Abstract

This article conducts a two-dimensional numerical model to simulate the ferrofluid droplet formation from microfluidic T-junction under inhomogeneous magnetic fields with diverse strengths. This external magnetic field is produced by two electric straight wires in a finite computational domain. A coupled volume-of-fluid and level-set interface tracking method (VOSET) is adopted to capture the evolution of two-phase interface. Meanwhile, a two-region computational domain method is designed for situations that the droplets are in close contact with the solid boundaries for the fluid flow. All 2-D numerical simulations are implemented by a self-developed CFD code, named as MHT (Multi-concept Heat Transfer). The numerical results show a significant inhibition effect in droplet formation at the presence of external magnetic field. With the increase of the current intensity, the magnetic force of the ferrofluid droplet increases and decreases periodically, especially when the electric current intensity is less than 60 A. The increasing current intensity enlarges the departure diameter and prolongs the departure period of ferrofluid droplet, especially when the current intensity in the range 12 A∼54A. In the cases of electric current within [12A, 54 A], the departure diameter growths monotonically and nearly in a quadratic manner with the increase of the current intensity. However, when the current intensity exceeds 60 A, the departure characteristic of ferrofluid will be changed due to ferrofluid droplet absorbed on the upper wall of the main channel.

摘要

本文进行了二维数值模型，以模拟在具有不同强度的非均匀磁场下，微流体T型结形成的铁磁流体液滴。此外部磁场是在有限的计算域中由两条直线产生的。采用流体体积和水平集接口耦合跟踪方法（VOSET）来捕获两相接口的演化。同时，针对液滴与流体流动的固体边界紧密接触的情况，设计了一种两区域计算域方法。所有二维数值模拟均通过自行开发的CFD代码（称为MHT（多概念传热））实现。数值结果表明在存在外部磁场的情况下，对液滴形成具有显着的抑制作用。随着电流强度的增加，铁磁流体液滴的磁力周期性地增大和减小，特别是在电流强度小于60 A时。增大的电流强度会增大铁磁流体液滴的发射直径并延长其发射周期，特别是当电流强度小于60 A时。当电流强度在12 A〜54A范围内时。在[12A，54 A]范围内的电流情况下，离场直径随着电流强度的增加而单调增长，并且几乎以二次方的方式增长。然而，当电流强度超过60 A时，由于吸收在主通道上壁的铁磁流体液滴会改变铁磁流体的离开特性。尤其是当电流强度小于60 A时。增加的电流强度会增大离液直径并延长铁磁流体液滴的离液期，尤其是当电流强度在12 A至54A范围内时。在[12A，54 A]范围内的电流情况下，离场直径随着电流强度的增加而单调增长，并且几乎以二次方的方式增长。然而，当电流强度超过60 A时，由于吸收在主通道上壁的铁磁流体液滴会改变铁磁流体的离开特性。尤其是当电流强度小于60 A时。增加的电流强度会增大离液直径并延长铁磁流体液滴的离液期，尤其是当电流强度在12 A至54A范围内时。在[12A，54 A]范围内的电流情况下，离场直径随着电流强度的增加而单调增长，并且几乎以二次方的方式增长。然而，当电流强度超过60 A时，由于吸收在主通道上壁的铁磁流体液滴会改变铁磁流体的离开特性。在[12A，54 A]范围内的电流情况下，离场直径随着电流强度的增加而单调增长，并且几乎以二次方的方式增长。然而，当电流强度超过60 A时，由于吸收在主通道上壁的铁磁流体液滴会改变铁磁流体的离开特性。在[12A，54 A]范围内的电流情况下，离场直径随着电流强度的增加而单调增长，并且几乎以二次方的方式增长。然而，当电流强度超过60 A时，由于吸收在主通道上壁的铁磁流体液滴会改变铁磁流体的离开特性。