Research on Turbulent Mixing Phenomena in T-junctions and Development of Efficient Method for Mixing Control

本文对方形截面T形三通中的湍流混合进行了实验研究，并根据测量结果提出了一种有效的促进和控制湍流混合的方法。我们已经测试了两种类型的混合T形接头。一种是逆流型T形管，主流和支流在T型管中迎头相撞，混合流体沿混合通道流下，该混合通道与主流和支流通道成直角连接。在逆流型T形结中，利用高时空分辨率的PIV和PLIF同时测量了混合后的速度场和浓度场的详细特征：罗丹明6G被溶解在主流道流体中作为荧光示踪剂。速度场的三维特征及其对…的影响进一步阐明了混合流中的浓度场。结果表明，混合通道内形成二次流，高浓度流体沿横向输送。通过POD分析，从瞬时场中提取出混合层沿展向的摆动运动和主流道流动的合理运动。此外，从湍流质量流量的角度分析了两种流体的湍流混合机理：在十字流型T形结中，主流道中的冷流和支流中的热流在T形结中合并。用PIV和热电偶RAKE分别测量了流动结构和平均温度分布。此外，还利用激光测温仪和冷线温度计相结合的方法测量了热混合层中的湍流热流密度。在T形结后，主流道中观察到较大的分离气泡，并在其周围的剪切层中产生强烈的湍流。然而，这种强烈的湍流出现在热力混合层以下，因此它不能有效地促进热流和冷流的湍流热混合。在此基础上，提出了在T型接头上游沿45°方向逆流吹入主流道的小射流促进和控制掺混的方法。这些射流可以增加热混合层上部的湍流强度，从而有效地促进冷热气流的湍流混合。此外，还可以通过改变射流速度来控制热混合程度。较少

Experimental studies have been conducted on the turbulent mixing in T-junctions with square cross sections and, based on the results of measurements, an effective method of its promotion and control has been developed. We have tested two types of mixing T-junctions. One is the counter-flow type T-junction in which the main and branch flows collide head-on in the T-junction and the mixed fluid flows down the mixing channel that is connected to the main and branch channels at right angles. The other is the cross-flow type T-junction, in which the branch is connected to the main channel at right angles.In the counter-flow type T-junction, detailed characteristics of the velocity and concentration fields after the mixing have been investigated using simultaneous measurements by PIV and PLIF with high spatial/time resolutions : Rhodamine 6G has been dissolved in the main-channel fluid as fluorescence tracer. The three-dimensional characteristics of the velocity field and its influence on th … More e concentration field in the mixed flow were clarified. It has been found that the secondary flows are formed in the mixing channel and the high-concentration fluid is transported in the transverse direction. By POD analysis, the wobbling motion of the mixing layer in the spanwise direction, and a rational motion of the main-channel flow have been extracted from instantaneous fields. Moreover, the mechanism of turbulent mixing of two fluids has been analyzed based on the turbulent mass fluxes.In the cross-flow type T-junction, the cold flow in the main channel and the hot flow in the branch merge in the T-junction. The flow structure and the mean temperature distributions have been measured by PIV and the thermocouple rake, respectively. Moreover, the turbulent heat fluxes in the thermal mixing layer have been measured by combining LDV and a cold-wire thermometer. A large separation bubble is observed in the main channel after the T-junction and strong turbulence is generated in the shear layer around it. This strong turbulence, however, appears below the thermal mixing layer, thus it does not contribute effectively to the turbulent thermal mixing of hot and cold flows. Based on these experimental results, we have proposed the mixing promotion and controlling method with small jets that are blown into the main channel at the upstream edge of the T-junction in the direction of 45 degrees against the main flow. Turbulence intensity in the upper part of the thermal mixing layer can be increased with these jets, and consequently the turbulent mixing of hot and cold airflows is promoted effectively. Moreover, the degree of thermal mixing can be controlled by changing the jet velocity. Less