Small-scale Combustion Research Articles

Transformation of NO in Combustion Gases by DC Corona

This study investigates the performance of DC corona discharge electrostatic precipitators (ESPs) for NO conversion to increase DeNOx technologies’ efficiency for small-scale biomass combustion systems. Experiments were conducted using a 5 kW automatic wood pellet domestic heat source with combustion gas treated in a specially designed ESP operated in both positive and negative corona modes, resulting in a reduction in NO concentrations from 130 mg/m3 to 27/29 mg/m3 for positive/negative polarities (at 0 °C and 101.3 kPa). NO conversion efficiency was evaluated across a range of specific input energies (SIEs) from 0 to 50 J/L. The results demonstrate that DC corona ESPs can achieve up to 78% NO reduction, with positive corona demonstrating a greater energy efficiency, requiring a lower SIE (35 J/L) compared to the negative corona mode (48 J/L). A detailed analysis of reaction pathways revealed distinct conversion mechanisms between the two modes. In positive corona, dispersed active species distribution led to more uniform NO conversion, while negative corona exhibited concentrated reaction zones with about 20% higher ozone production. The reactions involving O and OH radicals were more important in positive corona, whereas ozone-mediated oxidation dominated in negative corona. The research results demonstrate that ESP technology with DC corona offers a promising, energy-efficient solution for NOx control in small-scale combustion systems.

Stoichiometric Methane-Air Flame Propagation in a Diverging Hele-Shaw Channel

ABSTRACT Design and optimization of various small-scale combustors requires knowledge on the features of flame propagation in narrow spaces between solid walls. In the present work, experiments on laminar propagation of stoichiometric premixed methane-air flames in a planar channel formed by two parallel closely spaced (3–6 mm) glass plates are presented. The channel of 1.2 m length has straight side walls diverging at an angle between 5 and 25 degrees; ignition is performed near the closed narrow end of the channel. Video recordings of the propagating flames are taken and processed to obtain the flame coordinate, visible velocity, flame front shape and length, and cell width. For each channel geometry, visible flame propagation speed is obtained and compared with the normal flame speed. Positions of the angular points (cusps) separating the cells are obtained and analyzed in order to establish the statistical features of cellular flames. It is shown that flame propagation in the channels with the gap width exceeding 4.5 mm exhibits significant oscillations of the visible speed and flame length. These oscillations are of non-acoustic nature, their frequency is 4–8 Hz, and their amplitude increases with the gap width. For the developed cellular flames, the ratio of cell size to gap width is bounded to the range of 4–10. The cell size probability density functions are shown to obey the gamma distribution, except for the widest gap of 6 mm. The main novelty of the approach and its significance for practical applications is that flame propagation in narrow gaps is studied, with the effects of channel geometry and gas flow in the channel taken into account explicitly.

Long-term emission demonstration using pretreated urban non-woody biomass residues as fuel for small scale boilers

Long-term emission demonstration using pretreated urban non-woody biomass residues as fuel for small scale boilers

Experimental Study on Scale Effects of Kerosene Combustion in Scramjet Combustors

The combustion characteristics in two geometrically similar kerosene-fueled scramjet combustors with mass flow rates of 0.69 and 1.41 kg/s are experimentally investigated to explore the scale effects of flame stabilization at Mach 2.52 condition. As the equivalence ratio increases, the combustion usually changes from weak to intensive to blow-out mode. The weak combustion has little effect on the flow field, whereas the intensive combustion has the opposite effect. The transition combustion tends to occur between different modes. When the single injector is used, compared with the small-scale combustor, intensive combustion cannot occur in the large-scale combustor, and the flame stability range is also narrower. One probable reason is that as the combustor scale increases, the boundary layer becomes relatively thinner, resulting in a smaller low-velocity zone and a faster mainstream velocity at the downstream wall of the cavity, which is not conducive to the flame propagation upstream to form the intensive combustion. After shortening the isolator, all cases with intensive combustion in the small-scale combustor are transformed into weak combustion, further confirming the speculation. Compared to the single injector, the dual injector is required in the large-scale combustor to achieve intensive combustion and a wider flame stability range.

Combined control of PM and NOx emissions from small-scale combustions by electrostatic precipitation

Combined control of PM and NOx emissions from small-scale combustions by electrostatic precipitation

Application End Evaluation of Electrostatic Precipitation for Control PM and NOx Emissions from Small-Scale Combustions

Electrostatic precipitators (ESPs) have shown promise in reducing particulate matter (PM) emissions, but their potential for simultaneous NOx reduction in small-scale combustion systems remains underexplored. This study focuses on using non-thermal plasma generated in a corona discharge to reduce PM and NOx emissions from small-scale combustion. ESP was specifically designed for a commercially available 15 kW boiler with wood pellet combustion and used with both positive and negative discharge polarity to control emissions without any chemical additives. ESP performance was evaluated across a range of specific input energies (SIE) in terms of particle mass and number concentrations and NOx concentrations obtained by continuous gas analysis. ESP ensured the reduction in PM concentrations from 48 mg/m3 to the magnitude of PM content in the ambient air. The highest precipitation efficiency was observed for particles in the 20–200 nm range. Concurrently, NOx emissions were reduced by up to 78%, from 178 mg/m3 to 39 mg/m3. These results were achieved at specific input energies of 36 for positive and 48 J/L for negative corona, which is significantly lower than those reported for many existing separate PM and NOx control systems. This study demonstrates the potential of ESPs as a compact, energy-efficient solution for simultaneous PM and NOx removal in small-scale combustion systems, offering promising implications for improving air pollution control technologies for small-scale combustion systems.

On the scale effects of flame stabilization under different combustion modes in an ethylene-fueled scramjet combustor

On the scale effects of flame stabilization under different combustion modes in an ethylene-fueled scramjet combustor

On the Analysis of Scale Effects of Flame Stabilization in Scramjet Combustors

Scale effects of flame stabilization in scramjet combustors at Mach 2.52 are investigated through experiments and numerical simulations based on two geometrically similar combustors. As the equivalence ratio increases, the combustion mode will change from Scram to Dual to Ram. However, the larger combustor has a delayed mode transition. The scale effects of flame stabilization are also reflected in the changes of heat release, precombustion shock train length, jet penetration depth, and reaction zone location. The combustion flowfields of different-scale combustors exhibit numerous differences under different combustion modes. However, after shortening the isolator of the proportionally scaled small-scale combustor, the combustion flowfield is closer to that of the large-scale combustor. Further analysis reveals the reason for the inconsistent scale effects of flame stabilization under different combustion modes. The probable reason is that the boundary layer, which does not change proportionally with the combustor scale, has different effects on the combustion under the three modes. Compared with the Scram and Ram modes, the scale effect in the Dual mode is the most significant because the strong interaction between heat release and the boundary layer amplifies the slight difference in the relative thickness of the boundary layer.

Carbon monoxide (CO) and particulate matter (PM) emissions during the combustion of wood pellets in a small-scale combustion unit – Influence of aluminum-(silicate-)based fuel additivation

Carbon monoxide (CO) and particulate matter (PM) emissions during the combustion of wood pellets in a small-scale combustion unit – Influence of aluminum-(silicate-)based fuel additivation

Premixed flames in narrow heated channels of circular cross-section: Steady-state solutions, their linear stability analysis and the multiplicity of stable dynamic modes

Premixed flames in narrow heated channels of circular cross-section: Steady-state solutions, their linear stability analysis and the multiplicity of stable dynamic modes

Design and performance analysis of hydrogen-fueled micro combustion chamber with focus on back pressure minimization

The combustion chamber is a crucial component in power generation within a micro gas turbine. This paper prioritizes practical over theoretical considerations in designing an efficient, small-scale combustion chamber for micro gas turbine applications. The investigation covers the temperature profile within the combustion chamber, employing 19 reversible reactions and considering 9 different chemical species in reactive flow calculations. Preliminary experiments demonstrate hydrogen as a feasible fuel in a micro combustion chamber, generating approximately 1 kW of thermal power. Turbulence physics are assessed using the accurate k-Ɛ model. Findings indicate a reactant inlet temperature of 300 K and a primary zone temperature of 1750 K. This research suggests that minimizing the back pressure effect in a steady-state micro combustion chamber can improve turbine performance.

A Calculation Method of Heat Release Rate Based on Flame Spread Rate: Applied in a High-Speed Train Carriage Fire

ABSTRACT Heat release rate (HRR) is an important parameter for the fire safety assessment of high-speed trains and the design of tunnel ventilation systems. However, the existing theoretical calculation methods of the HRR do not consider the fire development process and can not accurately obtain the HRR of high-speed trains. This paper uses a combination of theoretical analysis and numerical simulation to study the HRR of high-speed train fires. Firstly, based on the flame spread rate, a calculation method of the HRR is established, called the spatial time-series superimposed method. The accuracy of the calculation method is verified through a small-scale combustion experiment. Then, the calculation model of the HRR of the high-speed train fire is obtained according to the heat release rate per unit area (HRRPUA) of the carriage material, the flame spread rate in the carriage, and the spatial time-series superimposed method. Afterward, the calculation model is modified according to the change in combustion intensity during the flame spread in the high-speed train carriage. Finally, the calculation method of the HRR established in this paper is compared with several existing calculation methods. The results show that the HRR curve obtained by the calculation model is consistent with that obtained by the FDS simulation. The maximum relative error is less than 25%, and the relative error of the peak heat release rate (PHRR) is 7.4%. Therefore, this study can provide a reference for the fast and accurate calculation of the HRR of the high-speed train carriage fire.

Low-Temperature-Solid Combustion Technology of Biomass for Pollution Reduction: Potentials and Necessary Fundamentals.

The high temperature of solid combustion in modern combustors leads to serious pollution in the combustion of biomass and solid wastes. The broad demands of relatively small-scale combustion of solid fuels and the imposition of increasingly strict emission limits require more economical methods for pollution reduction. Based on literature and our own work, low-temperature-solid combustion technology, which applies low temperature for solids but normal temperature for gas during combustion, was introduced in this paper. The potential of this technology in pollution reduction was analyzed, and necessary fundamentals for equipment design/operation were discussed. It showed that, for straws, more than 60% of deposit and particle emission can be reduced and 100% of ash can be recycled when the solid temperature is <600 °C. Fundamentals on rates of inorganic release, char oxidation, phase transformation in the condensed phase, and NOx/SOx release are necessary for better application of the technology.

Small-scale superadiabatic combustors with a two-step chain-branching chemistry model: Asymptotic models and the effect of two-dimensionality on lean mixtures burning

Small-scale superadiabatic combustors with a two-step chain-branching chemistry model: Asymptotic models and the effect of two-dimensionality on lean mixtures burning

A Computational Fluid Dynamics-Based Small-Scale Combustor Design Optimization Utilizing a Many-Objective Evolutionary Algorithm

Abstract This work demonstrates the capability of an open-source autonomous computational fluid dynamics (CFD) metamodeling environment (OpenACME) for optimizing small-scale combustor designs. OpenACME couples several object-oriented programing open-source codes for CFD-assisted design using a decomposition-based many-objective evolutionary algorithm. The CFD is based on steady, incompressible, three-dimensional simulations with k–ω SST RANS and flamelet/progress variable combustion model. There are five unconstrained design variables based on combustor liner dilution hole diameters. The CFD results are compared with existing experimental data in terms of combustion efficiency as a function of severity parameter. The comparison demonstrates that the CFD methods capture combustion efficiency trends. Next, more than 500 combustor designs are evaluated with OpenACME. A Pareto Frontier is generated in terms of combustion efficiency, pattern factor, and total pressure losses. Pseudo-weights are used to select a nondominated Pareto Frontier design point for future fabrication and experimental testing. OpenACME is demonstrated to be a viable tool for small-scale combustor design optimization.

Development of an Open-Source Autonomous Computational Fluid Dynamics Meta-Modeling Environment for Small-Scale Combustor Optimization

Abstract This work presents an open-source autonomous computational fluid dynamics (CFD) metamodeling environment (OpenACME) for small-scale combustor design optimization in a deterministic and continuous design space. OpenACME couples several object-oriented programing open-source codes for conjugate-heat transfer, steady-state, multiphase incompressible Reynolds averaged Navier-Stokes CFD-assisted engineering design metamodeling. There are fifteen design variables. Nonparametric rank regression (NPRR), global sensitivity analyses (GSA), and single-objective (SOO) optimization strategies are evaluated. The Euclidean distance (single-objective criterion) between a design point and the utopic point is based on the multi-objective criteria: combustion efficiency (η) maximization and pattern factor (PF), critical liner area factor (Acritical ), and total pressure loss (TPL) minimization. The SOO approach conducts Latin hypercube sampling (LHS) for reacting flow CFD for subsequent local constraint optimization by linear interpolation. The local optimization successfully improves the initial design condition. The SOO approach is useful for guiding the design and development of future gas turbine combustors. NPRR and GSA indicate that there are no leading-order design variables controlling η, pattern factor (PF), Acritical , and TPL. Therefore, interactions between design variables control these output metrics because the output design space is inherently nonsmooth and nonlinear. In summary, OpenACME is developed and demonstrated to be a viable tool for combustor design metamodeling and optimization studies.

Flame stabilization in narrow channels by a highly conductive wall segment: Application to small-scale combustion devices

The structure and dynamics of premixed flames in narrow channels in the presence of a highly conductive wall segment are investigated in this work. It is shown that these systems present a multiple steady-state regimes. A linear stability analysis applied to these steady solutions reveals that several steady-state solutions may be simultaneously stable. The stability analysis results are confirmed by time-dependent simulations, that show that the actual realization of one or another stable regime depends on the initial conditions. Finally, we show that nonlinear periodic and chaotic dynamics may appear under certain conditions in these system.

Superadiabatic small-scale combustors: Asymptotic analysis of a two-step chain-branching combustion model

An asymptotic study for a counterflow burner consisting of thin channels with heat exchange is proposed. The focus is made on the cases of ultra lean burning using a model of a two-step chain-branching chemistry. The ratio of the channel length to the thermal flame thickness is used as a natural large parameter. Then, the solution is constructed by the method of asymptotic expansions using this parameter. Finally, the obtained asymptotic for the flame position is satisfactorily compared with the exact solution of the problem. These explicit results have a clear advantage in that they facilitate considerably the parametric analysis. It is demonstrated that combustion of mixtures below the flammability limit can be carried out in the considered systems.

Combustion behaviour and slagging tendencies of pure, blended and kaolin additivated biomass pellets from fen paludicultures in two small-scale boilers < 30 kW

Pure, blended and additivated biomass pellets from four fen paludicultures were produced at TFZ and combusted in two small-scale biomass boilers (15 kW, 30 kW). Feedstocks derived from straw of Typha ssp., Phragmites australis , Phalaris arundinacea and Carex ssp. that were harvested during winter 2018, 2019 (used for pelletization) and 2020. Additivation of fuels with kaolin before pelletization or blending of fuels with EN plus wood pellets (A1 quality) after pelletization were applied. Straw and pellets were analyzed for physical and chemical fuel properties according to international standards for solid biofuels. Physical properties of pellets met the requirements of ISO 17225–6. Chemical properties of Typha indicated high TPM emissions due to high contents of K and Na in fuels while severe slagging was predicted for the other species by a high molar (Si + K + Al)/(Ca + Mg + P) ratio. During combustion in both boilers, CO and total particulate matter (TPM) emissions were high for Typha but slightly reduced by additivation with 2.3% kaolin. Blending of fuels significantly reduced NO X , SO X and HCl emissions due to lower N, S and Cl concentrations. Slagging was high for pure and additivated pellets of Phragmites , Phalaris and Carex with >50% of total ash consisting of particles >2 mm. No steady-state boiler operation could be achieved with either pure or additivated fuels. In conclusion, paludiculture pellets are challenging fuels for small-scale combustion plants. Their use cannot be recommended for the tested boilers. Technical solutions may be easier applied in medium sized combustions plants above 100 kW. • Fuel quality and combustion behaviour of paludiculture straw & pellets were analyzed. • Additivation (kaolin) and blending (wood) were applied to improve combustion. • CO & TPM emissions were high for Typha while slagging was high for all other fuels. • NO X emissions were significantly increased for paludicultures but improved by blending. • Kaolin had only minor effects on combustion due to overall low additivation levels.

Simulation numérique d'un bruleur à biogaz destiné aux moteurs Stirling

The design of a biogas burner for Stirling engines depends on its ability to provide a sufficient heat which is not produced by conventional burners because of the limitation of the contact surface between flame-hot head and the low methane content in the biogas. Therefore, a special microscale combustion chamber for this kind of burner is necessary. In this work, we studied numerically the biogas combustion by a Swiss-roll burner in a small-scale combustor. The temperature distribution versus the biogas injection rate is investigated. A significant increase of the flame temperature is obtained with an improvement of the uniformity of the temperature distribution and the burner efficiency.