The effects of injector nozzle geometry and operating pressure conditions on the transient fuel spray behavior

Effects of injector nozzle geometry and operating pressure conditions such as opening pressure, ambient pressure, and injection pressure on the transient fuel spray behavior have been examined by experiments. In order to clarify the effect of internal flow inside nozzle on the external spray, flow details inside model nozzle and real nozzle were also investigated both experimentally and numerically. For the effect of injection pressures, droplet sizes and velocities were obtained at maximum line pressure of 21 MPa and 105 MPa. Droplet sizes produced from the round inlet nozzle were larger than those from the sharp inlet nozzle and the spray angle of the round inlet nozzle was narrower than that from the sharp inlet nozzle. With the increase of opening pressure, spray tip penetration and spray angle were increased at both lower ambient pressure and higher ambient pressure. The velocity and size profiles maintained similarity despite of the substantial change in injection pressure, however, the increased injection pressure produced a higher percentage of droplet that are likely to breakup.     

Combustion of a single droplet in the presence of an oscillating flow

The two-dimensional, unsteady, laminar conservation equations for mass, momentum, energy and species transport in the gas phase are solved numerically in spherical coordinates. This is to study the heat and the mass transfer, and the combustion around a single spherical droplet. The droplet mass and momentum equations are also solved simultaneously with the gas phase equations in order to investigate the effects of droplet entrainment in the oscillating flow with and without a steady velocity. The numerical solution for a single droplet combustion gives the droplet diameter variation as well as the gas phase velocity, temperature and species concentrations as a function of time. The effects of frequency, amplitude of oscillating flow, velocity ratio of oscillating flow amplitude to the steady velocity, ambient temperature and initial droplet diameter on the droplet combustion are also investigated. The droplet burning history is not governed by thed ²-law in the presence of oscillating flow, unlike to the case under quiescent ambient conditions.     

Pressure drop and flow distribution characteristics of single and parallel serpentine flow fields for polymer electrolyte membrane fuel cells

This study numerically investigates pressure drop and flow distribution characteristics of serpentine flow fields (SFFs) that are designed for polymer electrolyte membrane fuel cells, which consider the Poiseuille flow with secondary pressure drop in the gas channel (GC) and the Darcy flow in the porous gas diffusion layer (GDL). The numerical results for a conventional SFF agreed well with those obtained via computational fluid dynamics simulations, thus proving the validity of the present flow network model. This model is employed to characterize various single and parallel SFFs, including multi-pass serpentine flow fields (MPSFFs). Findings reveal that under-rib convection (convective flow through GDL under an interconnector rib) is an important transport process for conventional SFFs, with its intensity being significantly enhanced as GDL permeability increases. The results also indicate that under-rib convection can be significantly improved by employing MPSFFs as the reactant flow field, because of the closely interlaced structure of GC regions that have different path-lengths from the inlet. However, reactant flow rate through GCs proportionally decreases as under-rib convection intensity increases, suggesting that proper optimization is required between the flow velocity in GCs and the under-rib convection intensity in GDLs.     

均匀液滴喷射过程仿真与试验研究

针对液滴喷射增材制造试验参数调整困难、实施难度较大的现存问题,基于流体体积(Volume of fluid,VOF)两相流模型,建立均匀液滴喷射过程流场的计算模型。采用数值模拟的方法,对液滴喷射过程中的液滴流形态、压力场和速度场及其影响因素进行了研究,揭示了形成均匀液滴流的内在变化规律,得到了均匀液滴喷射过程的最优频率。在模拟结果的基础上,建立了液滴喷射装置并配置了相应的高速拍照系统,对射流断裂形态、喷射过程、喷射速度进行了试验研究。结果表明,射流速度主要取决于喷射压强,液滴流均匀性主要取决于扰动频率和扰动振幅,射流的压力场则呈周期性变化。模拟结果与试验结果吻合较好,说明所提出的建模方法是可行的,为不同情况下射流内部流场的计算提供了实用的方法,也为液滴喷射增材制造技术的应用奠定了理论基础。     

Numerical and experimental study on hollow-cone fuel spray of highpressure swirl injector under high ambient pressure condition

A hybrid breakup model was proposed as a trustworthy prediction of hollow-cone fuel spray in the present study and the atomization process of the hollow-cone fuel spray of a high-pressure swirl injector in a Gasoline Direct Injection (GDI) engine under high ambient pressure conditions was studied by a new hybrid breakup model. The proposed hybrid breakup model is composed of the Linearized Instability Sheet Atomization (LISA) model as a primary breakup process. The Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model, instead of the Taylor Analogy Breakup (TAB) model, was used as a secondary breakup process. The effects of the droplet deformation on a droplet aerodynamic external force are considered in the APTAB model. In addition, we replaced the x ² distribution function used in previous the APTAB model by the Rosin-Rammler distribution function to improve the prediction precision. The Laser Induced Exciplex Fluorescence (LIEF) technique and the Phase Doppler Anemometry (PDA) system were used to produce a set of experimental data for the model validation. The estimation of the prediction ability of the LISA+APTAB model was carried out, and spray characteristics, which are difficult to obtain by experimental method, were calculated and discussed. The suggested hybrid breakup model showed better prediction capability compared with the previous model (LISA+TAB model). From the calculated results, the effect of the ambient pressure on the SMD (Sauter Mean Diameter) and droplet velocity could be discussed quantitatively.     

A compact vacuum UV excilamp on argon dimers

The energy and spectral characteristics of the argon dimer emission with the maximum at a wave-length of ∼126 nm under excitation by a pulse-periodic discharge in a gas flow with pressures higher than the atmospheric pressure are studied. A compact excilamp has been designed for obtaining radiation in the vacuum ultraviolet spectrum region. The minimal argon flow velocities are determined, at which cooling is effected by convective gas ejection from the discharge region. This allows one to form a diffusion discharge with a stable radiation power density. It is shown that, when the flow velocity is 0.5 m³/h, the radiation power density of an Ar₂* molecule is >100 μW/cm² behind a LiF window with ∼10% transmission at a wavelength of 126 nm. It is shown that the use of a windowless design and an increase in the gas flow velocity allows an increase in the radiation power density up to 10 mW/cm².     

Improved volume-of-fluid (VOF) model for predictions of velocity fields and droplet lengths in microchannels

Accurate measurement of flow in microchannels is imperative to better understand their flow behaviour, which aids in the design of microfluidic devices. In this work, we present an improved VOF model based on smoothing functions that can effectively minimise the issue of spurious velocities, which causes numerical simulations in microchannels to be less accurate. We use the smoothed VOF to simulate the velocity fields and droplet lengths in microchannels and compare the results with experimental data. The results show that the smoothed VOF is able to simulate flow in microchannels more accurately than the standard VOF model. Microchannel simulations using the standard VOF model are less accurate because the spurious velocities produces artificially higher velocity regions in the flow field results. The spurious velocities also induce a higher but non-physical shear stress during the droplet formation process, resulting in droplets forming prematurely with shorter lengths. Hence the smoothed VOF which resolves the issue of spurious velocities is shown to be a more viable tool in predicting the flow in microchannels by means of numerical simulations.     

Heat transfer of a spray droplet in a PWR pressurizer

Heat transfer rates to spray droplets under conditions corresponding to those of spray transients in a pressurizer of pressurized water reactor (PWR) have been predicted by a simple droplet model with internal thermal resistance and partial internal mixing. In those processes, the temperature distributions in the droplet have been obtained using the integral method, and the physical properties of the saturated steam-hydrogen gas mixture surrounding the droplets are estimated applying the concept of compressibility factor and using appropriate correlations. Results have been provided for the temporal variations of total heat flux with its convection and condensation heat transfer components, dimensionless droplet bulk temperature and droplet flight distance. The effects of ambient pressure, initial droplet size, concentration of hydrogen gas in the mixture, initial injection velocity, and spray angle on the heat transfer of spray droplets have been discussed.     

Comparison of experimental and predicted atomization characteristics of high-pressure diesel spray under various fuel and ambient temperature

The aim of this study is to investigate the effects of the fuel temperature and the ambient gas temperature on the overall spray characteristics. Also, based on the experimental results, a numerical study is performed at more detailed and critical conditions in a high pressure diesel spray using a computational fluid dynamics (CFD) code (AVL, FIRE ver. 2008). Spray tip penetration and spray cone angle are experimentally measured from spray images obtained using a spray visualization system composed of a high speed camera and fuel supply system. To calculate and predict the high pressure diesel spray behavior and atomization characteristics, a hybrid breakup model combining KH (Kelvin-Helmholtz) and RT (Rayleigh-Taylor) breakup theories is used. It was found that an increase in fuel temperature induces a decrease in spray tip penetration due to a reduction in the spray momentum. The increase of the ambient gas temperature causes the increase of the spray tip penetration, and the reduction of the spray cone angle. In calculation, when the ambient gas temperature increases above the boiling point, the overall SMD shows the increasing trend. Above the boiling temperature, the diesel droplets rapidly evaporate immediately after the injection from calculation results. From results and discussions, the KH-RT hybrid breakup model well describes the effects of the fuel temperature and ambient gas temperature on the overall spray characteristics, although there is a partial difference between the experimental and the calculation results of the spray tip penetration by the secondary breakup model.     

Characterization of the hydrodynamic flow regime in bubble columns via computed tomography

The current work evaluates the potential of Computed Tomography (CT) measurements for flow regime characterization. Experiments were carried out in a pilot scale (0.162 m diameter) bubble column using an air–Therminol LT system at ambient as well high operating pressures (0.4 and 1 MPa). The superficial gas velocities were varied from 1 to 20 cm/s at intervals of 1 cm/s. The steepness of the gas holdup radial profile was analyzed to demarcate the hydrodynamic flow regime. The regime transition velocities obtained from CT measurements are compared with the drift flux model. An increase in operating pressure was found to delay the regime transition and at higher pressures, a transition occurred over a range of superficial gas velocities. The current state of correlation prediction is evaluated against the experimental transition velocities.     

Transient liquid jet breakup model and comparison with phase doppler measurements

A new liquid jet breakup model is developed based on the transient breakup mechanism and incorporated into the KIVA-II code. Liquid column is considered as a chain of balls. Rayleigh-Taylor instability and Kelvin-Helmholtz instability was applied to the liquid jet column. Liquid jet column is continuously surveyed to apply breakup mechanisms. Once liquid droplets are separated from the main liquid jet column, these droplets are subjected to the single breakup mechanism. When Bond number is greater than a critical Bond number, single droplets continue to break up by Rayleigh-Taylor instability or Kelvin-Helmholtz instability. Computational results were compared with the PDPA measurement data. Gross behavior of the spray and detailed droplet sizes and velocities predicted by KIVA calculations which include proposed drop breakup model are compared with those produced from droplet size/velocity measurements.     

DYNAMIC ANALYSIS OF PARTICLE FLYING VELOCITY IN HIGH VELOCITY OXYGEN FUEL SPRAY

0INTRODUCTIONIn thermal spraying process, the flying velocity of the particles is one of the most important factors that control the quality of thermal sprayed coatings［1,2］. The coating deposited by the particles with higher flying velocity is dense and well bonded. Many researchers［1,2］ had identified that the flying velocity of the particles for depositing depends on the velocity and mass rate of flow of a HVOF gun, and the time or length of the particles being accelerated. Based…     

Oscillatory behaviors in initial film boiling: Implications on the triggerability of single droplet vapor explosions

A transient one-dimensional film boiling model was developed to study the film boiling dynamics that would occur when a hot spherical droplet is immersed in cold liquid. The focus of this study was to investigate the effects of noncondensible gas, liquid temperature, droplet temperature, and ambient pressure on film boiling during the initial growth phase. The results indicate that the film generally stablizes with more noncondensible gas present, higher liquid and lower droplet temperature. Small ambient pressurizations cause violent fluctuations of the film pressure while higher ambient pressure suppresses these oscillations. These qualitative behavior of film boiling around hot spherical droplet suggests that the spontaneous triggering of small-scale single droplet vapor explosions is led by the oscillatory characteristics of vapor film in its initial growth phase.     

A study on the spray characteristics of a piezo pintle-type injector for DI gasoline engines

We studied the spray flow initiated from a piezo pintle-type injector for DI gasoline engines in an environment supplied by a constant volume vessel by means of laser diagnostics. To fully grasp the effects of the characteristic parameters, including designed spray angle, needle lift, injection pressure (P _inj ) and ambient pressure (P _b ), on the spray atomization and mixture preparation, particle image velocimetry (PIV) and phase Doppler anemometry (PDA) are used in the experiment, respectively. The gas perpendicularly enters into the outer periphery of the conical spray injected through the pintle-type injector activated by piezo, which creates two large-scale vortices: the vortex A and vortex B. The velocity standard deviation of the spray field is introduced to analyze the gas flow motion in the vicinity of nozzle. The droplet information of spray field is also recorded by PDA in variable boundary conditions. The time dividing method is used to study the droplet characteristics in four parts of spray. The injector with 98° designed spray angle has smaller droplet mean diameter (D10 and D32), due to a larger spray distribution. When the droplet velocity of the spray field is close to 0 m/s, the D10 and D32 hold at around 10 μm and 20 μm, respectively, in atmospheric pressure condition, which are about 20 μm and 40 μm, respectively, at ambient pressure of 1.1 MPa.     

Investigation of design parameters for droplet generators driven by piezoelectric actuators

This paper presents a numerical analysis of a drop-on-demand (DOD) piezoelectric (PZT) actuated droplet generator. A finite difference numerical model was established to analyze the design parameters of droplet ejection. First, we discussed the influence of the driving conditions on the droplet ejection characteristics, such as the driving time and the driving volume change in the pressure chamber. The volume factor, an important design parameter, was proposed from the analysis. The ejected droplets can maintain the same ejection velocity at different nozzle diameters, as long as the volume factor remains the same. Two empirical formulas, based on the analysis data, are suitable for the design of PZT actuated droplet generator. The first empirical formula is a linear relationship between the droplet velocity and the volume factor with a slope of 0.3422 for different nozzle diameters. The second empirical formula defines the driving volume of PZT and nozzle diameter to eject the desired droplets. The geometrical design parameters of droplet generator, such as the nozzle thickness, the pressure chamber width and depth, as well as the driving conditions of the PZT actuator, are all included in the analysis. The sensitivity of geometrical design parameters which affect the droplet volume, the droplet velocity, and the lowest driving condition is established. The quantitative criterion for ejection of droplet, liquid jet, and no droplet is presented. The proposed empirical formula and figures provide easy-to-use tool for design of DOD PZT actuated droplet generators.     

脉冲激光焊接Hastelloy C-276合金的熔池流动传热特性分析

基于流体动力学方程和传热方程建立了三维瞬态模型,用于研究脉冲激光焊接0.5 mm厚Hastelloy薄板时熔池的流动行为及传热特性.应用Fluent软件,采用有限容积法(FVM)求解控制方程,用SIMPLE算法处理速度与压力的耦合.引入Pe来衡量焊接熔池中对流传热与传导传热的相对强弱,并以此分析焊接熔池的传热特性.结果表明:沿焊接方向,焊接熔池的流动速度随着离熔池中心距离的增加先增加后减小;在给定试验条件下,熔池流动速度在离熔池中心0.2 mm左右时出现最大值,且沿焊接方向前方稍大于后方,而后迅速减小为零;焊接熔池中对流的存在使得焊接熔池熔深较小而熔宽较大;最终的焊接形貌由对流传热与传导传热相互作用而成.对焊缝形貌的数值模拟结果与实验结果进行了比较,计算结果与实验结果吻合较好.此模型可为脉冲激光焊接Hastelloy C-276薄板时熔池流体流动行为的分析提供理论依据.     

A proposal for diesel spray model using a TAB breakup model and discrete vortex method

A hybrid model consisting of a modified TAB (Taylor Analogy Breakup) model and DVM (Discrete Vortex Method) is proposed for numerical analysis of the evaporating spray phenomena in diesel engines. The simulation process of the hybrid model is divided into three steps. First, the droplet breakup of injected fuel is analyzed by using the modified TAB model. Second, spray evaporation is calculated based on the theory of Siebers’ liquid length. The liquid length analysis of injected fuel is used to integrate the modified TAB model and DVM. Lastly, both ambient gas flow and inner vortex flow of injected fuel are analyzed by using DVM. An experiment with an evaporative free spray at the early stage of its injection was conducted under in-cylinder like conditions to examine an accuracy of the present hybrid model. The calculated results of the gas jet flow by DVM agree well with the experimental results. The calculated and experimental results all confirm that the ambient gas flow dominates the downstream diesel spray flow.     

An experiment analysis of MILD combustion with liquid fuel spray in a combustion vessel

Mild combustion is characterized by its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise, and thermal stress. Recently, many studies have revealed the potential of MILD combustion in various power systems but most studies have been focused on gas phase fuel MILD combustion. Therefore, further study on MILD combustion using liquid fuel is needed for the application to a liquid-fueled gas turbine especially. In this work, we studied experimentally on the formation of liquid fuel MILD combustion under the condition of high dilution by burnt gas generated from a first premixed flame in two stages combustor which consists of the first premixed burner and secondary combustor. In particular, the effects of burnt gas velocity and oxygen level of burnt gas on the formation of liquid fuel MILD combustion were investigated. The results show that as the burnt gas velocity through the nozzle becomes higher, the color of flames was changed from yellow to pale blue and flames became very short. The OH radical measured by ICCD camera was uniformly distributed on the pale blue flame surface and its intensity was very low compared to conventional liquid diffusion flame. As burnt gas velocity is increased, local high-temperature region appeared to be diminished and the flame temperature became spatially uniform. And CO emission was sampled around 1 ppm and NOx emission was measured around 10 ppm under the overall equivalence ratio of 0.8 to 0.98 for 40 mm or less diameter of velocity control nozzle. This low NOx emission seems to be attributed to maintaining the average temperature in secondary combustor below the threshold temperature of thermal NOx formation. In view of the uniform temperature distribution, low OH radical intensity and low NOx emission data in the secondary combustor, formation of stable MILD combustion using kerosene liquid fuel could be verified at high burnt gas velocity.

     

富氮气体模拟系统设计及实现

围绕机载惰化系统产生富氮气体流量及氧浓度指标，设计并实现了富氮气体模拟系统及其控制系统，在试验室模拟富氮气体流量和氧浓度。利用压缩空气和工业氮气混合，并基于电液伺服调节方法实现混合气体的流量和氧浓度的闭环控制。上位机采用LabVIEW软件及信号调理系统完成数据的采集、处理和输出，利用PID控制算法实现过程控制。基于压力反馈和继电器开关控制空压机启停，使得空气源出口的压力在合适范围，进而确保混合气体流量的静态稳定性。给出了几组混合罐内富氮气体压力、流量、氧浓度试验测试曲线，结果表明该系统模拟的富氮气体流量和氧浓度连续可调，控制精度满足指标要求。     

Experimental analysis and numerical modeling using lisa-ddb hybrid breakup model of direct injected gasoline spray

This paper presents the effect of injection pressure on the atomization characteristics of high-pressure injector in a direct injection gasoline engine both experimentally and numerically. The atomization characteristics such as mean droplet size, mean velocity, and velocity distribution were measured by phase Doppler particle analyzer. The spray development, spray penetration, and global spray structure were visualized using a laser sheet method. In order to investigate the atomization process in more detail, the calculations with the LISA-DDB hybrid model were performed. The results provide the effect of injection pressure on the macroscopic and microscopic behaviors such as spray development, spray penetration, mean droplet size, and mean velocity distribution. It is revealed that the accuracy of prediction is promoted by using the LISA-DDB hybrid breakup model, comparing to the original LISA model or TAB model alone. And the characteristics of the primary and secondary breakups have been investigated by numerical approach.