Motion of particles entrained in a plasma jet

The motion of small particles (glass microspheres, 30 to 140 microns in diameter) entrained in a free argon plasma jet was studied by means of high-speed cine streak photography. Radial temperature and velocity profiles as well as axial profiles of temperature, velocity, and argon concentration in the jet were experimentally determined by means of a plasma calorimetric probe. The system was found to be characterized by low relative Reynolds numbers (0.2 to 20) and extremely high deceleration rates (about –2,000 g). Under these conditions, an increase of drag coefficient over that predicted by the standard curve was experimentally observed. This increase was attributed to the nonsteady flow field around the particle (the so-called “history term” in the equation of motion). A general computer program has been proposed which predicts the particle velocity, acceleration and temperature along its trajectory.     

引入气相对D类颗粒间歇卸料过程特性的影响

通过测量并分析引入气相前后D类颗粒间歇卸料过程中示踪颗粒的流动状态、压力分布和颗粒流率,发现床体内气固两相流动特性不仅随时间变化,还受到气速（正负压差）的影响。据此将卸料过程按时间分为3个阶段：初始蓄压（PS）阶段、稳定卸料（SD）阶段和非满管流（PP）阶段;并给出各卸料阶段不同正、负压差和重力条件下的气固流动特性。在时间较长、流场较为稳定的SD阶段,发现卸料口颗粒阻力是影响颗粒流率的关键参数,通过修正De Jong公式、Beverloo公式,依次建立卸料口颗粒阻力、D类颗粒卸料流率预测模型,与实验值吻合较好,有望为引入气相调控D类颗粒卸料流率的方法提供参考。     

Analysis of thermophoretic deposition of aerosol particles from laminar-flow gas streams

Thermophoretic deposition of aerosol particles from fully-developed laminar flow was considered to investigate the applicability of the assumption of constant transverse temperature gradient in flow channels. From the numerical calculations, this assumption was found to hold only for particles depositing far from the entrance of the collecting channel. In particular, it is shown that the variation of gas viscosity with temperature can have significant effects on the deposition pattern of large particles.     

Effect of surface morphology on the flow of entrained fine particles by ascending gas through packed beds

Coarse particles of glass or cokes were packed in vertical columns, 1- and 2-m in height, and fine particles of glass or coke were entrained through the columns by the ascending gas flow. These systems were used as a model to investigate the flow of fine coal particles introduced into a blast furnace. The effects of properties of packed particles, as well as fines, on the static and dynamic holdups of fines were then investigated. The static holdup of fines was strongly affected by the surface roughness of the packed particles, while the dynamic holdup of fines was not. These results suggest that the fine particles are trapped in the form of static holdup in the isolated narrow spaces bounded with packed particles, and that they remain on the surface of the packed particles which are exposed to the gas flow for only a short period of time.     

Modeling percolative fragmentation during conversion of entrained char particles

A numerical model of carbonaceous particle conversion under chemical and external diffusion control is proposed. The model accounts for different classes of particles which undergo chemical conversion in parallel with percolative fragmentation. It applies to typical conditions of entrained flow reactors. The system of algebraic and differential equations has been numerically solved. Results include the total carbon conversion as well as the determination of particle properties along the reactor. The model correctly predicts the change of the conversion rate at varying temperature, initial oxidant concentration and excess oxidant ratio. The influence of percolation parameters is also relevant and claims further investigations for more accurate determination. A comparison with experimental data available in literature is also provided     

Motion and deposition of fibrous flexible particles in laminar gas flow through a pipe

The equation of motion of a flexible slender particle in a straight horizontal cylindrical pipe, under conditions of gas flow similar to those which occur in part of human airways, is presented. Particle orientation, deformation and limiting trajectory is computed. The deposition efficiency of particles due to sedimentation, inertia and interception for different gas velocities, particle mass and geometry is discussed.     

Coalescence model of particles in coaxial impinging streams

Coalescence is mainly affected by particle-to-particle collisions. A new model based on an equation expressing the condition for interparticle collision rate as well as the equation of motion of a single particle, has been developed for coaxial impinging streams. The model assumes that coalescence takes place only between small and large particles. In addition to the drag force usually influencing the behavior in impinging streams, the force acting on a large particle due to its collisions with small particles, has also been accounted for in the equation of motion. The effect of the change of the mass of the large particles on the acceleration due to coalescence with small particles, has also been included in the model. The model enables one to predict the composition of the product after coalescence from the composition of the feed and the operating conditions of the coalescentor.     

Design of stirred vessels with gas entrained from free liquid surface

In this review, the literature on surface aerators is critically analysed. In a number of studies, the critical speed for the onset of surface aeration has been correlated with the geometry of the system and the operating conditions. The studies on the rate of surface aeration, gas hold-up, mass transfer coefficients are inadequate for the design and scale-up. A comparison of the available designs, in terms of gas hold-up, and mass transfer coefficient is presented. A mathematical model has been proposed for surface aeration. Guidelines have been given for the selection of impeller configuration and overall design of surface aerators. Suggestions have been made for future studies in this area.     

Mercury removal from hydrogen gas streams

Type 13X molecular sieve and chromic acid on silica gel have been examined as sorbents for the removal of mercury vapour from hydrogen at atmospheric pressure and, chiefly, ambient temperature. The molecular sieve had a very low capacity for mercury. Chromic acid on silica gel had a much higher capacity but this was much reduced by the presence of water vapour in the hydrogen.     

Adsorptive purification of large waste gas streams

A two-bed process for the adsorption of parts per million concentrations of H₂S and CS₂ from humid exhaust air was investigated. In the first bed H₂S can be effectively converted to sulphur with an impregnated activated carbon. Regeneration is possible by extraction of sulphur. In the second bed the physisorption of CS₂ is reduced, because of competitive water adsorption. A linear driving force model was found to provide an acceptable simulation of measured breakthrough curves, provided the equilibrium data can be fitted.     

Size distribution of the particles entrained from fluidized beds: Gas humidity effects

Experimental results obtained with 65 μm sand and 69 μm cracking catalyst showed that the smallest particles were not the most easily elutriated from fluidized beds of mixed size particles. This is in contradiction with the assumption applied to existing models for the prediction of the flux and size distribution of particles elutriated from a fluidized bed. The smallest particles cannot be removed by elutriation from a mixture of particles of various sizes. The smallest particles may be agglomerated with larger particles. Increasing the gas humidity, which would presumably reduce electrostatic effects, did not greatly affect the size distribution of the elutriated particles.     

Dissolution and growth of entrained bubbles when dip coating in a gas under reduced pressure

This study assesses experimentally the role of gas dissolution in gas entrainment which hitherto has been speculated on but not measured. In this paper, we used dip coating as the model experimental flow and performed the experiments with a dip coater encased in a vacuum chamber in which we admitted various gases. An appropriate choice of gases (air, carbon dioxide and helium) coupled with low pressure conditions from atmospheric down to 75 mbar enables us to test whether gas solubility is a key determinant in gas entrainment. The data presented here track the evolution in time of the size of bubbles of gas entrained in the liquid (silicone oil) which we observed to always occur at a critical speed, immediately after the dynamic wetting line breaks from a straight line into a serrated line with tiny vees the downstream apices of which are the locations from which the bubbles stream out. The results suggest that permeability combining solubility and diffusivity as a single parameter dictates the rate of dissolution when at atmospheric pressure. Helium, despite its comparatively sluggish rate of dissolution/growth into silicone oil was observed to have a more enhanced gas entrainment speed than air and carbon dioxide. Thus, the hypothetical contention from previous work (Miyamoto and Scriven, 1982) that gas can be entrained as a thin film which breaks into bubbles before dynamic wetting failure occurs is not realised, at least not in dip coating. The data presented here reinforce recent work by Benkreira and Ikin (2010) that thin film gas viscosity is the critical factor, over-riding dissolution during gas entrainment. This finding is fundamentally important and new and provides the experimental basis needed to develop and underpin new models for gas entrainment in coating flows.     

颗粒在圆形偏心滚筒内的运动模式

实验研究了影响颗粒在圆形偏心滚筒内运动状态的因素,分析了2种偏心距、4种填充率和4种转速对运动模式的影响。结果表明:转速和填充率是影响颗粒在圆形偏心滚筒内运动模式变化的主要因素,偏心距对其基本没有影响;当转速为15r/min和25r/min时,填充率不会影响颗粒的运动模式;而在转速为5r/min时,颗粒运动模式随填充率由1/6增大到1/3,相应由阶梯模式变为滚动模式;在转速为40r/min时,颗粒运动模式随填充率由1/6增大到1/3,相应由滚动模式变为小瀑布模式;偏心距越大,颗粒在运动过程中产生的涡心越偏离滚筒中心;转速的增加使颗粒群的上表面和下落层两端的弧度发生变化;填充率对颗粒在圆形偏心滚筒中的运动模式的影响只发生在低转速和高转速下。     

Motion of a gas slug in inclined tubes

The velocities of a freely rising gas slug and a phase boundary (gas-liquid) under conditions of liquid discharge in tubes of various diameters are experimentally studied. It is shown that the nonmonotonic character of the dependence of measured velocities on the angle of inclination is determined only by the curvature in the vicinity of the critical point of the head of the slug or the phase boundary during liquid discharge. Experimental results are obtained by measuring the profiles of curvature of the bubble head in mutually perpendicular planes. For these purposes, an appropriate computer program was developed, and an immersion optical procedure was used that made it possible to eliminate optical “noises” associated with the thickness of a tube wall.     

Action of high temperature gas streams on zirconia refractories

Conclusions We investigated the erosion resistance of zirconia stabilized with CaO in the temperature range 1700–2350°C as a function of a gas current velocity of from 300 to 730 m/see.The erosion rate increases exponentially with temperature rise, and its dependence on the velocity of the current approximates to a linear rule. The highest erosion rate is noted in the first hours of the gas current action on the specimens, which is connected with the removal of impurities and weakly bonded grains, and also the formation of a surface layer of material.The erosion rate essentially depends on the composition and structure of the material.Incorporating potassium in the gas current increases the erosion rate 10–25%, but does not alter the nature of the relationship between the erosion and temperature of the current and its action time.Translated from Ogneupory, No. 6, pp. 45–50, June, 1968.     

Intermixing of fluidizing gas streams in a compartmented circulating fluidized bed

The intermixing of fluidizing gas between the compartments of a compartmented circulating gas fluidized bed fitted with a V-valve and riser combination has been experimentally studied. The intermixing of fluidizing gas that flows co-current (cross flow) as well as counter-current (back flow) to the circulating solids has been investigated. At low aerations, gas cross flow increased proportionally with the increase in solid circulation rate. But at high aerations, gas cross flow increased with aeration even when solid circulation decreased. Studies on back flow of gas revealed some interesting observations. At high fluidizing bed height on upstream side back flow of V-valve, gas has been negligible. But at low upstream bed heights and low aeration in V-valve and riser back flow of V-valve, gas has been substantial.     

Phoretic phenomena in tar vapor-particulate mixture separation from fuel gas streams

Analytical models were developed to predict the performance of a spray scrubber for separation of tars from gasifier off-gas streams. The models included heat transfer, mass transfer, condensation, nucleation, temperature and flow fields, and various phoretic phenomena involved in droplet-particle and droplet-vapor interaction and collection. The models indicate that for the tar particulates, both Brownian diffusion and inertial impaction, rather than diffusiophoresis and thermophoresis, are the more important phoretic forces causing the collection. For the limiting case of all tars being present only as vapor, the efficiency of their removal by condensation on water spray droplets could be extremely high. Scrubbing experiments were carried out on the hot gas/tar stream evolved from the simulated devolatilization section of a laboratory fixed bed gasifier. The combined overall collection efficiency for a mixed particulates and vapor stream compares satisfactorily with the model predictions.     

Removal of VOCs from gas streams via plasma and catalysis

Emission of volatile organic compounds (VOCs) has resulted in various environmental issues. Therefore, development of effective VOC removal technology is essential for reducing the adverse effects associated. This work provides a systematic review on VOC removal from gas stream via catalytic oxidation, plasma degradation, and plasma catalysis. For catalytic oxidation of VOCs, possible reaction mechanisms and how physicochemical properties of catalyst influences catalytic performance are presented and discussed, followed by plasma removal of VOCs, VOC degradation, and byproduct formation mechanisms. Next, interactions between plasma and catalyst are interpreted for comprehensive understanding. Last, perspectives are provided for further development of VOC removal technology.