Perfect Premixing Research Articles

Quantification of Mixing Quality Effects on Flashback Limits for H2-Rich Fuel Gases in Lean Premixed Combustion Using Laser-Induced Fluorescence of Acetone

Abstract One of the main challenges arising from hydrogen-rich fuel mixtures is to prevent flame flashback. In typical gas turbines, the fuel is commonly injected through holes in the axial swirler vanes to achieve a high mixing quality. However, this injection method is not perfect and can cause nonhomogeneous mixing regions, so that locally rich fuel clusters can significantly increase flashback propensity. This work aims to establish a link between the local mixing quality of fuel and air and flashback limits obtained experimentally under elevated pressure conditions. The nonreacting experiments have been conducted at an atmospheric mockup test rig and acetone-planar laser-induced fluorescence (PLIF) has quantified the local fuel concentration. Zones of high equivalence ratio are evident close to the center body wall. The near-wall equivalence ratio fields reveal that the critical probability of the local equivalence ratio being greater than the one for perfect premixing is between 20% and 35% for all hydrogen concentrations at the flashback limits observed. A probability of 35% is selected as a critical threshold to derive a correlation between the local and the global equivalence ratio in technical premixing. Even though the correlation is specific to the investigated burner configuration, the presented methodology offers valuable insights into the impact of the local mixing quality on flashback propensity, which can improve flashback prediction models formulated for perfect premixing conditions.

Role of the Equivalence Ratio on Soot Formation in a Perfectly Premixed Turbulent Swirled Flame: A Combined Experimental and Large Eddy Simulations Study

Abstract The understanding of processes that govern soot production in aero-engines is fundamental for the design of new combustion systems with low environmental impact. Many combustors, more specifically those used in aero-engines, feature rich flame regions typically exploited in the so-called rich-quench-lean (RQL) technology. Thus, it is important to consider rich turbulent flames operating in the premixed mode. To this purpose, a model scale swirled combustor, called EM2Soot, was designed at the EM2C laboratory to analyze soot production under perfectly premixed rich conditions. In this work, the effect of the equivalence ratio on soot production in this burner is experimentally characterized and numerically simulated. Measurements of Planar Laser Induced Fluorescence of polycyclic aromatic hydrocarbons (PLIF-PAH) were performed to examine soot precursors presence, whereas soot volume fraction is measured with planar laser-induced incandescence (LII). Large Eddy Simulations (LES) are carried out using models already established in literature. By considering a range of equivalence ratios, the soot volume fraction from the experiments was found to reach a maximum near 1.8, whereas a lower level of soot volume fraction was measured for lower and for higher equivalence ratios. The large eddy simulations are found to be in qualitative agreement with experimental data in terms of polycyclic aromatic hydrocarbons (PAHs) and soot location. The soot volume fractions fv are notably overestimated with respect to the LII measurements. However, the numerical results correctly retrieve a reduction of soot production for the highest considered equivalence ratio value and can be used to explain the experimental behavior.

Numerical Characterization of a Premixed Hydrogen Flame Under Conditions Close to Flashback

This work presents a numerical study of a technically premixed swirling combustor with central air injection at conditions close to flashback using large-eddy simulation with flamelet modelling. This burner has the characteristics of showing flashback at low equivalence ratios, so numerical simulations are set to identify the mechanisms behind the flashback formation. Experimental findings suggest the axial momentum ratio between fuel and air dominates the flame stabilization mechanism and flashback resistance over mixing and equivalence ratio fluctuations. This aspect is investigated here for two operating conditions with the same axial momentum ratio as in the experiment using a perfectly premixed assumption. The two test cases correspond to two stable operating points, far and close to the flashback point. The study shows the assumption of perfect premixing is valid during the stable operation of the burner up to flashback conditions. The experimental results are well predicted under inert and reacting conditions by using a perfectly premixed mixture. It is found that the non reacting flow field develops a self-excited oscillation in the form of a precessing vortex core. This oscillation is attenuated by the fuel injection due to the respective increase in axial momentum and it is ultimately suppressed in the reacting flow field. Both experiments and simulations confirm the same trends. The analysis of the flames have shown certain dynamics as the flashback point is approached. The flashback resistance of the burner is minimized due to an increase in the velocity deficit of the incoming mixture. The recirculation region is shifted upstream, the central recirculation is altered and the flame position is displaced towards the inlet of the reactants in the combustion chamber. The analysis of instabilities and flow dynamics suggest that the formation of flashback can be attributed to combustion induced vortex breakdown, which in turn is associated to the lower axial momentum introduced by the fuel jets in leaner conditions.

Interaction between velocity fluctuations and equivalence ratio fluctuations during thermoacoustic oscillations in a partially premixed swirl combustor

The combined effects of velocity fluctuations (VF) and equivalence ratio fluctuations (ERF) on thermoacoustic flame oscillations are studied experimentally in a gas turbine model combustor. The analysis is based on time-resolved simultaneous measurements of flow field, flame structure and fuel distribution using combined PIV, OH-PLIF and tracer-PLIF at a rate of 10 kHz. In order to separate the effects of VF and ERF, the combustor has been operated in two configurations: a technically (i.e. partially) premixed (TP) configuration where both ERF and VF occur, and a perfectly premixed (PP) configuration where only VF are present. A particular operating condition is selected where the VF are similar for both premixing modes. The corresponding fluctuations of heat release, by contrast, exhibit considerably different spatial patterns for the two modes. In particular, periodic widening and narrowing as well as extinction and re-ignition of the flame base, and a convective motion of the flame zone are observed in the TP flame but not in the PP case. An evaluation of periodic variations of flux rates and heat release shows that the feedback loop of the TP flame includes an additional convective delay of 50°. The time-resolved measurements reveal that this delay leads to a coincidence of fuel-lean unburned gas and high velocity that induces local flame extinction and narrowing of the flame base. When the equivalence ratio later increases, the flame base re-ignites and a widened zone of increased heat release forms that is convected downstream. For the PP case, by contrast, reaction at the flame base is stable and accordingly the flame responds earlier to VF compared to the TP case. The results show that interacting VF and ERF have a strong and complex impact on the thermoacoustic response of turbulent swirl flames, and this impact depends largely on their time-delay.

Large Eddy Simulation of combustion instabilities in a lean partially premixed swirled flame

This paper investigates one issue related to Large Eddy Simulation (LES) of self-excited combustion instabilities in gas-fueled swirled burners: the effects of incomplete mixing between fuel and air at the combustion chamber inlet. Perfect premixing of the gases entering the combustion chamber is rarely achieved in practical applications and this study investigates its impact by comparing LES assuming perfect premixing and LES where the fuel jets are resolved so that fuel/air mixing is explicitely computed. This work demonstrates that the perfect premixing assumption is reasonable for stable flows but is not acceptable to predict self-excited unstable cases. This is shown by comparing LES and experimental fields in terms of mean and RMS fields of temperature, species, velocities as well as mixture fraction pdfs and unsteady activity for two regimes: a stable one at equivalence ratio 0.83 and an unstable one at 0.7.

Impact of Swirl Fluctuations on the Flame Response of a Perfectly Premixed Swirl Burner

Combustion instabilities represent a long known problem in combustion technology. The complex interactions between acoustics and turbulent swirling flames are not fully understood yet, making it very difficult to reliably predict the stability of new combustion systems. For example, the effects of fluctuations of swirl number on the heat release of the flame have to be investigated in more detail. In this paper a perfectly premixed swirl stabilized burner with variable axial position of the swirl generator is investigated. In experiments, the position of the swirl generator has a strong impact on the dynamic flame response, although it does not influence the time-averaged distribution of the heat release significantly. This phenomenon is further investigated using computational fluid dynamics combined with system identification. The generation of fluctuations of swirl number, their propagation to the flame, and their effect on the dynamic flame response are examined. A simple model based on convective time lags is developed, showing good agreement with experiments.

Analysis of NOx Formation in a Hydrogen-Fueled Gas Turbine Engine

A commercially available natural gas fueled gas turbine engine was operated on hydrogen. Three sets of fuel injectors were developed to facilitate stable operation while generating differing levels of fuel∕air premixing. One set was designed to produce near uniform mixing while the others have differing degrees of nonuniformity. The emission performance of the engine over its full range of loads is characterized for each of the injector sets. In addition, the performance is also assessed for the set with near uniform mixing as operated on natural gas. The results show that improved mixing and lower equivalence ratio decrease NO emission levels as expected. However, even with nearly perfect premixing, it is found that the engine, when operated on hydrogen, produces a higher amount of NO than when operated with natural gas. Much of this attributed to the higher equivalence ratios that the engine operates on when firing hydrogen. However, even the lowest equivalence ratios run at low power conditions, higher NO was observed. Analysis of the potential NO formation effects of residence time, kinetic pathways of NO production via NNH, and the kinetics of the dilute combustion strategy used are evaluated. While no one mechanism appears to explain the reasons for the higher NO, it is concluded that each may be contributing to the higher NO emissions observed with hydrogen. In the present configuration with the commercial control system operating normally, it is evident that system level effects are also contributing to the observed NO emission differences between hydrogen and natural gas.

Modeling of Inhomogeneously Premixed Combustion With an Extended TFC Model

In many practical applications, so-called premixed burners do not achieve perfect premixing of fuel and air. Instead, fuel injection pressure is limited, the permissible burner pressure drop is small and mixing lengths are curtailed to reduce the danger of flashback. Furthermore, internal or external piloting is frequently employed to improve combustion stability, while part-load operation often requires burner staging, where neighboring burners operate with unequal fuel/air equivalence ratios. In this report, an extension of the turbulent flame speed closure (TFC) model for highly turbulent premixed combustion is presented, which allows application of the model to the case of inhomogeneously premixed combustion. The extension is quite straightforward, i.e., the dependence of model parameters on mixture fraction is accounted for by providing appropriate lookup tables or functional relationships to the model. The model parameters determined in this way are adiabatic flame temperature, laminar flame speed and critical gradient. The model has been validated against a test case from the open literature and applied to an externally piloted industrial gas turbine burner with good success.