| Evaporating hot water tower is the core equipment of the multi-nozzle opposite coal gasification process in the blackwater of heat recovery,using the flash-heat transfer integration technology.Steam directly contacts the cooling water in the form of bubbles inside of the water chamber over the tower plate for heat and mass transfer.Therefore,exploring the bubble characteristics on the tower plate is the basis of understanding the law of mass transfer in the hot water tower.The evaporating hot water tower was taken as the research object.Through building a visual chamber and using high-speed camera with computer aided software,the visualization research inside the tower was carried out.The bubble growth,movement characteristics and liquid film rupture of steam,nitrogen and carbon dioxide were deeply studied,as well as the influence of gas-liquid flow rate,solid particles and plate conditions on bubble characteristics.The simulation method was used to verify the experimental results.The research results provided theoretical guidance for the optimal design and operation of industrial hot water tower.And the conclusions were as follows:(1)Based on visual observation,the bubble movement cycle was divided into three stages:formation region,rising region and oscillation region.In the formation region,the equivalent radius and the centroid velocity of bubble increased rapidly,during which the aspect ratio of bubbles decreased.In the rising region,the bubble equivalent radius increased slowly.The equivalent radius of bubbles increased slowly and then decreased rapidly,while the aspect ratio changed rapidly during the oscillation region.The centroid velocity of the steam bubble in the Y direction increased firstly and then fluctuated steadily,while the velocity of the nitrogen bubble kept increasing.The velocity of the carbon dioxide bubble remained stable after it left the tower hole.The bubble formation time decreased with the addition of water and gas flow rate,and the bubble separation radius decreased with the water flow rate.The separation radius of carbon dioxide bubbles increased the most as the gas flow increased.The simulation model of single bubble movement was established,and the error between the simulation value of bubble radius and experimental value was less than 10%,which could predict the single bubble shape,velocity field and flow field direction accurately.The hollow phenomenon was found during the rising process inside the bubble both in experimental and simulation results.The increase in the proportion of non-condensable gas led to faster times of bubble formation and fragmentation,hindering the process of condensation and heat transfer.(2)The removal efficiency was calculated by bringing the solid coal particles into the hot water tower and capturing the solid particles at the air outlet and liquid outlet.Comparing the experimental results of two-phase and three-phase conditions,it was found that the entrainment of pulverized coal particles and the increase of non-condensable gas reduced the proportion of formation region time and increased the proportion of oscillation region,which indicated that the introduction of pulverized coal particles promoted the fragmentation of bubbles and strengthened the heat transfer in the tower.The removal efficiency of the tray for all particle sizes was greater than 95%,and the removal efficiency for the larger particle size(>20μm)was 100%.With the increasing nitrogen flow rate,the particle removal efficiency began to decrease,and the solid particles with size of 2-3 μm had the lowest removal efficiency.The decrease of α,which represents the mass flow ratio of steam to nitrogen,promoted heat and mass transfer.After the sieve plate was set,the temperature distribution of the whole tray changed in a stepped manner on the sieve plate,which meant adding the tray can enhance the heat and mass transfer in the tower.A heat transfer model was established for the micro-element section of the tower and the correlation formula between the number of heat transfer units and kinetic energy factor was obtained,NTUL=0.20·FP-1.34.(3)The visualization experiment of single-hole tray and double-hole tray in the hot water,double-hole bubble simulation model and force analysis of bubbles were established.The influence of gas-liquid flow rate,gas characteristics and hole spacing on the motion characteristics of coaxial double bubbles and double holes two bubbles were studied.The results showed that under the action of transverse water flow,the bubble trajectory rose to the side of the overflow weir.The increase of liquid flow strengthened the backflow effect of flow field and decrease the drift during bubble rising movement.The increase of air inlet gas velocity accelerated the bubble breaking and reduce the the separation time and size of the bubble.For coaxial bubbles,the velocity of the upper bubble increased rapidly in the early stage and tended to be stable after a period of time.For the lower bubble,the velocity increased and reach a velocity peak in the first half of the time,and then gradually decreased.The formation time of single-hole bubbles was longer than that of double-hole bubbles.The double-hole rising bubbles interacted with each other.The left bubble BL near the liquid phase entrance fluctuated more violently in the stage of bubble formation and the rising stage.When the bubbles approaching the liquid surface,both bubbles fluctuated violently.When the gas velocity was low,the double bubble trajectory presented a process of "close-away-close" to each other.The simulation results at low gas velocity agreed relatively well with the experimental results.BL had more vortexes in it,resulting in more changes in shape and velocity than BR.The aspect ratio of BR changed little under the influence of wall effect.As the gas velocity increased,the upward trajectory of the two bubbles was similar to some extent.The emergence time of left bubble BL was earlier,the detachment time was later,and the stability degree was weaker than that of right bubble BR.(4)By observing the rupture process of the bubbles rising to the free liquid surface,the breakage situation,the distribution of bursting points,the velocity of liquid film and the number distribution of film droplets were studied.The results showed that the bubble rose to the free liquid surface and was pressed back to the water surface after a fluctuation.In the process of bubble rupture,a rupture point was first generated in the liquid cap part of the bubble surface,and a circular liquid film was formed at the rupture point and then broken into droplets after continuous movement.There were three cases of bubble rupture location,which was located at the top,middle and bottom of the bubble.70%of the steam bubbles rupture occurred at the top of the bubble,resulting in fewer membrane droplets,and the middle and bottom of the bubble rupture accounted for less.The bursting points of nitrogen bubbles were mainly distributed in the middle and bottom,while the bursting points of the carbon dioxide bubbles were located at the top of 47%,decreasing in the middle and the bottom.The instantaneous velocity of liquid film was related to the membrane thickness,and the overall movement velocity of liquid film was inversely proportional to the bubble rupture radius.The fitting formula of film velocity and bubble rupture radiusis was in good agreement with the experimental results.The number of film droplets produced by bubble rupture was proportional to the bubble rupture radius R2,which meant the number of film droplets produced by breakage increased with the increase of bubble size.The relationship between droplet quantity and gas property is not obvious... |
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