Abstract:

A novel technique is proposed for heat and mass transfer enhancement in a microchannel by introducing a freely rotatable square cylinder. A numerical model is developed based on the finite volume method and moving mesh technique to investigate the flow past the freely rotatable cylinder and its influences on heat and mass transfer enhancement. It is discovered that the cylinder is almost stationary when Re is low (Re=10). And its influence on the flow field and heat and mass transfer is thus similar to that of the static cylinder. With the increase of Re, the freely rotatable cylinder starts oscillating periodically owing to the interaction between the fluid and the cylinder, which leads to alternating vortex shedding behind the cylinder at Re=50. When Re rises to 100, both the oscillation of the cylinder and the downstream vortex structures are significantly strengthened. Compared to a static square cylinder at the same Re, a freely rotatable cylinder can disturb the original Poiseuille flow in the microchannel more significantly and break the thermal boundary layers on the heated walls, thus improve the heat transfer efficiency. In addition, the horizontal flow of the vortices can promote the mixing of solute in the microchannel. At Re=100, the average Nu on the walls is increased by 17.5% and 29.6% compared to static cylinder case and the no cylinder case , respectively. And the mixing efficiency will be increased by 70.5% and 65500% compared to static cylinder case and the no cylinder case , respectively.

Key words: freely rotating cylinder, heat transfer, mass transfer, microchannels, computational fluid dynamics, moving mesh

摘要：

提出了一种基于自由摆动方柱提高微流体通道内传热传质效率的新方法。基于有限体积法并结合动网格技术，对微通道内液体流经自由摆动方柱（有旋转自由度、无平移自由度）时产生的扰流及强化传热传质现象进行了数值研究。研究显示，在低雷诺数（Re=10）下，自由摆动方柱几乎无运动，其对流动和传热传质的影响同固定方柱类似；随着雷诺数的增加，自由摆动方柱在流场作用下会自发产生周期性摆动，并在较低雷诺数（Re=50）下提前促使其后方产生交替性的涡脱落现象；随着雷诺数进一步增加（Re=100），方柱的自由摆动及其后方的涡街结构均显著增强。与同Re数下的固定方柱相比，自由摆动方柱能够更显著地扰动微通道内原有的泊肃叶流场，破坏通道内壁面处热边界层，提高其传热效率；同时通道内液体的横向流动可有效促进溶质混合，强化传质进程。当Re=100时，自由摆动方柱微通道内的平均换热努塞尔数Nu较固定方柱和无方柱时分别提高了17.5%和29.6%；同时，出口截面混合效率可较固定方柱和无方柱时分别提高了70.5%和65500%。

关键词: 自由摆动方柱, 传热, 传质, 微通道, 计算流体力学, 动网格