Non-premixed Combustion System Research Articles

Theoretical analysis of transient counter-flow combustion system fueled by porous biomass particles in a non-premixed configuration under non-adiabatic conditions

The current research aims to implement a reliable analytical model in order to scrutinize the transient behavior of a counter-flow non-premixed combustion system fueled by wet porous biomass particles under non-adiabatic conditions, including several processes such as phase change and dehumidification. A special term is added to the heat equation based on the law of conservation of energy as a means to assess the convective heat loss impacts. In order to analyze the structure of the diffusion flame and predict the time-dependent temperature and mass fractions of reactants distributions, transient forms of energy and mass equations are solved through the asymptotic method. Eventually, the influences of key factors such as equivalence ratio, porosity factor of particles, and reactants Lewis numbers on the flame structure and transient behavior of diffusion flames are elaborately investigated in the non-adiabatic condition, considering convective heat loss and compared to those in the adiabatic condition. The findings reveal that increasing the porosity factor not only improves flame temperature but also leads the flame to be located further from the fuel nozzle. In addition, when the system reaches its steady-state condition, the temperature of the flame formed in the adiabatic condition is about 20 % higher than that formed in the non-adiabatic condition.

Influence of side dilution jets on swirling and non-swirling pulverised biomass turbulent annular flows

The use of pulverised biomass in non-premixed combustion systems, featuring turbulent annular flows (swirling, non-swirling) with downstream air dilution jets, is of immense practical importance. The overall combustion performance and emissions heavily rely on the underlying flow dynamics, turbulence and dispersion behaviour of the pulverised biomass particles. Despite the practical significance of side dilution jets in aiding complete combustion and controlling pollutants, the fundamental understanding of side dilution jets influence on the biomass particle velocity field, turbulence, and dispersion characteristics is still lacking. However, the presence of a multi-phase (solid-gas) flow field, highly turbulent annular flows, swirl, and cross-wise turbulent dilution jets poses significant challenges to these investigations. This work employs a combination of two-dimensional planar Particle Image Velocimetry (PIV) experiments and three-dimensional multi-phase numerical modelling to resolve the influence of turbulent side dilution jets (Red = 18,000 and 27,000) on the particle flow and dispersion characteristics of turbulent annular jets equipped with a central pulverised biomass-laden jet. The numerical modelling uses Reynolds stress model (RSM) and discrete phase model (DPM) for the solution of gas and dispersed phases, respectively. Walnut flour based raw pulverised biomass is injected at a certain particle loading ratio (Φp=0.33 and 0.49) through a turbulent central jet (Rj = 4500) into non-swirling (S = 0) and swirling (S = 0.3) turbulent confined annular flows (Res = 35,500 and 17,800). Comprehensive Constant Temperature Anemometry (CTA) experiments are conducted to obtain well-defined boundary conditions for experiments as well as numerical simulations. Prior to numerical predictions, the models are duly validated against the experimental data.Results reveal that the characteristic flow feature of side dilution jets, i.e., peripheral recirculation zones (PRZ) significantly entrains pulverised biomass particles from the turbulent particle-laden (central jet) annular flows (both swirling and non-swirling) and alters the particle flow and turbulence characteristics. Under non-swirling conditions (S = 0), side dilution jets reduce mean particle axial velocity by 22.4% and increase turbulent fluctuating velocity by 52.6%. These figures elevate to 32.5% and 100% respectively for swirling conditions (S = 0.3), depicting enhanced turbulent mixing between solid particles and the gas phase. In terms of particle dispersion, side dilution jets significantly enhance lateral dispersion of biomass particles, particularly under swirling flow conditions. In the PRZ region, side dilution jets led to a 108% and 279% increase in normalised particle count for non-swirling and swirling cases, respectively. These findings suggest that swirl doubles the particle dispersion in the PRZ. Furthermore, a 50% increase in Red enables side dilution jets to entrain more biomass particles and disperse them into the PRZ. Conversely, a 50% increase in ṁp leads to a significant increase in particle volume fraction along the central axis of the nozzle.

Mathematical modeling of the production of magnetic nanoparticles through counter-flow non-premixed combustion for biomedical applications

PurposeThe widespread usage of magnetic nanoparticles (MNPs) requires their efficient synthesis during combustion process. This study aims to present a mathematical model for the oxidation of MNPs in a counter-flow non-premixed combustion system to produce MNPs, where the key sub-processes during the oxidation reaction are involved.Design/methodology/approachTo accurately describe structure of flame and determine distributions of temperature and mass fractions of both reactants and products, equations of energy and mass conservations were solved based on the prevailing assumptions that three regions, i.e. preheating, reaction and oxidizer zones exist.FindingsThe numerical simulation was first validated against experimental data and characteristics of the combustion process are discussed. Eventually, the influences of crucial parameters such as reactant Lewis numbers, strain rate ratio, particle size, inert gas and thermophoretic force on structure of flame and combustion behavior were examined. The results show that maximum flame temperature can achieve 2,205 K. Replacing nitrogen with argon and helium as carrier gases can increase flame temperature by about 27% and 34%, respectively. Additionally, maximum absolute thermophoretic force was found at approximately 9.6 × 10–8 N.Originality/valueTo the best of authors’ knowledge, this is the first time to numerically model the preparation of MNPs in a counter-flow non-premixed combustion configuration, which can guide large-scale experimental work in a more effective way.

Analytical modelling of counter-flow non-premixed combustion of coal particles under non-adiabatic conditions taking into account trajectory of particles

Due to the extensive use of coal particles as a fossil fuel, reliable studies on combustion of coal particles can optimize the efficiency, safety and cost of coal-fired systems. In this study, a mathematical analysis is performed to model the trajectory of fuel particles in a multi-zone counter-flow non-premixed combustion system using an asymptotic approach. For this purpose, drag, buoyancy and gravity forces are considered in momentum equation. To obtain the temperature and mass profiles of fuel and oxidizer, a system of mass and energy balance equations are solved considering appropriate boundary and jump conditions. The effects of convective and radiative heat losses are included in the energy equation. Preheating, drying, and pyrolysis processes, heterogeneous and homogeneous reactions are investigated. Eventually, trajectories of char, gas and ash, and coal particles are tracked using Lagrangian concept. Based on the results, trajectory of particles mainly depends on the Stokes number, Stk. The analysis reveals that the damped oscillations of ash occur and ash might cross the flame front several times before leaving the system. Maximum temperatures of the system under adiabatic and non-adiabatic (in presence of both radiative and convective heat losses) conditions are equal to 2500 K and 1935 K, respectively.

Flamelet Model for a Three-Feed Non-premixed Combustion System with a Diluent Stream: Analysis and Validation of Quasi-Two-Dimensional Flamelet (Q2DF) Models

Three formulations of quasi-two-dimensional flamelet (Q2DF) models are derived from the two-dimensional flamelet formulation on the basis of assumptions regarding the third stream (diluent), and these models are validated by means of a two-dimensional direct numerical simulation (DNS) of a three-feed non-premixed combustion system into which the diluent is injected as the third stream. DNS combined with the Arrhenius formation (ARF) together with a detailed mechanism is also performed as a reference case and compared with the present models. The characteristics of the flamelets in three-feed non-premixed combustion are discussed in detail for ARF in terms of the three mixture fractions, temperature, and major and minor species. Some discrepancies appeared between the ARF and the Q2DF models. The difference in the cross-scalar dissipation rates between Q2DF and the two-dimensional flamelet model is considered the major reason for the deviations. The cross-scalar dissipation rates for the reactor streams an...

Effects of differential diffusion on nonpremixed-flame temperature

This numerical and analytical study investigates effects of differential diffusion on nonpremixed-flame temperatures. To focus more directly on transport effects the work considers a single irreversible reaction with an infinitely fast rate, with Schab–Zel’dovich coupling functions introduced to write the conservation equations of energy and reactants in a chemistry-free form accounting for non-unity values of the fuel Lewis number LF. Different flow configurations of increasing complexity are analyzed, beginning with canonical flamelet models that are reducible to ordinary differential equations, for which the variation of the flame temperature with fuel-feed dilution and LF is quantified, revealing larger departures from adiabatic values in dilute configurations with oxidizer-to-fuel stoichiometric ratios S of order unity. Marble’s problem of an unsteady flame wrapped by a line vortex is considered next, with specific attention given to large-Peclet-number solutions. Unexpected effects of differential diffusion are encountered for S < 1 near the vortex core, including superadiabatic/subadibatic flame temperatures occurring for values of LF larger/smaller than unity as well as temperature profiles peaking on the oxidizer side of the flame. Direct numerical simulations of diffusion flames in a temporal turbulent mixing layer are used to further investigate these unexpected differential–diffusion effects. The results, confirming and extending previous findings, underscore the nontrivial role of differential diffusion in nonpremixed–combustion systems.

Low-frequency combustion instabilities of an airblast swirl injector in a liquid-fuel combustor

ABSTRACT Low-frequency combustion dynamics of a prefilming airblast injector were experimentally investigated in a laboratory-scale liquid-fuel combustor operated with Jet A-1 fuel and air at atmospheric pressure and elevated temperatures. Our measurements reveal that multiple modes – ranging from 45 to 292 Hz – can be excited in the non-premixed combustion system, including the Helmholtz, longitudinal, and hydrodynamic instabilities. The system's propensity for mode selection depends strongly on combustor length and pilot equivalence ratio. A strong Helmholtz mode with a peak-to-peak amplitude of ∼14 kPa occurs, provided that the combustor length is relatively short and the pilot equivalence ratio is high. A high-amplitude, intermittent burst at approximately 12 Hz always develops when the system is on the verge of transition to the Helmholtz instability. With a longer combustor length, on the other hand, the system transitions to the ¾-wave longitudinal mode via a discontinuous mode-hopping process. The system undergoes well-defined, limit cycle oscillations with an extremely large pressure amplitude of ∼30 kPa, but the global OH* fluctuation amplitude is limited to merely 8.3%. Phase-resolved flame imaging measurements demonstrate that a ring-like partially-premixed flame emerges periodically in the dump region, when the non-premixed flame wrapping around the central recirculation zone is lifted off the fuel nozzle. The mutual interaction between the partially-premixed and non-premixed reaction zones is the dominant mechanism for intense sound generation. Under certain conditions, superposition of the Helmholtz and longitudinal modes occurs, leading to nonlinear interactions manifested by additional spectral peaks at the sum and difference of the two frequencies. In contrast to the other two cases, this superimposed tone is not characterized by limit cycle behavior, but by a noise-driven unsteadiness.

배기가스 재순환을 적용한 하이브리드 연소시스템에 대한 기초 연구

Abstract - In this study confirmed the reduction effect of pollutant by applying Fi-EGR and FPI-EGR to hybrid combustion system realizing premixed flame and non-premxied flame at once. The results showed that NOx emission index decreased significantly in case of adopting EGR. Additionally, the hybrid combustion system with EGR resulted in a better performance compared to usual non-premixed combustion system such that it can reduce NO x emission at equivalent EGR ratios. Especially, in the case of 25% of FI-EGR ratio at hybrid combustion system that the ratio of non-premixed and premixed is 50 : 50, NOx emission index reduction rate was about 59% compared to NO x emission of non-premixed combustion system without EGR and in the case of 15% of FPI-EGR ratio at hybrid combustion system that the ratio of non-premixed and premixed is 70 : 30, NO x emission index reduction rate was about 48% compared to NO x emission of hybrid combustion system without EGR. Key words : Hybrid combustion, Exhaust gas recirculation (EGR), NO

Conditional Moment Closure for Turbulent Premixed Flames

The conditional moment closure (CMC) method has been successfully applied to various non-premixed combustion systems in the past, but its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modeling of conditional scalar dissipation rate of the conditioning scalar, the progress variable. A simple algebraic model for the conditional dissipation rate is validated using DNS results of a V-flame. This model along with the standard k–ϵ turbulence modeling is used in computations of stoichiometric pilot stabilized Bunsen flames using the RANS-CMC method. A first-order closure is used for the conditional mean reaction rate. The computed non reacting and reacting scalars are in reasonable agreement with the experimental measurements and are consistent with earlier computations using flamelets and transported PDF methods. Sensitivity to chemical kinetic mechanism is also assessed. The results suggest that the CMC may be applied across the regimes of premixed combustion.

Bicomponent droplets evaporation: Temperature measurements and modelling

Development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to reducing the production of green-house gases and to meeting air quality standards for other emissions. In non-premixed combustion systems, vaporization of the droplets and mixing of the vapour with the surrounding oxidizer control the overall rate of energy release. Droplet vaporization is affected by the nature of the liquid petroleum that contains species having extremely different volatilities. A fine understanding of multi-component droplet vaporization is therefore an important issue to optimize the combustion systems. This paper presents the application of a recently developed technique to measure the temperature of bi-component droplets. Based on the three-color laser-induced fluorescence (LIF) technique, droplet temperatures can be measured regardless of the composition. The method requires adding a small amount of a fluorescent organic dye to the fuel which is composed of ethyl-alcohol and acetone. The accuracy of the measurement is about ±1.3 °C. In this study, the emphasis is placed on the evaporation of the binary mixture in a hot air plume, the temperature of which (around 650 °C) largely exceeds the boiling point of the liquid components. An extensive study of the influence of the initial composition and droplet diameter is carried out. Finally, the experimental results are compared to an evaporation model based on the discrete components approach.

Direct numerical simulation of turbulence–radiation interactions in homogeneous nonpremixed combustion systems

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (turbulence–radiation interactions—TRI). TRI arises from highly nonlinear coupling between fluctuations in temperature and species composition with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in a statistically homogeneous nonpremixed system. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE). Radiation properties correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations have been found to be significant. Emission TRI has also been estimated analytically using a β PDF model. The β PDF results are in good agreement with DNS results. This suggests that relatively simple models for emission TRI in nonpremixed systems might be developed based on a modeled equation for mixture fraction variance.

On the analysis of temperature dissipation in a turbulent jet propane flame

The analysis of a turbulent jet flame of propane in still air is reported with the aim of contributing to a better understanding of the interaction between chemistry and the turbulent structure of the flow in non-premixed combustion systems. The emphasis is put on time-resolved two-point temperature measurements, from which the dissipation of the temperature fluctuations was obtained. The results quantify the correlation of thermal dissipation with the fluctuations of the temperature, which suggest the likely influence of large scales mixing on the anisotropy of the thermal dissipation rate. In addition, the experiments reported here demonstrate the ability of compensated fine wire thermocouples to measure the dissipation rate of temperature in strongly radiating flames, where the use of optical techniques may be limited.

Linear-Eddy modeling of nonequilibrium turbulent reacting flows with nonpremixed reactants

Interactions between turbulent mixing and finite-rate chemistry effects leading to highly nonequilibrium behavior of nonpremixed flames in incompressible, homogeneous turbulence are simulated by utilizing the linear-eddy model (LEM). Computations reveal that as turbulent mixing increases the local scalar dissipation rate, the mean temperature and reaction rate decrease rapidly at the flame location and, depending on the activation energy, the flame either reignites or extinguishes. The LEM results are also compared to predictions from the conditional moment closure (CMC) model and reveal interesting differences between these models near conditions of flame extinction. Specifically, for the chemistry under consideration and within the context of the comparisons to the LEM simulations, the results highlight the importance in the CMC approach of modeling the conditionally averaged mixture-fraction dissipation rate and accounting for conditional fluctuations in the chemical reaction rate term in nonpremixed combustion systems near extinction.

A laser anemometer seeding technique for combustion flows with multiple stream injection

In nonpremixed combustion systems, fuel and oxidant are injected into the reaction chamber via two or more separate streams. Velocity measurements by laser anemometry within the chamber may be biased by individual streams unless the seeding concentrations of both streams are uniform and constant. In order to meet this requirement, a seeding technique has been developed to disperse 1-μm metal oxide particles at a controllable rate. Evaluations of the steadiness of particle generation indicate rms fluctuations are within 1.5% of the particle generation rate. Demonstration of the technique has been conducted in both nonreacting and reacting flows.