A thermal inkjet printhead with a monolithically fabricated nozzleplate and self-aligned ink feed hole

A monolithic thermal inkjet printhead has been developed and demonstrated to operate successfully by combining monolithic growing of a nozzle plate on the silicon substrate and electrochemical etching of silicon for an ink feed hole. For the monolithic fabrication, a multiexposure and single development (MESD) technique and Ni electroplating are used to form cavities, orifices, and the nozzle plate. Electrochemical etching, as a back-end process, is applied to form an ink feed hole through the substrate, which is accurately aligned with the frontside pattern without any backside mask. The etch rate is nearly proportional to the current density up to 50 μm/min. Experiments with a 50-μm-diameter nozzle show ink ejection up to the operating frequency of 11 kHz with an average ink dot diameter of about 110 μm for 0.3-A, 5-μs current pulses     

Numerical investigation of cyclic variations in gasoline engines using a hybrid URANS/LES modeling approach

Cycle to cycle variations are an important aspect in the development and optimization process of internal combustion engines. In this study the feasibility of using a detached eddy simulation (DES) SST model, which is a hybrid URANS/LES model, to predict cycle to cycle variations is investigated. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS shear-stress transport (SST) model is used. In the other regions with higher grid resolution an LES model is applied. First, the numerical requirements associated with the hybrid URANS/LES and the employed solver are studied in detail. The numerical dissipation of the spatial scheme and the choice of the temporal scheme including the step size are evaluated. In addition, the accuracy of the solver for moving meshes, which are required for engine calculations, is assessed. The modeling constant linking the grid size to the DES filter length scale is determined by calculating a decaying homogeneous isotropic turbulence test case for different grid resolutions. The final applications of the model are two different engine cases with increasing complexity. The first case is the statistically stationary flow through an engine intake port. The time resolved flow structure predicted by the DES SST model is analyzed and the resulting time-averaged velocity fields are compared to experimental data at different locations. The second application is a motored multi-cycle simulation of a series production engine. The instantaneous flow development during the intake and compression stroke of one single cycle is studied and the ensemble-averaged and the instantaneous velocity fields as well as the resolved velocity fluctuations are compared to optical measurements. Special emphasis is placed on the cyclic differences of the velocity fluctuations at the time of ignition in the vicinity of the spark plug and the expected influence on the combustion process.     

Modeling and characterization of an industrial inkjet head for micro-patterning on printed circuit boards

A conceptual design using computational fluid dynamics (CFD) and micro-electro-mechanical systems (MEMS) fabrication has been performed to develop an industrial inkjet head for micro-patterning on printed circuit boards. The printhead has been fabricated with silicon and silicon on insulator (SOI) wafers by MEMS process and silicon to silicon bonding method. The measured displacement waveform from a piezoelectric actuator by laser doppler vibrometer (LDV) was used as input data for the three-dimensional flow solver to simulate the droplet formation. The mechanism of droplet ejection from piezoelectric-type inkjet heads was investigated by simulating two-phase flows of the air and metal inks. As a preliminary approach, liquid metal jetting phenomena are identified by simulating droplet ejection and droplet formation in a consequent manner. Parametric studies are followed by the design optimization process to deduce key factors to inkjet head performance: nozzle geometry, droplet size, ejecting speed, pulse amplitude, and ink viscosity. The present design tool, based on a two-phase flow solver and experimental measurements, has shown its promising applicability to various concept designs of industrial inkjet system for micro-patterning on electronic chips and boards.     

High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.     

Studying on water nanojet ejection and the wetting phenomena on the nozzle surface

In this research, molecular dynamics simulations of water nanojet ejection out of nozzle holes with various sizes under various pressing forces are performed. The water molecules are ejected out the nozzle by a back plate on which a constant force is applied. The results of MD simulations of water ejection show that after one ejection, about 1.3?C2.5% of total molecules accumulate on the nozzle plate surface. These molecules affect the ejection of water jet thereafter. The cause of the accumulation of wetting water is investigated by analyzing the trajectories of these molecules. It is found that in the firing chamber near the nozzle plate wall, the arrangement of water molecules is aligned by the surface topology of the metal wall. Water molecules are packed into filamentous structure and these lines stack up at equal distances to each other. Water molecules drift along these lines, the trajectories of these molecules are sinuous, the velocity directions of them are random; molecules drift along the parallel lines until they reach a region of low pressure beneath the nozzle opening. These molecules eject out through the edge of the nozzle, they fall back on to the nozzle surface and eventually deposit on the nozzle surface due to low ejection velocity.     

Effects of trapped air bubbles on frequency responses of the piezo-driven inkjet printheads and visualization of the bubbles using synchrotron X-ray

The reliability of drop formation is one of the key factors for the successful commercialization of inkjet printing applications. However, when the air bubble is entrapped from the nozzle exit, it leads to stop jetting of the droplets immediately. It has been known that the trapped air bubbles inside the chamber prevent a printhead from stable jetting. In this study the synchrotron X-ray has been used to visualize the air bubbles in the flow field inside of the printhead undergoing the standard jetting with the firing frequency of 1–20 kHz. An air pocket of bubble was formed repeatedly and reproduced well through the tested printheads at a certain jetting condition. To see the effects of the bubbles on the dynamic characteristics of the piezoelectric printheads, the piezoelectric velocity on the top of the pressure chamber was measured with a laser vibrometer. When the bubble was trapped at the nozzle exit orifice, the piezo velocity signals showed significantly different frequency peaks appearing in the spectrum and the high frequency components were identified with the frequency response measurements.     

瞬时流量法检测电子式燃气表计量性能

电子式燃气表通过传感器在采样周期测得瞬时流量,与采样周期相乘积得到累积流量。将各个采样周期内的数值累计,得到测试时间内的累积流量。按目前累积流量法测试误差,需要通过固定的脉冲当量计量累积流量,存在所采集累积值与实际值不一致、测试时间较长、标准装置累积值计算不够准确等问题。基于音速喷嘴气体流量标准装置和电子式燃气表输出瞬时流量的特点,标准装置通过光电头按照近红外通信协议与燃气表实现信息交互,实现瞬时流量法测试误差,有效解决了上述问题。描述瞬时流量法测试的方法原理,通过用0.5级音速喷嘴标准装置按两种方法测试G1.6规格超声波燃气表qmax、0.2qmax、qmin三个流量点误差各6次,两种方法测试误差的差异小于0.2MPE,瞬时流量法测试6次的标准偏差在0.12%以内,验证该测试方法的准确性;检测时间由30 min缩短为5 min,提升效率80%左右。该瞬时流量法值得推广使用,满足快速、准确、高效测试要求。     

一种测量气--液两相流的线列阵传感器设计

针对气—液两相流研究对含气率测量的需求,基于线列阵测量技术原理,设计了一种可移动式线列阵两相流测量传感器,该传感器具有较高的空间分辨率（3 mm）和极高的时间分辨率（2500 Hz）,设计了线列阵传感器标定和含气率算法,实现了瞬时二维局部含气率的测量。经过射流冲击试验验证表明：该线列阵传感器结构稳定,基于原始测量数据,采用标定和含气率求解算法,可计算气泡夹带现象在水平截面的平均含气率分布情况。     

注射成型弧形流道流场分析

针对在注射成型过程中,注射熔体的均匀程度以及其进入型腔的速度方向对制件的品质会产生很大影响的问题,主要对熔体进入型腔前的流动进行分析.采用弧形流道,用有限差分法离散连续性方程和动量方程,用超松驰迭代法求解离散后的代数方程组,进而求出弧形流道熔体流动的速度和压强分布.结果表明,在弧形流道中,外层熔体可能进入中心层甚至内层,局部有旋涡,内外层熔体有物质和能量交换,所以熔体经过弧形流道后密度和温度场分布更均匀.     

Fluid flow and heat transfer in the evaporating thin film region

The evaporating thin film region is an extended meniscus beyond the apparent contact line at a liquid/solid interface. Thin film evaporation plays a key role in a highly efficient heat pipe. A detailed mathematical model predicting fluid flow and heat transfer through the thin film region is developed. The model considers the effects of inertial force, disjoining pressure, surface tension, and curvature. Utilizing the order analysis, the model is simplified and can be numerically solved for the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate in the evaporating thin film region. The prediction shows that while the inertial force can affect the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate, in particular, near the non-evaporating region, the effect can be neglected. It is found that a maximum velocity, a maximum heat flux, and a maximum curvature exist for a given superheat, but the locations for these maximum values are different.     

Air-Flow-Based Single-Cell Dispensing System

We discuss a design and fabrication approach to increase the success rate of single-cell dispensing. Two pairs of capacitance sensors are placed in a biochip to detect the flow velocity of cells and air pressure is applied to eject cells by synchronizing the timing. A comprehensive design theory, which takes into account the back-pressure caused by the air pressure, the response time of the system, the sensor properties and the delay of the dispensing from the air pressure, is developed in order to minimize the disturbance of the system and maximize the throughput of the ejection system. Then, the system theoretically has a capability to eject 3 cells/s and the maximum flow velocity is 10 mm/s. The novelty of the system is that the biochip is disposable, which is unlike the conventional mechanical inkjet system; because the biochip is low cost and disposable this prevents contamination and means the drive system is reusable. Finally, we succeeded in automatic dispensing of a single polystyrene bead (100 μm) from a biochip to a culture well atmosphere using the developed cell ejection system with a success rate of 50%. Furthermore, we also succeeded in single swine oocyte dispensing by using the developed system.     

Waveform Design Methods for Piezo Inkjet Dispensers Based on Measured Meniscus Motion

Waveform design methods for piezo inkjet dispensers based on measured meniscus motion are presented. The meniscus motion is measured from charge-coupled-device camera images wherein strobe lights from light-emitting diodes are synchronized with the jetting signal. Waveforms for the piezo dispenser are designed such that the number of experiments can be significantly reduced compared to conventional methods. Furthermore, the designed waveform can also be evaluated by the measured meniscus motion since the motion is directly related to jetting behavior.     

Instability of Marangoni toroidal convection in a microchannel and its relevance with the flowing direction

The instability of the Marangoni toroidal flows in microchannels is of interest in various areas such as microfluidics and heat transfer. Using pure liquid as working fluid in this study, the phenomena of Marangoni symmetry-to-asymmetry transition which does not arise from the buoyancy was observed. The experiments used a vertical cylindrical channel and the meniscus was formed at the bottom outlet to minimize the buoyant influences. Two microscopes were used to have top view and side view of the meniscus simultaneously. The Marangoni flow field on the meniscus was obtained by means of tracing particles. It was observed that the Marangoni flow on a concave meniscus was always nearly symmetrical, while that on a convex meniscus was out of symmetry with only one single vortex occupying the whole channel. The experimental results were highly consistent to the simulation results of authors’ previous 3D numerical model (Pan and Wang in Microfluid Nanofluid 9:657, 2010). Theoretical analysis together with newly developed numerical models is employed to dig into the mechanisms. The inward (from the meniscus edge to the center) Marangoni flow is found not as stable as the outward one. Based on the heat transfer analysis, a concave meniscus always has a colder edge thus the flow is outward and stable; while a convex enough meniscus could have an inward flow thus not stable and tends to lose the symmetry. The amplification mechanism of the inward Marangoni flow is comprehensively explained. Two conditions are required for the inward flow to lose the symmetry, i.e., the bulk liquid must be warmer than the meniscus, and the Marangoni number must be above a certain small value.     

Experimental and numerical investigation of unsteady cavitating flows through a 2D hydrofoil

In the present study, firstly, the unsteady cavitating flows around a hydrofoil are studied based on the flow visualization and detail velocity measurement, a high-speed video camera is used to visualize the flow structures, and a particle image velocimetry (PIV) technique is applied to the measurement of the time-averaged and instantaneous velocity and vorticity fields. The results show that the unsteadiness of mass transfer process between the vapor and the two-phase regions is substantial, a self-oscilla...     

In situ ray tracing and computational steering for interactive blood flow simulation

Recent algorithm and hardware developments have significantly improved our capability to interactively visualise time-varying flow fields. However, when visualising very large dynamically varying datasets interactively there are still limitations in the scalability and efficiency of these methods. Here we present a rendering pipeline which employs an efficient in situ ray tracing technique to visualise flow fields as they are simulated. The ray casting approach is particularly well suited for the visualisation of large and sparse time-varying datasets, where it is capable of rendering fluid flow fields at high image resolutions and at interactive frame rates on a single multi-core processor using OpenMP. The parallel implementation of our in situ visualisation method relies on MPI, requires no specialised hardware support, and employs the same underlying spatial decomposition as the fluid simulator. The visualisation pipeline allows the user to operate on a commodity computer and explore the simulation output interactively. Our simulation environment incorporates numerous features that can be utilised in a wide variety of research contexts.     

Numerical investigation of meniscus deformation and flow in an isothermal liquid bridge subject to high-frequency vibrations under zero gravity conditions

This paper deals with meniscus deformation and flow in an isothermal liquid bridge maintained between two circular rods, when one rod is subject to axial monochromatic vibrations. It concerns a fundamental aspect of the problem of crystal growth from melt by the floating-zone technique which is often considered in weightlessness conditions. In the absence of vibrations the bridge is cylindrical; but due to vibration the mean shape of the meniscus is no more cylindrical and the meniscus oscillates around this mean shape. Two models are developed. First, we take into account the pulsating deformations of the meniscus (free surface), but we assume that the mean shape of meniscus remains cylindrical (i.e., we neglect the influence of vibration on this mean shape). For this simple case, a solution of the problem for the pulsating meniscus deformations and the pulsating velocity field is found in explicit form. For the mean flow, the problem is solved numerically by a finite-difference method. The calculations demonstrate the contribution of two basic mechanisms of mean flow generation due to vibrations, related to the generation of mean vorticity in the viscous boundary layer near the rigid boundaries and surface-wave propagation at a free surface. The intensity of the mean flow induced by surface waves is found to be sharply increasing when the vibration frequency approaches the resonance values that are determined from the explicit form of the solution of pulsation problem. In the second model, we take into account both pulsating and mean deformations of the meniscus. The governing equations for the potential of pulsating velocity and mean velocity, and for the pressure, are solved by using a finite-difference method and a boundary-fitted curvilinear coordinate system fitting the free surface.     

警用脉冲防暴水炮管外动力过程数值模拟

为研究脉冲防暴水炮发射后在管外的动力过程,在Open FOAM软件上利用LES和VOF相耦合的方法对脉冲防暴水炮气液两相流流动过程作数值模拟。对水炮发射过程及其在距端口1.5m范围内的压力分布和速度分布做了分析。用SAMPLE软件对流场数据进行了采样,得到了用其它手段难以获得的数据。模拟表明,液柱形状急剧变化,首先形成蘑菇形,接着被压缩成半球形,在球形内部压力的作用下,半球形液弹被压缩成弧形。压力的作用距离集中在距端口300mm范围内,并很快降到了0.4MPa以下,其间因为压力有一个二次突然释放的过程,造成了二次喷射现象。速度是一个急剧先增加后减小的过程。所采用的圆柱式喷嘴使压力和速度下降太快,不利于提高水炮射程,需要改用其它类型的喷嘴。     

Space approach to the hydro-magnetic flow of a dusty fluid through a porous medium

The technique of the state space approach and the inversion of the Laplace transformation method are applied to the non-dimensional equations of an unsteady laminar free convection flow of an incompressible, viscous, electrically conducting dusty fluid through a porous medium, which is bounded by an infinite vertical plane surface of constant temperature, in the presence of a constant magnetic field. The technique is applied to the thermal shock problem. The inversion of the Laplace transforms is carried out using a numerical approach. The numerical results of the dimensionless temperature, velocity, and induced magnetic and electric field distributions are given and illustrated graphically. The effects of the material’s parameters such as the Grashof number, the Prandtl number, the permeability parameter, the mass concentration of the particle phase, the Alfven velocity, the thermal relaxation time and the relaxation time of the particle phase on the temperature, velocity and the induced magnetic and electric fields are discussed.     

Full transient response of Taylor cones to a step change in electric field

We studied experimentally the complete transient response of Taylor cones subject to a step change in external electric field with the goal of finding optimal conditions to reduce the overall response time and achieve the highest possible switching bandwidth. The transient behavior of electrified menisci is of interest for many applications that would benefit from active control of on/off switching of the electrospray, such as femtoliter droplet-on-demand or novel fuel injectors in next generation internal combustion engines. We first investigated the transient behavior of ethanol, a typical solvent for droplet-on-demand. We then expanded the study to fuels such as JP-8 and E-30 biogas, a biofuel with 30% ethanol (vol.). The system response is a multi-stage process that can last from ~100?μs to ~100?ms. Potential bottleneck stages include liquid accumulation, meniscus oscillation, and cone relaxation, depending on the experimental conditions. A typical full response time is ~1?ms, and the shortest transient process observed is ~400?μs. For a given liquid, nozzle outer diameter (OD) and applied voltage are the two most important parameters to influence the full response time. Onset or near-onset voltage for the establishment of the cone jet often leads to a large number of oscillation cycles and should be avoided. Changes in conductivity and viscosity by less than a factor of 10 have negligible effects on the transient process. Using JP-8 or E-30 biogas, 90?μm OD nozzle with extractor, and flow rate of 0.4?mL/h, we can routinely achieve bandwidth of 1?kHz, corresponding to a full response time of 1?ms, after which quasi-monodispersed droplets of ~10?μm are generated. Adaptation of an inviscid model of a charged oscillating droplet to the oscillating meniscus satisfactorily explains several key phenomena observed in our experiments, such as the full response time and the overshoot of the meniscus height.     

An experimental and numerical study of the structure and stability of laminar opposed-jet flows

Experiments, simulations, and numerical bifurcation analysis are used to study the incompressible flow between two opposed tubes with disks mounted at their exits. The experiments in this axisymmetric geometry show that for low and equal Reynolds numbers, Re, at both nozzles, the flow remains symmetric about the plane halfway through the nozzle exits and the stagnation plane is located halfway between the two jets. When Re is increased past a critical value, asymmetric flow fields are obtained even when the momentum fluxes of the two opposed streams are equal. For unequal Re at the jet exits, when the fixed velocity (and the corresponding Reynolds number, Re₁) of one stream is low, the stagnation plane location, SPL, changes smoothly with the Re₂. For high enough Re₁, a hysteretic jump of SPL is observed. Particle Image Velocimetry and flow visualization demonstrate that within the hysteretic range, the two stable flow fields are anti-symmetric. The experimental setup is also studied with transient incompressible flow simulations using a spectral element solver. It is found that to accurately model the flow, we either need to extend the domain into the nozzles, or impose experimental velocity profiles at the nozzle exits. As in the experiments asymmetric flows are obtained past a critical Re. Finally, bifurcation analysis using a Newton-Picard method shows that the transition from symmetric to asymmetric flows results from the loss of stability of the symmetric flows at a pitchfork bifurcation.