PDPA experimental study of the hottest gas-solid t

PDPA experimental study on the flow of gas-solid two-phase fluid around a square cylinder

1 introduction

the flow around various cylinders is an important content of fluid mechanics research, and it is very common in engineering applications, such as aircraft development, bridge, chimney construction, heat exchanger design and so on. Mastering the flow characteristics of fluid (sometimes two-phase fluid) flowing through cylindrical obstacles is very important for many engineering design problems, which has attracted the interest of many researchers for a long time. Among them, the flow around a cylinder and a square cylinder is particularly important because they are not only the most widely used in engineering technology, but also the basis for understanding the flow around other cylindrical blunt bodies

most of these studies focus on the flow around unconstrained cylinders [1~3], including some related to the flow characteristics of particle phase in the cylindrical turbulent boundary layer [3]. Although the flow around a square cylinder also plays a very important role in various engineering applications, compared with a large number of research results and people's more in-depth understanding of the flow around a cylinder, people's research on the flow around a square cylinder is still relatively preliminary. The research in this area mainly focuses on the flow of single-phase gas or liquid around a square column, including the numerical simulation of the flow around the Lower Re number [4,5], and the experimental research on the flow around the higher Re number using various technologies (multi-purpose flow), such as FHWA (flight technology) [6], LDV technology [7] and PIV technology [8], as well as the research on the measurement of gas-solid two-phase flow field using PDPA technology [9]. There are also some numerical and experimental studies on the flow around gas-liquid two-phase fluid cylinders [10~12]

the flow of gas-solid two-phase fluid around the cylinder at medium or high Reynolds number is often encountered in engineering, such as bluff body pulverized coal burner, flue gas heat exchanger, etc. However, due to the irregularity of the flow field around the square cylinder, the distribution of particles in the flow field is very uneven, which makes its research very difficult. At present, no one has studied the flow around the square cylinder of gas-solid two-phase fluid, whether in experiment or numerical simulation. The work of this paper is to use three-dimensional Doppler particle dynamic analyzer (3d-pdpa) to study the flow field of gas-solid two-phase fluid flowing through a square cylinder in a square channel, and to explore the flow field structure, pulsation characteristics and the interaction between gas and solid phases

2 experimental device and test system

2.1 experimental section and device

the measurement object of this experiment is: a 12mm size is horizontally arranged in the middle of the experimental section with a square cross section × 12mm × 100mm long cylinder, blocking ratio d/h=0.12, the experimental section is placed vertically, and the air flow blows the cylinder horizontally from bottom to top, as shown in Figure 1. Because the performance of the column is stable and reliable, the length of the body is far greater than the side length of its cross-section, and the measuring surface is arranged on the vertical symmetrical plane downstream of the column, the influence of both ends of the column can be ignored and considered as the problem of flow around an infinite column

the front light transmission surface of the experimental section adopts fluorinated glass windows, and the back is made of plexiglass plate and painted black to reduce the reflected back ● the mainstay of high-strength steel automotive materials, the king light interference, and improve the quality of laser received signals. Other tubes are made of iron sheet. After the gas-solid two-phase flow passes through the experimental section, the cyclone separator separates and collects solid particles for recycling, and the exhaust flows to the induced draft fan. The whole test-bed system is shown in Figure 2. The measuring points are arranged in the flow field downstream of the square column, and only the left half of the flow field is selected for measurement in the experiment. Horizontal spacing of measuring points Δ Y=2 mm, 16 points are taken, that is, y=0~2.5 D. Six lines such as x/d=1, 1.5, 2, 2.5, 3.5 and 5.5 are selected longitudinally. Due to the limitation of light path, there are no measuring points in the area close to the square column

2.2 PDPA test system

the test system adopts the 3-D Doppler particle dynamic analyzer (PDPA) produced by Danish Dantec company introduced by the State Key Laboratory of clean coal combustion of Tsinghua University, and measures the three-dimensional velocity component, particle size, particle concentration, etc. of particles at the same time. The technical indicators of the system are shown in Table 1. The particles used in the experiment are glass beads, with an average particle size of 40~45 mm, a sphericity of 80%, and a refractive index of 1 The user expects to know that the material to be tested will break 5 after less cycles, the density is 2300 kg/m3, and the average powder feeding flow is 1.27 kg/min. For each measuring point, 1000 samples are taken within a time limit of 3 minutes. Except for the few points closest to the square column in the reflux area, 1000 valid data can be collected at other measuring points

the measurement is carried out under three incoming flow speed conditions. In view of the small change of Re number under the three conditions, here we take the working condition of speed u ∞=9.6 m/s as an example to analyze. The Reynolds number based on dimension D and incoming flow speed U0 is 7650, and other working conditions are used as a reference

3 analysis of experimental results

3.1 analysis of time average velocity field

select particles with particle size within the range of 0~10 mm to represent gas phase flow, so as to ensure better particle tracking; In order to compare and analyze particle motion, particles with a particle size range of 50~70 mm are selected as the representative of solid phase motion. Due to the extremely irregular flow behind the flow barrier, the data of a few measuring points in the reflux area are less, and the error may be relatively large

for an unstable flow field, the time average velocity is still an important flow analysis tool [7]. Here, the gas-solid two-phase data are separated to discuss the characteristics of the two-phase velocity flow field and analyze the interaction between the two phases

using the coordinate system shown in Figure 1, the length is dimensionless with the side length of the square column section d=12mm, and the velocity is dimensionless with the reference velocity U0. The average value is calculated from the time average velocity profile (0 ≤ y ≤ 2.5) of the airflow flowing to each position in the flow field