Flip-Flop Flow Mechanism in Diamond-Shaped Cylinder Bundles

在作者以前的研究中，已经揭示了钻石形圆柱束中的自诱导触发流现象。当管束用作热质交换设备时，触发式流动振荡促进了流体的混合，从而有助于强化对流换热和传质。本文采用二维粒子图像测速仪(PIV)和二维激光多普勒测速仪(LDV)对钻石形圆柱束内的流动现象进行了观察和监测，以期对触发流动机理进行研究。总结如下：以流程图的形式总结了钻石形圆柱束流道内发生触发流动振荡的机理。它由必要条件和辅助性条件组成。其必要条件包括分离涡的产生、冯·卡门涡片的涨落、科安达效应的发生和流动横截面的变化。考虑了交叉流、撞击力、能量源点和剪切层振荡等辅助条件。在气缸束中至少有三排，就会在第四排气缸的前缘出现边音现象(产生压力通道)，并在第三排气缸的后端发生连接分离涡对的往复迁移。后者在五排以上的菱形圆柱束中触发下游触发流动的波动和出口射流的稳定摆动。

A self-induced flip-flop flow phenomenon in diamond-shaped cylinder bundles has been disclosed, in previous studies by the authors. The flip-flop flow oscillation promotes fluid mixing and thus contributes to an enhancement in convective heat and mass transfer when the cylinder bundles are used as heat and mass exchanger equipment. In the present study, both two-dimensional particle image velocimetry(PIV) and two-dimensional laser Doppler velocimetry(LDV) were performed to observe and monitor flow phenomena in diamond-shaped cylinder bundles for the purpose of investigating flip-flop flow mechanisms. The following summary is derived from this study:The mechanisms for the occurrence of flip-flop flow oscillation in the flow channels of a diamond-shaped cylinder bundle are summarized in the form of a flow chart. It consists of necessary and auxiliary conditions. The necessary conditions include the generation of separation vortices, fluctuations of von Karman vortex sheets, occurrence of Coanda effect, and variation of flow cross-sectional areas. Intersecting flows, ramming effect, energy source point and shear layer oscillation are considered the auxiliary conditions. A minimum of three rows in a cylinder bundle warrants the occurrence of edge-tone phenomenon(generation of pressure wayes) at the leading edge of the fourth-row cylinder and the reciprocating migration of a conjunct separation vortex pair across the rear apex of the third-row cylinder. The latter triggers the fluctuation of flip-flop flow downstream in a diamond-shaped cylinder bundle of more than five rows and a stable swinging of exit jets from the bundle.