Experimental investigation on flow structures and mixing mechanisms of a gas–solid burner jet

The objective of this study was to investigate the flow structures and mixing mechanisms of a gas–solid two-phase jet from a burner employed in the utility boiler. Laboratory experiments were conducted in a 0.15 m internal diameter test facility. Gas–solid jet flow downstream of the burner exit was measured by a fiber optic probe. Local solid concentration and particle size distribution were obtained using the probe that was traversed over the cross-section of the jet. The effects of the side flow on the primary air gas–solid flow characteristics were also studied. The measurements showed the availability of fiber optic probe to investigate the gas–solid jet flow downstream the industrial burner nozzle with high solid concentration. Obvious slip between the gas phase and the solid phase in the gas–solid jet flow was found in the experiments. The side flow disturbed the solid concentration distribution in the jet, a more smooth solid concentration profile across the jet can be achieved after the side flow was introduced. The particle size distributions in the jet were similar with the distributions of the solid concentration, the particles with large diameters always appeared in the zones where the solid concentrations were high.     

二维喷嘴内稠密气固射流稳定性实验

利用高速摄像仪对二维喷嘴内稠密气固射流稳定性进行了实验研究,考察了颗粒粒径、料仓压力以及喷嘴收缩角等因素对射流流动模式及稳定性的影响。结果表明：对于颗粒粒径为78μm的气固射流,随着料仓压力的增大,射流出口速度增大,射流固含率降低,在料仓压力≥0.03MPa、射流速度≥4.82m/s、射流固含率≤0.168时,喷嘴内稠密气固射流出现气泡型的不稳定流动模式;随着颗粒粒径的增大,气固射流固含率降低,喷嘴内稠密气固射流从气泡模式转变为颗粒团不稳定流动模式;改变喷嘴收缩角对射流不稳定模式影响不大。利用微型压力传感器对喷嘴直管不同位置压力进行测量,结果表明压力脉动主要是由于气固射流中气泡及颗粒团的产生及演变导致的。研究表明,随着料仓压力增大,颗粒在喷嘴内向下运动过程中压降增加,渗透进颗粒流的气体分率增加,将导致喷嘴内气固相互作用增强,进而引起气固射流不稳定。     

Horizontal gas and gas/solid jet penetration in a gas–solid fluidized bed

In this paper, the real time, dynamic phenomena of the three-dimensional horizontal gas and gas/solid mixture jetting in a 0.3 m (12 in) bubbling gas–solid fluidized bed are reported. The instantaneous properties of the shape of the jets and volumetric solids holdup are qualified and quantified using the three-dimensional electrical capacitance volume tomography (ECVT) recently developed in the authors’ group. It is found that the horizontal gas jet is almost symmetric along the horizontal axis during its penetration. As the jet width expands, the total volume of the gas jet increases. A mechanistic model is also developed to account for the experimental results obtained in this study. Comparison of jet penetration length and width between the model prediction and ECVT experiment shows that both the maximum penetration length and the maximum width of the horizontal gas jet increase with the superficial gas velocity. When the horizontal gas jet coalesces with a bubble rising from the bottom distributor, it loses its symmetric shape and can easily penetrate into the bed. For the horizontal gas/solid mixture jet penetration in the bed, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which are different from the characteristics exhibited by the gas jet. The solids holdup in the core region of the gas/solid mixture jet is higher than that in the gas jet. The penetration length of the horizontal gas/solid mixture jet is also larger than that of the gas jet.     

Effect of suspension characteristics on the performance of thermal barrier coatings deposited by suspension plasma spray

This paper investigates the influence of suspension characteristics on microstructure and performance of suspensions plasma sprayed (SPS) thermal barrier coatings (TBCs). Five suspensions were produced using various suspension characteristics, namely, type of solvent and solid load content, and the resultant suspensions were utilized to deposit five different TBCs under identical processing conditions. The produced TBCs were evaluated for their performance i.e. thermal conductivity, thermal cyclic fatigue (TCF) and thermal shock (TS) lifetime. This experimental study revealed that the differences in the microstructure of SPS TBCs produced using varied suspensions resulted in a wide-ranging overall TBC performance. All TBCs exhibited thermal conductivity lower than 1 W/(m. K) except water-ethanol mixed suspension produced TBC. The TS lifetime was also affected to a large extent where 10 wt % solid loaded ethanol and 25 wt % solid loaded water suspensions produced TBCs exhibited the highest and the lowest lifetime, respectively. On the contrary, TCF lifetime was not as significantly affected as thermal conductivity and TS lifetime, and all ethanol suspensions showed marginally better TCF lifetime than water and ethanol-water mixed suspensions deposited TBCs.     

CFD simulation of flow pattern and jet penetration depth in gas-fluidized beds with single and double jets

A simple two-fluid model is validated by comparing single-jet fluidization experiments and numerical predictions. Subsequently, flow pattern and jet penetration depth are explored numerically in the bed with double jets under equal and unequal gas velocities. Glass balltoni with a density of 2550 kg/m³ and a diameter of 275 μm is employed as solid phase. The model used in this study considers the effect of the dispersed solid phase on both gas and particle momentum equations of the inviscid model A (Gidaspow, 1994). Numerical simulations are carried out in the platform of CFX 4.4, a commercial CFD code, together with user-defined FORTRAN subroutines. Both jet penetration depth and jet frequency predicted are in good quantitative agreement with measurements in an incipiently fluidized bed with a single jet. By combining solid volume fraction distribution and particle-phase velocity vector profile, three flow patterns (isolated, merged and transitional jets) are identified in the gas-fluidized bed with double jets, which depend more on the nozzle distance than the jet gas velocity. For the equal jet gas velocity, the jet penetration depth decreases with increasing nozzle distance in the merged-jet and transitional-jet regions, then reaches a minimum value in the transitional-jet region, and finally keeps steady in the isolated-jet region. For the unequal jet gas velocity, the merged jet penetration depth increases with increase in the velocity of one jet as the other jet gas velocity is fixed, whilst the jet penetration depths change a little in the transitional-jet region and remain a constant in the isolated-jet region.     

Solid entrainment rate into gas and gas—solid,two-phase jets in a fluidized bed

A mathematical model for solid entrainment into a permanent flamelike jet in a fluidized bed was proposed. The model was supplemented by particle velocity data obtained by following movies frame by frame in a motion analyzer. The experiments were performed at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings ranging from 0 to 2.75. The particle entrainment velocity into the jet was found to increase with increases in distance from the jet nozzle, to increase with increases in jet velocity and to decrease with increases in solid loading in the gas—solid, two-phase jet.     

Free-standing diamond films prepared by a dc-plasma method above the ethylene glycol solution

Free-standing diamond films were prepared using a plasma chemical vapor deposition method above the liquid surface through two routes. Diamond was deposited on the surface of a tungsten anode under a dc-plasma regime. The electric field near the anode surface was flattened by placing a sub-electrode and it brought uniform film deposition. The growth rate was 5 μm h^− 1 and the thickness increased with the deposition time up to 12 μm. A free-standing film removed from the tungsten anode showed translucency. A glassy carbon layer with a thickness of 100 nm existed between the diamond film and the anode surface, and it partly remained on the back side of the removed diamond film. Under a plasma-jet regime, diamond was deposited on a silicon substrate brown with a plasma jet expelled from a nozzle exit. A high growth rate of 100 μm h^− 1 was attained at the maximum with increasing discharge power and carbon concentration, but the thickness profile was quite uneven. The removed film was elliptical and was larger than the nozzle size. A 3C–SiC layer was formed on the back side of the removed film.     

MEASUREMENT OF CONTINUOUS PHASE VELOCITIES IN A CONFINED SOLID-LIQUID JET USING LDV

A two-component LDV was used to investigate how flow structures in a liquid jet are influenced by the presence of solid particles. Solid glass spheres with a density of 2.5 kg/dm³ and three different sizes (0.5 mm, 1 mm, and 2 mm) at different solids loading (0–6.2 vol.%) were used as particles, while water served as continuous phase. No significant influence of the solid particles on the rate of decrease in the centerline liquid velocity or the jet expansion ratio could be observed. Two flow regimes were identified: a stable jet characterized by a symmetric shear layer close to the nozzle, and a flow dominated by large-scale instabilities farther from the nozzle. The spreading of particles was dominated by interparticle collisions in the region close to the nozzle and by jet instability in the region farther from the nozzle. With increased solids loading increased liquid RMS values in the region close to the nozzle could be found. The effect on RMS values was larger with larger particles. For the 1 mm particles the RMS increased in this region, but decreased in the region farther from the nozzle. The presence of 2 mm particles acted to stabilize the jet, and the instability moved farther downstream. Suspensions with smaller particles had no effect on the instability, strength, or location.     

Electrohydrodynamic drop-on-demand patterning in pulsed cone-jet mode at various frequencies

The patterning of a series of drops was investigated by the electrohydrodynamic printing method in the drop-on-demand fashion. A positive pulse voltage was applied to the capillary nozzle periodically to eject a pulsed liquid jet. The ejected jet was directed to the moving substrate, to which DC bias voltage was applied. High-speed imaging revealed that a Taylor cone was established at the nozzle tip during the ejection of the liquid jet, and that the jet directly struck the substrate to form a drop without the jet break-up. The frequency of drop generation can be controlled precisely, because the frequency of the pulsed voltage was almost same as the pulsating frequency of the liquid in pulsed cone-jet mode. The deposited patterns showed a series of uniformly sized drops with a regular spacing. At the pulse voltage frequency of 25 Hz, the diameter of the drops was approximately 95 μm. Using this drop-on-demand method, it is feasible to produce a variety of patterns of dots and continuous/discontinuous lines.     

Analysis of flow pattern and heat transfer in direct contact condensation

In direct contact condensation (DCC) phenomenon, whenever steam (vapor) is injected with very high velocity in sub-cooled water, the momentum and the energy of the steam is transferred to the surrounding liquid, leading to generation of flow pattern, turbulent in nature. The turbulent flow pattern enhances the heat transfer coefficient at the interface of steam jet and water (vapor-liquid interface) as well as at the immersed surfaces (solid-liquid interface). The flow and the temperature pattern in DCC system have been measured using hot film anemometer (HFA). The values of heat transfer coefficient at the vapor-liquid and solid-liquid interface were estimated using the CCA module of the HFA. The nozzle diameter (d₀) was varied in the range of 1-2 mm and the nozzle upstream pressure in the range of 0.3-0.35 MPa (corresponding velocities in the nozzle were 286-304 m/s). The time series of velocity and temperature at the interface were analyzed to get the rates of surface renewal. A comparison has been presented between the predicted and the experimental values of heat transfer coefficient.     

Experimental investigation of the effect of fuel nozzle geometry on the stability of a swirling non-premixed methane flame

An experimental investigation on the stability of a swirling non-premixed methane flame is reported in this paper. Methane gas is supplied through a central nozzle, and combustion (co-flow) air is supplied through an annulus surrounding the nozzle. Two main parameters were varied independently, which are the nozzle geometry and swirl strength; however the exit velocity of the central (fuel nozzle) jet and co-airflow were also varied to provide a wide range of test conditions. Two nozzles were tested: a contracted circular (referred to hereafter as CCN) and a rectangular (referred to hereafter as RN), which have similar equivalent diameter, D_e (defined as the diameter of a round slot having the same exit area as the nozzle geometry). The contracted circular nozzle has a diameter of 4.82 mm, and the rectangular nozzle has a diameter of 4.71 with an aspect ratio of 2:1. The swirl strength of the co-flow was varied by changing the vanes’ angle. The main results obtained from this study show that the rectangular nozzle exhibits higher entrainment and jet spreading rates compared with its CCN counterpart. In addition, the results revealed that increasing the swirl strength creates a flow recirculation zone which is larger with the RN compared with that of the corresponding CCN. These flow features associated with the RN lead to an enhanced mixing which consequently promotes better flame stability compared with its CCN counterpart.     

Numerical simulation of microparticles penetration and gas dynamics in an axi-symmetric supersonic nozzle for genetic vaccination

This paper describes two phase (solid particles/gas) flow in a supersonic nozzle that is part of a device for micromolecular vaccine/drug delivery. It accelerates micro solid particles to high speeds sufficient to penetrate the viable epidermis layer to achieve the pharmaceutical effect. Helium is used as the driving gas for the solid particles because of its high compressibility factor. A numerical parametric study was performed for gas pressures ranging between 3 and 6 MPa and gold particles of diameters 1.8 μm and 5 μm. The computed results show that uniform particle velocity was achieved at standoff distance of 2 exit diameters (D_e) downstream of the device exit with particles concentrated on the supersonic core jet. Increasing the helium pressure from 3 to 6 MPa caused an increase in the particle velocity of 24% for particles with a diameter of 1.8 μm and 7% for particles of diameter 5 μm at the standoff distance. Furthermore increased gas pressure has adverse effect on particles concentration. As the inlet pressure increases, the particles are concentrated more at the core of the nozzle. Semi-empirical particle penetration calculation confirms the numerical results that the 5 μm particles penetration distance is 45-135 μm and the 1.8 μm diameter penetration is 35-95 μm beneath the skin. Comparison of different geometries has been done in order to understand each section function and to gain optimum performance.     

Heat transfer from a grid jet in a large fluidized bed

Heat transfer from a vertical grid jet within a 2 ft diameter and 4 ft deep fluidized bed of cracking catalyst was studied. The test nozzle diameter was varied from ¼ to 1 in. and the nozzle velocity from 50 to 250 ft /sec which is within the range of industrial practice. The axial temperature data have been related to a Froude, a Reynolds and a Nozzle number: In (δT/δT_o) = –58.1 Fr^?0.562 No^1.08 Re^?0.112 A simple jet quenching model yielded heat transfer coefficients between the fluid bed and grid jet which ranged from 300 to 1200 Btu/ft² hr.^o F.     

Coaxial nozzle for control of particle morphology in precipitation with a compressed fluid antisolvent

A coaxial nozzle was developed to achieve further control over the morphology of microparticles precipitated from solution by carbon dioxide as a compressed fluid antisolvent. The polymer solution was sprayed through the core of the nozzle and CO₂ through the annulus. For the coaxial nozzle versus a standard nozzle, polystyrene and poly(L-lactic acid) particles can be larger by a factor of 3–8 with less flocculation. A reduction in the Weber number reduces atomization and larger droplets are formed in the jet, delaying precipitation. However, because of the much higher Reynolds number for the high velocity CO₂, the mass transfer in the suspension outside of the jet is faster leading to less flocculation and agglomeration. For polyacrylonitrile, the delayed precipitation produces a transition from highly oriented microfibrils to microparticles. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2105–2118, 1997     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

喷嘴进料对提升管进料段内颗粒浓度分布的影响

在提升管冷模实验装置上考察了喷嘴进料对颗粒浓度径向分布的影响规律. 结果表明,提升管进料段内存在3种形式的颗粒浓度径向分布,在距喷嘴较近的轴向区域,颗粒浓度沿径向呈明显的W形分布,喷嘴进料对颗粒流动的影响很强;在距喷嘴较远的轴向位置,颗粒浓度沿径向呈环-核分布,喷嘴进料对颗粒流动的影响很弱;在二者之间,颗粒浓度沿径向呈弱W分布,喷嘴进料对颗粒流动具有一定影响. 随着喷嘴气速的增加或预提升气速的减小,颗粒浓度逐渐由W形分布转变为环-核分布,喷嘴进料对颗粒流动的影响逐渐减弱. 采用喷嘴射流动量与预提升来流动量比Mj/Mr考察了操作参数及装置结构尺寸对提升管进料段内颗粒浓度径向分布的综合影响. 在实验范围内,动量比对进料段内颗粒浓度径向分布及颗粒流动行为具有明显的影响规律,随着动量比的增加,颗粒浓度逐渐由W形分布转变为环－核分布,操作参数及装置结构尺寸对颗粒流动的影响逐渐减小. 在动量比小于4.21时,操作参数及装置结构尺寸对颗粒流动的影响在H=0.675~1.075 m间的轴向位置基本结束;在动量比增大为4.21时,操作参数及装置结构尺寸对颗粒流动的影响在H=0.375~0.675 m间的轴向位置便已基本结束.     

Improved DC arc-jet diamond deposition with a secondary downstream discharge

The efficiency of a polycrystalline diamond coating in a DC arc-jet setup with a power of 5–10 kW was enhanced by extension of a high temperature jet core. Two approaches were investigated. First, an expansion of the plasma jet anode channel was employed. It allowed us to increase the process efficiency from 5.5 to 10 mg/h/kW by optimizing values of pressure and CH₄ content in a reactionary Ar/H₂/CH₄ mixture. In the second approach, the extension of the hot jet core was achieved by an additional downstream discharge with a power of 2.5 kW. The secondary discharge was initiated between a positively biased auxiliary electrode and the arc-jet nozzle. This method allowed us to obtain a growth rate of 40 μm/h on an area of 12 cm², which corresponds to an efficiency of 16 mg/h/ kW.     

Encapsulation of irregularly shaped solid forms of proteins in a high-pressure fluidized bed

By combining the advantages of a high-pressure fluidized bed with the adjuvant properties of a supercritical fluid, namely RESS-process, solid proteins could be encapsulated by paraffin. Two different irregular shaped proteins, a model protein (bovine serum albumin, BSA) and a pharmaceutical protein (insulin) were investigated. Mixing the highly non-spherical protein particles with lactose allows its usage as bed material despite the unfavourable shape properties. The paraffin encapsulation of respective bed mixtures with BSA was successful with paraffin loadings up to 9.1 g/100 g_bed and yields up to nearly 100%, depending on process parameters. During the encapsulation process, breakage of the bed material occurred, resulting in mean sauter diameters of even less than 35 μm. Dissolution tests pointed out, that 100% dissolution after more than 180 min could be achieved. Thus, the particle breakage induced by the nozzle jet during the coating process seems to support the encapsulation of fines. A principle application of the process for insulin encapsulation for a selected set of process parameters has been demonstrated.     

Experimental measurements of a gas-solid jet downstream of a fuel-rich/lean burner with a collision-block-type concentrator

The mixing mechanisms in the gas-solid two-phase fuel-rich/lean jet from a burner with a collision-type coal concentrator has been experimentally investigated using a fiber optic measurement system in a continuous-two-phase-flow loop of internal diameter 150 mm. The local solid concentration and particle size distribution were measured to investigate the mixing performance of the parallel fuel-rich and fuel-lean streams, and also the process of development of the fuel-rich/lean jet. The measurement results indicated that the concentration ratio between the fuel-rich and fuel-lean streams changes greatly with the height of the collision block. The fuel-rich and fuel-lean streams do not mix together soon after leaving the burner; this leads to better ignition and combustion characteristics of the pulverized coal compared with an ordinary burner. A local zone of high fuel concentration can be achieved at the fuel-rich side, resulting in staged combustion, to control NO_x emissions. A deviation between the jet direction and the nozzle axis was found, resulting from the velocity difference between the fuel-rich and fuel-lean streams, and a larger deviation in the profile of the solid phase was also observed. Such a deviation of the high-concentration zone is favorable for flame stability and NO_x emission reduction.     

催化裂化再生器树枝状气体分布器喷嘴的射流特性

在催化裂化装置中再生器底部通常设置有树枝状管式气体分布器,通过分布器上的喷嘴分布气体。但在实际运行过程中喷嘴常出现布气不均和磨损问题,影响其自身的布气性能和使用寿命。为此,在二维床实验装置上针对喷嘴的射流特性进行了实验研究。实验物料为FCC催化剂颗粒,喷嘴出口气速范围为30～70 m·s^-1,喷嘴喷射角度范围为0°～67.5°。实验用摄影观察法测量喷嘴射流的射流长度和附近的流场流态。实验结果表明射流长度随喷嘴气速和喷射角度的增大而变长。射流气体在向上翻转过程中,在树枝状管式气体分布器两分支管之间产生旋转涡流现象,旋转涡流的大小与喷嘴出口气速和安装角度有密切关系。最后基于实验数据,建立了喷嘴射流长度的计算模型。