Theoretical Model And Flow Characteristics Of The Airlift For Underwater Mineral Slurry

As a result of drying up of shallow mineral resources, researches on the efficient and non-polluting slurry lifting technique focused on the deep resources mining are imminent. As a desirable method of lifting liquid or solids, the airlift pump have no moving parts, simple structure, small size, low cost, little pollution, scarce blockage and higher reliability. A special applicaton is to be used in borehole hydraulic jet mining(BHM), deep sea mining and river dredging for rising solid particles. Moreover, it has great potential for improving environment, solving energy problem and ensuring stainable development.At present, as the complexity and unpredictability of air-water-solid three-phase flow, investigations on the slurry airlift technology are still rudimentary. Especially, few investigations have been done on the theoretical model and flow field structure of airlift system. In this paper, a theoretical model of the airlift system, which has high-precision, high-efficiency and good generality, was established. Also, studies on the flow characteristics of mixture flow were conducted. Consequently, the external and internal characteristics of slurry airlift system was brought to light. The main contributions of this paper are described briefly as follows:(1) Considering the nonlinear variation of air phase along the axial of lifting pipe, the theoretical model of airlift system was proposed based on the combination of momentum theorem, phase volume fraction and pressure loss model. Thus, the disadvantage of the traditional model with many empirical formulas was efficiently solved. Also, the present model was no longer subject to the flow pattern. And the effect of operation and structure parameters on the airlift performance was obtained quantitively through computation. It also shows that as the superficial velocity of air or the pipe diameter increases, three critical performation parameters including the lifting efficiency, superficial velocity of water and solid increase at first. After reaching their maximums, they decrease eventually. However, it is also found that these parameters vary monotonically with the submergence ratio and height of air inlet. The volume fraction of each phase remains constant along the axis in the segment within which the flow pattern is liquid-solid two-phase flow. Even in the places, where air-liquid-solid three-phase flow is involved, the variation of the volume fraction of each phase is not obvious. But the volume fraction of each phase varies sharply at the air inlet. In addition, as the superficial velocity of air or the submergence ratio increases, the pressure drop in the two-phase flow remains almost constant. The pressure drop, however, varies dramatically in the three-phase flow..(2) By the combination of critical boundary, at which the particles can be lifted in liquid-solid two-phase flow, and air volume fraction formula, the critical model of air-liquid-solid three-phase flow was proposed and compared with that of liquid-solid two-phase flow. Thus it can be seen that the critical condition, under which solid particles are vertically lifted, depends solely upon the two-phase flow. Moreover, the critical boundary at which the solid particles were subject to chip hold down effect was discussed in detail. It is found that the chip hold down effect go against startup of solid particle. Sometimes it leads to failure of airlift system.(3) To test the validity of the theoretical model mentioned above, a small airlift system was set up. In this system, the total height of the vertical lifting pipe is 3m. In the experiment, which is performed in this system, the medium and small ceramic particles of sphere shape were used as testing solids. Also, the static pressure at measure points on the pipe was obtained with a pressure transducer. Then, the influence factors of basic capability, pressure drop characteristics and critical condition of the airlift system were discussed. Thereby, it has been found that the experimental data gives good fit to the model proposed in the present study under medium superficial velocity of air. However, the above theoretical model fails to predict the experiment value under too low or too high superficial velocity of air. Again, the prediction precision of model decreases as the considered submergence ratio decreases. In addition, the critical condition under which liquid or solid particles were vertically lifted was discussed from a practical viewpoint. It is indicated that the critical air flow rate for pumping water is indpendent of solid partices. However, the critical air flow rate for rising solid partice is greatly affected by its location and size. Also, through the experiment, the underwater chip hold down effect has been confirmed that it can decline the ability of the airlift.(4) A sparging air injection method having the ability of both traditionary airlift and jet pump was proposed. Comparing with the radial air injection method, the superficial velocity of solid and the efficiency of the proposed method have been increased. However, no significant difference of superficial velocity of liquid between the proposed injection method and the radial air injection method has been found. Furthermore, a dimensionless equation which described the flows in the pipe was presented based on the Bernoulli equation. It is compared with measurement results in the dimensionless form. From the comparative study, we found that the results obtained by these two injection method are in good agreement with each other. And the calculation result, which is obtained through the dimensionless model, also coincides with the experimental results. Moreover, the effect of water jet nozzles on the airlift performance is analyzed. It is found that the nozzles not only eliminate the chip hold down effect, but also make more solid particles move toward the bottom of airlift pump. By using the nozzles, solid particles also get into the air-liquid-solid three-phase flow more easily. Therefore, the airlift performance is greatly enhanced. It is worth noting that the arrangements of nozzles along the circumference of airlift pump should be a bit off-centre.(5) The air-liquid-solid upward three-phase flow regime in the lifting pipe was experimentally analyzed with the help of the high-speed photography tecnnolgy. Seven typical flow patterns such as:bubble flow, small slug flow, irregular slug flow, large slug flow (slug flow), churn flow, fine bubble flow and annular flow were firstly presented. Through the comparative studies between flow pattern and airlift performance curves mention above, it has been found that the optimum performance of airlift system occurs in the fine bubble flow. Furthermore, the gray histogram and probability graph could be obtained from the photo of different flow patterns by the image analysis technique. Then, the statistical characteristics of the gray histogram were extracted and finally a flow pattern identification method was proposed based on the standard deviation, which is one of the statistical characteristics. In this research, the high-speed photography were utilized to investigate the distribution and movement characteristics of air bubbles, solid particles and mixed flow. The results obtained by the series of experiments show that the air bubbles and solid particals gradually permeate into the center of mixtures with the increasement of airlift performance and move toward the pipe inner wall, exhibiting a discrete form. Also, each phase in mixtures exhibits a periodic motion, which makes the upward movement of air-liquid-solid three-phase flow oscillate. It was found in the study that the lifting ratio, which is difined as the rise time of solid particles and period of mixtures, is very small. Such drawback could account for the low performance of airlift. In view of this, solid particles were captured by high-speed photography and it was obvious that the lifting ratio in the sparging air injection method was larger than that in the radial air injection method. Moreover, we found that the collapsing shrink of one air bubble leads to the skip of its adjacent air bubbles or solid particles.(6) According to the special geology conditions of the Fe-Mn mining area which locates in Houjiangqiao town of Daoxian county in Hunan province, a pulsed water jet and airlift combined system was built. The system works very well in this area. It sets a precedent for mining mineral deposit with supergiant gushing water. The performance of this system illustrates the advantage of the airlift technology in in borehole hydraulic jet mining.