Search Results (265)

Multicomponent diffusion modeling based on the Maxwell–Stefan formulation is widely used in high-fidelity combustion simulations due to its superior physical accuracy compared with simplified diffusion models. However, the computational complexity of the Maxwell–Stefan model, which arises from the solution of coupled multicomponent transport equations, becomes a major performance bottleneck in large-scale CFD simulations. In this work, a high-performance computing optimization strategy for the Maxwell–Stefan diffusion model is developed within the OpenFOAM framework. The proposed method improves computational efficiency through block-based computation and vectorization-oriented data organization to better exploit modern CPU architectures and SIMD instruction capabilities. The optimized implementation enhances memory locality, increases data reuse efficiency, and reduces cache miss penalties. Numerical validation is performed using two-dimensional laminar counterflow flame cases and ammonia–hydrogen turbulent combustion cases, including both premixed and non-premixed jet flames. Results demonstrate that the optimized Maxwell–Stefan implementation preserves numerical accuracy while significantly improving computational performance. Speedups of 2.5×–4.5× are achieved depending on the number of chemical species. The developed approach provides an efficient solution for detailed combustion simulations involving large chemical mechanisms. The test cases and source code are openly shared. Full article

To address the problem of renewable energy curtailment and the need for operational economic optimization in integrated energy systems with high penetration of wind and solar power, a coordinated optimization method integrating thermoelectric decoupling, ammonia-blended combustion technology, and energy storage capacity leasing is proposed. First, a chaotic-improved Latin Hypercube Sampling (C-LHS) method, combined with an improved K-means clustering algorithm, is employed to generate representative wind–solar–load scenarios. This approach improves the efficiency of uncertainty scenario generation while reducing computational burden and maintaining solution accuracy. Secondly, by coordinating the operation of thermal energy storage and electric boilers, the “heat-led power generation” constraint is relaxed, and, in combination with ammonia-blended combustion in combined heat and power (CHP) units, the system’s flexibility and renewable energy accommodation capability are enhanced. Finally, with the objective of minimizing total operating cost, a day-ahead scheduling model incorporating electrical energy storage (EES) leasing optimization is established. For EES, under a shared energy storage market mechanism, the golden section search (GSS) algorithm is employed to optimize the day-ahead leasing capacity. The simulation results demonstrate that the proposed method improves renewable energy accommodation while maintaining economic performance, and effectively reduces the overall operating cost of the system. These findings confirm the effectiveness of the proposed strategy in enhancing both system flexibility and economic performance. Full article

With the advancement of low-carbon transition and deep load-following operation of coal-fired units, ammonia–coal co-firing is a retrofit-ready option for source decarbonization, but its coupled impacts on combustion and emissions remain to be quantified. A 350 MW corner-tangential pulverized-coal boiler at a 30% rated load was investigated using a three-dimensional ANSYS Fluent CFD model. Thirteen cases were designed by combining five ammonia shares (0–40%) with three injection locations (B, C, D). The temperature and key species fields were analyzed to track the reaction-zone shifts, and the outlet CO₂, SO₂, NO, and NH₃ were evaluated. Increasing ammonia reduced and contracted the high-temperature core, dispersed the flame, extended the ignition distance of the ammonia-laden primary jet, and shifted heat release downstream. CO₂ and SO₂ decreased with an ammonia substitution; at 40% co-firing, CO₂ fell by about 43% and SO₂ declined markedly. NO showed a nonlinear, location-dependent response: B and C injection may raise NO at low ratios, but reduce it at higher ratios under lower temperatures and stronger reduction, whereas D injection tends to maintain higher NO in the upper furnace. The findings guide coordinated selection of the co-firing ratio and injection location for low-load retrofits. Full article

The implementation of methanol-ammonia dual-fuel engines has the potential to contribute to a reduction in carbon emissions in the environment. The present study employs numerical simulations of the methanol-ammonia dual-fuel engine to investigate methanol direct injection pre-injection strategies. The impact of pre-main injection time interval and pre-injection quantity was investigated on output power, output torque, cylinder pressure and exhaust emissions such as NO_X, HC, CO, and CO₂. The results show that compared with the single methanol injection strategy, increasing the pre-injection strategy can effectively reduce soot emissions. Under certain pre-injection conditions, NO_X and soot emissions can also be significantly reduced. Compared with low pre-injection quantities, by using high pre-injection quantities, soot and NO_X emissions can be reduced by 36.91% and 35.31%, respectively. Under high pre-injection quantities, increasing the pre-main injection time interval can also significantly reduce NO_X emissions. Compared with the single methanol injection strategy, the pre-injection strategy leads to an increase in cylinder pressure peak and an advance in peak timing. As the pre-main injection time interval increases, both output power and output torque decrease. It is found that when the pre-injection quantity is 6 mg and the pre-main injection time interval is 25 °CA, with no substantial reduction in output power and output torque, the engine’s soot emissions can be reduced by 34.67%, and NO_X emissions can be reduced by 30.31%. Full article

Dual-fuel combustion is often proposed for diesel engines as a means to partially replace conventional diesel with cleaner and/or more sustainable alternatives, such as those derived from green hydrogen. However, the low reactivity of these fuels (i.e., methane, hydrogen, and ammonia) often leads to prolonged ignition delay (ID) and combustion instability. This challenge could potentially be overcome using nanomaterials, which are additives that could improve reactivity and compensate for autoignition deficiencies. Thus, this study evaluates the effect of carbon nanotubes (CNTs) dispersed in diesel fuel on the autoignition process under dual-fuel operation. CNTs were dispersed at a concentration of 100 mg/L and stabilized with surfactant sodium dodecylbenzene sulfonate (SDBS). The resulting nanofuels were then tested in a constant volume combustion chamber (CVCC) using methane, hydrogen, and ammonia as secondary fuels across various energy substitution ratios and temperatures (535 °C, 590 °C and 650 °C). The results show that the impact of CNTs on ID is negligible, especially at high temperatures. At the lowest tested temperature (535 °C) and 40% methane substitution ratio, only slight reductions in ID were obtained. Nevertheless, this effect is less significant at higher temperatures (590 °C and 650 °C). Regarding pressure gradient, the addition of CNTs and SDBS generally induced a decrease in pressure-peak of up to 15%. This trend is attributed to the trapping of fuel droplets within the CNT structures, which creates a physical barrier that delays vaporization. Results confirm that autoignition, which is expected to be the main phenomenon influenced by CNT addition, is not enhanced. Full article

Ammonia (NH₃) has emerged as a promising carbon-free fuel for next-generation green energy systems due to its high hydrogen density, ease of storage and transport, and compatibility with existing infrastructure. These attributes contrast with hydrogen, which presents major challenges related to storage, safety, and high-pressure handling. Thus, ammonia offers a more practical alternative for combustion-based applications. However, its low reactivity and complex vaporization behavior, particularly under flash-boiling conditions, pose challenges for accurate modeling. This study presents a comprehensive numerical investigation of liquid-ammonia spray behavior under a range of ambient pressures, encompassing both flash-boiling and non-flashing conditions. Simulations were conducted using the Lagrangian particle tracking method, coupled with various turbulence models (the renormalization group (RNG) family, k-ω family, $\varsigma -f$, V2F models) to evaluate their predictive performance. Validation against experimental data for liquid and vapor penetration demonstrated that the V2F model achieved the best overall balance between accuracy and computational efficiency. Under strong flash-boiling conditions (2 bar), rapid droplet breakup and notable cooling were observed, with droplet temperatures decreasing to approximately 235 K within a few millimeters of the nozzle. In contrast, the cooling effect was more moderate under non-flashing conditions at higher ambient pressures (10–15 bar). Although the current findings were based on numerical simulations, experimental studies are ongoing to validate and refine the modeling framework further. This work provided valuable insights into the coupled effects of turbulence, phase change, and thermal transport in superheated ammonia sprays. Future research will build upon these results by extending the model to NH₃/H₂ dual-fuel systems, refining turbulence-phase interaction models, and exploring the potential application of ammonia-based flash-boiling cooling systems for electric vehicle (EV) battery thermal management. Full article

The application of ammonia decomposition technology for hydrogen production enables hydrogen-enriched combustion in marine diesel-ignited ammonia engines. This study presents experimental and simulation investigations of a diesel-ignited ammonia engine operating with hydrogen-containing fuels derived from ammonia decomposition at various blending ratios. The combustion and emission characteristics of the engine were systematically examined, and a comparative analysis was conducted on the combustion behavior of the engine between using ammonia decomposition-derived hydrogen-containing fuel and pure hydrogen. The result shows that under constant engine output power, at 1200 rpm and 75% load, increasing the hydrogen energy rate results in largely unchanged cylinder pressure and heat release rate. The diesel substitution rate exhibits an initial increase followed by a decrease, while the energy consumption rate demonstrates the opposite trend. At 1500 rpm and 75% load, an increase in hydrogen enrichment leads to an earlier rise in cylinder pressure and heat release rate, a continuous increase in diesel substitution rate, and a consistent decrease in energy consumption rate. The early stage of in-cylinder combustion is dominated by diesel combustion, followed predominantly by the combustion of ammonia and hydrogen. Regarding the difference between using decomposition-derived hydrogen-containing fuel and pure hydrogen, within the hydrogen enrichment range of 0–20%, the discrepancies in intake composition and equivalence ratio between the two hydrogen-addition modes gradually widen but remain within 1.3%. Taking a hydrogen energy rate of 10.56% as an example, the differences in in-cylinder pressure and heat release rate between the two hydrogen-addition modes are not significant, indicating that the N₂ generated from ammonia decomposition has a relatively weak influence on the engine. With increasing hydrogen enrichment, NH₃ emissions gradually decrease, while NO emissions increase. For N₂O, hydrogen enrichment promotes its consumption, resulting in lower emissions. Under various hydrogen enrichment conditions, equivalent greenhouse gas emissions are mainly influenced by CO₂ emissions. Full article

This work numerically examines the premixed combustion of partially cracked ammonia/air in a cavity-stabilized micro-combustor. Effects of the equivalence ratio (Φ) and inlet temperature (T_in) on the combustion features, flame–wall heat transfer and nitrogen-containing emissions are investigated quantitatively at a cracking ratio of 0.6. Results show that increasing Φ from 0.8 to 1.2 shifts the high-temperature region downstream and causes it to elongate axially. This spatial expansion decreases peak temperatures and distributes heat release over a longer distance. Mean wall temperature and overall heat loss are thus decreased due to weakened near-wall thermal interaction. NO formation closely follows the high-temperature and OH-rich zones. However, at Φ = 1.2, oxygen limitation suppresses NO production and redirects fuel-bound nitrogen towards N₂O, enhancing its outlet emissions. As T_in increases from 300 K to 500 K, the improved reactivity of the mixture promotes an upstream shift of the main reaction zone. The reaction zone becomes more concentrated within the cavity. Such structural changes intensify NO formation but simultaneously compress the high-temperature zone, which reduces the wall-averaged temperature and overall heat loss. In the extended downstream post-flame region, lower temperatures and limited radical activity suppress NO₂ formation and N₂O decomposition. As a result, NO₂ emissions decrease monotonically, while N₂O emissions exhibit a gradual increase. These findings provide useful insights into the effects of operating parameters on combustion stability, heat transfer and nitrogenous pollutant evolution in microscale partially cracked ammonia flames. Full article

The growing need for effective air pollution control technologies has prompted significant interest in innovative flue gas treatment methods. This study investigates the plasma–chemical mechanisms and pollutant abatement performance of a pilot-scale microwave-assisted plasma reactor operating at 915 MHz and up to 75 kW for simultaneous removal of sulfur dioxide (SO₂), nitrogen oxides (NO_x), and carbon dioxide (CO₂) from combustion flue gas. Plasma treatment induced radical-driven oxidation of nitric oxide (NO), substantially enhancing the aqueous solubility of nitrogen oxides and thereby improving ammonia scrubbing efficiency. However, excessive plasma power resulted in thermal NO_x formation, governed by local gas temperature, highlighting the critical need for optimized specific energy input. A logarithmic correlation between plasma power and NO_x concentration was derived, enabling estimation of power thresholds necessary to suppress thermal NO formation. Complete or near-complete SO₂ removal and high CO₂ capture efficiency (50–100%) were achieved, demonstrating the synergistic coupling of plasma activation with alkaline scrubbing. These findings demonstrate the viability of microwave-assisted plasma technology as a flexible and efficient solution for integrated flue gas pollutant control with potential for industrial-scale deployment in coal-fired power plants and other combustion facilities. Full article

Ammonia has great potential as a clean energy alternative and can contribute to reducing carbon emissions from conventional fossil fuels. To investigate the combustion characteristics of ammonia-doped natural gas and to evaluate its feasibility for practical applications, this study experimentally and numerically examined the temperature and pressure variations of ammonia-doped natural gas mixtures under different initial pressures. In addition, the combustion products corresponding to different ammonia doping ratios were simulated and analyzed. The results indicate that, with increasing ammonia doping ratio, both combustion temperature and pressure decrease to varying degrees. Under atmospheric pressure, the combustion temperature generally decreases by approximately 25%, while the peak pressure reduction reaches up to 87.85% in certain cases. Furthermore, under negative pressure conditions, a relatively low ammonia doping ratio enhances the combustion intensity of the mixture, and the peak combustion temperature occurs at lower ammonia concentrations. From an environmental perspective, the variation in combustion products with ammonia doping ratio was further analyzed. The results show that the CO concentration in the combustion products decreases progressively by approximately 71.11% as the ammonia doping ratio increases. In contrast, the NO concentration increases to a maximum value and then remains nearly constant, whereas the NO₂ concentration initially increases and subsequently decreases after reaching a peak value of 0.813 ppm. Overall, these findings provide experimental and theoretical support for understanding the combustion characteristics of mixed gaseous fuels and offer a scientific basis for the application and safety assessment of ammonia-doped natural gas. Full article

This study investigates the combustion and emission characteristics of a marine low-speed two-stroke engine using diesel-ignited ammonia dual direct injection. Using a validated 3D CFD model, the impact of ammonia blending ratios (R_a) was systematically explored. Results indicate that the strategy of shifting energy from early diesel injection to late ammonia injection physically repositions the combustion phasing. Rather than ammonia delaying the heat release, this late injection strategy avoids the overly early combustion observed at low ammonia concentrations, thereby lowering peak in-cylinder temperatures while maintaining robust work extraction. Consequently, indicated power at the N90 condition increases by 3.5% (to 1689 kW) over the diesel baseline, with a minimum EISFC of 165.5 g/kWh. High-ratio ammonia blending achieves deep decarbonization: at N90, peak CO and soot emissions are reduced by over 90% and 95%, respectively. Additionally, NO_x emissions decrease by approximately 70% at N90 compared to the N20 peak, attributed to the thermal DeNO_x mechanism. However, the low-temperature environment introduces trade-offs, leading to increased ammonia slip (4 ppm at N90) and elevated N₂O emissions (peaking at N70). These findings clarify the mechanisms governing ammonia combustion and provide theoretical support for optimizing zero-carbon marine propulsion systems. Full article

Hydrogen-fueled gas turbines face challenges related to flashback risk, nitrogen oxide (NO_x) emissions, and operational flexibility. In this study, a Center-Graded Spiral Micromixing (CGSM) combustor was designed and experimentally investigated to enhance the robustness of fuel–air mixing under hydrogen-rich conditions. The proposed CGSM concept employs spiral microtubes to induce curvature-driven secondary flows, promoting mixing through airflow-controlled mechanisms rather than relying solely on fuel jet momentum. Numerical simulations were conducted to qualitatively analyze the internal flow and mixing characteristics of the spiral microtubes, followed by pressurized combustor experiments at an inlet pressure of 0.3 MPa and elevated air temperatures. The experimental results demonstrate stable combustion of pure hydrogen under lean conditions, with NO_x emissions being maintained below 25 ppm, corrected to 15% O₂, without observable flashback or combustion oscillations within the designated operating range (from ignition to full load). The combustor further exhibits stable operation with blended hydrogen–methane and hydrogen–ammonia fuels, enabling online fuel switching without hardware modification. Application tests on an 80 kW micro-gas turbine indicate that the CGSM combustor can support stable operation across the full range of load conditions, from ignition to full-load operation, under both simple- and reheat-cycle modes, with performance characteristics that are consistent with established operational standards for micro-gas turbines. These results suggest that the CGSM concept provides a feasible micromixing strategy for hydrogen and hydrogen-rich fuels at a moderate pressure and micro-gas turbine scale. Full article

To address the decarbonization requirements of the shipping industry, this study establishes an in-cylinder combustion simulation model for a medium–high speed four-stroke ammonia-fueled marine engine based on the CONVERGE v3.0 platform. A diesel combustion model was first developed and validated against experimental data. Building on this validated model, an ammonia/n-heptane dual-fuel combustion model was further developed by coupling a chemical kinetic mechanism for ammonia/n-heptane. To overcome the challenge of igniting pure ammonia, a combustion strategy employing intake port injection of n-heptane and direct in-cylinder injection of ammonia fuel was adopted, leveraging thermal compression ignition. The results indicate that under initial cylinder conditions of 1 bar and 350 K, misfire occurs when the ammonia energy proportion (AEP) reaches 70%, preventing stable ignition and combustion of ammonia. Based on an analysis of intake boundary conditions, the influence of intake supercharging coupled with intake heating on ammonia combustion characteristics was investigated. As the AEP increases further, the combustion of n-heptane deteriorates significantly. At a 90% AEP, the combustion efficiency of n-heptane is approximately 67% at an initial temperature of 350 K but drops to about 28% at 400 K. Full article

This study investigated premixed NH₃ combustion in a closed circular duct using two-dimensional numerical simulations. By varying the equivalence ratio and the oxygen volume fraction from 21% to 30%, the evolution of flame morphology, flame propagation velocity, flame surface area, as well as the temporal variations in duct-averaged temperature and pressure were systematically examined. In addition, sensitivity analysis and reaction-pathway analysis based on a detailed chemical kinetic mechanism were performed to clarify the coupling between local chemical reactions and global flow dynamics. The results showed that the flame generally evolves through a sequence of hemispherical, finger-shaped, wall-attached skirt, and planar finger- and tulip-shaped structures. Well-developed tulip flames are mainly observed under conditions close to stoichiometric composition with moderate to elevated oxygen enrichment, corresponding to an intermediate overall reactivity. As the oxygen volume fraction increases from 21% to 30%, flame propagation becomes markedly faster. The tube-averaged temperature and the peak overpressure show an overall increasing trend. This increase in overpressure is most pronounced at equivalence ratios of 1.0–1.2. This study identifies hazardous parameter ranges in oxygen-enriched NH₃ combustion that are prone to producing strong tulip flames and high overpressure, providing useful guidance for explosion risk assessment and safety-oriented design of NH₃-fueled combustion systems. Full article

Ammonia (NH₃) is a promising carbon-free energy carrier for low-carbon power generation. However, in turbulent ammonia–methane (NH₃-CH₄) premixed swirling flames, operating at lean conditions to limit NO_X, emissions can trigger strong thermoacoustic oscillations. This study investigates thermoacoustic oscillatory instability in an NH₃-CH₄ swirl-stabilized combustor using the chemiluminescence of CH*, OH*, and NH* over a wide range of ammonia fuel fraction (X_NH3). Combined spectral measurements and 2D chemiluminescence imaging are employed to obtain the global emission characteristics and spatial distributions of OH* and NH* in the UV band and CH* in the visible band. A custom-designed intensified CMOS (ICMOS) camera based on a high-gain UV–visible image intensifier with direct coupling is developed to enable sensitive OH* and NH* imaging (gain > 10⁴). Frequency analysis of continuous CH* imaging, together with morphology-based principal component analysis and k-means clustering of 46 image features, shows that oscillatory combustion occurs for X_NH3 < 0.40, whereas X_NH3 ≥ 0.40 leads to multimode, stable combustion. As X_NH3 increases, OH* and NH* fields progressively decouple from CH*, becoming more elongated and shifting downstream. These results demonstrate that UV radical chemiluminescence provides indispensable information on NH₃ reaction zones and should be combined with CH* diagnostics for reliable thermoacoustic analysis and control in practical NH₃-fueled combustion systems. Full article