Characteristics of strongly-forced turbulent jets and non-premixed jet flames

Previous researchers have demonstrated that strong pulsations of the fuel flow rate can significantly reduce the flame length and luminosity of laminar/transitional non-premixed jet flames. The physical mechanisms responsible for these changes are investigated experimentally in acoustically-forced jet flows where the peak velocity fluctuations are up to eight times the mean flow velocity. Both reacting and non-reacting flows were studied and Reynolds numbers, based on the mean flow properties, ranged from 800 to 10,000 (corresponding to peak Reynolds numbers of 1,450–23,000), and forcing frequencies ranged from 290 to 1,140 Hz. Both the first and second organ-pipe resonance modes of the fuel delivery tube were excited to obtain these frequencies. An analysis of the acoustic forcing characteristics within the resonance tube is provided in order to understand the source of the high amplitude forcing. Flow visualization of jets with first resonant forcing confirms the presence of large-scale coherent vortices and strong reverse flow near the exit of the fuel tube. With second-resonant forcing, however, vortices are not emitted from the tube as they are drawn back into the fuel tube before they can fully form. Increased fine-scale turbulence is associated with both resonant cases, but particularly at second resonance. The power spectra of the velocity fluctuations for a resonantly pulsed jet show the presence of an inertial subrange indicating that the flow becomes fully turbulent even for mean-Reynolds-number jets that are nominally laminar. It is shown that these pulsed jet flows exhibit strong similarities to synthetic jets and that the Strouhal number, based on the maximum velocity at the fuel tube exit, is the dominant parameter for scaling these flows. The Strouhal number determines the downstream location where the coherent vortices breakdown, and is found to provide better collapse of flame length data (both current and previous) than other parameters that have been used in the literature.     

Effect on flow field characteristics in methane–air non-premixed flame with steam addition

The present paper reports on an experimental study to determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame, and examines the dynamical behavior of the unsteady aerodynamic flow structures observed on a bluff-body burner at both humid and non-humid air combustion states. Particle image velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative velocity field. Streamlines and velocity contours analysis are used to identify specific flame structures and reveal the effect of steam added on the vortex structure. The results show both central fuel penetration limit and partially quenching limit in the humid air case reduce. The decrease in the critical penetration limit is primarily attributed to a reduction in momentum of the humid air. The flamelet concepts are applied to discuss the partially quenching limit in the blue neck region. The analysis reveals that the large decrease in the partially quenching limit is due to the increase in chemical reaction time of the humid air combustion.     

Quantitative three-dimensional imaging of soot volume fraction in turbulent non-premixed flames

A three-dimensional (3-D) imaging system for studies of reactive and non-reactive flows is described. It can be used to reveal the topology of turbulent structures and to extract 3-D quantities, such as concentration gradients. Measurements are performed using a high repetition rate laser and detector system in combination with a scanning mirror. In this study, the system is used for laser-induced incandescence measurements to obtain quantitative 3-D soot volume fraction distributions in both laminar and turbulent non-premixed flames. From the acquired data, iso-concentration surfaces are visualised and concentration gradients calculated.     

Influence of pulverized coal properties on heat release region in turbulent jet pulverized coal flames

The effects of pulverized coal properties, volatile matter, particle size and moisture content, on the heat release region in turbulent jet pulverized coal flames were investigated experimentally. To understand the accuracy of line of sight measurement in the two-dimensional (2-D) visualization, point measurements of chemiluminescence intensity by cassegrain optics were also conducted. The heat release region for the structure of pulverized coal flame was observed through visualization by CH^∗ chemiluminescence image with an intensified high-speed camera, and by CH^∗ chemiluminescence intensity for local point measurements. The streamwise length of the heat release region based on 2-D visualizations was about 11.4% longer than that of point measurements and increased proportionally to the volatile matter content. The temperature rise for 35-45 μm coal particles was faster than that for 75-90 μm particles, which resulted in a shift of reaction region toward upstream direction. The length of heat release region depends upon the particle size and the volatile matter. However, the coal moisture content less than 15% shows minor effect on the heat release region comparison to volatile matter and coal size within our experimental conditions.     

Stabilization of non-premixed flames in supersonic reactive flows

A Lagrangian framework is set out to describe turbulent non-premixed combustion in high speed coflowing jet flows. The final aim is to provide a robust computational methodology to simulate, in various conditions, the underexpanded GH2/GO2 torch jet that is used to initiate combustion in an expander cycle engine. The proposed approach relies on an early modelling proposal of Borghi and his coworkers. The model is well suited to describe finite rate chemistry effects and its recent extension to high speed flows allows one to take the influence of viscous dissipation phenomena into account. Indeed, since the chemical source terms are highly temperature sensitive, the influence of viscous phenomena on the thermal runaway is likely to be all the more pronounced since the Mach number values are high. The validation of the extended model has been recently performed through the numerical simulation of two distinct well-documented experimental databases. Only a brief summary of this preliminary validation step is provided here. The main purpose of the present work is to proceed with the numerical simulation of geometries that bring together the essential peculiarities of the underexpanded GH2/GO2 torch. The behavior of the corresponding supersonic coflowing jet flames for various conditions is discussed in the light of computational results. To cite this article: J.-F. Izard, A. Mura, C. R. Mecanique 337 (2009).     

Laminar and turbulent burning velocities of propane in spherical vessels

Laminar and turbulent burning velocities of C₃H₈/air mixtures under moderately elevated temperature and pressure conditions have been determined experimentally using spherical combustion bombs. The results are reported here and the effects of temperature, pressure, turbulence intensity and stoichiometric ratio are included. The results obtained are compared with those of other researchers. The laminar burning velocity obtained from the pressure-time curves without compensation for the reaction zone thickness is lower than those determined using burners. A reaction zone thickness correction which reduces the difference is suggested and this relies on an assessment based on the different histories found experimentally when different initial temperatures were used for the isentrope through the standard state. For mixtures with initially low turbulence levels, both the turbulent burning velocity and the burning velocity ratio, turbulent to laminar, increase as the combustion progresses. The rate of increase becomes larger with increase in the initial and instantaneous turbulence intensity, temperature and stoichiometric ratio. Deviation from the Damköhler relationship is noted.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Numerical simulation of methane-air turbulent jet flame using a new second-order moment model

A new second-order moment model for turbulent combustion is applied in the simulation of methane-air turbulent jet flame. The predicted results are compared with the experimental results and with those predicted using the wellknown EBU-Arrhenius model and the original second-order moment model. The comparison shows the advantage of the new model that it requires almost the same computational storage and time as that of the original second-order moment model, but its modeling results are in better agreement with experiments than those using other models. Hence, the new second-order moment model is promising in modeling turbulent combustion with NO_x formation with finite reaction rate for engineering application. The project sponsored by the Foundation for Doctorate Thesis of Tsinghua University, and the National Key Project in 1999–2004 sponsored by the Ministry of Science and Technology of China     

Experimental criteria for the determination of fractal parameters of premixed turbulent flames

The influence of spatial resolution, digitization noise, the number of records used for averaging, and the method of analysis on the determination of the fractal parameters of a high Damköhler number, methane/air, premixed, turbulent stagnation-point flame are investigated in this paper. The flow exit velocity was 5 m/s and the turbulent Reynolds number was 70 based on a integral scale of 3 mm and a turbulent intensity of 7%. The light source was a copper vapor laser which delivered 20 nsecs, 5 mJ pulses at 4 kHz and the tomographic cross-sections of the flame were recorded by a high speed movie camera. The spatial resolution of the images is 155 × 121 m/pixel with a field of view of 50 × 65 mm. The stepping caliper technique for obtaining the fractal parameters is found to give the clearest indication of the cutoffs and the effects of noise. It is necessary to ensemble average the results from more than 25 statistically independent images to reduce sufficiently the scatter in the fractal parameters. The effects of reduced spatial resolution on fractal plots are estimated by artificial degradation of the resolution of the digitized flame boundaries. The effect of pixel resolution, an apparent increase in flame length below the inner scale rolloff, appears in the fractal plots when the measurent scale is less than approximately twice the pixel resolution. Although a clearer determination of fractal parameters is obtained by local averaging of the flame boundaries which removes digitization noise, at low spatial resolution this technique can reduce the fractal dimension. The degree of fractal isotropy of the flame surface can have a significant effect on the estimation of the flame surface area and hence burning rate from two-dimensional images. To estimate this isotropy a determination of the outer cutoff is required and three-dimensional measurements are probably also necessary.     

An experimental investigation of the interaction of swirl flow with partially premixed disk stabilized propane flames

The present work describes the experimental investigation of reacting wakes established through fuel injection and staged premixing with air in an axisymmetric double cavity arrangement, formed along three concentric disks, and stabilized in the downstream vortex region of the afterbody. The burner assembly is operated with a co-flow of swirling air, aerodynamically introduced upstream of the burner exit plane, to allow for the study of the interaction between the resulting partially premixed recirculating afterbody flames with the surrounding swirl. At low swirl the primary afterbody disk stabilizes the partially premixed annular jet in the downstream reacting wake formation region. As swirl increases, a system of two successive vortices emerges along the axis of the developing wake; the primary afterbody vortex is cooperating with an adjacent, swirl induced, central recirculation zone and this combination further promotes turbulent mixing in the hot wake.Complementary measurements of the counterpart isothermal turbulent velocity fields provided important information on the near wake aerodynamics under the interaction of the variable swirl and the double cavity produced annular jet stabilized by the afterbody. Under reacting conditions, measurements of turbulent velocities, temperatures and statistics together with an evaluation of the exhaust emissions were performed using LDV, thin digitally-compensated thermocouples and gas analyzers. A selected number of lean and ultra-lean flames were investigated by regulating the injected fuel and the air supply ratio, while the influence of the variation of the imposed swirl on wake development, flame characteristics and emission performance was documented for constant fuel injections. The differences and similarities between the present partially premixed stabilizer and other types of axisymmetric configurations are also highlighted and discussed.     

Experimental investigation of a free-surface turbulent jet with Coanda effect

The deviation of a jet from the straight direction due to the presence of a lateral wall is investigated from the experimental point of view. This flow condition is known as Coanda jet (from the Romanian aerodynamicist Henry Marie Coanda who discovered and applied it at the beginning of XXth century) or offset jet. The objective of the work is to detail the underlying mechanisms of such a phenomenon aiming to use it as a flow control method at polluted river flows mouth. To do this, a large laboratory free-surface tank with an incoming channel has been set up and velocity field measurements are performed by Optical Flow methods (namely Feature Tracking). Preliminary tests on the well-known free jet configuration without any marine structure (i.e. lateral wall) are performed to allow comparison with free jet scaling and self-similar solutions. The presence of the free-surface gives rise to centerline velocity decay which is lower than in free unbounded plane or circular jets due to the vertically limited ambient fluid entrainment. In the second part of the paper, the effect of a lateral wall on the jet configuration is examined by placing it at different lateral distances from the jet outlet. The resulting velocity fields clearly show an inclined Coanda jet with details which seems to depend on the lateral wall distance itself. The analysis of self-similarity along the inclined jet direction reveals that for wall distances larger than 5 jet widths this dependence almost disappears.     

Experimental analysis of transverse intermittency in a turbulent jet flow

The statistical properties of the velocity differences are experimentally investigated in a turbulent jet-flow at moderate Re _λ by X-probe hot wire anemometry measurements. It is found that the traverse velocity components show a more intermittent behavior with respect to the longitudinal ones. This result is obtained by the analysis of the longitudinal and transverse intermittency exponents measured by the Extended Self-Similarity form of scaling, and by the comparison of the longitudinal and transverse velocity difference Probability Distribution Functions. Received: 25 March 1996/Accepted: 15 August 1996     

Spatial resolution effects on the measurement of scalar variance and scalar gradient in turbulent nonpremixed jet flames

The effect of spatial averaging is important for scalar gradient measurements in turbulent nonpremixed flames, especially when the local dissipation length scale is small. Line imaging of Raman, Rayleigh and CO-LIF is used to investigate the effects of experimental resolution on the scalar variance and radial gradient in the near field of turbulent nonpremixed CH₄/H₂/N₂ jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and B) and in piloted CH₄/air jet flames at Reynolds numbers of 13,400, 22,400 and 33,600 (Sandia flames C/D/E). The finite spatial resolution effects are studied by applying the Box filter with varying filter widths. The resulting resolution curves for both scalar variance and mean squared-gradient follow nearly the same trends as theoretical curves calculated from the model turbulence kinetic energy spectrum of Pope. The observed collapse of resolution curves of mean squared-gradient for nearly all studied cases implies the shape of the dissipation spectrum is approximately universal. Fluid transport properties are shown to have no effect on the dissipation resolution curve, which implies that the dissipation length scale inferred from the square gradient is equivalent to the length scale for the scalar dissipation rate, which includes the diffusion coefficient. With the Box filter, the required spatial resolution to resolve 98% of the mean dissipation rate is about one−two times of the dissipation cutoff length scale (analogous to the Batchelor scale in turbulent isothermal flows). The effects of resolution on the variances of mixture fraction, temperature, and the inverted Rayleigh signal are also compared. The ratio of the filtered variance to the true variance is shown to depend nearly linearly on the probe resolution. The inverted Rayleigh scattering signal can be used to study the resolution effect on temperature variance even when the Rayleigh scattering cross section is not constant. The experimental results also indicate that these laboratory scale turbulent jet flames have small effective Reynolds numbers, such that there is some direct interaction of the large (energy containing) and small (dissipative) scalar length scales, especially for the near field case at x/d = 7.5 of the piloted Sandia flames C/D/E.     

Experimental investigation of flame/solid interactions in turbulent premixed combustion

An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.     

Experimental investigation of turbulent natural convection flow in a channel

This paper presents the results of velocity measurements of natural convection in symmetrically heated vertical channel using the particle image velocimetry (PIV) system. Velocity measurements were conducted at three different sections on the horizontal plane to validate the flow two-dimensionality and at three different heights in the vertical plane to establish vertical mean velocity profiles. The results indicate a considerable influence of the Rayleigh number and aspect ratio on the mean velocity profile. The results also indicate significant diffusion rates of the vertical mean velocity component and normal Reynolds stresses towards the center of the channel.     

Modelling and simulation of nonpremixed turbulent flames

A computational model for nonpremixed turbulent flames is presented. It is based on the conserved scalar approach and on a convenient specification of the probability density function, which allows the mean density to be recovered in closed (algebraic) form. The k-1 model is adopted for turbulence, and the resulting equations for parabolic flows are solved via a block implicit algorithm. The computed results are compared with experimental data and other authors' predictions.

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Sommario Si propone un modello per fiamme turbolente nonpremiscelate. Esso si basa sull'approccio dello scalare conservato e su una conveniente specificazione della funzione densità di probabilità, che permette di ottenere la densità media in forma chiusa (algebrica). Per la turbolenza si adotta il modello k-1, ed il sistema di equazioni risultante per flussi parabolici è risolto attraverso un algoritmo implicito a blocchi. I risultati calcolati sono confrontati con dati sperimentali e previsioni di altri autori.

     

Experimental investigation on the similarity of a 3D rectangular turbulent jet

 The paper presents an experimental investigation of turbulent jets issuing from rectangular nozzles. Nozzles with aspect ratios between 3 and 10 were used. Eight different initial conditions were studied. The following jet parameters were measured and evaluated: mean velocity components, jet boundaries, jet momentum, jet entrainment, turbulence intensities and Reynolds stresses. A DISA 55M thermoanemometer and a data acquisition system BE256 were used. The influence of the initial conditions on the similarity of the flow was determined with respect to the mean axial velocity, turbulence intensity and the Reynolds stresses. A significant influence of the initial conditions on the flow structure was observed. The possibility for jet control is discussed and suggestions are given about the need to investigate different parameters. Received: 25 November 1996/Accepted: 30 October 1997     

Calculation of turbulent combustion of propane in furnaces

An evaluation of some numerical methods for turbulent reacting flows in furnace-like geometries is carried out. The Reynolds averaged Navier–Stokes equations and the two-equation k–? model together with either finite-rate or infinite-rate reaction models are solved numerically. Either single- or multiple-step reactions together with the ‘eddy dissipation concept’ (EDC) are used to model reacting flows with finite reaction rates. The numerical scheme is finite difference based, together with a multi-grid method and a local grid refinement technique. These methods have been used to calculate the combustion of propane in a single- and multiple-burner configurations. In the former case, the sensitivity of the solution to variations in some model parameters (determining the reaction rate) and numerical parameters (mesh spacing) has been studied. It is noted that different dependent variables exhibit different levels of sensitivity to the variation in model parameters. Thus, calibration and validation of models for reacting flows require that one compares the most sensitive variables. For engineering purposes, on the other hand, one may calibrate and validate models with respect to the most relevant variables. Our conclusion is that since sensitivity of the temperature distribution is relatively mild, one can still use EDC-like methods in engineering applications where details of the temperature field are of minor importance.