Ignition Front Propagation Research Articles

The Formation of a Flame Front in a Hydrogen–Air Mixture during Spark Ignition in a Semi-Open Channel with a Porous Coating

An experimental study of ignition and flame front propagation during spark initiation in a hydrogen–air mixture in a semi-open channel with a porous coating is reported. The bottom surface of the channel was covered with a porous layer made of porous polyurethane or steel wool. The measurements were carried out for a stoichiometric mixture (equivalence ratio ER = 1.0) and for a lean mixture (ER = 0.4) of hydrogen with air, where ER is the molar excess of hydrogen. The flame front was recorded with a high-speed camera using the shadow method. Depending on the pore size, the velocity of the flame front and the sizes of disturbances generated on the surface of the flame front were determined. Qualitative features of the deflagration flame front at ER = 0.4, consisting of disturbances resembling small balls of flame, were discovered. The sizes of these disturbances significantly exceed the analytical values for the Darrieus–Landau instability. The effect of coatings made of porous polyurethane or steel wool is compared with the results obtained for an empty smooth channel. Depending on the hydrogen concentration in the hydrogen–air mixture, the velocity of the flame front compared to a smooth channel was three times higher when the channel was covered with steel wool and five times higher when the channel was covered with porous polyurethane.

A numerical investigation on heterogeneous combustion of aluminum nanoparticle clouds

This study investigated the heterogeneous combustion of nano-sized aluminum dust clouds. A numerical model was developed to capture the combustion properties of aluminum nanoparticle clouds by employing multiphase flow approach and combustion model of nano-aluminum particle. The simulation framework adopts an accurate modeling of heat convection in the transition and free-molecular regimes besides considering inter-particle radiation and surface reaction. The numerical model was validated by comparison against predicted/experimental results. The flame structure of nano-aluminum dust-air mixture has been demonstrated, whose flame thickness is thinner than their micron-sized counterparts. The ignition front propagation speed increases with raising equivalence ratio in the lean mixture and almost maintains a constant under fuel-rich conditions; and it raises with decreasing particle size and increasing oxygen concentration. The oxygen content plays a significant role on flame behaviors than the particle concentration and size. The flame temperature and feedback of heat from reaction zone to preheat zone plays a crucial role in ignition front propagating process.

Thermochemical conversion of coal and biomass blends in a top-lit updraft fixed bed reactor: Experimental assessment of the ignition front propagation velocity

Co-thermochemical conversion of coal and biomass can potentially decrease the use of fossil carbon and pollutant emissions. This work presents experimental results for the so-called top-lit updraft fixed bed reactor, in which the ignition front starts at the top and propagates downward while the gas product flows upwards. The study focuses on the ignition front propagation velocity for the co-thermochemical conversion of palm kernel shell and high-volatile bituminous coal. Within the range of assessed air superficial velocities, the process occurred under gasification and near stoichiometric conditions. Under gasification conditions increasing coal particle size from 7.1 to 22 mm decreased ignition front velocity by around 26% regardless of the coal volume percentage. Furthermore, increasing coal volume percentage and decreasing coal particle size result in product gas with higher energy content. For the operation near stoichiometric conditions, increasing coal volume percentage from 10 to 30% negatively affected the ignition front velocity directly proportional to its particle size. Additional experiments confirmed a linear dependence of ignition front velocity on air superficial velocity. Further steps in the development of the top-lit updraft technology are implementing continuous solids feeding and variable cross-sectional area and optimizing coal particle size distribution.

Numerical study of preheating primary air on pinewood and corn straw co-combustion in a fixed bed using Eulerian-Eulerian approach

The effect of preheating primary air on the co-combustion characteristics of a 50–50% blend of pinewood and corn straw in a fixed bed. The primary air temperatures were assessed from 20 to 130 °C. The co-combustion characteristics were included the co-combustion behaviors and emissions. In order to reveal the features of the combustion process in the porous bed, a two-dimensional unsteady state model was employed to investigate the combustion process in a fixed bed of blended biomass on the combustion process in a fixed bed reactor. Conservation equations of the bed were implemented to describe the combustion process. The gas phase turbulence was modeled using the k-ε turbulent model and the particle phase was modeled using the kinetic theory of granular flow. Results showed that by increasing primary air temperature the residual mass on bed decreased, while the average burning rates and ignition front propagation velocity increased At the primary air temperature of 85 °C the smallest unburned carbon was left in the ash, and the emissions of nitrogen-compounds were relatively small. In contrast, the primary air temperature of 85 °C was found to be well-operating condition, which can be suggested for industrial boiler during blend co-combustion. The simulation results were then compared with experimental data for different temperature, which shows that the combustion process in the fixed bed is reasonably simulated. The simulation results of solid temperature, gas species and process rate in the bed are accordant with experimental data.

Effects of initial temperature on ignition and flame propagation of dual-fuel mixture in mixing layer

The study aims to identify the characteristics of ignition and flame propagation in an n-heptane/methane–air dual-fuel mixture based on a specified mixing layer scheme using a group of numerical simulations by incorporating a skeletal chemistry mechanism. The effects of different temperature stratifications and uniform temperature distribution on autoignition kernel development, flame propagation, and flame structure in the dual-fuel combustion have been comprehensively investigated. The results indicate that with the increase in the oxidizer stream temperature, the high-temperature ignition (HTI) location in the mixing layer changes non-monotonously, as determined by the most reactive mixture fraction in the mixture fraction–temperature (ξ−T) space obtained from zero-dimensional simulation. The HTI points located in the negative temperature coefficient (NTC) region shift to a richer mixture with temperature increase, whereas the points located in the high temperature region shift to the smaller mixture fraction region. Additionally, the results demonstrate that for the ignition points in the NTC region, the shift to the richer mixture weakens the combustion rate, resulting in a prolonged time interval for steady flame propagation although the oxidizer stream temperature is increased. Furthermore, the low-temperature ignition (LTI) front propagation is driven by the gradient of the ignition delay time, and the HTI propagates as a spontaneous ignition front at first and then transforms to a diffusion-driven flame. The results also demonstrate that the maximum temperature of the LTI propagates to the richer mixture with temperature stratification, in contrast to that with a uniform temperature. Lastly, the comparative analyses of chemical explosion mode (CEM) and flame structure are conducted for two representative cases. A wide non-CEM region induced by low-temperature chemistry (LTC) occurs in the oxidizer stream when its temperature is relatively lower. And double peaks of heat release rate (HRR), that represent the HTI of n-heptane and post-combustion of methane respectively, are observed within the ignition front at the oxidizer side.

Numerical simulation of a mixed-mode reaction front in a PPC engine

The ignition process, mode of combustion and reaction front propagation in a partially premixed combustion (PPC) engine running with a primary reference fuel (87% iso-octane, 13% n-heptane by volume) is studied numerically in a large eddy simulation. Different combustion modes, ignition front propagation, premixed flame and non-premixed flame, are observed simultaneously. Displacement speed of CO iso-surface propagation describes the transition of premixed auto-ignition to non-premixed flame. High temporal resolution optical data of CH2O and chemiluminescence are compared with simulated results. A high speed ignition front is seen to expand through fuel-rich mixture and stabilize around stoichiometry in a non-premixed flame while lean premixed combustion occurs in the spray wake at a much slower pace. A good qualitative agreement of the distribution of chemiluminescence and CH2O formation and destruction shows that the simulation approach sufficiently captures the driving physics of mixed-mode combustion in PPC engines. The study shows that the transition from auto-ignition to flame occurs over a period of several crank angles and the reaction front propagation can be captured using the described model.

A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y′) and wavelength (0.01 mm<λ<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y′, λ) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion–reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called β-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y′, λ) parameter space while the β-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the β-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified β-curve for turbulent cases. The practical output of the paper is the β-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations.

Fixed-bed Biomass Combustor: Air Mass Flow Rate and Particles Size Effects on Ignition Front Propagation of Solid Olive Waste

ABSTRACT This study analyzed the biomass flame behavior in a vertical fixed-bed combustor. Experimental tests were carried out with olive waste (OW) as solid fuel in different combustion conditions. The effect of air mass flow (AMF), particles size and bulk density on mass ignition rate (MIR), maximum temperature, ignition front velocity (IFV) and bed height was investigated. Comparison with other experimental results given in the literature is carried out. The results showed the strong effect of AMF detected on the conversion behavior of tested biomass. It is shown that the increase in the AMF distinguishes three different phases. Firstly increasing in maximum temperature, IFV and MIR, it is the acceleration phase. Secondly corresponding to convective cooling phase where all parameters decrease lightly and the last quasi-constant phase, correspond to the reaction-limited phase. Finally, The five experimental tests with different particles size and various bulk densities were conducted. It is shown that the large particles have higher IFV than small particles.

Front velocity in the combustion of blends of poultry litter with straw

The paper presents experimental studies on the burning characteristics of poultry litter and its blends with straw in the context of their potential usage in small-scale heat and power generation. For comparison purposes classic fuel such as wooden pellets was also investigated. The samples were analyzed in terms of water content, chemical composition and devolatilization, heating values, ash composition and ignition temperatures. An experimental analysis was performed of the combustion process of various sample beds in a small laboratory reactor that covered the temperature measurements and visual observations. This resulted in estimation of the speeds of ignition front propagation in the studied fixed beds, which is of general importance in the design of grate-fired furnaces. The combustion dynamics was discussed in terms of the physical properties of the bed, such as bulk density and porosity. The authors concluded that the main issue regarding the efficiency of poultry litter combustion was closely connected to a low bulk density of the fuel bed, which led to faster combustion front propagation but lower temperatures compared to a more dense fuel bed such as a wood pellet bed.

Thrust generation experiments on microwave rocket with a beam concentrator for long distance wireless power feeding

Experiments using a 1 MW-class gyrotron were conducted to examine a beamed energy propulsion rocket, a microwave rocket with a beam concentrator for long-distance wireless power feeding. The incident beam is transmitted from a beam transmission mirror system. The beam transmission mirror system expands the incident beam diameter to 240 mm to extend the Rayleigh length. The beam concentrator receives the beam and guides it into a 56-mm-diameter cylindrical thruster tube. Plasma ignition and ionization front propagation in the thruster were observed through an acrylic window using a fast-framing camera. Atmospheric air was used as a propellant. Thrust generation was achieved with the beam concentrator. The maximum thrust impulse was estimated as 71 mN s/pulse from a pressure history at the thrust wall at the input energy of 638 J/pulse. The corresponding momentum coupling coefficient, Cm was inferred as 204 N/MW.

Numerical Modeling and Simulation of Combustion Phenomena Related to Thermal Ignition and Flame Fronts

In this study, we consider problems of thermal ignition and flame front propagation. Ordinary and partial differential equations with proper initial and boundary conditions are solved for generic cases. Numerical schemes are tested and results are discussed. Comparisons with the literature for benchmark cases are favorable.

Numerical study of the ignition front propagation of different pelletised biomass in a packed bed furnace

In the current study, a 3D numerical model for biomass combustion in a packed bed furnace has been developed to study combustion of four types of biomass fuels (wood pellet, poplar pellet, olive stone and pine shavings) with different chemical composition and physical properties. A number of sub models representing various processes of the fuel thermal conversion were included through user defined subroutines coupled to commercial CFD software AVL Fire. Ignition front propagation velocity, maximum flow temperature, effect of excess air and moisture content have been studied with the change of air flow rate and compared with an experimental analysis of biomass fixed bed combustor. It is found that, air mass flow greatly affects ignition front velocity and maximum values are achieved under sub-stoichiometric condition. Highest ignition front velocity is achieved by pine shavings due to its smaller particle size and bed solid fraction. The numerical model reasonably predicted ignition front velocity in the oxygen-limited regime. In the reaction limited regime, simulation slightly over estimates the ignition front velocities except for the fuel pine shavings, but it followed the measured trend. Beyond the reaction limited regime, further increase in air mass flow forces combustion to extinguish due to increased convection cooling and a downward trend of the ignition front velocity is observed. Reason for the over predictions in reaction limited and cooling by convection regimes for the larger diameter fuels is explained.

Investigation of the combustion characteristics of Bati Raman oil with sand

In situ combustion (ISC) is one of the thermal oil recovery processes that has been applied in a variety of reservoirs. The energy is generated within the reservoir by a series of multi-step reactions that provide energy to heat the reservoir and reduce oil viscosity. Also, the oil is upgraded in situ by burning heaviest fractions.In this study, combustion characteristics of Bati Raman crude are investigated. The Bati Raman oil field is the largest known oil field in Turkey. Ramped temperature oxidation (RTO) experiments with effluent gas analysis (EGA) are performed with oil-sand mixtures having different sand particle size. The results are analyzed using isoconversional kinetic approach, which is a model-free method, based on effluent gas analysis and temperature measurements. Effective activation energy values are estimated for the sample with two different sand particle size. Besides, for the same samples, laboratory combustion tube experiments are conducted to evaluate ignition and combustion front propagation characteristics. Asphaltene content and API gravity of the oil sample prior to experiments and produced oil are measured and compared. Results indicate that the oil is upgraded through a reduction in asphaltene content.A complete set of kinetics and combustion experiments are provided for Bati Raman oil. There are only limited number of studies in the literature to link kinetic experiments to proposed reaction models of ISC. This work presents a way to link a reaction model to isoconversional fingerprint obtained through kinetic experiments. In addition, it is shown that Bati Raman oil exhibits good combustion characteristics and it is associated with its isoconversional fingerprint.

ReaxFF based molecular dynamics simulations of ignition front propagation in hydrocarbon/oxygen mixtures under high temperature and pressure conditions.

In this paper, we present the first atomistic-scale based method for calculating ignition front propagation speed and hypothesize that this quantity is related to laminar flame speed. This method is based on atomistic-level molecular dynamics (MD) simulations with the ReaxFF reactive force field. Results reported in this study are for supercritical (P = 55 MPa and Tu = 1800 K) combustion of hydrocarbons as elevated pressure and temperature are required to accelerate the dynamics for reactive MD simulations. These simulations are performed for different types of hydrocarbons, including alkyne, alkane, and aromatic, and are able to successfully reproduce the experimental trend of reactivity of these hydrocarbons. Moreover, our results indicate that the ignition front propagation speed under supercritical conditions has a strong dependence on equivalence ratio, similar to experimentally measured flame speeds at lower temperatures and pressures which supports our hypothesis that ignition front speed is a related quantity to laminar flame speed. In addition, comparisons between results obtained from ReaxFF simulation and continuum simulations performed under similar conditions show good qualitative, and reasonable quantitative agreement. This demonstrates that ReaxFF based MD-simulations are a promising tool to study flame speed/ignition front speed in supercritical hydrocarbon combustion.

Direct numerical simulations of autoignition in stratified dimethyl-ether (DME)/air turbulent mixtures

In this paper, two- and three-dimensional direct numerical simulations (DNS) of autoignition phenomena in stratified dimethyl-ether (DME)/air turbulent mixtures are performed. A reduced DME oxidation mechanism, which was obtained using rigorous mathematical reduction and stiffness removal procedure from a detailed DME mechanism with 55 species, is used in the present DNS. The reduced DME mechanism consists of 30 chemical species. This study investigates the fundamental aspects of turbulence-mixing-autoignition interaction occurring in homogeneous charge compression ignition (HCCI) engine environments. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. The computational configuration corresponds to a constant volume combustion vessel with inert mass source terms added to the governing equations to mimic the pressure rise due to piston motion, as present in practical engines. DME autoignition is found to be a complex three-staged process; each stage corresponds to a distinct chemical kinetic pathway. The distinct role of turbulence and reaction in generating scalar gradients and hence promoting molecular transport processes are investigated. By applying numerical diagnostic techniques, the different heat release modes present in the igniting mixture are identified. In particular, the contribution of homogeneous autoignition, spontaneous ignition front propagation, and premixed deflagration towards the total heat release are quantified.

Biomass Fuel and Combustion Conditions Selection in a Fixed Bed Combustor

The biomass market has experienced an increase in development, leading to research and development efforts that are focused on determining optimal biofuel combustion conditions. Biomass combustion is a complex process that involves divergent parameters and thus requires the use of advanced analysis methods. This study proposes combining grey relational analysis (GRA) and error propagation theory (EPT) to select a biofuel and its optimal combustion conditions. This research will study three biofuels that are currently used in a region of South Europe (Spain), and the most important variables that affect combustion are the ignition front propagation speed and the highest temperature that is reached at the fixed bed combustor. The results demonstrate that a combination of both theories for the analysis of solid-state thermochemical phenomena enables a fast and simple way of choosing the best configuration for each fuel.

Direct numerical simulation of lean hydrogen/air auto-ignition in a constant volume enclosure

This paper reports on two-dimensional (2D) and three-dimensional (3D) direct numerical simulations (DNSs) of the auto-ignition process of a lean H2/air mixture with temperature stratification in a constant volume enclosure. Detailed chemistry and transport properties are taken into account in the simulations. The combined propagation of spontaneous ignition front and deflagration front is identified and the relation between the reaction front displacement speed and the temperature gradient is verified. The difference between 2D- and 3D-DNS is investigated by comparing the evolutions of global combustion parameters such as the averaged heat release rate, total reaction front area and the averaged displacement speed of the reaction front. The extra spatial dimension in 3D-DNS has been shown to cause a higher velocity strain rate to enhance the heat transfer process, which leads to a delayed but more rapid ignition of the mixture than the 2D-DNS cases. The 3D reaction front surfaces are examined based on the local mean and Gaussian curvatures. By introducing a cutoff Gaussian curvature two types of 3D surface elements, the small sphere fronts and the strong saddle fronts, are defined. The effect of these fronts on the combustion process is studied in terms of their contribution to the total reaction front area, fuel consumption rate and curvature-induced stretch.

Experimental analysis of several biomass fuels: The effect of the devolatilization rate on packed bed combustion

An experimental analysis of the ignition front propagation of eight biomass fuels in a one-dimensional fixed-bed combustor (batch reactor) is carried out and these biomass materials are also researched by thermogravimetry to study their thermal behaviour during devolatilization. The analysis of the thermogravimetric data has been centered on the results obtained for 50 °C · min−1 (maximun heating rate of the thermogravimetric (TG) equipment), taking into consideration that it is the nearest heating rate value to the one at which the heating process is carried out in the batch reactor. The influence of the TG-determined devolatilization rate in the burning behaviour of biomass in situations of packed bed combustion is analyzed. This study reveals that the biomass fuels with higher devolatilization rates during pyrolysis are the ones with a higher velocity of propagation of the ignition front during combustion processes in the fixed-bed combustor. The thermal decomposition characteristics during devolatilization are also evaluated, revealing that the most reactive biomass fuels upon pyrolysis are the ones which also produce the higher volatile yield during TG experiments at 50 °C · min−1.

Study of a Fixed-Bed Biomass Combustor: Influential Parameters on Ignition Front Propagation Using Parametric Analysis

This paper presents a study of the parameters influencing the combustion behavior of an experimental counter-current fixed-bed combustor. An innovative parametric analysis has been conducted to examine a wide range of parameters from very different biomasses. This analysis based on the parametrization of front propagation velocity versus the airflow rate curve not only allows fuel properties to be compared in terms of maximum or minimum values of ignition or air flux but also enables the shape of their performance curve to be examined. The study of the data has identified the moisture content and particle size as the most influential parameters. These factors strongly determine the operational range of the fuel in terms of air mass flux and ignition front propagation velocity. Furthermore, a new parameter called char stoichiometry is defined to examine fuel combustion regimes. This factor represents the stoichiometric ratio of the air and char fraction (assuming the fuel is pure carbon). The results obtai...

Modelling of solid fuel stoves

This work presents a detailed theoretical study of solid fuel combustion in a domestic stove (make Harsha, RRL, Bhubaneswar, India). In this work different steady state as well as unsteady state combustion models have been formulated, which include the description of all the chemical and physical processes taking place during the solid fuel combustion inside the stove. Steady state models involve the calculation of effective maximum flame temperature, suction of combustion air created by hot flue gases inside the stove and the propagation of ignition front inside the stove. Unsteady state mathematical model involves all the processes occurring simultaneously or sequentially during the solid fuel combustion such as moisture evaporation, devolatilization, pyrolysis and homogeneous and heterogeneous combustion reactions. This unsteady state model gives the temperature profiles at different locations inside the stove and fuel mass loss (combustion) rate, which can be further used to calculate the power delivery by the stove during combustion. The model shows good agreement with the experimental results. These models can be used to find the effect of stove geometry and fuel properties on the stove performance parameters such as effective maximum flame temperature, suction created inside the stove, propagation of ignition front inside the stove, and fuel burn rate, which play an important role in the design of such stoves for maximum thermal efficiencies.