Juan-Carlos Rolon

Dynamics of flame/vortex interactions

Abstract Vortex interactions with flames play a key role in many practical combustion applications. Such interactions drive a large class of combustion instabilities, they control to a great extent the structure of turbulent flames and the corresponding rates of reaction, they occur under transient operations or when flames travel in ducts containing obstacles. Vortices of various types are often used to enhance mixing, organize the flame region, and improve the flame stabilization process. The analysis of flame/vortex interactions has value in the development of our understanding of basic mechanisms in turbulent combustion and combustion instability. The problem has been extensively investigated in recent years. Progress accomplished in theoretical, numerical and experimental investigations on flame/vortex interactions is reviewed in this article.

Experiments on the interaction between a vortex and a strained diffusion flame

Flame-vortex interactions constitute a basic problem in the analysis of turbulent combustion. Vortex rollup is also found to be one of the major driving mechanisms of combustion instabilities. While there are many analytic and numerical studies of the process, the number of detailed experiments is relatively limited. In particular, the nonpremixed case has not been explored, apparently because the experimental configuration is less easily designed. It is shown here that this case may be examined by employing a counterflow burner. A steady diffusion flame is established in this geometry and a vortex ring is generated from a cylindrical tube installed in one of the combustor nozzles. The vortex impinges on the flame from the oxidizer side entraining the reactive layer and producing a hole in the initial flame. The apparatus is described and initial results of visualizations and velocity measurements by laser doppler anemometry are discussed. It is found that the interaction leads to different outcomes. Strong vortices produce flame extinction and a subsequent blowout. In contrast, the flame is recovered after an interaction with a weak vortex.

Study of radiative effects on laminar counterflow H2/O2N2 diffusion flames

Abstract: The effects of radiative transfer on the structure and extinction limits of counterflow H-2/O-2/N-2 diffusion flames are studied numerically using detailed kinetics and transport properties. The radiative properties of the main emitting species, H2O and OH in these flames, are represented using a statistical narrow-band model. The radiative transfer equation and flow governing equations are solved in a coupled manner. The model is first validated by comparing numerical results with Rayleigh temperature and total flame radiative emission measurements. It is then applied to the numerical study of radiative effects on flame structure and properties. These effects, i.e., a decrease in flame temperature, flame width and production of minor species, are found to be the most important for high values of inlet H-2 mass fraction and for low strain rates. Quantitative values of radiative low strain rate extinction limits are given. The limits of validity and the discrepancies due to the optically thin medium approximation are also investigated.

Laminar counterflow spray diffusion flames: A comparison between experimental results and complex chemistry calculations

Abstract Experimental and numerical studies of laminar flames formed by the counterflow of a monodisperse fuel spray with an air stream are reported in this article. In this simple configuration it is possible to analyze the influence of the phase transfer terms on the flame structure. The experimental setup used to produce such laminar spray diffusion flames is first described. A set of experiments are carried with liquid heptane fuel sprays. The flame is characterized with a laser sheet imaging system and with a particle sizing apparatus based on laser light diffraction. Results of a numerical study are then presented. The two phase-reacting flow equations are solved through Newton iterations and adaptative gridding using detailed transport and complex chemistry. An iterative procedure is devised to solve the gas- and liquid-phase balance equations. Comparison between experimental and numerical values of the diameter are found to be in good agreement.

Structure of cryogenic flames at elevated pressures

Abstract: This paper presents new experimental results on cryogenic jet flames formed bb a coaxial injector at a pressure of 70 bar, which approaches the pressures found in rocket engines. This element, fed with liquid oxygen and gaseous hydrogen, is placed in a square combustion chamber equipped with quartz windows. The flame is examined via spectroscopy, OH* emission, and backlighting, the aim being to provide basic information on the flame structure. It is found that some of the OH* emission is absorbed by the OH radicals present in the flame. A detailed examination of this effect is presented, in which it is shown that, for this turbulent flame, the Abel transform gives the position of the intense reaction region, whether or not absorption is signficant. The flame is attached to the oxygen injector as at low pressure. At high pressure, flame expansion is reduced compared with low pressure and is also less dependent on the momentum flux ratio between the hydrogen and the oxygen streams. An analysis of the relevant Damkohler numbers suggests that this is because the rate of combustion is mainly controlled by large-scale turbulent mixing at high pressure, and it is dominated by jet break-up, atomization, and vaporization at low pressures. Jet break-up is particularly dependent on the momentum flux ratio. Finally; the mean volumetric heat release rates and flame surface density in the experimental facility are estimated.

Regimes of non-premixed flame-vortex interactions

Detailed studies of flame-vortex interactions are extremely valuable to improve our understanding of turbulent combustion regimes. Combined experimental and numerical studies have already been performed in the premixed case during previous investigations. Therefore, we decided to carry out a detailed experimental investigation on the regimes observed during in teraction of a vortex ring and a non-premixed, diluted, hydrogen/air, laminar counterflow flame. To obtain the needed information, several optical diagnostic techniques have been used, in particular, planar laser-induced fluorescence (PLIF) of acetone to quantify vortex structure and speed, simultaneous OH PLIF and Rayleigh measurements, and simultaneous OH PLIF and particle-imaging velocimetry (PIV) measurements. A post-processing of the results combined with direct simulations using detailed chemistry and transport models to check the quality of the postprocessing procedures has led to the construction of a spectral interaction diagram. Eight interaction types were found, emphasizing the relative importance of competing physical phenomena such as straining, curvature, wrinkling, roll-up, and extinction. In particular, we observe two different types of extinction, one due to the combined action of curvature and straining, and the other purely due to straining effects. It was also observed that many vortices are too small or dissipate too rapidly to influence the flame. In other cases, the vortex ring can lead to the formation of pockets of oxidizer burning in the fuel part of the domain. These regimes and the limits between them have important implications for the modeling of turbulent non-premixed combustion.

Extinction processes during a non-premixed flame-vortex interaction

Studies of flame-vortex interactions are quite valuable in the analysis of turbulent combustion. As turbulence may be viewed as a collection of vortices with different scales and intensities, the interaction of isolated vortical structures with flames defines the elementary process by which turbulence acts on flames. Experiments and interpretation are thus simplified because the unperturbed flame and the incoming vortex may be controlled with precision. We here investigate the influence of vortex velocity (directly related to its induced strain rate) and of global mixture ratio on the extinction limits. Three vortex types with different velocities interact with a non-premixed diluted hydrogen-air flame. The global mixture ratio of this flame has been varied between 0.5 and 1.2. Four different kinds of interaction are described, and the limits of the connected-flame regime, relevant for flamelet modeling, are identified. The growth of the flame surface during the interaction is also examined, showing very different effects depending on vortex velocity and global mixture ratio. The increase in flame surface area is maximum for slow vortices and intermediate values of the mixture ratio. The main features of the interaction and the relative importance of the increase in flame surface are then explained in the light of characteristic times and extinction strain rates obtained by asymptotic analysis. The extinction of the flame front is finally examined using direct numerical simulations of flame-vortex interactions, including complex chemistry, detailed thermodynamics, and multicomponent diffusion velocities. The relative importance of the strain rate acting on the flame front and of mixing effects is assessed, proving that unmixedness is not responsible for the extinction.

Influence of droplet number density on the structure of strained laminar spray flames

This article describes an investigation of the structure of strained diffusion flames formed by a spray of monodisperse liquid droplets and established in a counterflow. In a first part, a comparison between experimental and numerical results in the case of n-Heptane spray is presented. The experimental setup using ultrasonic atomization for the spray production is briefly described. Laser sheet images and size measurements by laser light diffraction indicate that a sharp vaporization front is formed in the vicinity of the reaction front. The burning mode is of “external group combustion” type. This configuration is calculated numerically by solving a coupled set of equations for the liquid and gaseous phase, including complex reaction and transport mechanisms. A good agreement is found between experimental results and prediction of the location of the vaporization front. In a second part, this numerical technique is used to study the case of flames formed by a spray of liquid oxygen impinging on a stream of gaseous hydrogen. The flame exhibits the same vaporization front as in the case of hydrocarbon in air. Some results concerning the influence of droplet density on the flame structure are presented in this configuration.

Structure of a non-premixed flame interacting with counterrotating vortices

Flame-vortex interactions constitute a problem of fundamental interest for turbulent combustion modeling. Flames rolled up in vortices may also be found in a variety of practical applications. Results of a theoretical and experimental study concerning the interaction of vortical structures with a nonpremixed flame are reported. Numerical computations are performed using direct numerical simulation with detailed models for the chemistry, transport processes, and thermodynamical properties. The accuracy of the time-dependent computation has been carefully checked. Due to the cost of the simulations, the calculations were carried out on parallel vector supercomputers. The experimental results are obtained using a steady, nonpremixed counterflow flame with hydrogen as the fuel. A vortex ring, which is generated from a tube installed in one of the combustor nozzles, impinges on the flame. A detailed description of the numerical results is given, showing a global enhancement of the chemical reactions due to the interaction with the vortices. Qualitative comparison between theoretical and experimental results concerning the flame structure during the interaction process are also described. Numerical results are used to complement the experimental measurements and help explain typical features of the roll-up, while experimental results may be employed to assess the validity of the computational procedure.

Sensitivity of temperature and concentration measurements in hot gases from FTIR emission spectroscopy

Abstract The IR emission spectra of combustion gases in a quasi-two-dimensional burner have been measured and processed for temperature and concentration determinations. A Fourier transform spectrometer with a spectral resolution up to 0.02 cm −1 has been used in the spectral range 1700– 4300 cm −1 . An original data reduction procedure based on the adjustment of the measured spectra and calculated ones at low spectral resolution has been developed and is shown to be efficient for temperature, CO2 and H2O measurements. The temperature was also deduced from the ratio of the intensities of a suitable pair of CO lines and CO concentration was determined from individual CO line intensities. The sensitivity of the different data reduction procedures is discussed and FTIR results are also compared to probe measurements.