涡流运动降低柴油机混合气浓度及碳烟排放的数值分析

为了揭示涡流运动对柴油机混合气形成及碳烟排放的影响规律，采用经过实验验证的喷雾及湍流模型，用CFD数值分析软件对某车用柴油机燃烧室内不同涡流条件下柴油喷雾的混合气浓度、速度矢量场、燃油液滴空间分布及油束特性进行了模拟计算．模拟结果表明，当涡流比从0到5．0依次增大时，过喷孔轴线的铅垂面内喷雾浓度场局部浓区燃空当量比逐渐降低，而过喷孔轴线且与铅垂面垂直的平面内喷雾浓度场局部浓区的燃空当量比则先降后升，而不是逐渐下降．只有合理选择剖切平面，即选择过喷孔轴线且与铅垂面垂直的平面，才能正确评价涡流运动对燃烧室内混合气浓度分布及碳烟形成区域分布的影响规律．组织燃烧室内气流运动，须兼顾与气缸轴线垂直的水平面内的涡流运动和过气缸轴线的铅垂面内的湍流或滚流运动．涡流比太大，铅垂面内的湍流或滚流太弱，会削弱喷雾射流对燃烧室底部空气的卷吸，降低燃烧室底部的空气利用率．随涡流比增大，射流顺涡流方向的弯曲度增大，不同喷孔的油束会发生相互干涉，在靠近气缸中心的区域内形成局部浓混合气，不利于降低碳烟排放．对具体的燃烧室结构和喷油系统，合理匹配涡流运动十分必要．     

OPTIMIZED DESIGN OF A NEW GASOLINE DIRECT SWIRL INJECTOR

The internal flow characteristics of a gasoline direct injector (GDI) have been studied to improve fuel economy and reduce exhaust emissions. Computational fluid dynamics (CFD) is used to examine the internal flow of the GDI with the purpose of designing the optimum geometry of the injector. The design parameters include orifice length, cone angle, swirl angle, orifice diameter, and needle lift. Numerical results show that optimum sizes of the orifice length, cone angle, swirl angle, orifice diameter, and needle lift are 0.8 mm, 140°, 120°, 0.8 mm, and 70 m, respectively. The size of the lift does not affect the formation of the air core significantly near the tip of the needle compared with the ball-type needle. The vena contracta phenomenon near the orifice inlet can be released by smoothing the edge.     

Numerical study on the effect of jet spacing on the Swirl flow and heat transfer in the turbine airfoil leading edge region

ABSTRACT

In this paper, flow and heat transfer of a swirl chamber that models an internal cooling passage for a gas turbine airfoil leading edge is studied with numerical simulation. The geometry consists of a circular pipe, and rectangular section inlets that lead inlet flow to impinge tangentially on the circular pipe. The effects of the ratio of jet spacing to swirl chamber radius and Reynolds numbers on swirl cooling performance are investigated. The results indicate how the pressure loss and globally averaged Nusselt number on the swirl chamber wall increase with increases of Reynolds number and the ratio of jet spacing to swirl chamber radius. A Nusselt number correlation on these parameters is suggested. Also shown is how Nusselt numbers on the swirl chamber surface increase with the ratio of jet spacing to swirl chamber radius.     

火焰筒扩张角与旋流匹配对燃烧特性的影响

为研究旋流器流量分配对干式低排放(Dry Low Emission,DLE)燃烧室燃烧特性的影响规律,针对单头部中心分级旋流燃烧室,以天然气作为燃料,在保持旋流数不变的前提下开展两级旋流器不同空气分配比例下的试验测试和数值模拟,获得不同结构参数条件下燃烧室的综合燃烧性能以及污染物排放等变化规律。研究表明：随主燃级/预燃级旋流器流量比增大,燃烧室中心回流区变小、回流区长度变短;预燃级局部当量比的增大造成燃烧室出口CO排放增加,主燃区燃烧加剧,热力型NOx排放也增加;同时,燃烧室中心高温区域向燃烧室出口方向扩张,出口温度分布均匀性变差。     

缸内空气运动对直喷式柴油机喷雾特性影响的研究

本文采用高速摄影，利用激光衰减法，研究了小型直喷式柴油机缸内燃油雾特民生，特别是在缸内空气运动对燃油喷雾特性影响方面，进行了比较详细的研究，研究结果表明，在柴油机狭小的燃烧室内，自由喷雾得不到充分发展便得快与燃烧室壁面发生碰撞，故油束的锥角比较小，燃油的密集程度比较高，受空气涡流吹拂而发生弯曲的程度很小，较强的空气涡流会降低喷雾的贯穿速度。     

涡流燃烧室碰撞喷雾模拟分析

应用FIRE软件,对一种吊钟型涡流室式柴油机的压缩、喷雾过程进行模拟,考察气流运动、喷雾方向对混合气形成、分布、扩散过程的影响.对于喷雾过程,主要通过缸内的燃空当量比分析了燃油的分布情况;由油滴在涡流室内的粒径云图观察喷雾的形态.模拟结果表明:涡流室内强烈的气流运动对燃油的雾化作用很大,由于气流的阻尼作用,油滴不断破碎...     

An Investigation on the Effect of Air Swirler Vane Angle on Liquid Fuel Combustion Characteristics

This is an experimental and numerical study on the effect of air swirl vane angle on combustion characteristics of liquid fuel burners. The swirl vane angle varied in a range from 0° to 75° and the values of the dependent variables were determined. The flame temperature was measured by an S‐type thermocouple and a Testo 350 XL gas analyzer was used to determine the NO and CO pollutant concentrations. Also, sprint CFD code along with suitable models was used in analytical modeling. The results indicate that there is an optimum angle for the swirl vane (approximately 45° for the case study). At the optimum angle, the average temperature of the flame increases as much as 12.5% and 28.5% in comparison with small and large angles, respectively. Therefore, combustion efficiency reaches its maximum level and CO emission is at an extremely low level. The results also demonstrate that large swirl angles decreases NO emission.     

基于VOF-DPM模型的离心喷嘴中的燃油流动及雾化特征研究

针对燃油在离心喷嘴中的内部流动及其外部雾化过程,采用VOF-DPM模型对其进行了数值模拟研究。分析了压力对喷嘴出口处空气芯大小和液膜厚度的影响,得到了液膜破碎长度和雾化锥角等雾化特性,应用实验测试结果对数值模拟进行了验证,并与流体体积函数法(VOF)和离散相追踪法(DPM)进行了对比。结果表明：VOF-DPM模型可以真实反映离心喷嘴的内部流动和外部雾化特性,研究发现了与实际雾化过程符合的液膜破碎存在孔洞破碎和边缘破碎两种形式;捕捉到了在液膜表面的波动及气动力共同作用下液膜失稳破碎形成液滴的过程;燃油流动及雾化特性随着压力增加发生变化,喷嘴内空气芯直径增大,出口处液膜厚度减小,液膜的破碎长度下降。     

Experimental investigation of producer gas burner for thermal application

This paper deals with the performance evaluation of a premixed-type burner with producer gas, in terms of emission, axial and radial flame temperature. In this study, a burner of 150 kWth capacity was tested on an open core downdraft-type gasifier. The developed burner is a concentric tube-type where the air is supplied through a central tube, which is surrounded by another one. The burner consists of a swirl vane for mixing the air and producer gas, mixing tube and bluff body for flame stabilization. Swirl angle and bluff body diameter were kept constant throughout the study. The burner was evaluated with an open core downdraft-type gasifier. The temperature evaluation and emission testing was done for three flow rates and air–fuel ratio. The study shows low NO _x and CO emission at 125 Nm³ h^?1 when compared with that of 75 and 100 Nm³ h^?1. Maximum flame temperature (753 °C) was recorded at 10 cm axial and 10 mm radial distance.     

柴油机涡流室内空气流动特性的LDA测试及数学模型

本报导了采用激光多普勒测速仪（ＬＤＡ）首次在实机上对柴油机涡流室内空气运动规律测试的研究结果。研究表明：涡流室内涡流在一定的半径范围内是刚体涡流，在涡流室周边区域可近似看作势涡流。     

Effects of Variable Jet Nozzle Angles on Cross-Flow Suppression and Heat Transfer Enhancement of Swirl Chamber

This paper investigated the effects of variable jetting nozzle angles on the cross-flow suppression and heat transfer enhancement of swirl cooling in gas turbine leading edge. The swirl chamber with vertical jet nozzles was set as the baseline, and its flow fields and heat transfer characteristics were analyzed by 3D steady state Reynolds-averaged numerical methods to reveal the mechanism of cross-flow weakening the downstream jets and heat transfer. On this basis, the flow structure on different cross sections and heat transfer characteristics of swirl chamber with variable jetting nozzle angels were compared with the baseline swirl chamber. The results indicated that for the baseline swirl chamber the circumferential velocity gradually decreased and the axial velocity gradually increased, and the cross-flow gradually formed. The cross-flow deflected the downstream jets and drawn them to the center of the chamber, thus weakening the heat transfer. For swirl chamber with variable jetting nozzle angles, the air axial velocity is axial upstream, opposite to the mainstream, so that the impact effects of cross-flow on the jets were reduced, and the heat transfer was enhanced. Furthermore, with the increase of axial velocity along the swirl chamber, the jetting nozzle angle also gradually increased, as well as the effect of cross-flow suppression, which formed a relative balance. For all swirl chambers with variable jet nozzle angles, the thermal performance factors were all larger than 1, which indicated the heat transfer was enhanced with less friction increment.     

旋流喷嘴缩放型出口的流动特性

利用VOF(volume of fluid)方法对旋流式喷嘴气液两相流动进行了数值模拟.分析了缩放型出口结构功能和其内的流场特征.结果表明,喷嘴出口处液膜厚度和速度分布存在周向不均匀性.旋流室与出口段的结构连接方式较充分地利用了旋流室的加速功能,但也会造成流场方向强烈扭曲.随入口压力增大,出口液体的径向、切向和轴向速度均增大,而液体速度方向变化微小.收缩段和喉部上部对液体有加速作用;喉部能较平滑的引流;扩张段损耗了液体速度,但足够长的扩张段有控制液体喷淋方向作用.     

中心射流对旋流燃烧器热声振动的影响

在CH4与空气的化学当量比为0.7、空气量为25 m/s时进行了旋流燃烧器热声振动试验.分析了不同中心风量下旋流燃烧的热声不稳定特性.结果表明:在中心风量为10%时,旋流核心区明显受到中心射流的影响,热声振动振幅明显下降,最大峰值脉动压力下降216 Pa;中心射流对热声振动的影响存在一个阈值,提高射流速度可使中心射流作用于旋流核心区,从而影响燃烧与声波的耦合,以实现对热声振动的被动控制.     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

An experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10067     

Mass Transfer and Particle Separation by Swirl-Chamber and Swirl-Tube Devices

The goal of this investigation is to formulate, implement, and definitively validate a numerical simulation model to predict particle separation and concomitant mass transfer by means of a swirl chamber or swirl tube. The separation model encompassed three-dimensional turbulent fluid flow abetted with a particle transport model. All components of the model were used in their native form, without tuning or adaptation to accommodate the problem in question. Experiments were performed which were a precise replica of the numerical simulations in order to provide an unequivocal basis for evaluating the efficacy and validity of the simulation model. The experiments involved air flow as the carrier fluid. The particle-laden flow was ducted through a straight pipe to a three-bladed swirl generator from which the emergent swirling flow entered a settling chamber where the centrifugal forces propelled the particles toward the chamber walls. At the downstream end of the chamber, provision was made for the wall-adjacent particles to be collected. The non-collected particles exited through a central aperture. The investigation was performed for air velocities between 5 and 36 m/s. The key results that were extracted from both the numerical simulations and the experiments are the collection efficiency and the pressure drop. Comparison of the experimental and simulation results revealed very good agreement. This outcome lends unequivocal support to the numerical simulation model as a predictive tool for particle separations in swirl-chamber or swirl-tube devices.     

The study of key design parameters effects on the vortex tube performance

In this paper, energy separation effect in a vortex tube has been investigated using a CFD model. The numerical simulation has been done due to the complex structure of flow. The governing equations have been solved by FLUENT code in 2D and 3D compressible and turbulent model. The effects of geometrical and thermo-physical parameters have been investigated. The results have shown that the optimum length to diameter ratio is from 25 to 35. Increasing the number of nozzles from 2 to 4 with convergent shape is found to be an efficient configuration for the swirl generator. The optimum value of orifice diameter to tube diameter ratio, for the maximum cold air temperature difference and efficiency, has been determined to be around 0.58. The results show that if the inlet pressure increases up to a critical value, the efficiency will increase. Nevertheless, if it increases to higher values, the efficiency will decrease. Moreover, it is found out that increasing the cold fraction decreases the cold temperature difference and efficiency.     

Analysis on lean blowout of swirl cup combustor at atmospheric pressure condition

The experimental data of lean blowout fuel/air ratio of a rectangular swirl cup combustor with different inlet temperatures was obtained at atmospheric pressure condition. Numerical simulations both burning and non-burning were performed corresponding to the experimental data at lean blowout. Results indicated that the size of the recirculation region in the primary zone was obviously smaller when burning than non-burning, but the locations of the cores of their recirculation regions were almost the same. The increase of inlet air temperature didn??t mean the rise of the temperature of recirculation region core. The location of the maximum temperature in the primary zone was not the same as that one of the core temperature of the recirculation region. Further more, the reasons were analyzed how the lean blowout fuel/air ratio changed with the inlet temperature increasing under the actions of factors both positive and negative to combustion, and this would be helpful to deepen the understanding of the lean blowout process of swirl cup combustor.     

生物乙醇喷雾蒸发和预混过程的数值模拟研究

基于生物乙醇燃料的贫燃预混、预蒸发燃烧技术(Lean Premixed Pervaporation,LPP),采用数值模拟方法,研究了预混室内生物乙醇雾化蒸发流场,分析了预热空气温度为500、600和1 000 K以及旋流数为0.47、0.8和1.41时的生物乙醇蒸发和气体混合特性的规律。研究表明:在LPP预混室旋流流场中,中心回流区宽度随预混室距离的增加先增大后减小,并且会受喷雾射流的影响拉伸变长,中心回流区随旋流强度的增大更贴近喷雾出口,角回流区的长度随旋流强度增大而缩短直至消失,旋流强度对液雾整体蒸发速率影响不大,但会影响液雾分布;进气温度增加会增大进气速度,提高液滴蒸发速率,缩短液雾炬长度;液滴蒸发过程存在一定程度上的压力振荡,会对LPP不稳定燃烧过程产生一定影响。     

进气涡流比对直喷式柴油机油束碰壁过程影响的研究

本采用高速摄影技术，研究了小型直喷式柴油机缸内空气运动对油束碰壁过程的影响。研究结果表明，在小型直喷式柴油机中，燃油壁面喷射的反溅作用是燃油雾化过程中的重要阶段。油束在碰壁过程中，其锥角及贯穿速度均发生变化。不同的进气涡流强度，壁面油束的形状及其发展速度均不同，顺涡流方向壁面油束的扩展速度较快，随着涡流强度的增加，壁面油束只出现在顺涡流方向。空气涡流对燃油与空气混合的促进作用主要发生在油束与燃烧     

Research on a Refrigeration Dehumidification System with Membrane-Based Total Heat Recovery

An independent air dehumidification system is helpful to improve indoor air quality and decrease energy consumption by heating, ventilation, and air conditioning (HVAC). A refrigeration dehumidification system with membrane-based total heat recovery is the key equipment to realize this goal. The system comprises two subsystems: a membrane total heat recovery and a direct expansion refrigeration system. The total heat exchanger has a membrane core where the incoming fresh air exchanges moisture and temperature simultaneously with the exhaust air. In this manner, the total heat or enthalpy from the exhaust air is recovered. Then the fresh air flows through a cooling coil where it is dehumidified below the dewpoint. Finally, the cold and dry air is supplied to indoors. A prototype of practical application is designed and fabricated. Experiments are conducted under variable operating conditions in the psychrometric calorimeter chamber. The effects of varying operating conditions like temperature and air humidity on the air dehumidification rate, cooling power, coefficient of performance, and compressor power are evaluated with indoor exhaust air dry bulb 27°C, wet bulb 19°C, and fresh air flow rate 200 m³/h. In comparison with a conventional refrigeration dehumidification system, the coefficient of performance and air dehumidification rate of the prototype are 2.3 times and 3 times higher, respectively. The performance of the prototype is rather robust under a hot and humid environment.