François Lacas

Stabilization of a Turbulent Premixed Flame Using a Nanosecond Repetitively Pulsed Plasma

A nanosecond repetitively pulsed plasma (NRPP) produced by electric pulses of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25-kW lean turbulent premixed propane/air flame (ReD=30000) at atmospheric pressure. We show that, when placed in the recirculation zone of the flow, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. Stabilization is obtained with a very low level of plasma power of about 75 W, or 0.3% of the maximum power of the flame. In addition, they find that at high flow rates, where the flame should normally blow out, the NRPP allows the existence of an intermittent V-shaped flame with significant heat release, and at even higher flow rates the existence of a small dome-shaped flame confined near the electrodes that can serve as a pilot flame to reignite the combustor. Optical emission spectroscopy measurements are presented to determine the temperature of the plasma-enhanced flame, the electron number density, and to identify the active species produced by the plasma, namely O, H, and OH

Experimental and numerical study of chemiluminescence in methane/air high-pressure flames for active control applications

Chemiluminescence of excited OH * , CH * , and C 2 * radicals was investigated as a tool for combustion control. A parametric study in premixed methane/air flames is presented regarding the effects of pressure (1 to 10 bar) and equivalence ratio (0.6 to 1.1). The experimental geometry corresponds to a Bunsen-type burner, with pilot flames to achieve steady combustion at very lean conditions. The burner was set in a pressurized vessel to control ambient pressure. The chemiluminescence was spatially measured using an intensified CCD camera with interference filters centered on the three radical emission bands. A monochromator and a low-resolution spectrometer were used to obtain spectrally resolved data. The three diagnostic techniques show good agreement. The experimental results show that the chemiluminescence of the radicals investigated has different dynamics for given pressure and equivalence ratio conditions. The OH * radical seems more suitable for lean flames, while CH * and C 2 * have a more monotonic behavior and stronger dynamics for richer flames. A numerical simulation with complex chemistry and transport modeling based on the PREMIX code was performed for two different kinetic schemes including OH * and CH * . A comparison is presented for integrated chemiluminescence (both spectrally and spatially), as well as for local excited radical concentration trends within the range of experimental conditions. Good qualitative agreement is found with the experimental results except for rich flames, where disagreements due to kinetic schemes are observed. As a conclusion, a new strategy for flame sensing using chemiluminescence over several wavelengths is proposed.

Large eddy simulation and experimental study of flashback and blow-off in a lean partially premixed swirled burner

Lean premixed prevaporized (LPP) combustion is a widely used concept for reducing pollutant emissions in gas turbines. In LPP systems, a mixing tube is added between the injector and the combustion chamber to promote mixing and combustion efficiency. These devices are efficient to reduce pollutant emissions but can be sensitive to complex transient phenomena such as blow-off or flashback which are still beyond the prediction capabilities of most numerical tools. The present study describes a joint experimental and numerical study to evaluate the capacities of large eddy simulation (LES) for the prediction of flame dynamics in a swirl-stabilized LPP burner operated with propane. Combustion regimes are first identified experimentally: compact flames (the normal regime for LPP and also flashback regimes where the flame is stabilized in the mixing tube) as well as lean blow-off situations are encountered. LES is then used to investigate each regime as well as the transition and the hysteresis phenomena betwee...

Closed-loop equivalence ratio control of premixed combustors using spectrally resolved chemiluminescence measurements

A control system based on a multiwavelength chemiluminescence sensor has been developed and tested to adjust the operating point of an optically accessible, lean premixed methane/air burner operated under pressures up to 20 bar and equivalence ratios ranging from 0.5 to 1.2, as typically found in premixed gas turbine combustors. The sensor comprises a broadband, low-resolution grating spectrometer scanning the light emitted by the burner in the range 250–850 nm. Spectral distributions, are analyzed in real time to determine the equivalence ratio according to a novel method described in this paper. Radical emission data in the form of intensity ratios OH * /CH * and CO 2 * /CH * are first measured during an, off-line combustor identification step carried out with the same monochromator. These data are collected in a lookup table, which is then used to monitor the equivalence ratio and control the fuel mass flow rate. Closed-loop control using this chemiluminescence sensor has been tested over a broad range of, operating conditions. The performance of the sensor and control system is assessed by simulating changes in fuel quality and by operating the system with contaminated windows. Equivalence ratio control was successfully achieved under all of these conditions, showing that a suitable processing of multiwavelength data allows premixed gas turbine combustion control.

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.

Transfer function measurements in a model combustor: Application to adaptive instability control

Due to its good performance in terms of low pollutant emissions, the lean premixed, prevaporized combustor design has received a special interest in the field of gas turbines. This article reports an experimental study of a laboratory-scale, premixed, prevaporized, swirl-stabilized combustor operated with preheated air and fed with liquid heptane as fuel. Under certain operating conditions, it exhibits well-established combustion oscillations at frequencies near 400 Hz. To understand and control these instabilities, it is useful to determine the burner response to external modulations. A novel actuator relying on secondary air modulation was designed and integrated in the injector to allow measurements of the combined transfer function of the burner and actuation system. Two different techniques were used for this determination: direct measurement between command and pressure on photo-multiplier signals based on spectral and cross-spectral densities, or identification through a finite impulse response numerical filter. The two techniques are in good agreement. This identification is then successfully applied to active control of combustion instabilities. The mean pressure oscillation amplitude is reduced by about 50% (from 650 Pa to 400 Pa, the noise level being 130 Pa). This modest reduction is due to the fact that the actuator operates at a frequency that is slightly greater than its cutoff frequency and that its effectiveness is therefore degraded. This highlights the difficulties associated with the bandwidth limitations of practical actuators.

On PAH formation in strained counterflow diffusion flames

The structural response of methane/air and methane-nitrogen/air counterflow diffusion flames to strain was investigated by measurements and computations. The numerical predictions were found to be in reasonably good agreement with the experiments. Different reaction pathways leading to PAH formation are examined computationally to obtain a deeper understanding of the process of soot precursor formation in strained diffusion flames. Both experimental and computational results indicate that the concentration of C2H2 and C3H3, as well as that of the PAH, leading candidates for soot precursor formation, diminish with increasing strain rates. The decrease of the PAH is caused by a depletion of the benzene precursors. In looking to find control parameters for strained reactive flows, it is suggested to image strain rates based on the CH2O, respectively CHO, to C2H2 ratio.

One-Dimensional Propagation of a Premixed Turbulent Flame With a Balance Equation for the Flame Surface Density

Abstract A transport equation for the flame surface density is used to describe premixed turbulent combustion in the simple case of a one-dimensional propagation in a homogeneous mixture. An analytical method of the type devised by Kolmogorov, Petrovski and Piskunov, as well as numerical simulations, are exploited to study the influence of turbulence and laminar flame speed on the turbulent flame speed and on the turbulent flame thickness. It is shown that the model exhibits steadily propagating turbulent flames and that flame speed is proportional to the square root of the turbulent viscosity multiplied by the effective strain rate of the flow. If these two quantities are evaluated with classical expressions one finds that the turbulent flame speed is proportional to the square root of the turbulent kinetic energy (St = λAu′). This result agrees well with other experimental and theoretical expressions and correlations available in the literature. The comparison with experiments yields one of the constant...

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.

Instability Mechanisms in a Premixed Prevaporized Combustor

Instabilities in a premixed prevaporized 150-kW model scale combustor are investigated experimentally. The injector fed with liquid heptane and preheated air features two sets of swirling blades that induce flow rotation in the same direction (corotative) or in the opposite direction (counter-rotative). The flame is stabilized with swirl behind a dump. Instabilities occur in the low-frequency range around 400 Hz corresponding to a quarter-wave mode acoustic coupling of the system. Simultaneous measurements of pressure and heat-release oscillations and phase-locked CH chemiluminescence images are used to characterize the combustion dynamics. In both corotative (COS) and counter-rotative (CNS) cases, the reaction region moves closer to the injector when the flame becomes unstable by about one-third of the stabilization distance under normal operation. Experiments indicate that the two swirl configurations have distinct domains of instability. The instability boundary separating stable and unstable regions can be defined in terms of a critical velocity v c , which depends on the equivalence ratio Φ, air injection temperature T i n j , and swirl geometry. In the coswirl configuration, instabilities occur when the injection velocity is lower than the critical velocity [u v c (Φ, T i n j ; CNS)]. In a range of conditions corresponding to low injection velocities, reduced eqivalence ratio, and for the coswirl configuration, an unsteady flashback takes place in which the flame moves periodically in and out of the fuel premixer. This mechanism is related to the existence of a low-velocity region near the injector exit plane. Observations of the space-time development of the heat release under unstable operation indicate that the oscillations are significantly influenced by the swirl geometry and are caused by different mechanisms. The coswirl configuration features a central recirculation, which gives rise to periodic vortex roll-up, convection, and sudden release of heat when the vortices impinge on the lateral walls. In the counterswirl geometry there are no identifiable flow structure, but the heat-release pattern is convected periodically in the chamber. Estimates of the delay times associated with the two mechanisms support the view that coswirl instabilities are driven by vortex roll-up, whereas counterswirl instabilities are probably sustained by equivalence-ratio inhomogeneities.