Combustion Systems Research Articles

Combustion Characterization and Kinetic Analysis of Coupled Combustion of Bio-Syngas and Bituminous Coal

The coupled combustion of bio-syngas and bituminous coal is an effective technology to reduce pollutant emissions from coal-fired power plants and improve the utilization rate of biomass energy. However, the effects of bio-syngas blending on bituminous coal combustion characteristics and reaction kinetics remain unclear, restricting the development of bio-syngas and bituminous coal combustion technology. In this study, the experimental studies on the coupled combustion of bio-syngas and bituminous coal under different bio-syngas lower-heating-value-based blending ratio (BLBR) were carried out by Micro-Fluidized Bed Reaction Analyzer (MFBRA), the coupled combustion characteristics and kinetic reaction mechanism of coupled combustion of bio-syngas and bituminous coal were analyzed. The results indicated that the nucleation and growth model (F1 Model) provided a reasonable description of the bituminous coal combustion process on the coupled combustion of bio-syngas and bituminous coal in MFBRA. The blending of bio-syngas significantly reduces the apparent activation energy and pre-exponential factor of bituminous coal combustion reaction. In particular, when the BLBR is 5, 10, 20, and 30%, the apparent activation energy is 48.70, 45.45, 46.75, and 42.35 kJ·mol−1, and the pre-exponential factor is 9.26, 7.10, 8.95, and 6.70 s−1, respectively. This research is helpful for improving the coupled combustion efficiency and ensuring the efficient operation of the coupled combustion system.

Numerical investigation of combustion characteristics of hydrogen-enriched low calorific value landfill gas for energy applications

Landfill gas is regarded as a promising renewable energy resource. However, Low calorific value landfill gases with less than 40 vol% CH4 are not well suited for energy applications and therefore disposed by flaring or direct emission to atmosphere causing environmental problems. In the present work, the potential of using hydrogen-enriched low calorific value landfill gas for energy applications has been explored through numerical investigation of combustion characteristics of different blends of fuels. The ignition delay of fuel mixtures decreased with the increase of hydrogen fraction (H2 enrichment from 0 to 50 vol%) at stoichiometric conditions. At high initial pressure conditions, ignition delay showed a weak relationship with the pressure. The laminar burning velocity increased with the increment of hydrogen fraction for the range of equivalence ratios considered (φ = 0.7 – 1.4) and also with the increase of initial temperature. However, the laminar burning velocity decreased significantly with the increasing initial pressure and hydrogen enrichment has insignificant effect at high initial pressures. The sensitivity analysis shows that the main elementary reactions inhibit the laminar burning velocity more with increasing initial pressure. The combustion performance of low calorific value landfill gas with 30 vol% CH4 enriched with 30 vol% hydrogen shows comparable performance to biogas with 60 vol% CH4 for a range of operating conditions. The numerical investigation results show that hydrogen-enriched low calorific value landfill gas could be used for thermal applications with the use of suitably designed combustion systems operating at low initial pressures and relatively high initial temperatures.

Waste plastic pyrolysis oils as diesel fuel blending components: detailed analysis of combustion and emissions sensitivity to engine control parameters

Efforts to reduce greenhouse gas emissions and ensure energy security have led to increased interest in renewable energy technologies, particularly pyrolysis for converting waste into fuel. This study examines the use of polypropylene (PPO) and polystyrene (PSO) pyrolysis oils as diesel fuel (DF) blending components. Single-cylinder engine tests were conducted on PPO and PSO, each blended with DF in mass proportions of 20%, 40% and 60%. The engine featured a state-of-the-art compression ignition combustion system, including an 8-hole high-pressure injector and precisely controlled air and exhaust gas recirculation paths. Results proved that both PPO and PSO could be efficiently combusted in modern compression ignition systems, provided their admixture with reference DF did not exceed 60%. For optimal performance, the engine control map needed to be re-calibrated to prevent over-homogenization of the pilot spray, caused by higher volatility and reduced fuel reactivity. When maximising waste-derived components, PPO provided optimal emission results (1.37 g/kWh NOX and 0.04 g/kWh PM) when combined with heavy EGR and advanced injection timing. In this case, the total emissions overlimit value was 3.7, compared to 7.3 in the best-case DF scenario (a 50% reduction in cumulative emissions). For minimizing emissions, small additions (20%) of highly volatile PSO were found to offer the greatest potential for co-optimization. Compared to the optimal DF configuration, a cumulative emissions reduction of 81% was achieved in the same calibration map region as for PPO.

Investigation of NO emission characteristics from co-combustion of methane and ammonia at high-altitude areas

The high-altitude areas of China are abundant in renewable energy and have a natural advantage in ammonia production. Based on this advantage, this paper proposes a co-combustion strategy for methane and ammonia to reduce carbon emissions in these areas. However, the NO emission characteristics associated with this strategy remain uncertain. A custom-designed combustion system capable of simulating high-altitude environments was used to investigate the effect of ammonia mixing ratio, equivalence ratio, and pressure on NO emission in methane/ammonia/air flames. Additionally, chemical kinetic calculations were conducted to explore the mechanisms of how sub-atmospheric pressure influences NO emission. The results indicate that for stoichiometric flames, NO increases with the ammonia mixing ratio. In fuel-rich flames, NO remains nearly constant once the ammonia mixing ratio exceeds 10%. Sub-atmospheric pressure leads to higher NO, particularly in fuel-rich flames, where the increase can reach up to 24.4%. Analysis of nitrogen reaction pathways and key radical concentrations reveals that sub-atmospheric pressure has a minimal effect on nitrogen conversion pathways. The variation in NO is achieved by altering the pathway contributions and the concentrations of H, NH, and N. This work provides direction and guidance for improving the application of methane and ammonia co-combustion in high-altitude areas.

The Role of Micro Propulsion in Enabling Autonomous Operations of Nanosatellites

Micro-Electro-Mechanical Systems (MEMS)-based propulsion devices have significant potential for providing high specific power. The application of Micro Power Generation technology to space propulsion systems can enhance orbit and station-keeping capabilities, potentially at lower costs. New-generation spacecraft and satellites are becoming smaller and require micro-propulsion systems for all aspects of their operations. The development of micro-propulsion systems with associated combustors for micro-electricity generation, utilizing liquid fuel, presents major challenges in fuel injection, mixing, combustion, and chemical reactor systems. A micro-spacecraft with high-accuracy station-keeping and attitude control capabilities needs to have low mass and deliver small impulses. Each nanosatellite will weigh a maximum of 10 kg, including the propellant mass. Provisions for orbital manoeuvres, attitude control, multiple sensors, and instruments, along with full autonomy, will result in a highly capable miniaturized satellite. All onboard electronics will endure a total radiation dose of 100 k rads over a two-year mission lifetime. Nanosatellites designed for in-situ measurements will be spin-stabilized and equipped with a complement of particles and fields instruments. Nanosatellites intended for remote measurements will be three-axis stabilized and equipped with imaging and radio wave instruments. Key technologies under development include: advanced, miniaturized chemical propulsion systems; miniaturized sensors; highly integrated, compact electronics; autonomous onboard and ground operations; miniaturized onboard orbit determination methods; onboard RF communications capable of transmitting data to Earth from significant distances; lightweight and efficient solar array panels; lightweight, high-output battery cells; a miniaturized heat transport system; lightweight yet strong composite materials for nano-satellite and deployer-ship structures; and simple, reusable ground systems.

Integrated DNN and CFD model for real-time prediction of furnace waterwall slagging of coal-fired boiler

Slagging of furnace waterwall adversely affects the thermal performance and increases the operation and maintenance costs of coal-fired boilers. A real-time slagging prediction model is of great significance for the plant operators to monitor the furnace slagging conditions and make appropriate operation of soot blowers to maintain the cleanliness of waterwall. Three-dimensional (3D) computational fluid dynamics (CFD) can predict detailed furnace slagging distributions, but its high computational cost has been the major impediment hindering its application in large-scale combustion systems that require real-time information. To resolve this problem, a CFD-based deep neural network (DNN) model framework was developed in this study to realize the real-time prediction of furnace wall slagging distribution. A 3D CFD model was first developed and employed to generate the slagging database under typical boiler operating parameters. Then this database was utilized to train the DNN model to capture the mapping relationship between boiler operating parameters and wall slagging distributions. The core idea of this CFD-based DNN model is to generalize the wall slagging distributions under different boiler operating conditions based on this mapping relationship learned by the DNN model. This model was applied to a 350 MW coal-fired boiler. The prediction results by the CFD model demonstrated that the boiler operating parameters, such as boiler load and coal burner swirling blade angle, have strong effects on furnace wall slagging distributions, while the DNN model could precisely capture their relationship and make predictions with deviation from the CFD results below 5 %. More importantly, the DNN model could give the prediction results within seconds that is three to four orders of magnitude faster than the CFD model. Thus, it can respond to the rapidly changing boiler operating conditions and realize the real-time prediction of furnace wall slagging distributions.

Determination of Plastic Pollutants in Solid Biofuels

Many countries widely use biomass for household heating and heat production in district heating systems. Unfortunately, the steady increase in annual plastic waste production has a negative impact on the quality of solid biofuels. This is due to the increasing contamination of these fuels with wastes from plastic and wastes from furniture production, such as laminates and medium-density fiberboard made from wood fibers, among others. The design of specialized biomass combustion systems does not allow for the burning of waste fuel, or the reduction in hazardous organic compounds emitted when burning contaminated biofuels. The study demonstrated the detection of polymeric impurities in solid biofuels through analytical pyrolysis (Py-GC-MS). The study was conducted on model samples that contained increasing proportions of plastic waste, ranging from 0.1 to 10.0% w/w to biomass. Markers were identified and described to indicate contaminated fuel, and the interactions between the sample matrix and plastic were studied. Unique markers were detected that indicate the presence of contamination, even at low concentrations like 0.1% w/w of plastic waste in solid biofuel. These results suggest that direct analytical pyrolysis of solid biofuels, which are already on the market but not covered by the relevant regulatory system and are contaminated with polymeric ingredients, is a method that is not only possible but also gives quick confirmation.

Effects of noise intensity on early warning indicators of thermoacoustic instability: An experimental investigation on a lean-premixed combustion system

In this work, we experimentally investigate the noise-induced dynamics of a lean premixed combustion system operating on natural gas–air mixtures that exhibit thermoacoustic instability via a subcritical Hopf bifurcation. The investigation is done before the bistable region with equivalence ratio (ϕ) as the control parameter. We analyze the acoustic pressure oscillations (p′) in the combustor and fluctuations in the heat release rate (q′) from the laminar quasi-flat flame at increasing levels of white noise. We show the effects of noise intensity on the reliability of various types of early warning indicators (EWIs) to predict the onset of the impending thermoacoustic oscillations. We investigate the indicators based on statistical measures (variance, skewness, and kurtosis), autocorrelation and spectral properties (coherence factor), system identification (growth/decay rates of p′), multi-fractality (Hurst exponent), and time series complexity (permutation entropy and Jensen–Shannon complexity). The coherence factor, variance, and decay rates of p′ always increases as the system approaches thermoacoustic instability, indicating their robustness as an EWI under most noise levels. An increase in kurtosis cannot be employed as an EWI. Implementing autocorrelation, skewness, Hurst exponent, permutation entropy and Jensen–Shannon complexity as effective EWIs has limitations: they can be estimated accurately only from pressure oscillations (p′) data and work only above a particular threshold value of noise intensity. Our results have direct implication on early prediction and control of thermoacoustic instability in practical gas turbine combustors.Novelty and significance statementDeveloping effective early warning indicators (EWIs) to anticipate the onset of thermoacoustic instability is crucial for preventing potential damage and ensuring the reliable operation of lean premixed gas turbine combustion systems. In such systems, inherent noise dynamics undergo variations with changing operating conditions and combustor designs. Specifically, noise intensity increases as the system becomes more turbulent. In this study, we demonstrate that the inherent noise dynamics in a lean premixed combustion system play a crucial role in influencing the trends observed in early warning indicators of thermoacoustic instability. We address several key questions, including (a) whether comparative reliability assessments of different classes of EWIs exist, (b) the effect of variations in noise properties on the efficacy of EWIs, and (c) which EWIs are most reliable considering that noise characteristics depend on the system and may even change with operating conditions within a specific system. This information is critical for engine designers/users, facilitating the development of robust and effective monitoring systems for gas turbine combustion systems.

Flame emission spectroscopy measurement and comprehensive characterization of various spray flames in a turbulent swirl burner

In this study, a spray swirling combustion system focusing on five liquid fuels: methanol, ethanol, heptane, aviation kerosene, and sustainable aviation kerosene is built, and the combustion characteristics of spray swirling flames based on OH*, CH*, and C2* chemiluminescence characterization are further investigated. Subsequently, a quantitative model relating the chemiluminescence intensities of OH*, CH*, and C2* to the flame parameters is established, providing essential data for studying spray swirling flames. Results showed that the combustion efficiencies of the spray swirling flame for five liquid fuels positively correlate with the equivalence ratio and the airflow exit velocity. Yet the linearity between the chemiluminescence intensities of OH*, CH*, and C2* and the equivalence ratio is relatively low, and the efficacy of the intensity ratios OH*/CH*, OH*/C2*, and CH*/C2* in characterizing the equivalence ratio is limited. The correlation coefficients of the quadratic polynomial fitting between the chemiluminescence intensity ratio of OH*·C2*/CH* and the equivalence ratio exceed 0.98 under any airflow exit velocity condition, making it the optimal indicator for characterizing equivalence ratio in spray swirling flames. For the heat release rate, a power-law model based on the chemiluminescence intensity is proposed, demonstrating significantly higher effectiveness compared to the traditional linear models. The three-species power-law model utilizing the OH*, CH*, and C2* chemiluminescence intensities achieves the fitting accuracy exceeding 98% across the five fuels, providing a robust quantitative characterization for heat release rate in spray swirling flames. However, employing the chemiluminescence of single or two of the three radical species to determine heat release rates in liquid fuels lacks universality.

Analysis of Excited Species Formation across the Flame of Various Ammonia‐Hydrogen Fired Combustor Geometries

Although ammonia can be used as a fuel, it also presents drawbacks that require further investigation before the chemical can overtake fossil fuels in combustion systems. The main barriers are the low flammability in combination with high NOx emissions. Although the first barrier can be surpassed by doping ammonia with hydrogen, the second becomes more challenging under these conditions, as hydrogen increases NO emissions due to the increase in H radicals in the chemical pool of species. How the change in radicals impacts the stability of the flame, its reactivity, and emissions profile is of the greatest concern for the use of these net zero fuels. Thus, the work herein presented shows the trends of excited species such as NH*, NH2*, and OH* when using ammonia–hydrogen at 70%–30% (vol) blending. Various equivalence ratios are employed from lean to rich conditions. Results denote that there is a continuous displacement of radicals across the field, with NH2* relocating closer to the centerline of the burner as equivalence ratio increases, while NH* tends to raise its location while dislocating from the production/consumption of OH* radicals. The results can be used to target desirable radicals for the mitigation of emissions and flame control.

Evaluation of combustion system for organically bound tritium analysis using dried fish samples.

The combustion process of the organically bound tritium (OBT) analysis method is difficult and requires sophisticated techniques. For ease, safety, and reproducibility a semi-automatic combustion system was developed. An arbitrary combustion program that prevents sudden ignition was devised and combustion tests were conducted. Analytical test runs were conducted using fish standard samples to confirm the validity of the developed system. The test values were compared with values obtained with another analysis method, and the recovery rate of the combustion water was evaluated. The results confirmed that the analytical values obtained with this combustion system were equivalent to those obtained with another analytical method and were valid for OBT analysis.

Modeling two-phase gas-solid flow in axisymmetric diffusers using cut cell technique: an Eulerian-Eulerian approach

Axisymmetric diffusers find wide applications in various industrial processes, including combustion systems, pneumatic conveying, and fluidized bed reactors. Understanding and accurately modeling the behavior of two-phase flows in these devices are critical for optimizing their performance and efficiency. Various aspects of the two-phase gas-solid flow, including particle-particle and particle-wall interactions, interphase momentum transfer, and phase segregation, are investigated. Our currently developed technique, which uses a cut-cell technique, is an established new method to handle the inclined wall of an axisymmetric diffuser. The near-wall cells are treated as five faces for the new grid; one is the inclined wall. This helps treat the boundary condition at the wall in an accurate physical way. The code FORTRAN, developed in-house, was used to solve the axisymmetric diffuser using a cut-cell technique. The results of this study will provide valuable insights into the behavior of gas-solid two-phase flows in axisymmetric diffuser. A parametric study of the impact of particles diameters (100,200,and300μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$100,\\ 200,\\ \ ext{and} \\ 300\\mu m$\\end{document}), the solid volume loading ratios (0.005,0.008,0.01)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$(0.005,\\ 0.008,\\ 0.01)$\\end{document}, and cant angle (4.5∘,7∘,9.5∘)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$(4.5^{\\circ},\\ 7^{\\circ},\\ 9.5^{\\circ}) $\\end{document} effect of axisymmetric diffuser on the local skin friction, pressure, velocity, turbulent kinetic energy, and separation zone.

Numerical modeling and parameter optimization of the combustion chamber in a tower-type zinc refining furnace

With the implementation of “coal-to-gas” projects, the tower-type zinc refining furnace (TZRF) has encountered a mismatch between high-calorific value fuel and the primary combustion system. To address this issue, this study conducted a full-scale modeling of the combustion and heat transfer processes in the TZRF combustion chamber using computational fluid dynamics. An in-depth analysis and condition diagnosis of the TZRF were performed, elucidating the key causes of deteriorating furnace conditions, reduced efficiency, and decreased production capacity following fuel replacement. This study shows that the flow ratios of gas outlets are unevenly distributed, leading to poor mixing of natural gas and air, resulting in an uneven temperature field in the TZRF. A low-temperature zone is clearly observed below the second-layer burner, where significant accumulation of natural gas fails to combust completely. Although tilting the first-layer burner upward slightly enhances the overall heat transfer performance of the tray, it detrimentally affects the overall combustion of natural gas in the TZRF. The deployment of third-layer air outlets substantially eliminates low-temperature zones, expands the range of high-temperature zones, and significantly improves the furnace condition. The optimal flow ratio of the third-layer air outlets is 29.57 %.

Long-term emission demonstration using pretreated urban non-woody biomass residues as fuel for small scale boilers

There is currently a lack of non-woody biogenic solid biofuels to replace fossil fuels to expand renewable heat energy due to their combustion-critical behavior (slagging and emissions). For this reason, this study demonstrates a best practice under which non-woody solid biofuels can be put into practice. For this demonstration, a homogeneous non-woody solid biofuel was produced from urban park foliage in accordance with ISO 17225–6:2021 on industrial scale (>50 t). Long-term combustion tests were performed in a small-scale combustion system (80 kW) under field test conditions. Compliance with all emission thresholds of the legal framework was demonstrated (Carbon monoxide (CO): 44.9 mg/m³, Total particulate matter (TPM): 11.9 mg/m³, Nitrogen oxides (NOx): 279.6 mg/m³, Polychlorinated dibenzodioxins and dibenzofurans + Dioxin-like polychlorinated biphenyls (PCDD/F + dl-PCB): 0.01 ng/m³ and Benzo[a]pyrene (B(a)P): 7.3*10−6 mg/m³). Additionally, nutrient cycles can be closed by applying the bottom ash as soil fertilizer additive. Based on the study, greenhouse gas-neutral fuels from non-woody biomass residues for comparable applications can be produced based on the demonstration.

Future technological directions for hydrogen internal combustion engines in transport applications

The paper discusses some of the requirements, drivers, and resulting technological paths for manufacturers to develop hydrogen combustion engines for use in two types of market application – on-road heavy- and light-duty. One of the main requirements is legislative certainty, and this has now been afforded – at least in the major market of Europe – by the European Union's recent adoption into law of tailpipe emissions limits specifically designed to encourage the uptake of hydrogen engines in heavy-duty vehicles, giving manufacturers the confidence they need to invest in productionized solutions to offer to customers.It then discusses combustion systems and boosting systems for the two market types, emphasizing that heavy-duty vehicles need best efficiency throughout their operating map while light-duty ones, since they are rarely operated at full load, will mainly primarily need efficiency in the part-load region. This difference will likely cause a divergence in solutions, with heavy-duty engines running very lean everywhere and light-duty ones likely operating at the stoichiometric air-fuel ratio, at least for most of the map. The impacts of the strategies on engine systems and vehicle integration are discussed.It is postulated that due to reasons of preignition avoidance and efficiency hydrogen engines will rapidly adopt direct injection and that the long-term heavy-duty types will migrate towards the typical current spark-ignition-type cylinder head architecture where tumble, rather than swirl, will ultimately be needed for air motion in the cylinder for these reasons. They may also adopt active pre-chamber technology to ignite extremely lean mixtures for maximum efficiency and minimum emissions of oxides of nitrogen.It is suggested that light-duty engines will evolve less from their current gasoline architectural norm since they already contain all of the necessary fundamentals for hydrogen combustion. However, since part-load efficiency will be important, some new strategies may become desirable. Developing dual-fuel light-duty engines could accelerate their uptake as the heavy-duty market simultaneously accelerates the creation of the fuel supply infrastructure.The likely technological evolution suggests that variable valve trains, and specifically cam profile switching technology, would be extremely useful for all types of hydrogen engine, especially since they are readily available in different gasoline engines now. New operating strategies afforded by variable valve trains would benefit both heavy- and light-duty engines, and these strategies will become more sophisticated. There will therefore likely be a convergence of technologies for the two markets, albeit with some key differences maintained due to their vehicle applications and their differing operation in the field.

Convective heat transfer parameters in the film cooling with thermal radiation

Thermal radiation is a large contributor to the total heat transfer of combustion systems so that it cannot be neglected, with the improvement of gas temperature and infrared stealth requirements. This paper deeply investigated the convective parameters in film cooling with thermal radiation through theoretical analysis and numerical simulation. A method to obtain the convective driving temperature under radiation was proposed and demonstrated. Research found that the film cooling effectiveness with radiation should be expressed by the mixing temperature rather than the adiabatic wall temperature. Although heat transfer and mass transfer under radiation cannot be theoretically analogized due to the radiant source term in the energy equation, the concentration cooling effectiveness without and with radiation remains approximately the same. The numerical results indicate that although conjugate coupling plays a crucial role in determining convective and radiative heat fluxes, the coupling of radiation and convective parameters is very weak. Radiation has little influence on the film cooling effectiveness (η) and the convective heat transfer coefficient (hf). Convective parameters can be decoupled from radiation with negligible loss of accuracy. This allows the non-radiation convective parameters η and hf to be directly used for calculating the convective heat flux under radiation, with relative deviations of less than 5%.

Advanced image processing techniques for multi-level characterization of significant flame features in carbon-neutral combustion

This research presents an advanced image processing framework designed for the multi-level characterization of significant flame features in carbon-neutral combustion systems. The study introduces an optimized neural network based on a brainstorming algorithm and an enhanced support vector machine (SVM) algorithm, which utilizes an improved particle swarm optimization (PSO) technique. The framework is capable of accurately processing complex flame image data, achieving flame classification accuracy rates of up to 100 %. The proposed system combines detailed analysis of flame brightness, texture, and morphological features, contributing to the comprehensive monitoring of combustion states. Additionally, the paper describes the design of a clean burner that promotes the full combustion of zero-carbon gases. The integration of this burner with the advanced monitoring system significantly enhances safety and efficiency in industrial applications. This work addresses the challenges associated with the transition to renewable gases and provides a robust solution for maintaining stable and efficient combustion processes.

Numerical investigation of turbulent fuel jet diffusion and its influence on the auto-igniting diffusive flame development

Abstract Turbulent auto-igniting diffusion flames are common in combustion systems, where the mixing of the fuel jet with the outer oxidant flow is the fundamental mechanism for the flame ignition and development. The paper presents a critical analysis on the Reynolds-Averaged-Navier-Stokes (RANS) simulation of a methane-air flame, done with a two equations turbulence model, in comparison with detailed experimental data. In the first part, the non-reactive jet is simulated with the standard k − ε model and a modified version, obtained with motivated changes of the Cμ and σk constants. In the second part the reactive jet is simulated, introducing a consistent modification of the Cγ constant, in the eddy dissipation concept (EDC) model. The comparison with experimental data confirms the validity of the proposed model and highlights the effect of the proper turbulent jet development on the mechanisms of the diffusive flame ignition and propagation.

Volumetric and Entropic Sound Sources of Helmholtz Resonators with Different Temperatures

This study proposes an acoustic analogy model for the Helmholtz resonator (HR)–combustor system, considering the temperature difference between the bias flow from the HR and the grazing flow inside the combustor duct. The model highlights how the mass flux of the bias flow and the temperature difference serve as mass and entropic sound sources, respectively, influencing the HR’s sound absorption performance. To validate the model, we conducted numerical simulations using the Reynolds-averaged Navier–Stokes shear stress transport model to obtain the mean flow, followed by solving the frequency-domain linearized Navier–Stokes equations in COMSOL. The model was tested with HRs attached to combustor ducts under open–open and open–closed boundary conditions. Theoretical results agree well with numerical simulations, confirming the model’s accuracy. Results indicate that when the bias flow temperature is lower than the grazing flow temperature, the entropic sound source negatively affects duct acoustics, reducing sound absorption performance. Conversely, higher bias flow temperatures enhance sound absorption by superimposing entropic and mass-related sound sources. Additionally, the overall effect of the mass-related and entropic sound sources is equal to a volumetric source which depends on the mean sound speed and density of the HR instead of those of the combustor.

The Effect of Entropy Waves Features on the Indirect Noise Generation within an Aeronautical High-Pressure Turbine stage

Abstract First stages of aeronautical high-pressure turbine are subjected to inlet distortions generated by the combustor systems. Such disturbances, characterized by velocity and temperature fluctuations, are referred to as vorticity and entropy waves and they are convected downstream by the main flow. Such disturbances interact with the turbine stages affecting the stage efficiency, heat exchange and generating noise emissions. This work summarizes a comprehensive numerical campaign on the effect of different Entropy Waves features (i.e. radial position, clocking and swirl direction) on the indirect noise generation of engine-representative high pressure turbine stage tested at the Politecnico di Milano LFM (Laboratory of Fluid Dynamics of Turbomachinery). The numerical campaign is based on URANS CFD simulations with time-varying inlet distortions, carried out with the University of Florence in-house TRAF code. A Discrete Fourier Transform (DFT) based post-processing tool has been used to extract the low frequency entropy wave content to assess indirect noise emissions. The extensive validation of the TRAF code in previous campaigns grants the fidelity of the results in terms of aerodynamic and acoustic quantities. The numerical results provide a deeper insight of the effect of entropy waves features on indirect noise generation. The outcome of the results may be used by the designer to better understand the combustor-turbine interaction, also considering the possible introduction of innovative fuels such as hydrogen or Sustainable Aviation Fuel (SAF).