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

在垂直管内进行了氢氟醚(HFE-7200)或全氟碳(FC-72)不相容液滴下落的向上流动实验。对HFE-7200液滴的速度场、温度场进行了可视化和图像处理。(1)HFE-7200液滴在湍流上升流近壁区有明显的摆动。另一方面，比重高于HFE-7200的液滴在中心区下降，摆动幅度较小。在液滴通过区测量到了湍流强度的增加和平均速度的降低。液滴尾迹流中的涡流引起了湍流统计的变化。HFE-7200液滴流动引起的向外或向壁流动影响着近壁的共格结构。(2)液滴运动的斯特劳哈尔数可以从上升气泡的经验中估计出来。该Strouhal数小于St…实心球体脱落的旋涡数量较多。(3)尾迹流区观察到流体的圆周运动。这种运动被认为是非定常Ω形涡管的横截面。热尾流区小于尾流中通过Ω涡管的区域。这是因为普朗特数较高，而且管外高温流体被带入涡管包围的尾流区。(4)漏斗与管内壁面射流中的高速流动减慢了液滴的下落速度。通过跳线产生的壁法向脉动速度，液滴很容易在中心区输送。布置在试验段底部附近的漏斗和跳线可以很好地收集所有下降的液滴。对具有4个不相容液滴的两个加热壁间的湍流上升流动进行了直接数值模拟。得到的主要结论如下：(1)液滴诱导了多种类型的二次流。这些流动增加了雷诺剪切应力积。这些流动在流向上被相邻的流动衰减。液滴附近的小尺度流向涡减弱。二次流使壁法向速度脉动和温度脉动的乘积增大。这表明液滴对湍流换热有强化作用。较少

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