Search Results (10)

The air compression ratio in a modern aero engine has been significantly increased to enhance the engine’s thermal efficiency, thereby leading to high-pressure combustion, with the combustor pressure exceeding the fuel’s critical pressure (~23 atm for the aviation kerosene). In this work, large eddy simulations are conducted to investigate the effect of two air swirling injections on the flow dynamics and turbulent combustion of kerosene–air in a dual-swirl model combustor at a supercritical pressure of 4 MPa. The flamelet progress variable (FPV) model is applied to handle turbulent/chemistry interaction, and the extended corresponding states (ECS) method is adopted to evaluate thermophysical property variations. The results indicate that the inner air swirler controls flow and chemical reactions inside the injector, while the outer air swirler exerts a strong impact on the flow and flame characteristics in the combustor. A precessing vortex core (PVC) is generated by the inner swirling flow, and its frequency increases significantly as the inner air swirler angle varies from 25° to 40°. A modified Strouhal number is proposed for PVC frequency analyses, which reveal that the PVC frequency is influenced by the inner swirl number and the maximum axial velocity in the inner injector. The results obtained herein should help in developing a fundamental understanding of swirling flow and flame dynamics at high pressures. Full article

The global trend towards net-zero carbon emissions from burning fuels in combustion engines alerts us to the alternative role of biodiesel. The manufacturing cost of biodiesel hinders the fast development of various types of biofuels. Feedstock cost is one of the major determining factors of biodiesel cost and thus the extent of its competitiveness in the fuel market with other available alternative fuels or fossil fuels. Some low-cost feedstocks such as high-acid oil, which is produced from the acidifying processes of soybean soapstock, frequently contain high contents of free fatty acids (FFAs) and water. Hence, those feedstocks cannot be used to produce biodiesel through strong alkaline catalyst transesterification on an industrial scale. In contrast, the water can be converted to hydroxyl radicals to enhance the formation of esters from the dissociation of the FFA in a supercritical reacting tank. Hence, cheap high-acid oils containing high amounts of water and FFAs were used to produce biodiesel through a supercritical transesterification reaction system. The engine emission characteristics of using the biodiesel produced in this study were analyzed and compared with those of commercial biodiesel and super-low sulfur diesel (SLSD). A naturally aspirated, direct-injection, four-stroke, four-cylinder marine diesel engine associated with an eddy-current dynamometer was used to carry out the engine emission measurement. In comparison with super-low sulfur diesel (SLSD), the biodiesel had lower CO₂ and CO emissions and black smoke opacity but higher emissions of O₂ and NO_x. The higher engine speed caused lower emissions of O₂ and NO_x but higher CO₂ emissions. The supercritical-transesterification biodiesel appeared to be a competitive alternative fuel to fossil diesel. Full article

The accumulation of energy from non-programmable renewable sources is a crucial aspect for the energy transition. Using surplus electricity from renewable energy sources, power-to-gas plants allow to produce a substitute natural gas (SNG) that can be injected in the existing infrastructure for large-scale and long-term energy storage, contributing to gas grid decarbonisation. The plant layout, the method used for carbon dioxide capture and the possible cogeneration of electricity can increase the efficiency and convenience of SNG synthesis plants. In this work, a system for the simultaneous production of SNG and electricity starting from biomass and fluctuating electricity from renewables is proposed, using a plant based on the Allam thermodynamic cycle as the power unit. The Allam power cycle uses supercritical CO₂ as evolving fluid and is based on the oxycombustion of gaseous fuels, thus greatly simplifying CO₂ capture. In the proposed system, oxycombustion is performed using biomass syngas and electrolytic oxygen. The CO₂ generated by means of oxycombustion is captured, and it is subsequently used together with renewable hydrogen for the production of SNG through thermochemical methanation. The system is also coupled with a solid oxide electrolyser and a biomass gasifier. The whole plant was analysed from an energy-related point of view. The results show overall plant efficiency of 67.6% on an LHV basis (71.6% on an HHV basis) and the simultaneous production of significant amounts of electricity and of high-calorific-value SNG, whose composition could be compatible with the existing natural gas network. Full article

Mixing under high pressure conditions plays a central role in several engineering applications, such as direct-injection engines and liquid rocket engines. Numerical flow simulations have become a complementary tool to study the mixing process under these conditions but require complex thermodynamic modeling as well as validation with accurate experimental data. For this reason, we use experiments of supercritical single-phase jet mixing from the literature, where the mixing is quantified by the mixture speed of sound, as a reference for our work. We here focus on the thermodynamic modeling of multi-component flows under high pressure conditions and the analytical calculation of the mixture speed of sound. Our thermodynamic model is based on cubic equations of state extended for multi-components. Using an extension of OpenFOAM, we perform large-eddy simulations of hexane and pentane injections and compare our results with the experimentally measured mixture speed of sound at specific positions. The simulation results show the same characteristic trends, indicating that the mixing effects are well reproduced in the simulations. Additionally, the effect of the sub-grid scale modeling is assessed by comparing results using different models (Smagorinsky, Vreman, and Wall-Adapting Local Eddy-viscosity). The comprehensive simulation data presented here, in combination with the experimental data, provide a benchmark for numerical simulations of jet mixing in high pressure conditions. Full article

The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a supersonic crossflow were investigated by experimental and numerical methods, respectively. The schlieren system and acetone planar laser-induced fluorescence (PLIF) optical system were used to capture the flow-field structural characteristics and instantaneous plume. The mixture and real gas models were employed to calculate the interaction of a transverse jet and supersonic crossflow and reveal a good accuracy with the experimental results. The mixing efficiency and total pressure loss were analyzed based on the numerical results. The results indicate that the supercritical-state EHF directly changes to a gaseous state as it enters the supersonic crossflow from the injector. The EHF jet plume boundary increases with the increasing momentum flux ratio (q). As the streamwise and spanwise distance increases, the traverse heights and expand width increase, and the EHF jet plume presents a semicircle shape in the cross-sectional plane. With the increase in the traverse direction, the concentration distribution shows a fast and then slow power exponential decreasing law; the highest concentration point starts from the near-wall region and rises in the transverse direction with the flow distance increasing. For the same injection condition, the higher the inflow Mach number, the higher the mixing efficiency. For the same Ma, the mixing efficiency is better for the case with low injection pressure and high injection temperature. The total pressure loss is greater in the higher Ma, and high injection pressure conditions cause greater total pressure loss. Full article

The successful design of a liquid rocket engine is strictly linked to the development of efficient cooling systems, able to dissipate huge thermal loads coming from the combustion in the thrust chamber. Generally, cooling architectures are based on regenerative strategies, adopting fuels as coolants; and on cooling jackets, including several narrow axial channels allocated around the thrust chambers. Moreover, since cryogenic fuels are used, as in the case of oxygen/methane-based liquid rocket engines, the refrigerant is injected in liquid phase at supercritical pressure conditions and heated by the thermal load coming from the combustion chamber, which tends to experience transcritical conditions until behaving as a supercritical vapor before exiting the cooling jacket. The comprehension of fluid behavior inside the cooling jackets of liquid-oxygen/methane rocket engines as a function of different operative conditions represents not only a current topic but a critical issue for the development of future propulsion systems. Hence, the current manuscript discusses the results concerning the cooling jacket equipping the liquid-oxygen/liquid-methane demonstrator, designed and manufactured within the scope of HYPROB-NEW Italian Project. In particular, numerical results considering the nominal operating conditions and the influence of variables, such as the inlet temperature and pressure values of refrigerant as well as mass-flow rate, are shown to discuss the fluid transcritical behavior inside the cooling channels and give indications on the numerical methodologies, supporting the design of liquid-oxygen/liquid-methane rocket-engine cooling systems. Validation has been accomplished by means of experimental results obtained through a specific test article, provided with a cooling channel, characterized by dimensions representative of HYPROB DEMO-0A regenerative combustion chamber. Full article

In the present numerical study, implicit large eddy simulations (LES) of non-reacting multi-components mixing processes of cryogenic nitrogen and n-dodecane fuel injections under transcritical and supercritical conditions are carried out, using a modified reacting flow solver, originally available in the open source software OpenFOAM^®. To this end, the Peng-Robinson (PR) cubic equation of state (EOS) is considered and the solver is modified to account for the real-fluid thermodynamics. At high pressure conditions, the variable transport properties such as dynamic viscosity and thermal conductivity are accurately computed using the Chung transport model. To deal with the multicomponent species mixing, molar averaged homogeneous classical mixing rules are used. For the velocity-pressure coupling, a PIMPLE based compressible algorithm is employed. For both cryogenic and non-cryogenic fuel injections, qualitative and quantitative analyses are performed, and the results show significant effects of the chamber pressure on the mixing processes and entrainment rates. The capability of the proposed numerical model to handle multicomponent species mixing with real-fluid thermophysical properties is demonstrated, in both supercritical and transcritical regimes. Full article

To manage greenhouse gas emissions, directives on renewable energy usage have been developed by the European Commission with the objective to reduce overall emissions by 40% by 2030 which presents a significant potential for renewable energy sources. At the same time, it is a challenge for these energy technologies which can only be solved by integrated solutions. Carbon capture and storage combined with geothermal energy could serve as a novel approach to reduce CO₂ emissions and at the same time facilitate some of the negative impacts associated with fossil fuel-based power plants. This study focuses on the technical and economic feasibility of combining these technologies based on a published model, data and market research. In the European Union, Germany is the most energy intensive country, and it also has an untapped potential for geothermal energy in the northern as well as the western regions. The CO₂ plume geothermal system using supercritical carbon dioxide as the working fluid can be utilized in natural high porosity (10–20%) and permeability (2.5 × 10⁻¹⁴–8.4 × 10⁻¹⁶ m²) reservoirs with temperatures as low as 65.8 °C. The feasibility of the project was assessed based on market conditions and policy support in Germany as well as the geologic background of sandstone reservoirs near industrialized areas (Dortmund, Frankfurt) and the possibility of carbon capture integration and CO₂ injection. The levelized cost of electricity for a base case results in € 0.060/kWh. Optimal system type was assessed in a system optimization model. The project has a potential to supply 6600/12000 households with clean energy (electricity/heat) and sequester carbon dioxide at the same time. A trading scheme for carbon dioxide further expands potential opportunities. Full article

The similarities and differences in the combustion and emission characteristics of supercritical- and subcritical-state fuel injection conditions of an internal combustion engine was clarified. The effects of fuel state on temperature, pressure, turbulent kinetic energy, heat release rate, NO, and soot in the cylinder during the operation of the internal combustion engine were simulated. Ignition occurred faster, and the peak temperature in the cylinder was achieved in shorter time under the supercritical-state fuel injection condition than under the subcritical condition. The cylinder pressures in both states peaked at the same time, but the value of pressure in the supercritical fuel state was larger than that in the subcritical state. Furthermore, the turbulence in the supercritical fuel state was more intense than that in the subcritical state. The intense turbulence was beneficial to fuel and air mixing. NO emission increased, and soot emission decreased in the supercritical fuel state. The results show that supercritical fuel can be fully mixed with air to reduce the local concentration area in the cylinder, improve the combustion performance of the engine, and greatly reduce pollutant emissions. Full article

Semi-coke is one of the principal by-products of coal pyrolysis and gasification, which features the disadvantages of ignition difficulty, low burnout rate, and high nitrogen oxides (NO_x) emission during combustion process. Co-firing semi-coke with coal is a potential approach to achieve clean and efficient utilization of such low-volatile fuel. In this paper, the co-firing performance of semi-coke and lean coal in a 600 MW supercritical wall-fired boiler was numerically investigated which has been seldom done previously. The influences of semi-coke blending ratio, injection position of semi-coke, excess air ratio in the main combustion zone, the co-firing method, and over fire air (OFA) arrangement on the combustion efficiency and NO_x generation characteristics of the utility boiler were extensively analyzed. The simulation results indicated that as the blending ratio of semi-coke increased, the NO_x emission at furnace outlet decreased. The blending methods (in-furnace versus out-furnace) had certain impacts on the NO_x emission and carbon content in fly ash, while the in-furnace blending method showed more flexibility in co-firing adjustment. The injection of semi-coke from the upper burners could significantly abate NO_x emission at the furnace outlet, but also brought about the rise of carbon content in fly ash and the increase of outlet temperature. Compared with the condition that semi-coke and lean coal were injected from different burners, the burnout ratio of the blend premixed outside the furnace was higher at the same blending ratio of semi-coke. With the excess air ratio in the main combustion zone increased, NO_x concentration at the furnace outlet was increased. The excess air ratio of 0.75 in the main combustion zone was recommended for co-firing 45% semi-coke with lean coal. The operational performance of the boiler co-firing semi-coke was greatly affected by the arrangement of OFA as well. The amount of NO_x generated from the supercritical wall-fired boiler could be reduced with an increase of the OFA height. Full article