Mixing of high speed coaxial jets

In this study, five different supersonic nozzles – conical, elliptical, tabbed, radially lobed and two-dimensional lobed – are compared experimentally for their mixing performance. With the background of studies by various groups conducted on the above nozzles, the present paper aims to provide an experimental comparison of their respective mixing performances with that of a conventional conical nozzle under identical operating conditions. The mixing of the supersonic stream coming from such nozzles with a coaxial sonic stream is investigated. The investigation is performed non-intrusively, using digital image processing of planar Mie-scattering images of the flow field. The results of these investigations reveal the superiority of mixing performance of the two-dimensional lobed nozzle over conventional circular and other non-conventional nozzles. Received: 15 July 1999/Accepted: 3 July 2000     

Studies on freejets from nozzles for high-speed mixing applications

In this study four different supersonic nozzles – circular, elliptical, tabbed, and radially lobed nozzles are compared experimentally for their freejet mixing performance. With the background of studies by various groups conducted on elliptical, tabbed, and radially lobed nozzles, the present paper aims at a comparative experimental study to compare their mixing performance with that of a conventional circular nozzle under identical operating conditions. The investigation is performed non-intrusively, using digital image processing of the planar Mie scattering images of the flow field. The results of these investigations reveal the superiority of the mixing performance of the lobed nozzles over conventional circular and other non-conventional nozzles. Received: 29 July 1998/Accepted: 5 November 1999     

Studies on freejets from radially lobed nozzles

The freejet characteristics of radially lobed nozzles were studied experimentally. Schlieren photographs of the freejets revealed that the lobes of the nozzles gave rise to alternate expansion-compression regions. For two of the lobed nozzles the potential core was reduced by a factor of three relative to the conical nozzle tested. The jet spread and the shear layer thickness of the lobed nozzles were considerably higher than those for the conical nozzle. The jet width and the shear layer thickness were higher in the major plane; however, the shear layer growth rate was higher in the minor plane of the lobed nozzles. Jet axial thrust measurements revealed a thrust loss of 14% for two of the lobed nozzles tested.     

Influence of strain rate on a premixed turbulent flame stabilized in a stagnating flow

In order to describe the influence of strain rate on the behaviour and on the characteristics of premixed turbulent combustion, a methane-air flame stabilized by a stagnation plate is studied experimentally. The plate is set at a fixed distance from the nozzle and the strain is varied by changing the exit velocity at the nozzle. At low strain rates, the evolution of profiles of mean axial velocity along the centreline agrees with classical results, and these results are used to characterise the flame. The variation of these characteristics with parameters such as plate temperature, equivalence ratio and strain rate is investigated. At the highest strain rates, the shape of the axial velocity profiles along the stagnation line is modified. This change emphasises a critical strain rate K _C that has to be considered as well as the extinction strain rate K _EX. Measurements also demonstrate the existence of a virtual stagnation point that moves towards the plate as the strain rate increases. The axial and transverse fluctuating components of the velocity are analyzed along the centreline and very close to the wall. The results show the importance of the critical strain rate K _C , which is linked to a drastic change in the evolution of the axial and transverse velocity fluctuations. Received: 15 January 1998/Accepted: 7 February 1999     

Dual-plane stereoscopic particle image velocimetry: system set-up and its application on a lobed jet mixing flow

 The technical basis and system set-up of a dual-plane stereoscopic particle image velocimetry (PIV) system, which can obtain the flow velocity (all three components) fields at two spatially separated planes simultaneously, is summarized. The simultaneous measurements were achieved by using two sets of double-pulsed Nd:Yag lasers with additional optics to illuminate the objective fluid flow with two orthogonally linearly polarized laser sheets at two spatially separated planes, as proposed by Kaehler and Kompenhans in 1999. The light scattered by the tracer particles illuminated by laser sheets with orthogonal linear polarization were separated by using polarizing beam-splitter cubes, then recorded by high-resolution CCD cameras. A three-dimensional in-situ calibration procedure was used to determine the relationships between the 2-D image planes and three-dimensional object fields for both position mapping and velocity three-component reconstruction. Unlike conventional two-component PIV systems or single-plane stereoscopic PIV systems, which can only get one-component of vorticity vectors, the present dual-plane stereoscopic PIV system can provide all the three components of the vorticity vectors and various auto-correlation and cross-correlation coefficients of flow variables instantaneously and simultaneously. The present dual-plane stereoscopic PIV system was applied to measure an air jet mixing flow exhausted from a lobed nozzle. Various vortex structures in the lobed jet mixing flow were revealed quantitatively and instantaneously. In order to evaluate the measurement accuracy of the present dual-plane stereoscopic PIV system, the measurement results were compared with the simultaneous measurement results of a laser Doppler velocimetry (LDV) system. It was found that both the instantaneous data and ensemble-averaged values of the stereoscopic PIV measurement results and the LDV measurement results agree well. For the ensemble-averaged values of the out-of-plane velocity component at comparison points, the differences between the stereoscopic PIV and LDV measurement results were found to be less than 2%. Received: 18 April 2000/Accepted: 2 February 2001     

Mixing characteristics of turbulent water vapour jets measured using an isokinetic sampling probe

 Horizontal turbulent water vapour (steam) jets were discharged into ambient air from a circular convergent nozzle under unchoked/choked and saturated/superheated nozzle exit conditions, resulting in two-phase (liquid and vapour), two-fluid (air and water) condensing free jets. Flow properties and mixing characteristics have been measured with the aid of an isokinetic sampling probe arrangement. Radial and axial profiles of air and steam mass flow rates and mass fractions were measured from which entrainment, centreline decay and half-width spreading rates were calculated and compared with data from the literature. Overall, the mixing characteristics of the condensing jets are very similar to those of non-condensing jets extensively reported in the literature. Received: 30 September 1996 / Accepted: 19 May 1997     

Vortex dynamics and mass entrainment in turbulent lobed jets with and without lobe deflection angles

Passive control of jet flows in order to enhance mixing and entrainment is of wide applicative interest. Our purpose is to develop new air diffusers for HVAC systems, by using lobed geometry nozzles, in order to ameliorate users the thermal comfort. Two turbulent 6-lobed air jets with and without lobe deflection angles were studied experimentally and compared with a reference circular jet having the same initial Reynolds number. The main objective was to analyze the modifications occurring in the vortex dynamics of the flow, firstly by replacing a circular tube with a straight lobed tube, and secondly by a lobed tube having a double inclination of the lobes. Rapid visualizations of the flows and hot-wire measurements of the streamwise velocity spectra allow understanding the vortex roll-up mechanisms. Unlike the circular jet, where the primary rings are continuous, the Kelvin–Helmholtz vortices in the lobed jet flows were found to be discontinuous. The resulting “ring segments” detach at different frequencies whether they are shed in the lobe troughs or at the lobe sides. One explanation relies on the strong variation of the exit plane curvature. Additionally, a speculative scenario of the vortical dynamics is advanced by the authors. The discontinuous nature of the K–H vortices enables the development of secondary streamwise structures, non-influenced by the passage of the primary structures as in the case of the circular jet. Thus, the momentum flux transport role played by the streamwise structures is rendered more efficient and leads to a spectacular increase in the entrainment rate in the initial region. The amount of fluid being entrained in the lobed jet by the streamwise structures is drastically amplified by the double inclination of the nozzle exit boundary.     

Near field vortex dynamics in axially forced,laminar, co-flowing jets: a descriptive study of the flow configurations

An experimental study is presented of the vortex structures that appear in the shear layer of laminar, co-flowing air jets subjected to strong axial forcing. A set of flow visualisation experiments has been performed to elucidate the nature of the different structures and the mechanisms leading to their appearance and further interactions. The axial forcing sets the axisymmetric instability to prescribed values of amplitude and frequency (and thus wavelength) and produces a strong effect in the lateral spreading of the inner jet. It is shown that the near field development of the flow can be explained via inviscid vortex dynamics arguments, involving three vortex structures. Due to the strong axial forcing, all these vortices already appear as developed concentrations of vorticity in the surroundings of the nozzle exit. An azimuthal perturbation is added to the flow in the form of a lobed nozzle exit, in order to lock the azimuthal organisation of the vortices. The results are discussed and some representative configurations are examined. Each configuration appears for a given range of the forcing parameters. A tentative model of the near-field vortex dynamics is developed, but quantitative measurements are still necessary.     

Transverse diffusion and heat conduction in a granular layer

The process of non-steady-state transverse diffusion of a passive additive in a granular layer described by a cellular model is investigated. The general results obtained in [1] are applied to an analysis of concrete transport processes of matter and heat in a granular layer. The following four cell models are treated: (1) ideal mixing cells without stagnation zones; (2) ideal mixing cells with stagnation zones; (3) ideal mixing cells with diffusive stagnation zones; (4) ideal mixing cells with diffusive stagnation zones having a finite exchange rate between the free volume and the stagnation zone. The conditions of applicability for each of the above models are found. The time to establish a normal distribution in the transverse diffusion process is determined for all the models. This quantity is then connected with the physical characteristics of transport processes of matter in a layer of nonporous and porous particles, the transport of heat in a granular layer, and the transport of matter in a layer of particles which adsorb an additive.     

Experimental investigation of jets from rectangular six-lobed and round orifices at very low Reynolds number

This article presents an experimental study conducted on a six-lobed rectangular jet at a very low Reynolds number of 800. The near-exit flow dynamics is compared to the reference counterpart circular jet with same initial conditions. Flow dynamics is analyzed using time-resolved flow-visualizations, hot-wire anemometry and laser Doppler velocimetry. In the round jet, flow motion is dominated by large primary Kelvin–Helmholtz (K–H) structures. In the six-lobed rectangular jet, the K–H vortices are very thin compared to the large secondary vortices generated by the high shear at the lobed nozzle lip. The inspection of mean-velocity profiles and streamwise evolutions of the spreading rates in the major and the minor planes of the lobed jet confirm the absence of the switching-over phenomenon not observed on flow images. The streamwise structures that develop in orifice troughs render the volumetric flow rate significantly higher than that of the reference circular jet. Comparison of the obtained results to available data of the literature of similar rectangular six-lobed jets investigated at very high Reynolds numbers reinforces the notion that the three-dimensional flowfields at very low and very high Reynolds numbers are similar if the geometry of the lobed nozzle is conserved. However, important variations in flow dynamics might occur if one or several geometric parameters of the lobed nozzle are modified.     

Effectiveness of secondary tabs for supersonic mixing

The effect of vortex generators, in the form of small tabs projecting into the flow at the nozzle exit, aided by secondary tabs on either side, on the mixing characteristics of an axisymmetric jet at Mach number 1.7 is investigated. Experimental studies on the basic features of the jet from a nozzle with secondary tabs are conducted to assess the free jet characteristics as well as the momentum and thermal mixing behavior. The secondary tabs were found to increase the jet spread and distort the jet cross-section and were found to cause substantial enhancement of mixing of supersonic jets. Jet structure is observed using flow-visualization techniques. LLS images are employed to obtain cross-sectional views of the jet with the introduction of secondary tabs. The ability of secondary tabs to eliminate the screech noise of the supersonic jet is also observed. Received: 3 February 2000/Accepted: 8 February 2001     

Jet flow field during screech

Measurements were made of the flow field structure and the near field parameters of a jet exhausting from a sonic nozzle with a 1.27 cm exit diameter. Compressed air was used for obtaining stagnation pressures up to 5 atmospheres. The jet exhausted vertically from a settling chamber into an acoustically insulated room and through an insulated duct out through the roof. Measurements were made with several different reflecting surfaces at the nozzle exit as well as an insulating surface. Schlieren pictures at 500,000 frames/s were taken. Overall sound pressure level, impact pressure level downstream, and sound frequency analyzer measurements were made.It was found that with a reflecting surface there was a radial oscillation of the jet which had the same frequency as the dominant sound (screech) frequency emitted by the jet. No axial motion of the inviscid part of the flow structure was detected. The insulated surface at the nozzle exit appeared to shift the dominant frequencies of the sound generated into the region above the audible (>16 KHz). A reflecting surface yielded pure tones (screech) with one or two harmonics. The dominant (screech) frequency decreased as the stagnation pressure increased. The screech frequency was found to be approximately inversely proportional to the length of the first shock cell.Nomenclature C ₀ speed of sound in ambient gas - D diameter of nozzle exit - f frequency of pure tone (screech frequency) - L ₁ length of first cell, distance between nozzle exit plane and intersection of shock with shear layer - M Mach number based on isentropic expansion to ambient pressure - P ₀ stagnation chamber pressure - P _a ambient pressure - P _i impact pressure - R _LB distance from nozzle centerline to left boundary of jet - R _RB distance from nozzle centerline to right boundary of jet - t time - period of screech, 1/f - X _E axial distance of eddy from nozzle exit plane - X _I axial distance of third cell shock intersection from nozzle exit plane - Y _I transverse distance of third cell shock intersection from nozzle centerline     

High order numerical simulation of the thermal load on a lobed strut injector for scramjet applications

In this paper, the thermal load on an actively cooled lobed strut injector for scramjet (supersonic combustion ramjet) applications is investigated numerically. This requires coupled simulations of the strut internal and external flow fields together with the heat conduction in the solid injector body. In order to achieve a fast mixing, the lobed strut is positioned at the channel axis to inject hydrogen into the core of a Mach 3 air stream. There it is exposed to the extremely high temperatures of the high speed flow. While the external air and hydrogen flows are supersonic, the strut internal hydrogen flow is mainly subsonic, in some regions at very low Mach numbers. To enable a simulation of the internal flow field which ranges from very low to very high Mach numbers (approximately Mach 2.25 at the nozzle exit), a preconditioning technique is employed. The compressible finite‐volume scheme uses a spatially fourth order multi‐dimensional limiting process discretization, which is used here for a first time to simulate a geometrically and fluid mechanically highly complex problem. It will be demonstrated that besides its high accuracy the multi‐dimensional limiting process scheme is numerically stable even in case of demanding practical applications. The coupled simulation of the lobed strut injector delivers unique insight into the flow phenomena inside and outside the strut, the heat fluxes, the temperature distribution in the solid material, the required hydrogen mass flux with respect to cooling requirements and details concerning the conditions at the exit of the injector. Copyright © 2016 John Wiley & Sons, Ltd.     

Jet flow field during screech

Measurements were made of the flow field structure and the near field parameters of a jet exhausting from a sonic nozzle with a 1.27 cm exit diameter. Compressed air was used for obtaining stagnation pressures up to ～5 atmospheres. The jet exhausted vertically from a settling chamber into an acoustically insulated room and through an insulated duct out through the roof. Measurements were made with several different reflecting surfaces at the nozzle exit as well as an insulating surface. Schlieren pictures at 500,000 frames/s were taken. Overall sound pressure level, impact pressure level downstream, and sound frequency analyzer measurements were made. It was found that with a reflecting surface there was a radial oscillation of the jet which had the same frequency as the dominant sound (screech) frequency emitted by the jet. No axial motion of the inviscid part of the flow structure was detected. The insulated surface at the nozzle exit appeared to shift the dominant frequencies of the sound generated into the region above the audible (>16 KHz). A reflecting surface yielded “pure tones” (screech) with one or two harmonics. The dominant (screech) frequency decreased as the stagnation pressure increased. The screech frequency was found to be approximately inversely proportional to the length of the first shock cell.     

Schlieren measurement of turbulent structure in a diffusion flame

The regular and random mixing structures in a turbulent diffusion flame were investigated using the quantitative, dynamic crossed-beam schlieren method. Evidence was found close to the nozzle relating to the vortexlike structure of eddies surrounding the central fuel jet flow. The observations also make possible resolution of turbulent intensity, scales, convection, and spectra within the diffusion flame without the use of seeding or intrusion of measuring probes. It is found that length scales and other turbulence parameters in the diffusion flame progressively revert to values similar to those expected and observed in scalar passive mixing as the combustion reaction intensity reduces with axial distance from the nozzle system.     

圆柱形充液室中4股贴壁燃气射流扩展特性的实验研究

为了探索高温高压周向均布4股贴壁燃气射流在受限空间中的扩展特性,设计了贴壁燃气射流在圆柱形充液室内扩展的实验装置,借助数字高速录像系统,观察了4股贴壁燃气射流在充液室中的扩展过程,发现由Kelvin-Helmholtz不稳定性引起的表面不规则一直存在于整个射流扩展过程;通过处理拍摄记录的射流扩展序列图,获得不同时刻射流扩展的轴向和径向位移; 对比了不同破膜喷射压力和喷孔结构参数对4股贴壁燃气射流扩展过程的影响。实验结果表明:喷孔面积越大,贴壁射流初期轴向扩展速度越大,但由于径向扩展达到交汇的时间较早,湍流掺混和干涉强烈,衰减也越快;破膜喷射压力越高,射流径向扩展到达交汇的时间越短; 破膜喷射压力从12 MPa升高到20 MPa,射流轴向扩展速度大幅增加,气液湍流掺混效应增强。     

An investigation of the heat transfer and static pressure on the over-tip casing wall of an axial turbine operating at engine representative flow conditions. (I). Time-mean results

The over-tip casing of the high-pressure turbine in a modern gas turbine engine is subjected to strong convective heat transfer that can lead to thermally induced failure (burnout) of this component. However, the complicated flow physics in this region is dominated by the close proximity of the moving turbine blades, which gives rise to significant temporal variations at the blade-passing frequency. The understanding of the physical processes that control the casing metal temperature is still limited and this fact has significant implications for the turbine design strategy. A series of experiments has been performed that seeks to address some of these important issues. This article reports the measurements of time-mean heat transfer and time-mean static pressure that have been made on the over-tip casing of a transonic axial-flow turbine operating at flow conditions that are representative of those found in modern gas turbine engines. Time-resolved measurements of these flow variables (that reveal the details of the blade-tip/casing interaction physics) are presented in a companion paper. The nozzle guide vane exit flow conditions in these experiments were a Mach number of 0.93 and a Reynolds number of 2.7 × 10⁶ based on nozzle guide vane mid-height axial chord. The axial and circumferential distributions of heat transfer rate, adiabatic wall temperature, Nusselt number and static pressure are presented. The data reveal large axial variations in the wall heat flux and adiabatic wall temperature that are shown to be primarily associated with the reduction in flow stagnation temperature through the blade row. The heat flux falls by a factor of 6 (from 120 to 20 kW/m²). In contrast, the Nusselt number falls by just 36% between the rotor inlet plane and 80% rotor axial chord; additionally, this drop is near to linear from 20% to 80% rotor axial chord. The circumferential variations in heat transfer rate are small, implying that the nozzle guide vanes do not produce a strong variation in casing boundary layer properties in the region measured. The casing static pressure measurements follow trends that can be expected from the blade loading distribution, with maximum values immediately upstream of the rotor inlet plane, and then a decreasing trend with axial position as the flow is turned and accelerated in the relative frame of reference. The time-mean static pressure measurements on the casing wall also reveal distinct circumferential variations that are small in comparison to the large pressure gradient in the axial direction.     

The ultrasonic velocity profile measurement of flow structure in the near field of a square free jet

 Coherent structures in the near field of a three-dimensional jet have been investigated. Experiments were carried out for a free jet issuing from a square nozzle using a water channel. Instantaneous velocity profiles were obtained in the axial and radial directions by using an ultrasonic velocity profile (UVP) monitor. Axial variations of dominant time-scales of vortex structures were examined from one-dimensional wavelet spectra. Wavenumber-frequency spectra were calculated by two-dimensional Fourier transform along the axial direction in a mixing layer, and it was found that a convective velocity of flow structures was nearly constant independently of their scales in space and time. Coherent structures in the axial direction were investigated in terms of proper orthogonal decomposition (POD). Eigenfunctions are similar to a sinusoidal wave, and reconstructed velocity fields by the lower-order and higher-order POD modes demonstrate large-scale and smaller-scale coherent structures, respectively. Received: 8 May 2000/Accepted: 23 January 2001 Published online: 29 November 2001     

Velocity and Strain-Rate Characteristics of Opposed Isothermal Flows

Velocity measurements in the isothermal flows created by an opposed nozzle configuration are reported with emphasis on the axis, stagnation plane and the distributions of mean and instantaneous strain rates. The instrumentation comprised particle image velocimetry (PIV) with silicon oil droplets added to the flows upstream of both nozzles with the laser sheet passing through the axis between the nozzles. The results identify the regions of high strain rates and quantify the development of the mean and turbulent components of the flow from the nozzle exits as a function of bulk velocities from 3 to 8.2 m/s and nozzle separations from 0.4 to 1.0 diameters. Results show, for example, the rise in the values of axial and radial normal stress towards the stagnation plane with values increasing by up to 300% and 160% respectively. The maximum mean strain rate occurred just over one nozzle radius from the axis at the smallest separation and with values that increased from 450 to 950 s⁻¹ with decreasing separation at a bulk velocity of 3.0 m/s. Probability density functions were near Gaussian and hence much larger instantaneous strain rates were observed. The PIV image size had the advantage that it allowed the entire flow field to be viewed in terms of velocity vectors and derived quantities include mean strain rates. Small asymmetry of the flow and the higher strain rates at finite distances from the nominal impingement plane were observed. The experimental results permitted the domain of applicability of different modelling approaches to be defined more accurately and calculations were performed with different turbulence models. The results showed that two-equation turbulence models did not represent turbulence intensities close to impingement and that Reynolds stress closures produce superior agreement. It was further shown that ad hoc modifications to the dissipation equation, such as those based on the ratio of the turbulent to mean strain time scale, can improve results at the expense of generality. It is also shown that mean flows are well reproduced by a Reynolds stress closure for all nozzle separations. Comments are included on the implications of the results for investigations of reacting flows and extinction.