收缩扩张管内液氮空化流动演化过程试验研究

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. 

EXPERIMENTAL STUDY ON THE EVOLUTION OF LIQUID NITROGEN CAVITATING FLOWS THROUGH CONVERGING-DIVERGING NOZZLE

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 T_throat ≈ 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.