| Energy is an important material basis for human survival and development,as well as an important factor restricting economic and social development.Energy saving and emission reduction has become the best choice for green and sustainable development in various countries.Among the energy consumption of various industries in my country,industrial energy consumption ranks first,and more than half of it is converted into industrial waste heat of different carriers and different temperatures.Make effective use to improve industrial economic efficiency.In order to study the particle flow characteristics in the heat exchanger,this paper establishes a multi-zone heat exchanger,uses the discrete element method to establish a numerical calculation model of the heat exchanger,and proves its accuracy through experimental comparison.For the particle flow process in the multi-zone heat exchanger,through the statistical law of multi-scale structure,analyze the wear position and strength of the vanadium particles in the heat exchanger on the heat exchange tube,as well as the particle group coverage area,residence time,force chain structure change,and contact The force,force balance state,the position of the shear layer of the particle bypass heat exchanger and the critical blockage state and its influencing factors.The effect of vanadium particle physical parameters(particle-to-particle static friction coefficient,particle size)and heat exchanger operation and structural parameters(discharge speed,internal heat exchanger tube diameter,heat exchanger spacing)on the particle flow characteristics are systematically studied.And draw the following conclusions:(1)The particle flow in the heat exchanger is divided into a near-wall area and a central beam area near the heat exchange tube.The uniformity of particle flow in the central beam area between the heat exchange tubes is better than that of the heat exchanger tube wall area;the particles form a boundary layer area near the boundary layer,and the particle flow at this time will produce an obvious “wall effect”,that is,some particles will be here.The area forms a hanging wall,and some particles are attached to the wall surface.The particle size range near the wall surface is about 4-6 times,and it is strongly disturbed.With a reasonably distributed tube spacing,the MFI value of each partition is greater than 0.3,the average MFI value of the particle flow in the central beam zone of the heat exchange tube is 0.867,and the particle flow is a bulk flow.The particle flow will be disturbed by the heat exchanger,and the radial direction of the heat exchange tube affects the range of about 8-10 times the particle size.The rotation potential energy of the particles in this affected zone accounts for more than 85%,and decreases rapidly after leaving the zone.The flow rate of particles in the range of about 15 times the particle size above the heat exchange tube is reduced,and the accumulation “dead zone” will be formed above the heat exchanger tube,and the accumulated particles will form a cavity area under the heat exchanger.(2)The transition of particle flow mode behavior largely changes with the change of the force chain structure between particles.With the increase of the friction coefficient between particles,the force chain system is denser,and the strength of the force chain increases.The number of force chain lengths of 6-8 particles is obviously higher than that of the low friction coefficient.At the same time,the number of force chains in the direction of 45° in the central flow zone gradually increases,and the particle flow is more prone to arch structure;the increase in particle size will reduce the number of long force chains(greater than 6)from 76 to 34,which is a decrease.Reaching 42%,it is not easy to form an arched strong chain between the internal heat exchangers and the walls of the heat exchanger,and the speed fluctuation range gradually increases;the larger the particle size,the average residence time of the particle group on the heat exchanger tube wall is reduced from 2.15 s By 1.73 s,the drop rate reaches 20%,and the heat exchange time between the particles and the heat exchanger tube is shortened;the faster the discharge speed,the void ratio near the tube wall decreases from 0.832 to 0.762,a drop of 12%,and the particle system is looser and more difficult.Form an arch structure.(3)The flow velocity has no obvious influence on the particle flow pattern and stability,and the force chain structure.When the control flow rate increases from 2 mm/s to 10 mm/s,the standard deviation of the particle vertical velocity pulsation gradually increases,and the velocity pulsation frequency increases from 0.08-0.13 KHz to 0.09-0.15 KHz,and the particle velocity pulsation becomes more intense.As the diameter of the heat exchanger tube increases,the influence range of the tube expands,the particle flow uniformity decreases slightly,and the direction of the strong chain in the heat exchanger does not change significantly.There is no significant difference in the probability density distribution of the ratio of the contact force between particles to the average contact force.The larger the heat exchanger tube diameter,the larger the cavity area under the heat exchanger tube,the area where the particles accumulate on the flow surface of the heat exchanger tube,and the coverage area will become larger,and the return flow of particles in the central area will collapse more obviously.It is easier to trigger the transmission of new velocity waves,making the particle pulsation more violent.When the dimensionless value N is around 0.25-0.5,there will be a greater probability of clogging.When the distance is maintained at 1.5-2.0 times the pipe diameter,the particle flow is relatively easy to reach a steady state,and the possibility of clogging is greatly reduced,and exchange Heater wear is relatively small.(4)The wear position of the heat exchanger is hardly affected by the material characteristics(particle size,friction coefficient,movement speed,etc.).Under the same particle size,the wear position will increase as the ratio of the heat exchanger spacing value to the pipe diameter increases.-45°～45° gradually reduced to-20°～20°,and the wear strength gradually weakened,and the uniformity of flow in the pipe was greatly improved.The larger the particle size,the more serious the abrasion of the heat exchanger tube wall is.Due to the "dead material" accumulation area on the upstream surface of the heat exchanger tube wall,the abrasion strength is very weak,and the positions on both sides of the heat exchanger tube wall the particles affected by the shear band have a serious abrasion effect on this location. |
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