Transition Mechanism Based on Load Imbalance of Liquid Metal Film

According to the lubrication theory, when the bearing capacity of the liquid metal film (LMF) is not enough to support the external load, the film will be unstable and at the risk of break. In this article, the criterion of the instability for the hydrodynamic lubrication at the armature/rail (A/R) interface is established, and the transition mechanism at current down-slope is proposed that the break of the LMF to degrade the electrical continuity across the A/R interface will lead to the transition. A 2-D Reynolds equation considering the self-acceleration of the film is derived. By coupling the electromagnetic field, stress, and fluid field, a magneto-elastohydrodynamic (MEHD) model is established completely. Based on this, the dynamic characteristics of the bearing capacity and external load are explored. The influence of the slope of the current drop on the transition moment and transition critical velocity is analyzed. The results show that: 1) at a certain moment of current down-slope, the bearing capacity of LMF is indeed less than the external load; 2) the larger the slope (absolute value) of the current drop, the earlier the transition occurs, and the lower the critical velocity of transition; and 3) the model gives an acceptable magnitude for the transition critical velocity that matches well with the experimental data, which indicates the criterion of the instability for the hydrodynamic lubrication and the transition mechanism at current down-slope are reasonable.