Combustion Research Research Articles

Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion Applications

Decarbonising the energy grid through renewable energy requires a grid firming technology to harmonize supply and demand. Hydrogen-fired gas turbine power plants offer a closed loop by burning green hydrogen produced with excess power from renewable energy. Conventional dry low NOx (DLN) combustors have been optimized for strict emission limits. A higher flame temperature of hydrogen drives higher NOx emissions and faster flame speed alters the combustion behavior significantly. Micromix combustion offers potential for low NOx emissions and optimized conditions for hydrogen combustion. Many small channels, so-called airgates, accelerate the airflow followed by a jet-in-crossflow injection of hydrogen. This leads to short-diffusion flames following the principle of maximized mixing intensity and minimized mixing scales. This paper shows the challenges and the potential of an economical micromix application for an aero-derivative industrial gas turbine with a high-pressure ratio. A technology transfer based on the micromix combustion research in the ENABLEH2 project is carried out. The driving parameter for ground use adaption is an increased fuel orifice diameter from 0.3 mm to 1.0 mm to reduce cost and complexity. Increasing the fuel supply mass flow leads to larger flames and higher emissions. The impact was studied through RANS simulation and trends for key design parameters were shown. Increased velocity in the airgates leads to a higher pressure drop and reduced emissions through faster mixing. Altering the penetration depth shows potential for emission reduction without compromising on pressure loss. Two improved designs are found, and their performance is discussed.

Development and Status of Spontaneous Coal Combustion Research in China

Spontaneous coal combustion is the result of physical and chemical interactions between coal and oxygen over a long period of time. It is a complex physical and chemical process that only occurs under certain conditions (such as low-temperature oxidation and heat accumulation), during which the temperature of the coal rises, eventually reaching the ignition point. It is also a common cause of major disasters in coal mines, as it threatens mine safety, coal production, and coal applications. In order to deeply grasp the research and development status of coal spontaneous combustion in China and point out the direction for future research, firstly, this paper introduces the energy status of China's coal resources, then expounds the development process and theory of coal spontaneous combustion, summarizes the characters and related problems of different spontaneous combustion test methods, and thirdly introduces the main institutions and personnel of coal spontaneous combustion and the corresponding results, finally this paper discusses the existing problems and future development direction of coal spontaneous combustion in China.

Low-carbon alcohol fuels for decarbonizing the road transportation industry: a bibliometric analysis 2000-2021.

The application of low-carbon alcohols (LCA fuels) in internal combustion engines has become one of the most important topics in road transport decarbonization. This paper aims to identify the trends and characteristics of LCA combustion research for the period 2000-2021 through bibliometric analysis. Citation analysis is used to evaluate the influence of most productive journals, countries/regions, authors, institutions, and relevant literature, while collaborative network between various authors, countries/regions, institutions, and the co-occurrences among different keywords are discussed. A dataset of 2250 publications was extracted from the Web of Science Core database and analyzed with CiteSpace and Biblioshiny. The extracted documents involve 429 journals of publications by 4782 authors from 1434 institutions across 83 countries/regions. The results reveal that the research output in this field has undergone three main stages of development, i.e., initial development (2000-2007), slow development (2008-2015), and rapid development (2016-2021). Currently, the research field is growing at an annual growth rate of 9.24%, with most of the contributions by authors and institutions originating from China. The analysis from relevant keywords and literature suggests that the core of this research field centers on the combustion, performance, and emission characteristics of LCA-fueled engines. The current study helps keep the scientific community informed of the latest paradigms in the LCA combustion research field.

Mini Review of Current Combustion Research Progress of Biodiesel and Model Compounds for Gas Turbine Application

Biodiesel is one of the most promising low-carbon fuels and has been applied in internal combustion engines. In recent years, there have been emerging interests in the application of biodiesel in gas turbines, accompanied by the growing need for carbon reduction in the gas turbine industry. This mini review focuses on the fundamental combustion research of biodiesel and its model compounds, as well as its current application in gas turbines, mainly in the last 15 years. As for fundamental combustion research, selected advances in combustion chemistry and swirl combustion are reviewed. Diversity in molecular structures of biodiesel and its model compounds includes a vast amount of research on fuel molecular structure effects on combustion characteristics, including the effects of the functional group, chain length, and isomeric structure. Swirling flame studies of biodiesel are also reviewed, which focus on the flame shape, flame stability, atomization performance, and pollutant emissions. Combustion performances of biodiesel in aircraft and land-based gas turbine are also highlighted. Furthermore, some prospects of future research of biodiesel combustion with the purpose of gas turbine application are provided.

Photoelectron Photoion Coincidence Spectroscopy Provides Mechanistic Insights in Fuel Synthesis and Conversion

Clean combustion, i.e., the reduction of NOx and soot emissions, and carbon neutrality, achieved in part by biofuel synthesis, are major milestones in the transition to a sustainable future. To overcome empiric and time-consuming process optimization steps, we need detailed reaction mechanistic and chemical insights on these processes. Be it in combustion or in catalysis, highly reactive intermediates, such as radicals, carbenes, and ketenes drive chemical reactions. Knowing the fate of these species helps develop strategies to optimize chemical energy conversion processes. This calls for advanced mass spectrometric tools, which enable the detection of transient species. In this review, we report on the application of state-of-the-art photoelectron photoion coincidence (PEPICO) spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation as advanced diagnostic tools in catalysis and combustion research. We discuss reaction mechanisms in biomass conversion to sustainable fuels, where we report on the pyrolysis of wood samples probed using VUV photoionization mass spectrometry (PIMS) and obtain deep mechanistic insights in the (non)catalytic pyrolysis of lignin model compounds with PEPICO detection. PEPICO is also shown to contribute to the mechanistic understanding of catalysis by unveiling catalytic alkane valorization mechanisms. We discuss how PEPICO detection advances combustion diagnostics, thanks to the application of photoelectron spectroscopy and velocity map imaging. We report on mechanistic aspects of ignition, such as fuel radical formation and oxidation to peroxy species, and discuss reaction pathways of pollutant formation. In addition, we zoom into the elementary reactions of combustion and discuss isomer-selective kinetics experiments on radical oxidation. Newly revealed reaction pathways to polycyclic aromatic hydrocarbon (PAH) formation are also detailed. Finally, we describe current instrumental developments to improve PEPICO detection and report on innovative sources, reactors, and reaction sampling approaches to be combined with this technique.

Broadband shock-associated noise modelling for high-area-ratio under-expanded jets.

Broadband shock-associated noise (BBSAN) is an important component of supersonic jet noise for jets at off-design conditions when the pressure at the nozzle exit is different from the ambient. Two high-area-ratio under-expanded supersonic jets at nozzle pressure ratios (NPRs) 3.4 and 4.2 are considered. The jets correspond to conditions of the experiment in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) in the Supersonic Jet Facility of Monash University. Flow solutions are obtained by the large eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) methods. The solutions are validated against the particle image velocimetry (PIV) data. For noise spectra predictions, the LES solution is combined with the time-domain Ffowcs Williams-Hawkings method. To probe the accuracy of the reduced-order method based on acoustic analogy, the RANS solutions are substituted in the Morris and Miller BBSAN method, where different options for modelling of the acoustic correlation scales are investigated. The noise spectra predictions are compared with the experimental data from the non-anechoic LTRAC facility and the NASA empirical sJet model. Apart from the low frequencies influenced by the jet mixing noise, the RANS-based acoustic predictions align with those from LES for most frequencies in the range of Strouhal numbers (St) 0.4 < St < 2 within 1-2 dB.

Numerical analyses of MILD and conventional combustions with the Eddy Dissipation Concept (EDC)

Moderate or intensive low-oxygen dilution (MILD) combustion is a novel combustion technology with high efficiency and low emissions. The turbulence-chemistry interaction is crucial in the numerical research of MILD combustion. In this paper, the MILD and conventional combustions are investigated numerically with the original and the modified Eddy Dissipation Concept (EDC) parameters. The results show that the original EDC constants are suitable for conventional combustion prediction. While, the predicted temperature, and therefore the NOx production in MILD combustion, are overestimated with the original EDC constants. MILD combustion can be reasonably predicted with the modified EDC constants. However, it is worth noting that, the numerical results of conventional combustion with the modified EDC parameters are similar to those of MILD combustion, so it is difficult to judge whether the combustion mode is MILD combustion only with the numerical results at this state. In addition, although conventional combustion with the modified EDC parameters is incorrectly predicted for the distributions of temperature and species, the flow field is basically unchanged with different EDC parameters. Therefore, the flow characteristics can be used to numerically distinguish between MILD combustion and conventional combustion.

Experimental Investigation of Self-Excited Combustion Instabilities in a LOX/LNG Rocket Combustor

Two types of combustion instabilities were observed in a multi-injector research combustor with liquid oxygen and liquefied natural gas (LOX/LNG) propellants operating at conditions similar to an upper stage rocket engine. The first was identified as an excited tangential acoustic mode of the combustion chamber and can be explained by coupling with the LOX injectors, similar to the mechanism previously observed in tests with propellants. The other type is characterized by longitudinal acoustic oscillations with peak-to-peak amplitudes of at least 40% of the mean chamber pressure. Analysis of the test data suggests that sudden upstream displacement of a lifted flame may be the trigger mechanism for the second type of instabilities.

Gaussian process regression for direct laser absorption spectroscopy in complex combustion environments

Tunable diode laser absorption spectroscopy (TDLAS) has been proved to be a powerful diagnostic tool in combustion research. However, current methods for post-processing a large number of blended spectral lines are often inadequate both in terms of processing speed and accuracy. The present study verifies the application of Gaussian process regression (GPR) on processing direct absorption spectroscopy data in combustion environments to infer gas properties directly from the absorbance spectra. Parallelly-composed generic single-output GPR models and multi-output GPR models based on linear model of coregionalization (LMC) are trained using simulated spectral data at set test matrix to determine multiple unknown thermodynamic properties simultaneously from the absorbance spectra. The results indicate that compared to typical data processing methods by line profile fitting, the GPR models are proved to be feasible for accurate inference of multiple gas properties over a wide spectral range with a manifold of blended lines. While further validation and optimization work can be done, parallelly composed single-output GPR model demonstrates sufficient accuracy and efficiency for the demand of temperature and concentration inference.

Pyrolysis of Waste Tires in a Twin-Auger Reactor Using CaO: Assessing the Physicochemical Properties of the Derived Products

The authors would like to express their gratitude to COLCIENCIAS for the financial support given by the research project (1210-715-51742). F.C. also acknowledges COLCIENCIAS for the Ph.D. scholarship (757-2016). F.C. expresses gratitude to the Clean Combustion Research Center at King Abdullah University of Science and Technology for the research internship. Some of the measurements presented herein were performed in the KAUST Core Laboratories with assistance from Wen Zhang, Abdul-Hamid Emwas, and Eman Barradah. The authors are also deeply grateful to Cindy N. Arenas for her valuable support with the characterization of some samples

Chemical Kinetics of Asphaltene Pyrolysis

The research reported in this publication was partially supported by the Saudi Electricity Company (SEC) and the Clean Combustion Research Center (CCRC) of the King Abdullah University of Science and Technology (KAUST). The authors thank Drs. Long Jiang and Aiping Chen for their work on asphaltene separation and analytical chemistry. This research used resources of the Core Labs at the KAUST. We also thank Dr Wen Zhang for the FT ICR-MS analysis and Dr Salvedin Telalovic for the TGA–MS.

Development trend analysis of variable oxygen concentration combustion research based on bibliometrics

Based on CiteSpace, 955 journal literature related to variable oxygen concentration combustion research collected in the web of Science (WOS) core database were analyzed visually. The results show that: 1 Literature publication trends. According to the total number of published literature, it can be divided into three stages: initial development, steady development, and rapid development. 2. Analysis of the author’s co-occurrence map. The author’s cooperation network in this field is obvious, but the overall cooperation network presents a “core-edge” structure, and the cooperation of academic teams is scattered. 3. Research hotspots, the related research mainly focused on the combustion characteristics of polymer synthetic materials, and chemical cycle combustion related to the field of energy and power engineering. 4. Research on the frontier trend. At present, “mechanical property” and “Nanoparticles” are the keywords representing the latest frontier trend in this field.

A new technique for real-time measurements of potassium and sodium aerosols based on field-reversal surface ionization

A new method for real-time measurements of potassium and sodium containing aerosol particles is described and verified. The method is based on surface ionization technique and may be used to explore the alkali chemistry related to high temperature chemistry processes. The measurement device is a further development of the simple and cost-effective surface ionization detector previously used for online alkali measurements in combustion and gasification research. The discrimination between sodium and potassium is possible due to differences in their surface desorption kinetics and facilitated by rapidly reversing the field potential between the ion source and the nearby collector. The instrument is evaluated in a series of laboratory experiments using size-selected alkali salt particles containing KCl, NaCl, K2SO4, Na2SO4, KNO3 and NaNO3. The filament temperature was found to be a key influencing factor in order to optimize the strength and Na–K deviation of the observed ion current. The ability to simultaneously report absolute concentrations of Na and K makes the instrument attractive for solid fuel conversion of alkali-rich fuels such as low-grade biomass and to explore behavior deviations of Na and K in high temperature processes.

Identification of In-Cylinder Aerosol Flow Induced Emissions due to Piston Ring Design in a DISI Single Cylinder LV Engine Using Oxygenated Synthetic Fuels

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;In the near future, pollutant and GHG emission regulations in the transport sector will become increasingly stringent. For this reason, there are many studies in the field of internal combustion research that investigate alternative fuels, one example being oxygenated fuels. Additionally, the design of engine components needs to be optimized to improve the thresholds of clean combustion and thus reduce particulates. Simulations based on PRiME 3D® for dynamic behaviors inside the piston ring group provide a guideline for experimental investigation. Gas flows into the combustion chamber are controlled by adjusting the piston ring design. A direct comparison of regular and synthetic fuels enables to separate the emissions caused by oil and fuel. This study employed a mixture of dimethyl carbonate (DMC) and methyl formate (MeFo). These two components have no C-C bonds, and the mixture displayed extremely good performance in terms of the particle number (PN) emissions on an ambient level published in previous studies. This fuel property is employed in this study to identify oil induced, engine-out PN-emissions, while the combustion process remains almost identical to that of conventional gasoline. The PN-emissions are measured and subdivided into two ranges: larger than 10 nm and larger than 23 nm.&lt;/div&gt;&lt;div class="htmlview paragraph"&gt;It was demonstrated that merely changing the piston ring design has an impact on raw PN engine emissions and gas flow behavior in the piston assembly. An increase in PN-emissions and lower blow-by level could only be detected by changing the piston ring design. With reduced, predicted fluid flows into the combustion chamber, lower VOC emissions could be observed during motored runs. The adaptations in the tested piston ring design demonstrate that it is possible to improve particulate emissions by modifying the piston ring group.&lt;/div&gt;&lt;/div&gt;

30 Years of Dry-Low-NOx Micromix Combustor Research for Hydrogen-Rich Fuels—An Overview of Past and Present Activities

Abstract This paper presents an overview of the past and present of low-emission combustor research with hydrogen-rich fuels at Aachen University of Applied Sciences (AcUAS). In 1990, AcUAS started developing the dry-low-NOx Micromix combustion technology. Micromix reduces NOx emissions using jet-in-crossflow mixing of multiple miniaturized fuel jets and combustor air with an inherent safety against flashback. At first, pure hydrogen as fuel was investigated with lab-scale applications. Later, Micromix combustor prototypes were developed for the use in an industrial gas turbine Honeywell/Garrett GTCP-36-300, proving low NOx characteristics during real gas turbine operation, accompanied by the successful definition of safety laws and control system modifications. Further, the Micromix was optimized for the use in annular and can-combustors as well as for fuel-flexibility with hydrogen-methane-mixtures and hydrogen-rich syngas qualities by means of extensive experimental and numerical simulations. In 2020, the latest Micromix application is demonstrated in a commercial 2 MW-class gas turbine can-combustor with full-scale engine operation. This paper discusses the advances in Micromix research over the last three decades.

Special Issue: Advances in Combustion Research

Special Issue: Advances in Combustion Research

The Reattachment Process of a Lifted Jet Diffusion Flame by Repetitive DC Pulse Discharges

On research of plasma assisted combustion, effects of electric and plasma discharges in DC, AC and pulse forms on reattachment of a lifted flame have attracted extensive attention. However, the detailed plasma assisted reattachment process and mechanism and roles of induced corona discharge and corona-induced ozone on the reattachment process are still unclear and undocumented. The forced reattachment process of a lifted diffusion jet flame by repetitive DC electric pulse discharges was experimentally investigated in this study using high-speed flame imaging, conditioned particle image velocimetry (PIV), and planar ozone concentration imaging. The forced reattachment process can be divided into three stages in sequence: ionic wind prior to corona initiation, corona initiation, and corona enhancement propagation. The conditioned PIV results showed that the instantaneous flame base propagation velocity is sufficiently enhanced beyond the laminar burning velocity for high pulse-repetition-frequency (PRF) cases at the instant of pulse discharge (on pulse) due to the enhanced oxidation of the corona induced ozone. By observing the dynamic flame-base behavior and evolution characteristics of the short-lived corona induced ozone for various PRFs, the novel forced reattachment process and mechanism of a lifted jet flame induced by repetitive DC electric pulse discharges is proposed.

Demonstration of a uniform, high-pressure, high-temperature gas cell with a dual frequency comb absorption spectrometer

Accurate absorption models for gases at high pressure and temperature support advanced optical combustion diagnostics and aid in the study of harsh planetary atmospheres. Developing and validating absorption models for these applications requires recreating the extreme temperature and pressure conditions of these environments in static, uniform, well-known conditions in the laboratory. Here, we present the design of a new gas cell to enable reference-quality absorption spectroscopy at high pressure and temperature. The design centers on a carefully controlled quartz sample cell housed at the core of a pressurized ceramic furnace. The half-meter sample cell is relatively long compared to past high-pressure and -temperature absorption cells, and is surrounded by a molybdenum heat spreader that enables high temperature uniformity over the full length of the absorbing gas. We measure the temperature distribution of the sample gas using in situ thermocouples, and fully characterize the temperature uniformity across a full matrix of temperatures and pressures up to 1000 K and 50 bar. The results demonstrate that the new design enables highly uniform and precisely known temperature and pressure conditions across the full absorbing path length. Uniquely, we test the new gas cell with a broadband (~2500 cm−1), high-resolution (0.0066 cm−1) dual frequency comb spectrometer that enables highly resolved absorption spectroscopy across a wide range of temperature and pressure conditions. With this carefully characterized system, we measure the spectrum of CO2 between 6800 and 7000 cm−1 at pressures between 0.2 and 20 bar, and temperatures up to 1000 K. The measurements reveal discrepancies from spectra predicted by the HITRAN2016 database with a Voigt line shape at both low- and high-pressure conditions. These results motivate future work to expand absorption models and databases to accurately model high-pressure and -temperature spectra in combustion and planetary science research.

Emissions Performance of Staged Premixed and Diffusion Combustor Concepts for an NH3/Air Flame With and Without Reactant Humidification

Abstract Renewably generated ammonia offers a form of carbon-free chemical energy storage to meet the differences between uncertain supply and fluctuating demand and has the potential to support future energy requirements. The storage/transportation characteristics of NH3 are favorable compared with H2; however, there are combustion research challenges to enhance fuel reactivity while reducing harmful emissions production. The purpose of this work was to evaluate different fuel delivery concepts for a representative gas turbine combustor. An experimental and numerical comparison was made between swirl-stabilized premixed and diffusion NH3–air flames at elevated inlet temperature (473 K). The exhaust NOx and NH3 emissions generated from each concept were quantified to optimize combustor performance. High-speed OH* and NH2* chemiluminescence was employed to characterize the change in flame topology with variation in fuel–air equivalence ratio, and the resultant influence on measured emission concentrations. Chemiluminescence intensities were shown to elucidate changes in sampled exhaust emissions, enabling detailed analysis of intermediate chemistry. A comparison was made between experimental data and kinetic simulations, demonstrating the sensitivity of NOx emissions to premixed fuel–air equivalence ratio. A comparison was also made between exclusive primary airflow, and the staged introduction of secondary air, to quantify the change in NOx production between each configuration and improve fuel burnout. Secondary air loadings were incrementally increased through the combustor. Finally, reactant humidification was employed as a secondary process for NOx reduction, having shown favorable performance with NH3–H2 mixtures, with the efficacy compared for both premixed and diffusion configurations.

Studies on gas concentration reconstruction methods for simultaneous temperature and H2O concentration tomographic imaging in a flame based on tunable diode laser absorption spectroscopy

Analysis of spatial temperature and gas concentration distributions is of great importance for combustion research. Algebraic reconstruction technique (ART) and adaptive algebraic reconstruction technique (AART) based on tunable diode laser absorption spectroscopy are both widely used methods for temperature tomographic imaging in a flame. After being obtained the temperature tomography through iterative method, the gas concentration reconstruction could be determined from a simple division equation (direct gas concentration algorithm) or iterative algorithm (ART gas concentration algorithm or AART gas concentration algorithm) for the second time. For the latter, iterative algorithm was used twice through the whole process. A series of numerical simulations were carried out by three gas concentration reconstruction methods mentioned above. The results demonstrated that AART method had better robustness for different phantoms and stability in the case that measured projections contained different levels of noise. Experimental measurements in a flat methane/air flame on a McKenna burner under three different equivalence ratios (1.0, 0.9, and 0.8) were conducted using AART gas concentration algorithm. The reconstructed results coincide well with those results obtained by thermocouple and theoretical calculation. Both simulation and experiment demonstrate that the AART gas concentration algorithm would be reliable and suitable for simultaneous temperature and H2O concentration tomographic imaging during combustion. As a consequence, this method has unrivalled potential for two-dimensional combustion diagnosis.