liquid layer. The attempt is made in this two-part article to reproduce different granular sand flows in two-fluid modeling (TFM) by changing the COR value. Instead of the original definition as a vector of the ratio between impact and rebound velocity in particle-particle collision microscopically, COR works as a scalar degree in continuum model to focus on its ability to represent the energy dissipation for granular sand flows with different characteristics attributed from the particle properties and wetting conditions macroscopically. Based on this, COR is measured and compared for industrial-grade sand in a free-fall experiment by tracking the movement of particles in the stream. Its transient explanation is illustrated in 2D TFM simulation accordingly. Comparison is made for sand materials with different impact velocity, grain properties, content level of binder and additive. The experiment result shows that in the tested velocity range COR value measured from liquid-coated sand with 1.2 wt% binder is 0.70 +/- 0.05, while that for dry sand ranges from 0.88 to 0.98. The simulation also demonstrates the significant difference in sand flow performance due to the variation in COR, which is of great importance for modeling of core-making process in foundry industry. The analysis using 1D simple approach with equation similar to Bernoulli's model on the flow path indicates that theoretically COR can have either positive or negative correlation with granular temperature. The correlation of COR and sand mixing condition as well as the flow performance is further verified in parametric study on lab core shooter at a given blow pressure using a customized test box, which is reported in Part II of this two-part article.