Study of Rectangular Underexpanded Microjets

The underexpanded microjet emerging from a rectangular convergent nozzle with a high aspect ratio at the nozzle exit is investigated numerically using the Reynolds-averaged Navier-Stokes (RANS) simulation with the Menter’s shear stress transport (SST) k-ω turbulence model. The simulation is performed at the nozzle pressure ratio of 5.0 to produce a strong shock and it is validated by a comparison with a rainbow schlieren picture of the microjet. The three-dimensional structure of the shock-containing rectangular microjet is demonstrated using the isopycnic surface and bright-field schlieren representations.

Laser-induced microjet-assisted ablation for high-quality microfabrication

Liquid-assisted laser ablation has the advantage of relieving thermal effects of common laser ablation processes, whereas the light scattering and shielding effects by laser-induced cavitation bubbles, suspended debris, and turbulent liquid flow generally deteriorate laser beam transmission stability, leading to low energy efficiency and poor surface quality. Here, we report that a continuous and directional high-speed microjet will form in the laser ablation zone if laser-induced primary cavitation bubbles asymmetrically collapse sequentially near the air-liquid interface under a critical thin liquid layer. The laser-induced microjet can instantaneously and directionally remove secondary bubbles and ablation debris around the laser ablation region, and thus a very stable material removal process can be obtained. The shadowgraphs of high-speed camera reveal that the average speed of laser-induced continuous microjet can be as high as 1.1 m sin its initial 500 μm displacement. The coupling effect of laser ablation, mechanical impact along with the collapse of cavitation bubbles and flushing of high-speed microjet helps achieve a high material removal rate and significantly improved surface quality. We name this uncovered liquid-assisted laser ablation process as laser-induced microjet-assisted ablation(LIMJAA) based on its unique characteristics. High-quality microgrooves with a large depth-to-width ratio of 5.2 are obtained by LIMJAA with a single-pass laser scanning process in our experiments. LIMJAA is capable of machining various types of difficult-to-process materials with high-quality arrays of micro-channels, square and circle microscale through-holes. The results and disclosed mechanisms in our work provide a deep understanding of the role of laser-induced microjet in improving the processing quality of liquid-assisted laser micromachining.

Comparative investigation of microjetting generated from monocrystalline tin surface and polycrystalline tin surface under plane impact loading

With considering the scattering effect of grain boundary and the grain orientation,the molecular dynamics is used for the first time to comparatively investigate microjetting generated by monocrystalline tin surface and polycrystalline tin surface under plane impact loading in this work.The research results show that when the impact velocity is low,the scattering effect of grain boundary and different grain orientations in a polycrystalline tin will cause the sample to melt inhomogeneously,leading the shock wave front to attenuate,meanwhile,the inhomogeneous melting can result in jet deviating.Comparing with monocrystalline tin,the jet head velocity,jet velocity coefficient,and jet mass coefficient of polycrystalline tin at low impact velocity are all low.Moreover,as the impact velocity increases,this influence decreases and the microjetting results of polycrystalline tin and monocrystalline tin tend to be consistent with each other.

Numerical Analysis of a Microjet-Based Method for Active Flow Control in Convergent-Divergent Nozzles with a Sudden Expansion

A method based on microjets is implemented to control the flow properties in a convergent-divergent nozzle undergoing a sudden expansion.Three different variants of this active control technique are explored numerically by means of a finite-volume method for compressible fluid flow:with the first one,the control is implemented at the base,with the second at the wall,while the third one may be regarded as a combination of these.When jets are over-expanded,the control is not very effective.However,when a favourable pressure gradient is established in the nozzle,the control becomes effective,leading to an increase in the base pressure.

Multi-Schlieren CT Measurements of Supersonic Microjets from Circular and Square Micro Nozzles

Instantaneous three-dimensional (3D) density distributions of a shock-cell structure of perfectly and imperfectly expanded supersonic microjets escaping into an ambient space are measured. For the 3D observation of supersonic microjets, non-scanning 3D computerized tomography (CT) technique using a 20-directional quantitative schlieren optical system with flashlight source is employed for simultaneous schlieren photography. The 3D density distributions data of the microjets are obtained by 3D-CT reconstruction of the projection’s images using maximum likelihood-expectation maximization. Axisymmetric convergent-divergent (Laval) circular and square micro nozzles with operating nozzle pressure ratio 5.0, 4.5, 4.0, 3.67, and 3.5 have been studied. This study examines perfectly expanded, overexpanded, and underexpanded supersonic microjets issued from micro nozzles with fully expanded jet Mach numbers Mj ranging from 1.47 - 1.71, where the design Mach number is Md = 1.5. A complex phenomenon for free square microjets called axis switching is clearly observed with two types “upright” and “diagonal” of “cross-shaped”. The initial axis-switching is 45° within the first shock-cell range. In addition, from the symmetry and diagonal views of square microjets for the first shock-cells, two different patterns of shock waves are viewed. The shock-cell spacing and supersonic core length for all nozzle pressure ratios are investigated and reported.

Multi-frequency focusing of microjets generated by polygonal prisms

We systematically investigate the power distribution characteristics of microjets generated by prismatic scatterers with different shapes at sub-THz region(λ = 8.57 mm). Among these prismatic scatterers, the hexagonal-type one shows better focusing feature than the others. Aiming at the hexagonal-type one, we propose a double-layer scatterer composed of a Teflon hexagonal prism as an outer layer and a semiconductor cuboid as an inner layer. Aiming at the double-layer scatterer, we further study the effects of refractive index, size, and shape of the inner cuboid on microjet’s features. The study allows us to present an optimized double-layer scatterer, which has a side length λ/2(λ) and a refractive index 2.0(1.4) for the inner(outer) layer. We show that the optimized scatterer can produce an ultra-strong, ultra-narrow microjet with a power enhancement of;0 and a full width at half maximum(FWHM) of;0.26λ, and the microjet is just located at the output face. The microjet keeps compact within the distance range of λ from the output face. These features and effects are explained from the viewpoint of ray optics theory. According to the optimized double-layer scatterer, we further study the multi-frequency focusing features of the microjets, and find that the microjet remains good features at harmonic frequencies 2f_(0) and 3f_(0). In addition, we investigate the effect of an Au sphere presence in the center of the microjet on the power distribution. The results show that a spherical dark spot with a size similar to that of the Au sphere emerges in the area where the Au sphere is placed. The feature can be used to measure the size of a metallic particle.

Direct numerical simulation of shock wave/boundary layer interaction controlled by steady microjet in a compression ramp

Shock wave/boundary layer interaction in a 24°turning angle of the compression ramp at Mach number 2.9 controlled by steady microjet is investigated using direct numerical simulation.Three different jet spacings which are termed as sparse,moderate and dense are considered,and the induced vortex system and shock structures are compared.A moderate jet spacing configuration is found to generate counter-rotating vortex pairs that transport high-momentum fluid towards the vicinity of wall and strengthen the boundary layer to resist separation,reducing the separation region.The dense jet spacing configuration creates a larger momentum deficit region,reducing the friction downstream of the corner.Analysis of pressure and pressure gradient reveals that dense jet spacing configuration reduces the intensity of separation shock.The impact of varying jet spacings on the turbulent kinetic energy transport mechanism is also investigated by decomposing the budget terms in the transport equation.Furthermore,the spectral characteristics of the separation region are studied using power spectral density and dynamic mode decomposition methods,revealing that moderate jet spacing configuration suppresses low-frequency fluctuations in the separation region.

Mitigation of Combustion Instability and NO_(x)Emissions by Microjets in Lean Premixed Flames with Different Swirl Numbers

Swirl combustion serves as a helpful flame stabilization method,which also affects the combustion and emission characteristics.This article experimentally investigated the effects of CO_(2)microjets on combustion instability and NO_(x)emissions in lean premixed flames with different swirl numbers.The microjets’control feasibility was examined from three variables of CO_(2)jet flow rate,thermal power,and swirl angles.Results indicate that microjets can mitigate the combustion instability and NO_(x)emissions in lean premixed burners with different swirl numbers and thermal power.Still,the damping effect of microjets in low swirl intensity is better than that in high swirl intensity.The damping ratio of pressure amplitude can reach the maximum of 98%,and NO_(x)emissions can realize the maximum reduction of 10.1×10^(−6)at the swirl angle of 30°.Besides,the flame macrostructure switches from an inverted cone shape to a petal shape,and the flame length reduction at low swirl intensity is higher than that of high swirl intensity.This research clarified the control differences of mitigation of combustion instability and NO_(x)emissions by microjets in lean premixed flames with different swirl numbers,contributing to the optimization of microjets control and the construction of high-performance burners.

基于超高压处理的猕猴桃原浆制备研究

为优化猕猴桃超高压灭菌条件下原浆制作工艺,对3种不同原浆制作工艺分别进行单因素优化试验,筛选出两种产品色泽和稳定性较好的工艺,并对这两种工艺所制作样品进行4℃储藏40 d的品质对比。结果表明,在最大程度保留猕猴桃原浆绿色,提高其稳定性的条件下,3种工艺的优选次序为:对辊式挤压制浆>胶体磨制浆>高压微射流制浆;与胶体磨工艺相比,对辊式挤压工艺制作的产品在4℃条件下储藏30 d内无明显的自然分层,产品VC损失较少;优化后的制浆工艺为:采用对辊式挤压工艺,挤压次数为1次。按照该工艺制作的猕猴桃原浆色泽相对均一,以青绿色为主,VC含量为40~50 mg/100 g。

Effect of the interval between two shocks on ejecta formation from the grooved aluminum surface

This work focuses on the effect of the interval between two shocks on the ejecta formation from the grooved aluminum(Al_(1100))surface by using smoothed particle hydrodynamics numerical simulation.Two unsupported shocks are obtained by the plate-impact between sample and two flyers at interval,with a peak pressure of approximately 30 GPa for each shock.When the shock interval varies from 2.11 to 7.67 times the groove depth,the bubble velocity reduces to a constant,and the micro jetting factor R_(J) from spike to bubble exhibits a non-monotonic change that decreases initially and then increases.At a shock interval of 3.6 times the groove depth,micro jetting factor R_(J) from spike to bubble reaches its minimum value of approximately 0.6.While,the micro jetting factor R_(F) from spike to free surface decreases linearly at first,and stabilizes around 0.25 once the shock interval surpasses 4.18 times the groove depth.When the shock interval is less than 4.18 times the groove depth,the unloading wave generated by the breakout of the first shock wave is superimpose with the unloading part of the second shock wave to form a large tensile area.

动态高压微射流对山楂果渣膳食纤维理化和结构特性的影响

采用动态高压微射流(dynamic high pressure microfluidization,DHPM)技术处理,探究其对山楂果渣膳食纤维结构及理化特性的影响。结果表明,经不同条件下DHPM处理后,山楂果渣中可溶性膳食纤维(soluble dietary fiber,SDF)含量比CK组提高了34.59%~73.54%,亚硝酸盐吸附能力和阳离子交换能力显著增强(P<0.05);在200MPa循环3次时,其持水力、持油力、膨胀力达到最大,较CK组分别提高了2.08、8.34、2.25倍(P<0.05);热重和流变学分析表明,DHPM处理后表观黏度呈下降趋势,热稳定性降低;粒径和扫描电子显微镜图分析表明,经DHPM处理后粒径先减小后增大,表面粗糙,结构疏松,且压力为250 MPa时颗粒发生团聚现象;红外光谱和13C核磁共振分析表明,结晶度减小,部分纤维素、半纤维素和果胶发生降解。因此,DHPM处理能提高可溶性膳食纤维含量、水合特性,增强其亚硝酸盐吸附能力和阳离子交换能力,但热稳定性降低,粒径、表观黏度和结晶度减小。该研究为山楂果渣的高值化利用提供了理论依据。

微射流撞击形成液膜的形态演变特征

利用高速相机从正面和侧面两个视角拍摄了不同速度微射流撞击形成液膜的流动形态,以深入理解液膜形态演变的物理机制.实验中的射流韦伯数在6.3~404.5之间变化,结果表明:随着微射流速度的增加,射流的流动形态经历了从层流到湍流的变化,所形成的液膜经历了液体链条、封闭液膜、边缘失稳和波动液膜等多种流动形态.侧面视角观察到了撞击点附近射流表面存在静止毛细波,其波长随韦伯数增加呈现快速减小后缓慢减小的趋势.还观察到液膜顶端存在界面不稳定状态,其摆动导致沿液膜边缘产生向下传播的界面扰动,进而在液膜边缘形成液珠,液珠又可发展成指状液丝.指状液丝的生成带走了液膜中的部分流体,导致液膜在指状液丝根部上方撕裂.当射流中出现速度脉动后,会激励起液膜表面波动.若速度脉动间歇出现,液膜波动会在扰动消失后很快衰减;而持续的速度脉动会激励产生剧烈且持续的液膜摆动.这一发现证实了速度脉动引入的有限大小的扰动是激发液膜波动的必要条件.此外,液膜波动加速了液珠形成以及液珠向指状液丝的转变过程,还影响了指状液丝的空间分布.液膜摆动使两侧的撕裂点向液膜中心汇聚,形成下游的长液丝,完成了从液膜到液丝再到液滴的雾化过程.研究还利用纹影法获得了射流从层流向过渡流演变过程中液滴直径数据,发现液滴大小概率密度分布由多峰分布逐渐转变为符合Gamma函数的单峰分布.本研究得到的微射流撞击形成液膜的演变规律和机制,为相关应用研究提供了理论支持和定量认识.

高压微射流参数对虾油凝胶构建的影响及油凝胶的应用

该文旨在研究射流压力和循环次数对构建虾油凝胶的影响,再将制备的油凝胶按不同添加比例替代复合龙虾肠中的动物脂肪。结果表明,随着射流压力从40 MPa增加至160 MPa,乳液粒径和油凝胶油损失率逐渐减小,电位绝对值和油凝胶硬度逐渐增大,射流压力为120 MPa和160 MPa时各指标无显著性差异;随着循环次数从1次增加至7次,乳液粒径和油凝胶油损失率呈现先减小后增大的趋势,电位绝对值和油凝胶硬度呈现先增大后减小的趋势,在循环次数为5次时各指标效果达到最佳,油凝胶硬度为(13.38±0.48)N,油损失率为6.30%;通过对复合龙虾肠的凝胶强度、持水性、蒸煮损失、质构、色泽、流变特性、感官品质等进行测定,对比油凝胶替代动物脂肪的4种不同比例对复合龙虾肠品质的影响。结果表明,替代比例为25%时,各项指标与对照组无显著性差异,适宜作为复合龙虾肠中动物脂肪的替代比例。

High-repetition-rate(kHz) targets and optics from liquid microjets for high-intensity laser–plasma interactions

High-intensity laser–plasma interactions produce a wide array of energetic particles and beams with promising applications.Unfortunately,the high repetition rate and high average power requirements for many applications are not satisfied by the lasers,optics,targets,and diagnostics currently employed.Here,we aim to address the need for high-repetition-rate targets and optics through the use of liquids.A novel nozzle assembly is used to generate highvelocity,laminar-flowing liquid microjets which are compatible with a low-vacuum environment,generate little to no debris,and exhibit precise positional and dimensional tolerances.Jets,droplets,submicron-thick sheets,and other exotic configurations are characterized with pump–probe shadowgraphy to evaluate their use as targets.To demonstrate a highrepetition-rate,consumable,liquid optical element,we present a plasma mirror created by a submicron-thick liquid sheet.This plasma mirror provides etalon-like anti-reflection properties in the low field of 0.1%and high reflectivity as a plasma,69%,at a repetition rate of 1 k Hz.Practical considerations of fluid compatibility,in-vacuum operation,and estimates of maximum repetition rate are addressed.The targets and optics presented here demonstrate a potential technique for enabling the operation of laser–plasma interactions at high repetition rates.

液体束光电子能谱测量水溶液相胸苷光电子谱:综合实验误差分析

水溶液中的光电子发射过程易受到由弹性及非弹性电子散射、流动势和空间电荷效应引起的误差和影响,然而,直接评估这些误差中各自贡献的大小仍是一项艰巨的挑战。本文以综合实验误差的形式评估了这些误差如何影响真实的光电子动能谱,利用飞秒液体束光电子能谱技术研究不同水溶液流速和激光能量下水溶液相胸苷的紫外光电子动能(或电子结合能)谱.实验中,利用液态水的1b_(1)轨道的精确的竖直电子结合能值(11.33eV)实现对观测到的水溶液相胸苷的电子结合能谱的校准,并估算出液态水的1b_(1)轨道的精确的竖直电子结合能的综合实验误差约为200meV.在不同的流速和激光能量条件下,水溶液相胸苷阳离子基态D_(0)(π^(-1))和阳离子第一激发态D_(1)(n^(-1))的竖直电子结合能的平均综合实验误差被估计为小于3%,即误差范围在几十毫电子伏特至200meV之间.

基于自激振动激励器的流动控制汽车减阻研究

为了降低汽车行驶时的空气阻力,文章采用一种基于内部结构流固耦合的自激振动激励器以产生高速、高频的脉冲微射流进行主动流动控制。通过ANSYS双向流固耦合仿真模拟激励器的工作过程,分析其产生的脉冲射流速度和频率,再从不同工作参数和结构参数对其激励特性的影响进行研究;将该激励器沿Ahmed类车体模型尾部的分离边缘布置,研究其对车体尾迹区流动拓扑结构的影响,并通过空气阻力系数分析其减阻效果。结果表明,自激振动激励器产生的脉冲微射流可以有效地控制减阻,最高可达7.12%的减阻效果。该研究为主动流动控制减阻提供了新思路。

玫瑰天竺葵精油纳米脂质体的制备影响因素考察

采用薄膜分散法结合高压微射流均质法制备玫瑰天竺葵精油纳米脂质体,并利用正交实验设计选择出最优制备工艺进行质量评价考察。利用粒度电位仪测定精油纳米脂质体粒径,利用红外光谱仪测定精油是否与辅料产生影响。最佳制备工艺下制备的玫瑰天竺葵精油纳米脂质体粒径为142.30 nm,这进一步表明了玫瑰天竺葵精油已成功与氢化卵磷脂原料形成脂质体包合物,基本符合载药纳米脂质体的制备标准。本工艺制备的玫瑰天竺葵精油纳米脂质体稳定性较好且包裹处理后的玫瑰天竺葵精油对小鼠皮肤内胶原蛋白的促生成效果优异,在化妆品领域具有一定的应用前景。

表面微结构对阵列微射流沸腾换热的影响

与单相射流相比,阵列式微射流沸腾换热耦合了分布式射流与气液相变两种高效传热模式,在高热通量电子器件冷却领域具有重要的应用前景。本文创新性提出一种具有顶部浸入式阵列射流柱与底部微针肋阵列结构耦合的微射流沸腾换热系统,采用无水乙醇为工质,研究了入口过冷度、入口Re、热通量对射流沸腾换热影响特性;采用电刷镀制备了镍/石墨烯微纳复合结构,研究了该复合结构对微针肋阵列表面射流沸腾换热的影响规律,揭示了镍过渡层引入的附加热阻以及蘑菇状微纳复合结构对气泡脱离的抑制是换热削弱的主要原因。为克服上述弊端,采用激光对镍/石墨烯微纳复合结构表面进行了刻蚀,发现激光刻蚀消除了镍/石墨烯微纳复合结构导致的附加热阻及其气泡脱离抑制效应,其最大传热系数达到30787.0W/(m^(2)·K),较镍/石墨烯微纳复合结构表面和针肋阵列光滑表面传热系数分别提高了140.7%和119.8%。本文的研究结果表明,微纳复合结构对沸腾换热的影响取决于制备工艺及其结构形貌,激光刻蚀较电刷镀形成的微纳复合结构在微射流沸腾换热强化方面更具优势,为表面微纳结构强化沸腾换热系统设计、制备和运行提供科学参考。

水介质对超声空蚀纳米生成物形貌的影响

为探究水介质下超声空蚀纳米结构的生成机制,研究不同水介质条件下超声空蚀纳米生成物的形貌特征。利用超声振动空蚀实验装置,在4种不同水介质中分别对45钢样品进行超声空蚀实验,通过激光共聚焦显微镜、扫描电子显微镜对实验后样品表面空蚀纳米生成物形貌进行分析。结果表明:45钢在不同水介质中空蚀生成的纳米微结构有很大差异;在去离子水和Na Cl溶液中空蚀坑环状区域纳米结构呈现为不规则絮状结构,在自来水和Na_(2)SO_(4)溶液中生成的空蚀坑周围形成了纳米多层片状结构;在自来水中,随着超声时间的增加,纳米单层片状结构先是长度方向尺寸增大,后逐渐叠加成纳米多层片状结构,总厚度增大。45钢在自来水中超声空蚀生成的纳米多层片状结构的尺寸,与实验时间和介质中离子有关,源于空蚀-腐蚀耦合作用产生;自来水中的SO_(4)^(2-)等离子也为片状纳米层间的组装起到促进作用。

微纳CL⁃20颗粒悬浮型炸药油墨的3D微喷打印成型及性能

为了解决六硝基六氮杂异伍兹烷(CL‑20)基全液型炸药油墨微喷成型的主体炸药转晶和成型效率低等问题,以聚乙烯醇(PVA)的水溶液为胶体悬浮液,以微纳CL‑20颗粒为悬浮颗粒,设计并配制了与3D微喷打印相兼容的悬浮型炸药油墨,并采用3D微喷技术对炸药油墨进行打印成型。通过密度计、激光共聚焦显微镜、扫描电镜、X射线衍射仪、纳米压痕仪对成型样品的性能进行表征,并测试了样品的撞击感度、摩擦感度和爆速,在此基础上研究了微纳CL‑20颗粒含量与微型传爆药性能之间的关系。结果表明:随着炸药油墨中微纳CL‑20颗粒含量的增大,墨滴成型后“咖啡环”现象逐渐明显,药线单层沉积厚度增加,打印成型效率增大,但孔隙率则逐渐增大。主体炸药CL‑20在3D微喷打印过程中没有晶型的转变,仍为ε型。当成型样品中微纳CL‑20颗粒的含量与黏结剂的比例为9∶1时,样品的实测密度为1.638 g·cm^(-3)(86.19%TMD),弹性模量为5.43 GPa;其撞击感度、摩擦感度和爆速分别为4 J、240 N和7689 m·s^(-1),展现了较好的安全性能及微尺度传爆能力。