Experimental research of drop‐on‐demand droplet jetting 3D printing with molten polymer

Three‐dimensional (3D) printing technology has become an effective method for parts manufacturing and got a certain application in many fields. Now, drop‐on‐demand droplet jetting 3D printing appears as a new method of manufacturing technology which has a proven research progress for metal, colloid, and liquid resin materials. However, there are hardly any researches of droplet jetting 3D printing with molten polymer. So, considering molten polymer as the jetting material with droplet jetting method is an explorative direction. In order to attain the molten polymer droplets and achieve droplet jetting 3D printing with molten polymer, the 3D printing technology of differential melt (3DPDM) is developed independently. According to 3DPDM, a complete set of drop‐on‐demand droplet jetting 3D printer have been developed. In this work, PP (6820) was chosen as the experimental material. Under the different print parameters such as the rotation speed of screw, nozzle diameter, mechanical impact frequency, heating temperature, the space between nozzle and platform, the form, and deposition of droplets were studied. Furthermore, the optimal print parameters were summarized. By printing models with the optimal print parameters, it turned out that the 3DPDM is able to achieve drop‐on‐demand droplet jetting 3D printing with molten polymer. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45933.     

Droplet formation at megahertz frequency

Droplet formation has been a fascinating subject to scientists for centuries due to its natural beauty and importance to both scientific and industrial applications, such as inkjet printing, reagent deposition, and spray cooling. However, the droplet generation frequency of common drop‐on‐demand (DOD) jetting techniques is mostly limited to ～10 kHz. This article presents an investigation of the possibility of jetting at megahertz frequencies to boost the productivity of DOD material deposition by ～100 times. The focus of this article is to understand the limitations of generating droplets at a megahertz frequency and to explore possible solutions for overcoming these limitations. A numerical model is first developed for the simulation of droplet formation dynamics. The numerical model is validated against available experimental data from the literature. Aided by insights gained from scaling analysis, the validated model is then used to study the effects of different parameters on high frequency jetting. The study finds energy density input to the nozzle is the key to achieve megahertz frequency droplet breakup. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2367–2377, 2017     

Visualization study of emulsion droplet formation in a coflowing microchannel

An experimental visualization study is conducted to investigate the hydrodynamic characteristics of emulsion droplet formation in a coflowing microchannel. Both monodisperse and polydisperse patterns of drop formation are observed, including dripping regime, jetting regime (widening jetting and narrowing jetting). Especially, two dripping-to-jetting transition regimes and wavy regime with no individual droplet produced are captured and analyzed. A corresponding phase diagram is provided to characterize the transitions between different emulsification patterns through the control of flow rate of continuous phase. In addition, the dependence of generated droplet size on the Capillary number of the continuous phase (Ca) and the Weber number of the dispersed phase (We) is presented. It is indicated that, when Ca is below 3, the generated droplet size is sensitive to the viscous force and the drop formation regime is widening jetting and dripping. However, when Ca exceeds 3, the generated droplet size is approximately independent of Ca, and the droplet formation regime is thinning jetting.     

Hydrodynamics of spray column for enzymatic reactions of oil

The hydrodynamic characteristics of a spray column for oil–water system have been investigated in the present work in a column of 5 cm inner diameter and 250 cm height, operated in a semi batch manner. The effect of dispersed phase flow rate and nozzle diameter on different hydrodynamic parameters such as drop diameter, dispersed phase hold up, terminal rise velocity and jetting velocity has been studied. It has been observed that the drop diameter, dispersed phase hold up, terminal rise velocity and jetting velocity increase with an increase in nozzle diameter. Also, dispersed phase hold up increases with dispersed phase velocity whereas terminal rise velocity and drop diameter show a marginal dependency. Correlations have been developed, using nonlinear regression, for the prediction of drop diameter, dispersed phase hold up, terminal rise velocity and jetting velocity.     

Electrically stressed water drops in oil

The deformation of water droplets in an electric field has been studied in hydrocarbon liquids. A water drop will elongate in an electric field due to the electrostatic pressure, and becomes unstable when a critical field limit is reached. By applying different voltage waveforms it is possible to measure both transient, time varying and static effects. An experimental method has been developed to study the rheological properties of the water droplet. The experimental results in surfactant-free model oil are in good agreement with classic Taylor theory. In diluted crude oil a correlation was found between the drop behavior in an electric field and the interface elasticity modulus, measured by dilational interfacial rheology. The static drop deformation was reduced with increasing elastic modulus due to adsorption of polar surfactants to the water/oil interface. Drop surface oscillations will be damped by the viscosity of the bulk liquid. The dynamic drop behavior was studied in viscosity standards prepared from different concentrations of polystyrene in toluene. The drop oscillations were modeled as damped oscillator, and the influence of viscosity on the eigen-frequency and damping coefficient was studied for different drop sizes. The oscillations can also explain the premature break-up of water droplets sometimes observed in the experiments.     

陶瓷过滤器脉冲反吹系统引射性能的实验测定(Ⅰ)──喷吹气体流量的测定和分析

采用热线风速仪测量陶瓷过滤器脉冲反吹喷嘴出口的流场得到了喷吹气体流量，分析了喷吹压力和脉冲宽度对喷吹气体流量的影响。实验结果表明，喷吹气体流量随喷吹压力的增加而显著增加，而脉冲宽度的增加对喷吹气体流量的影响较小，只是增加了气体喷吹持续时间。同时证明由热线风速仪测得的一次脉冲反吹的压缩气体耗量与将储气罐排气过程简化为绝热排气过程所计算出的结果非常吻合，推荐采用绝热排气过程估算压缩气体耗量。     

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.     

Breakup of relatively low-concentrated O/W emulsion jets

The breakup of emulsion jets ejected downward into air from a nozzle has been investigated experimentally, and the breakup length of jets and the drop size for kerosene-water emulsions have been measured. The concentration of the dispersed phase was less than 50 wt%, so all experimental emulsions exhibited Newtonian flow. Photographic observation indicated that the emulsion jet resembled the jet of normal homogeneous liquids. The breakup length of the jet and the drop size from the jet are in good agreement with predictions from stability analysis for normal liquids.     

陶瓷过滤器脉冲反吹系统引射性能的实验研究(Ⅱ)——动态引射比的测定与分析

通过测定陶瓷过滤器引射器喉管处的瞬态流场得到了脉冲反吹过程中引射气体流量随时间的变化规律 ,分析了喷吹压力和脉冲宽度对引射气体流量的影响。依据喷吹气体流量的实验结果 ,得到了不同喷吹距离时的动态引射比随时间的变化规律。结果表明 ,喷吹距离是影响引射性能的一个重要参数。     

Experimental study on drop formation in liquid-liquid fluidized bed

Drop formation in liquid-liquid fluidized bed was investigated experimentally. The normal water was injected via a fine-capillary spray nozzle into the co-flowing No. 25 transformer oil with jet directed upwards in a vertical fluidized bed. Experiments under a wide variety of conditions were conducted to investigate the instability dynamics of the jet, the size and size distribution of the drops. Details of drop formation, drop flow patterns and jet evolution were monitored in real-time by an ultra-high-speed digital CCD (charge couple device) camera. The Rosin-Rammler model was applied to characterize experimental drop size distributions. Final results demonstrate that drop formation in liquid-liquid system takes place on three absolutely different developing regimes: bubbling, laminar jetting and turbulent jetting, depending on the relative Reynolds number between the two phases. For different flow domains, dynamics of drop formation change significantly, involving mechanism of jet breakup, jet length pulsation, mean size and uniformity of the drops. The jet length fluctuates with time in variable and random amplitudes for a specified set of operated parameters. Good agreement is shown between the drop size and the Rosin-Rammler distribution function with the minimum correlation coefficient 0.9199. The mean drop diameter decreases all along with increasing jet flow rate. Especially after the relative Reynolds number exceeds a certain value about 3.5×10⁴, the jet disrupts intensely into multiple small drops with a diameter mainly ranging from 1.0 to and a more and more uniform size distribution. The turbulent jetting regime of drop formation is the most preferable to the dynamic ice slurry making system.     

Break-up of a drop in a stirred tank

The drop break-up mechanism was studied in a stirred tank containing two immiscible liquids. The daughter drops formed by break-up of a single drop of known size were recorded photographically. From the experiments at constant agitator speed the following results were obtained. There is a critical drop size under which drops do not break up under given conditions. The break-up frequency increases approximately linearly with increase in drop volume. The number of daughter drops, v, is a random variable with a mean v > 2 which increases with the volume of the mother drop. The relative volume of a daughter drop has a β-distribution.     

INTERACTING JETS IN A FLUIDIZED BED

This work extends the Pitot-tube probe technique to the study of jetting phenomena in a three-dimensional, high-temperature gas-solid fluidized bed. For an isolated jet, the jet height was defined as the intersection of two momentum flux profiles, one along the jet axis and the other in the emulsion phase. A study of two adjacent jets confirmed that the measured jet heights were close to the visual jet heights observed through a window in the bed.

In two adjacent jets, the jets behave like two isolated jets at low nozzle velocities. As the nozzle velocity increases, the jet heights reach a maximum height in the transition zone. The jets begin to interact after the transition zone, and the jet height becomes a constant.

Similar results are obtained for two kinds of perforated-plate distributors. The maximum jet height for multiple, interacting jets is a function only of distributor geometry and is well predicted by a simple geometric model.     

Experimental and numerical study of fluid dynamic parameters in a jetting fluidized bed of a binary mixture

The solid circulation pattern, the voidage profile, and the jet penetration height have been investigated experimentally and computationally in a cold-flow model of jetting fluidized beds (JFBs) of a binary mixture in this paper. This rectangular two-dimensional bed is 0.30 m wide and 2.05 m high with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized-bed coal gasifier. A video camera and coloured particle tracer method were employed to explore the fluid dynamics in the bed. In terms of the average physical properties of binary mixtures, a hydrodynamic model describing the gas-solid flow characteristics in a jetting bed is resolved by using a modified Semi-Implicit Method for Pressure-Linked Equation (SIMPLE) algorithm. This paper focuses on three features of the fluid dynamics—solid circulation pattern, voidage profile, and jet penetration height. The solid circulation pattern is composed of three regions: the jetting region, the bubble street, and the annular region. Above the central nozzle the time-averaged isoporosity contours are almost elliptic, while near the walls of the bed, the voidage in high solid concentration region is approximately equal to that at the minimum fluidization state. The jet penetration height increases with increasing jet gas velocity and with decreasing average particle diameter. The increase in weight percentage of the lighter component in the binary system reveals that reduction of average density causes the enlargement of jet penetration height. The simulated results show good agreement with the experimental data.     

EXPERIMENTAL AND NUMERICAL STUDY ON GAS FLOW IN THE GRID ZONE OF JETTING-FLUIDIZED BEDS

A grid model describing the gas flow and interchange in the grid zone of jetting fluidized beds is proposed. Based on this model, longitudinal gas concentration profiles in the jet and annulus are calculated. The longitudinal gas concentration distribution is also experimentally investigated in a jetting fluidized bed with an inside diameter of 50 mm at the ambient temperature, and a jetting fluidized bed with an inside diameter of 80 mm at high temperatures. Comparison between the calculated and experimental results has shown that the experimental profiles can be qualitatively predicted by the grid model. The results indicated that the concentration in the grid zone depends on the gas exchange between the jet and the annulus, and the net gas flow from the jet to the annulus. The gas exchange rate is mainly affected by the inlet gas velocity from the nozzle. The present study is thought to be helpful to understand the grid gas behavior in the jetting fluidized bed coal gasifier.     

单组分两维射流床内空隙率分布的数学模拟

根据基本守恒方程建立了描述气－固两相流动的数学模型，将用于单相流计算的ＳＩＭＰＬＥ算法改进后，成功地开发了预测流化床内流体动力学的ＣＡＳＩＣＣ软件包。进而详细地模拟了单组分射流床空隙率分布特性，结果表明时均空隙率随轴向位置依次分为：浓相区、颗粒夹带区和稀相区。所得结论与实验观察相一致，可以为该类反应器的设计和放大提供有益的建议     

Effects of polymer properties on jetting performance of electrohydrodynamic printing

We perform systematic study on the jetting performance of electrohydrodynamic (EHD) with an insulating polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). EHD printing applies electrostatic field to ink droplet hang on nozzle tip, which causes the deformation of the meniscus to generate discrete droplets or continuous jet stream. Although EHD jetting mechanism has been frequently investigated with conducting or semiconducting materials, there still needs to elucidate EHD jetting of insulating polymer materials for producing controllable droplets. In the present study, we focused on how the physical/chemical properties (conductivity, dielectric constant, and molecular weight) of an insulating polymer affect jetting behavior of EHD printing (especially, the deformation of the meniscus and the corresponding morphology of the printed one). The relationship between the printing parameters and applied voltage is also investigated, thereby allowing the optimization of EHD printings for PS and PMMA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45044.     

蒸汽喷射式抽空器变工况性能及分析

炼油厂减压蒸馏装置中的减压塔一般采用蒸汽喷射式抽空器来实现塔内的真空。在实际生产运行中 ,工作蒸汽的参数是不稳定的 ,而且处理量也是变化的。针对这种状况 ,推导了绝热指数的计算公式并建立了蒸汽抽空器的计算模型 ,用蒸汽作为引射气体进行了计算 ,并对变工况条件下蒸汽抽空器的性能及进行了分析。     

Photorealistic ink‐jet digital printing – factors influencing image quality,image stability and print durability

Digital cameras have now replaced film‐based cameras as the most popular method of still image capture. As a consequence, there has been a rise in growth of the photo‐realistic digital‐printer market as many amateur and professional photographers choose to produce their own hard copy images. The most popular digital printing technology for producing photo‐realistic images is currently drop‐on‐demand ink jet. There has been much research into key factors influencing the quality, stability and durability of images produced using this, and other, digital printing technologies. A key area of study in achieving photo‐realistic images from digital printing systems has been ink‐receivable layers and dye/pigment colorants, and most importantly compatibility between the two. As with any new technology it is important to achieve an acceptable standard of performance and, to this end, research work has been instigated by the International Standards Institute since the mid‐1990s, to achieve a set of standards appertaining to areas such as light fastness, water fastness, thermal stability, humidity fastness and pollution susceptibility. This paper reviews the current state regarding the aforementioned areas with respect to their influence on print quality, stability and durability.     

Numerical simulation of gas-assisted polymer-melt electrospinning: Parametric study of a multinozzle system for mass production

In this study, we developed a multiphysics model for simulation of a gas-assisted melt-electrospinning (GAME) process, focusing on jet formation and propagation behavior. By numerically calculating the stresses acting on the jet during a single-nozzle GAME process, the shear viscous stress was identified as the main factor with respect to jet stretch; thus, the relationship between shear viscous stress and jet thickness was investigated. The jet stretch ratio increased sharply when shear viscous stress reached the level at which jet sharpening occurred, leading to stable jet formation. We defined this stress as the critical shear viscous stress to determine stable spinnability. By imposing an electric field distribution calculated for a multi-nozzle array (number of nozzles, tip-to-tip distance, and applied voltage) on the boundary condition of the single-nozzle GAME simulation model, multinozzle GAME was simulated; this enabled proposal of a spinnability diagram for stable spinning.     

The formation of uniformly sized drops by vibration-atomization

The effects of external vibrations on the disintegration of a liquid jet are described. Conditions for the formation of uniformly sized drops are discussed and results presented to illustrate the effect of amplitude and frequency of vibration on the drop size and break-up length of the jet. Liquids of various viscosity have been studied, 60-1400 c.p., and the results compared with existing theories. This method can be used with standard pressure nozzles and may be of use in research experiments when dispersions of uniform drops are required. It also has application in the formation of uniform prills.