Detached eddy simulation on the structure of swirling jet flow field

The improved delayed detached eddy simulation method with shear stress transport model was used to analyze the evolution of vortex structure,velocity and pressure fields of swirling jet.The influence of nozzle pressure drop on vortex structure development and turbulence pulsation was investigated.The development of vortex structure could be divided into three stages:Kelvin-Helmholtz(K-H)instability,transition stage and swirling flow instability.Swirling flow could significantly enhance radial turbulence pulsation and increase diffusion angle.At the downstream of the jet flow,turbulence pulsation dissipation was the main reason for jet velocity attenuation.With the increase of pressure drop,the jet velocity,pulsation amplitude and the symmetry of velocity distribution increased correspondingly.Meanwhile the pressure pulsation along with the axis and vortex transport intensity also increased significantly.When the jet distance exceeded about 9 times the dimensionless jet distance,the impact distance of swirling jet could not be improved effectively by increasing the pressure drop.However,it could effectively increase the swirl intensity and jet diffusion angle.The swirling jet is more suitable for radial horizontal drilling with large hole size,coalbed methane horizontal well cavity completion and roadway drilling and pressure relief,etc.

Heat transfer in turbulent tube flow inserted with loose-fit multi-channel twisted tapes as swirl generators

Heat transfer and flow behaviors in three-dimensional circular tubes with loose-fit multiple channel twisted tapes were numerically studied. The investigation was examined for Reynolds numbers （Re） ranging from 5000 to 15,000, by using air as testing fluid. Effects of the multiple channel number （N = 2, 3, and 4）, clearance ratio （CR = 0.0, 0.025, 0.05, and 0.075） on heat transfer enhancement and flow friction were examined. The numerical results indicate that the tubes with loose-fit multiple channel twisted tapes perform higher heat transfer rates than the plain tube. The enhanced heat transfer rate is escorted with larger pressure drop. Both heat transfer and pressure drop increase with increasing multiple channel number （N） and decreasing clearance ratio （CR）. Heat transfer augmented by the loose-fit multiple channel twisted tape with N = 4 is higher than those enhanced by the ones with N = 2 and 3 by around 9.5-17.8% and 5.8-7.8%, respectively. In addition, the loose-fit multiple channel twisted tapes with clearance ratio of 0.025, 0.05, and 0.075 give lower heat transfer rates than the one with CR = 0.0 by around 8.4%, 17.5%, and 28.8%, respectively.

Heat Transfer and Friction Factor Studies in a Circular Tube Fitted with Twisted Tape Consisting of Wire-nails

This paper deals with the experimental investigation on Nusselt number,friction factor and thermal en-hancement factor of a double pipe heat exchanger equipped with twisted tape consisting wire nails(WN-TT) and plain twisted tapes(P-TT) with three different twist ratios of y 2.0,4.4 and 6.0. Test runs are conducted using the water as the working fluid with Reynolds number range between 2000 and 12000 for WN-TT and P-TT. It is found that Nusselt number,friction factor and thermal enhancement factor in the tube equipped with WN-TT appreciably higher than those in the tube fitted with P-TT and plain tube. Over the range considered Nusselt number,friction factor and thermal enhancement factor in a tube with WN-TT are respectively,1.08 to 1.31,1.1 to 1.75 and 1.05 to 1.13 times of those in tube with P-TT. The better performance of WN-TT is due to combined effects of the follow-ing factors:(1) common swirling flow generated by P-TT,(2) additional turbulence offered by the wire nails. Em-pirical correlations for Nusselt number,friction factor and thermal enhancement factor are also formulated from the experimental results of WN-TT and P-TT.

The Current Situation of the Study on Twisted Tape Inserts in Pipe Exchangers

Heat transfer enhancement is used in many applications including heat exchangers, air conditioning, and refrigeration systems; hence many researchers have conducted experimental and numerical researches on heat transfer enhancement and have developed various techniques and methods. As a passive heat transfer technique, twisted tapes are widely used in various industries for their cost savings, lower maintenance requirements and the fact that they are easily set up. This paper introduces the principle of heat transfer enhancement of twisted tapes and reviews some of the experimental works done by researchers on this technique in recent years. The variously modified twisted tape inserts are widely researched and used to enhance heat transfer efficiency for heat exchangers. Twisted tapes perform better in low Re conditions and in square tubes. However, they could also cause higher pressure drops. Twisted tapes have great potential and profound implications if they can be used in traditional heat exchangers. Besides this, some correlations between the Nusselt number and friction factors are presented in this paper.

Numerical investigation of confined swirling gas-solid two phase jet

This paper presents a \%k\|ε\|k\-p\% multi\|fluid model for simulating confined swirling gas\|solid two phase jet comprised of particle\|laden flow from a center tube and a swirling air stream entering the test section from the coaxial annular. A series of numerical simulations of the two\|phase flow of 30 μm, 45 μm, 60 μm diameter particles respectively yielded results fitting well with published experimental data.

Laws of Jet Mixing of the Swirled Flows in a Pipe

Results of experimental research of the mixing process of coaxial flows in a pipe with swirled peripheral jet are presented in this paper. Distribution of temperature and concentration of gases on the axis and wall of the channel under the influence of such factors as the regime flow, ratio of density of flows and swirl degree of the peripheral jet are studied. Research of temperature, swirl angle, circulation in cross sections along with the channel have shown that their distributions have the jet-like character and are described by known dependences for the layer of mixture.

基于旋流射流的清管器结构研究

针对管道污垢容易在清管器皮碗底部形成楔形堆积,造成清管器卡堵等问题,提出一种基于旋流射流的清管器结构。基于计算流体力学方法,对旋流射流清管器进行流场数值模拟,研究导流管直径、叶片安装角、翼型折角对叶轮力矩和旋流射流平均速度的影响,分析导流管气流对皮碗底部污垢吹扫效果;采用正交试验法确定最优结构参数。研究表明:导流管直径在65~75 mm之间时,叶轮力矩基本达到峰值;增大翼型折角或叶片安装角都能显著提高叶轮力矩,但叶片安装角对旋流射流平均速度影响较小;导流管中射到皮碗底部的气流速度在40~60 m/s,湍动能大小在6 m^(2)/s^(2)左右,且影响范围广,能解决由皮碗底部污垢堆积而产生的卡堵问题;最优结构参数为导流管直径45 mm、叶片安装角40°、翼型折角33°。研究可为研发新型清管器以及解除卡堵问题提供参考。

LYZH型急冷箱数值模拟与结构优化

为适应加氢反应器大型化趋势,开发了集旋流、撞击及挡板分流等功能于一体的LYZH型急冷箱,其结构由上至下依次为旋流冷氢管、冷氢盘、喷射管和接液盘。采用CFD软件对LYZH型急冷箱进行数值模拟研究,结果表明LYZH型急冷箱具有优秀的传热性能及气液混合性能。为使LYZH型急冷箱的性能得到进一步提升,对其结构进行了优化,最优结构的旋流冷氢管中径为2218.9 mm、喷射管直径为526.9 mm以及接液盘到冷氢盘高度为175.4 mm。与原始结构相比,最优结构的传热性能提高7.61%,压力降降低14.4%,LYZH型急冷箱的综合性能得到较大改善。

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.

旋转磨料射流喷嘴叶轮参数优化设计

为增强旋转磨料射流在硬岩地层高效破岩效果,喷射破岩形成较大孔径径向水平井井眼,对喷嘴叶轮参数展开了优化设计。对径向水平井井下工况旋转磨料射流流场进行模拟,研究了叶轮扭转角、长度、直径、叶片厚度和叶片数量等参数对流场速度的影响。结果表明:叶轮扭转角度为影响流场旋流强度的主要因素;减小叶轮直径或增加叶片厚度会缩小流道的过流面积,增加射流的旋转强度,直径影响最大切向速度相差15.1%,厚度影响相差44.6%;叶片数量较多会降低射流冲击速度。在本文条件下,给出了建议性优化喷嘴叶轮结构参数:叶轮扭转角度为630°,长度为30 mm,直径为18 mm,叶片厚度为4 mm,叶片数量为3。所得结果可为径向井旋转磨料射流喷嘴优化设计提供依据。

Characteristics of annular impinging jets with/without swirling flow by short guide vanes

Annular jets impinging on a uniformly heated flat plate with or without swirling flow by short guide vanes are experimentally characterized. With the Reynolds number fixed at a relatively low value, the variation of jet flow structures with impinging distance is characterized using the technique of particle image velocimetry (PIV). Correspondingly, the distributions of wall pressure and heat transfer on the plate are measured. At sufficiently large impinging distances, without swirling flow, the obtained flow and wall pressure/heat transfer data are consistent with the classical observation for a conventional annular impinging jet, showing the transition from annular impinging jet flow to single circular impinging jet-like flow. In contrast, no such transition occurs in the presence of flow turning by short guide vanes. At short and intermediate impinging distances, flow turning causes more non-uniform distributions of wall pressure and heat transfer on the target plate and the local heat transfer rates higher than those of the conventional annular jet. This is attributed to the vortical flow structures shed and convected downstream from the short guide vanes. In sharp contrast, at large impinging distances, the larger momentum loss due to flow turning results in lower heat transfer rates on the plate.

射吸式旋流排水采气装置结构参数优化

针对天然气开采过程中产生的井底积液问题,本文提出一种射吸式排水采气装置,并通过数值模拟对其内部流程进行仿真分析,对比分析了旋流工具结构参数对排水采气效果的影响,并根据其仿真结果对旋流工具的结构参数进行优化,得到了确定工况下最佳的结构参数。

直旋混合射流内流场的特性研究

自进式钻头是径向射流钻井技术中的一个不可或缺的组件。然而,目前国内外对直旋混合射流进行的研究多数是关于其外流场对破岩效率的,而对组合射流的形成机理的研究较少。通过在非淹没条件下进行了新型直旋混合射流的内部流场仿真,得到了内流场的混合特性:轴向速度靠近轴心处速度最大,切向速度则是沿着射流的不断混合呈现正弦曲线分布的规律。结果可为进一步研究钻头的破岩特性、机理和优化设计提供理论依据。

内置旋转扭带换热管的传热强化机理

针对换热设备的低效率和污垢沉积问题,研究开发具有强化传热和污垢在线清洗双重功效的旋转清洗扭带技术。提出旋转扭带强化传热的机理包括:①换热管当量直径减少效应强化。②近管壁区域流速加大效应强化。③螺旋线流动流速加大效应强化。④二次流流速增大效应强化。对旋转扭带的这些强化传热机理进行理论分析,建立湍流工况下强化传热的努塞尔数预测关联式。研究结果表明:当扭率大于等于10时,当量直径减小效应和近管壁区域流速加大效应是传热强化的主要控制机理;当扭率小于10时,螺旋线流动流速加大效应是强化传热主要控制机理。二次流流速加大效应对强化传热的贡献与其他三种机理相比相对较弱,只有在扭率小于等于1时,对强化传热的贡献份额才比较大。对内置旋转扭带换热管的努塞尔数预测值与试验值进行了比较,两者吻合较好。

同轴直射流与旋转射流组合的双射流湍流流场数值模拟

直射流与旋转射流分别具有能量传递距离长与能量传递分布面积大的优点,为了尽可能的提高射流能量利用,本文提出了结合两者优点的同轴直射流和旋转射流组成的新型双射流。对淹没条件下的组合双射流流场特性,进行数值模拟研究。采用RNGk ε模型和高精度QUICK离散格式,得出了不同速度组合下双射流流场的直射流轴心速度衰减和旋转射流旋转速度衰减规律,速度矢量图,量化分析了射流结构对流场结构的影响。计算结果表明,在喷嘴压降一定时,存在三种轴心速度衰减曲线,当旋转射流轴向速度高于门限速度的情况下,射流衰减速度最慢。另外,直射流与旋转射流轴心衰减曲线存在很大差异,要提高双射流能量利用,内外喷嘴的流量分配至关重要。

管内周期性自旋流强化传热的结构优化

在圆管内间隔插入多个旋流片的流道中,流体流动呈现周期性自旋流.用数值模拟方法对空气湍流情况的传热和流阻性能受相邻旋流片间距Lp、旋流角β和旋转角α3个因素的影响进行了分析,并得出旋流片的优化结构,而且实验验证了数值模拟预测的正确性.结果表明:α越大,传热性能越好,阻力越大;Lp越小,传热性能越好,阻力越大;β越大,传热性能越好,而阻力损失因受到摩擦阻力和形体阻力的综合影响而呈现非单调变化.旋流片的最佳结构参数为:旋流角β为20·3°,旋转角α为180°,相邻旋流片间距Lp为33d.与α相比,β对传热和流阻性能的影响更显著.

喷射角度和喷嘴数对旋流冷却流动与传热特性的影响

针对喷射角度和喷嘴数影响旋流冷却流动和传热特性的问题,采用数值方法进行了研究。研究时冷气通过不同的喷嘴进口进入旋流腔并经旋流腔出口流出,当变化喷嘴数时,保持喷嘴进口在轴向上均匀分布。研究结果表明:冷气从喷嘴射入旋流腔,冲刷壁面并与轴向主流强烈混合,形成了高传热区域;换热强度在轴向和周向沿下游逐渐减弱,高传热区域在下游向出口偏移。喷射角度远离90°时,冷气旋流运动减弱,传热强度减小;随着喷嘴数的增多,冷气喷射速度减小,高传热区换热强度减小,冷气周向速度和靶面传热强度分布更为均匀;平均努塞尔数随着喷射角度和喷嘴数的增大而先增大后减小,在喷射角为90°、喷嘴数为9时平均努塞尔数最大;总压损失系数随着喷射角度和喷嘴数的增大而增大。与简单圆管旋流冷却模型相比,喷射角为90°、喷嘴数为9的旋流腔结构的换热特性更加优良。

同轴旋转双射流冲蚀岩石机制与实验研究

同轴旋转双射流是一种新型高效射流,它综合旋转射流破岩面积大和直射流破岩深度大的优点,并在较高围压下能够利用空蚀提高破岩能力。运用实验的方法研究常压淹没和围压条件下喷距、射流压力以及围压对射流破岩能力的影响。实验结果表明,双射流在冲击破岩时存在最优喷距范围,该次实验条件下最优喷距约为喷嘴当量直径的13倍;增大射流压力可大幅提高双射流的破岩体积。压力超过15MPa时,破岩体积大幅增长。双射流的破岩体积和破岩孔深均随围压的增大而急剧减小,破岩孔径随围压的增大而减小的幅度相对较小。双射流冲蚀破碎岩石机制包括空蚀破坏、直射流冲击破坏、剪切破坏以及压力波动破坏。

关于加权组合旋流强度的研究

通过理论分析表明,传统的组合旋流强度定义不能充分体现组合旋流的流体动力学规律.在冷态实验中,用热线风速仪测量研究了旋流燃烧器喷口的流场特性.通过对实验结果的分析,严格证明了组合旋转射流的各层射流对流场的贡献并不相同.对射流的最大回流率而言,燃烧器内层的射流贡献大于外层的射流.在此基础上,提出了"加权组合旋流强度"的概念,使用权函数来体现各层风对流场特性影响的作用大小.实验数据的分析证明,加权组合旋流强度可以将实验范围内各工况的最大回流率分布拟合为一个线性的公式,相关系数的数值约为0 9,这优于使用传统的组合旋流强度拟合的公式.

旋流喷头的特性和用于溅渣护炉的分析

通过流体力学测试和冷态模拟实验，证明旋流铸铜喷头（三孔或四孔）具有良好的特性。安钢和酒钢使用旋流喷头后，化渣效果好，起渣时间提前１～２ｍｉｎ，冶炼过程平稳易控制，不易发生喷溅，供氧时间缩短近３ｍｉｎ，耗氧量降低５．７５ｍ３／ｔ。从冲击动力、喷溅量和流股特性等方面分析了旋流喷头的优点，可以满足溅渣护炉的要求，并能够提高溅渣的均匀性和效果。