【作者】 李海军；

【导师】 沈胜强；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2004， 博士

【摘要】 作为一种流体机械,喷射器在许多工业领域中都有应用。但由于喷射器内部的流动过程复杂,在很长一段时期内,喷射器理论多半是作为研究和详细分析一定用途喷射器的计算方法而得到发展的,设计分析一般都是建立在一维分析基础上的半经验公式。而针对喷射器内复杂流场的实验观察、分析模拟,以及喷射器结构对喷射器性能影响等具体细节的研究方面,由于受实验水平、计算能力、流体力学理论发展水平的限制,是不充分的。尤其是对以蒸汽为工质的喷射器,由于蒸汽物性的远离理想气体状态,使简单的采用理想气体的状态方程所得出的结果与实际偏离很大。因而,尽管蒸汽喷射器的应用范围很广,对其内部流动机理的研究还不够深入,在提高蒸汽喷射器性能方面受到很大的限制。 本文借助PHOENICS软件平台,通过对蒸汽喷射器内流动进行数值模拟,详细观察其内部的流场,深入分析各因素对蒸汽喷射器性能的影响,对掌握喷射器工作机理、优化喷射器结构,提高喷射器效率具有重要的意义。主要研究工作如下: 1、建立物理上合理、计算上可行的蒸汽喷射器模拟的数学模型。本文借助商用软件平台优势,采用多种湍流模型及索科洛夫一维分析方法,对实验用的蒸汽喷射器进行模拟计算,并将各种模型所得到的结果与实验结果进行比较,按照与实验值相符、收敛速度快、流场分布合理的原则,得出Chen-Kim修正的k-ε两方程湍流模型可用来模拟喷射器内的复杂流动过程。 2、引入蒸汽的真实物性,对数学模型进一步完善。本研究考虑到蒸汽离液态很近,在蒸汽喷射器中又存在超音速流、两股汽流混合等复杂的流动过程,使蒸汽喷射器内蒸汽存在蒸汽的过热状态、饱和状态、及部分凝结等不同的集态,以往的研究都是关于理想气体喷射器的计算分析,不能合理给出蒸汽喷射器内的蒸汽物性。本文在PHOENICS软件中加入了蒸汽的真实流体物性,使模拟更接近真实情况,这对蒸汽喷射器流场的给出,以及对蒸汽喷射器中相变现象等复杂现象的研究意义非常重大。 3、详细分析、总结了工作参数对蒸汽喷射器性能的影响。在所采用的模型及计算方法已经证明适用于蒸汽喷射器的模拟计算的前提下,针对不同结构、不同工况下蒸汽喷射器流场进行大量的模拟计算,得出工作流体压力、引射流体压力、喷射器出口压力、喷嘴出口马赫数大小对蒸汽喷射器性能的影响规律。在对喷射器性能研究的过程中,引入膨胀比(工作流体压力与引射流体压力之比)和临界压缩比(临界出口压力与引射流体压力之比)两个参数,给出了一定结构喷射器的膨胀比与临界压缩比、膨胀比与最大喷射系数的关系式,以及喷射器最大膨胀比与喷嘴出口马赫数之间的幂指数关系。这两个参数的使用可以使模拟所得出的结论不局限于具体的参数值,使其具有很好的通用性。 4、详细分析喷射器各部分结构对喷射器性能的影响规律。研究充分利用数值模拟的灵活性、可重复性、低成本等优势,不断调整喷射器的各部分的结构形状、尺寸,模拟计算不同结构下喷射器的最大喷射系数及临界压缩比的变化,并对模拟结果进行分析整理,得出了喷射器各部分结构对喷射器性能的影响规律。研究发现,混合室结构对喷射器性能影响很大,并可根据需要在两种不同结构方式间选择。当混合室 结构不同时,喷嘴位置对喷射器的影响规律也是不同的。另外,尝试采用中心调节 针来改变喷嘴喉口面积的方法来改变喷射器的喷射系数和临界压缩比的值,结果表 明,这种方法可以方便地改变喷射器的性能,是改善喷射器性能,扩大喷射器应用 范围的一个可行的方法。总之,研究结果证明,改善喷射器的结构,可以提高喷射 器的喷射系数,是提高喷射器效率的一个重要途径。5、捕捉蒸汽喷射器内特殊的流动现象,并对其进行理论分析。通过对流场的观察,证 明了喷嘴出口后的射流会形成钻石形网格模式。在喷射器内激波出现在两个位置, 一是高马赫数的工作流体在喷嘴出口产生压缩波(膨胀波),经两股流体间的混合层反 复反射而沿流动方向传递,并由于两汽流的混合而不断地衰减直至被耗尽;另一处 是在扩压器的起始端,从混合室来的超音速流在扩压器入口引起强的斜激波,超音 速流经激波的压缩而使压力突增,以达到出口压力,保证顺利流出。扩压器处的激 波的产生使混合室中的混合流体发生雍塞,即工作流体及引射流体的流量不随出口 压力的变化而变化。激波强度随膨胀比的增大而增大,激波的存在会增加机械能的 损失,由于喷射器中超音速流动的存在,激波的产生几乎是不可避免的,但当喷射 器在最佳工况下运行时,会降低这种损失,提高喷射器的效率。6、超音速流中相变现象的定量分析。通过在计算中引入蒸汽真实物性,使本研究可以 通过计算喷射器中凝结水的百分含量,捕捉到蒸汽喷射器中普遍存在、但用传统的 理论分析方法不能得到反映的相变现象,即工作流体、引射流体发生凝结相变。从 流场中凝结水的百分含量分布,可以直观地观察到在流动核心上,凝结水的百分含 量分布规律。结合喷射器内激波分布规律,可以很容易地理解,凝结水量波动递减 及在扩压器入口处再次出现的原因是超音流经激波压缩突然增压,导致流体温度低 于饱和温度而出现凝结的,提高工作及引更多还原

【Abstract】 As a fluid machine, ejector is widely applied in industries, such as being used as a heal pump to enhance pressure or take advantage of waste heat or as a compressor in an ejectoi refrigeration/cooling system. Limited by experimental equipments and computer capability, not enough work has been done to obtain the ejector inside flow profile of the complex flowing and mixing process. Until now the design and evaluation of an ejector usually base on 1-D analysis. There is limitation on inspecting insight flow field and mastering the effects of operating condition parameters and geometric parameters of an ejector on its entrainment ratio and critical compression ratio So it is difficult to improve the COP of an ejector.In this paper, three-dimensional Navier-Stokes equations are solved with Chen-Kim k-εturbulent model to simulate the insight complex flow field of a steam ejector. Through observing flow field and analyzing the simulation results, many relations between ejector COP and operating parameters or ejector structures are mastered. The main work and results of this paper are given as follows:1 Selecting a turbulence model which is suitable for ejector numerical simulation. Four popular turbulence models and 1-D Sokolov analytical method were used in simulating the flow process in a steam ejector which was used in an experiment. The numerical simulation results show that the results gained with Chen-Kim k-ε model are moreidentified with that of experimental results. Considering that the calculation time with Chen-Kim k-ε model is the shortest, Chen-Kim k-ε turbulence model was selected toanalyse the compressible characters of the supersonic working flow in the ejector and the effect of the structure on its performance.2 Improving the mathematical model for steam ejector simulation by introducing steam property relations. Since overheat steam, saturated steam and condensed water coexist in a steam ejector, it is not reasonable to regard steam as an ideal gas during numerical analysis. So in this paper, real gas related expressions of steam properties was adding into the PHOENICS software. This improvement adequately considers the compressible characteristics of the supersonic steam in the ejector, so it makes the simulation results be more identified with real conditions and makes it possible to investigate phase transition phenomenon which cannot be avoided in a steam ejector but hard to master through measurement.3 Summing up the relationships between the ejector performance and its operating condition parameters systematically. From the computational results, how the motive fluid pressure, the suction fluid pressure and the discharging fluid pressure influence the ejector entrainment ratio and critical discharging pressure was concluded. Two parameters, namely expansion ratio and critical compression ratio, are introduced to conclude the relationships between this three pressures, such as the related expression of expansion ratio and critical compression ratio for a given ejector and the related expression of themaximum expansion ratio and Mach number at the nozzle outlet. These two relative expressions can be used to design a suitable ejector for certain working parameters or analyse a given ejector parameter characteristics in order to correctly apply and operate it.4 Analyzing the effect of ejector structures on entrainment ratio and critical compression ratio. Taking advantages of the flexibility, easy repetition, and low cost of numerical simulation, the effect on the maximum entrainment ratio and critical compression ratio of each component configuration of the ejector, viz. the nozzle, the absorber, the mixing-chamber, and the diffuser, is analysed respectively. Optimal shape and dimension of each component is concluded. A novel nozzle, a nozzle with an adjustable needle, is used in the ejector as a method of adjusting ejector performance. These results can be used as guidelines for ejector designing to enhance the ejector COP.5 Comprehending the reason of shocks emerging, their positions, and their e更多还原