EXPERIMENTAL STUDY ON THE EVOLUTION OF LIQUID NITROGEN CAVITATING FLOWS THROUGH CONVERGING-DIVERGING NOZZLE
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摘要: 本文基于低温空化试验平台研究了收缩扩张流道内液氮非定常空化流动的演化过程. 试验采用高时空分辨率的高速摄像机对77 K液氮在不同空化数σ下空穴结构的演变进行了精细化的分析和研究. 利用试验得到的空穴长度和面积等数据, 定量分析了液氮空化流动的非定常特性与时空演变规律. 研究结果表明: (1)在相似来流速度和温度条件下, 随着空化数的减小, 液氮空化流动呈现四种典型流型, 空穴长度在2.5 h以内为初生空化、空穴长度在2.5 h ~ 7.5 h之间为片状空化、空穴长度在7.5 h ~ 15 h之间为大尺度云状空化, 空穴长度超过15 h为双云状空化, 且在大尺度云状空化和双云状空化阶段均捕捉到了回射流现象; (2)液氮空化流动从初生空化到双云状空化, 脱落空穴的尺度逐渐增大, 空穴面积脉动的幅值和准周期均有所增加. 同时, 在大尺度云状空化与双云状空化阶段, 喉口处堵塞效应对空化流动的影响显著增强; (3)相比于初生空化, 片状空化、大尺度云状空化以及双云状空化中脱落空穴的移动距离依次增加了0.97倍、2.65倍与2.68倍, 溃灭时间依次增加了1.18倍、3.59倍与4.47倍, 但溃灭速度依次减小了0.10倍、0.20倍与0.30倍. 除此之外, 对于双云状空化阶段, 存在两种显著不同的脱落空穴演化过程.Abstract: The objective of this paper is to investigate the unsteady characteristics of liquid nitrogen cavitating flow in a converging-diverging (C-D) nozzle via a cryogenic experimental facility. A high-speed camera with high resolution was employed to study the evolution of cavity with varying cavitation numbers σ under Tthroat ≈ 77 K. In order to quantitatively analyze the unsteady characteristics and temporal-spatial evolution, processed data such as the length and area of cavitation based on experimental images were obtained. The results show that: (1) As the cavitation number decreases and under similar free-stream velocity and temperature, the liquid nitrogen cavitation shows four typical flow patterns, with the cavitation length within 2.5 h for incipient cavitation, between 2.5 h and 7.5 h for sheet cavitation, between 7.5 h and 15 h for large-scale cloud cavitation, and over 15 h for double-cloud cavitation, Additionally, a significant phenomenon of re-entrant jet is captured in the large-scale cloud cavitation and double-cloud cavitation; (2) as the liquid nitrogen cavitating flow evolves from incipient cavitation to double-cloud cavitation, the scale of shedding cavity increases gradually, meanwhile, the amplitude and quasi-period of cavity area pulsation is getting longer. In addition, it is observed that the blockage effect on the cavitation flow at the throat is significantly enhanced in the large-scale cloud cavitation and double-cloud cavitation; (3) compared with incipient cavitation, the travel distance of shedding cavities increases by 0.97, 2.65 and 2.68 times in sheet cavitation, large-scale cloud cavitation and double-cloud cavitation, and the collapse time increases by 1.18, 3.59 and 4.47 times, respectively. For the double-cloud cavitation, there are two significantly different evolutions of shedding cavity.
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Key words:
- liquid nitrogen /
- cavitating flows /
- cavitation patterns /
- unsteady evolution /
- experimental observation
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图 1 低温空化测试平台的总体示意图(1.运行罐2.试验段3.涡轮流量计4.收集罐5.真空隔热层6.高速相机7.LED灯8.数据采集模块)
Figure 1. Schematic of cryogenic cavitation test rig (1. run tank 2. test section 3. turbine flowmeter 4. catch tank 5. vacuum insulation chamber 6. high speed camera 7. LED lamp 8. data collection module)
图 2 收缩扩张流道的示意图(左)与实物图(右)
Figure 2. Schematic (left) and physical (right) picture of the converging-diverging (C-D) nozzle
图 3 高速摄像机采集的试验图像与时均空穴结构图
Figure 3. Typical flow visualization captured camera and time-averaged cavity images
图 4 典型液氮空化流动可视化图像及图像处理步骤(黑色阴影为空穴)
Figure 4. Images of liquid nitrogen cavitation flow and image processing (black shadow is the cavity)
图 5 空穴图像沿选定的三条线上的灰度分布得到的时空处理结果
Figure 5. Spatio-temporal processing results obtained by analyzing grayscale distribution along the selected three lines on the cavity image
图 6 空穴长度时均值随空化数σ的变化(Tthroat ≈ 77 K)
Figure 6. The variations of time-averaged cavity lengths with cavitation number σ (Tthroat ≈ 77 K)
图 7 一个准周期内空穴形态的演化过程(Case1 ~ Case4, Tthroat ≈ 77 K)
Figure 7. Evolution of cavitating flow during a quasi-cycle (Case1 ~ Case4, Tthroat ≈ 77 K)
图 8 双云状空化中上附着空穴的演变过程
Figure 8. Evolution of the upper attached cavity in double cloud cavity
图 9 空穴面积随时间的脉动及其平均值(Case1 ~ Case4)
Figure 9. Fluctuation of cavity area with time and its average value (Case1 ~ Case4)
图 10 5 ms内空穴面积随时间的变化(起始时刻t0为图9中的t0 + 12.5 ms时刻)
Figure 10. Temporal evolution of cavity area during a period of 5 ms (the starting moment t1 is the t0 + 12.5 ms moment in Fig. 9)
图 11 t0 ~ t20时刻分别对应的瞬态空穴图像(Case1 ~ Case4)
Figure 11. The transient cavity images corresponding to the moments t0 ~ t20 respectively (Case1 ~ Case4)
图 12 选定三条直线上灰度强度在30 ms内的时空分布
Figure 12. Spatio-temporal processing results obtained by analyzing grayscale distribution along the selected three lines on the cavity image during 30 ms
图 13 脱落空穴沿x轴的移动距离Δxci、运动时间Δtci以及平均移动速度∆
$ \stackrel{-}{v} $ ci随空化数的分布(i = 1 ~ 4分别代表Case1 ~ Case4, 用4′, 4″分别代表Case4中两种脱落机制Mode Ⅰ与ModeⅡ)Figure 13. Distribution of the distance, the movement time and the average movement velocity of detaching cavities moving along the x-axis with cavitation number (i = 1 ~ 4 represent Case1 ~ Case4 respectively, 4′ and 4″ are used to represent the two shedding mechanisms Mode I and Mode II in Case4, respectively)
图 14 不同空化流型中脱落空穴脱落的演化过程
Figure 14. Shedding process and mechanism of cloudy cavity in different cavitation flow patterns
表 1 低温试验台的整体运行参数[42]
Table 1. The overall operating parameters of the cryogenic cavitation test rig[42]
Pressure/Pa Temperature/K Velocity/(m·s−1) Cavitation number Reynolds number/105 30 ~ 300 68 ~ 86 20 0.1 ~ 0.9 0.6 ~ 2.6 
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表 2 所选工况的试验条件与试验结果
Table 2. Experimental conditions and results in selected cases
Case Throat tempera-
ture Tthroat/KCavitation number σ Reynolds
number
Re/105Velocity Uthroat/(m·s−1) Up-stream
pressure pup/kPaDown-stream
pressure
pdown/kPaVapor pressure
pthroat/kPaTime-averaged
cavity lengths lcavity/hTime-averaged
cavity area Scavity/h21 77.42 0.60 1.93 15.51 185.82 160.41 101.86 2.25 1.03 2 77.36 0.38 1.92 15.43 172.83 138.14 101.38 5.77 6.42 3 77.50 0.18 1.95 15.61 156.21 120.85 103.07 14.15 24.70 4 77.35 0.11 1.92 15.49 149.83 112.14 101.27 16.32 26.83
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