环形射流泵内部流动机理及结构优化研究

发布时间：2018-07-26 09:35

【摘要】：射流泵具有结构简单、可靠性高、运行和维护成本低等优点。与传统中心射流泵相比,环形射流泵的吸入通道没有阻碍且被吸流体不用改变流向,因此特别适合于抽吸含有大颗粒固体(活鱼、矿石、胶囊、产业废弃物等)的混合流体,在工程领域中有着广阔应用前景。然而,关于其内流机理和结构优化方面的研究并不充分。环形射流泵内部流动是受限空间环形壁面射流在有逆压梯度环境下的混合与发展,流场内存在逆压梯度、射流剪切层、壁面边界层以及可能的回流区等复杂现象,流动机理十分复杂。为此,本文基于大涡模拟,结合湍流统计理论和拟序运动理论等对环形射流泵内部流动机理进行了系统研究,并采用非传统结构来实现其性能的优化。主要工作和研究成果如下： (1)为验证本文LES方案的可靠性以及加深对环形射流泵的直观认识,首先对面积比m=1.72,2.26和3.33的环形射流泵进行了试验研究。结果表明：环形射流泵内流动与传统中心射流泵一样存在自模性,且高效区较宽,而其最优结构尺寸随着面积比改变；随着面积比增加,性能曲线向较大流量比方向移动。 (2)采用LES对m=1.72和3.33的环形射流泵在不同工况下进行了计算,并对比分析了不同的网格数、网格布置与亚格子模型的影响。通过与试验数据对比,结果表明本文所采用的LES方案能够较为准确地预测环形射流泵的时均外特性和内部流动。通过对流场中监测点的瞬时压力系数进行频谱分析,获得各个工况下的涡脱落频率以及特征St数。当m=1.72时,St数在0.2～0.22之间,随着流量比增加,St数降低,而当m=3.33时,St约为0.23,并几乎不随流量比变化。 (3)对LES所获得的环形射流泵内时均流场进行分析发现：随着流量比的增加,工作和被吸流体势流核心长度近似线性增加；吸入室内射流半宽接近线性增长,且流量比越小增长越快；而吸入室内边界层厚度增长速率与流量比无关,但在喉管内部,流量比越小边界层越厚,且增长速率越大；剩余能量系数可描述泵内能量的沿程变化情况,该系数沿流向整体呈下降趋势,流量比越大,下降速率越小；回流区在瞬时和时均情况下差别较大,瞬时回流区的形状及分布较不规则,甚至不连续；随着流量比增大,回流区缩小并向下游移动,相比再附点,分离点向下游移动较大。 (4)通过采用压力判据、涡量和Q准则等拟序结构辨识方法有效提取流场中的拟序结构,结果表明：相比Q准则,压力判据对于较小尺度的拟序结构,尤其对大量存在于涡辫区内的肋状涡的辨识度较低；拟序结构主要由混合层、边界层和回流区内产生并交互作用；混合层内涡结构在增长时,对吸入室壁面边界层进行压迫,诱发吸入室壁面边界层内涡环结构的产生,由于二者转向相反,在二者相接处的区域流向相同,相互作用较弱；拟序结构中的流向涡和展向涡由于产生机理不同,导致其强度、形态和演化方式不同,尽管流向涡涡量与展向涡涡量相比较小,然而流向涡促进了展向涡的扭曲和破碎,并对流场中卷吸和混合的贡献较大。流场内流向涡与展向涡之间、混合层内涡结构与边界层内涡结构甚至与回流之间的相互作用,使得环形射流泵内部流场较为复杂。然而亦正是这些复杂的演变过程和相互作用,主宰着泵内流场的湍流特征、流体间的掺混与传能甚至环形射流泵的外特性。 (5)鉴于LES计算成本较高,因此采用RANS湍流模型验证本文所提出的结构优化方案。首先对射流泵模拟中常用的六种RANS湍流模型进行了验证,结果表明：RNG k-ε模型能够较为准确地预测环形射流泵的外特性和内部流动的平均特征；标准k-ε模型低估了环形射流泵的外特性,而对外特性和壁面压力系数的变化趋势预测较为准确。 (6)提出利用等速度变化和等压力变化方法来设计环形射流泵的扩散器,可降低锥形扩散器内由于速度或压力变化不均匀导致的流动损失,并推导出简洁易用的设计公式。数值模拟表明这两种设计方法确实能使扩散器内速度或者压力变化更为均匀,其中等压力变化扩散器内压力的确能够呈线性增长,从而改善环形射流泵的性能；特别在扩散器较短或扩散角度较大时表现更优,因此较为适用于泵的空间和质量受限的场合。相比等速度变化扩散器,等压力变化扩散器在大流量比工况下表现较优,但在小流量比下较差。 (7)提出了新型夹心式环形喷嘴,使得高速环形工作流体内外侧均为被吸流体,避免了高速射流贴壁流动产生的较大摩擦损失,并由于增大了与被吸流体的接触面积,使得泵的最高效率从35.8%提升至45.1%。工作喷嘴与壁面的距离以及内外被吸流体之间的速度比对采用该喷嘴的新型泵性能具有较大影响,且在取不同的工作喷嘴与壁面距离时,相应的最优速度比不同。在最优速度比未知的情况下,选择1/1可获得较好结果。
[Abstract]:The jet pump has the advantages of simple structure, high reliability and low cost of operation and maintenance. Compared with the traditional central jet pump, the suction channel of the annular jet pump is not hindered and is absorbed by the fluid without changing the flow direction, so it is especially suitable for the suction of mixed fluid containing large particle solids (living fish, mineral stone, capsule, industrial waste and so on). However, the research on the internal flow mechanism and structural optimization is not sufficient. The internal flow of the annular jet pump is the mixing and development of the annular wall jet under the reverse pressure gradient environment, and the flow field is in the reverse pressure gradient, the jet shear layer, the wall boundary layer and the possible reflux area. In this paper, the internal flow mechanism of the annular jet pump is systematically studied in this paper based on the large eddy simulation, combined with the turbulence statistical theory and the theory of the pseudo order motion, and the performance optimization is realized by using the non traditional structure. The main work and research results are as follows:
(1) in order to verify the reliability of the LES scheme and to deepen the intuitive understanding of the annular jet pump, the experimental study on the annular jet pump with an area ratio of m=1.72,2.26 and 3.33 is first studied. The results show that the inner flow of the annular jet pump has the same self mode as the traditional central jet pump, and the high efficiency zone is wide, and the optimum structure size is along with the surface. As the area ratio increases, the performance curve moves to a larger flow ratio direction.
(2) the m=1.72 and 3.33 annular jet pumps are calculated under different working conditions with LES, and the effects of grid number, grid arrangement and subgrid model are compared and analyzed. By comparing with the experimental data, the results show that the LES scheme used in this paper can accurately predict the time average characteristics and internal flow of the annular jet pump. Through the spectrum analysis of the instantaneous pressure coefficient of the monitoring point in the flow field, the vortex shedding frequency and the characteristic St number are obtained under each condition. When m=1.72, the number of St is between 0.2 and 0.22, with the increase of the flow ratio, the number of St decreases, and when m=3.33, the St is about 0.23, and almost does not change with the flow ratio.
(3) the analysis of the time averaged flow field in the annular jet pump obtained by LES shows that with the increase of the flow ratio, the core length of the flow and the flow is approximately linearly increased, and the half width of the jet in the suction chamber is close to linear growth, and the smaller the flow ratio is, the faster the increase of the flow ratio; but the growth rate of the inner boundary layer thickness in the suction chamber is not related to the flow ratio. In the interior of the throat, the smaller the flow ratio, the thicker the boundary layer and the greater the growth rate. The residual energy coefficient can describe the change of the energy in the pump, and the coefficient decreases along the flow direction, the greater the flow ratio, the smaller the drop rate, the larger the instantaneous and the time, the shape and distribution of the instantaneous reflux area. The rule is even discontinuous. As the flow ratio increases, the recirculation zone shrinks and moves downstream.
(4) the pseudo sequence structure identification method, such as pressure criterion, vorticity and Q criterion, is used to effectively extract the pseudo sequence structure in the flow field. The results show that, compared with the Q criterion, the pressure criterion has a lower identification degree to the smaller scale quasi order structure, especially a large number of ribbed vortices in the vortex zone; the pseudo sequence structure is mainly composed of mixed layer, boundary layer and return layer. In the flow area, the vortex structure in the mixed layer increases, and the boundary layer of the suction chamber wall surface is compressed and the vortex ring structure in the boundary layer of the suction chamber wall is induced. As the two turns opposite, the flow direction is the same and the interaction is weak, and the flow vortices and the spreading vortices in the quasi order structure are produced due to the formation of the flow vortex and the spreading vortex in the quasi sequence structure. The mechanism is different, which leads to the different intensity, morphology and evolution mode. Although the flow vorticity is smaller than the spread vorticity, the flow vortex promotes the distortion and breakage of the spreading vortex, and contributes greatly to the volume absorption and mixing in the flow field. The inner vortex structure and the inner vortex structure in the mixed layer and the boundary layer vortex structure are even between the flow flow and the flow vortex. The interaction between reflux makes the internal flow field of the annular jet pump more complex. However, it is the complex evolution process and interaction that dominates the turbulent characteristics of the internal flow field of the pump, the mixing of the fluid and the external characteristics of the annular jet pump.
(5) in view of the high cost of LES calculation, the RANS turbulence model is used to verify the structural optimization scheme proposed in this paper. First, six kinds of RANS turbulence models commonly used in the jet pump simulation are verified. The results show that the RNG k- epsilon model can accurately predict the external characteristics of the annular jet pump and the average characteristics of the internal flow, and the standard k The external characteristic of the annular jet pump is underestimated by the - E model, while the variation trend of external characteristic and wall pressure coefficient is more accurate.
(6) to design the diffuser of the annular jet pump by using the equal velocity change and the equal pressure change method, it can reduce the flow loss caused by the uneven velocity or pressure variation in the conical diffuser, and deduce the simple and easy to use design formula. The numerical simulation shows that the two methods do make the diffuser speed or pressure inside the diffuser. The change is more uniform, in which the pressure in the equal pressure variable diffuser does increase linearly, thus improving the performance of the annular jet pump, especially when the diffuser is short or the diffusion angle is larger, so it is more suitable for the space and the limited quality of the pump. It performs better under large flow rate than under low flow ratio.
(7) a new type of sandwich annular nozzle is proposed, which makes the inner and outer sides of the high speed annular working fluid be suction fluid, avoiding the large friction loss caused by the high-speed jet flow, and because of the increase of the contact area with the absorbed fluid, the maximum efficiency of the pump is raised from 35.8% to the distance between the 45.1%. working nozzle and the wall and the inside and outside. The velocity ratio between the absorbed fluid has a great influence on the performance of the new type of pump using the nozzle, and the optimum velocity ratio is different when the distance between the different working nozzles and the wall is taken. In the case of the unknown optimal velocity ratio, the selection of 1/1 can obtain better results.
【学位授予单位】：武汉大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TH38

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