Dynamics of PIV seeding particles in turbulent premixed flames

Particle image velocimetry (PIV) estimates the fluid velocity field measuring the displacement of small dispersed particles between two successive instants separated by a small time interval. The accuracy of the measurements depends on the ability of the particles to accommodate their velocity to the fluid fluctuations. When the fluid is subjected to extreme accelerations, the small but finite inertia prevents the particles from following the fluid, originating a substantial relative velocity. This effect is shown to be crucial for applications of PIV to turbulent premixed combustion, particularly in the product region at locations just behind the instantaneous flame front. The issuing inaccuracy may easily spoil the estimate of certain statistical observables which are of crucial importance in the theory of turbulent premixed combustion. By exploiting the direct numerical simulation of a model air/methane flame, a suitable criterion for proper particle seeding is validated and compared with the corresponding experiments with a combined PIV/OH-LIF (laser-induced fluorescence) system. The proposed parameter, the flamelet Stokes number, depends on particle properties and thermochemical conditions of the flame and substantially restricts the particle dimensions required for a reliable estimate of the relevant flow statistics.     

Rayleigh scattering for density measurements in premixed flames

Rayleigh scattering measurements for molecular number density in turbulent, premixed CH₄-air flames are discussed, and data for both flamelet passage time distributions and power spectral density functions are reported and compared to the recent predictions of Bray, Libby and Moss (1984). Measurement problems associated with variations in mixture-averaged Rayleigh scattering cross section, index of refraction fluctuations, finite spatial and temporal resolution and with scattering from particles are discussed. It is concluded that these effects are relatively minor in the reported experiments. Correction procedures are suggested for the effects of cross section variation and of finite resolution. Passage time and spectral data support the Bray, Libby and Moss hypothesis for the passage time distribution function. Furthermore, model predictions for the variation across the flame brush of mean passage times for both reactant and product eddies are in reasonable agreement with experiment. Finally, the data suggest that these mean times scale in part with ū and λ in the reactant flow.     

Heat and mass transfer study of impinging turbulent premixed flames

 Impinging jet combusting flows on granite plates are studied. A mathematical model for calculating heat release in turbulent impinging premixed flames is developed. The combustion including radiative heat transfer and local extinction effects, and flow characteristics are modeled using a finite volume computational approach. Two different eddy viscosity turbulence models, namely the standard k–ɛ and the RNG k–ɛ model with and without radiation (discrete transfer model) are assessed. The heat released predictions are compared with experimental data and the agreement is satisfactory only when both radiative heat transfer and local extinction modeling are taken into account. The results indicate that the main effect of radiation is the decrease of temperature values near the jet stagnation point and along the plate surface. Radiation increases temperature gradients and affects predicted turbulence levels independently of the closure model used. Also, the RNG k–ɛ predicts higher temperatures close the solid plate, with and without radiative heat transfer. Received on 13 November 2000 / Published online: 29 November 2001     

Temperature response of turbulent premixed flames to inlet velocity oscillations

Flame–turbulence interactions are at the heart of modern combustion research as they have a major influence on efficiency, stability of operation and pollutant emissions. The problem remains a formidable challenge, and predictive modelling and the implementation of active control measures both rely on further fundamental measurements. Model burners with simple geometry offer an opportunity for the isolation and detailed study of phenomena that take place in real-world combustors, in an environment conducive to the application of advanced laser diagnostic tools. Lean premixed combustion conditions are currently of greatest interest since these are able to provide low NO _x and improved increased fuel economy, which in turn leads to lower CO₂ emissions. This paper presents an experimental investigation of the response of a bluff-body-stabilised flame to periodic inlet fluctuations under lean premixed turbulent conditions. Inlet velocity fluctuations were imposed acoustically using loudspeakers. Spatially resolved heat release rate imaging measurements, using simultaneous planar laser-induced fluorescence (PLIF) of OH and CH₂O, have been performed to explore the periodic heat release rate response to various acoustic forcing amplitudes and frequencies. For the first time we use this method to evaluate flame transfer functions and we compare these results with chemiluminescence measurements. Qualitative thermometry based on two-line OH PLIF was also used to compare the periodic temperature distribution around the flame with the periodic fluctuation of local heat release rate during acoustic forcing cycles.     

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.     

Temperature characteristics of turbulent,premixed flames stabilised on a step

The paper presents fine-wire thermocouple measurements of temperature and temperature fluctuations in a confined, two-dimensional, premixed methane-air flame of 0.8 equivalence ratio, stabilised by a backward-facing step, and with bulk flow Reynolds numbers ranging from 2800 to 13000. The results indicate a narrowing of the flame front with decrease in Reynolds number. The temperature fluctuation measurements were compensated for thermal inertia and the probability density functions were bimodal with the probability density at the two extremes of the temperature range increasing with fall in Reynolds number. Temperature power spectra indicated that the preferred frequencies corresponded to the acoustic frequencies of the combustor system, although the influence of vortex shedding was evident at the lowest Reynolds number.     

Time-resolved measurements of hydroxyl in stable and extinguishing partially premixed turbulent opposed-jet flames

Quantitative hydroxyl time-series measurements from a set of stable and extinguishing turbulent opposed-flow partially premixed CH₄/air flames are used to investigate the effect of Reynolds number and fuel-side equivalence ratio on the structure of turbulent partially premixed flames. The hydroxyl (OH) integral time scale, computed from the autocorrelation function, is used to characterize OH fluctuations and is found to reach a minimum at the axial location of peak OH. Analyses of the duration of and period between bursts in the OH time series are used to examine the dynamics of flame-front motion. In general, with increasing Reynolds number (Re), the distribution in OH burst times shifts towards smaller time scales. A hydroxyl intermittency parameter is also defined from the bursts to quantify the presence or absence of OH. For flames with the same fuel-side equivalence ratio, the hydroxyl intermittency at peak OH remains almost constant when going from stable to extinguishing flames. However, histograms portray an increase in burst separation times for flames displaying occasional extinction events. Hydroxyl time series for a partially premixed flame at a fuel-side equivalence ratio of 2.0 and Re = 6650 are synthesized by using mixture-fraction simulations based on calculated state relationships for OH versus mixture fraction (f). The laminar-flamelet model is employed to explore relations between OH and f so as to predict trends in mixture-fraction time scales.“Time-Series Measurements in Turbulent Opposed-Jet Flames" is submitted for consideration as a full length article to Flow Turbulence and Combustion.     

Error estimates for Rayleigh scattering density and temperature measurements in premixed flames

Rayleigh scattering has become an accepted technique for the determination of total number density during the combustion process. The interpretation of the ratio of total Rayleigh scattering signal as a ratio of densities or temperatures is hampered by the changing composition through a flame, since the average Rayleigh scattering cross-section depends on the gas composition. Typical correction factors as a function of degree of reaction, fuel and equivalence ratio were calculated. The fuels considered were H₂, CH₄, C₂H₄, C₂H₆ and C₃H₈. Factors as low as 0.7 and 0.56 were found for the heaviest hydrocarbon fuel at large equivalence ratio for interpreting the Rayleigh scattering intensity as gas density and inverse temperature, respectively. This is primarily due to the presence of CO and H₂ as intermediates. As CO and H₂ are subsequently oxidized to CO₂ and H₂O, these factors approach 1.0. Conversely, the worst case, when using H₂ as a fuel, occurs in the post flame zone. However, the correction factors for H₂ are near 1.0 and the errors involved will, in general, remain within the expected experimental accuracy of a typical Rayleigh scattering system. Linear correlations of correction factors with equivalene ratio and with the product of equivalence ratio and fuel molecular weight were found and presented. The interpretation of Rayleigh scattering as temperature was found to have larger errors than the interpretation as density. Corrections for changes in gas composition were applied to Rayleigh scattering temperature measurements in the post flame region of CH₄ and C₃H₈ flames with equivalence ratios of 0.75 and 1.0. The corrected temperatures were in excellent agreement with thermocouple measurements.List of symbols A ₁, A ₂ correlation coefficients - B ₁, B ₂ correlation coefficients - C ₁, C ₂ correlation coefficients - D ₁, D ₂ correlation coefficients - C calibration constant for Rayleigh scattering optics - H total enthalpy - Î I _R /I _RO - I _i incident laser intensity - I _R Rayleigh scattering intensity - I _R0 Rayleigh scattering intensity at reference condition - N total number density of gas - N ₀ total number density of gas at reference condition - n _i index of refraction of species i - – T/T _O - T temperature - T _a adiabatic flame temperature - T ₀ reference temperature - t time - W/W ₀ - W mean molecular weight - W ₀ mean molecular weight at reference condition - W _ij rate of production of species i by reaction j - X _i mole fraction of species i - degree of reaction (T – T ₀)/(T _a – T ₀) - laser wavelength - ₀ Loschmidt number - /₀ - density - ₀ density at reference condition - dimensionless mean Rayleigh scattering cross-section - _Ri Rayleigh scattering cross-section of species i - scattering angle measured from the electromagnetic field vector - equivalence ratio     

Imaging of premixed flames in microgravity

A laser schlieren system which uses video recording and digital images analysis has been developed and applied successfully to microgravity combustion experiments performed in a drop-tower. The optical system and the experiment are installed within a small package which is subjected to free-fall. The images are recorded on video tape and are digitized and analyzed by a computer-controlled image processor. The experimental results include laminar and turbulent premixed conical flames in microgravity, normal positive gravity (upward), and reverse gravity (downward). The procedures to extract frequency information from the digitized images are described. Many gross features of the effects of gravity on premixed conical flames are found. Flames that ignite easily in normal gravity fail to ignite in microgravity. Buoyancy driven instabilities associated with an interface formed between the hot products and the cold surrounding air is the mechanism through which gravity influences premixed laminar and turbulent flames. In normal gravity, this causes the flame to flicker. In reverse gravity, -g, and microgravity, g, the interface is stable and flame flickering ceases. The flickering frequencies of +g flames vary with changing upstream boundary conditions. The absence of flame flickering in g suggest that g flames would be less sensitive to these changes.This work is supported by NASA Microgravity Sciences and Applications Divisions under contract No. C-32000-R through the U.S. Department of Energy Contract No. DE-AC03-76F00098. Technical support is provided by NASA Lewis Research Center. Project Scientist is Dr. Karen J. Weiland. The authors would like to acknowledge Dr. Liming Zhou for his contribution to early testing of the schlieren system, and to Mr. Gray Hubbard for writing the image analysis software     

Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner

Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH₄, CO, CO₂, H₂ and N₂, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.     

A-priori testing of an eddy viscosity model for the density-weighted subgrid scale stress tensor in turbulent premixed flames

In this study, we report on the direct measurement of the density-weighted subgrid scale (SGS) stress tensor in turbulent premixed flames. In large-eddy simulations (LES), this unresolved tensor is typically modelled using eddy viscosity approaches. Additionally to the direct measurement, we provide a pure experimentally based a-priori test of the commonly used eddy viscosity model suggested by Smagorinsky. For two turbulent premixed V-shaped methane–air flames, a statistical analysis is presented where the correlation between the directly measured SGS stress tensor and the eddy viscosity model following Smagorinsky is tested. The measurement strategy is based on the application of a dual-plane stereo-PIV technique which enables the measurement of the 3D flow field in two parallel planes. This allows the determination of velocities as well as velocity gradients in all three directions. Here, a vector resolution of 118 μm was achieved. For a priori testing, the data are subjected to a spatial filtering procedure that reproduces the application of the filter function in LES. The calculation of velocity gradients is performed after the application of this spatial averaging. Additionally to the velocity field, the flame front position is deduced from the clearly observable step in the tracer particle number density between burnt and unburnt regions of the flame. This facilitates the direct single-shot-based evaluation of all components of the density-weighted SGS stress tensor. Additionally, the model expressions related to these terms can be determined, which is done in this first study for the static Smagorinsky model. With that, the instantaneous local comparison between directly measured stress terms and modelled terms is possible, based on the instantaneous local evaluation procedure. The measurement procedure is described, and first results are presented and discussed. They show a rather poor performance of the static form of the Smagorinsky model (with fixed Smagorinsky constant). Our future aims are to use the directly measured SGS data for the a-priori comparison with more advanced models.     

Dynamics of premixed confined swirling flames

Considerable effort is currently being extended to examine the fundamental mechanisms of combustion instabilities and develop methods allowing predictions of these phenomena. One central aspect of this problem is the dynamical response of the flame to incoming perturbations. This question is examined in the present article, which specifically considers the response of premixed swirling flames to perturbations imposed on the upstream side of the flame in the feeding manifold. The flame response is characterized by measuring the unsteady heat release induced by imposed velocity perturbations. A flame describing function is defined by taking the ratio of the relative heat release rate fluctuation to the relative velocity fluctuation. This quantity is determined for a range of frequencies and for different levels of incoming velocity perturbations. The flame dynamics is also documented by calculating conditional phase averages of the light emission from the flame and taking the Abel transform of these average images to obtain the flame geometry at various instants during the cycle of oscillation. These data can be useful to the determination of possible regimes of instability. To cite this article: P. Palies et al., C. R. Mecanique 337 (2009).     

Tulip flames: changes in shape of premixed flames propagating in closed tubes

 The experimental results that are the subject of this communication provide high-speed schlieren images of the closed-tube flame shape that has come to be known as the tulip flame. The schlieren images, along with in-chamber pressure records, help demonstrate the effects of chamber length, equivalence ratio, and igniter geometry on formation of the tulip flame. The pressure/time records show distinct features which correlate with flame shape changes during the transition to tulip. The measurements indicate that the basic tulip flame formation is a robust phenomenon that depends on little except the overall geometry of the combustion vessel. Received: 22 April 1997/Accepted: 7 July 1997     

Scale similarity based models and their application to subgrid scale scalar flux modelling in the context of turbulent premixed flames

The performance of a variety of scale similarity (SS) type models for closure of sub-grid scalar flux in the context of Large Eddy Simulations (LES) of premixed turbulent combustion has been assessed. In addition to the well-known SS models, a more recent development by Anderson and Domaradzki (2012) is included in the analysis and also further model extensions and improvements are discussed. The work is based on a priori analysis of two Direct Numerical Simulation (DNS) databases of freely propagating turbulent premixed flames with a range of different Lewis and turbulent Reynolds numbers. Depending on the balance between the effects of flame normal acceleration due to heat release and the effects of turbulent velocity fluctuations, as well as the filter size, the subgrid-scalar flux exhibits both local gradient and counter-gradient transport which presents a considerable modelling challenge. The assessment is based on a correlation analysis and on the magnitude of the model expressions conditional on the Favre averaged reaction progress variable in comparison to the value obtained from DNS. Despite the fact that most of the models have been developed in the context of momentum transport in non-reactive flows they show either comparable or better performance in comparison to more conventional models used for reactive scalar flux closure. It is found that some models are sensitive to the test filter width and recommendations are provided in this regard. Further it is observed that the use of a Favre test filter substantially increases the correlation strength in direction of mean flame propagation where effects of heat release are most pronounced.     

Simultaneous velocity and scalar measurements in premixed recirculating flames

 The use of a laser-Doppler velocimeter has been extended to the analysis of turbulent heat transfer in a strongly sheared disc-stabilised propane-air flame through its combination with either laser Rayleigh scattering or digitally-compensated fine-wire thermocouples. The laser velocimeter was based on a conventional forward scattering system from the green light of a 5W Argon-Ion laser, while the Rayleigh signals used the blue line of the same laser. The procedure for the numeric compensation of the thermocouple signals included analysis of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to allow combined velocity–temperature samples acquired by a purpose-built digital interference with a frequency up to 2000 Hz, without deterioration of the thermocouple by particle accretion. The maximum effective data rate for the combined Rayleigh/LDV system is shown to be around 130 Hz, which corresponds to a data rate of valid Doppler signals around 400 Hz and statistics based on more than 15 000 measurements is made possible. The results obtained with the two systems agree qualitatively, although the use of thermocouples attenuates the measured velocity-temperature correlations. The results are used to assess the extent to which turbulent mixing in flames is altered by the accompanying heat release and quantify the processes of non-gradient diffusion in a strongly recirculating premixed flame. Received: 15 November 1996/Accepted: 2 September 1997     

Fractal analysis of turbulent premixed flame surface

The fractal-like character of the laminar flamelet surface in turbulent premixed combustion of lean methane/air mixtures was studied by using the laser tomography technique to visualize the instantaneous flame surface in the two-dimensional section cut by the laser sheet. The fractal analysis of the surface revealed that the surface actually exhibits a self-similarity behavior in a narrow range of scale, and the value of fractal dimension can be defined. The inner cutoff scale was the laminar flame thickness, while the outer cutoff scale was the flame size. The fractal dimension was found to depend on the orientation of the section, and to increase towards downstream. It is suggested that the observed fractal-like character is not directly connected to approach flow turbulence, but should represent certain aspects of the flamelet itself.     

CH double-pulsed PLIF measurement in turbulent premixed flame

The flame displacement speeds in turbulent premixed flames have been measured directly by the CH double-pulsed planar laser-induced fluorescence (PLIF). The CH double-pulsed PLIF systems consist of two independent conventional CH PLIF measurement systems and laser beams from each laser system are led to same optical pass using the difference of polarization. The highly time-resolved measurements are conducted in relatively high Reynolds number turbulent premixed flames on a swirl-stabilized combustor. Since the time interval of the successive CH PLIF can be selected to any optimum value for the purpose intended, both of the large scale dynamics and local displacement of the flame front can be discussed. By selecting an appropriate time interval (100–200 μs), deformations of the flame front are captured clearly. Successive CH fluorescence images reveal the burning/generating process of the unburned mixtures or the handgrip structures in burnt gas, which have been predicted by three-dimensional direct numerical simulations of turbulent premixed flames. To evaluate the local flame displacement speed directly from the successive CH images, a flame front identification scheme and a displacement vector evaluation scheme are developed. Direct measurements of flame displacement speed are conducted by selecting a minute time interval (≈30 μs) for different Reynolds number (Re _λ = 63.1–115.0). Local flame displacement speeds coincide well for different Reynolds number cases. Furthermore, comparisons of the mean flame displacement speed and the mean fluid velocity show that the convection in the turbulent flames will affect the flame displacement speed for high Reynolds number flames.