Search Results (30)

Wood used in construction varies in density, leading to differences in strength and rigidity. Wood densification has recently emerged as a promising technique to address these limitations and enhance material performance. This study explores the potential of two abundant and low-cost invasive hardwood species in South Africa—Prosopis glandulosa (Honey Mesquite) and Acacia mearnsii (Black Wattle)—as sources for producing densified wood. A range of strengthening methods, including chemical, pressure, and heat treatments, were applied and compared. After partial delignification and hot pressing, sample thicknesses were reduced by 40% for Prosopis and 50% for Acacia, yielding substantial increases in flexural strength of 216% (22.61 MPa) for Prosopis and 334% (24.65 MPa) for Acacia. In addition to anatomical imaging, analyses of lignosulphonate content, and thermogravimetric profiling, the study also evaluated several practical, carpentry-relevant mechanical properties. These included comparative tests for flexural and compressive strength, nailing and sanding performance, as well as assessments of water absorption, electrical resistivity, and flame-holding capacity. Full article

To address the issues of narrow ignition limits and low combustion efficiency in ramjet combustors under low-temperature and low-pressure conditions, caused by low reactivity of liquid fuel and slow chemical reaction rates, a multi-channel gliding arc plasma excitation activation method for fuel–air mixtures is proposed. This method generates gaseous small molecules and highly active radicals. Focusing on the vaporizing flame holder of a subsonic ramjet combustor, this study investigates the fuel–air activation characteristics under different carrier gas flow rates, fuel flow rates, and numbers of discharge channels. The mechanism by which multi-channel gliding arc discharge plasma enhances fuel–air activation, ignition, and combustion performance is revealed. Full article

The global concern regarding the reduction of carbon emissions has led to the development of hydrogen as a clean, carbon-free fuel for combustion systems. The present work combines Particle Image Velocimetry flow field measurements and Reynolds-Averaged Navier–Stokes numerical simulations to investigate the reactive flow downstream of a newly developed flame holder as part of a hydrogen-fueled afterburner system. The obtained numerical results are in reasonable agreement, for a RANS simulation, with the PIV measured data. According to the results presented in this article, it can be seen that ignition occurs, the flame is attached to the flame holder, and vortices develop downstream of the flame holder. These vortices facilitate the mixing of hydrogen with the flue gas coming from the gas generator. The recirculation zone generated by the flame holder in the flow measures approximately 100 mm, with the peak negative velocity reaching around 10 m/s. Downstream of the recirculation zone, the far-field free stream velocity on the centerline reaches 20 m/s. Outside the recirculation region, in the radial direction, the free stream is accelerated to an experimentally measured value of approximately 40 m/s, at 20 mm downstream from the flame holder, and 35 m/s at 100 mm downstream of the flame holder. The information gathered thus far will aid further investigation of the presented hydrogen-fueled afterburner system. Full article

Differing from traditional isobaric combustion, a pulse detonation-based ramjet (PD-Ramjet) was proposed in this study to enhance the efficiency of traditional ramjets. By using a two-dimensional numerical simulation method, the filling process and detonation initiation process of the hydrogen/air stoichiometric mixture in channels equipped with typical flameholders were studied under the inflow condition of a ramjet combustor, and the influences of the typical flameholders on the filling process and detonation initiation process were analyzed. Single cavity, sudden expansion cavity, central cavity, and V-shaped groove were chosen as typical ramjet flameholders. The simulation and analysis results indicated that the flameholders would affect the filling effect, and the blocking ratio had a great influence on the filling process. The hydrogen volume discharged from the outlet of the channel and the time for mixed gas to reach the outlet were related to the blocking ratio and the cavity aft wall inclination angle. The detonation initiation process revealed that the flameholders promoted the generation of detonation waves. Contrastingly, the detonation wave could not be initiated in the channel without flameholders despite the better filling effect. Moreover, different flameholders would change the position of high-pressure point formation and the time for generating the stable detonation wave. On the whole, the sudden expansion cavity had a lower blockage ratio and also gave consideration to the filling effect and detonation initiation characteristic, making it the most suitable flameholder structure for PD-Ramjet in this study. Full article

Scramjet operation requires a comprehensive understanding of the internal flowfield, encompassing fuel–air mixing and combustion. This study investigates transient flow and flame development within a HIFiRE-2 scramjet engine combustor, which features two opposing cavities and dual sets of fuel injectors—the upstream (primary) and downstream (secondary) injectors. These cavities function as flameholders, creating circulating flows with elevated temperatures and pressures. Shock waves form both ahead of fuel plumes and at the diverging and converging sections of the flowpath. Special attention is given to the interactions among these shock waves and the shear layers along the supersonic core flow as the system progresses towards a quasi-steady state. Driven by increased backpressure, bow shocks and disturbances induced by the normal, secondary fuel injection and the inclined, primary fuel injection move upstream, amplifying the cavity pressure. These shocks generate adverse pressure gradients, causing near-wall flow separation adjacent to both injector sets, which enhances the penetration and dispersion of fuel plumes. Once a quasi-steady state is achieved, a feedback loop is established between dynamic wave motions and combustion processes, resulting in sustained entrainment of reactive mixtures into the cavities. This mechanism facilitates stable combustion in the cavities and near-wall separation zones. Full article

The stability of hydrogen-fueled flames in afterburner systems is crucial for advancing clean energy technologies but is challenged by intense turbulence and flow variability. This study uniquely integrates advanced particle image velocimetry (PIV) techniques to investigate the flow dynamics around a V-gutter flame holder fueled with 100% hydrogen. Detailed velocity measurements were conducted to analyze the standard deviation of V_y, average V_y, average V, and uncertainty of V_y, as well as the mean swirling strength and mean vorticity profiles across multiple horizontal and vertical lines. The results reveal significant flow variability and turbulence intensity near the flame holder, with standard deviation peaks of up to 12 m/s, indicating zones of high turbulence and potential flame instability. The mean swirling strength, peaking at 850,000 [1/s²], and vorticity values up to 5000 [1/s] highlight intense rotational motion, enhancing fuel–air mixing and flame stabilization. The average V_y remained stable near the centerline, ensuring balanced flow conditions, while lateral deviations of up to −10 m/s reflect vortical structures induced by the flame holder geometry. Low uncertainty values, typically below 1 m/s, validate the precision of the PIV measurements, ensuring a reliable representation of the flow field. By providing a detailed analysis of turbulence structures and their impact on hydrogen combustion, this study offers novel insights into the interplay between flow dynamics and flame stability. These findings not only advance the understanding of hydrogen-fueled afterburner systems but also demonstrate the critical role of rotational flow structures in achieving stable and efficient combustion. By addressing key challenges in hydrogen combustion, this study provides a foundation for designing more robust and environmentally sustainable combustion systems, contributing to the transition toward clean energy technologies. Full article

This study is dedicated to an in−depth analysis of the combustion characteristics of extruded polystyrene (XPS) as a building insulation material with the aim of accurately assessing its fire risk in the built environment. Innovatively, this research employed a cone calorimeter equipped with a self−designed insulating sample holder to conduct a systematic experimental study. Additionally, it performed a comprehensive analysis of the ignition characteristics, heat release rate, fire hazard, smoke release, and toxic gas emission of XPS materials. The experimental results revealed that the combustion behavior of XPS is influenced by multiple factors, including the content of flame retardants and external heat flux, which significantly affect the fire hazard of XPS. When the thermal radiation intensity escalates from 25 kW/m² to 55 kW/m², the peak heat release rate of XPS−B1 rises from 428 kW/m² to 535 kW/m², marking an increase of 25.00%. Conversely, the peak heat release rate of XPS−B2 surges from 348 kW/m² to 579 kW/m², reflecting a substantial increase of 66.38%. This research not only provides a solid theoretical foundation and detailed experimental data for the fire behavior of XPS materials but also holds significant practical importance for enhancing the fire safety of buildings. Overall, this research contributes to the scientific understanding of XPS insulation materials and supports the development of more effective fire prevention measures in construction. Full article

A considerable number of Combined Heat and Power (CHP) systems continue to depend on fossil fuels like oil and natural gas, contributing to significant environmental pollution and the release of greenhouse gases. Two V-gutter flame holder prototypes (P1 and P2) with the same expansion angle, fueled with pure hydrogen (100% H₂) or hydrogen–methane mixtures (60% H₂ + 40% CH₄, 80% H₂ + 20% CH₄), intended for use in cogeneration applications, have been designed, manufactured, and tested. Throughout the tests, the concentrations of CO₂, CO, and NO in the flue gas were monitored, and particle image velocimetry (PIV) measurements were performed. The CO, CO₂, respectively, and NO emissions gradually decreased as the percentage of H₂ in the fuel mixture increased. The NO emissions were significantly lower in the case of prototype P2 in comparison with prototype P1 in all measurement points for all used fuel mixtures. The shortest recirculation zone was observed for P1, where the axial velocity reaches a negative peak of approximately 12 m/s at roughly 50 mm downstream of the edge of the flame holder, and the recirculation region spans about 90 mm. In comparison, the P2 prototype has a length of the recirculation region span of about 100 mm with a negative peak of approximately 14 m/s. The data reveal high gradients in flow velocity near the flow separation point, which gradually smooth out with increasing downstream distance. Despite their similar design, P2 consistently performs better across all measured velocity components. This improvement can be attributed to the larger fuel injection holes, which enhance fuel–air mixing and combustion stability. Additionally, the presence of side walls directing the flow around the flame stabilizer further aids in maintaining a stable combustion process. Full article

Vortices belong to the most important phenomena in fluid dynamics and play an essential role in many engineering applications. They can act detrimentally by harnessing the flow energy and reducing the efficiency of an aerodynamic device, whereas in other cases, their presence can be exploited to achieve targeted flow conditions. The control of the vortex parameters is desirable in both cases. In this paper, we introduce an optimization strategy for the control of vortices in the wake of a bluff body. Flow modelling is based on RANS and DES computations, validated by experimental data. The algorithm for vortex identification and characterization is based on the triple decomposition of motion. It produces a quantitative measure of vortex strength which is used to define the objective function in the optimization procedure. It is shown how the shape of an aerodynamic device can be altered to achieve the desired characteristics of vortices in its wake. The studied case is closely related to flame holders for combustion applications, but the conceptual approach has a general applicability to vortex control. Full article

A new afterburner installation is proposed, fueled with pure hydrogen (100%H₂) or hydrogen–methane mixtures (60% H₂ + 40% CH_4, 80% H₂ + 20% CH₄) for use in cogeneration applications. Two prototypes (P1 and P2) with the same expansion angle (45 degrees) were developed and tested. P1 was manufactured by the classic method and P2 by additive manufacturing. Both prototypes were manufactured from Inconel 625. During the tests, analysis of flue gas (CO₂, CO, and NO concentration), PIV measurements, and noise measurements were conducted. The flue gas analysis emphasizes that the behavior of the two tested prototypes was very similar. For all three fuels used, the CO₂ concentration levels were slightly lower in the case of the additive-manufactured prototype P2. The CO concentration levels were significantly higher in the case of the additive-manufactured prototype P2 when 60% H₂/40% CH₄ and 80% H₂/20% CH₄ mixtures were used as fuel. When pure H₂ was used as fuel, the measured data suggest that no additional CO was produced during the combustion process, and the level of CO was similar to that from the Garrett micro gas turbine in all five measuring points. The NO emissions gradually decreased as the percentage of H₂ in the fuel mixture increased. The NO concentration was significantly lower in the case of the additive-manufactured prototype (P2) in comparison with the classic manufactured prototype (P1). Examining the data obtained from the PIV measurements of the flow within the mixing region shows that the highest axial velocity component value on the centerline was measured for the P1 prototype. The acoustic measurements showed that a higher H₂ concentration led to a reduction in noise of approximately 1.5 dB for both afterburner prototypes. The outcomes reveal that the examined V-gutter flame holder prototype flow was smooth, without any perpendicular oscillations, without chaotic motions or turbulent oscillations to the flow direction, across all tested conditions, keeping constant thermal power. Full article

As modern aeroengine combustors advance towards high temperatures, afterburners are inevitably affected by diminished oxygen content in incoming flows, thus affecting combustion efficiency, instability, and flammability limits. In this study, the dynamic combustion characteristics of V-shaped bluff body-stabilised diffusion flames were investigated using a large eddy simulation method with an oxygen mass fraction range of 14–23% and temperatures ranging from 900 to 1100 K. The results show the significant effects of oxygen content and inflow temperature on the flame/flow behaviours downstream of the bluff-body flame holder. In a separated shear layer, two distinct modes of flow/flame shedding are observed when varying the oxygen content and inflow temperature. The results show that BVK instability governs the far-field wake flow/flame features, whereas the oxygen concentration and temperature significantly affect their oscillation amplitudes. In addition, variations in the incoming oxygen content and temperature shift the axial position of the transition from KH instability to BVK instability. Finally, a spectral analysis is conducted to investigate the characteristics of pressure and heat release pulsations under different scenarios. This study highlights the importance of oxygen content on the combustion dynamics of bluff body-stabilised diffusion flames at various temperatures, which is essential for optimising combustion efficiency and stability in practical applications. Full article

In order to integrate renewable electricity into the power grid, it is crucial for coal-fired power plant boilers to operate stably across a wide load range. Achieving steady combustion with low nitrogen oxide (NO_x) emissions poses a significant challenge for boilers burning low-volatile coal in coal-fired power plants. This study focuses on developing a decoupling combustion technology for low-volatile coal-fired boilers operating at low loads. A three-dimensional numerical simulation is employed to analyze and optimize the geometrical parameters of a burner applied in a real 300 MW pulverized coal fired boiler. Detailed analysis of the burner’s decoupling combustion characteristics, including stable combustion ability and NO_x reduction principles, is conducted. The results indicate that this burner showed three stages of coal/air separation, and the flame holder facilitates the stepwise spontaneous ignition and combustion of low-volatile coal. By extending the time between coal pyrolysis and carbon combustion, the burner enhances decoupling combustion and achieves low nitrogen oxide emissions. Based on optimization, a flat partition plate without inclination demonstrates excellent performance in terms of velocity vector field distribution, coal air flow rich/lean separation, combustion, and nitrogen oxide generation. Compared with the initial structural design, the average nitrogen oxide concentration at the outlet is reduced by 59%. Full article

This paper presents the results of testing and optimization of a plasma-assisted combustion scheme based on a pylon for fuel injection equipped with a plasma actuator. Electrodes were installed behind the stern of the pylon for the creation of Q-DC discharge with voltage U = 200–2500 V and current I = 3–7.5 A. The experiments were performed in the PWT-50 supersonic wind tunnel of the JIHT RAS under the following conditions: Mach number M = 2, static pressure ~200 Torr, stagnation temperature T₀ = 300 K. Gaseous fuel ethylene was used and the fuel mass flow rate was 0.5–4 g/s. The pylon had a streamlined shape that prevented the formation of a stagnant zone; plasma-assisted combustion was performed under more difficult conditions compared to plasma-assisted combustion on a flat wall, where separated flows near the wall are easily formed by discharge. In this work, two new geometries of pylon equipped with electrodes were proposed and experimentally tested. A second version providing a longer discharge length demonstrates stable ignition and intense combustion in a fully discussed fuel mass flow rate. The process of ignition in a supersonic flow and flame front pulsations was described. A reduction in the energy input in comparison with the previously considered configurations of plasma-assisted combustion was also demonstrated. Full article

In order to explore plasma-assisted turbulent mixing in aerospace engines, the dielectric barrier discharge plasma actuation for the turbulent mixing of fuel droplets and oxidant air in a ramjet combustor was studied using computational fluid dynamics. A two-way coupling of turbulent air and discrete droplets was realized by Eulerian–Lagrangian simulation, and the dielectric barrier discharge plasma action on flow was modeled by body force. The results show that the plasma actuation can rearrange the recirculation zone behind the evaporative V-groove flameholder, and the main mechanism of actuation is to increase the local momentum of the fluid; the actuation dimension, actuation intensity, and actuation position of the dielectric barrier discharge plasma have strong effects on the turbulent mixing of fuel droplets and oxidant air; and a relatively optimal turbulent mixing can be achieved by adjusting the actuation parameters. Full article

The aviation sector is facing a massive change in terms of replacing the currently used fossil jet fuels (Jet A, JP5, etc.) with non-fossil jet fuels from sustainable feedstocks. This involves several challenges and, among them, we have the fundamental issue of current jet engines being developed for the existing fossil jet fuels. To facilitate such a transformation, we need to investigate the sensitivity of jet engines to other fuels, having a wider range of thermophysical specifications. The combustion process is particularly important and difficult to characterize with respect to fuel characteristics. In this study, we examine premixed and pre-vaporized combustion of dodecane, Jet A, and a synthetic test fuel, C1, based on the alcohol-to-jet (ATJ) certified pathway behind an equilateral bluff-body flameholder, spray combustion of Jet A and C1 in a laboratory combustor, and spray combustion of Jet A and C1 in a single-sector model of a helicopter engine by means of numerical simulations. A finite rate chemistry (FRC) large eddy simulation (LES) approach is adopted and used together with small comprehensive reaction mechanisms of around 300 reversible reactions. Comparison with experimental data is performed for the bluff-body flameholder and laboratory combustor configurations. Good agreement is generally observed, and small to marginal differences in combustion behavior are observed between the different fuels. Full article