磁射流抛光的射流稳定性分析

Analysis on Jet Stability of Magnetorheological Jet Polishing

磁射流抛光利用喷嘴出口附近的外加局部轴向磁场稳定磁流变液射流,产生稳定、汇聚的射流束进行远距离确定性精密抛光。为了优化该碰射流抛光工艺,研究了磁流变液射流在外加轴向磁场作用下的稳定性。首先推导了磁流变液射流的运动学模型,分析磁流变效应对射流运动学特征的影响;然后应用有限元方法,基于Maxwell方程和磁流变液射流运动方程,用数值计算的方法模拟了磁流变效应对磁流变液射流的稳定作用;其次通过实验观察了外磁场对磁流变液射流结构的稳定作用;最后结合连续媒质电动力学知识分析了轴向磁场对磁射流的稳定作用。实验结果与理论分析具有较好的一致性:磁流变液在局部轴向磁场作用下形成的纵向链状纤维结构导致其表观黏度迅速增大,能抑制初始动力扰动对射流结构的破坏,从而可以产生长距离稳定、汇聚的射流束。磁射流对距离不敏感的特性在确定性精密抛光高陡度的凹形光学零件和内腔等复杂形面中具有潜在的独特优势。

In magnetorheological jet polishing（MJP）, the stable and collimated jet of magnetorheological（MR） fluid magnetized by a local axial magnetic field near the exit of the nozzle was used to polish the remote surface in a deterministic precision process. But the theoretical considerations on the mechanism of jet stabilization were required for optimization of the technology. Therefore, the stabilization of MR jet under the external axial magnetic field was analyzed. Firstly, a kinematic model was developed to analyze the MR effects on the jet flow. Secondly, the stabilization of MR effect was numerically simulated by finite element methods based on the Maxwell equations and the kinematic equation of MR jet. Thirdly,experiments were conducted to observe the jet structure of the MR jet under the external magnetic filed. Finally, the stabilization of axial magnetic field on MR jet was analyzed based on the knowledge of continuous medium. And the results are consistent with the experimental ones. The longitudinal chain--like fibrillation and subsequently high apparent viscosity within the MR jet induced by a local axial magnetic field can suppress iet breakup in the stage of initial dynamic disturbances and result in a stable and collimated jet remaining for a relatively long distance. Due to the insensitivity to the offset distance MJP has potential advantages in deterministic precision polishing of complex shapes of steep concave optics and cavities.