喷嘴参数对喷气涡流纺内流场特性的影响研究

2022-04-21 14:13王青叶明露梁高翔盛晓超高帅
丝绸 2022年4期
关键词:数值模拟

王青 叶明露 梁高翔 盛晓超 高帅

摘要: 为明确喷气涡流纺纱的纺纱机理,研究喷嘴中喷孔数量、喷孔倾角和供气压力对喷气涡流纺内流场的影响情况,文章采用Fluent软件进行喷气涡流纺的内流场模拟分析。通过结果对比分析发现:喷孔数量和供气压力对喷嘴内流场的气流旋转运动影响较为显著,随着喷孔数量和供气压力的增加,喷嘴内气流的旋转运动显著增强,对纱线自由端纤维的加捻效果也增强;喷孔倾角大小对喷孔出口气流速度影响较小,但随着倾角增大,进入涡流管内的气流切向速度分量增大,对自由端纤维的加捻效果增强,纱线包缠越紧密,其强度也越高;喷孔数量、倾角和供气压力的变化都不会改变流场速度分布规律,只是改变了其值的大小,且随着气流的螺旋推进运动,气流速度不断衰减,说明气流在对纱线加捻的过程中要消耗大量的动能。

关键词: 喷气涡流纺;喷嘴参数;数值模拟;流场特性分析;纺纱机理;加捻效果

中图分类号: TS103.2文献标志码: A文章编号: 10017003(2022)04003906

引用页码: 041106DOI: 10.3969/j.issn.1001-7003.2022.04.006(篇序)

喷气涡流纺是一种在气流加捻腔内利用高速旋转气流加捻自由端纤维成纱的新型纺纱技术,该纺纱技术相对传统环锭纺,省去了粗纱环节,集细纱、络筒、卷绕成形工序于一体,缩短了纺纱流程。因此,近年来日益受到纺纱企业的青睐,是目前世界上最先进的纺纱技术,也是一种非常具有市场前景的纺纱新技术。

针对喷气涡流纺纱技术,国内外众多学者开展了一系列研究工作。Tyagi等[1-2]采用正交分解实验,研究了前罗拉钳口到空心锭子入口距离与成纱结构的相关性;Ortlek等[3]和Kuthalam等[4-5],Basal等[6]研究了喷气涡流纺空心锭子直径对成纱性能的影响;Naylor等[7]分析了试纺纤维长度与气流加捻过程中落纤率的相关性;Eldeed等[8]以瑞士立达喷气涡流纺纱机的喷嘴结构为对象,采用数值方法对喷嘴内流场进行了分析。陈彩红等[9],任玉斌[10]研究了喷孔数量、倾角对喷嘴内流场的影响;邹专勇等[11-12]研究了喷嘴内流场特性,初步解释了喷气涡流纺纱机理;尚珊珊等[13-14]研究了初始引纱过程和正常稳定纺纱过程中流场流动特性,并通过对喷嘴内高速旋转气流动力学特性及纱体运动三维数值模拟分析,揭示了喷气涡流纺纱过程中喷嘴内高速旋转气流的流动规律等;韩晨晨等[15-17]采用有限元方法分析了纤维在流场中的运动轨迹,且提出了一种自捻型喷气涡流纺的创新技术;郭臻等[18]建立纤维的三维运动模型,分析了纤维在流场中的运动和变形情况。综上分析,国内外学者对喷气涡流纺进行了大量研究,主要集中于对喷气涡流纺中喷嘴、空心锭子的部分结构和工艺参数对流场特性及成纱性能的影响分析,纤维在流场中的运动特性分析,以及单独的喷孔数量、倾角对流场的影响分析,而同时针对喷孔倾角、数量和供气压力大小等参数对喷气涡流纺内流场影响的研究较少。鉴于此,本文基于数值方法,详细研究喷孔数量、喷孔倾角和供气压力大小对喷气涡流纺内流场的影响情况,为喷气涡流纺喷嘴的设计研究提供一定的理论参考。

1模型建立

1.1喷嘴结構建模

参考日本村田公司的MVS型喷气涡流纺纱机喷嘴结构模型,本文建立喷嘴结构模型,包含喷气孔(简称喷孔)、空心锭子、涡流管(喷嘴和空心锭子间形成的气流流动空间)等结构,如图1所示。高压气流经过喷孔进入喷嘴内部,在涡流管内部形成高速旋转气流,完成对纱线的加捻作用。

1.2网格划分

本文采用ICEM软件进行喷嘴内流场的网格划分,考虑到喷孔、涡流管等结构比较复杂,因此划分网格为非结构网格(即四面体网格),如图2所示。同时对喷孔和涡流管等结构尺寸小、且内部流场变化最为剧烈的区域,为精确地捕获纺纱过程中的气流特性,本文采用密度盒加密方法进行局部网格加密处理,如图3所示。

1.3边界条件设置

根据气流流动特点,设置喷嘴入口为压力入口1,喷孔入口为压力入口2;设置喷嘴出口为压力出口1,空心锭出口为压力出口2,如图4所示。

2流场数值仿真和结果分析

设基准参数为:喷孔数量5、喷孔倾角(图1中θ角)70°、供气压力0.5 MPa,采用单一变量法分别研究这三个参数对喷嘴内流场的影响情况。参考文献[10,13],确定具体研究方案如表1所示。

根据表1中三种方案,本文针对七个状态分别进行结构建模、网格划分,以及数值模拟仿真。

2.1喷孔数量对喷嘴内流场的影响分析

喷孔数量不同对应的仿真结果如图5—图7所示。由气流静压、动压与速度之间的关系可知:气流速度高时,气流静压低,即由气流速度分布可以间接得到静压分布,因此本文后续仅对气流速度进行分析。

分析图5—图7发现:1) 由于喷孔内部和涡流管之间存在巨大压力差,使得气流在喷孔内加速,在喷孔出口处达到最大,且已达超音速,为463 m/s左右(图5)。2) 喷孔数量从4个增加到6个时,喷孔出口处的气流速度差很小,原因在于虽然喷孔数量增加,但是各喷孔的供气压力一样大,且喷孔尺寸规模一样大,因此气流在各喷孔中加速性相当(图6)。3) 喷孔数量从4个增加到6个时,喷嘴内气流的旋转运动显著增强(图7),对自由端纤维的加捻效果也增强。因此,在喷嘴结构强度满足要求的前提下,可以尽量增加喷孔的数量。

2.2喷孔倾角对喷嘴内流场的影响分析

由于各种状态的速度云图、速度矢量图等比较类似,且速度流线图同时包含了较多速度云图和速度矢量图信息,因此限于篇幅,下文仅给出速度流线图,而速度云图和速度矢量图不再给出。图8为喷孔角度不同时涡流管内气流的速度流线图。

分析图8发现:1) 当喷孔倾角逐渐增大时,喷孔内部流场的速度峰值在458~472 m/s逐渐增大,且增幅较小。分析其原因在于随着喷孔倾角增大,喷孔长度略有增加,气流加速段略有增长,喷嘴出口气流速度随之增大。2) 喷孔倾角越大,气流从喷孔进入涡流场时沿着喷嘴周向的气流速度分量越大,因此对纱线的加捻特性越好,纱线包缠的越紧密,纱线强度越高。但是喷孔倾角进一步增大,会导致气流沿着喷嘴周向的气流速度分量进一步增大,轴向气流速度减小,纱线包缠更加紧密,使得纱线表现较硬,断裂伸长率降低,纱线易断裂。因此,倾角最佳数值的确定,应结合实验,并综合考虑喷孔数量、供气压力等参数的影响。

2.3供气压力对喷嘴内流场的影响分析

图9为不同供气压力条件下,涡流管内气流的速度流线图。分析图9可知:1) 当供气压力从0.3 MPa提高到0.5 MPa时,喷孔出口气流速度从396 m/s增大到464 m/s,即随着供气压力的增大,气流速度峰值显著增大。这是因为供气压力越大,喷孔内部和涡流管之间压差越大,喷孔中气流加速性越好。2) 供气压力越大,涡流管内气流速度越高,气流旋转运动也越强,对纱线的加捻效果提高,因此供气压力对喷嘴内部的流场特性影响显著,应该在考虑耗气量的前提下尽可能提高供气压力。

此外,综合分析图7—图9发现:随着喷孔数量、倾角和供气压力大小的变化,涡流管中气流速度分布规律基本不变;气流在向喷嘴出口螺旋式推进的运动过程中,气流速度逐渐衰減,说明气流在对纤维加捻过程中,需要消耗大量的动能。

3结论

本文基于数值方法,采用单一变量法依次研究了喷孔数量、喷孔倾角和供气压力对喷气涡流纺喷嘴内流场特性的影响情况,可得出结论:1) 气体经喷孔进入涡流管之后,在喷嘴内部形成高速旋转气流,该旋转气流对自由端纤维实现了加捻作用,且旋转气流在向前推进运动过程中,速度不断衰减,说明气流在对纤维加捻的同时需要消耗大量的动能;2) 随着喷孔倾角、喷孔数量和供气压力的不断增大,涡流管中气流的旋转运动均增强,对自由端纤维的加捻效果提高。因此,在喷气涡流纺纱机喷嘴结构设计时,在保证喷嘴结构强度的前提下,尽量设计较多的喷孔,同时增大喷孔倾角和供气压力,可有效提高对自由端纤维的加捻效果。

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Influence of nozzle parameters on the characteristics of the internal flow field in air-jet vortex spinningWANG Qing, YE Minglu, LIANG Gaoxiang, SHENG Xiaochao, GAO Shuai(School of Mechanical and Electrical Engineering, Xi’an Polytechnic University, Xi’an 710048, China)

Abstract: Air-jet vortex spinning is a new spinning technology, which uses high speed rotating airflow to twist free-end fibers into yarns in the air twisting chamber. This technology emitting the roving process integrates spinning, winding and winding molding processes, shortening the spinning process and making itself the most advanced and promising new spinning technology in the world. At present, air-jet vortex spinning machines are mainly imported from Japan. The main reason is that the twisting mechanism of air-jet vortex spinning has not been fully grasped by Chinese. Thus, many scholars have carried out relevant studies which mainly focus on the influence of structure and process parameters of nozzles and hollow spindles on the flow field characteristics and yarn forming performance, the motion characteristics of the fibers in the flow field, as well as the influence of the numbers and inclination angles of jet orifices on the flow field. There are few studies on the influence of numbers, inclination angles of jet orifices and air supply pressures on the internal flow field of air-jet vortex spinning. In view of this, the influence of these three parameters on the internal flow field of air-jet vortex spinning is studied in detail in this paper. It can provide a theoretical reference for the design of air-jet vortex spinning nozzles.

Based on numerical method and single variable method, the influence of numbers and inclination angles of jet orifices as well as air supply pressures on the internal flow field of nozzles and twist characteristics were studied. Values of the three parameters of the reference configuration selected in this study were 5, 70° and 0.5 MPa. And the three parameters could change as [4, 5, 6], [65°, 70°, 75°] and [0.3, 0.4, 0,5] MPa, respectively. The structure modeling, meshing and numerical simulation of the seven combined states were carried out respectively, and the velocity vector diagram and flow diagram obtained by simulation were compared and analyzed. There are four conclusions obtained. Firstly, as the number of jet orifice increases from 4 to 6, the velocity difference at the outlet of jet orifices is quite small. The reason is that: although the jet orifices increase, the air supply pressures, and the length of each jet orifice remain unchanged. As a result, the airflow accelerates equally in each jet orifice. However, with the increase of jet orifices, the rotational motion of airflow in the nozzle as well as the twisting effect on the free-end fiber are enhanced. Thus, the jet orifices can be increased as many as possible on the premise that nozzles have sufficient structure strength. Secondly, when the inclination angles increase, the speed of the internal flow field in the nozzle increases from 458 m/s to 472 m/s gradually. And the velocity component along the circumferential direction increases when the airflow enters the vortex tube from the jet orifice. Thus, the yarn twists better, wraps tighter and the strength of yarn is higher. Whereas, when the inclination angle further increases, the circumferential airflow velocity along the nozzle will further increase, and the axial airflow velocity will decrease. As a result, the yarn will be more tightly wrapped, which makes the yarn harder and easier to fracture. In consequence, the number of jet orifices, air supply pressures and other parameters should be considered comprehensively to determine the optimal value of inclination angle combined with the experiment. Thirdly, the higher the air supply pressure, the higher the airflow velocity in the vortex tube, the stronger the airflow rotation motion, and the better the yarn twisting effect. Therefore, the air supply pressure has a significant influence on the flow field characteristics inside the nozzle. And the air supply pressure should be increased as much as possible when considering the gas consumption. Fourthly, with the change of jet orifice numbers, inclination angles and air supply pressures, the flow velocity distribution in the vortex tube is basically unchanged.

In conclusion, when we design the nozzle structure of air-jet vortex spinning machines, jet orifices should be designed as many as possible on the premise of sufficient nozzle structure strength, and inclination angles of jet orifices and air supply pressures should be increased at the same time. In this way, the twisting effect of free-end fiber can be improved effectively.

Key words: air-jet vortex spinning; nozzle parameters; numerical simulation; flow field characteristic analysis; spinning mechanism; twisting effect

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