The Transition And Control Of Critical Phenomena In Multi-phase Flow Systems

The key factor of accurate determination of operation parameters in complicated multi-phase flow systems is how to use appropriate technique and how to extract useful signals. Extracting characteristic information based on fluctuating signals representing multi-phase flow hydrodynamics in order to determine the crucial parameters is one of the significant ways in the multi-phase research. There are various critical phenomenal parameters in multi-phase flow systems, which are the important control principle of reactors operation. Therefore, taking the critical phenomenon as a special issue to study and analyzing the appearance and transition of critical phenomenon based on the direct relationship between the fluctuating signals and critical parameters play a crucial role to understand the flow condition and supply useful advice to solve the specific problems in industrial process, which is also a big challenge.Our study employed acoustic emission (AE) measurement that is a reliable, safe, on-line, accurate and well-established technique for machinery condition and has been proved sensitive, environment friendly and non-invasive to study all the critical phenomenon and key parameters in multi-phase flows. We studied the temporal and spatial critical phenomenon and obtained the direct relationship between AE signals and critical parameters in multi-phase flows including gas-solid flow and liquid-solid flow by using single-channel and multi-channel measurement based on FFT analysis, wavelet analysis and multi-scale analysis. Firstly, the criterion to determine critical fluidized velocity, critical turbulent velocity and critical suspension speed was presented by analyzing single-channel AE signals varied temporally. Secondly, the criterion to detect particle fluidization pattern, the agglomeration and slurry suspension height was presented by analyzing multi-channel AE signals varied spatially. Finally, we established the control rule of particle fluidization pattern based on relationship between fluidization pattern and flow quality in gas-solid fluidized bed. By studying, specific cases such as the influences of improved distributor structure, various gas distribution styles, added agitated equipment on fluidization pattern and different bed structure, we got important operation guidance to the industrial process. The main achievements of this paper are as follows.1. Based on single-channel AE measurement, we studied the critical phenomenon varied temporally in multi-phase flows. By temporal transition of AE signals, we presented the criterion including the phenomenal parameters such as critical fluidized velocity and critical turbulent velocity that represent the flow regime transitions in gas-solid fluidized bed and the critical suspension speed that represent the flow regime transition in liquid-solid stirred tank respectively.1) In gas-solid fluidized bed, we obtained the mechanism and characteristic of AE signals from impact between particles and wall based on FFT analysis. It appears that the average energy E increased with superficial velocity Ug and sudden increase took place when Ug=Umf or Ug=Uc distinguishingly. Moreover, by the definition of energy ratio RE, a criterion to determine the flow regime transition based on AE method was obtained. That is, the velocity was the critical fluidized velocity when the ratio of the AE energy reached the maximum first, while it secondly reached the maximum the corresponding velocity was the critical turbulent velocity. The influence of different particle Geldart classification was also considered, in order to analyze the different fluidization characteristic.2) In a liquid-solid stirred tank, a characteristic frequency band (S1 scale) of AE signals which represented the motion of solid particles was obtained by using FFT, wavelet transform wavelet and R/S analysis. Then it was observed that the energy fraction of AE signals on different frequency changed regularly with the increasing impeller speed in a stirred tank. Taking the energy ratio of characteristic frequency band (S1 scale) as variable, a new method for determining the flow regime critical transition (critical suspension speed) was presented based on this regular behavior of energy ratio in S1 scale as speed increases. When the AE energy fraction in the characteristic frequency band decreased rapidly and began to level off, the corresponding impeller speed was the critical suspension speed. We also investigated the influences of particle size, slurry concentration and impeller height on the critical suspension speed. It showed that the critical suspension speed increases with the particle size, slurry concentration and impeller height.2. Based on multi-channel AE measurement, we studied the critical phenomenon varied spatially in multi-phase flows. By spatial transition of AE signals, we presented the criterion to determine spatial critical phenomenon including particle fluidization pattern and flow quality that represent the flow quality and slurry suspension height that represent the mixing degree m liquid-solid stirred system.1) In gas-solid fluidized bed. sensors were located along the bed in order to detect the particle fluidization pattern under different conditions based on AE energy analysis. It was found that the particle fluidization structure performs multi-circulation pattern based on the AE energy axial profile, including main circulation in upper bed, smaller circulation above the distributor and stagnant-zone between them. The location of stagnant-zone can be detected by AE energy profile, which was a minimum in the energy value. In stagnant-zone, particles have less motion energy, where agglomeration often forms. Furthermore, the effects of operating variables such as particle size, superficial gas velocity, static bed height and particle classification were considered. The results show that particle size, particle classification and superficial gas velocity have greater effect on the fluidization pattern while static hold-up has little influence contrastively. The particle fluidization pattern varied between single-pattern and multi-pattern. It was concluded that, the particle fluidization pattern and location of stagnant-zone obtained from the AE energy spatial distribution under different operation conditions can supply advice to the stable operation of industrial process, in order to improve the flow quality.Moreover, experiments were taken out by adding different mass particle agglomeration in the fluidized bed in order to investigate the influence of agglomeration size on particle fluidization pattern. It showed that the single circulation flow pattern of polyethylene particles in fluidized bed would change to the multi-circulation flow pattern when the ratio of agglomeration reached to certain amount. The location of agglomeration was corresponding to the location of stagnant zone. That is to say, the influence area became larger and the particle distribution and movement change if the agglomeration size reached to certain amount.We also addressed issues related to granular temperature with respect to particle velocity as well based on particle fluidization pattern in gas-solid fluidized bed. Through the mechanism of particle impact on wall and characteristic of granular temperature, we established a model to quantitatively detect the particle motion energy based on granular temperature and AE technique. By analyzing of spatial energy distribution, we got the formula to calculate granular temperature in different zones (main zone and stagnant-zone). Consequently, by the definition of critical granular temperature and critical operation velocity, we presented a criterion to monitor the fluidization quality in gas-solid fluidized bed, namely, particles are properly active and there is no agglomeration, indicating good fluidization quality when the granular temperature in all regions satisfies T*>T*cr. In addition, the definition of critical operation velocity was introduced which provide significant direction to production in pilot-scale.2) In a liquid-solid stirred tank, we discussed the AE signals spatial distribution by setting multi-channel sensor along the bed height. We presented the characteristics frequency band of AE signals on behave of the particles movement by spectrum analysis, wavelet transform and R/S analysis based on the mechanism of interaction between particles and the wall of tank. A criterion of slurry suspension height in stirred tank was established based on the relationship between acoustic emission energy on the characteristics frequency band from the collision between the moving particles and inside wall and the height of the investigation location in stirred tank. The criterion was that the location with according to the sharp decrease of the AE energy on the characteristics frequency band or the ratio of the energy on the characteristics frequency band at one location and the average energy of all of the samples was the slurry suspension height of stirred tank. By using this criterion, a on-line measurement method based on AE technique to determine the liquid-solid mixing degree was established. The criterion was also suitable with different particle size and impeller structure. The relative error of both impeller structures were 6.5% and 8.9% respectively, which were well agree with the result of observation.3. Concerning the gas-solid fluidized bed, the transition and control rule of particle fluidization pattern under various conditions such as added agitated set up, various gas flow distribution, various distributor structure and different bed structure were observed in order to establish some useful experience to practical production and reactor design.1) Agitator was added in a fluidized bed to monitor the particle fluidization pattern, which showed great influence. The particles were brought to the bubble inside and the bubbles were broken continuously with the effect of impeller, leading to the decrease of huge bubble, channeling and slugging and good fluidization condition. By comparing with the condition without impeller, the addition of impeller intensified the fluidization and mixing between bubbles and particles, which decreased the stagnant-zone and unstable fluidization.2) Particle fluidization pattern was studied by changing gas flow distribution style in the distributor with equal area of inside and outside parts. The results showed that, superficial velocity and gas distribution (inside/outside gas ratio) would affect the fluidization pattern. Both the conditions of various outside gas fluxes with constant inside gas flux and various inside gas flux with constant outside gas flux generated complex pattern. Moreover, the particle fluidization direction and bubble distribution under various gas distributions were obtained based on the AE energy spatial distribution. The inside and outside gas flow form different fluidization structure respectively.3) The influence of distributor structure on fluidization pattern was observed by AE technique. By analyzing the AE signals in the distributor, the particle distribution and fluidization quality were also presented, which gave the result of anti-deposit distributor with good fluidization quality.4) We studied the AE signals in a 2D gas-solid fluidized bed in order to investigate the influence of bed structure. The results showed that, the fluidization pattern in 2D was obviously different with 3D since the influence of wall effect in 2D with bigger bubbles developing in 2D space. Often,2D fluidized bed easily produces slugging and unsteady fluidization and has no multi-circle structure as in 3D fluidized bed. Experiments were taken out to observe AE energy profile under different velocity and static bed height. It showed that AE energy increased with velocity and static bed height, and the symmetry results proved that AE measurement can represent the multi-phase flow information inside the reactor.