CFD simulations of droplet and particle agglomeration in an industrial counter-current spray dryer

In this study, CFD simulations of particle and droplet agglomeration in an industrial counter-current spray dryer are presented. For this purpose, a modified form of the stochastic collision agglomeration model is proposed. This model takes into account droplet–droplet collision as well as wet and dry particle interaction. These events are coupled with heat, mass and momentum transfer. A comprehensive moisture evaporation model based on the concept of characteristic drying curve (CDC) was applied to predict the drying kinetics of the detergent slurry. Due to high instability in air flow inside the drying chamber, simulations were carried out under transient condition. A comparison between time-averaged simulation results and measurements, which were performed on an industrial spray drying installation, shows a good agreement. This finding proves the correctness of the developed agglomeration and drying models. The presented methodology of CFD simulations of agglomeration can be used to design or optimise spray-drying installations and to predict the final particle size distribution of the product.     

Oscillation behavior of a high-temperature silicon droplet by the electromagnetic levitation technique superimposed with a static magnetic field

When a static magnetic field was applied to a Si droplet levitated by an electromagnetic force, only one peak was observed to remain in the frequency spectrum. It was the objective of this work to clarify whether this peak can be assigned to the m = ± 2 oscillation or to the rotation of the droplet. By analyzing the behavior of the deflection angle of the droplet in a top view, we conclude that this peak is not due to the surface oscillation of the droplet but to the droplet rotation.     

Combustion of a liquid fuel droplet in an acoustic field

The rate of combustion of a liquid fuel droplet in an acoustic field is investigated analytically for certain limiting values of the dimensionless numbers characterizing the process. An expression relating the burning rate and the acoustic parameters is derived.     

Containerless solidification of germanium by electromagnetic levitation and in a drop-tube

Containerless solidification of germanium has been realized by electromagnetic levitation and drop-tube processing, respectively. The effect of undercooling in the range of 40–426 K on the as-solidified structures of levitation melted Ge drops (∼8.4 mm diameter) was investigated. For undercoolings less than 300 K, the lamellar twins were grown, whereas a microstructural transition to equiaxed grains was observed at undercoolings ≥300 K. Further increasing the undercooling to 400 K, a significant reduction in grain size was achieved. In addition to a similar microstructural development among the particles solidified during free fall in a 8.5 m drop-tube, high-undercooling-induced single crystals were found for some droplets less than 200 μm in diameter. The results on the transition from twins to fine equiaxed grains are accounted for by theories of solidification kinetics and a dendrite break-up model. This revised version was published online in November 2006 with corrections to the Cover Date.     

液氮核态池沸腾CFD模拟和可视化实验

为探究低温流体池内核态沸腾机理,对液氮池内核态沸腾进行了计算流体力学(CFD)建模及实验研究。除了探究过热度和热流关系,重点分析过热度对气泡脱离直径和频率影响。根据实验观测,将核态沸腾过程分为3个阶段:低热流阶段;过渡沸腾阶段;完全核态沸腾(FDNB)阶段。基于得到的沸腾过程气泡直径及频率,构建了核态沸腾CFD数值模型,得到的过热度及热流密度关系,与实验测量得到的数据吻合。     

高频电磁力作用下金属液内电磁超声波的生成

为解决由机械振动产生的超声波无法在高温环境中成功应用的难题，提出了利用静磁场和交流电流、静磁场和交流磁场的相互作用分别在金属液内直接生成高强度电磁超声波的方法，运用超声波压力传感器对用这两种方法在金属液内生成的电磁超声波的强度进行了测量。研究表明：电磁超声波的强度与所施加的高频电磁力的强度成正比，频率与电磁力的频率相同；实验结果和理论解析结果基本一致；利用高频电磁力的局部作用在金属液全场范围内生成的电磁超声波可以在金属精炼及凝固过程中获得广泛应用。     

Electrophoresis of a charged droplet in a dielectric liquid for droplet actuation

Electrophoretic motion of a charged droplet in a dielectric fluid under an electric field has been investigated experimentally for use as a microdroplet actuation method. The effects of the droplet size, electric field strength, and electrolyte concentration and ion species on the charging of an aqueous droplet have been examined. The amount of electrical charging has been measured by two different methods: indirect measurement using the image analysis of droplet motion and direct measurement using the electrometer. Quantitative comparison of the droplet charge measured experimentally and the theoretical value of a perfectly conductive sphere shows that an aqueous droplet is less charged than the corresponding perfectly conductive sphere. The limiting effect on electrical charging is more significant for an electrolyte droplet, and the effect is positively correlated to the electrolyte concentration rather than the ion species. This implies that the low electrical conductivity of water is not a major cause of the limiting effect. The scaling law of the charging amount for a deionized water droplet nearly follows that of the perfect conductor, whereas for an electrolyte droplet, the scaling law exponent is slightly higher. Some advantages and potentials of the current droplet actuation method are also discussed in comparison with the conventional ones.     

Performance of droplet generator and droplet collector in liquid droplet radiator under microgravity

The Liquid Droplet Radiator (LDR) has an advantage over comparable conventional radiators in terms of the rejected heat power-weight ratio. Therefore, the LDR has attracted attention as an advanced radiator for high-power space systems that will be prerequisite for large space structures. The performance of the LDR under microgravity condition has been studied from the viewpoint of operational space use of the LDR in the future. In this study, the performances of a droplet generator and a droplet collector in the LDR are investigated using drop shafts in Japan: MGLAB and JAMIC. As a result, it is considered that (1) the droplet generator can produce uniform droplet streams in the droplet diameter range from 200 to 280 [μm] and the spacing range from 400 to 950 [μm] under microgravity condition, (2) the droplet collector with the incidence angle of 35 degrees can prevent a uniform droplet stream, in which droplet diameter is 250 [μm] and the velocity is 16 [m/s], from splashing under microgravity condition, whereas splashes may occur at the surface of the droplet collector in the event that a nonuniform droplet stream collides against it.     

Performance of droplet generator and droplet collector in liquid droplet radiator under microgravity

The Liquid Droplet Radiator (LDR) has an advantage over comparable conventional radiators in terms of the rejected heat power-weight ratio. Therefore, the LDR has attracted attention as an advanced radiator for high-power space systems that will be prerequisite for large space structures. The performance of the LDR under microgravity condition has been studied from the viewpoint of operational space use of the LDR in the future. In this study, the performances of a droplet generator and a droplet collector in the LDR are investigated using drop shafts in Japan: MGLAB and JAMIC. As a result, it is considered that (1) the droplet generator can produce uniform droplet streams in the droplet diameter range from 200 to 280 [μm] and the spacing range from 400 to 950 [μm] under microgravity condition, (2) the droplet collector with the incidence angle of 35 degrees can prevent a uniform droplet stream, in which droplet diameter is 250 [μm] and the velocity is 16 [m/s], from splashing under microgravity condition, whereas splashes may occur at the surface of the droplet collector in the event that a nonuniform droplet stream collides against it.     

The frequency effect on electromagnetic confinement and shaping of liquid metal

Electromagnetic confinement and shaping is a kind of newly developing solidification technology. With the electromagnetic field imposed, the metal is melted to some superheating degree by Joule heating and is confined and shaped to the desired form by the electromagnetic force at the same time. The frequency effect of the electromagnetic field on the electromagnetic confinement and shaping is investigated. The relationship between the Joule heat, the electromagnetic force and the frequency is revealed from the viewpoint of the magnetohydrodynamics theory. The results show that, given the condition of the same magnetic field strength, the time-averaged volume electromagnetic force at the melt surface is approximately proportional to the field frequency, whilst at the same time the volume Joule heating rate is approximately proportional to the square of the field frequency. However, as the frequency increases, the shaping stability and the surface quality decrease due to the violent surface electromagnetic stir. The frequency effect is also correlative to the sample's size and electric conductivity. The bigger is the sample, the lower ought to be the frequency and vice versa. The bigger is the electric conductivity, the lower can be the frequency and vice versa. Thus there is an optimum frequency range for each different sample, which is tens to hundreds of kilohertz for the small-size or medium-size steel sample. The experimental research is carried out with aluminium, copper and stainless steel samples, and the results show good agreement with the theoretical predictions.     

The frequency effect on electromagnetic confinement and shaping of liquid metal

Electromagnetic confinement and shaping is a kind of newly developing solidification technology. With the electromagnetic field imposed, the metal is melted to some superheating degree by Joule heating and is confined and shaped to the desired form by the electromagnetic force at the same time. The frequency effect of the electromagnetic field on the electromagnetic confinement and shaping is investigated. The relationship between the Joule heat, the electromagnetic force and the frequency is revealed from the viewpoint of the magnetohydrodynamics theory. The results show that, given the condition of the same magnetic field strength, the time-averaged volume electromagnetic force at the melt surface is approximately proportional to the field frequency, whilst at the same time the volume Joule heating rate is approximately proportional to the square of the field frequency. However, as the frequency increases, the shaping stability and the surface quality decrease due to the violent surface electromagnetic stir. The frequency effect is also correlative to the sample's size and electric conductivity. The bigger is the sample, the lower ought to be the frequency and vice versa. The bigger is the electric conductivity, the lower can be the frequency and vice versa. Thus there is an optimum frequency range for each different sample, which is tens to hundreds of kilohertz for the small-size or medium-size steel sample. The experimental research is carried out with aluminium, copper and stainless steel samples, and the results show good agreement with the theoretical predictions.     

Modeling of an isolated liquid hydrogen droplet evaporation and combustion

In this model, diffusive governing equations of Liquid Hydrogen (LH) evaporation and combustion were solved. The simulation reveals that, there exists a critical radius (a_cri) where radiation heat is equal to conduction heat (Q_rad = Q_cond) and a_cri is a function of ambient temperature during LH droplet evaporation process. Under pure evaporation condition, for large liquid hydrogen droplets (a > a_cri) radiation heat is dominant at a given environment temperature, but as liquid droplet size decreases, radiation heat becomes insignificant and thermal conduction will be dominant for liquid evaporation. When LH droplet is burned in a cold environment (T_∞ = 300 K), there are two films above the LH surface, Film I is from LH surface to flame front within which a dense hydrogen gas cloud is formed; Film II is from flame front to the free stream where oxygen is diffused inward to react with hydrogen. The flame front is located about 95 times of the droplet radius (r_f = 95a) and the flame temperature could rise up to 2077 K. When an LH droplet is immersed in a hot environment (T_∞ = 2050 K), the flame front is located at a similar distance to the LH droplet (r_f/a = 114) and flame temperature could go up to 3769 K.     

Preparation of metals in ultra high vacuum by electromagnetic levitation

The conditions for dynamic instabilities of electromagnetically levitated metal specimens have been investigated. Methods which eliminate the instabilities have been developed and were used to decarburize and degas a levitated niobium sphere in ultra-high vacuum for a period of 16 hours.     

Analysis of liquid droplet deformation in a gas flow

The small perturbation method is used to obtain equations describing the dynamics of a liquid droplet in a flow of ideal incompressible gas. The stability criteria and droplet disintegration time are determined.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 50, No. 5, pp. 743–748, May, 1986.     

液态金属双频电磁成形中磁感应强度和电磁压力的数值计算

对液态金属双频电磁成形中三种圆形截面感应圈配置形式中磁感应强度分布进行了计算 ,结果发现 :(1 )直筒和直筒感应圈组合形式中磁感应强度和电磁压力在轴向上易造成两个峰值分布 ,而斜筒和直筒 ,斜筒和斜筒感应圈组合形式可得到与液态金属静压力分布趋势相同的磁感应强度和电磁压力分布 ;(2 )在径向上 ,离双感应圈内壁越近 ,三种配置形式感应圈产生的磁感应强度和电磁压力越大 ,液态金属双频电磁成形形状的稳定性也就越高 .     

Numerical simulations of an electromagnetic acoustictransducer-receiver system for NDT applications

The development of a multistage numerical model of an electromagnetic acoustic transducer (EMAT), with particular emphasis on an EMAT receiver, is presented. The model includes five separate modeling states: static magnetic field simulation of an electromagnet; pulsed eddy current distribution of a generic meander-line coil suspended over a conducting specimen; Lorentz force distribution due to the interaction of the static magnetic field with the eddy current distributions; acoustic wave generation and propagation based on the dynamic Lorentz forces; and acoustic wave detection by an EMAT receiver. In particular, it is shown how the transient particle displacement fields are converted into an induced voltage response as part of the EMAT receiver system. Numerical simulations show that the voltage response is dependent on the wire spacing of the receiver coil     

电磁悬浮熔炼系统的结构及其悬浮力的研究

电磁悬浮熔炼技术是伴随着对特种纯净材料的需求以及航天等高新技术中对微重力研究等而产生和发展起来的，因其重要的科学技术地位而受到关注。本文从电磁基本理论从发，对轴对称螺线锥管电磁悬浮熔炼系统的结构进行分析，得到悬浮系统几何参数与悬浮力的关系；对悬浮系统中平衡线圈的结构进行改进，计算结果表明，改进结构具有较大的悬浮力。据此研制的试验系统实现了电磁全悬浮。