半封闭狭窄通道内甲烷/空气预混火焰传播不稳定性实验研究

针对贫油预混预蒸发燃烧室主燃级中横喷液雾现象进行研究,综合考虑RP-3航空煤油横喷液雾的雾化、蒸发和自燃过程构建自燃预测模型,基于CH基团随时间的变化规律对自燃延迟时间进行预测。结合试验测试结果对模型进行校验,并进一步分析温度、压力、流速、射流动量比等变量对自燃延迟时间的影响规律。结果表明：对于直射式喷嘴形成的横喷液雾,其下游的油气分布主要受射流动量比和流动速度的影响,射流动量比决定了液雾的总体油气比,流动速度则主要影响液滴的粒径及其蒸发时间;随着压力、射流动量比及气流速度的增加,自燃延迟时间均会缩短,相比于预混燃料液雾的自燃延迟时间受负温度效应的影响较弱。     

燃气轮机燃烧室液雾自燃延迟时间预测方法

为减少一体化加力燃烧室内支板火焰稳定器高度与进口试验参数较高所导致的昂贵基础试验成本,采用经试验数据验证的数值计算方法,对不同高度的一体化模型加力燃烧室燃烧性能进行数值模拟,分析模型加力燃烧室高度变化和侧壁边界层效应对一体化加力燃烧室回流区、总压恢复系数以及燃烧效率的影响。在保持空间油雾场分布均匀与阻塞比一致的前提下,简化扇形加力燃烧室模型为矩形加力燃烧室模型,其中模型加力燃烧室高度H分别为200,150和100 mm,总长L=1 480 mm,宽B=125 mm。结果表明：模型加力燃烧室高度的降低对燃烧性能影响较小,其中回流率最大降幅为0.16%,总压恢复系数最大降幅为0.15%,燃烧效率的最大降幅为1.9%;模型加力燃烧室侧壁面边界的引入对燃烧性能影响较小,回流率、总压恢复系数最大降幅均小于1%,燃烧效率的最大降幅仅为0.7%;可以采用单支板火焰稳定装置降低高度的方法简化试验件设计。     

Measurements and numerical simulations for optimization of the combustion process in a utility boiler

A three‐dimensional computational fluid dynamics code was used to analyse the performance of 550MW pulverized coal combustion opposite a wall‐fired boiler (of IEC) at different operation modes. The main objective of this study was to prove that connecting plant measurements with three‐dimensional furnace modelling is a cost‐effective method for design, optimization and problem solving in power plant operation. Heat flux results from calculations were compared with measurements in the boiler and showed good agreement. Consequently, the code was used to study hydrodynamic aspects of air–flue gases mixing in the upper part of the boiler. It was demonstrated that effective mixing between flue gases and overfire air is of essential importance for CO reburning. From our complementary experimental‐numerical effort, IEC considers a possibility to improve the boiler performance by replacing the existing OFA nozzles by those with higher penetration depth of the air jets, with the aim to ensure proper mixing to achieve better CO reburning. Copyright © 2004 John Wiley & Sons, Ltd.     

Study on main flow and fuel injector configurations for Scramjet applications

A parametric study of combustor inlet configuration for supersonic combustion ramjet (Scramjet) engine has been conducted by solving two-dimensional full Navier–Stokes equations. The main stream is air of Mach 5 entering through the configured inlet of the combustor and gaseous hydrogen is injected from the configured jet on the side wall. The parameters included are air stream angle and injection angle. On the effect of air stream angle, strong interaction between main and injecting flows can be observed for smaller angle causing sharp increase in mixing efficiency on the top of injector. Also high momentum of air stream towards the side wall causes no recirculation at the upstream of injector and the system becomes unable for flame holding. For the variation of injection angle, results show that in upstream of injector the mixing is dominated by recirculation and in downstream the mixing is dominated by mass concentration of hydrogen. Upstream recirculation is dominant for injecting angle 60° and 90°. Incorporating the various effects, perpendicular injection shows the maximum mixing efficiency and its large upstream recirculation region has a good flame holding capability.     

Characteristics and mixing enhancement of a self-throttling system in a supersonic flow with transverse injections

A three-dimensional self-throttling system is proposed in a scramjet combustor with transverse fuel jet, and investigated by Reynolds-averaged Navier-Stokes (RANS) simulations with the k-ω SST turbulence model. Numerical validation has been carried out against experiment and LES results. The effects of the jet-to-cross-flow momentum flux ratio and the throttling angle on mixing performance, fuel jet penetration depth and total pressure losses are all addressed. Through the proposed throttling system, the higher pressure upstream of the transverse fuel injection can drive part of the low momentum mainstream air into the downstream lower pressure region. The flow structures and the interactions between the shock waves and boundary layer are significantly changed to improve the mixing performance. The enhancement of mixing efficiency in the self-throttling system is closely related to the magnitude of the jet to crossflow momentum flux ratio, and a smaller throttling angle is found to further improve the mixing. On the other hand, the self-throttling system has a good performance in reducing the total pressure losses.     

Flow and impingement cooling heat transfer along triangular rib-roughened walls

Experiments are performed to study slot air jet impingement cooling flow and the heat transfer along triangular rib-roughened walls. Both flow visualization and local heat transfer measurements along the ribbed wall are made. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 2 and 4, respectively. The widely opened cavity between neighboring ribs make more intense transport of momentum between the wall jet and cavity flow so that recirculation cell in the cavity is hardly observed. This leads to a higher heat transfer around the cavity wall than in the case with rectangular ribs. However, in the region of laminar wall jet, a number of air bubbles enclosing the cavities are formed which prevent penetration of the wall jet into the cavities. This leads to a significant reduction in the heat transfer. The geometric shape of the triangular ribs is more effective in rebounding the wall jet away from the wall than in the case with rectangular ribs. The rebound of the jet away from the wall causes a significant reduction in the heat transfer. A comparison and correlations of the stagnating point Nusselt number under different conditions are presented and discussed. During the experiments, the Reynolds number varies from 2500 to 11,000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16.     

Effect of hybrid coaxial air and hydrogen jets on fuel mixing at supersonic crossflow

In this research study, a computational method is applied to examine the impacts of coaxial hybrid air and fuel jets on fuel mixing at the supersonic cross-flow of Mach = 4. This study examined the coaxial air and fuel jet effects on main parameters i. e. circulation, mixing efficiency, and fuel penetration. Computational Fluid Dynamic is employed for the modelling of the coaxial jet at cross supersonic flow. Reynolds Average Navier-Stocks equations with SST turbulence model for achieving hydrodynamic feature of the main model. Impacts of air-jet pressure and nozzle configurations on fuel distribution are also presented and the main effective factors for efficient fuel mixing condition are explained. Our results disclosed that injection of coaxial air and fuel jets at supersonic cross airflow significantly improves the fuel penetration and mixing inside the combustion chamber. Flow study analysis shows that the coaxial injector augments the spiral feature of the fuel jet, which surges fuel mixing downstream. Our circulation analysis confirms that circulation strength increases in far away from an injector by the injection of a coaxial air jet.     

Numerical investigation of spray combustion in jet mixing type combustor for low NOx emission

The present paper describes a numerical investigation of spray combustion in a jet mixing type combustor. In this combustor, kerosene spray was injected with a pressure atomizer, and high speed combustion air was introduced towards the spray flow through some inlet air nozzles to improve mixing of the spray and the air. In the numerical simulation, the conservative equations of mass, momentum and energy in the turbulent flow field were solved in conjunction with the k–ε two equation turbulence model. The effects of the diameter and the number of air inlet nozzles on the combustion behavior and NO emission were numerically investigated. When the diameter of the inlet air nozzle decreased from 8 to 4 mm, the calculated NO mole fraction in the exhaust gas was drastically decreased by about 80%. An increase in the inlet velocity resulted in improvement of the mixing of the spray and the air, and hence, the high temperature region where thermal NO was formed became narrow. As a result, the exhaust NO mole fraction decreased. Furthermore, a decrease in exhaust NO mole fraction was explained by a decrease in the residence time in the high temperature region above 1800 K.     

Aerodynamic characteristics of a 350-MWe supercritical utility boiler with multi-injection and multi-staging: Effects of the inner and outer secondary air distribution in the burner

Prior to commercial operation of the first-ever 350-MWe supercritical down-fired boiler incorporating multiple injection and multiple staging combustion technology, cold modeling experiments varying the inner-to-outer-secondary air (ISA/OSA) ratio (applying ratios of 0:10, 2:8, 5:5, 6:4 and 10:0) were performed within a 1:13 scale model of the boiler furnace. The aim of these trials was to establish an optimal inner and outer secondary air distribution model for the burner in the full-scale furnace. At ratios of 0:10 and 2:8, the fuel-rich flow apparently diffused toward the furnace center zone after leaving the corresponding nozzle outlet, resulting in the formation of relatively small recirculation zones below the arches and independent fuel-rich and fuel-lean flow streamlines. In contrast, at ISA/OSA ratios greater than or equal to 5:5, the strengthened carrying effect of the inner secondary air significantly reduced the diffusion of the fuel-rich flow. Consequently, the fuel-rich and fuel-lean flow streamlines combined in the middle region of the lower furnace and the recirculation zones under the arches enlarged. As the ISA/OSA ratio was increased from 0:10 to 10:0, the line fitting slopes for the downward airflow decay curves increased from 1.8 to 3.0, meaning that the downward airflow decayed more rapidly, such that the penetration depth was reduced. Finally, at ISA/OSA ratios of 0:10 and 2:8, the initial flow-field deflection was observed to gradually adopt a symmetrical pattern.     

大流量单混合孔Y型喷嘴的雾化特性

对大流量单混合孔Y型喷嘴的雾化性能进行了实验研究,分析了其流量特性以及气耗率对雾化粒径的影响.结果表明,单混合孔Y型喷嘴设计流量能够达到1000kg/h以上,且具有较细的雾化粒径;在气压一定时,随着水压的增大其水流量增大,气耗率减小;气耗率对雾化粒径的影响较明显,但当粒径减小到一定程度后,继续增大气耗率对雾化粒径的影响不明显;单水孔与多水孔Y型喷嘴的雾化性能无明显差别;改进的Y型喷嘴液膜随机破碎模型可较好地用于大流量单混合孔Y型喷嘴雾化粒径的预报.     

MODELING OF JET-GENERATED RECIRCULATING FLOWS IN A MINIATURE COMBUSTOR

The complicated flow fields in miniature annular combustors are investigated numerically. The flow parameters studied include the jet tube length and momentum ratio of the primary jet. From the numerical results it is found that four different recirculations, namely, the primary, second, third, and dilution recirculations, might be generated in the combustor owing to the interaction between the mainstream and the lateral jets. In all cases studied, the primary recirculation can be observed, although its strength, size, and location are changed with the flow parameters. The jet tube length produces a significant effect on the recirculation flow structure in the primary zone. The increase of the momentum ratio of the primary jet results in the suppression and enhancement of the primary and second recirculations, respectively.     

Mechanism of fuel mixing of hydrogen multi-jet in presence of upstream convergent/divergent ramp at supersonic flow

The enactment of the fuel mixing structure is crucial for the advance of supersonic vehicles. All-inclusive efforts have been done to disclose the impacts of different parameters on instrument of the fuel combination with air within the combustion chamber. In the present work, comprehensive computational investigations have been done to explore the importance of oblique ramp upstream on the fuel mixing process of hydrogen multi-jets at supersonic cross airflow. The primary attention of the current study is to compare the role of interaction of air and fuel by the existence of an oblique ramp upstream of four cross jets. Flow analysis is also done to unveil the main difference of convergent and divergence ramps located upstream of each injector. For simulation of the proposed models, Computational Fluid Dynamics (CFD) is employed to resolve RANS equations with the SST turbulence model in high-speed free stream. The main significant factors i.e. mixing efficiency and circulation factor are also compared in our work for comparison of the flow parameters and mixing concepts. According to our investigations, the presence of the upstream oblique jet meaningfully enhances the fuel mixing as flow moves downstream of injectors. The outcomes also showed that productivity of the divergent ramp is higher than that of the convergent one due to high jet diffusion in the depth of the domain by the creation of a strong horseshoe vortex.     

考虑冷气掺混的涡轮S_2流面计算方法

本文在S_2流面流函数线松弛解法的基础上提出了考虑冷气掺混的涡轮气动计算方法,阐述该方法对直接计入质量和能量掺混而间接计入动量掺混的影响,通过一个算例证实了该方法的可行性和必要性。     

Optimization of ammonia injection grid in hybrid selective non-catalyst reduction and selective catalyst reduction system to achieve ultra-low NOx emissions

The uniformity of NH₃ in the flue gas is the main factor improving the efficiency during the hybrid selective non-catalytic reduction (SNCR)-selective catalytic reduction (SCR) denitrification process. This work presents the optimized ammonia injection grid (AIG) to enhance the mixing performance and optimize concentration uniformity. A testing ammonia injected facility with a scale of 1:1 is established to experimentally simulate the NH₃ mixing behavior in the tail flue gas duct employing the tracer gas. A novel injection grid with multiple inclined nozzles is proposed to replace the conventional direct-injection type. The flow field and tracer gas concentration field are studied under the three conditions with various inclined jet angles including 0°, 30° and 45°. From results, mixing enhancement between jet and crossflow for the inclined injection grid is achieved. It is also indicated that the inclined angle plays an important effect on jet rigidity and tracer gas dilution. The flow field and tracer gas mixing performances are quantitatively analyzed by using the root mean square deviation coefficient. Multiple jets with an inclined angle of 30° achieved the best mixing performance. Consequently, optimized AIG design has shown effective NO_x control performance in industrial utility furnace.     

Evaluation of a jet plate solar air heater

To achieve higher heat transfer from the absorber plate to the flowing air stream with an intention to increase the amount of the collected energy, and hence, to improve the efficiency of an air-based solar collector, a unique jet impingement concept has been advanced for evaluation in the present study. To investigate the effects of various geometrical parameters such as the ‘hole’ or ‘nozzle’ diameter on the jet plate, their interspacings, the nozzle height, the distance between the absorber and the jet plate and the operational parameter such as the velocity of air impinging out of the holes/nozzles on to the back side of the absorber surface on the performance parameters of the jet impingement concept air heater, a detailed theoretical parametric analysis has been made on the design for different mass flow rates of air and different lengths of air channel. A parallel study has also been carried out on a conventional parallel plate air heater in order to compare its air temperature increment and performance efficiency with those of the jet plate air heater. The gain in air temperature increment and performance efficiency of the jet-concept air heater over that of the parallel plate air heater with duct depth 10 cm and length 2 m is 15.5°C to 2.5°C and 26.5% to 19%, respectively, for air flow rates in the range 50 to 250 kg/hm².     

应用激光多普勒测速仪研究湍流有旋自由射流

用激光多普勒测速仪对旋流器产生的强湍流有自由射流的速度场和湍流场进行了实验研究。实验结果表明,这种旋流器十分有利于中心回流区的形成,射流的平均速度分布沿轴向快速地衰减,有旋自由射流呈现湍流各向异性,特别是在回流区域中,本文对流场中湍流动量传递的方向进行了讨论。     

天然气柔和燃烧锅炉结构参数影响模拟研究

在天然气锅炉中引入柔和燃烧技术将大大降低NOx排放,高速未燃气卷吸高温烟气回流并与之快速掺混再燃烧是柔和燃烧的重要特征,因此,开展天然气锅炉关键结构参数优化设计以组织流场形成柔和燃烧所需的高温低氧反应气氛非常必要。基于天然气锅炉的工况特征,设计了热负荷15kW的模型燃烧室,采用数值模拟手段详细研究了燃烧室高度、喷嘴孔径、喷嘴相对位置及烟气出口尺寸对燃烧室流场、组分场及关键参数——烟气回流比的影响规律,并最终确定了燃烧室结构优选方案,对天然气锅炉柔和燃烧机设计提供理论基础数据。     

Effects of incident shock wave on mixing and flame holding of hydrogen in supersonic air flow

The effects of incident shock wave on mixing and flame holding of hydrogen in supersonic airflow have been studied numerically. The considered flow field was including of a sonic transverse hydrogen jet injected in a supersonic air stream. Under-expanded hydrogen jet was injected from a slot injector. Flow structure and fuel/air mixing mechanism were investigated numerically. Three-dimensional Navier–Stokes equations were solved along with SST k-ω turbulence model using OpenFOAM CFD toolbox. Impact of intersection point of incident shock and fuel jet on the flame stability was studied. According to the results, without oblique shock, mixing occurs at a low rate. When the intersection of incident shock and the lower surface is at upstream of the injection slot; no significant change occurs in the structure of the flow field at downstream. However when the intersection moves toward downstream of injection slot; dimensions of the recirculation zone and hydrogen-air mixing rate increase simultaneously. Consequently, an enhanced mixing zone occurs downstream of the injection slot which leads to flame-holding.     

Influence of primary air ratio on flow and combustion characteristics and NOx emissions of a new swirl coal burner

Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MW_e wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O₂ and NO_x concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly.     

Gas entrainment in an evaporating spray jet

Gas entrainment induced by a spray jet can be significantly affected by the spray evaporation rate. In this study, we have directly measured the air entrainment induced by a liquid nitrogen spray jet into an unbounded and stagnant room air. It is realized that the air entrainment is proportional to the axial gradient of oxygen mass flow in a pure nitrogen spray jet. Hence, the air entrainment can be determined by a combined measurement of local cross-sectional distributions of oxygen concentration, gas temperature and gas velocity along the jet path. These measurements are directly obtained using an in situ oxygen concentration analyzer, a thermocouple system, and a Laser Doppler Velocimeter. In order to evaluate the effect of evaporation rate, direct measurements and numerical simulations of the air entrainment by a cold gaseous jet of nitrogen (at a temperature slightly above that of liquid nitrogen) into room air are also performed. Measurements of the entrainment rate and flow similarity of the gaseous jets without droplets compared very well against those from the single-phase jet theories and numerical simulation, which validates our experimental approach and analysis method. Our experimental results indicate rough flow similarities exist in evaporating spray jets with round nozzles. Although the air entrainment by the liquid nitrogen spray is found significantly increased, as compared to that by the cold gaseous jet of nitrogen from the same nozzle and at the same jetting velocity, the increased ratio is far less than the equivalent momentum ratio of the liquid nitrogen spray to the gas nitrogen jet. This experimental finding suggests that the evaporation of spray markedly weakens the gas entrainment. In this study, a parametric model is also developed to provide a theoretical basis of the data analysis for the cross-section averaged spray evaporation rate within the spray jet region.