Determination of principal characteristics of turbulent swirling flow along annuli: Part 2: Measurement of turbulence components

Turbulence data for swirling flows along the annulus formed between two co-axial tubes are presented. The swirl was generated by a set of inlet guide vanes which produced, after settling and removal of wall boundary layers, a nominally free vortex flow over the complete entry plane of the test section. The work complements that described in Part 1 of the paper which considered the behaviour of time mean values under the same entry conditions¹. A hot wire anemometer was used to detect turbulence quantities and techniques have been developed for the measurement of these using the minimum number of simple probe geometries. The theory leading to the derivation of the separate turbulence parameters from hot wire measurements is described. A series of radial profiles are given as representative examples of the extensive data collected in addition to longitudinal variations of friction factor and shear stress. A brief discussion is given on the determination of eddy diffusivity as a function of radial and axial location.     

Correlation of swirl number for a radial-type swirl generator

An experimental investigation was undertaken to derive a new correlation for the swirl number of a radial-type swirl generator under various Reynolds numbers and various vane angle conditions. A radial-type swirl generator with 16 rotary guide vanes was used to generate an annular swirling jet flow. The Reynolds numbers ranged from 60 to 6000, and the vane angles from 0° to 56°. Quantitative measurements for the velocities were made by using an optical method of laser-Doppler anemometry (LDA). Three-component velocity profiles of axial, radial, and azimuthal components at the swirling jet exit were measured for various flow conditions. A flow visualization method using smoke-wire and still photography was also applied to observe the flow patterns of the recirculation region behind the circular bluff body. Under low Reynolds number conditions, the swirl strength was found to be strongly dependent on the Reynolds number as well as on the guide vane angle. Based on the experimental results, a modified swirl number S is derived to characterize the swirling flow, which is useful for the design of a swirl generator.     

Prediction of Pressure Oscillations in a Premixed Swirl Combustor Flow and Comparison to Measurements

The most common and reliable technique used for flame stabilization of industrial combustors with high thermal loads is the application of strongly swirling flows. In addition to stabilization, swirl flames offer the possibility to influence emission characteristics by simply changing the swirl intensity or the type of swirl generation. Despite of these major advantages, swirling flows tend to evolve flow instabilities, that considerably constitute a significant source of noise. In general, noise generation is substantially enhanced, when such a swirling flow is employed for flames. Thus, the minimization of the resulting noise emissions under conservation of the benefit of high ignition stability is one major design challenge for the development of modern swirl stabilized combustion devices. The present investigation makes an attempt to determine mechanisms and processes to influence the noise generation of flames with underlying swirling flows. Therefore, a new burner has been designed, that offers the possibility to vary geometrical parameters as well as the type of swirl generation, typically applied in industrial devices. Experimental data has been acquired for the isothermal flow as well as swirl flames by means of 3-D-LDV-diagnostics comprising the components of long-time averaged mean and rms-velocities as well as spectrally resolved velocity fluctuations for all components. The noise emission data was acquired with microphone probes resulting in sound pressure levels outside the zone of the perceptible fluid flow. Along to the experiments, numerical simulations using RANS and LES have been carried out for isothermal cases with different burner outlet geometries. The results of the measurements show a distinct rise of the sound pressure level, obtained by changing both the test setup from the isothermal into the flame configuration as well as the geometrical parameters. This is also resembled by the LES simulation results. Furthermore, a physical model has been developed from experiments and verified by the LES simulation, that explains the formation of coherent flow structures and allows to separate their contribution to the overall noise emission from ordinary turbulent noise sources.     

LDV measurements in complex swirling flows,their physics and a database for CFD evaluations

In an earlier paper in this journal (Mocikat et al. in Exp Fluids 34:442–448, 2003) LDV measurements in a simple geometry but for a complex flow have been provided as a database for CFD evaluation purposes. With special inflow devices swirl could now be added to the flow. By changing the exit position of the test section in order to get a non-symmetric flow field, a steady swirling flow without instability induced precessing motions could be established. This flow can be interpreted as a superposition of a swirling motion to an otherwise swirl-free flow by introducing “swirl influence factors” for various aspects of the flow field. With a modified inflow device a periodically unsteady flow with swirl emerged. The turbulence features of this flow are distinctively different from the steady flow case with swirl. For all flows under consideration the three time-averaged components of the velocity vector and all components of the Reynolds stress tensor are measured in selected cross sections and provided as a data base for CFD calculations.     

Determination of principal characteristics of turbulent swirling flow along annuli: part 4: an asymptotic solution

An asymptotic solution of the momentum equation is given that describes the decay of swirling flow passing along the annulus formed between two concentric, straight, circular-section pipes having a common starting point. The flow is considered turbulent and approximations are made consistent with the notion of fully developed flow conditions. Applications of this approach are reviewed and shortcomings highlighted. A series of calculations are presented and compared with experimental and theoretical data previously obtained by the authors. It is shown that acceptable predictions of the overall flow behavior can be obtained over a wide range of initial conditions provided the calculations procedure is applied in regions of validity, which has not been the case in some published work. Substantial errors are found if, for example, the procedure is allowed to commence at inlet to the annulus owing to the inconsistency of the assumptions in the initially developing-flow region, which for this work extends at least five outer-pipe diameters downstream from inlet. The authors' previous numerical integration scheme may be used to predict flows satisfactorily in the developing-flow region and the present asymptotic solution used subsequently to reduce computation time and cost.     

An experimental study of fluid flow and heat transfer in decaying swirl through a heated annulus

The mean and turbulent structures of turbulent swirling flow in a heated annulus have been measured. Both forced and free vortex swirling flows were generated, and the outer wall of the test section was heated uniformly. The maximum swirl number was 1.39, Reynolds numbers were up to 200000, and heat input was 10.5 kW. Mean and turbulent velocity components, air and wall temperatures, and wall static pressures were all measured. Hot-film techniques were developed to measure turbulence. From these parameters, the flow and temperature fields, pressure distribution, and heat transfer coefficients were determined. The mechanisms of heat transfer were identified.     

阶梯型加速段对旋流喷嘴雾化特性的影响

旋流内芯是压力旋流式喷嘴最主要的旋流发生构件, 其几何特征直接影响压力旋流式喷嘴的喷雾特性.目前采用平滑型加速段的旋流内芯导流效率较低.为减小高流量条件下的能量损失, 使喷嘴旋流内芯加速段对喷雾介质产生预旋效应, 增强旋流强度, 本文设计喷嘴旋流内芯加速段为阶梯型, 其下段阶梯相对上段阶梯旋转15°, 旋向与喷嘴旋流槽方向相同.利用粒子动态分析仪(particle dynamics analysis system, PDA) 和高速摄影(charge coupled device, CCD)系统实验研究了加速段结构改进前后喷嘴的喷雾流量、雾场索特尔平均直径(Sauter mean diameter, SMD)、雾滴速度以及喷雾锥角, 并分析了SMD、 雾滴速度的轴向和径向分布特性. 结果表明, 背压差0.08\mathrm{~} 0.46 MPa 范围内, 阶梯型加速段对喷雾介质具有较好的预旋效果.喷嘴的流量提高了48.0% ~ 51.8%; 喷雾的轴向速度提升了31.4% ~ 32.8%, 径向速度提升了1.6% ~ 16.8%; 喷雾锥角减小了4.21°~6.57°; 较高背压差下喷雾下游的SMD减小了9.8%.与平滑型加速段相比, 阶梯型加速段的设计有效地提高了喷嘴的雾化质量.      

Reynolds stress modelling of jet and swirl interaction inside a gas turbine combustor

Influences of the inlet swirl levels on the interaction between the dilution air jets and the swirling cross‐flow to the interior flow field inside a gas turbine combustor were investigated numerically by Reynolds stress transport model (RSTM). Due to the intense swirl and jet interaction, a high level of swirl momentum is transported to the centreline and hence, an intense vortex core is formed. The strength of the centreline vortex core was found to depend on the inlet swirl levels. For the higher swirling inlet, the decay of the swirling motion causes strong streamline variation of pressure; and consequently leads to an elevated level of deceleration of its axial velocity. Predictions contrasted with measurements indicate that the stress model reproduces the flow correctly and is able to reflect the influences of inlet swirl levels on the interior flow structure. Copyright © 1999 John Wiley & Sons, Ltd.     

Particle image velocimetry investigation of the flow-field of a 3D turbulent annular swirling decaying flow induced by means of a tangential inlet

Particle image velocimetry (PIV) has been used in order to measure the three mean components and turbulence intensities of the velocity vector in a swirling decaying flow induced by a tangential inlet in an annulus. This kind of flow motion is found to be very complex, exhibiting three-dimensional and non-axisymmetric characteristics coupled with a free decay of the swirling intensity along the flow path, thereby making it difficult to study. A method allowing the measurement of the three components of the velocity flow-field with a standard PIV system with two-dimensional acquisitions, is presented. The evolution of each velocity component between the inlet and the outlet of the annulus is obtained. Furthermore, the PIV technique is extended to the measurement of turbulent characteristics such as turbulent intensities and dimensionless turbulent energy. The main characteristics of the swirling flow are discussed and the swirl number is estimated as a function of the axial distance from the tangential inlet. Received: 6 July 1998/Accepted: 20 March 1999     

Effect of swirling flow on unburned ratio and nitrogen oxide concentration in a spray combustion system

The effect of swirling flow on the unburned ratio and NO concentration in exhausted gas was studied for slurry [coal-water mixture (CWM)] spray combustion with variations of swirl numbers. A numerical analysis for CWM combustion was performed for axisymmetric flow in a cylindrical geometry. First, to check the performance of three previous k-ε turbulence models modified with swirling flow, velocity components of isothermal swirling jets were measured by laser-Doppler anemometry (LDA) and compared with predicted results. The two modified models gave more reliable results than the conventional one. Next, as the swirl number could not be estimated by the angular momentum derived from the vane angle of the circular swirler, the reduction rate of the tangential momentum flux through the tube of the circular swirler was measured and calculated. Both measured and predicted results showed that when the swirl number S′ given by the vane angle was 2.0, the effective swirl number S_eff decreased by about 60% to S′. To take the results mentioned above into consideration, effects of swirl number on both the exhausted NO concentration and unburned ratio were investigated. The predicted unburned ratio showed good agreement with the experimental results. Both experimental and calculated results showed that the optimum operating conditions controlling the exhausted NO concentration and unburned ratio in this spray combustion system were obtained when the swirl number S_eff was about 0.5.     

Turbulent flow in converging nozzles,part one: boundary layer solution

The boundary layer integral method is used to investigate the development of the turbulent swirling flow at the entrance region of a conical nozzle. The governing equations in the spherical coordinate system are simplified with the boundary layer assumptions and integrated through the boundary layer. The resulting sets of differential equations are then solved by the fourth-order Adams predictor-corrector method. The free vortex and uniform velocity profiles are applied for the tangential and axial velocities at the inlet region, respectively. Due to the lack of experimental data for swirling flows in converging nozzles, the developed model is validated against the numerical simulations. The results of numerical simulations demonstrate the capability of the analytical model in predicting boundary layer parameters such as the boundary layer growth, the shear rate, the boundary layer thickness, and the swirl intensity decay rate for different cone angles. The proposed method introduces a simple and robust procedure to investigate the boundary layer parameters inside the converging geometries.     

LDV Measurements of the Response of the Swirling Turbulent Flow in a Pipe to a Rapid Temporal Change in Swirl

The present paper describes an experiment to study the response of the swirling flow in a pipe at a high Reynolds number to a rapid temporal change in swirl. The measurements have been conducted with 3D-LDV in a facility of special design that operates on the principle of refractive-index matching and in which swirl is generated in the flow by a tube-bundle in the pipe rotating about its own axis. The temporal change in swirl is effected by a computer controlled change of the angular velocity of the swirl generator. Measurement data from both cases, of increase and decrease of swirl, are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

旋流燃烧室内湍流燃烧速度场的实验研究

建立了采用分级进风方式的同轴射流旋流燃烧室实验装置，选用耐高温的氧化铝细粉作为示踪粒子，实现了用三维激光粒子动态分析仪(PDA)测量湍流旋流燃烧的热态瞬时速度场．在分级进风比率和旋流致不同的3组实验工况条件下，得到了气体时均轴向与切向速度、轴向与切向脉动速度均方根值和轴向—切向脉动速度二阶关联量的分布．     

Computations of strongly swirling flows with second‐moment closures

The present study is concerned with simulating turbulent, strongly swirling flows by eddy viscosity model and Reynolds stress transport model variants adopting linear and quadratic form of the pressure–strain models. Flows with different inlet swirl numbers, 2.25 and 0.85, were investigated. Detailed comparisons of the predicted results and measurements were presented to assess the merits of model variants. For the swirl number 2.25 case, due to the inherent capability of the Reynolds stress models to capture the strong swirl and turbulence interaction, both the linear and quadratic form of the pressure–strain models predict the flow adequately. In strong contrast, the k–ϵ model predicts an excessively diffusive flow fields. For the swirl number 0.85 case, both the k–ϵ and Reynolds stress model with linear pressure–strain process, show an excessive diffusive transport of the flow fields. The quadratic pressure–strain model, on the other hand, mimics the correct flow development with the recirculating region being correctly predicted. Copyright © 1999 John Wiley & Sons, Ltd.     

Considerations for the design of swirl chambers for the cyclone cooling of turbine blades and for other applications with high swirl intensity

The focus of this study lies on turbulent incompressible swirling flows with high swirl intensity. A systematic parameter study is conducted to examine the sensitivity of the mean velocity field in a swirl chamber to changes in the Reynolds number, swirl intensity and channel outlet geometry. The investigated parameter range reflects the typical kinematic flow conditions found in heat transfer applications, such as the cooling of the turbine blade known as cyclone cooling. These applications require a swirl intensity, which is typically much higher than necessary for vortex breakdown. The resulting flows are known as flow regime II and III. In comparison to flow regime I, which denotes a swirling flow without vortex breakdown, these flow regimes are characterized by a subcritical behavior. In this context, subcritical means that the flow is affected by the downstream channel section. Based on mean velocity field measurements in various swirl chamber configurations, it is shown that flow regime III is particularly sensitive to these effects. The channel outlet geometry becomes a determining parameter and, therefore, small changes at the outlet can produce entirely different flow patterns in the swirl chamber. In contrast, flow regime II, as well as flow regime I and axial channel flows, are much less sensitive to changes at the channel outlet. The knowledge about the sensitivity of the flow in different flow regimes is highly relevant for the design of a cyclone cooling system. Cooling systems employing flow regime III can result in a weakly robust flow system that may change completely over the operating range. As a remedy, the swirl intensity needs to be decreased so that flow regime III cannot be reached, which, however, reduces the maximum achievable heat transfer in the cooling system. Alternatively, the flow has to transition back from flow regime III to flow regime II or I before the flow leaves the swirl chamber. Two practical methods are presented. These findings can be directly applied in the design processes of future cyclone cooling systems, and other applications of swirling flow.     

Numerical and experimental study of an axially induced swirling pipe flow

In-line flow segregators based on axial induction of swirling flow have important applications in chemical, process and petroleum production industries. In the later, the segregation of gas bubbles and/or water droplets dispersed into viscous oil by swirling pipe flow may be beneficial by either providing a pre-separation mechanism (bubble and/or drop coalescer) or, in the case of water-in-oil dispersions, by causing a water-lubricated flow pattern to establish in the pipe (friction reduction). Works addressing these applications are rare in the literature. In this paper, the features and capabilities of swirling pipe flow axially induced by a vane-type swirl generator were investigated both numerically and experimentally. The numerical analysis has been carried out using a commercial CFD package for axial Reynolds numbers less than 2000. Pressure drop, tangential and axial velocity components as well as swirl intensity along a 5 cm i.d. size and 3 m long pipe were computed. Single phase flow experiments have been performed using a water–glycerin solution of 54 mPa s viscosity and 1210 kg/m³ density as working fluid. The numerical predictions of the pressure drop were compared with the experimental data and agreement could be observed within the range of experimental conditions. The experiments confirmed that swirl flow leads to much higher friction factors compared with theoretical values for non-swirl (i.e. purely axial) flow. Furthermore, the addition of a conical trailing edge reduces vortex breakdown. Visualization of the two-phase swirling flow pattern was achieved by adding different amounts of air to the water–glycerin solution upstream the swirl generator.     

Experimental investigations into vortex flow in a straight,annular section channel

Results of experimental velocity measurements in air at several cross-sections along a straight annular section channel comparable to a hydraulic machine admission duct are presented. The flow analysis focuses on the axial and tangential velocity components and their modification with changes in the intensity of swirl along the channel. Particular prominence is given to an anomalous flow, the ‘dead water core’, occurring at the greatest swirl intensities. An earlier method of evaluating the core size of this anomalous flow on the basis of a vortex-type schematization is tested; Strscheletzky's method is also analysed. Finally a definition of the dead water core dimensions based on the axial velocity component distribution is proposed     

An experimental study and CFD analysis towards heat transfer and fluid flow characteristics of decaying swirl pipe flow generated by axial vanes

In this presentation, influences of axial vane swirler on heat transfer augmentation and fluid flow are investigated both experimentally and numerically. The swirl generator is installed at the inlet of the annular duct to generate decaying swirling pipe flow. Three different blade angels of 30°, 45° and 60° were examined. Meanwhile, flow rate was adjusted at Reynolds numbers ranging from 10000 to 30000. Study has been done under uniform heat flux condition and air was used as working fluid. Experimental results confirm that the use of vane swirler leads to a higher heat transfer compared with those obtained from plain tubes. Depending on blade angle, overall Nusselt augmentation is found from 50% to 110% while friction factor increases by the range of 90–500%. Thermal Performance evaluation has been done for test section and test section together with swirler. In both cases, thermal performance increases as vane angle is raised and decreases by growth of Re number. When increasing the blade angle, higher decay rate has been observed for local Nusselt number. In CFD analysis, time-averaged governing equations were solved numerically and RSM model was applied as the turbulence model. Here, the simulation results of axial and tangential velocities, turbulent kinetic energy, wall stresses and swirl intensity are provided. They illustrate the effect of swirling pattern on mean flow and turbulence structure, as well as on improving heat transfer enhancement in the annular duct.     

Investigation of large-scale structures of annular swirling jet in a non-premixed burner using delayed detached eddy simulation

Delayed detached eddy simulation (DDES) is accompanied with Stereo-PIV measurements to study the non-reacting flow field of a non-premixed swirl burner in this paper. Comparisons of experimental and numerical data show that the DDES results are capable of predicting the mean swirling flow features adequately. The instantaneous flow field is found to be strongly affected by the Kelvin–Helmholtz instability. The flow near the injector involves a complex behavior including a recirculation zone. The 3D flow structure at the burner exit, visualized by the iso-surface of Q-criterion, displays four instability types. The dominant instabilities are vortex ring structures induced by the Kelvin–Helmholtz instability, and finger structures induced by the swirling instability. Pressure fluctuation signal recorded in the swirling jet region show that the computational flow passes through transition instants from RANS to DDES equations. After that, the swirling jet becomes fully developed with an oscillation frequency of 222 Hz.