Measurement of opposing heated line jets discharged at an angle to a confined crossflow

An experimental study is presented for the discharging of opposing heated line jets at an angle to a confined crossflow. Parametric variations characterizing the mixing processes such as the velocity and temperature trajectories, and the circulation length, are correlated in terms of the momentum flux ratio, the incident angle, and the downstream distance. Measurements show that the turbulence level is high within the region where the mean velocity gradient is steep, and the vertical temperature profiles can be expressed in self-similar forms. Better thermal mixing can be achieved at higher momentum flux ratio and incident angle.     

Design consideration for cross jet air mixing in municipal solid waste incinerators

In mass-burning municipal solid waste incinerators, overfire air injection plays a key role in the improvement of mixing and reaction between oxygen and incomplete combustion products and/or pollutants. However, the design parameters of overfire air nozzles are not well understood and sometimes confusing. In this paper, major design parameters concerning cross jet air nozzles are discussed along with flow simulation results for simplified furnace geometry. The overall performance of jet air mixing and the effects of design parameters are quantitatively evaluated. The flow simulation results are interpreted in terms of the penetration depth of the jet into the main flow, the size of the recirculation zone and the ratio of the unmixed portion of the gas flow. The momentum flux ratio J of the jet to the cross flow strongly affects the penetration depth of the jet and the mixing of two flow streams. As the inter-nozzle distance S (in non-dimensional form) decreases, the penetration depth decreases but the size of the recirculation zone increases and the resultant mixing deteriorates. The degree of mixing of the jet with the cross gas stream is evaluated in terms of the mass-averaged probability distribution of the relative concentration. Fresh air disperses more efficiently into the gas stream as J and S increase. The momentum flux ratio and the inter-nozzle distance are considered as important design parameters, and optimum values of these variables can be chosen for the given furnace conditions. This numerical evaluation also provides a basis for similarity considerations in cold flow model tests and the validity of the two-dimensional idealization. © 1997 by John Wiley & Sons, Ltd.     

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.     

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。     

A COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION OF TURBULENT JET AND CROSSFLOW INTERACTION

The flowfield induced by a single circular jet exhausting perpendicularly from a flat plate into a crossflow has been investigated numerically. The flow regime investigated corresponds to that encountered in a modern gas-turbine combustor. Reynolds-averaged solutions were obtained using a pressure-based Navier-Stokes solver. The standard k -epsilon turbulence model with and without nonequilibrium modification was employed. Two different momentum flux ratios, J, between the jet and the free stream are investigated, namely, J = 34.2 and J = 42.2. To aid the evaluation of the computational capability, experimental information also has been obtained, including mean and root-mean-square (RMS) velocity distributions downstream of the jet, and the detailed velocity profile at the jet exit. An evaluation of the different convection schemes reveals that the second-order upwind scheme does a noticeably better job than the first-order scheme to predict the velocity profile at the jet exit while predicting less mixing than the experimental measurement during the jet and free stream interaction. It appears that turbulence modeling primarily is responsible for the deficiency the accounting for the physics of the jet and free stream interaction.     

Non-Boussinesq turbulent buoyant jet resulting from hydrogen leakage in air

This paper is devoted to introduce a numerical investigation of a vertical axisymmetric non-Boussinesq buoyant jet resulting from hydrogen leakage in air as an example of injecting a low-density gas jet into high-density ambient. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the binary gas mixture is assumed to be of a linear mixing type. Also, it is assumed that the rate of entrainment to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. On the other hand, the local rate of entrainment may be considered to be consisted from two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Firstly, the integral models of the mass, momentum and concentration fluxes are obtained and transformed to a set of ordinary differential equations using some non-dimensional transformations known as similarity transformations. The given ordinary differential system is integrated numerically and the mean centerline mass fraction, jet width and mean centerline velocity are obtained. In the second step, the mean axial velocity, mean concentration and mean density of the jet are obtained. Finally in the third step of this article, several quantities of interest, including the cross-stream velocity, Reynolds stress, velocity-concentration correlation (radial flux), turbulent eddy viscosity and turbulent eddy diffusivity, are obtained. In addition, the turbulent Schmidt number is estimated and the normalized jet-feed material density and the normalized momentum flux density are correlated.     

Measurements of turbulent mass transport of a circular wall jet

The results of a laboratory investigation on the turbulence characteristics of a circular three-dimensional turbulent wall jet are presented. Measurements were taken up to 50 nozzle diameters using combined particle image velocimetry and planar laser induced fluorescence. The results showed that the induced turbulence was still evolving in the present range and had not achieved similarity. While the turbulent intensity for both velocity and concentration increased downstream, the turbulent mass transport showed a decline over distance for both the streamwise and spanwise directions, implying weakening dispersion from the jet core.     

Turbulent mixing and molecular transport in highly under-expanded hydrogen jets

Highly under-expanded hydrogen jets releasing in quiescent air atmosphere are studied using highly resolved numerical simulations accounting for complex multicomponent molecular transport phenomena. In a first step of the analysis, the main overall features of the hydrogen jet structure are described and compared to those of the classical under-expanded air jet at the same nozzle pressure ratio (NPR). Even if the global flow topology remains quite similar in both cases (i.e., hydrogen and air discharges), the modification of both mean density and mean velocity gradients leads to different relative energy levels for each velocity component. The corresponding change of fluid properties mainly leads to an enhanced mixing at the jet periphery. In comparison to the air case, the turbulence development within the internal part of the under-expanded hydrogen jet surrounding the subsonic core also yields a different structure. While a significantly higher peak of streamwise turbulent stress is observed downstream of the reflected shock, the vorticity dynamics is dampened by viscous diffusion and velocity divergence (i.e., volumetric expansion) contributions. Then, the performance of the simplified Hirschfelder and Curtiss approximation of the multicomponent molecular diffusion phenomena is evaluated with respect to the detailed multicomponent transport representation, as deduced from the EGLIB library. The detailed representation of molecular phenomena is shown to have a significant influence on the estimated local levels of hydrogen mass flux, leading to a non-negligible alteration of the global jet structure.     

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。     

Reynolds number effects within the development region of a turbulent round free jet

In this work, flying and stationary hot-wire measurements were made to investigate the effect of the Reynolds number on the near- and intermediate-fields region (0 ? x/D ? 25) of a round free jet. Measurements were carried out over a range of Reynolds numbers, based on the jet exit mean velocity and the nozzle diameter, that span the mixing transition. The specific Reynolds numbers tested were 6000, 10,000 and 30,000. The objective of this study was to determine what differences there were in mean velocity profiles, turbulence intensity profiles, and velocity spectra. Results revealed a close coupling between the mean velocity distribution and the turbulence intensities and the Reynolds shear stress. From those data obtained, it was concluded that the inertial sub-range frequency span increases with distance downstream from the jet inlet and the mixing transition seems to occur at the appearance of the inertial sub-range rather than at the transition from the inertial to dissipation range.     

LES analysis of flow and turbulent mixing in an unsteady jet

The flow and mixing process of unsteady jets are fundamentally analyzed by large eddy simulations. The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between the nozzle fluid and ambient fluid were investigated. The results show that a toroidal‐shaped vortex, which emerges around the jet tip, primarily accelerates the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages mixing in the jet without changing the jet‐contour. Furthermore, when the rise‐up time of the initial nozzle velocity is elongated, turbulent mixing is suppressed. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 303–313, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20158     

双平行平面射流输运特性研究

平行射流输运特性研究具有较重要的理论意义和工程价值,对水平浓淡煤粉燃烧也有较重要的意义。本文实验研究了双平行平面射流动量、能量和质量输运,结果表明：速度分布表现为双射流相互吸引并最终汇合成单一射流;温度分布反映了冷射流为加热射流所卷吸;浓度分布反映了惯性力的占优作用。三种变量分布反映了动量、能量和质量输运性质的根本性区别。     

Computational investigation of multi-cavity fuel injection on hydrogen mixing at supersonic combustion chamber

Enhancement of the mixing inside the combustor is a significant process for increasing the efficiency of the scramjet. This work applied the computational method for the investigation of the depth of the cavity on the flow feature of the multi hydrogen jet in the supersonic crossflow. The main focus of this research is to evaluate the depth of the cavity on the mixing rate of the hydrogen jets inside the combustion chamber. CFD method with the SST turbulence technique is applied for the simulation of the fluid flow inside the domain. The impact of the depth of the cavity, the pressure of the fuel jet and the number of the jet are comprehensively explained in this study. Our findings show that the rising of the cavity enhances the mixing inside the domain due to more fuel distribution along the spanwise direction. Our results clearly demonstrate that replacing the single jet with 8 equivalent multi jets increases the mixing rate of more than 45% in the vicinity of the jet injection. Attained results revealed that increasing the jet space develops the mixing in far downstream. Obtained results also show that mixing intensifies 15% when jet space of 8 microjets is increased from 4 dj to 10 dj.     

Numerical Analysis of Jet Impingement Heat Transfer at High Jet Reynolds Number and Large Temperature Difference

Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer was investigated at a jet Reynolds number of 1.66 × 10⁵ and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up to about 100% were observed. Furthermore, the variation in stagnation point heat transfer was examined for jet Reynolds numbers in the range from 1.10 × 10⁵ to 6.64 × 10⁵. Based on the investigations, a correlation is suggested between the stagnation point Nusselt number, the jet Reynolds number, and the turbulence intensity at the jet inlet for impinging jet flows at high jet Reynolds numbers.     

Interaction of Two Unequal Turbulent Jets with Different Temperature

This paper examines the control and heat transfer process of the mixing of a strong jet and a weak jet. The control of the jet diffusion may be required in many devices such as cooling systems, combustors, gas turbines, heating or cooling surfaces, cooling problem techniques, mixing flows, etc. A steady three‐dimensional problem is solved by the finite volume method using RANS modeling. The validation confirms that both RSM and k‐ε realizable models predict more accurately this flow configuration than the standard k‐ε model. A parametric study is conducted on the basis of ratios of the momentum of the two jets. Due to the Coanda effect, the deviation of the weak jet has been highlighted for several momentum ratios. In the fully developed flow region, after the confluence point, the structure of a single jet flow is recovered. However, the point of maximum velocity is shifted toward the side of the strong jet. For a given velocities ratio of 0.25, the temperatures ratio between the two jets is checked. The location of maximum temperature of the jet is almost constant for diverse values of 0.8 ≤ λ_T ≤ 1. The location of the point of maximum temperature varies linearly according to λ_T.     

Laser doppler velocimetry investigation of the turbulence structure of axisymmetric diffusion flames

Measurements of mean velocity components, turbulent intensities, velocity probability density functions, power spectra and autocorrelation functions of axial velocity fluctuation, and spatial turbulence macroscale, are reported in a turbulent round jet flow, issuing vertically into stagnant air, in non-combusting and combusting situations. The fuel density (a mixture of methane and argon) is chosen to be equal to the cold flow gas density (a mixture of air and helium) in order to minimize cold fuel/cold gas mixture density difference effects on measured turbulence properties. The objectives are to study the influence of the combustion process on the turbulence structure of the combustible jet flows considered, and to provide data against which results of numerical prediction methods for such flows embodying various turbulence and combustion models can be compared, with a view to improving our understanding of relevant transport processes and on guiding modelling and prediction efforts of such flows. A one-dimensional laser velocimeter operating in forward scatter differential Doppler mode was used to obtain the measurements. Gas temperatures were measured by thermocouples. A visual study by schlieren photography has also been conducted. It is found that the existence of the flame suppresses turbulence in the upstream region of the jet flow and enhances it in the downstream region, where turbulence intensities are substantially higher than in the corresponding cold jet flow. However, the relative intensities, i.e. the ratio of the local turbulent intensity to the local mean velocity, are smaller in the jet diffusion flame and become comparable to relative turbulent intensities found in the cold jet flow in the downstream region of the flow. Turbulence in the jet diffusion flame is appreciably more anisotropic than in the corresponding cold jet in all regions of the flow, suggesting the eventual desirability of multi-stress models of turbulence for the prediction of such flames. The combustion process has been found to have also a marked influence on the turbulence macroscale. It is significantly smaller than in the cold jet flow in the upstream region and increases appreciably at downstream distances, the rate of this increase closely following the rate of temperature increase. The experimental results obtained will guide the development of an improved prediction method for such combusting systems.     

Classification of turbulent jets in a T-junction area with a 90-deg bend upstream

Many nuclear power plants report high cycle thermal fatigue in their cooling system, which is caused by temperature fluctuation in a non-isothermal mixing area. One of these areas is the T-junction, in which fluids of various temperatures and velocities blend. The objective of this research is to classify the turbulent jet mechanics in order to examine the flow-field structure under various operating conditions. Furthermore, this research discovers the optimum operating conditions of the mixing tee in this piping system. An experimental model, including the T-junction with a 90-deg bend upstream, is operated to analyze this mixing phenomenon based on the real operation design of the Phenix reactor. The temperature and velocity data show that a 90-deg bend has a strong effect on the fluid mixing mechanism and the momentum ratio between the main velocity and the branch velocity of the T-junction, which could be an important parameter for the classification of the fluid mixing mechanism. By comparing their mean velocity distributions, velocity fluctuations, and time-series data, the behavior of the branch jet is categorized into four types of turbulent jets; sorted from the highest to the lowest momentum ratios, the jets are categorized as follows: the wall jet, the re-attached jet, the turn jet, and the impinging jet. Ultimately, the momentum ration of the turn jet was selected as the optimum operating condition because it has the lowest velocity and the lowest temperature fluctuations near the wall of the mixing tee.     

主支管动量比对T型管道流体混合过程影响

采用大涡模拟(large-eddy simulation,LES)的方法对T型管道内主管与支管不同动量比的流体混合过程的流动情况进行了数值模拟,采用时均值和均方根值来描述速度的平均大小和波动强度。通过改变主管和支管的速度比即动量比,将流体分为三类:碰撞射流、偏射流和壁面射流,研究其对速度的平均值和波动的影响,并研究其所反映的惯性力对流动的影响。该研究揭示了流体混合过程中动量比对波动的影响规律,对预测和校核管壁疲劳失效具有重要的指导意义。     

Liftoff of turbulent jet flames—assessment of edge flame and other concepts using cinema-PIV

Three theories of the liftoff of a turbulent jet flame were assessed using cinema-particle imaging velocimetry movies recorded at 8000 images/s. The images visualize the time histories of the eddies, the flame motion, the turbulence intensity, and streamline divergence. The first theory assumes that the flame base has a propagation speed that is controlled by the turbulence intensity. Results conflict with this idea; measured propagation speeds remains close to the laminar burning velocity and are not correlated with the turbulence levels. Even when the turbulence intensity increases by a factor of 3, there is no increase in the propagation speed. The second theory assumes that large eddies stabilize the flame; results also conflict with this idea since there is no significant correlation between propagation speed and the passage of large eddies. The data do support the “edge flame” concept. Even though the turbulence level and the mean velocity in the undisturbed jet are large (at jet Reynolds numbers of 4300 and 8500), the edge flame creates its own local low-velocity, low-turbulence-level region due to streamline divergence caused by heat release. The edge flame has two propagation velocities. The actual velocity of the flame base with respect to the disturbed local flow is found to be nearly equal to the laminar burning velocity; however, the effective propagation velocity of the entire edge flame with respect to the upstream (undisturbed) flow exceeds the laminar burning velocity. A simple model is proposed which simulates the divergence of the streamlines by considering the potential flow over a source. It predicts the well-established empirical formula for liftoff height, and it agrees with experiment in that the controlling factor is streamline divergence, and not turbulence intensity or large eddy passage. The results apply only to jet flames for Re<8500; for other geometries the role of turbulence could be larger.     

Mixing augmentation induced by the combination of the oblique shock wave and secondary recirculation jet in a supersonic crossflow

The mixing process of fuel-air in the supersonic crossflow is a pivotal technology for the scramjet engine. In this paper, numerical simulation of the transverse sonic hydrogen jet into a supersonic Mach 3 crossflow with the mixing augmentation strategy induced by the combination of the oblique shock wave and secondary recirculation jet has been carried out. Detailed flow field structures, hydrogen mass fraction distributions, vortex structures, heat flux and some parameters have been explored in order to investigate its mixing enhancement mechanism. Results of the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two-equation shear stress transport (SST) κ-ω turbulence model show that the combined strategy of the oblique shock wave and secondary recirculation jet device can effectively improve the mixing speed and mixing efficiency with little total pressure loss. Also, the secondary recirculation jet device can reduce the peak of the heat flux effectively. In this study, the case with the single bleed hole owns the best effect with improving the mixing efficiency by 82.75% locally and reducing the maximum heat flux by 15.24% respectively。