Scale Combustion Research Articles

Computational assessment of performance parameters of an aero gas turbine combustor for full flight envelope operation

Abstract This paper presents the computational study carried out on an aero gas turbine combustor to assess important performance parameters. The CFD results are compared with experimental dataobtained from the full scale combustor tested at ground test stand simulating various operational conditions. The CFD predictions have agreed very well with the experimental data. The model is then extended to predict combustor exit temperature pattern factors, pressure loss, and combustion efficiency and exhaust gas constituents over a wide range of operating pressure and temperature conditions. The paper also presents the studies carried out on the effect of atomizer spray cone angle, particle size and fuel flow variations expected due to manufacturing tolerances in various flow passages as well as due to operational degradations on temperature pattern factors. The pattern factors are also analyzed on cold and hot day environment. The radial pattern factor (RPF) at mid height is found to increase as altitude increases from sea level to 12 km. Spray cone angle is found to have a predominant effect on temperature non-uniformity at exit, lower cone angle increasing both radial and circumferential pattern factors. The findings of this study are valuable inputs for engine performance estimation.

The efficiency and toxicity of dodecafluoro-2-methylpentan-3-one in suppressing lithium-ion battery fire

Currently, the effective and clean suppression of lithium-ion battery (LIB) fires remains a challenge. The present work investigates the use of various inhibitor doses (Xin) of dodecafluoro-2-methylpentan-3-one (C6F12O) in 300 Ah LIBs, and systematically examines the thermal and toxic hazards of the extinguished batteries via real scale combustion and gas analysis. The inhibitor is shown to be completely effective. The inhibition mechanism involves a combination of chemical inhibition and physical cooling. While the chemical inhibition effect tends to saturate with increasing Xin, the physical cooling remains effective at higher inhibitor doses. However, extinguishing the battery fire with a high Xin of C6F12O is found to incur serious toxicity problems. These results are expected to provide a guideline for the design of inhibitor doses for the suppression of LIB fires.

Particle formation during suspension combustion of different biomass powders and their fast pyrolysis bio-oils and biochars

The fly ash formation during suspension combustion of five different biomass powders (stem wood, bark, forest residue, willow, and reed canary grass) and the corresponding products from fast pyrolysis (bio-oil and biochar) of the powders was investigated. The fifteen fuels were burned in a drop tube furnace under normal (20 vol-% O2) and oxygen-enriched combustion conditions (40 vol-% and 60 vol-% O2). The trends in the data were used to discuss differences in combustion behavior and devise recommendations for the use of the fuels. There was a general difference in fly ash formation mechanism between the solid fuels (biomass and biochar) and the bio-oil fuels, which was attributed to parts of the ash-forming elements in bio-oil being dissolved in the oil. Oxygen-enrichment did not affect the release of inorganic elements to the gas phase for bio-oil combustion. Since the bio-oils generate lower fly ash during combustion, ~100 times compared to the original biomasses, they should be reserved for combustion technologies demanding fuels with very low ash content, whereas the biochar should be used in large scale combustion facilities with advanced gas cleaning technology operated by teams with experience of handling ash related operational problems.

Lean blow-out prediction in an industrial gas turbine combustor through a LES-based CFD analysis

The use of lean premixed flames to control combustion temperature and limit nitric oxides (NOx) formation is a common practice in all the applications of land-based gas turbines irrespective of their size and power production capacity. Despite the capability of lean premixed combustion to nominally reduce NOx emissions well below the most strict international regulations, such technology still requires improvements to overcome the impact on whole engine operability due to possible onset of combustion instabilities and flame lean blow-out (LBO). Both phenomena involve highly unsteady processes resulting in challenging numerical modeling. Particularly critical is the prediction of lean blow-out mechanisms where the role of finite rate chemistry and turbulence cannot be easily simplified. The current study aims at proving the capability of a CFD methodology based on Large Eddy Simulation (LES) and on an extended version of the Turbulent Flame Closure (TFC), to describe and properly catching the dynamics of flame extinction at the leanest operating conditions in typical turbulent lean premixed flames adopted in gas turbine combustors. The methodology is first validated on a laboratory test case where detailed experimental results are available. The extended TFC model well describes the LBO process observed in an atmospheric swirl stabilized burner investigated at the University of Cambridge. The proposed approach is then applied to the investigation of a real scale combustor operated with a swirl lean burner, representing the standard device adopted by Baker-Hughes on the heavy-duty 16 MW class engine Nova LT16. An accelerated numerical procedure is used to trigger the LBO, considering both constant firing temperature and constant pilot split conditions. An excellent agreement is observed with respect to the data obtained on a full-annular test rig, confirming the validity of the developed LBO modeling strategy which can be used in design phase to improve the overall engine operability.

Experimental investigation and optimal combustion control of untreated landfill gas via fuzzy logic rule knowledge based approach

Optimal combustion control of untreated landfill gas is proposed for an effective usage and a low-cost solution in waste to energy technologies. Variations of methane concentration in untreated landfill gas over time cause undesired performance of combustors in thermal efficiency and gas emission. In this work, the experimental investigation on variables of combustion process is systematically presented to determine an inherent performance index of a combustor, reflecting actual thermal efficiency and gas emission for optimal control. Those quantitative findings can be implemented via fuzzy logic rule knowledge based approach to combustion control. From the experimental results of a can-type combustor, it is confirmed that the optimal combustion control of untreated landfill gas yields the desired exhaust gas temperature with maximum thermal efficiency and minimum gas emission under varying methane concentrations or changes of operating conditions. The proposed methodology can be generalized to optimal combustion control of untreated landfill gas via the fuzzy logic rule knowledge based approach, which is the most suitable to characteristics of each combustor. For industrial applications, the experimental results at a laboratory scale can be extended by increasing the capacity of the studied combustor to obtain the large scale combustion in practical production where the optimal combustion control of the untreated landfill gas is effectively accomplished without cost of gas treatment for usage in conventional processes.

Design and investigation of a central air jet flameless combustor

Due to strict regulations on harmful emissions such as NOx and CO, many novel combustion techniques have been investigated in order to decrease emissions from gas turbines. One of these novel techniques is colorless distributed combustion (CDC) or flameless combustion (FC). CDC has been examined under reacting conditions using ANSYS Fluent at 27 different operating cases for the aim of developing design equations and a 3D flameless combustion map that includes the effect of air diameter, confinement size and excess air ratio on recirculation ratio to allocate flameless combustion regions. A simple lab scale combustor with central air jet arrangement is employed for the analysis. Eddy dissipation concept (EDC) either global or with detailed mechanism (DRM19) fails to predict the effect of excess air ratio in flameless combustion. A numerical approach for calculating the recirculation ratio is presented. As the air jet diameter decrease 40% there is about 54% increase in the recirculation ratio and about 90% decrease in the NOx emissions. It is found that above a recirculation ratio of 3 there is an insignificant effect on NOx emissions. Reducing the combustor diameter by a constant value for a given air diameter is found to result in a decrease in the recirculation ratio and the percentage of decrease is higher at high thermal intensities combustors. Moreover, it is observed that as the combustor diameter decrease to half of its value at constant air jet diameter and constant excess air ratio, there is an 80–90% increase in the NOx emissions inside the combustor. Furthermore, design equation is obtained to predict the appropriate air diameter that if combined with the available combustor diameter achieves a minimum recirculation ratio suitable to sustain flameless combustion.

Synthesis of MoO <sub>3</sub> by Pilot Scale Combustion Reaction: Evaluation in Biodiesel Production from Residual Oil

Synthesis of MoO <sub>3</sub> by Pilot Scale Combustion Reaction: Evaluation in Biodiesel Production from Residual Oil

Prognostics of Combustion Instabilities from Hi-speed Flame Video using A Deep Convolutional Selective Autoencoder

The thermo-acoustic instabilities arising in combustion processes cause significant deterioration and safety issues in various human-engineered systems such as land and air based gas turbine engines. The phenomenon is described as selfsustaining and having large amplitude pressure oscillations with varying spatial scales of periodic coherent vortex shedding. Early detection and close monitoring of combustion instability are the keys to extending the remaining useful life (RUL) of any gas turbine engine. However, such impending instability to a stable combustion is extremely difficult to detect only from pressure data due to its sudden (bifurcationtype) nature. Toolchains that are able to detect early instability occurrence have transformative impacts on the safety and performance of modern engines. This paper proposes an endto- end deep convolutional selective autoencoder approach to capture the rich information in hi-speed flame video for instability prognostics. In this context, an autoencoder is trained to selectively mask stable flame and allow unstable flame image frames. Performance comparison is done with a wellknown image processing tool, conditional random field that is trained to be selective as well. In this context, an informationtheoretic threshold value is derived. The proposed framework is validated on a set of real data collected from a laboratory scale combustor over varied operating conditions where it is shown to effectively detect subtle instability features as a combustion process makes transition from stable to unstable region.

Large Eddy Simulation of Lean Premixed Swirling Flames via Dynamically Thickened Flame Model Coupling with the REDIM Chemistry Table

In this paper, the so-called REDIM–DTF sub-grid scale combustion model is proposed to improve the well-known dynamically thickened flame (DTF) combustion model by combining the DTF modeling strategy with the reaction-diffusion manifold (REDIM) chemistry table, which is generated from a detailed reaction mechanism. The new model is then used to calculate two lean premixed swirling flames in the PRECCINSTA combustor via large eddy simulation. The radial profiles of velocity, temperature, and species concentration, as well as the flame surface area and vortex structure are analyzed. The results are in good overall agreement with the corresponding experiment, and the performance of the REDIM–DTF model is very similar to that of the DTF model. The predicted CO mass fraction profiles, however, show a relatively larger discrepancy between the original DTF model and the proposed REDIM–DTF model, which is thought to be due to different reaction mechanisms used. As a smaller number of species transport equations are solved in the REDIM–DTF model, its computational efficiency is about 15% higher than that of the original DTF model.

REDIM-PFDF modelling of turbulent partially-premixed flame with inhomogeneous inlets using top-hat function for multi-stream mixing problem

Present study focuses on numerical investigation of two important partially-premixed inhomogeneous types of flames using REDIM-PFDF sub-grid scale combustion model. REDIM-PFDF model is based on the reaction-diffusion manifold (REDIM) technique and presumed filtered density function (PFDF) method. The detailed chemical kinetics is reduced into a two-dimensional chemistry table which is using the mass fractions of CO2 (YCO2) and N2 (YN2) as the reduced coordinates. The shapes of the Filtered Density Function (FDF) for YCO2 and YN2 are presumed to be clipped Gaussian and top-hat, respectively. We find that the top-hat function, in context of LES, is a suitable option to deal with multi-stream mixing problem. The comparison between LES and experimental data shows that the mixture fraction and mass fractions of several species are in a good agreement with the experimental data. The characteristics of two flames are also analyzed by comparing their OH mass fractions and CO2 production rates.

Characterization of laminar flame using high speed camera and spectrometer

In order to improve flame stabilization in small scale combustors, there have been many research works going on non-premixed as well as on premixed flame. The present experimental study characterizes colour and shape of the premixed and non-premixed flames through a laminar co-flow burner. Various flame behaviors are observed and their stabilization limits are investigated at different fuel–air ratios. The spectroscopic approach becomes suitable method for the measurement of chemiluminescensce for both premixed and diffusion flames. Thus, we can predict the behavior of a combustion process. We observe also the intensity variation of chemiluminescence species, OH*, CH*, and C2* for different fuel-air flow combinations. Apart from that, we have an analysis of colour variance of the premixed flame for different flame locations.

Modal dynamics of high-frequency transverse combustion instabilities

This paper investigates the modal dynamics of self-excited high frequency transverse instabilities in a multi-nozzle, swirl-stabilized can combustor. It utilizes results from multiple pressure sensors to decompose the disturbance into clockwise (CW) and counter-clockwise (CCW) waves, enabling a reconstruction of the pressure amplitude, phase, anti-nodal line location, and spin ratio. While these types of decompositions have been demonstrated on small scale combustors, particularly annular combustors, this is the first study to analyze these results for a high power, multi-nozzle can combustor. Its particular focus is in characterizing the system evolution in phase space and the nature of the system attractors under linearly unstable conditions; results are presented in spin ratio-phase difference space, so that limit cycle oscillations appear as fixed points. Results are shown for conditions where three different attractors dominate (standing, CW, and CCW) but are taken for very long time periods (∼50,000 cycles) so that the system's interaction with these attractors can be observed. For the standing mode-dominated case, the phase portrait shows spiral trajectories converging to a single fixed point. For the spinning mode dominated cases, however, multiple fixed points and saddle points were observed, and the trajectories showed a more complicated structure. It was also shown that even though the CW mode is dominant, this mode intermittently switches to standing or CCW modes for short period. These results suggest that the strength of each attractor depends on test conditions and that random noise stochastically perturbs the system so that large regions of the phase space are explored. Finally, one interesting observation is that the angular velocity of the anti-nodal line linearly varies with the spin ratio under standing-mode dominant conditions.

Investigation of novel scaling criteria on a reverse-flow combustor

Abstract The present study investigates the scaling of a 3.3-kW laboratory scale combustor (baseline) to 25-kW using Detached Eddy Simulation (DES) with detailed chemistry. The combustor operates on syngas under atmospheric conditions. Two novel scaling criteria, namely, constant volume to jet momentum ratio (CM) and constant volume to jet kinetic energy ratio (CK) have been introduced and applied to the 3.3-kW combustor. Additionally, the constant velocity (CV) and constant residence time (CRT) scaling criteria have also been investigated for comparison. Flow, mixing, temperature, and their effect on the reaction zone and emissions have been compared for the scaled-up combustors with the laboratory scale combustor. The scaling criteria were observed to significantly influence the location and volumetric distribution of the reaction zone. In terms of performance parameters, the CV criterion entails the lowest pressure drop and CO emissions. The performance of the CK and CM criteria closely follows that of the CV criterion, thus offering a promising scaling methodology for reverse-flow combustors. The chamber scaled by the CRT criterion recorded the highest pressure drop and CO emissions. The insights from the present study will aid in sizing lab-scale combustors to industrial requirements.

REDIM-PFDF Sub-grid Scale Combustion Modeling for Turbulent Partially-premixed Flame: Assessment of Combustion Modes

ABSTRACT The focus of the present study is to evaluate the capability of reaction-diffusion manifold (REDIM) technique combined with presumed filtered density function (PFDF) method to calculate the important characteristics of turbulent partially-premixed flame with near-homogeneous inlets and inhomogeneous inlets. The REDIM approach reduces detailed chemical kinetics into two-dimensional chemistry table, where mass fractions of CO2 and N2 are used as reduced coordinates. The fluctuation of the scalars within the LES filter volume is modeled by the PFDF method. Higher shear generated turbulent intensity at the mixing layer of the fuel and air streams provides the near-homogeneous inlet conditions. The radial statistics analysis of the mixture fraction, temperature and species mass faction, and their comparison with the experimental data show the good performance of the REDIM-PFDF model. Scatter plots show the better agreement for the inhomogeneous case. The near-homogeneous case exhibits little discrepancy with the measured distribution because of the limited ability of the present model to capture local extinction effects. Flame index parameter identifies the multiple combustion modes present at different locations. This parameter shows the presence of premixed and diffusion dominated types of combustion near the main jet exit plane in the inhomogeneous and near-homogeneous cases, respectively.

Review of effect of meso scale combustor wall material, Catalytic material, & Porous media on flame Stabilization

In recent trends demand of energy for portable devices like notebook, microsatellite, laptops, UAV are increasing at high rate, a high energy density hydrocarbon fuel creates opportunity to alternate chemical Li-ion batteries with micro combustion based power generation system. Mainly, this review focus on experimental, mathematical modelling techniques, and numerical simulation performed on heat recirculating combustor to find the effect of combustor wall material, catalytic material and porous media. It also summarizes the electricity generation possibilities with micro combustor turbine, thermo electric material & thermo-photovolatic combustion system. The effect of combustor wall material, catalytic material, and porous media on flame stabilization are enlisted with its summary.

Large eddy simulation of a partially pre-vaporized ethanol reacting spray using the multiphase DTF/flamelet model

Spray reactive flow finds application in various technical devices, and due to the complex nature, their optimization is very challenging, requiring proper modeling of turbulence/chemistry interactions as well as of the contribution from spray evaporation. This work presents a study of sub-grid scale combustion models, where relevant assumptions on multiphase coupling and their effects are analyzed in detail. For this purpose, two different flamelet approaches, i.e. progress variable spray flamelet and multi-regime gas flamelet are examined in an implementation coupled with the dynamic thickened flame model, along with which the impact of inlet inhomogeneities condition and droplet evaporation taking into account internal temperature gradient is also investigated. The numerical evaluation is carried out in large eddy simulations of a benchmark ethanol spray flame with partial pre-vaporization, where an Eulerian-Lagrangian numerical framework is adopted. The analysis demonstrated that the flame dynamics under consideration is governed by a close coupling between spray evaporation, turbulent dispersion and unsteady flame propagation at upstream shear layers. Results show that the spray flamelets built from counterflow partially-premixed spray flames achieved a better agreement with experiments, capturing the flame structure in terms of gas-phase temperature, OH mass fraction as well as spray statistics.

An a priori DNS analysis of scale similarity based combustion models for LES of non-premixed jet flames

In this work, recently developed finite-rate dynamic scale similarity (SS) sub-grid scale (SGS) combustion models have been a priori assessed and compared with the Eddy Dissipation Concept (EDC) and “no model” approaches based on a Direct Numerical Simulation (DNS) database of a temporally evolving non-premixed jet flame. Two different filter widths, one placed in the inertial range and the other in the near dissipation range, have been used. The analyses were carried out in two time instants corresponding to instants of maximum local extinction and re-ignition. Conditional averaged filtered chemical source terms, conditioned on different parameters in the composition space, have been presented. Improvements are observed using the dynamic SS models compared to the two other approaches in the prediction of filtered chemical source terms of individual species while using larger filter widths. However, discrepancies still exists using the dynamic SS model on the turbulent/non-turbulent interfaces of the jet, mainly in the prediction of the oxidizer consumption rate.

Effects of non-thermal plasma on the lean blowout limits and CO/NOx emissions in swirl-stabilized turbulent lean-premixed flames of methane/air

This study investigates experimentally the effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the characteristics of swirl-stabilized turbulent lean-premixed methane/air flames in a laboratory scale combustor by systematically varying the applied AC voltage, VAC, and frequency, fAC. Especially, it is elucidated how the NTP influences the lean blowout (LBO) limits and the characteristics of CO/NOx emissions depending on flame configuration. Without applying the NTP as the mixture equivalence ratio, ϕ, decreases from the stoichiometry to an LBO limit, the flame configuration changes from an M-flame (Regime I) to a conical flame (Regime II) and to a columnar flame (Regime III) for the whole range of the mixture nozzle exit velocity, U0, (4–10 m/s). With the NTP, however, it exhibits only Regimes I and II at relatively-low U0 range (4–6 m/s), while all three regimes at relatively-high U0 range (7–10 m/s). For both velocity ranges, the LBO limits are significantly extended by the NTP enhancing the flame stability. Under the relatively-low U0 range, streamers induced by the DBD reactor play a critical role in stabilizing the flames such that the degree of extension of the LBO limit depends linearly on VAC and fAC. Under the relatively-high U0 range, however, ozone generated by the DBD reactor in Regime III is found to be a major reason in extending the LBO limit, which is substantiated by another flame regime diagram with ozone addition only, and hence, the extension of LBO limit minimally depends on fAC. Simultaneously, the NTP considerably reduces CO emission, while slightly increases NOx emission near the LBO limits due to the enhanced combustion by ozone.

Pocket formation and behavior in turbulent premixed flames

Pocket formation is an important characteristic of turbulent premixed flames and understanding pocket behavior is key to developing high-fidelity numerical combustion models. In this study, a dual-burner experiment is used to study pockets in single- and dual-flame configurations and synchronized high-speed OH-planar laser-induced fluorescence and stereoscopic-particle image velocimetry imaging techniques are implemented to track flame pockets and the surrounding flow field. Statistical analysis of pocket origin and fate is performed using a novel tracking algorithm incorporating non-rigid image registration. Results show that pocket formation rates increase as a function of increasing inlet turbulence level; reactant pocket formation increases as a function of downstream distance, whereas product pocket formation decreases. Tracking reactant pocket lifetime shows that a majority of these pockets burn out and displacement speeds are characterized. Product pockets usually merge with the main flame surface, which could have an impact on local flame structure and propagation. Results presented in this study show that pocket behavior in turbulent flames can change local flame dynamics and it is important to capture these effects in sub-grid scale combustion models to accurately predict flame behavior.

Application of Non-Thermal Plasma for NOx Reduction in the Flue Gases

Over the years, ever more stringent requirements on the pollutant emissions, especially NOX, from combustion systems burning natural gas are introduced by the European Union (EU). Among all NOX reduction methods, the flue gas treatment by plasma is widely applied and could be used for both small scale and domestic combustion systems. However, the removal efficiency depends on concentrations of oxygen, water vapor, traces of hydrocarbons, and nitrogen oxides in flue gas. In order to analyze the application of the NOX reduction for small-scale or domestic combustion systems, experiments of NOX reduction by non-thermal plasma from real flue gases originating from premixed methane combustion at different equivalence ratio (ER) values were performed. It was determined that the residual oxygen in flue gas plays an important role for improvement of NO to NO2 oxidation efficiency when O2 concentrations are equal to or higher than 6%. The power consumption for the plasma oxidation constituted approximately 1% of the burner power. In the case of ozone treatment, the addition of O3 to flue gas showed even more promising results as NO formed during combustion was fully oxidized to NO2 at all ER values.