窄通道自然循环临界热流密度的非线性分析

通过自然循环流动实验，取得5 mm间隙窄矩形通道的自然循环临界热流密度（CHF）发生时的可视化图片，以及流量、壁温和实验段压差信号，并运用非线性分析技术对CHF发生过程进行了定性和定量研究。研究发现：自然循环压差时间序列的功率谱在半对数坐标中呈指数下降；自相关系数逐渐下降；三维吸引子相图表现出奇怪吸引子的特点。这表明了自然循环系统CHF的发生过程具有非线性混沌特性。自然循环CHF发生的初始阶段，由于流量脉动和流型往复变迁，流动和换热表现出一定的周期性；随着热流密度的提高，周期性减小，随机性增大，但总能达到一个确定的状态，体现了混沌运动的特点。

Nonlinear Analysis of Natural Circulation Critical Heat Flux in Narrow Channels

The visual images of dryout, signals of flow, wall temperature, and pressure differences before and after the critical heat flux (CHF) were obtained in 5 mm clearances channels through natural circulation experiment, respectively. The qualitative and quantitative researches were conducted by applying nonlinear analysis techniques. The research results show that the power spectra of time series of pressure difference drop exponentially in semilogarithmic coordinates, the autocorrelation coefficient descends gradually, and the phase diagram exerts the characteristic of strange attractor, which proves that the nonlinear chaos characteristics exist in the natural circulation system when CHF occurs. During the initial period of CHF occurrence, the flow and the heat transfer represent periodic characteristics due to the flow oscillation and flow pattern changed periodically. With the increment of heat flux of heating channel, the periodicity decreases, while the randomness increases. A certain state of the system will be achieved, which embodies the characteristics of chaos movement.