【作者】 费立森；

【导师】 徐胜利；

【作者基本信息】 中国科学技术大学 ， 流体力学， 2007， 博士

【摘要】 本文研究煤油在超声速气流中的喷射、雾化和混合现象。该问题是煤油超声速燃烧的基本现象,也是研制煤油超燃冲压发动机(特别是燃烧室设计)的关键技术难题。在学术方面,该问题伴随超声速流动、激波、湍流、射流破碎与雾化、两相流非平衡效应和雾化流场光学诊断等学科交叉,急需解决超声速气流(即大速差)气流中的射流破碎、湍流两相流和雾化参数在线测量等问题。因此,本文研究有重要的工程应用前景和学术意义。自60年代以来,煤油超燃雾化问题的研究一直得到重视,但由于两相湍流和射流破碎雾化等核心问题的理论研究没有取得实质性进展,因此,该问题的理论分析、数值模拟与实验结果对比还是相差甚远,人们对该问题的现象认识也尚待深入。为此,本文采用实验研究方法,主要说明如下:(1)以煤油雾化参数在线测量为目标,为避免尺度效应的影响,建造了和单模块超燃发动机燃烧室和喷嘴尺寸相同的直连式煤油超燃冷态雾化实验台,力图获得和实际发动机相近的流场特征、雾化现象和参数分布。(2)以先进光学诊断为核心,以平面激光诱导荧光(PLIF)和激光散射、高速纹影测量为主要手段,以雾化参数的定量和半定量测量为目标,开展煤油超燃雾化现象的测量研究。其中,采用高时间/空间分辨率的PLIF方法研究煤油的喷射和扩散范围是本文主要特色,得到的煤油液滴SMD(Sauter mean diameter)分布也是国际上首次公布的在线测量数据。(3)采用先进的光学测量方法,首次较为系统地研究了不同结构和几何参数的凹槽流动机理及其对煤油超燃雾化、混合现象的影响。本文主要内容概述如下:(1)第一章指出本文研究背景、相关研究进展,介绍了本文主要内容和特色。(2)为开展实验研究,第二章主要介绍本文研制的直连式煤油超燃冷态雾化实验台以及相关的测量系统。重点介绍了喷管、实验段设计和加工以及配套的光学测量系统。结果表明:实验段内气流较为均匀、杂乱波系较弱,可满足煤油超燃雾化现象的实验要求。(3)针对直通道(平板)燃烧室,第三章主要介绍煤油在超声速气流中喷射、混合的实验研究,给出了煤油扩散范围、穿透深度和液滴直径分布等流场图象和数据。结果表明:射流和来流的动压比对穿透深度、射流柱破碎和雾化有重要影响。动压比越大,穿透深度增大,促进了煤油射流的破碎和雾化,但射流激波变强、总压损失增大。表面波是引起射流柱破碎的重要因素。当动压比较小,射流的表面波发展受抑制,只有经过较长平滑初始段后才出现明显的表面波。增大动压比,在射流柱初始弯曲处出现表面波并迅速向下游发展,表面波振幅也增大。表面波是三维、非定常的。(4)针对两种典型的开式和闭式凹槽,第四章介绍了煤油在含凹槽通道内的喷射和雾化现象的实验和测量研究。重点研究不同几何参数和结构(前后缘角、导流槽和波状后缘)对闭式凹槽的流场特征和煤油喷射、混合现象的影响。结果表明:●对开式凹槽(L/D=3),其产生的波系较弱,流场总压损失小,自激振荡较弱,但槽内流向和横向漩涡较强,可增强煤油与主气流的横向输运和掺混,煤油射流的雾化情况(激波和液滴SMD)与平板相近,但穿透深度降低。与凹槽激波相比,射流激波可以忽略不计。●对于闭式凹槽(L/D=15),无煤油喷射的凹槽激波较强,导致了上壁面边界层的分离,总压损失较大,凹槽后缘流场的自激振荡较强。减小后缘倾角和采用波状后缘,可减弱闭式凹槽的后缘激波。减小前缘倾角和采用导流槽,闭式凹槽的剪切层和前缘激波变化不大。当有煤油喷射时,射流激波和凹槽激波相比可忽略不计。相对于开式凹槽,闭式凹槽的后缘气流偏转使煤油射流穿透深度显著增大。采用波状后缘,闭式凹槽的后缘激波和上壁面边界层分离均消失。改变凹槽几何结构和煤油喷射参数,除了凹槽内的煤油略有改变外,对主气流的煤油射流扩散、混合影响不大。进一步分析还表明:在含凹槽燃烧室中,由于当地气流速度很大,煤油射流的雾化不是难点,重点是要减弱凹槽产生的波系和增强煤油的扩散混合。凹槽类型不仅取决于几何特征参数(长深比),而且和凹槽结构、来流参数等相关。●在超声速气流中,通道内是否有无凹槽,对煤油雾化后液滴SMD几乎无影响,但对煤油的扩散混合影响较大,特别是闭式凹槽。本文主要特色和创新之处为:(1)本文利用煤油自身受激荧光(磷光)辐射特性和PLIF方法,研究煤油在超声速气流中的扩散混合问题,为煤油超燃雾化问题的测量研究提供了新的手段。众所周知,PLIF方法在燃烧流场小分子自由基诊断方面得到了广泛运用,国外也将PLIF方法用于内燃机的柴油喷雾流场测量研究,但针对煤油在超声速气流中的雾化流场测量研究方面,本文是公开文献首次给出该方法的运用和测量结果。与高速纹影等传统方法相比,PLIF可准确地反应煤油扩散边界及其表面波的三维非定常特性,给出的穿透深度等数据也更准确。基于纹影方法,美国在70～80年代给出的射流穿透深度拟合经验公式存在较大误差,不能作为煤油超燃发动机燃烧室设计的数据。可以预计,PLIF方法将成为有潜力的煤油超燃雾化场测量研究方法。(2)基于在线测量和实际燃烧室尺度等条件,本文给出了有、无凹槽的煤油雾化液滴直径分布,这也是公开文献首次给出的煤油在超声速气流中雾化的在线测量数据。利用PDPA,尽管LIN测量了水自直径2mm喷孔在超声速气流中喷射的液滴SMD分布,但其实验条件与超燃发动机工况相差较大。本文测量数据表明:由于气液相对速差大,因此,煤油在超声速气流中的雾化不是决定燃效率的主要因素,降低煤油点火和燃烧的化学延时是关键。凹槽等强化措施有助于增强煤油的扩散和混合,但对雾化后的液滴直径分布无明显贡献。(3)通过煤油扩散的PLIF图象和液滴直径测量数据,为煤油超燃冷态雾化现象提供了新的认识。特别是有关凹槽流动机理及其凹槽几何结构的影响,这些认识校正了过于基于压力测量和直觉想象得到的不正确判断,为研究煤油超燃现象和设计超燃发动机燃烧室提供了新的数据和思路。要说明的是:相关看法或结论还需要更多的实验图象和数据进行验证。更多还原

【Abstract】 In this thesis, experimental and numerical studies were carried out on kerosene injection, mixing and atomization in a supersonic air-stream. The phenomenon is crutially important not only for understanding kerosene-fueled supersonic combustion but also related to spray strategy of a scramjet combustor. In such a topic, complex flow physics are involved among supersonic flow, shock waves, shear layer, turbulence, jet breakup and atomization, phase exchanges of mass, momentum and energy as well as optical diagnostic on spray and atomization. Theoretical consideration on jet breakup in high slip velocity flows and spray measurement on line are especially challenged due to scramjet development and academic interests. Although some experiments have been conducted on jet breakup and atomization in a supersonic air-stream since 1960’s, no anticipated progresses have been achieved in jet breakup model and numerical methods. Thus, this thesis focus on experimental studies on liquid spray in a supersonic air-stream by planar laser induced fluorescence (PLIF) and laser scattering. The purpose is to obtain more high resolution images and droplet size distribution on line for deep understanding the kerosene spray in a supersonic flow. The experiments are summarized as follows:(1) A test rig based on supersonic wind tunnel was established for experimental studies on spray in a Mach 2.2 supersonic flow. The test section is almost identical to a single scramjet combustor to avoid the scale effects and satisfy requirements of online measurement. Also, some optic measurements and fuel supply ranged in variety of different injection pressure and orifice diameter are combined to this direct-connected facility.(2) Optical diagnostic including PLIF, Mie scattering and schlieren with high-speed photography were employed to experimentally study the kerosene atomization in a supersonic flow. The quantitative or semi-quantitative data are emphasized in the spray measurement. Different from schiliren images, PLIF ones with high temporal and spatial resolution can supply database on kerosene jet breakup and spreading. The unsteady and three dimensional surface waves appear on PLIF images of kerosene jet. Thus, jet penetration and spreading can be precisely measured from these images by kerosene fluorescence. Meanwhile, droplets’ SMD (Sauter mean diameter) distribution was also got in these experiments. (3) By the afro-mentioned laser measurements, the flow features with and without kerosene injection were systemically studied when a cavity embedded in a duct with different geometry and aspect ratio to show whether it can enhance jet mixing in a supersonic flow. The main works are described as following(1) Chapter one presents problem description and related progress on this topic. The thesis is outlined and innovations are listed either.(2) Chapter two describes design and manufacture of this direct-connected test rig for kerosene atomization in a cold supersonic flow, including supplementary system, such as fuel supply and laser instrumentation. The results show that the flow in the test section is quite uniform, the disturbance waves are weak enough to satisfy the requirements of spray experiments.(3) Chapter three shows the results on kerosene injected into a supersonic flow in a duct. The photographs, schiliren and PLIF images demonstrate the jet atomization and spreading in a supersonic flow. The jet penetration, droplets’ SMD distribution and spreading were obtained in the experiments. The data imply that jet-to-free stream dynamic pressure ratio is a dominant parameter in determining the jet penetration, breakup and atomization. High dynamic pressure ratio increases the penetration and accelerates the breakup and atomization of the jet column. But it induces strong shock wave and large loss of the total pressure. Three-dimensional unsteady surface waves locate on jet surface and the waves enhance the jet breakup. The surfaces wave emit from column initial curvature and wave altitudes increase while they march along jet downstream. When dynamic pressure ratio is low, the surface waves are suppressed and observed obviously after a certain length of the jet column.(4) Chapter four demonstrates the experiments conducted in a duct embedded a cavity with different geometry and aspect ratio. The experiments emphasize the flow characteristics and kerosene jet mixing to show the effects of different geometries of a closed cavity, such as angles of front and rear wall, leading slot and wavelike rear wall. Some conclusions are yielded as followsThe open cavity (L/D=3) induces weak shock waves, low loss of the total pressure. Moreover, the cavity self-sustained oscillation can be neglected due to its low amplitude. The longitudinal and transverse vortex are strong inside the cavity and they enhance the kerosene mixing both inside and outside the cavity. The distributions of droplets’ SMD in a duct with and without a cavity are almost identical but penetration height decreases slightly. The jet shock can be ignored relative to those induced by the cavity.In contrasted to the open cavity, the closed cavity (L/D=15) induces strong shock waves that lead to boundary layer separation on the opposite wall. Meanwhile, loss of total pressure is high and the self-sustained oscillation is observed obviously. Such flow oscillation can be suppressed by reducing the rear wall angle or employing wavelike rear wall. The front wall angle and leading slot have no significant effects on the shear layer and the induced shock near front wall. With kerosene injection, the jet shock can still be ignored relative to the cavity shock waves. At the same time, shear layer and free stream deflects upwards the rear wall and penetration height increases greatly. Shock wave upwards the rear wall and boundary layer separation on the opposite wall both disappear when the wavelike rear wall is employed. Furthermore, jet atomization is not difficult due to the high slip velocity, but the problem is how to weaken the cavity shock and enhance kerosene spreading in a supersonic flow. The cavity flow pattern depends on aspect ratio, the cavity geometry together with jet and incoming flow conditions.The distribution of droplets’ SMD is not related to the duct with or without cavity in a supersonic flow. But a closed cavity enhances kerosene spreading and mixing much more than those of an open one.The innovations in thesis are as follows:(1) PLIF and fluorescence (phosphorescence) of kerosene excited by UV laser were employed to show jet spreading and mixing. These approaches have been a new way of studying kerosene atomization in supersonic combustion. PLIF has extensively applied in combustion diagnostics for measuring molar fraction of small radicals or temperature as well as spray detection on gasoline or diesel engine, but it is firstly used to study kerosene atomization of in a supersonic flow. In contrast to shadowgraph and schlieren, PLIF can detect jet mixing zone and surface waves accurately. Therefore, empirical formulas of penetration height of liquid jet based on schlieren images are not so precisely as to compare to PLIF ones. Therefore, updated database are suggested to build up by these fresh PLIF images. It is anticipated PLIF has great potential application in studying kerosene atomization in a supersonic airstream.(2) With or without a cavity, the distribution of droplets’ SMD in a duct which is identical to a single scramjet combustor is presented in this thesis. Although LIN got data of droplets’ SMD when water is injected into a supersonic flow from an orifice which diameter is 2mm, the flow filed is quite different from that of a scramjet combustor configuration. The results imply that kerosene atomization in a supersonic flow is not a dominant in determining combustion efficiency, but the key is to reduce time delay of ignition and combustion. Cavity contributes much more to kerosene spreading and mixing, but less to decreasing droplets’ SMD.(3) Based on obtained PLIF images and droplet’s SMD, physical understanding has been improved to know the phenomena of kerosene spreading and mixing in a supersonic airstream, especially on the flow patterns in a duct embedded a cavity with different geometry and aspect ratio. Some knowledge has been updated or revised which are from previous wall pressure measurement or intuition. The methods in this thesis provide a way to study the supersonic combustion of kerosene and development of a scramjet combustor.更多还原