BASIC STUDIES ON THE DEVELOPMENT OF HIGHLY EFFICIENT COMPACT HEAT EXCHANGER WITH LOW ENVIRONMENTAL LOAD

在垂直管道中进行了向上流动的实验，实验中有不混溶的氢氟醚（HFE-7200）或全氟碳（FC-72）液滴下落。进行了速度场、温度场的可视化和图像处理。(1) HFE-7200液滴在湍流向上流动的近壁面区域有明显的摆动。另一方面，比重高于hFE-7200的液滴在中心区域以小幅度摆动下降。在液滴通过的区域测量了湍流强度的增加和平均速度的降低。液滴尾流中的涡旋引起了湍流统计量的变化。HFE-7200液滴流动引起的向外或向壁面流动影响了近壁相干结构。(2)液滴运动的Strouhal数可由气泡上升经验估计。这个斯特罗哈尔数低于从实心球体脱落的涡的斯特罗哈尔数。(3)在尾流区观察到圆形流体运动。这个运动被认为是不稳定Ω-shaped涡旋管的横截面。热尾迹区小于尾迹流过Ω-shaped涡管的面积。这是因为普朗特数很高，而且由于管外的高温流体被带入了涡管包围的尾流区。(4)漏斗与管道内壁之间壁面射流中的高速流动使液滴下落速度减慢。由起扣线产生的壁向波动速度使液滴容易在中心区域输送。布置在测试段底部附近的漏斗和脱扣线能够很好地收集所有下降液滴。对带有4个不混相液滴的两加热壁间湍流向上流动进行了直接数值模拟。得到的主要结论如下。(1)液滴诱导了几种类型的二次流。这些流动增加了雷诺剪应力积。这些流动在流向上被相邻的水流减弱。小尺度流向涡在液滴附近减弱。大尺度发展的流向涡被液滴变形。(2)二次流增加了壁面法向速度波动与温度波动的乘积。这显示了液滴湍流传热的增强。少

The experiments were conducted for the upward water flow in a vertical pipe with falling immiscible droplets of hydrofluoroether (HFE-7200) or perfluorocarbon (FC-72). The visualization of velocity field and temperature field and image processing were carried out.(1) The HFE-7200 droplets fall with noticeable wobbling in the near-wall region of the turbulent upward water flow. On the other hand, the droplets whose specific weight is higher than that of hFE-7200 fall with small-amplitude wobbling in the central region. The increases in the turbulence intensities and the decrease in the mean velocity are measured in the region of droplet passing. The vortices in the wake flow of the droplets cause the change in the turbulence statistics. The outward or wallward flow induced by the flow of HFE-7200 droplets affects the near-wall coherent structure.(2) The Strouhal number for the droplet motion can be estimated from the empirical of rising bubbles. This Strouhal number is lower than the St … More rouhal number of vortex shedding from a solid sphere.(3) The circular fluid motion was observed in the wake flow region. This motion is considered to be the cross sections of the unsteady Ω-shaped vortex tube. The thermal wake region is smaller than the area of passing the Ω-shaped vortex tube in the wake flow. This is because the Prandtl number is high and because high-temperature fluid outside the tube was carried into the wake flow region surrounded by vortex tube.(4) The high velocity flow in the wall jet between the funnel and the pipe inner wall decelerated the droplet falling velocity. The droplet was easily transported in the central region by the wall-normal fluctuating velocity generated by the tripping wire. The funnel and tripping wire allocated near the bottom of the test section worked well for collecting all the descent droplets.The direct numerical simulation was carried out for turbulent upward flow between two heating walls with four immiscible droplets. The main conclusions obtained are as follows.(1) The droplet induced several types of secondary flows. These flows increased the Reynolds shear stress product. These flows were attenuated by the adjacent in the streamwise direction. The small-scale streamwise vortices were attenuated near the droplets. The large-scale developed streamwise vortices were deformed by the droplets.(2) The product of wall-normal velocity fluctuation and the temperature fluctuation increased by the secondary flows. This shows an enhancement of turbulent heat transfer by droplets. Less