Behavior of microdroplets in diffuser/nozzle structures

This paper investigates the behavior of microdroplets flowing in microchannels with a series of diffuser/nozzle structures. Depending on the imposed flow direction, the serial structures can act either as a series of diffusers or nozzles. Different serial diffuser/nozzle microchannels with opening angles ranging from 15° to 45° were considered. A 2D numerical model was employed to study the dynamics of the microdroplet during its passage through the diffuser/nozzle structures. The deformation of the microdroplet was captured using a level set method. On the experimental front, test devices were fabricated in polydimethylsiloxane using soft lithography. T-junctions for droplet formation, diffuser/nozzle structures and pressure ports were integrated in a single device. Mineral oil with 2% w/w surfactant span 80 and de-ionized water with fluorescent worked as the carrier phase and the dispersed phase, respectively. The deformation of the water droplet and the corresponding pressure drop across the diffuser/nozzle structures were measured in both diffuser and nozzle configurations at a fixed flow rate ratio between oil and water of 30. The results show a linear relationship between the pressure drop and the flow rate. Furthermore, the rectification effect was observed in all tested devices. The pressure drop in the diffuser configuration is higher than that of the nozzle configuration. Finally, the pressure measured results with droplet and without droplet were analyzed and compared.     

Study of subsonic�Csupersonic gas flow through micro/nanoscale nozzles using unstructured DSMC solver

We use an extended direct simulation Monte Carlo (DSMC) method, applicable to unstructured meshes, to numerically simulate a wide range of rarefaction regimes from subsonic to supersonic flows through micro/nanoscale converging–diverging nozzles. Our unstructured DSMC method considers a uniform distribution of particles, employs proper subcell geometry, and follows an appropriate particle tracking algorithm. Using the unstructured DSMC, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number on the flow field in micro/nanoscale nozzles. If we apply the back pressure at the nozzle outlet, a boundary layer separation occurs before the outlet and a region with reverse flow appears inside the boundary layer. Meanwhile, the core region of inviscid flow experiences multiple shock-expansion waves. In order to accurately simulate the outflow, we extend a buffer zone at the nozzle outlet. We show that a high viscous force creation in the wall boundary layer prevents any supersonic flow formation in the divergent part of the nozzle if the Knudsen number exceeds a moderate magnitude. We also show that the wall boundary layer prevents forming any normal shock in the divergent part. In reality, Mach cores would appear at the nozzle center followed by bow shocks and expansion region. We compare the current DSMC results with the solution of the Navier–Stokes equations subject to the velocity slip and temperature jump boundary conditions. We use OpenFOAM as a compressible flow solver to treat the Navier–Stokes equations.     

Application of a one-fluid model for large scale homogeneous unsteady cavitation: The modified Schmidt model

It is found that the one-fluid cavitation model developed by Schmidt et al. [Schmidt DP, Rutland CJ, Corradini ML. A fully compressible, two-dimensional model of small, high speed, cavitating nozzles. Atomiz Sprays 1999;9:255–76] (Schmidt Model) does not work consistently when applied to simulate the unsteady transient cavitating flows with a large vapor to liquid density ratio or under the condition of a low surrounding pressure. In this work, the apparent difficulties of the Schmidt model are analyzed and a modified Schmidt model is proposed for greater robustness and consistency. The modified Schmidt model is then applied to study the creation, evolution and collapse of transient cavitation commonly observed in underwater explosions and industrial pipe flow. The model is firstly verified by simulating several cavitating flows where analytical, experimental or numerical results are available for comparison, and then applied to multi-dimensional transient cavitating flows generated by underwater explosions. The numerical results show that the modified Schmidt model can overcome the difficulties associated with the (original) Schmidt model and be applied to both small and large scale transient cavitating flows to predict the pressure surge caused by cavitation collapse regardless of the surrounding pressure.     

Novel no-moving-part valves for microfluidic devices

This study characterizes and analyzes the performances of micro diffusers/nozzles with five types of enhancement structures and one of conventional micro nozzle/diffuser valve. The pressure drops across the designed micro nozzles/diffusers are found to be increased considerably when the obstacle and fin structure are added. Further, the micro nozzle/diffuser having added circular area reveals the lowest pressure drop, owing to the hydraulic diameter is increased by circular area and lower interface friction. The maximum improvement of the loss coefficient ratio is about 16% for an added 3-fin structure operated at a Reynolds number around 70. Upon this situation, the static rectification efficiency improves 4.43 times than the conventional nozzle/diffuser. Experimental results indicate the performance peaks at a Reynolds number around 70, and an appreciable decline is encountered when the Reynolds number is reduced. It is due to the efficiency ratio of conventional micro nozzle/diffuser significant increases with the Reynolds number.     

Numerical simulation of vortical flows in the near field of jets from notched circular nozzles

The vortex dominated flows in the near field of jets from notched circular nozzles are investigated using direct numerical simulation. The nozzles studied include a normal circular nozzle, a V-shaped notched nozzle, and an A-shaped notched nozzle, all with the same circular cross-section. The vortical structures resulting from these different circular nozzles are visualized by using a numerical dye visualization technique. Results for the V-shaped notched nozzle are compared with available experimental measurements using laser-induced fluorescence techniques. In addition to azimuthal vortex rings created because of the shear-layer between the jet and the ambient fluid, the computations also reveal streamwise vortex pairs both inside and outside the vortex rings that spread outward as the vortex rings move downstream. Comparisons of the three different nozzles show that, unlike in the case of the circular nozzle where the streamwise vortex pairs emerge evenly along the nozzle lip, streamwise vortex pairs for the notched circular nozzles are produced only at peak and trough locations. Analysis of the mixing characteristics of the three types of nozzles shows that the notches in the nozzle exit significantly enhance jet mixing.     

Design of maximum thrust plug nozzles with variable inlet geometry

An optimization analysis is presented for axisymmetric plug nozzles with varible inlet geometry. The analysis is based on the governing gas dynamic relations for a rotational flow of a frozen or equilibrium gas mixture. The problem is formulated to maximize the axial thrust produced by the plug nozzle for a general isoperimetric constraint, such as constant nozzle length or constant nozzle surface area. The effects of base pressure and ambient pressure are included in the thrust expression to be maximized. The governing gas dynamic equations and the differential and integral constraints that the solution must satisfy are incorporated into the formulation by means of Lagrange multiples. The formalism of the calculus of variations is applied to the resulting functional to be maximized. The results of the optimization analysis are a set of partial differential equations for determining the Lagrange multipliers in the region of interest and a set of equations for determining the necessary boundary conditions for the solution. The complete set of equations for the gas dynamic properties and the Lagrange multipliers are system of first order, quasi-linear, non-homogeneous partial differential equations of the hyperbolic type, which can be treated by the method of charac- teristics. The characteristic and compatibility equations for the system are presented. A numerical solution procedure is presented to determine wether or not a given plug nozzle geometry is an optimal solution. An iteration technique is developed which systematically adjusts the plug nozzle geometry until the optimal solution is obtained. Selected parametric studies are presented. These studies illustrate the effect of the specific heat ratio, the design pressure ratio and the base pressure model on the thrust peformance and nozzle geometry of optimal, fixed length, plug nozzles.     

固体火箭发动机喷管热防护设计

为保证固体火箭发动机(Solid Rocket Motor,SRM)尾翼机构工作的可靠性,进行热防护设计以控制喷管外壁温度.基于三维有限元法对喷管工作时的温度场进行数值仿真,获取发动机喷管在温度和压力载荷联合作用下的温度场分布,结果表明原喷管的热防护不满足要求,通过对喷管各组成结构的重新设计,获得符合设计要求的喷管结构,并且发动机点火试验表明热防护设计满足要求,所得方法可为喷管的设计、试验和生产提供参考.     

A SVD-GFD scheme for computing 3D incompressible viscous fluid flows

A generalized finite difference (GFD) scheme for the simulation of three-dimensional (3D) incompressible viscous fluid flows in primitive variables is described in this paper. Numerical discretization is carried out on a hybrid Cartesian cum meshfree grid, with derivative approximation on non-Cartesian grids being carried out by a singular value decomposition (SVD) based GFD procedure. The Navier-Stokes equations are integrated by a time-splitting pressure correction scheme with second-order Crank-Nicolson and second-order discretization of time and spatial derivatives respectively. Axisymmetric and asymmetric 3D flows past a sphere with Reynolds numbers of up to 300 are simulated and compared with the results of Johnson and Patel [Johnson TA, Patel VC. Flow past a sphere up to a Reynolds number of 300. J Fluid Mech 1999;378:19-70] and others. Flows past toroidal rings are also simulated to illustrate the ability of the scheme to deal with more complex body geometry. The current method can also deal with flow past 3D bodies with sharp edges and corners, which is shown by a simple 3D case.     

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.     

喷嘴-挡板式液压控制元件力特性的实驗研究

在液压随动系统中,广泛采用了喷嘴-挡板式液压放大器.本文通过一系列的实验,研究了喷嘴-挡板元件的力特性.实验中采用了不同内径的尖锐稜边喷嘴及具有不同端面外径与内径比值(相对直径)的非尖锐稜边喷嘴. 实验和理论分析结果表明,对于尖锐稜边的喷嘴,作用在挡板上的力与工作压力成比例关系,而与喷嘴和挡板之间的位移关系很小;对于非尖锐稜边喷嘴,力、位移和工作压力之间的实验特性曲线是非线性的和不对称的. 作者认为在功率控制液压放大器中,采用尖锐稜边的喷嘴是最适宜的.     

Implementation of synthetic turbulence inlet for turbomachinery LES

Large eddy simulation (LES) has the potential to model complex separated flows, where Reynolds Averaged Navier–Stokes (RANS) based methods often fail. An important aspect of LES is specifying correlated turbulent fluctuations at the inlet boundary. This is particularly important in turbomachines, where turbulence length scale and intensity play a key role in the correct prediction of component performance.In this work, a method is implemented into an unstructured Computational Fluid Dynamics (CFD) solver to impose correlated turbulent fluctuations in a compressible form. It is shown that compressibility effects are particularly important in turbomachinery and must be taken into account. The method uses a pre-processing method to generate a cube of isotropic, homogeneous turbulence. The velocity fluctuations so obtained are used to determine a fluctuating Mach number in order to evaluate the instantaneous total pressure and temperature fluctuations at domain inlet. In the authors knowledge this is one of the first attempts to define correlated fluctuations in a compressible form.The method is successfully applied to two turbomachinery related flows. Firstly, the jet flow from a propelling nozzle is investigated. Following this, the flow over a low pressure (LP) turbine blade is predicted. Results from the LES simulations show that modifications to the inlet conditions can significantly affect flow development. For the jet, changes in the shear layer and peak shear stress are shown, important in the context of high frequency sideline noise generated by the jet. Despite what is suggested in the literature the differences in shear stresses are important also in a non-swirling jet.For the LP turbine, incoming turbulent fluctuations modify the onset of transition and the extent of separation bubble. Without imposed turbulence fluctuations, loss is overpredicted by up to 50%. Moreover it is important to use a compressible solver. Despite the fact that the majority of the results proposed in literature on LP turbine is using incompressible solvers, the difference in terms of pressure coefficient, Cp, is comparable to turbulence contribution.     

基于FDM工艺结构的3D打印机高温喷头结构优化研究

打印机喷头结构不合理,会出现温度场分布不均现象,导致喷头堵塞,影响打印速度。为此,本文提出基于FDM工艺结构的3D打印机高温喷头结构优化研究。设计三维CAD模型,控制喷头移动,结合喷头结构,计算热传导遵循的傅里叶定律与稳定热传导规律;从模型“台阶效应”、挤出丝材两方面探究影响喷头打印效果的主要因素,在确保打印质量前提下,利用磁制冷方法,经过多次调试,确定最佳压力值,优化送丝机构,设计制冷装置,实现高温喷头结构优化。仿真实验证明,优化后的高温喷头温度分布更为合理,有效避免喷头堵塞现象,可实现高速度持续打印。     

气力式水稻芽种直播精量排种器关键部件的设计与仿真

针对气力式排种器窝眼及吸孔内部的气流场难以进行精确计算和分析的 问题,论文结合生产实际确定了排种器窝眼滚筒的结构参数和工作转速,并应用Pro/E 软件 分别对排种器的窝眼滚筒、窝眼及吸孔内的气流场进行三维建模。利用CFD 软件对3 种不 同结构参数的窝眼及吸孔内的气流场进行仿真模拟,其结果表明：半球型窝眼的吸种性能理 论上优于锥孔型和直孔型;吸孔直径越大,吸种能力越强,吸种效果越好。此方法适于复杂 气流场的精确计算和仿真。     

Evaluation of flow maldistribution in air-cooled heat exchangers

Investigation of the evaluation of flow maldistribution in air-cooled heat exchangers is presented. The flow field in the inlet and return headers was obtained through the numerical solution of the governing partial differential equations including the conservation equations of mass and momentum in addition to the equations of the turbulence model. The effects of the number of nozzles, nozzle location, nozzle geometry, nozzle diameter, inlet flow velocity and the incorporation of a second header on the flow maldistribution inside the tubes of an air-cooled heat exchanger were investigated using a 3-D computational method. The results are presented in terms of the standard deviations of the mass flow rate and static pressure in addition to the distributions in the static pressure inside the inlet header of the air-cooled heat exchanger. The results indicate that reducing the nozzle diameter results in an increase in the flow maldistribution. 25% increase is obtained in the standard deviation as a result of decreasing the diameter by 25%. Increasing the number of nozzles has a significant influence on the flow maldistribution. A reduction of 62.5% in the standard deviation of the mass flow rate inside the tubes is achieved by increasing the number of nozzles from 2 to 4. The results indicate that incorporating a second header results in a significant reduction in the flow maldistribution. Fifty percent decrease in the standard deviation is achieved as a result of incorporation of a second header of 7 holes. The results indicate that the mass flow rate and the static pressure distributions become uniform at the inlet of the second pass.     

航空发动机喷口分油活门位移传感器容错控制研究

分析了出现喷口分油活门位移传感器故障时,通过开环设置电液伺服阀电流控制喷口面积存在的问题,在此基础上提出了一种容错控制方法,根据喷口面积期望值与实际测量值之间的误差闭环运算喷口控制电液伺服阀电流。通过全数字仿真和半物理模拟试验对该容错控制方法进行了验证,结果表明,该容错控制方法能够保证全权限数字电子控制系统稳定工作,并具有较好的稳态和动态性能,且实施方便,对提高航空发动机全权限数字电子控制系统的工作可靠性具有重要作用。     

Nozzle size effects on the nanoelectrospraying of Au nanocolloid in a fully voltage-controlled form

Nozzle size effects on the nanoelectrospraying of Au nanocolloid were investigated in a pure voltage controlled form. The nanoelectrospraying system was set up in a nozzle-to-plate geometry and the spray current was monitored to evaluate the performance of the nozzles during the atomization of Au colloid. Current–voltage characteristic shows that nanoelectrospray properties strongly depend on the nozzle size. In the spraying of Au nanocolloid using nozzles sized from 4 to 30 μm in diameter, two well-distinguished regimes of pulsation and cone-jet were observed with an increasing of voltage. The onset voltages for entering both modes were found to increase with the nozzle size. In the pulsation mode, pulsating frequency ranged from several tens kHz up to hundred kHz was witnessed and current with low oscillation amplitude was detected in the case of small nozzles, indicating the rate of ejected charges decreases with a fine nozzle. Meanwhile, the DC equivalent resistance derived from the best-fit model was determined to be in the range of 0.89–3.16 GΩ. In cone-jet region the equivalent resistance, representing an electrical equivalence of the spray gap, was measured to be between 0.49 and 1.41 GΩ. The relatively low resistance at high field implies better efficiency of spray in cone-jet mode compared with in pulsating mode. In both modes the equivalent resistances fall with increasing nozzle size, reflecting large nozzles easier process high volume ejections.     

Fabrication of chamber for piezo inkjet printhead by SU-8 photolithography technology and bonding method

The chamber is an important part of the inkjet printhead. However, the present fabrication methods of chamber suffer from a low alignment resolution between nozzle plates and piezoelectric structure and residual SU-8 removing problems during chamber fabricating process. In this paper, a SU-8 chamber was fabricated by using ultraviolet (UV) photolithography and SU-8 thermal bonding method. By this method, the infilling problem of the chamber during thermal bonding process was solved, and low alignment resolution problem of conventional UV exposure system during assembly process was avoided. The thickness of the SU-8 nozzle plate was optimized, and the influence of bonding parameters on the deformation of chamber was analyzed. The simulation results show that the optimal thickness of the SU-8 nozzle plate is 40 μm and the optimal bonding parameters are bonding temperature of 50 °C, bonding pressure of 160 kPa and bonding time of 6 min. The tensile test results show the bonding strength of the SU-8 chamber is 2.1 MPa by using the optimized bonding parameter.     

基于消光断层法的喷雾分布不均匀度测量技术

燃油喷嘴喷雾分布不均匀度作为衡量喷嘴喷雾性能的重要参数之一,直接影响发动机的燃烧效率以及污染排放。为了验证消光断层法激光式分布器(Statistical Extinction Tomography Scan Optical Patternator,SETscan)在喷雾分布不均匀度方面的测量能力,利用离心式喷嘴在不同燃油工况条件下开展燃油喷嘴雾化性能实验,并与传统机械式测量方法进行了对比。结果表明,相比于传统机械式测量方法,激光式测量方法稳定性更好、实验效率更高、实验数据更加直观,具有更高的时间和空间分辨率,对激光遮挡率为10%~90%的燃油喷雾能够得出合理有效的浓度分布实验结果,同时由于激光式测量方法属于非接触测量,不会受到喷雾本身特性和安装位置等因素的影响。     

固体火箭发动机喷管两相流场与结构温度场一体化数值模拟与软件实现

为准确、高效地进行固体火箭发动机的喷管结构设计与性能分析,考虑两相流场和结构温度场之间的相互影响,采用有限差分方法求解二维轴对称两相欧拉方程组,进行喷管气体-颗粒两相无黏流动数值模拟;采用有限元法求解二维轴对称瞬态导热微分方程,进行喷管复合结构温度场数值模拟;根据面向对象编程思想,采用VC+ +以人机交互的工作方式实现喷管内型面和结构设计,并完成有限差分网格和有限元网格的自动划分和显示;通过数值模拟方法与面向对象软件设计方法的有效结合,实现二维轴对称喷管两相无黏流场和复合结构温度场的一体化数值模拟. 数值模拟结果表明,该方法有助于在统一的软件平台上充分利用计算机辅助技术完成喷管性能与结构的综合评估,可以用于固体火箭发动机喷管的工程设计.