| In the operation of the industrial gasifier,slender particles with certain aspect ratios bridged and deposited on the internals in the scrubbing-cooling chamber.In some serious cases,this may result in the fluctuated liquid level and purgative crude coal gas with excess black water,thereby affecting the stable operation of the gasifier.Based on these backgrounds,in this study,the short nylon fiber particles were used as simulants in the gas-liquid-solid three-phase system.The cold experimental studies on the multi-phase distribution characteristics in the slender-particle containing scrubbing-cooling chamber were deeply carried out and aimed to provide theoretical reference and engineering practical guidance for the fluid dynamics researches in the scrubbing-cooling chamber containing slender particles.(1)The local distr:ibution characteristics of slender particles and gas phase in the scrubbing-cooling chamber were studied by the direct sampling method,dual-tip probe conductivity probe method and high-speed imaging method.The results showed that the lower liquid-solid flowing region of the scrubbing-cooling chamber was divided into three regions:gas phase turbulence region,recirculation region and secondary flow region.Under the effects of hindrance and agglomeration of slender particles,the axial distribution of local solid concentration fluctuated while the radial distribution was affected by the lateral drift velocity of slender particles.The local radial solid concentration fluctuated greatly in the gas phase turbulence region and recirculation region while the secondary flow region showed a circular flow,in which the local radial solid concentration presented higher in the center but lower near the wall due to the "wall effects"of the fluid and slender particles.The upper gas-liquid-solid mixing region in the liquid bath of the scrubbing-cooling chamber was divided into the downcomer spout region,bubble-breaking plate region and foam region.Under the effects of reverse buoyancy and negative pressure gradients,in downcomer spout region,the gas phase flowed upward in the form of plumes along the outer wall of the downcomer.The radial distribution of local gas holdup in the bubble-breaking plate region showed obviously proximate core peak distribution,which was closely related to the internals and flow patterns.The foam region consisted of bubble coalescence and breakup as well as droplets formation and splashing.(2)The cross-section averaging and global gas holdups were obtained by crossing sectional and volume averaging methods,respectively.Local flow regime map was obtained by the maximum dimesionless bubble chord length and local gas holdup.And the effects of the superficial gas velocity,fiber volume fraction and fiber aspect ratio on the global gas holdup were studied.The results showed that the cross-section averaging gas holdup at different fiber volume fractions and fiber aspect ratios can be effectively predicted by the modified Kataoka&Ishii bubbly flow semi-empirical correlation.Both of the local gas holdup and bubble chord length had great effects on the local flow regime.Bubbly flow formed near the inner wall of the liquid bath(region Ⅰ)due to the bubble breakup caused by the wall shear stress while the other annulus regions(regions Ⅱ and Ⅲ)showed cap-bubbly flows due to the emergence of cap bubbles.The global gas holdup increased with the increase of the superficial gas velocity and the increasing rate gradually reduced under the coupling effects of two factors:the increasing rate of bubble numbers and bubble coalescence.At lower superficial gas velocities,with the increase of fiber volume fraction,the global gas holdup slightly increased due to the suppressed bed turbulence and bubble loading.At higher superficial gas velocities,with the increase of the fiber volume fraction,the global gas holdup decreased due to the increased apparent liquid viscosity and vortex shedding on the particles.As the fiber aspect ratio increased,global gas holdup gradually increased due to the suppressed small-scale velocity fluctuation in the fiber suspensions.A modified drift-flux model containing crowding factor and fiber number density was proposed and the global gas holdup models for the scrubbing-cooling chamber(with and without fibers)were established,which were in good agreement with the experimental data.(3)The chord length distribution was transformed into the size distribution of bubbles touching the probe by the decomposition of measured chord length distribution and estimation of bubble shape factor.The effects of superficial gas velocity,fiber volume fraction and fiber aspect ratio on the rising,descending and total bubble size distributions were studied.The results showed that rising bubble size distributions showed wide unimodal and bimodal distributions in regions Ⅰ and Ⅱ,respectively,while the sharp unimodal distribution was observed in region Ⅲ.Descending bubble size distributions showed sharp distributions in the three regions and the total bubble size distributions were the same as that of rising bubbles.The demarcation of the equivalent bubble diameter between the small spherical and large non-spherical bubbles was 2 mm.The variation of superficial gas velocity affected the liquid turbulence,shear stress between the fluid and the wall,and liquid backmixing,which finally changed the size distributions of rising,descending and total bubbles.The different bubble size distributions in different regions changed with the variation of fiber volume fraction,which was affected by the extent of turbulence suppression and fiber isolation effect.The fiber number density characterizing the variation of the fiber aspect ratio with different fiber diameter and length was used and the bubble size distributions at different aspect ratios in different regions were affected by interactions among fibers,bubbles,the fluid and the wall.(4)Bubble velocity distribution was obtained by the kernel density estimation.The interfacial area was calculated by the size distribution of total bubbles in the bed which was transformed from that of total bubbles touching the probe by an analytical transformation method.The effects of superficial gas velocity,fiber volume fraction and fiber aspect ratio on the distributions of rising and descending bubble velocities and interfacial areas were studied.The results showed that mean rising bubble velocities significantly decreased at r/R=0.55 while the minimum values were obtained at r/= 0.85.As the superficial gas velocity increased,the radial fluctuation of mean descending bubble velocity was gradually reduced.Rising bubble velocity distributions gradually transformed from wide peak distri’butions in region Ⅰ to sharp peak distributions in region III while descending bubble velocity distributions showed sharp peak distributions in the three regions.With the increase of superficial gas velocity,mean rising and descending bubble velocities increased in regions I and Ⅱ.While,in region III,mean rising and descending bubble velocities decreased and fluctuated with increased superficial gas velocity,respectively.Mean rising and descending bubble velocities were determined by the mean bubble diameters at different fiber volume fractions.However,for small descending bubbles,the flow regime and fiber volume fraction both had significant effects.At different fiber aspect ratios,the variations of mean rising and descending bubble velocities were inversely proportional to those of the fiber number density,but the mean descending bubble velocity showed an opposite trend in region II.Interfacial areas showed central and wall peak distributions at low superficial gas velocity(Ug= 0.074 m/s)and high superficial gas velocity(U-=0.3 7 m/s),respectively.Interfacial areas increased with the increase of superficial gas velocity in the three regions and they were mainly affected by total bubble Sauter diameters at different fiber volume fractions and fiber aspect ratios.A modified Besagni&Inzoli interfacial area correlation was developed by introducing fiber volume fractions,fiber aspect ratios and dimensionless radius. |
