Combustion Process Research Articles

Investigation on H2-fueled combustion characteristics and improvement of thermal performance in a micro-planar with arranged inlet-fins and bluff-bodies for micro-thermophotovoltaic

To improve the energy conversion and efficiency, a micro-planar with arranged inlet-fins and bluff-bodies is proposed for H2-fueled micro thermophotovoltaic system. Effects of inlet-fins and bluff-bodies setting and combustor dimensions on combustion properties, heat transfer and energy conversion are investigated and analyzed under various operating conditions. The results illustrate that the flame stability and heat transfer in combustion chamber with arranged inlet-fins and bluff-bodies are enhanced, where heat recirculation is formed and radiation temperature is boosted. According to the field synergy analysis, coupling effects of inlet-fins and bluff-bodies on burning and thermal process can be improved by the optimization of bluff-bodies setting, and the flow limit is also expanded. There is a balance of the burner dimensions effects on the augment of radiation surface area and the reduction of radiation temperature for micro power generation. Particularly, the micro-planar with La/L = 0.62 contributes to achieve a higher radiation density, and the highest electrical output Pe = 3.16 W is adopted in combustor C2-0.62 at mf = 6.751 × 10−5 kg/s. It is beneficial to promote energy output of micro-planar and the energy efficiency of micro-thermophotovoltaic system.

Numerical study of NOx emission reduction by MILD and air staged synergistic combustion in a new type of coke oven flue

In order to address the issue of high emissions in the coke industry, we require innovative solutions. This article designs a coke oven flue with an array type secondary air nozzle, achieving the synergistic occurrence of MILD combustion and air staged combustion, which can significantly reduce nitrogen oxide emissions. However, further research is needed on the optimal control methods for synergistic combustion effects. First, Analyzed the mixing state of air and fuel under different air mass flow ratios. Next, by taking into account the oxygen distribution law, analyze the temperature distribution in both the primary and secondary combustion zones. Finally, analyze the distribution of NOx concentration. Study the impact of temperature differences caused by air mass flow ratio in synergistic effects on NOx emissions. The results indicate that by adjusting the air flow ratio, the air vortex intensity in the collaborative combustion process can be enhanced. This method can reduces flame temperature, achieving the objective of reducing NOx concentration. In the new type of coke oven flue, when the air mass flow ratio is 0.4, the combustion state in the primary and secondary combustion zones is optimal, and the NOx emission concentration is 312.88 mg/m3, representing a reduction of 38 %.

Spontaneous ignition and flame propagation in hydrogen/methane wrinkled laminar flames at reheat conditions: Effect of pressure and hydrogen fraction

Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼10% bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows.Novelty and significanceThe reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels.

Investigation on the effect of the leading angle of the strut on the stabilization characteristics of lifted jet flames

To achieve stable combustion within supersonic combustors, the strut-based mixing augmentation techniques are commonly employed. Large Eddy Simulations (LES) are employed to capture the intricate details of the combustion process in this study, focusing on the influence of shock waves induced by the leading edge of the strut and recirculation zones formed at the trailing edge on flame structure and stability. The effects of five struts with different leading edge angles on the stabilization characteristics of lifted jet flames were investigated, and a novel wedge-shaped strut was proposed. The results reveal that variations on the leading-edge angle of the struts lead to significant differences in the morphology of the jet flames. Under conditions permitting stable combustion, a strut with a 5° leading edge half-angle exhibits the highest combustion efficiency, though the combined effects of shock waves and recirculation zones in the 7° leading edge half-angle strut are also noteworthy. Based on these findings, a novel wedge-shaped strut design is introduced, which combines the characteristics of both strut types. The novel wedge-shaped strut achieves a combustion efficiency of 97 % at the slight cost of additional total pressure loss, providing a foundation for further optimization.

Composition changes and micro explosion characteristics of burning droplets formed by combustion of micrometer iron wires

To analyze the composition changes during the combustion process of iron droplets using the energy balance equation, iron wires with diameters of tens of micrometers were burned to form temperature-stable iron droplets with diameters of hundreds of micrometers. High-speed two-color imaging pyrometry was employed to measure the spatially resolved surface temperature and volume changes of the droplets under varying oxygen concentrations. Experimental results showed that the droplet's temperature stabilized between the melting points of wüstite and iron. Quantitative analysis revealed the presence of bubbles within the droplets, primarily sourced from the oxidation of carbon in the iron to carbon dioxide. Simulation results of the droplet combustion process revealed that at the experimental combustion temperatures, the transport of oxygen in molten iron oxides was the rate-limiting step. The gas responsible for micro-explosions originates from the release of dissolved excess oxygen before the solidification of molten iron oxides, with rough estimates indicating that the mass of the released gas is only 0.02% to 0.03% of the droplet mass. The impact of reaction kinetics parameters on the simulation results, as well as the role of the micro-explosion mechanism in the self-sustained combustion of iron wires, were also discussed.

Diagnostics of the ionization processes in hydrocarbon flame with the use of the current-voltage characteristics

The possibility of estimating the ionization parameters of high-temperature gas mixtures formed as a result of combustion processes is considered on the basis of the current-voltage characteristics measured using electrodes that generate an external electric field in the media under consideration.

Oxyfuel combustion based carbon capture onboard ships

Reducing greenhouse gas emissions in the shipping sector is a challenging task. While renewable fuels stand out as the most promising long-term solution, their near- and mid-term viability is hampered by limited availability and high costs. An alternative approach is onboard carbon capture, which can reduce emissions from new ships as well as retrofitted vessels. This paper examines the techno-economic potential of oxyfuel combustion based carbon capture on ships. The oxyfuel concept uses an oxygen-rich atmosphere in the combustion process, resulting in a mixture of carbon dioxide and water. After the condensation of water, the carbon dioxide rich gas can be directly stored on board. Various onboard oxygen supply concepts are investigated, including different technologies for onboard air separation and liquid oxygen bunkering. Influences on the ship energy system are studied by system simulation of a deep-sea container vessel. Benchmarked against a technologically mature post-combustion carbon capture system, the results show that the oxyfuel concepts have limited competitiveness because of reduced engine efficiencies and high energy demands for onboard oxygen supply. Avoiding onboard oxygen supply by using liquefied oxygen as a byproduct from onshore electrolysis increases energy efficiency and the competitiveness of oxyfuel combustion but requires additional storage space. Sensitivity analyses highlight that the engine combustion concept and engine efficiency are the most critical influences on the techno-economic performance.

Effect of Microwave Antenna Material and Diameter on the Ignition and Combustion Characteristics of ADN-Based Liquid Propellant Droplets

Microwave-assisted ignition is an emerging high-performance ignition method with promising future applications in aerospace. In this work, based on a rectangular waveguide resonant cavity test bed, the effects of two parameters (material and diameter) of the microwave antenna on the ignition and combustion characteristics of ADN-based liquid propellant droplets were investigated using experimental methods. A high-speed camera was used to record the droplet combustion process in the combustion chamber, the effect of the microwave antenna on the propellant combustion response was analyzed based on the emission spectroscopy method, and finally, the loss of the microwave antenna was evaluated using a scanning electron microscope. The experimental results show that the droplet has the lowest critical ignition power (179 W) when the material of the microwave antenna is tungsten, but the ignition delay time is higher than that of copper. A finer diameter of microwave antenna is more favorable for plasma generation. At a microwave power of 260 W, the ignition delay time of the droplet with a microwave antenna diameter of 0.3 mm is 100 ms lower than that of 0.8 mm, which is about 37.5%. In addition, this study points out the mechanism of microwave discharge in the droplet combustion process. The metallic microwave antenna not only collects the electrons escaping from the gas discharge, but also generates a large amount of metallic vapor, which provides charged particles to the plasma. This study provides the possibility for the application of microwave-assisted liquid fuel ignition.

Preparation of fully physically crosslinked double-network gel and its fire prevention mechanism

It is important to gain an in-depth understanding of the structure of fire prevention and extinguishing materials during self-heating of coal. In this paper, an Al3+-CMC/PAM-MMT fire prevention and extinguishing gel with double-network was prepared using sodium carboxymethyl cellulose (CMC) and polyacrylamide (PAM) as raw materials, and aluminum citrate (AlCit) and sodium montmorillonite (Na-MMT) as auxiliary materials, by introducing the fully physically crosslinking effect. The optimal amounts of CMC, PAM, AlCit and MMT were determined to be 2 %, 3 %, 8 %, and 3 %, respectively, based on gelation time, stability, and resistance of the gel. The micromorphological image of the double-network gel exhibited a laminar structure of parallel-aligned fibrous networks, where the pores and channels exhibited more complex shapes and became smaller in size, highlighting the superiority of the double-network architecture. The rheological tests showed that the double-network gel had superior structural stability and viscosity under varying shear rates, making it more effective in covering the fire source, enhancing extinguishing efficiency. Additionally, its higher storage and loss moduli indicated stronger elasticity and mechanical properties compared to the single-network gel, emphasizing its advantages in energy storage and recovery. The characterization of active functional groups showed that there was an effective suppression of reactions involving hydroxyl, hydrocarbon, and carbonyl functional groups, during the spontaneous combustion process of coal, following treatment with the gel. Additionally, a proposed mechanism for the gel formation was presented. In the sealing performance test, the pressure-bearing capacity of the double-network gel was approximately twice that of the single-network gel. It indicated that the multi-level structure formed by the gel led to higher density of physical cross-linking points, which provided better sealing performance. Thermogravimetric analysis showed significant decrease in total calorific value and increase in activation energy of coal, after treatment with the gel, rendering it more difficult for spontaneous coal combustion. In the small-scale fire extinguishing performance test, the double network of Al3+-CMC/PAM-MMT gel demonstrated effective suppression of spontaneous combustion of coal and reduced the susceptibility to re-ignition. The fire prevention mechanism of the gel was further explored experimentally.

Evaluation of the ducted fuel injection concept for medium duty engines and multi-hole nozzles: An optical analysis

Ducted Fuel Injection (DFI) is a strategy still in development, which has proved to be effective in reducing soot emissions in compression ignition engines. It works by driving the spray, formed by a high-pressure fuel injection, through a small duct co-axial to the spray itself, which is expected to affect the mixture formation and combustion process, in turn leading to noticeable reduction in soot formation. This strategy has been mostly deployed in spray vessels or in some cases in heavy duty engines consisting of mostly 2-to-4-hole nozzle injectors. For this reason, the work here is aimed to study the potential of DFI in a medium-duty single-cylinder optical engine fueled with conventional diesel having an 8-hole nozzle injector. Two different optical techniques including OH* chemiluminescence and 2-color pyrometry have been utilized to perform the analysis regarding combustion evolution and soot formation. A parametric analysis regarding different geometrical parameters including stand-off distance, diameter and length of duct has been carried out regarding the DFI performance. Results indicate that DFI does decrease the soot emissions in the context of this study and the duct geometrical parameters influence combustion evolution and soot formation ultimately affecting the device's performance. However, the scale of soot reduction is not as high as reported in previous studies, which is limited by specific boundary conditions including combustion chamber design, piston geometry utilized in this study.

Study on Soot and NOx Formation Characteristics in Ammonia/Ethylene Laminar Co-Flow Diffusion Flame.

The formation of soot and NOx in ammonia/ethylene flames with varying ammonia ratios was investigated through experimental and numerical analysis. The spatial distribution of the soot volume fraction and NOx concentrations along the flame central line were measured, and the mechanism of soot and NOx formation during ammonia/ethylene co-combustion was analyzed using CHEMKIN 17.0. The experimental results indicated that the soot volume fraction decreases with an increase in ammonia ratio, with the soot peak concentration occurring in the upper region of the flame. The distribution of NOx is complex. In the initial part of the flame, a higher concentration of NOx is generated, and the lower the ammonia ratio, the higher the concentration of NOx. As the combustion process progresses, the concentration of NOx initially decreases and then subsequently increases rapidly, with higher ammonia ratios leading to higher concentrations of NOx. The addition of ammonia results in a decrease in CH3, C2H2, and C3H3, and an increase in CN concentration. This leads to a transformation of carbon atoms within the combustion system, reducing the available carbon for soot formation and suppressing its generation. A higher ammonia ratio increases the likelihood that NH3 will be oxidized to N2, as well as increasing the probability that any generated NO will undergo reduction to N2 through the action of the free radicals NH2 and NH.

Thermodynamic analysis and multi-objective optimization of different gas turbine configuration strategies for an atmospheric SOFC system

Solid oxide fuel cells (SOFCs) play a critical role as a key technological component in the utilization of hydrogen energy. Extensive research has been conducted on pressurized SOFC hybrid systems, focusing on performance enhancement and novel system configurations, etc. However, the planar SOFC suitable for commercial application exists limitations that confine its operation solely to atmospheric pressure. Moreover, SOFC exhaust contained considerable potential for energy recovery. In light of these limitations, two hybrid power generation systems of SOFC and GT have been proposed: direct and indirect utilization of SOFC cycle exhaust. Parameter and exergy analysis has been conducted to investigate the characteristic of SOFC-GT system. Multi-objective optimization is performed by setting exergy efficiency and output power as objective. The results reveal that the combustion process contributes significantly to system losses, with exergy efficiency reaching a maximum of 49.48% and 41.61%, respectively. Additionally, the system's maximum output power reaches 92 kW. Notably, the indirect SOFC-GT system demonstrates an 8% increase in exergy efficiency compared to the direct SOFC-GT system at the same output power. To improve the overall system fire efficiency, it is recommended to develop planar SOFCs suitable for high pressure and slow down the reaction process inside the combustion chamber.

Optical study on spray mixing, flame propagation and jets evolution within visible methanol active pre-chamber for turbulent jet ignition

Methanol spray impingement, mixing, flame propagation and jet evolution within the narrow-confined active pre-chamber (APC) with complex walls were experimentally observed by optical method for the first time. The effects of injection pressure and duration was investigated on an actual-geometry optical APC assembled in constant volume combustion chamber (CVC). Exploration on APC internal processes within static environments forms the basis for optimization of APC and analysis of jet formation. Mie scattering coupled with flame natural luminosity imaging were used to record spray mixing and combustion. The results show that in the APC with narrow-confined space, methanol spray undergoes three wall impingements, inducing a tumble that promotes mixing. High injection pressure enhances rapid spray tip penetration, while long injection duration slows methanol diffusion. In longer injection duration cases, the spray tip after the third impingement interferes with subsequent sprays. For higher injection pressure, this interference occurs in the APC cone section due to a larger spray cone angle, whereas for lower pressure, it happens in the APC duct section. Higher injection pressure also causes more spray to impact the APC bottom after the first wall impingement, hindering flame acceleration in the duct section. Irregular flame propagation is observed in the APC cone section due to wider space and uneven mixture conditions, while the convergent geometry from the APC throat to the duct section significantly accelerates flame propagation. The APC jet appears due to the pressure difference between the APC and the main chamber, with the brightness and length of the visible flame jet increasing with APC flame luminosity and pressure difference. A Mach ring structure is observed initially, but the jet velocity decreases as the Mach ring disappears during the APC inner flame extinguishing process. Through this study, the actual spraying and combustion process in APC is experimentally captured and revealed, which are an important contribution for APC optimization and development.

Local Burning Behavior of Wind-Driven Flames under the Influence of Mixed-Convective Turbulent Flow Conditions

ABSTRACT Experiments were conducted to investigate the burning behavior of wind-driven turbulent flames stabilized over a condensed fuel surface. A controlled turbulent crossflow environment was established to examine the impact of external flow velocity and turbulence intensity on the flame profile parameters and mass burning rates. To address the complexities of practical fire scenarios, a mixed-convection parameter ξ x in the form of G r x 1 / ψ x 2 n was defined, encapsulating the combined effects of momentum, buoyancy, and freestream turbulence. Furthermore, the property of fuel (in terms of mass transfer number B) was integrated into the mixed-convection parameter ( ξ x ) to establish a fuel-dependency factor within the formulation . The interdependence of flame characteristics and the mass burning process was observed using ξ x , that yielded several meaningful correlations. In this regard, a power-law trend was obtained between flame standoff distance and the dimensionless parameter ξ x . Finally, the local mass burning rates were estimated and plotted against mixed-convective parameter ( ξ x ), resulting in a unified local mass burning rate correlation. This correlation incorporates the fuel-dependency aspect in its formulation and is applicable in both laminar and turbulent crossflow environments.

Ignition and combustion of AlH3-nanoparticles: A molecular dynamics study

AlH3 has tremendous potential for hydrogen energy storage and energetic applications. The structure and thermodynamic properties of core-shell AHNP with different oxidation degrees from ignition to combustion are studied using ReaxFF molecular dynamics simulations. Two models covered by different alumina shells are built. Results indicate that the thinner shell is ready to form an Al-rich environment, which is crucial for accelerating ignition and subsequent combustion processes. In ignition period, the influence of shell is investigated by comparing surface oxygen adsorption rates, oxidation degree of Al and H atoms, and stress evolution of AHNP. AHNP with thinner shell exhibits a higher O adsorption rate to form an O-rich environment, which accelerates the oxidation process of Al atoms. A thicker shell significantly inhibits the thermal diffusion of core Al and H atoms, while H atoms are still able to penetrate it with a low diffusion rate. Stress analysis shows the thickness of the shell directly affect the thermal diffusion rate of Al atoms. The combustion of AHNP can be divided into three stages: rapid, slow, and self-sustaining combustion. The combustion process is controlled by the particle and external O atoms diffusion behaviors. A detailed structural evolution process during AHNP combustion has also been clarified through the displacement analysis. The findings provide a theoretical foundation for the diffusion-dominated combustion mechanism of AHNP from an atomic view. Novelty and Significance StatementAlH3 has tremendous potential for hydrogen energy storage and energetic applications. At present, the combustion mechanism of AlH3-nanoparticles (AHNP) is still not well understood, and the atomic-scale evolution process and reaction mechanism are also poorly studied, which is difficult to obtain in experiments. In this study, the ignition and combustion mechanisms of AHNP is revealed based on the ReaxFF molecular dynamics simulations, and the influence of oxidation degree on the ignition and combustion of AHNP is also clarified, which can provide theoretical guidance for the application of AHNP as a clean fuel and preparing for the combustion research of AHNP coated with different materials.

Accuracy of EVC Method for the PiN Diode Pattern on SiC Epi-Wafer

In the previous report, we proposed the EVC (Expansion-Visualization-Contraction) method (Fig. 1) that effectively screens for malignant BPDs (basal plane dislocations) in the epi layer and near substrate interface, which expand to SSFs (Shockley-type stacking faults), leading to forward voltage degradation. The method intentionally utilizes the REDG (recombination enhanced dislocation glide) mechanism by UV (ultraviolet) irradiation in wafer sorting to replace the so-called burn-in (accelerated current stress) process, which is time-consuming during mass production. In this report, to verify the effectiveness of this method, we compared the SSFs expanded by forward biasing the PiN diode (Fig.3) on a wafer with the SSFs expanded by UV irradiating at the same PiN diode area where the metal electrode was removed by etching. The accuracy of the EVC method requires that SSFs expanded by forward biasing should be detected in the same positions as those of SSFs expanded by UV irradiation. Not all BPDs expand at the same time, but the number of expanded SSFs increases over time under constant forward current conditions. In this experiment, the current density was 400 A/cm2 for 8 minutes, and the excessive UV irradiation conditions was 143 W/cm2 for 20 minutes to avoid missing. Missing means the inability to check the SSFs expanded by forward biasing against the SSFs expanded by UV irradiation (Fig.2). For each diode electrode window, the presence or absence of SSFs were determined, and as shown in Table 2, 2 out of 49 window areas were missing, with the EVC method accuracy rate of 96 %.

Combustion and energy performance of multiple aluminum-based alloy particles

The long ignition delay time, high ignition temperature and agglomeration prevent the aluminum (Al) particles from fully releasing the chemical energy stored within them during combustion. Al-based alloy fuels with high energy density and excellent combustion performance have become a potential candidate for replacement with pure Al. Therefore, understanding the combustion and energy performance of Al-based alloys is beneficial for the development of Al-based energetic materials. Herein, the combustion and energy performance of pure Al and three Al-based alloys were comparatively investigated by means of a thermal analysis system, an ignition and combustion test apparatus, and a variety of physicochemical property characterizations. The results showed that all three Al-based alloys contained at least one intermetallic compound. Al-boron (B) and Al-magnesium (Mg)-B alloys had higher theoretical energy densities than pure Al, while Al-Mg did not. The thermal oxidation properties of all three alloys were significantly better than those of pure Al. Compared with pure Al, the three alloys showed higher burning rate, shorter ignition delays, more vigorous combustion, and higher burnout at room pressure in air. This is still true after adding AP. According to the compositions of the condensed combustion products, the possible chemical reactions during the combustion of the alloys were speculated. Grasping the combustion mechanisms of the alloys is challenging due to the relative lack of thermodynamic data on intermetallic compounds. Finally, a comprehensive comparison of the combustion processes of pure Al and alloys was made. Although the Al-Mg alloy does not have the same theoretical energy density as pure Al, the more outstanding combustion performance and fewer two-phase flow losses may make up for the gap in theoretical energy density.

Oxygen-rich combustion behaviors and mechanisms of a new Ni-Cr-based superalloy fabricated by selective laser melting

The mechanical and combustion behavior of a new Ni-Cr-based superalloy, fabricated by selective laser melting in an atmosphere with an oxygen concentration greater than 99.5 %, was investigated through the analysis of microstructural morphology, elements distribution and combustion kinetics. The results show that heat treatment eliminated the anisotropy of the as-built microstructures and improved the tensile strength. The studied superalloy can be ignited with a combustion threshold of about 2.5 MPa–3 MPa. The burning length and burning rate increase with the increase of pressure. The selective combustion of Al, Ti, Nb, Cr, Fe, Mo, and Cu elements occurred, resulting in the dissolution of γ′, γ", and δ precipitates and formation of holes at the grain boundaries in the heat-affected zone. The microstructures exhibit large elongated grains in the melting zone and column grains in the heat-affected zone along the heat dissipation direction. The combustion products are a mixture of oxides exhibiting dendritic structures in terms of selective combustion, density, and melting point. The findings from this study enhance our comprehension of the combustion process in nickel-based superalloys, offering valuable data and guidance for the advancement of new oxygen-rich combustion-resistant superalloys.

Numerical investigation of the physical-thermal-chemical behaviours of particles during coal combustion and desulfurization processes in a CFB combustor

The desulfurization process has been practised for the clean utilization of coal in chemical engineering fields, yet the physical, thermal, and chemical characteristics of particles are still lacking understanding. In this work, a comprehensive numerical model based on the Eulerian-Lagrangian framework was established to study the particle behaviours during coal combustion and desulfurization processes in a pilot-scale circulating fluidized bed (CFB) combustor. After model validation, the effects of several crucial operation parameters (e.g., excess air ratio, calcium to sulphur ratio, and calcium oxide size) on particle behaviours are illuminated. The results show that density-induced segregation causes coal particles to accumulate in the upper part of the riser. The sand, coal, and CaO particles have time-averaged Reynolds numbers of 15, 6.5, and 0.5, respectively. The particle temperature is higher in areas with lower solid concentrations. Increasing excess air ratio (ϕg) decreases the temperature of sand particles but elevates that of coal and CaO particles. The average particle heat transfer coefficients (HTCs) for sand, coal, and CaO particles are 205 W/m2·K, 172 W/m2·K, and 275 W/m2·K, respectively. With the increase in ϕg, the HTC of sand particles increases but that of CaO particles remains unchanged, additionally, the mass of coal particles decreases and the mass of carbon in coal particles decreases along with the riser. The influence of the Ca/S ratio and CaO size on the axial distribution of coal particle mass and carbon mass fraction is negligible. For each particle species, the axial dispersion coefficient (Dz) is two orders of magnitude greater than the radial ones (Dx, Dy), indicating that the vertical introduction of the fluidizing gas dominates the dense gas-solid flow in the riser.

The effects of acoustic disturbances and mechanical vibration on laminar premixed flames: A comparative study

The energy conversion between the combustion process and acoustic waves is very complex in high-performance propulsion systems, which may trigger large-amplitude combustion instabilities, potentially leading to severe vibration or structural damage. Both acoustic disturbances and mechanical vibration are found to be able to cause the combustion instability, but their differences on the influence of laminar premixed flames are still unknown. In this paper, a premixed methane flame burner was disturbed by either a loudspeaker or an electromagnetic vibration exciter, and the effects of their forcing frequencies and amplitudes on the dynamic responses of the premixed methane flame were evaluated. Specifically, the unsteady heat release rate was measured by a photomultiplier tube, and the flame dynamic behaviors were captured by a high-speed camera with image intensifier. Furthermore, the acoustic disturbances upstream of the flame was measured by the two-microphone method, and the structure vibration was measured by an accelerometer. Then, the flame transfer functions were obtained at various equivalence ratios and mean flow velocities. The results demonstrated that the premixed flame exhibited obvious band-pass filtering characteristics when subjected to external forcing, whether through a loudspeaker or an electromagnetic vibration exciter. In addition, increasing either acoustic or structure excitation amplitude to a large extent can make the flame respond in a nonlinear manner. However, when the flame was subjected to mechanical vibration, the peak frequency corresponding to the maximum gain of the flame transfer function had minimal sensitivity to variations in equivalence ratios and mean flow velocities. In contrast, the acoustic disturbances were more likely to induce the nonlinear flame responses and flame front wrinkling.