Search Results (136)

This study investigates the effects of secondary air supply on the combustion characteristics of briquettes with different moisture contents using a coupled FLIC–Fluent framework. FLIC was employed to simulate drying, pyrolysis, volatile release, and char oxidation in the fixed bed, and the resulting bed outlet temperature, velocity, and gas composition were transferred to Fluent as inlet boundary conditions for three-dimensional furnace simulations. At a constant total air supply, all-primary-air operation was compared with staged primary–secondary air supply for briquettes with moisture contents of 10%, 20%, and 30%. Increasing moisture content prolonged drying and preheating, reduced the bed outlet temperature and gas velocity, and weakened fixed-bed combustion. Without secondary air, the furnace’s high-temperature region progressively decreased, while relatively low-temperature regions expanded, and the mean furnace temperature declined from 1125 to 960 K as moisture content increased from 10% to 30%. Although CO release from the fuel bed decreased at higher moisture content, lower furnace temperatures and insufficient gas mixing suppressed subsequent CO oxidation, increasing furnace outlet CO from 820 to 1780 ppm. Redistributing 23.6% of the total combustion air as secondary air improved oxygen–fuel mixing and gas-phase burnout, with a stronger effect at higher moisture content. At 30% moisture, secondary air increased the mean furnace temperature from 960 to 1045 K, reduced outlet CO from 1780 to 980 ppm by 44.9%, and increased the volume fraction above 900 K by approximately 21%. Model validation showed relative errors below 1.0% for mean furnace temperature and below 5.0% for outlet CO and O₂, confirming the reliability of the coupled model. These results demonstrate that the investigated air-staging configuration can effectively mitigate the deterioration of furnace combustion caused by high briquette moisture content. Full article

This study addresses the lack of quantitative understanding regarding vent-gas combustion feedback in thermal runaway propagation of manganese-based prismatic battery modules. We develop a coupled multiphysics model integrating solid heat transfer, runaway kinetics, gas ejection, and turbulent combustion, informed by constant-volume reactor tests on a single cell that yield a total gas release of 10.6 mol per cell, with H₂ and CO comprising about 49% of combustibles. Model predictions are validated against three-cell module propagation experiments, capturing sequential failure with simulated inter-cell intervals matching experimental repeats that range from 71 to 105 s. The combustion model assumes auto-ignition-based initiation and therefore does not address the stochastic ignition delays observed in the experiments. It is intended for quantifying propagation acceleration once combustion has commenced. Quantitative heat-flow analysis reveals that gas-phase convective and radiative heating contributes substantially to the total heat flux on adjacent cells during venting, and direct comparison between simulations with and without combustion shows that combustion reduces inter-cell propagation intervals by 8–11%, confirming that combustion actively accelerates propagation. Simulations further resolve the combustion zone extending up to 0.4 m laterally. This framework provides a predictive tool and mechanistic basis for vent-gas management and flame-mitigation strategies in battery energy storage systems. Full article

This study presents a systematic mapping of methanol spray autoignition, lift-off, and flame development across an engine-relevant range of ambient temperatures (1000–1200 K), injection pressures (70–130 MPa), and O₂ concentrations (21–15 vol.%), using a single fixed injector and optical configuration. In addition, the study reports a dual-fuel strategy to address the low-temperature instability challenges highlighted by the mapping. Within this dataset, ignition delay increases with a lower ambient temperature, reduced injection pressure, or a lower O₂ concentration, while the lift-off length increases with a lower temperature and higher injection pressure. Schlieren imaging consistently captures ignition in the mid-axial region of the jet, softening of spray-head gradients before high-temperature ignition, and occasional upstream ignition sites during the diffusion-controlled phase that affect the flame base position. At the lowest tested temperature of 1000 K, methanol autoignites over a wide ignition-delay range (±1.33 ms), with combustion occurring outside the chamber’s field of view. The corresponding heat-release profile cannot be interpreted conclusively under the current test configuration. Introducing a pilot jet at this condition enables methanol to ignite shortly after the start of injection over a much narrower range (∼±0.10 ms). The resulting combustion event remains within the field of view and occurs much closer to the nozzle compared with its autoignition counterpart. Full article

Based on the decoupled n-dodecane skeletal mechanism and the computational fluid dynamics (CFD) numerical framework, a multilayer feedforward neural network surrogate model was developed to predict ignition delay in a constant-volume combustion vessel. The Levenberg–Marquardt optimizer with adaptive damping coefficients was used for model training, with mean squared error as the loss function and an inherent early stopping mechanism to prevent overfitting without additional weight decay regularization. To eliminate random interference from initial parameter settings, the surrogate model underwent 1000 repeated training trials, each with random weight re-initialization. The effects of hidden neurons, data partition strategy, normalization scheme, and sample size on predictive performance were systematically examined. The optimal configuration—three hidden neurons, a 70:15:15 data split, and a 105-sample training set—showed low sensitivity to data normalization. The resulting surrogate model is concise and sample-efficient, maintaining satisfactory prediction accuracy at 800 K and 1100 K while substantially reducing computational overhead. It provides a practical and reliable tool for subsequent combustion prediction and uncertainty quantification of hydrocarbon fuels. The feedforward neural network surrogate model substantially cuts the computational overhead for fuel combustion prediction to merely 15–20 min for every batch of 60 samples. Full article

This study investigated the effects of partial substitution of diesel fuel with hydrogen in dual-fuel operation on the engine operating characteristics, energy performance, and exhaust emissions of a compression-ignition passenger car engine. Hydrogen was supplied into the intake manifold, and the vehicle was tested on a chassis dynamometer at a constant vehicle speed under varying load conditions. To avoid the onset of abnormal combustion, the maximum stable H₂ mass fraction under diesel–hydrogen dual-fuel operation had to be reduced from 19% to 11% as engine load increased over the brake mean effective pressure (BMEP) range of 0.38–0.88 MPa, while the corresponding H₂ volume fraction in the intake air increased from 4.2% to 6.3%. Hydrogen addition reduced diesel fuel consumption and CO₂ emissions, although its effect depended strongly on engine load. At a 10% H₂ mass fraction, diesel fuel consumption decreased by 20–25%, and CO₂ emissions decreased by 17–21%. At low load, hydrogen addition improved brake thermal efficiency (BTE) and reduced NO_x emissions. At higher loads, reaching the hydrogen flammability limit reduced the excess-air ratio, deteriorated diesel combustion quality, and increased NO_x emissions and the exhaust smoke absorption coefficient. Higher H₂ concentrations reduced combined NO_x + HC emissions at low load but increased them at medium and high loads. Overall, hydrogen enrichment was most effective at low and medium loads, below the abnormal combustion onset limit, indicating that load-sensitive hydrogen dosing is required for efficient and environmentally favourable diesel–hydrogen dual-fuel operation. Full article

To investigate the early-stage flame propagation and pressure response of premixed H₂-O₂ combustion under ultra-high-pressure constant-volume conditions, a transient CFD model was developed for a large-volume confined chamber. The numerical framework combines a density-based solver, the Peng–Robinson real equation of state, large eddy simulation, and a reduced H₂-O₂ chemical kinetic mechanism. Simulations were conducted at initial pressures of 30 and 40 MPa, H₂/O₂ molar ratios of 8:1 and 12:1, and three-, four-, and five-point ignition configurations. The results show that increasing the initial pressure from 30 MPa to 40 MPa advances the pressure rise onset from approximately 1.65 ms to 1.28 ms and increases the maximum pressure rise rate from 18.6 MPa·ms⁻¹ to 27.4 MPa·ms⁻¹ under the H₂/O₂ = 8:1 and three-point ignition condition. Under the investigated fuel-rich conditions, increasing the H₂/O₂ molar ratio from 8:1 to 12:1 delays the pressure rise onset from approximately 1.28 ms to 1.46 ms and reduces the maximum pressure rise rate from 27.4 MPa·ms⁻¹ to 21.1 MPa·ms⁻¹. For the 30 MPa and H₂/O₂ = 8:1 cases, the four-point ignition case produces the largest pressure rise rate of approximately 23.5 MPa·ms⁻¹, whereas the five-point ignition case shows a lower pressure fluctuation amplitude of approximately 3.6 MPa. The present conclusions are based on CFD quantitative engineering predictions and should be further validated using quantitative experimental measurements. Full article

In this work, Direct Numerical Simulation (DNS) investigates the combustion behaviour of a reactive transverse lean premixed jet of an ammonia blend (10% NH₃, 11% H₂, 16% O₂ and 63% N₂ by volume) injected through a rectangular nozzle in a pre-heated non-vitiated air crossflow at a pressure of 5 bar. The configuration has been chosen from a Reynolds-Averaged Navier–Stokes (RANS) test campaign to ensure low NO and low unburned fuel, while maintaining a high temperature profile at the turbine inlet. The DNS shows that the flame stabilises on the leeward side of the rectangular jet, within and downstream of the recirculation region, while high scalar dissipation and short residence times prevent persistent anchoring on the windward side. Joint statistics reveal that the reaction does not follow a constant equivalence ratio path, since intermediate progress states are shifted towards leaner mixtures by entrainment, dilution and differential diffusion. The strongest heat-release and displacement-speed events occur in localised regions where mixture state, stretch and flame-front geometry act jointly. The displacement-speed budget is mainly controlled by the chemical source term, with diffusion reducing the net propagation speed and stratification-induced cross terms remaining small. Under intense stretch, positively curved flame elements exhibit larger displacement speeds, indicating a coupled effect of curvature, preferential diffusion and local radical transport. NO formation is dominated by fuel-nitrogen chemistry: HNO and NH₂ are the main NO-producing routes, whereas N₂ and N₂O provide the dominant NO-sink channels. The DNS predicts an outlet-averaged NO level of 400 dppm, while extended-domain RANS calculations indicate that longer residence times could reduce it below 100 dppm. Full article

In response to the current limitation where conventional constant volume combustion apparatuses are generally confined to pressure ratings of 5–20 MPa, insufficient for the demands of ultra-high-pressure combustion fundamental research, this study designs and verifies a high-pressure-resistant constant volume combustion apparatus with a rated working pressure of 250 MPa. The strength design and safety factor calculation for the combustion chamber main body were conducted based on the Lame thick-walled cylinder elastic theory. A finite element numerical simulation method was systematically employed to perform static analysis, transient impact response analysis, and high-cycle fatigue-life assessment of the key components of the apparatus. The results indicate that under a 250 MPa design internal pressure load, the maximum circumferential stress at the inner wall of the combustion chamber main body is 328.0 MPa, with a safety factor greater than 1.5, complying with relevant safety codes for high-pressure vessels. Under transient loading simulating combustion impact, the maximum equivalent stress of all structural components is below the material yield strength, with a maximum elastic deformation of less than 0.06 mm, demonstrating excellent structural stiffness and impact resistance. Fatigue assessment with a design-life target of 1.0 × 10⁶ pressure cycles shows that the cumulative damage values for all components are significantly less than 1.0, meeting the reliability requirements for long-term cyclic service. This apparatus integrates functional modules such as high-pressure precision gas mixing, high-energy reliable ignition, high-speed transient parameter acquisition, and safe product collection, providing a stable, controllable, and safe experimental platform for in-depth research on the combustion mechanisms of gaseous fuels under ultra-high-pressure conditions. Full article

Recently there has been notable interest in the reduction in emissions of the shipping industry via the substitution of the currently used fossil fuels with alternative green fuels. One such alternative studied presently could be the use of pure vegetable oils, which are cheaper and easier to produce than other proposed fuels. In this study, pure vegetable oils were tested in a constant volume combustion chamber to assess their ignition quality via the measurement of their Estimated Cetane Number (ECN) and to compare it with that of heavy fuel oils (HFOs). Moreover, the effect of vegetable oil composition on ignition quality was investigated. It was found that all the vegetable oils tested possessed significantly higher ignition quality than standard heavy fuel oils. Vegetable oil ignition quality was found to be most impacted by their degree of unsaturation. The results of the present study indicate that from the point of view of ignition quality, vegetable oils are a viable alternative to fossil fuels, being expected to lead to an increase in the ignition quality of standard heavy fuel oils. Full article

This study experimentally investigated the effect of characteristic chamber length, $L^{\ast }$, on combustion efficiency and stability in CAMUI-type hybrid rockets using 70 wt% and 80 wt% hydrogen peroxide under non-catalytic spray-injection conditions. Combustion tests were conducted by systematically varying $L^{\ast }$ through changes in the nozzle throat diameter while maintaining the combustor volume constant. For both oxidizer concentrations, the characteristic exhaust velocity efficiency, ${\eta }_{c^{\ast }}$, increased with increasing $L^{\ast }$. The 70 wt% cases required a larger $L^{\ast }$ than the 80 wt% cases to achieve comparable efficiency, and flame blowoff occurred in the low-$L^{\ast }$ region. The normalized RMS pressure fluctuation was also larger for the 70 wt% cases, particularly in the low-$L^{\ast }$ region, indicating lower combustion stability. These results indicate that reducing the hydrogen peroxide concentration increases the $L^{\ast }$ required to maintain stable and efficient combustion. As a key outcome of this study, stable and efficient combustion of 70 wt% hydrogen peroxide was demonstrated without catalytic assistance when a sufficiently large $L^{\ast }$ was provided. These results demonstrate the capability of the CAMUI-type combustor to extend stable operation toward lower oxidizer concentrations and experimentally clarify the concentration-dependent $L^{\ast }$ requirement as a practical design guideline for catalyst-free hydrogen peroxide hybrid rockets. Full article

In spite of the intense research interest in the integration of Pressure Gain Combustion (PGC) systems with a turbomachinery module, limited studies have been conducted regarding the experimental investigation of the strong spatio-temporal perturbations of these unconventional machines’ outflow. This paper focuses on experimentally characterizing the perturbing exhaust flow of a Constant-Volume Combustor (CVC). Preceding numerical analysis offers a transition duct able to attenuate the CVC’s produced unsteadiness and connect this PGC with a turbomachinery module. In fact, the transition duct is manufactured, while a pair of windows are introduced allowing for high-frequency Particle Image Velocimetry (PIV) analysis. In addition, fast-response pressure sensors in the combustion chamber, upstream and downstream of the transition duct, are implemented. A parametric analysis of the rotational frequency of the inlet–outlet rotary valve pair is conducted. The perturbing outflow of this PGC is characterized and experimentally visualized for the first time. Moreover, the attenuation performance of the transition duct on the CVC’s produced unsteadiness is evaluated for different cycle frequencies. The transition duct is proved to be able to alleviate the spatial and time-dependent unsteadiness by CVC, offering crucial evidence and conclusions for the future industrial integration of the CVC with a High-Pressure Turbine stage. Full article

The use of hydrogen direct injection (DI) plays a crucial role in decarbonizing internal combustion engine (ICE) technology. However, a suitable characterization of the injection process is required to control the mixture preparation before combustion, especially in the case of late injection timing. CFD modeling represents a useful tool to support experiments in addressing this goal. This study presents a numerical investigation of hydrogen DI using a swirled-type injector, seated in a constant-volume vessel. First, the selected numerical setup is validated against optical measurements of the jet penetration, demonstrating the reliability of the approach. Then, the analysis compares swirling and non-swirling configurations under different nozzle pressure ratios (nPRs) to evaluate the interaction between swirl-induced mixing and under-expanded jet structures. Results show that at lower nPR, swirl significantly alters the momentum distribution, reducing axial penetration. Instead, at higher nPR, where the H2 jets exhibit strong shock structures, the effects of swirl become negligible, with penetration and plume morphology nearly identical to non-swirling conditions. Analysis of the scalar dissipation rate showed the presence of a redistribution of mixing characteristics at low nPR due to swirl, while shock structures dominate at high nPR. This could have a significant impact on combustion and NO_x emissions in ICE operated with late injection strategies, where lower nPR are found. Full article

A one-pot green combustion route was developed for the synthesis of ashy single-crystalline NiO nanoparticles using date molasses as a biogenic fuel and complexing medium. The obtained DM–NiO showed phase-pure cubic NiO with an average crystallite size of about 18 nm, a mesoporous texture with a BET surface area of 68.9 m² g⁻¹, a pore volume of 0.59 cm³ g⁻¹, an average pore diameter of 17.6 nm, and a mean particle size of 43.6 ± 8.13 nm. Optical characterization revealed defect-mediated light absorption with an energy gap of 3.11 eV, supporting solar-light-driven activity. In the photocatalytic degradation of pyronin Y, the catalyst exhibited strong pH dependence, reaching its best H₂O₂-free performance at pH 11 with a pseudo-first-order rate constant of 0.0072 min⁻¹, nearly six times higher than that at pH 3. The introduction of H₂O₂ markedly intensified the process, and at 9 mM H₂O₂, the rate constant increased to 0.048 min⁻¹, representing more than a sixfold enhancement over photocatalysis alone, while complete disappearance of the main visible absorption band was achieved within 38 min under solar illumination. Radical trapping experiments identified photogenerated holes and hydroxyl radicals as the dominant oxidative species. The catalyst also retained high activity over four successive cycles, with degradation efficiencies decreasing only slightly from 91.8% to 85.7%. These results demonstrate that date-molasses-assisted combustion synthesis provides a sustainable route to defect-active mesoporous NiO with highly enhanced solar photo-Fenton-like performance for dye-contaminated wastewater treatment. 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

This study experimentally investigated the ignition and combustion characteristics of marine gas oil as a pilot fuel in dual-fuel marine engines using a constant-volume combustion chamber. In-cylinder temperature, pressure, and injection duration were the primary experimental variables. Results showed that temperature is the dominant factor governing ignition delay: increasing temperature from 520 °C to 580 °C reduced ID by 46.7% and its standard deviation by 62.8%. Increasing pressure shortened ID by 24.5% and reduced variability by 28.8%. In contrast, injection duration minimally affected ignition timing but increased accumulated heat release and maximum heat release rate by 37% and 20%, respectively. The time interval between ID and main combustion delay remained constant at approximately 0.30 ms across all conditions, indicating simultaneous advancement of ignition and combustion development. These findings demonstrate that ignition timing control (via temperature management) and combustion intensity control (via injection quantity) can be independently optimized, providing fundamental experimental data for the development of robust combustion-control strategies in dual-fuel marine engines. Full article