Swirling Spray Flames Research Articles

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Computational prediction of thermoacoustic instabilities arising in gas turbine and aero-engine combustors still remains a challenge especially if fuel is injected in a liquid spray form. This study shows that, in LES of such a combustor, the treatment of the liquid fuel film created on the walls of the injection system affects the mean flame weakly, but modifies the flame dynamics strongly. The configuration used for this work is the experimental setup SICCA-spray available at EM2C laboratory in Paris. First steady spray flame measurements are used to validate the LES Euler-Lagrange approach. Two modelling strategies for the interaction between the liquid fuel and the injector walls are tested with a negligible impact on the flame shape and structure. In the second part the same comparison is applied to another operating condition where a self-sustained thermo-acoustic limit-cycle is experimentally observed. In that case resonant coupling is achieved with LES, confirming the adequacy of the approach but only when the film layer is taken into account. Indeed, contrarily to the stable configuration, the difference between the two Lagrangian boundary conditions is shown to have a major impact on the feedback mechanism leading to the thermoacoustic oscillation.

Experimental study on NOx emission correlation of fuel staged combustion in a LPP combustor at high pressure based on NO-chemiluminescence

Experimental investigations on NOx emissions of a single-cup, Lean Premixed Prevaporized (LPP), module combustor were carried out at elevated inlet temperature and pressure up to 810 K and 2.0 MPa, close to the real operating conditions of aero-engine combustors. This LPP combustor adopts centrally staged fuel injections which could produce separated stratified swirling spray flame. In the NOx emissions measurements, the ranges of dome equivalence ratio and fuel stage ratio were from 0.55 to 0.58 and 8% to 24%, respectively. The optical diagnosis on separated stratified swirling spray flame were carried out with fuel stage ratio changing from 15% to 30%. Therefore, NO* and OH* chemiluminescence images were obtained. The results show that NOx emissions increase with the increase of the fuel stage ratio. And from the chemiluminescence images, the main flame and pilot flame are found weakly coupled. The pilot flame plays a significant role in NOx emission production because of its higher adiabatic flame temperature. Based on the results of chemiluminescence optical tests, a new NOx emission prediction model is proposed based on the Lefebvre’s single flame model. The estimate of local equivalence ratio of the pilot stage’s non-premixed flame is modified considering the characteristics of spray combustion, and a “PLUS” emission prediction model suitable for separated stratified swirling spray flame is obtained. Compared to the experimental data, the “PLUS” model exhibits a good prediction in a range of ±13% of deviation.

Temperature measurements in confined swirling spray flames by vibrational coherent anti-stokes Raman spectroscopy

A gas turbine model combustor for swirling spray flames has been operated at atmospheric pressure with n-hexane, n-dodecane and kerosene Jet A-1. Temperature measurements were performed using single-shot broadband shifted vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS). Series of 1200 single-shot measurements were performed at different radial and vertical locations in the flames from which the temperature distributions were deduced. In regions with high droplet load a significant number of CARS spectra were discarded due to large signal background from laser-induced breakdown effects. Results from the flames burning different fuels were compared and revealed considerable differences in the temperature profiles. The temperature measurements are part of a comprehensive research program that aims at the design of alternative fuels for aero engines and stationary gas turbines. In addition to the experimental characterization of the spray flames, the datasets are used for the validation and improvement of computational models.

Measurements in swirling spray flames at blow-off

Various characteristics of swirling spray flames of ethanol, n-heptane, n-decane, and n-dodecane have been measured at conditions far from and close to blow-off using phase Doppler anemometry and OH* chemiluminescence, OH-planar laser-induced fluorescence, and Mie scattering at 5 kHz. The blow-off transient has also been examined. The OH* showed that the two main heat release regions lie around the spray jet at the inner recirculation zone and along the outer shear layer between the inner recirculation zone and the annular air jet. The heat release region is shortened and more attached as the flame approached blow-off. Mie images and phase Doppler anemometry data showed a wider dispersion of the ethanol spray compared to the other fuels. Similar spatial distributions of the Sauter mean diameter were observed for the four fuels for identical flow conditions, with the Sauter mean diameter value increasing with decreasing fuel volatility, but with the exception of significant presence of droplets in the nominally hollow cone for the ethanol spray. The OH-planar laser-induced fluorescence measurements showed an intermittent lift-off from the corner of the bluff body and the average lift-off height decreased with increasing air velocity, with less extinction along the inner flame branch especially for the heavier fuels. At the blow-off conditions, local extinctions appeared at both flame branches. The blow-off process followed a gradual reduction of the size of the flame, with the less volatile fuels showing a more severe flame area reduction compared to the condition far from blow-off. The average blow-off duration, [Formula: see text], calculated from the evolution of the area-integrated OH* signal, was a few tens of milliseconds and for all conditions investigated the ratio [Formula: see text] /( D/ UB) was around 11, but with large scatter. The measurements provide useful information for validation of combustion models focusing on local and global extinction.

Effect on O2 enrichment and CO2 dilution on rapeseed oil combustion in a stationary burner

Application of oxygen-enhanced and CO2 diluted combustion to existing fossil fuel/biofuel energy systems to facilitate CO2 capture presents several challenges. This paper investigates the combustion characteristics of rapeseed oil and its blends with diesel oil in swirling spray flames. It focuses on the stability of flames, NOx, CO and particle emissions. We observed that the oxygen enrichment produced an increase of NOx emission due to the higher furnace temperature, while CO emissions slightly increased. Moreover, it did not modify the modes of the particle size distribution functions, but it tended to reduce the total amount of particle emissions by one order of magnitude. On another hand, CO2 dilution decreased the NOx emission (up to 20 times) due to the lower furnace temperature at fixed oxygen concentration. Although the dilution of combustion does not produce significant effect on the shape of the size distribution functions, this modifies significantly the absolute concentrations of the emitted particles. Finally, the produced oxygen-enhanced/diluted flames were stable and more luminous than the air/fuel flames. The increase of O2 concentration caused that the flames were longer by about 20% compared with base line flame.

Spray flame structure of rapeseed biodiesel and Jet-A1 fuel

The spray combustion characteristics of rapeseed methyl esters (RME) were compared to Jet-A1 fuel using a gas turbine type combustor. The swirling spray flames for both fuels were established at a constant power output of 6kW. The main swirling air flow was preheated to 350°C prior to coaxially enveloping the airblast-atomized liquid fuel spray at atmospheric pressure. Investigation of the fundamental spray combustion was performed via measurements of the fuel droplet sizes and velocities, gas phase flow fields and flame reaction zones. The spray flame droplets and flow fields in the combustors were characterised using phase Doppler anemometry (PDA) and particle imaging velocimetry (PIV) respectively. Flame chemiluminescence imaging was employed to identify the flame reaction zones. The highest droplet concentration zone extends along a 30° angle from the symmetry axis, inside the flame zone. Only small droplets(<17μ) (<17μm)are found around the centreline region, while larger droplets are found at the edge of the spray outside the flame reaction zone. RME exhibits spray characteristics similar to Jet-A1 but with droplet concentration and volume fluxes four times higher, consistent with the expected longer droplet evaporation timescale. The flow field characteristics for both RME and Jet-A1 spray flames are very similar despite the significantly different visible characteristics of the flame reaction zones.

LES/CMC of Blow-off in a Liquid Fueled Swirl Burner

Large Eddy Simulations of two-phase flames with the Conditional Moment Closure combustion model have been performed for flow conditions corresponding to stable and blow-off regimes in a swirl n-heptane spray burner. In the case of stable flame (i.e. low air velocity), the predicted mean and r.m.s. velocities and the location and shape of the flame agree reasonably well with experiment. In particular, the presence of localised extinctions is captured in agreement with experiment. Using model constants previously calibrated against piloted jet methane flames (Sandia F) with localised extinction, we obtain that at the experimentally determined blow-off velocity of the swirling spray flame, the predicted flame also blows off, demonstrating that the LES-CMC approach can capture the global extinction point in a realistic configuration.

Spray Combustion Characteristics of Palm Biodiesel

The potential of palm methyl esters (PME) as an alternative fuel for gas turbines is investigated using a swirl burner. The main air flow is preheated to 623 K, and a swirling spray flame is established at atmospheric pressure. The spray combustion characteristics of PME are compared to diesel and Jet-A1 fuel under the same burner power output of 6 kW. Investigation of the fuel atomizing characteristics using phase Doppler anemometry (PDA) shows that most droplets are distributed within the flame reaction zone region. PME droplets exhibit higher Sautermean diameter (SMD) values than baseline fuels, and thus higher droplet penetration length and longer evaporation timescales. The PME swirl flame presents a different visible flame reaction zone while combusting with low luminosity and produces no soot. NOx emissions per unit mass of fuel and per unit energy are reduced by using PME relative to those of conventional fuels.

Passive Control of Forced Combustion Instability in a Swirl-Stabilized Spray Combustor

A coannular swirl-stabilized burner, equipped with a pulsation mechanism to introduce controlled periodic instability into the fuel flow to the nozzle, was used to examine instability mechanisms and demonstrate passive control of combustion instability. The effects of swirl configuration and airflow distribution on forced, unstable combustion were examined. Fuel pulsation was used to excite a dynamic response from the combustor that would mimic the behavior of instabilities encountered in many commercial, military, and industrial systems. The response of a spray flame to pulsations of different frequencies was examined using a sound spectrum analyzer and particle image velocimetry. The effect of swirl distribution in the burner, including co- and counterswirl distribution, on the amplitude response of three different flames to low-frequency fuel pulsation was examined. An analysis of the flame structure at different points during the cycle of a forced, low-frequency instability is presented for three swirling spray flames. The effect of airflow distribution on the dynamic response of one flame obtained with a particular swirl configuration is presented. Changes in swirl distribution, which affected recirculation and mixing, were found to reduce the magnitude of the flame response to forcing. Changes in airflow distribution, which affected residence times of product gases in the combustion region, were also found to reduce the combustor response to forcing. Changes in swirl and airflow distribution in a burner of this type were shown to provide effective passive control of combustion instabilities.

In-Situ Calibration Technique for Laser-Induced Fluorescence Measurements of Nitric Oxide in High-Pressure, Direct-Injection, Swirling Spray Flames

We report spatially resolved laser-induced fluorescence (LIF) measurements of nitric oxide (NO) in a preheated, two-atmosphere, lean direct-injection (LDI) spray flame. The spray is produced by a hol-low-cone, pressure-atomized nozzle supplied with liquid heptane. NO is excited via the Q2(26.5) transition of the γ(0,0) band. Detection is performed in a 2-nm region centered on the γ (0,1) band. A complete scheme is developed by which quantitative NO concentrations in high-pressure LDI spray flames can be measured by applying linear LIF. Standard excitation and detection scans are performed to assess possible interferences and to verify a non-resonant wavelength for subtracting the influence of oxygen interferences and Mie scattering in the NO detection window. NO is doped into the reactants and convected through the flame with no apparent destruction, thus allowing an NO fluorescence calibration to be taken inside the flame environment. The in-situ calibration scheme is validated by comparisons to a reference flame. Relative axial calibration slopes are utilized in order to obtain radial profiles of absolute NO concentrations. These quantitative NO profiles are presented and analyzed so as to better understand the operation of lean-direct injectors for gas turbine combustors.

An experimental study on swirling spray flame structure by air-blast nozzle

Detailed experimental study has been made of air blast kerosene spray flames with and without swirl in combustion air flow. Phase-Doppler detect technique is used to measure Sauter mean diameter, axial component mean and rms velocity, size-velocity correlation, and number density. These measurements are obtained for both nonreacting and reacting cases under several stable flame conditions. The results show that the introduction of swirl to the combustion air modifies the spatial distribution of droplet size, velocity, and number density, and thus alters the flame structure. However, due to the weak swirl intensity, the overall structure of swirling flames are essentially same as that of nonswirling flames. Physical model of structure of air blast atomized spray flames is projected to show that spray flames are composed of three distinct regions: the two-phase mixture region, the main reaction and the intermittent combustion region. Near the atomizer, two phase mixture of droplet and air is formed in the core region. This dense spray region is characterized by high droplet number density and the strong convective effect. There follows the main combustion region where the main flame penetrates within the spray boundary. Main reaction region of these flames are governed by internal group combustion mode. Finally there exists the intermittent combustion region where local group burning or isolated droplet burning occurs.

Fuel property effects on the structure of spray flames

The effect of fuel properties on the structure of swirling spray flames has been investigated. Droplet size, number density and velocity measurements have been carried out in pressure-atomized spray flames using phase/Doppler interferometry. Four fuels with different physical properties were studied, namely n-heptane, methanol, 50/50 methanol/1-dodecanol mixture, and kerosene. The results indicate that droplet mean size and velocity are influenced primarily by the fuel viscosity. No detectable trend could be attributed to changes in surface tension. Fuel volatility seems to have some effect on the spray flame structure, especially in the methanol flame. Flame luminosity is found to increase with increasing C/H ratio and fuel heating value. Droplet size and velocity distributions near the nozzle are found to be quite broad, and include regions of negative velocities. Some evidence has also been found to indicate the occurrence of microexplosions in the methanol/dodecanol mixture flame.