Simulation Of Combustion Research Articles

Particle-resolved numerical simulations of char particle combustion in isotropic turbulence

In the present study, two-dimensional particle-resolved numerical simulations of char particle combustion are performed. A series of cases are considered, with varying turbulence intensities and heterogeneous reaction rates. The impact of turbulence and Stefan flow on char particle combustion is examined. It is shown that the cases with higher turbulence intensity and heterogeneous reaction rate exhibit higher mean gas-phase temperatures and reaction rates. The range of the Stefan flow Reynolds number (ReS) increases as the value of heterogeneous reaction rate increases. Additionally, higher turbulence intensity is associated with an increased range of ReS. The drag force coefficient of burning particles is analyzed. The results show that turbulence induces fluctuations of drag force coefficient, and increasing the turbulent velocity broadens the drag force coefficient fluctuation amplitude. The particle drag force is increased with increasing heterogeneous reaction rate. The pressure and viscous components of the drag force are examined. It is revealed that the increase of heterogeneous reaction rate alters the pressure distribution on the particle surface, resulting in the increase of pressure drag force, while the viscous drag force remains almost unchanged.

Numerical model of restrikes in gliding arc discharges

Abstract Direct current (DC) electric arcs are of particular interest because they can produce large volumes of thermal plasmas with controlled energy deposition. When such discharges are applied in a gas flow, convection displaces the top of the arc downstream while the arc roots remain attached to the electrodes, thus increasing the length of the arc over time. However, this growth is limited by a restrike phenomenon, which starts from streamers appearing in high electric field regions and shortcutting the long, stretched electric arc. From a numerical point of view, DC arcs can be efficiently simulated with a resistive magneto-hydrodynamics (MHD) model, with numerical requirements in terms of spatial and temporal discretization that are compatible with classic fluid dynamics and combustion simulations. However, arc restrikes rely on the propagation of streamer discharges that are highly non-neutral phenomena, whereas classical MHD assumes neutrality. To tackle this problem, we propose in this paper a model of restrike that can be used in an MHD approach. After describing the ideas of the model, we perform a parametric study of the input parameters to examine its influence on the discharge dynamics.

Coupled simulation on flue gas and steam deviation of final reheater in a 600 MW tangentially fired boiler with reversed separated overfire air under different loads

Flue gas and steam deviation of the final reheater is an inevitable problem in the tangentially fired boiler due to the remaining gas spinning at the furnace exit. An in-house one-dimensional process simulation code coupled with comprehensive three-dimensional combustion simulation was adopted to accurate investigate the characteristics of the flue gas and steam maldistribution of the final reheater in a 600 MW supercritical tangentially fired boiler. Firstly, the combustion simulation result was validated by the in-house fire-side heat transfer calculation data, then the effect of boiler loads and reversed separated overfire air (SOFA) deflection angles on the gas temperature deviation, velocity deviation and outlet steam temperature of reheater were conducted. The results showed that the deviation of flue gas temperature and velocity increases as the boiler load decreases. The reversed SOFA deflection angle plays a crucial role in improving the gas distribution under low loads, and the deviation of flue gas temperature and velocity is the smallest when the angle is -18? under 100% boiler maximum continuous rating (BMCR), 75% and 50% turbine heat acceptance (THA), but it is the smallest when the angle is -9? under 35% BMCR load. Moreover, the maximum outlet steam temperature difference between parallel heating surfaces of the final reheater increases from 84.68 K to 106.48 K as the boiler load decreases from 100% BMCR to 75% THA, and it is mainly affected by the flue gas temperature deviation rather than the mass flow rate maldistribution when the deflection angle changes from -9? to -18?.

Numerical investigation of radiative transfer during oxyfuel combustion of hydrogen and hydrogen enriched natural gas in the container glass industry

The substitution of natural gas with hydrogen is one way to eliminate direct CO2 emissions. However, oxyfuel combustion of hydrogen or hydrogen enriched natural gas leads to different exhaust gas properties due to a changed composition compared to conventional combustion. In combustion simulation, the emissivity of a gas mixture is usually approximated using a Weighted Sum of Gray Gases (WSGG) model. Most of the existing WSGG models have been validated for natural gas combustion with air or oxyfuel and are therefore not applicable to hydrogen-oxyfuel combustion. CFD simulations showed, that none of the investigated WSGG models is able to predict the radiative heat transfer for all considered combustion scenarios with appropriate accuracy. In addition, in container glass manufacturing more than 95% of the heat flux to the glass surface is transferred by radiation because of the high process temperatures. Due to the changed gray gas emissivity, the high content of water vapor leads to a different emission spectrum of the exhaust gas. The influence of the changed emission spectrum on radiative heat transfer and the penetration depth of radiation in the glass melt is investigated using a simulation model of a pilot plant and non-gray modelling of the radiation transport. The CFD simulations show slightly enhanced radiative heat transfer to the glass and a slightly deeper penetration depth especially for wavelength below 2.2 μm for hydrogen-oxyfuel combustion.

Spectral analysis of soot dynamics in an aero-engine model combustor

The dynamics of soot evolution in a swirl-stabilized model aero-engine combustor are studied using Large-Eddy Simulation (LES) and state-of-the-art combustion and soot models. The simulated combustor is a dual-swirl combustor with and without secondary oxidation air at a pressure of 3 bar. The comparison with experimental data shows that the simulation accurately captures the gas-phase statistics. Overall, soot statistics are well captured, particularly in the shear layers, although underpredicted in other locations. Three locations in the combustor: Shear Layer (SL), Inner Recirculation Zone (IRZ), and Outer Recirculation Zone (ORZ) are probed to obtain velocity, temperature, and soot volume fraction signals. Lomb–Scargle spectral analysis on the probed signals reveals that soot evolution in the SL is characterized by high-frequency dynamics, whereas in the IRZ and ORZ, it is characterized by low-frequency dynamics. Within the SL, couplings between the soot and flow dynamics are observed, with the soot volume fraction and velocity sharing a common dominant frequency. However, in the IRZ and ORZ, such couplings are not evident. Additionally, the wavelet transform is applied to the probed signals to study the temporal distribution of the frequencies. The analysis indicates that, in the SL, the dominant frequency of the velocity occurs continuously throughout the entire time series, while higher frequencies occur in short bursts. Conversely, for the soot volume fraction, both the dominant frequency and higher frequencies appear in short bursts. In the ORZ and IRZ, the soot volume fraction scalogram shows that low frequencies dominate and occur continuously throughout the time series. Finally, the phase space reconstructions show that the trajectory of the soot dynamics shows a circular pattern, indicative of periodic behavior. The center of attraction remains stationary over a relatively large time scale, suggesting stability in the dynamics as they evolve.

Numerical simulation of oxygen-enriched combustion in a precalciner using coal gangue-blended pulverized fuel

This study emphasizes the potential role of coal gangue as an alternative fuel and oxygen-enriched combustion technology within TTF precalciners(Twin-Tank Furnace) in mitigating carbon emissions from cement production. The research primarily unfolds in two segments. Initially, the investigation addresses the influence of blending varying proportions of coal gangue under an air atmosphere on the internal temperature field and raw material decomposition components within the precalciner, aiming to discern the optimal blending ratio. Subsequently, the study simulates combustion under different oxygen-enriched atmospheres at the ideal coal-gangue blending ratio, establishing the combustion patterns under these conditions. Although the mixed fuel prompts symmetry in the flow field and temperature field, the distinct combustion characteristics of coal gangue and coal powder-following a 20% coal gangue blend-lead to an accelerated mainstream velocity and abbreviated fuel residence time. Consequently, the exit temperature and CO2 concentration diminish with increasing blending content, reaching an optimal raw material decomposition rate of 91.12% within the precalciner when blended with 20% coal gangue. Furthermore, in oxygen-enriched combustion, as the oxygen content escalates, both the average temperature at the precalciner's exit and the raw material decomposition rate witness an upsurge, whereas the average CO2 concentration at the outlet experiences a decline.

Simulation study on effect of nozzle geometry on flameless combustion of non-preheated methane gas

In this paper, the effects of different burner configurations on the characteristics of flameless combustion were evaluated by comparing the temperature field, NOx, OH, and H2CO at different burner inlet velocity and angles through a combination of experimental and numerical simulations. The results show that increasing the burner inlet angle and gas velocity is imperative in achieving the flameless combustion, increasing the re-circulation rate in the furnace, making the temperature distribution in the furnace uniform, and reducing the emission of NOx at the end of the furnace. During the simulation of flameless combustion, it was found that OH radicals and H2CO radicals were well correlated with the reaction exothermic zone, and the Reynolds number was positively correlated with the re-circulation rate in the furnace. With the increase of Reynolds number, the entrainment rate of flue gas increases, and the combustion state is closer to flameless combustion. When re-circulation rate Kv >2, combustion becomes flameless. Through the summary analysis of the data, it can be found that there is a critical Reynolds number for the burner to achieve flameless combustion, and flameless combustion occurs only when the Reynolds number is greater than 1.0?104.

Numerical Simulation of Combustion in 660 MW Tangentially Fired Pulverized Coal Boiler on Ultra-Low Load Operation

Numerical Simulation of Combustion in 660 MW Tangentially Fired Pulverized Coal Boiler on Ultra-Low Load Operation

A comprehensive study on the design and computational analysis of an air swirl burner

This paper presents a comprehensive study on optimizing the design of a vane-type air swirl burner, along with a computational analysis comparing the combustion of diesel and hydrogen fuel within the same burner. For the combustion simulations, a coupled approach was employed involving a Lagrangian approach with the Discrete Phase Model to track the trajectories of fuel particles providing a comprehensive understanding of the particulate dynamics within the system. This research aimed to analyze hydrogen's potential as a sustainable energy carrier in comparison to its conventional counterparts such as diesel fuel. The results showed that the contours of hydrogen fuel revealed a well-defined and intense flame front whereas it was not the same in the case of diesel fuel which had a less intense flame front.

Biomass particle-radiation-interaction and the effect of shape and structure simplifications

Particle radiation significantly contributes to the overall heat transfer in pulverized solid fuel combustion. For a lower computational effort in numerical simulations, the particles are often assumed to be spherical without an internal structure. In particular, biomass particles deviate strongly from this shape assumption and consist of a structured porosity. To investigate the influence of these aspects, pulverized beech wood particles are modeled as realistic as possible, considering the asymmetric three-dimensional shape and tube-like pores. The numerically created particle is then used to calculate single-scattering properties (absorption-, scattering cross-section, and scattering phase function) by applying the discrete dipole approximation (DDA). Afterward, the shape and internal structure of the representative biomass particle are step-wise simplified, while comparing the impact on the radiation interaction with the detailed modeled particle.The calculations reveal that simplifications of the internal structure are acceptable as long as a similar shape is maintained. Here, an extruded ellipsis with no internal structure and an effective complex index of refraction indicates the best alternative and is recommended for future particle radiation simulations. While the deviations of the radiation cross-section between the extruded ellipsis and the realistic particle are below 10%, the deviations of the radiation cross-section between a sphere and the realistic particle are around 50%. The orientation averaged phase function of all particle models scatters more than 99.5% of the scattered radiation forward. Thus, a purely forward scattering model in combustion simulations is recommended.

Numerical Modeling of Chemical Kinetics, Spray Dynamics, and Turbulent Combustion towards Sustainable Aviation

With growing interest in sustainable civil supersonic and hypersonic aviation, there is a need to model the combustion of alternative, sustainable jet fuels. This work presents numerical simulations of several related phenomena, including laminar flames, ignition, and spray flames. Two conventional jet fuels, Jet A and JP-5, and two alternative jet fuels, C1 and C5, are targeted. The laminar burning velocities of these fuels are predicted using skeletal and detailed reaction mechanisms. The ignition delay times are predicted in the context of dual-mode ramjet engines. Large Eddy Simulations (LES) of spray combustion in an aeroengine are carried out to investigate how the different thermodynamic and chemical properties of alternative fuels lead to different emergent behavior. A novel set of thermodynamic correlations are developed for the spray model. The laminar burning velocity predictions are normalized by heat of combustion to reveal a more distinct fuel trend, with C1 burning slowest and C5 fastest. The ignition results highlight the contributions of the Negative Temperature Coefficient (NTC) effect, equivalence ratio, and hydrogen enrichment in determining ignition time scales in dual-mode ramjet engines. The spray results reveal that the volatile alternative jet fuels have short penetration depths and that the flame of the most chemically divergent fuel (C1) stabilizes relatively close to the spray.

Iron oxide fluxing and precipitation in sulphate deposits during heat-resistant alloy corrosion in simulated combustion gas

Two ferritic alloys, Fe-25Cr and Fe-25Cr-2Mn-1Si, and two austenitic alloys, Fe-25Cr-20Ni and 310SS, coated with a thin film of Na2SO4-K2SO4 mixture were exposed to a flowing wet CO2 gas at 650 and 750 °C for up to 300 h. In addition to oxide scaling, a new manifestation of Fe oxide fluxing into the salt layer was observed. The effects of alloy composition and reaction temperature on this phenomenon were determined. The process is attributed to formation of a low melting Na2O-Na2SO4 solution in the early stages of reaction, followed by basic fluxing of FeO and/or Fe3O4.

A novel machine learning based lumping approach for the reduction of large kinetic mechanisms for plasma-assisted combustion applications

The development of skeletal mechanisms has become essential for multi-dimensional simulations of plasma-assisted combustion (PAC). However, reduction tools developed for traditional combustion applications are not always applicable to PAC, due to the complex interplay between non-equilibrium plasma and combustion kinetics. Plasma direct relation graph with error propagation (P-DRGEP) is a recent plasma-specific reduction method developed in order to incorporate plasma energy branching in the reduction. In the first part of this work, the applicability of P-DRGEP to large kinetic mechanisms is investigated. A detailed isooctane/air plasma mechanism containing 2805 species and 18457 reactions is reduced to 415 species and 4716 reactions, keeping errors on ignition time within 3% for a wide range of initial conditions: from 750 K to 1200 K, 10 atm and equivalence ratios from 0.75 to 1.50. The second part focuses on isomer lumping, which is another reduction technique widely used in combustion. When applied to PAC, it is shown that the resulting lumped mechanism produces poor results. A novel plasma-specific isomer lumping strategy using machine learning is proposed instead. With the supervised algorithm of gradient boosting, predictive regression models are generated, which describe rate coefficients of lumped reactions adequately. These models are trained with simulation data. Leveraging this newly proposed lumping approach on the reduced mechanism, allows for an additional 28% reduction in the number of species and 19% reduction in the number of reactions. Two different versions are presented: in the first one the models are trained using one input feature (1D), while in the second one, two input features are selected (2D). The resulting lumped mechanism is shown to produce accurate predictions of PAC over the entire parameter space of interest, while significantly decreasing the computational time. Indicatively, with the 1D version the maximum error on ignition time in this range of conditions is 6%. The 2D approach produces even lower errors, which do not exceed 3%.Novelty and significance statementIn this work, a novel approach for isomer lumping, in plasma-assisted combustion mechanisms, is demonstrated. This plasma-specific approach, uses predictive machine learning regression models to describe the complex evolution of lumped reaction rate coefficients. Combining it with the plasma direct relation graph with error propagation, a powerful reduction framework is created, which is successfully demonstrated on a detailed isooctane/air plasma kinetic mechanism, via zero-dimensional ignition simulations. This framework constitutes a useful tool towards the creation of highly accurate skeletal mechanisms, which significantly reduce the computational costs of simulations.

Sustainable Energy from Waste: A Feasibility Study in Miri, Malaysia

The growth of urban populations, industrialization, and economic development has led to a surge in solid waste production. When local recycling infrastructure falls short, much of this waste ends up in landfills, causing environmental and social challenges. This study aims to assess the feasibility of converting municipal solid waste (MSW) into energy, with a focus on combustion chamber modeling in Miri, Sarawak. Data on MSW composition are obtained from secondary sources. Ansys Fluent software is used to model the combustion chamber, and simulations are conducted to explore temperature, turbulence, and species distribution. MSW composition illustrates higher substantial fractions, with 39.8% being food waste, followed by 20.7% plastic/rubber. Calorific values range from 4652 kJ/kg for food waste to 32564 kJ/kg for plastic/rubber. Combustion simulations result in maximum flue gas temperatures of 1500 °C, 1200 °C, and 1800 °C under varying air inlet conditions. Turbulence intensities on the grate range from 125% to 174% for these air inlet configurations. The study concludes that moisture content significantly affects calorific value and heat generation during combustion. Higher turbulence intensities lead to increased reaction rates and heat generation, improving the energy efficiency of the process.

Analysis of direct and indirect noise in a next-generation aviation gas turbine combustor

A large-eddy simulation (LES) of a next-generation combustor is performed to examine effects of this combustor concepts on direct and indirect combustion noise characteristics. Direct noise is computed considering the unsteady heat release predictions while entropy fluctuations in the downstream part of the combustor are used to estimate indirect noise through the transfer function of the outlet nozzle. A low-order acoustic reconstruction technique, which utilizes the Green’s function of the configuration, is developed to compute the acoustics inside the combustor. Comparisons to experimental results are reported, showing the reasonable accuracy of the LES and combustion noise computations. The direct noise spectrum peaks at frequencies around 3 kHz because of the fast timescales associated with the chemical reactions inside the combustor. In addition, the indirect noise is dominant at low frequencies, i.e., less than 400 Hz, because of the characteristics of the entropy spectrum at the outlet nozzle. Compared to a conventional rich-quench-lean (RQL) combustor configuration, significant differences in the acoustics are observed, and are explained by two specific technological novelties. Firstly, the direct noise spectrum peaks at significantly higher frequencies due to the combustor’s compactness. Secondly, the entropy inhomogeneities downstream of the combustion region are an order of magnitude smaller in amplitude due to the lack of dilution with cold air. This leads to a significant reduction in the indirect noise amplitude at low frequencies, where it is the dominant source of noise. These results suggest opportunities for advanced combustion technologies to achieve substantial reductions in combustion-related noise.

The importance of unsteady phenomena of ammonia/methane combustion in an experimental swirl burner: Comparison of steady-state and transient simulation results

Ammonia is a potential advanced green fuel. Combustion simulations play a crucial role in facilitating the usage of ammonia in existing combustion systems as a drop-in fuel. Choosing between the unsteady and steady simulation methods should be done carefully depending on the investigated phenomena. In this study, steady-state and transient Computational Fluid Dynamics simulations were performed using detailed chemistry of the Okafor mechanism for a wide range of ammonia/methane blend combustion under lean conditions. The results revealed that the steady-state simulations could provide close values to the unsteady mean results at the outlet in magnitude, while outlet temperature and intermediate species concentration slightly differ with ammonia concentration-dependent extents. The unsteady simulations provided valuable insights into the temporal evolution of the combustion and flow field characteristics, and the Root Mean Square and mean properties were evaluated comprehensively, indicating that the oscillations highly depend on the fuel composition. The model was validated using measurement data from the literature, including Particle Image Velocimetry, OH* chemiluminescence, and pollutant emission. The velocity and OH* distribution matched acceptably, while the CO, NO, and NO2 emissions were underpredicted, which implies necessary reaction model improvements.

Quantitative studies of NO emissions from various reaction pathways in swirling combustion of hydrogen-enriched methane

Hydrogen-enriched methane is a promising alternative fuel for carbon dioxide reduction. Large eddy simulations of turbulent combustion and NO emission are carried out using a new numerical method, which incorporates an improved NO prediction model and a recently-developed NO reaction pathway separation approach. Quantitative analyses of NO formations from all reaction pathways are conducted in an open swirl burner (combustion in the open environment) with hydrogen-enriched methane at a constant thermal power of 10.85 kW at the atmospheric pressure. Results indicate that different NO reaction pathways response very differently with hydrogen addition. In the open burner with hydrogen enrichment up to 60 %, the prompt NO makes the largest (around 50 %) contribution to the total NO emission, but its relative contribution decreases with hydrogen addition. The thermal NO strongly increases with hydrogen addition, but it accounts for only around 25 % of the total NO emission in hydrogen-enriched cases, because of the reduced high-temperature zone by co-flow air dilution. NO formation from the NNH pathway largely increases and contributes around 20 % to the total NO emission with hydrogen enrichment. Results would provide quantitative understanding of NO emission in the bluff-body stabilized swirling combustion of hydrogen-enriched methane in an open burner.

Modelling of Combustor Non-Uniformities Evolution Through a High-Pressure Turbine Stage

Abstract In modern gas turbines, the reduction of pollutant emissions can be achieved by employing lean-burn combustors. At the combustion chamber outlet, the flow is non-uniform and characterized by a residual swirl superimposed to steady (hot streak) and unsteady (entropy waves) temperature disturbances. During the transport from the combustor outlet to the turbine inlet, these disturbances are weakly dissipated and persist at the turbine inlet. Therefore, the interaction between the combustor non-uniformities and the turbine has to be deeply studied. To study combustor-turbine interaction experimentally, a common practice is to install combustor simulators on non-reactive turbine test facilities. For this purpose, a combustor simulator was designed and installed at Politecnico di Milano turbine test facility. This device can generate a combined steady/unsteady temperature disturbance and swirl profile at the turbine inlet. Using this layout, several experimental campaigns have been carried out changing the type of injected disturbance, the injection position and the turbine operating condition. In this paper, the data collected from these experiments have been used to develop simplified models to predict the transport and dissipation of combustor perturbations through a turbine first stage. In the open literature, few attempts are discussed regarding the modelling of combustor-turbine interaction that - in authors' opinion - represents an important tool for preliminary turbine design.

Study on the dynamic numerical simulation of flow and combustion in hybrid rocket motors based on a discrete phase model

Numerical simulation based on the discrete phase model and dynamic mesh model can more accurately simulate the working process for hybrid rockets, including the motion and evaporation process of particles and regression process of the fuel grain. The dynamic numerical simulation of combustion and flow in hybrid rocket motors based on a discrete phase model is carried out. Aluminum and aluminum hydride are added to the internal flow field as discrete droplets. Then, a hybrid rocket is designed with 95 % hydrogen peroxide as oxidizer and 38 % aluminum hydride+20 % aluminum+42 % hydroxyl-terminated polybutadiene as the fuel and used to verify the model by firing test. A high degree of agreement between experiment and simulation findings is found. The average deviations for thrust and pressure are 3.6 % and 5.9 %, respectively. It offers the required backing for using the simulation method to investigate the flow and combustion process for the hybrid rockets. The results also demonstrate the transient characteristics of the inner flow field, including the separation and merging of vortexes in the pre-chamber. It is found that a peak of the regression rate appears at the front of the grain and moves backward. This is mainly because the vortex appears here and enhances its heat transfer. Through the discrete phase model, it is worth noting that the aluminum droplet content gradually decreases at the nozzle over time, which is consistent with the firing test. Further research on the impact of aluminum hydride content on the performance of the hybrid rockets has been carried out. The results show that the higher aluminum hydride content can lead to better performance for the hybrid rockets. The thrust, pressure, specific impulse, and characteristic velocity enhance with the increase of aluminum hydride content. An interesting discovery is that the average regression rate is maximum at 48 % aluminum hydride content. This research innovatively couples dynamic mesh and discrete phase models, which can help better understand the working process for hybrid rockets and provide important guidance for the selection of fuel formula.

Effects of dimethyl ether blending on H2-fueled combustion characteristics and thermal improvement in fuel-lean condition

Micro-combustors play a pivotal role in micro powering system due to the potential for high energy density. However, addressing challenges such as incomplete combustion and high heat dissipation losses are essential for optimizing the working performance. So, the blended DME is proposed to micro-combustion, which acts as a promising oxygenated fuel with significant attention due to its higher reactivity, lower pollutant emissions, and carbon–neutral properties. However, the lack of dynamic models applicable to H2/DME fueled combustion in micro condition, so an optimized mechanism for blended H2/DME combustion is developed and validated. The results demonstrate that a reduced mechanism comprising 29 species and 106 reactions (M29-106*) can be commendably employed in micro H2/DME/air combustion simulation, according to the sensitivity analysis and reaction modification. Besides, investigation is conducted to explore micro-combustion characteristics of H2/DME/Air. The result shows that burning process and heat transfer are notably influenced by DME addition, leading to shifts in flame positions and adjustments in combustion reaction rates. So, DME addition enhances energy efficiency and modifies flame location through fluid field optimization and reduced heat loss. The thermal performance of the combustor is significantly improved with DME blending, resulting in higher radiative power and radiation efficiency. However, the choice of DME blending ratio must be carefully considered, as high blending ratios may compromise flame stability and lead to increased exhaust heat losses under fuel-lean conditions. Specifically, burning with 30 % DME addition achieves a radiative power of 20.24 W and a radiative efficiency of 40.57 % at the same chemical energy Ec = 49.9 W, which is 5.57 W and 11.17 % higher than that of pure H2-fueled combustion, respectively. These findings offer valuable insights for the design and optimization of micro-combustors, where compact and efficient power sources are required.