Experimental Investigation on Flow and Drying Characteristics of Impinging Stream Drying

Based on the experimental investigation of one-stage semi-circular impinging stream drying, the experiments with the two-stage semi-circular, as well as the vertical and semi-circular combined impinging stream drying were conducted. The variations of system pressure drop, the mean residence time of particles with the mass flow-rate ratio and air velocity etc, were determined. The influences of inlet air temperature, mass flow-rate ratio, initial moisture content of particles and air velocity etc. on drying characteristics were also studied. The results indicate that the vertical and semi-circular combined impinging stream drying can make full use of the advantages of both the vertical and semi-circular impinging stream drying. Reasonable mass flow-rate ratio, air velocity, and higher inlet air temperature should be used for less energy consumption and cost during drying process.

Numerical Study on Heat Transfer and Flow Characteristics for Laminar Flow in a Circular Tube with Swirl Generators

This study investigated the heat transfer and flow characteristics of one kind of swirlgenerator in a circular heat exchanger tube through a numericalsimulation. The swirlflow induced by this type of swirlgenerator can obtain a high heat transfer rate with minimalpressure drop penalty. The simulations were carried out to understand the physicalbehavior of this kind of mesoscale heat enhancement component. By visualizing the heat transfer and flow characteristics, it is found that the swirlflow is induced by swirlgenerator in the circular tube couples with the impinging jet effect. After passing through the swirlgenerator, the localfriction factor of liquid can quickly return to lower levelmore quickly, while the localNusselt number maintains higher values for a distance; thus, the evaluation criterion of localperformance is improved. Single-factor optimization is used for three geometric parameters, i.e., the angle of swirlgenerator(25o, 45o, and 60o), the length of swirlgenerator(0.005, 0.01, and 0.02 m), and the center rod radius(1, 2, and 3 mm). The optimum parameters of the swirlgenerator for laminar flow of air in a circular tube are obtained, which should be 60o, 0.005 m, and 3 mm, respectively.

Drying characteristics, functional properties and in vitro digestion of purple potato slices dried by different methods

The drying characteristics,physico-chemical and functional properties,as well as starch digestibility,of purple potato slices dried using different methods(such as,vacuum freeze-drying,VFD;hot-air drying,HAD;air-impingement jet drying,AIJD;and far-infrared assisted heat-pump drying,FIHPD)were investigated.Drying rate was the highest(3.0 g 100 g^-1 min^-1)using AIJD,followed by FIHPD and HAD,and the rate of VFD was the lowest one(0.3 g 100 g^-1 min^-1).Drying data were fitted to 12 thin-layer drying models,with the Midilli model giving the best predictions.Moreover,AIJD showed higher diffusivity(5.5×10^-10 m^2 s^–1)and energy efficiency(55 J g^-1)than any other drying method used in this study.With reference to the samples dried by VFD,the starch granules of the samples obtained by HAD,FIHPD,and AIJD exhibited different extent of disruption,which significantly increased their water absorption capacity,swelling power,and in vitro digestibility,but decreased the peak viscosity.The sample resulting from AIJD had the greatest water absorption capacity(7.9 g g^-1)and solubility(21.6%),but the smallest syneresis rate(48%).Good correlation coefficients(R^2>0.98)implied that the pseudofirst order kinetic model adequately described the rate and extent of starch digestion of dried potato flours.Samples from AIJD and FIHPD showed the highest digestibility percentages,reaching to 72.4 and 72.5%.Based on the drying rate,specific energy consumption,functional properties and digestibility,AIJD appeared to be quite effective and suitable to be transferred on the industry scale.

Impact characteristics and stagnation formation on a solid surface by a supersonic abrasive waterjet

A computational fluid dynamics(CFD)study of the impact characteristics and stagnation formation on a solid target surface by an abrasive waterjet at supersonic velocities is presented to understand the impact process.A CFD model is developed and verified by experimental water and particle velocities and then used to simulate the jet impact process.The trends of the stagnation formation and its effect on the jet flow with respect to the jetting and impacting parameters are amply discussed.It is found that stagnation formation at the impact site increases with an increase in the impact time,nozzle standoff distance and nozzle diameter,while the initial peak velocity at the nozzle exit has little effect on the size of the stagnation zone.It is shown that stagnation markedly changes the water and particle flow direction,so that the particle impact angle is varied and the jet impact area is enlarged.The jet structure may be classified to have a free jet flow region,a jet deflection region with a stagnation zone and a wall jet region.Furthermore,the stagnation affects significantly the waterjet and particle energy transferred to the target surface.The average particle velocity across the jet is reduced by approximately one third due to the damping effect of the stagnation under the conditions considered in this study.

基于振荡射流的撞击流反应器流动及混合特性研究

为了研究振荡射流撞击流反应器内流场特性,利用SST k-ω湍流模型,建立振荡射流撞击流反应器数学模型且与试验进行对比。通过分析速度场、湍流动能分布、压力场和湍流黏度分布规律来研究撞击流反应器内部流动特性,揭示混合性能变化规律。结果表明:在轴向上,速度、湍动能和压力呈多峰分布趋势且随着入口速度的上升而不断增大。在径向上,振荡器混合室内部涡旋中心受喷嘴小尺度涡旋干扰不断变化,从而影响流体振荡,使流场内产生4种流型。该反应器在35 s后混合均匀,混合强度达到0.95以上。研究结果丰富了流体振荡特性理论,为新型反应装置的开发提供理论参考。

Roe格式在水滴欧拉方程数值求解中的应用

目前普遍采用欧拉法求解水滴运动控制方程以模拟结冰表面水滴撞击特性。水滴欧拉方程近似没有压力项的空气欧拉方程,但已应用于空气欧拉方程的空间离散格式多数不能直接应用于水滴欧拉方程,有研究者将经典Roe格式应用于水滴欧拉方程时出现了奇点问题。通过对水滴欧拉方程进行形式改造,基于中心格式验证改造方法的有效性,并应用Roe格式结合线性重构与限制器函数实现对水滴欧拉方程二阶精度的求解。将NACA23012翼型计算结果与FENSAP ICE软件进行对比,两者水滴收集系数分布基本一致。

不同偏流板角度的冲击射流噪声特性

舰载机尾喷流冲击偏流板(挡焰板)产生高强度噪声。为研究不同偏流板角度对噪声的影响,在全消声室进行射流冲击实验。实验时收缩喷口直径D=56.4 mm,受冲击的偏流板长、宽均为600 mm,射流马赫数Ma=1.01。在偏流板角度β=45°、55°和65°条件下,利用传声器和流动显示对远场噪声和冲击流动进行观测。结果表明,不同冲击角度下壁面射流结构明显不同,总声压级及噪声辐射特性存在明显区别,随着冲击角度的增加,上游和下游极角总声压级分别出现上升和下降,β=65°时,下游总声压级最多降低7 dB,上游相应增加15 dB。流动和频谱结果表明下游主要受到后缘分离噪声影响,该噪声来自壁面射流从偏流板后缘分离产生的大尺度涡结构,低频占主导,并且互相关分析表明,偏流板角度越小,该噪声指向性越明显。上游噪声被冲击噪声所主导,虽然偏流板角度显著影响冲击噪声强度,但是相关分析表明,冲击噪声向各个极角辐射的指向性结构较为稳定。此外偏流板对边线形成低通高阻效应,边线噪声频谱最为陡峭,而且边线观测到的总声压级、频谱在不同冲击角度的差别均较小。本研究可为航母甲板噪声治理提供一定参考。

Experimental Study on Impingement Processes of Fuel Sprays on Biomimetic Structured Surfaces

To improve the controllability of the wall-wetting process after the fuel spray-wall impingement in internal combustion engines,the methods of laser etching,chemical etching and surface free energy modification are used to prepare biomimetic structured surfaces with different wettability.The impingement processes of diesel and n-butanol sprays on the walls under different conditions are experimentally investigated.As the surface oleophilicity increases,the spreading radius of wall-impinging sprays decreases.At about 5 s after the fuel injections,the fuel spray droplets hit the walls for the first time,and the secondary breakup and rebound occur.The mixture concentrations of different fuels hitting the various walls reach the peak value.Under a higher surface temperature,the peak value of the mixture concentration is mainly related to the heat flux to the fuel droplets in different boiling regimes from the metal surfaces.The concentration of the air–fuel mixture in the near wall region increases with increasing surface oleophilicity,increasing wall temperature and decreasing ambient pressure.Compared with diesel,n-butanol presents a higher air–fuel mixture concentration in the near wall region.

迎风格式在水滴欧拉方程数值求解中的应用

数值模拟是研究飞机结冰的重要手段,为获取水滴撞击特性,通常采用欧拉法求解水滴运动方程。液态水含量在空间分布上存在间断,会导致水滴欧拉方程的求解极易发散;若采用一阶格式,虽然稳定性得到提高,但计算精度难以保证满足工程设计需求;有研究者通过添加扩散项以提升收敛性,但会人为增加数值耗散。针对这一问题,本文采用一阶迎风格式对水滴欧拉方程的对流通量进行离散,并通过线性重构与限制器函数保证计算精度达到二阶,建立一种基于迎风离散格式的水滴欧拉方程二阶求解方法。通过对比NACA0012、NACA23012和NLF0414翼型水滴撞击特性计算结果与FENSAP-ICE软件的结果,验证了该方法的有效性与计算精度。

基于IDDES方法的受限撞击流反应器流动及混合特性

采用基于SSTk-ω模型的IDDES方法对CIJR内部的流动及混合过程进行数值模拟研究,较完整地识别了流场不同区域的大尺度涡结构,探讨了涡结构的生成和演化机制。讨论涡结构对相平均温度场和热通量场的影响,揭示了混合机理。研究结果表明:2股射流向圆顶区的交替偏转导致动量不相等,射流剪切层的自持振荡伴随涡旋的周期性运动,因此驻点出现周期性偏移现象。下游发展区K-H不稳定性的发展卷起大尺度展向涡。撞击区的流向涡对混合的促进作用局限在腔室轴线附近,下游发展区的展向涡能够激发大规模的热量输运,从而有效强化混合。

不等径开放式撞击流雾化特征及二乙二醇/水体系混合研究

采用粒子图像测速技术(PIV)对不等径开放式撞击流中质量分数60%的甘油‑水溶液雾化性能进行了研究,探讨韦伯数We(51≤We≤1605)、入射速度u(2.12 m·s^(-1)≤u≤6.37 m·s^(-1))、喷嘴直径(喷嘴1:左侧喷嘴直径(D_(1))=1.5 mm,右侧喷嘴直径(D_(2))=2 mm;喷嘴2:D_(1)=2 mm,D_(2)=3 mm)对液膜破碎特征及液滴行为的影响,并对复合含能材料粘结剂溶剂二乙二醇与水混合过程的液滴分布进行了研究。结果表明:随着We增大,液膜破碎长度先增大,当液膜破碎模式到达无边缘模式后减小,且液膜厚度与液滴直径逐步减小,液滴速度逐步增大;随喷嘴直径增大,液膜破碎模式变化不显著,液膜破碎长度、厚度、液滴直径均增大,液滴速度则逐步减小。同时,拟合得到液膜破碎长度、液膜厚度、液滴索特平均直径D[3,2]与喷嘴直径、韦伯数间的经验关联式,采用二乙二醇与水体系验证后发现,随入射速度增大,二乙二醇与水撞击混合后液滴直径减小,分布变窄,D[3,2]数值在经验关联式±15%误差范围内与理论预测结果一致。

航空发动机分流环水滴撞击特性研究

为了研究航空发动机分流环的防冰需求,利用LEWINT软件对分流环的简化结构进行了水滴撞击特性计算。结果表明:分流环前缘区域水滴局部撞击率最高,上表面靠近前缘的区域和下表面前段有水滴撞击;随着来流速度增大,分流环总撞击率逐渐增大,撞击极限先增大然后缓慢减小;随着来流攻角增大,下表面局部撞击率逐渐增大,上表面撞击极限逐渐减小,分流环总撞击率逐渐增大,撞击极限逐渐减小,来流攻角超过6°撞击极限几乎不再变化;随着水滴直径增大,分流环局部撞击率、总撞击率、撞击极限逐渐增大;与水滴直径S-S分布相比,L-D分布的前缘局部撞击率偏小,分流环撞击极限偏大,总撞击率随水滴直径增大先偏大后偏小。分析结果可为分流环防冰设计提供参考。

Experimental and Theoretical Analysis of the Impinging Stream Drying

The experiments of one-stage semi-circular and two-stage semi-circular impinging stream drying as well as the vertical and semi-circular combined impinging stream drying were carried out. The velocity distribution and the mean residence time of particles, and the influence of various factors on drying characteristics were studied. A mathematical model of granular material drying in a semi-circular impinging stream dryer was proposed, in which the flow characteristics as well as the heat and mass transfer mechanisms were considered. Reasonable numerical methods were used to solve the equations. Under various conditions, the calculated results agree well with the experimental data. The unsteady-state drying dynamic equation, as well as the variations of drying rate and moisture content versus time were obtained. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate semi-circular stage number and curvature radius, flow-rate ratio, air velocity, and higher inlet air temperature should be used for enhancing the drying process.

Heat transfer characteristics in a narrow confined channel with discrete impingement cooling

Heat transfer characteristics in a narrow confined channel with discrete impingement cooling were investigated using thermal infrared camera. Detailed heat transfer distributions and comparisons on three surfaces with three impact diameters were experimentally studied in the range of Reynolds number of 3000 to 30000. The experimental results indicated that the strong impingement jet leaded to a high strength heat transfer zone in the ΔX=±2.5D;range of the impact center,which was 1.3–2.5 times of the average heat transfer value of the impingement wall. With the same coolant mass flow rate, small diameter case had lower heat transfer coefficient on both inner wall and outside wall, while the impingement wall was insensitive to the impact diameter. The surface averaged Nusselt number of inner wall was only 43%–57% of impingement wall, while the outside wall can reach up to 80%–90%. The larger the diameter, the higher heat transfer enhancement and the smaller the channel flow resistance was observed in term of Reynolds number. The surface averaged Nusselt numbers were developed as the function of Reynolds number and the impingement height-to-diameter for further engineering applications.

Simulation on spray evaporation processes in Multi-spout bed by modified impinging wall method

Basing on some researchers' experimental results,the droplet impinging wall model was modified,and We Number and K Number were adopted to determine the mode of droplet impinging wall.The simulation results indicated that the mode of impinging droplet is sticking on the wall in Muti-spout bed.The droplet impinging wall model modified can describe the droplet impinging wall phenomenon accurately in the multistage spouted fluidized flue gas desulfurization reactor.Basing on the study above,the distributions of temperature and relative humidity in the reactor were attained.In atomization region,the distribution of temperature exhibits "M" shape.The relative humidity exhibits"W"shape.The mass concentration of droplet reduces along axis of the reactor.Due to the multi-spouted configuration,droplet concentration increases obviously in the intersection region between the 1st and 2nd stage reaction region.The evaporating intensity reaches its peak value in the area of 200 mm away from the sprayer,and droplets are completely evaporated at 1500 mm.

飞机结冰中水滴撞击特性的欧拉法准确性分析

过冷水滴撞击特性计算是飞机结冰冰形预测与防除冰系统性能分析的基础,常用方法为欧拉法与拉格朗日法,2种方法的结果通常是一致的,但在某些部件上会出现差异。通过对欧拉法与拉格朗日法进行比较,分析2种方法结果的异同,进而讨论欧拉法对于飞机结冰中水滴撞击特性计算的准确性。以NACA 0012翼型、冰风洞风道、S形进气道与发动机气膜帽罩为对象,采用欧拉法与拉格朗日法计算获得水滴运动及局部水收集系数。结果表明:当水滴运动未受到上游效应影响时,欧拉法与拉格朗日法的结果一致;当水滴发生偏转后,欧拉法速度的单一性使水滴流线不能相交,而拉格朗日法能捕捉水滴轨迹的交叉,导致2种方法的预测产生差异,且欧拉法结果与水滴不碰撞及聚并的假设存在冲突;水滴在上游部件空气绕流或气流吹袭作用下都会发生偏转,使得欧拉法与拉格朗日法得到的下游表面水收集系数不相符,欧拉法对于S形进气道与发动机气膜帽罩的水滴运动及撞击特性计算存在误差。研究成果对飞机结冰冰形的准确预测及防除冰系统的设计有重要参考价值。

面向过冷大水滴适航验证的结冰探测技术研究

过冷大水滴(Supercooled large droplet,SLD)结冰超出了常规防除冰系统的防护范围,是一种更为严重的结冰情形,极大地影响着飞行安全。在美国联邦航空管理局14 CFR 25.140条款中明确提到为保证飞机在SLD结冰条件下的安全运行,首先且必须要对SLD结冰气象环境进行探测。由于水滴破碎会改变水滴运动轨迹和表面撞击水分布,使水滴撞击极限变小,而水滴飞溅对结冰极限位置影响不大,因此本文通过研究大、小粒径水滴在飞机上收集范围不同的特点,提出一种可以满足SLD结冰适航符合性的结冰探测技术。该结冰探测技术采用常规结冰探测器,根据水滴收集范围将其布置于不同敏感结冰部位,可以实现全剖面飞行的结冰环境探测。

柴油双喷射撞击喷雾和燃烧可视化研究

利用高速相机基于纹影法获取上置喷油器和侧置喷油器在单独喷射和垂直喷射状态下的喷雾和燃烧火焰发展图像。柴油喷射压力设置为80、120、140 MPa,环境背压设置为4 MPa,环境温度在喷雾和燃烧研究中分别设置为500 K和800 K。结果表明:随着喷射压力的增加,不管是单独喷射还是垂直喷射,喷雾贯穿距和面积均增大;垂直喷射状态下,两油束碰撞后损失部分能量,导致其喷雾面积小于两单独喷射状态下的喷雾面积之和。采用双色法研究不同工况下燃料燃烧特性,结果表明:在喷射后1.50~3.50 ms的燃烧稳定时期内,随着喷油压力的提高,各喷射状态下的火焰面积均增大,燃烧温度提高,碳烟生成量降低;垂直喷射状态下的火焰面积小于两单独喷射状态下的火焰面积之和,燃烧温度和碳烟生成量均高于两单独喷射状态。

异形喷嘴撞击流混合器涡特性及混合性能数值模拟

为了研究撞击流混合器不同形状喷嘴的流场涡特性及混合效果,利用数值模拟方法对不同工况下的流场进行分析,得到不同喷嘴撞击流混合器的速度场、涡量场、涡结构以及浓度变异系数的变化规律,揭示不同形状喷嘴对撞击流混合器混合效果的影响。结果表明:喷嘴出口截面为圆形、等边三角形和正方形的撞击流混合器轴向速度为“V”形分布;喷嘴截面为圆形和正方形的撞击流混合器径向速度为“M”形分布。等边三角形截面喷嘴的撞击流混合器产生的流向涡数量多、强度大并且产生的涡旋结构的连续性更高,分布范围更广。相同工况下等边三角形截面喷嘴的撞击流混合器混合效果明显优于圆形喷嘴和正方形喷嘴混合器,最快达到混合均匀。

壁面温度和撞壁距离对柴油喷雾燃烧特性的影响

根据撞壁距离L与自由喷雾液相贯穿距L_(liquid)之比,将喷雾撞壁模式分为严重湿壁、轻微湿壁、临界湿壁、未湿壁.通过高温高压定容弹试验和OpenFOAM仿真,研究了高、低壁温条件下,4种喷雾撞壁模式对着火特性、火焰发展特性以及燃油分布特性的影响.结果表明:提高壁面温度可以促进喷雾着火及燃烧过程,且其促进作用随L/L_(liquid)增大而减小;高壁温条件下,一定程度的喷雾撞壁有利于油气混合和燃烧,低壁温条件下则相反.