Suppression/Enhancement of Natural Convection by Use of Thermal Screen

通过实验和数值研究了热筛的中间冷却或加热对浮力对流和马兰戈尼对流的影响。中间冷却或加热采用多孔筛网，本研究称之为热筛，由一排热管置于流体层中构成。通过将液晶颗粒迁移到测试液中进行视觉观察。首先，研究了热筛对从下面加热的流体中自然对流的影响。研究发现，热屏使水平温度分布具有较强的均匀性，可以改变浮力，有效抑制浮力对流。除了筛网的这种热效应外，达西阻力阻碍了通过多孔筛网的流动。因此，根据温度水平和筛网渗透率的不同，对流模式要么以贯穿整个层的延迟大尺度对流的形式发生，要么以局限在多孔筛网所限定的空间内的局部对流的形式发生。此外，数值和实验结果表明，局部对流夹带了不同温度下筛网另一侧的流体，产生了随时间变化的周期运动。其次，研究了在自由表面加热细导线引起的马兰戈尼对流。研究还发现，热筛可以降低水平温度梯度，特别是在受热丝附近，马兰戈尼对流减弱。再次，需要注意的是，热筛的达西阻力会造成流量衰减的影响。通过热筛的中间冷却或加热可以有效地控制浮力对流和马兰戈尼对流。

An experimental and numerical study has been made concerning the effects of intermediate cooling or heating by use of thermal screen on both buoyant and Marangoni convection. A porous screen, which is called as a thermal screen in this study and is constituted with a row of heat pipes placed in a fluid layer, is adopted for the intermediate cooling or heating. Visual observations are made by using liquid crystal particles migrated into test liquid.First, the effects of thermal screen on natural convection in a fluid heated from below are examined. It is found that the thermal screen introduces a rather strong uniformity in horizontal temperature distributions which can modify the buoyancy forces and can substantially suppress the buoyant convection. In addition to this thermal effect of the screen, the Darcy resistance retards flow through the porous screen. Thus, convective patterns were found to occur in the form of either the retarded large-scale convection through the whole layer or the local convection confined to a space bounded by the porous screen depending on the temperature level and the permeability of the screen. Further, the numerical and experimental results show that the local convection entrains fluid of the other side of the screen with different temperatures, which can yield time-dependent periodic motions.Secondly, Marangoni convection induced by heating a thin wire on the free surface is examined. It is also found that the thermal screen can reduce horizontal temperature gradients particularly near the heated wire and the Marangoni convection is weakened. Again, it is noted that the Darcy resistance of the thermal screen can cause an effect of attenuation of flow.Then, it is concluded that the intermediate cooling or heating by the thermal screen can provide a very efficient method of control of both buoyant and Marangoni convection.