流化床压力波动多尺度多分形特征

Characteristics of Multi-scale and Multi-fractal of Pressure Signals in a Bubbling Fluidized Bed

对流化床不同测量位置的压力波动信号用Daubechies二阶小波在1～9尺度下进行分解,并分别对分解的信号进行R/S分析.研究发现,分解的信号可由多尺度方法得到较好的理解.1、2尺度下的细节信号只有一个明显的Hurst指数H,且H＜0.5,表现为反持久性的更随机的特征,说明1、2尺度下的细节信号主要反映了气流与颗粒之间的微尺度作用.3～9尺度下的细节信号都具有两个Hurst指数H,分别大于0.5和小于0.5,表现为多分形特征,H＞0.5代表了具有正持久性的气泡相的运动;H＜0.5代表了具有反持久性的乳化相的运动,说明3～9尺度的细节信号反映了介尺度作用的乳化相和气泡相的相互作用.而9尺度下的概貌信号只有一个Hurst指数H,且H＞0.5,表现为正持久性,说明9尺度下的概貌信号主要反映了颗粒流体系统与外界边界之间的宏尺度作用.各尺度信号的能量分布表明,压力波动信号主要体现了介尺度的乳化相和气泡相之间的相互作用.

The complexity of fluctuation dynamics in a bubbling fluidized bed with 0.3 m in diameter and 3 m in height was studied by using multi-scale and R/S analysis method. The Daubechies second order wavelet was firstly applied to decompose pressure signals at different measurement locations to 1-9 scale detail signals and 9 scale approximation signal. Then, Hurst analysis was used to analyze multi-fractal characteristics of different scale signals. The results show that 1-2 scale detail signals have only one Hurst exponent smaller than 0.5 corresponding to micro-scale interaction between particles and fluid and particles; 3-9 scale detail signals have two Hurst exponents one smaller than 0.5 and one larger than 0.5 corresponding to meso-scale interaction of the dilute phase and the dense phase, respectively; 9 scale approximation signal has only one Hurst exponent which larger than 0.5 corresponding to macro-scale interaction of the particle-fluid suspension within its boundaries. Therefore, measured pressure signals were resolved to multi-scale signals such as micro-scale signals, meso-scale signals and macro-scale signals. Micro-scale and macro-scale signals are of mono-fractal characteristics, and meso-scale signals are of bi-fractal characteristics. By analyzing energy profile of different scale signals, we found that pressure fluctuations mainly reflect meso-scale interaction between the emulsion phase and the bubbling phase.