Normal Diffusion Flame Research Articles

Effects of ammonia addition on the soot nanostructure and oxidation reactivity in n-heptane/toluene diffusion flames

Co-firing NH3 with conventional hydrocarbon fuels is an important approach for reducing CO2 emissions in existing combustion systems. Besides CO2, the blending of NH3 would also notably affect soot formation and its oxidation behaviors. In the present study, we focus on the effects of NH3 on the nanostructure and oxidation characteristics of soot produced in diffusion flames of n-heptane/toluene mixtures. Two configurations of laminar co-flow diffusion flame, including both normal and inverse diffusion flames (NDF and IDFs), were used for investigation. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (Raman), and Thermogravimetric analysis (TGA) were employed for soot characterization. The HRTEM and Raman spectra showed that with the increase of NH3 blending ratio, the fringe length (La) and the degree of graphitization decreased while the microcrystal tortuosity (Tf) increased. The results are in consistent with TGA analysis which suggests the promoting effects of NH3 on the soot oxidation reactivity. Difference between NDF and IDF with respect to the soot nanostructure and oxidation activity were discussed. It is our hope that the present results could deepen our understanding on the effects of NH3 on soot nanostructure and oxidation behavior and benefit the design of particulate filters for combustion devices fueled with hydrocarbon/NH3 mixtures.

Numerical investigation of normal diffusion flame of bio-hythane

Numerical investigation of normal diffusion flame of bio-hythane

Assessing PAHs-based soot inception models in various laminar non-premixed flame configurations

Accurate soot modeling is challenging due to the complex underlying physical and chemical processes. To improve the fidelity of the soot prediction in inverse coflow flame configurations, an empirical reactive soot inception model with radical involvement was developed in our previous work [Guo et al. Combust Flame 254 (2023) 112,853]. In this study, the model is further assessed by comparing the predictions of polycyclic aromatic hydrocarbons (PAHs) and soot parameters, including volume fraction, number density, and particle size, with measurements in various laminar diffusion flame configurations. These configurations encompass counterflow diffusion flames (CDF), normal coflow diffusion flames (NDF), and inverse coflow diffusion flames (IDF). The empirical reactive inception model is also compared to the reported irreversible and reversible inception models. The results show that the empirical reactive inception model with radical involvement most accurately capture axial soot behavior in IDF configuration, while irreversible and reversible models predict a sustained increase in soot mass, inconsistent with measurements. In CDF configurations, the empirical reactive model and the reported reversible model have better performance than the irreversible inception model, regarding the predictions of spatial distributions of soot and PAHs. Moreover, in NDF configuration, the soot peaks predicted by the reactive and reversible models are in reasonable agreement with measurements within a factor of 2–3. For the soot spatial distribution in NDF, the difference in the predictions is mainly reflected in the flame wings, which is caused by the temperature dependences of the inception rate provided by the different models.

Modelling of the Flame Synthesis of Single-walled Carbon Nanotubes in Non-premixed Flames with Aerosol Catalyst

The use of aerosol catalyst in the flame synthesis of carbon nanotube (CNT) is known to yield single-walled CNT (SWCNT) that is useful for various applications. Modelling works are needed to optimize operating conditions for SWCNT growth but are unavailable. Therefore, a baseline model for the aerosol-catalyst system in flames is developed and the effect of oxygen on SWCNT growth is investigated. A baseline flame model for a normal diffusion flame with 24% oxygen concentration at the inlet is established via Computational Fluid Dynamic simulation. A dispersed phase model (DPM) is employed to simulate the entrainment of catalyst particles. The flame model is coupled with a published CNT growth rate model to predict the CNT growth rate at each particle. Inlet oxygen concentration is varied from 19% to 27% to study the effect of oxygen on SWCNT growth. Satisfactory validation of the baseline flame shape and temperature is established. Results show that particle 3 for the baseline case yields the highest CNT length compared to other particles due to the suitable path for the synthesis. The particles are classified based on the shortest time residence, moderate and longest time residence. Increasing oxygen concentration from 19% to 27% results in a 30% decrease in CNT length for particle 3 for each inlet condition due to lower carbon precursor and composition in the flame. Furthermore, the results showed that regardless of burner operating conditions, high SWCNT growth is consistently predicted between 120-140 mm HAB, which indicates the existence of an optimum range of species concentration for SWCNT growth in aerosol-based flame synthesis. Thus, it can be inferred that SWCNT growth in the aerosol–based method is highly dependent on carbon source and moderately dependent on temperature

Effects of equivalence ratios on the normal and inverse diffusion flame of acid gas combustion in the pure oxygen atmosphere

The experimental studies on the effect of equivalence ratios to the acid gas (H2S and CO2) combustion in the pure oxygen atmosphere was presented in a coaxial jet double channel burner. Three equivalence ratios (Φ = 0.8, 1.0 and 1.5) are examined to analyze the distribution of the flame temperature and gas composition in the normal and inverse diffusion flame along central axial (R = 0.0) and axial line at 3 mm (R = 0.75) in radial direction. The results revealed that acid gas combustion mainly occurred chemical decomposition of H2S and oxidation of H2S and H2 at R = 0.0, while mainly occurred H2S and H2 oxidation at R = 0.75 in the normal diffusion flame. Reducing Φ increased the flame temperature and it is higher at R = 0 than that at R = 0.75 because of heat loss. It also increased the volume fraction of CO, H2 and COS in the flame combustion area, while decreased downstream reactor because of occurring oxidation. CO was formed by the reaction of CO2 and H, and H2 primarily derived from chemical decomposition of H2S. COS was generated by the reaction of CO2 with SH, H2S and S as well as the reaction of CO with SO and SH at R = 0.0, while was mainly formed by the reaction of CO with SO and SH at R = 0.75. H2S mainly occurred the oxidation in the inverse diffusion flame. The temperature at R = 0.0 was still higher than that at R = 0.75, and it was higher than that in the normal diffusion flame in the combustion area. Increasing Φ promoted the formation of CO, H2 and COS, and each gas under Φ of 1.5 was higher significantly. The Φ had no significant effect on the distribution of SO2 compared to the normal diffusion flame, but changed the distribution of CO, H2 and COS. It can be inferred that the content of CO, H2 and COS will be more higher under Claus condition in the inverse diffusion flame.

Assessment of physical soot inception model in normal and inverse laminar diffusion flames

Despite the extensive studies, accurate and reliable modeling of the soot inception process, especially at high pressure conditions, amenable to multi-dimensional flame simulations remains a challenge. In this study, the physical inception model was comprehensively evaluated in the fully-resolved simulations of laminar normal diffusion flame (NDF) and inverse diffusion flame (IDF) at elevated pressures. The effects of inception models on polycyclic aromatic hydrocarbons (PAHs) and soot predictions were quantitatively analyzed, including the selection of soot precursors and collision efficiency models. The results show that the quantitative PAH predicted by different collision efficiency models can differ by an order of magnitude. Compared to the constant efficiency, the temperature-dependent collision efficiency was found to improve the quantitative PAH predictions and the prediction of the spatial soot distribution in NDF, with an increased level of soot on the flame centerline. The inclusion of small-sized PAH species (such as A2, A2R5, and A3) as soot precursors was also found to improve the quantitative prediction of soot volume fraction. The physical inception model performs well in NDF using the optimal parameters. Moreover, simultaneous measurements of PAH and soot were performed in IDF configuration for the evaluation of the physical inception model. Contrary to NDF, PAHs and soot are formed on the outer side of the flame and cannot be oxidized in IDF. The experiment observed that the PAHs concentration increased in the post-flame region, while the soot concentration remained unchanged. However, the opposite trend was obtained in simulations, that is, the PAHs concentration decreased while the soot concentration increased, because the physical inception model predicts the inception behavior in the post-flame area, resulting in persistent transformation of PAHs into soot particles. To improve the predictions in IDF, the radical effects in the inception process need to be considered in the model.

Soot production in high pressure inverse diffusion flames with enriched oxygen in the oxidizer stream

Catalyst degradation due to soot formation is one of the main issues in the autothermal reforming (ATR) process, which is widely regarded as the future technology for hydrogen production from natural gas. In this work, soot formation under conditions similar to ATR was systematically investigated, focusing specifically on the effects of pressure on soot formation in inverse diffusion flames (IDFs) under oxygen rich conditions. Methane was diluted with carbon dioxide; the oxygen content in the oxidizer stream varied from 55 to 70%-by-mol. Polycyclic aromatic hydrocarbon (PAH) and soot concentrations in the flames were measured by laser-induced fluorescence (LIF) and laser-induced incandescence (LII) respectively. Flame images showed that, as the pressure increased, the luminous region of the IDFs moved downward to mask the blue reaction region, and the flames became narrower. The degree of flame narrowing in the IDFs was milder than normal diffusion flames (NDFs). LIF measurements showed that increasing the pressure promoted PAH formation, which also subsequently promoted soot formation. Both PAH and soot formation increased linearly with pressure. The linear relationship was different from that of the NDFs. Flame simulations suggested that the promotion of soot formation with pressure was largely driven by PAH adsorption. Under the conditions of this study, lowering the oxygen content promotes soot formation in the IDFs. The results of this work contribute to the understanding of soot formation in IDFs at elevated pressures and the optimization of the ATR process.

Co-flow jet diffusion flames in a multi-slot burner: Flow field and emissions

Motivated by the application of internally air-assisted flares and recent anomalous data that black carbon (BC) emissions changes are not monotonic with the amount of air added inside the fuel stream, a burner was designed to study the flow, emissions, and stability aspects of the existence of both normal and inverse jet diffusion flames in close proximity. Since the radius of curvature of a burner affects all aspects of combustion, such as dynamics, stability, flame structure, and emissions, a slot burner configuration, inspired by the Wolfhard-Parker burner, was adopted. This multi-slot burner consists of five parallel rectangular slots (i.e., the central slot for the inner air, sandwiched between two fuel slots, and surrounded by two outer air slots). When operated with laminar fuel and air flows, this burner produced flame sheets at each fuel-air mixing layer with open optical access to all the flows. Flow fields were characterized using two-dimensional two-component particle image velocimetry, while simultaneous single-lens reflex photography was used to establish the overall height and the location of the flames. Emission measurements specifically targeted BC, a precursor of soot, and oxides of nitrogen (NOx). The experimental test conditions involved constant flows of outer air and propane, and variable inner air flow. At one extreme, i.e., zero inner air flow, two flame bases were produced at the interfaces between the fuel and outer air streams. These flames merged to produce a normal diffusion flame. With the addition of inner air, two more flame bases appeared (referred to as the inner flame) and formed an inverse diffusion flame. With increasing inner air flow, the inner flame transitioned from a closed-tip flame, to an open-tip flame, and eventually to a lifted flame. With increasing inner air flow, the NOx emissions remained constant while the BC emissions, which were 3 g/kg-fuel with no inner air, rose by an order of magnitude as it became an open-tip flame. Only when the inner flame finally lifted off due to the inner air flow, did the BC emissions collapse to near zero. Phenomenological models associated with the importance of partial premixing were proposed to explain this collapse, thereby generalizing this finding to other combustion systems when attempting to reduce BC emissions through secondary internal air addition.

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁtotal, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, Tad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁtotal from 2.3 to 11.3 mg/s, and Tad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m2-s for normal flames, and an oxygen-based mass flux of 0.68 g/m2-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)

Experimental investigation of soot formation in inverse diffusion flames of CO2 and N2 addition: Isolation of dilution and thermal effects

ABSTRACT This experimental study compares the flame structure, temperature, and soot formation characteristics of normal (NDFs) and inverse diffusion flames (IDFs), evaluates the effect of CO2 addition, and isolates the thermal and dilution effects of CO2 and N2 addition on soot formation in IDFs. A hyperspectral imager using the TR-GSVD algorithm measures IDFs with different CO2 addition (XD) in the oxidant, with N2 addition as the comparison. In contrast to NDFs, IDFs have an opened tip, and the visible flame height and flame width decrease as XD grows. IDFs have a higher peak temperature than NDFs, but their peak soot volume fraction is substantially lower. Compared with N2 addition at the same XD, CO2 addition narrows IDFs and makes the reaction zone visible, and the peak temperature of CO2 addition is 350 K lower than that of N2 addition. The suppression effect of CO2 on soot formation is more potent than N2, with a soot formation rate of 30% that of N2 addition and a soot loading of 20–60%. The activation energy of soot formation reaction with CO2 addition is higher than N2 addition. The isolating results reveal that the thermal effect contributes to the main suppression effect of CO2 addition on soot formation, while the dilution effect of N2 addition is stronger than its thermal effect.

An investigation of thermal radiation from laminar diffusion flames in a tri-coflow burner with central oxygen

In natural gas combustion processes, oxygen-enriched combustion is often used to improve heat transfer and thermal efficiency. In this investigation, a triple-port burner is employed to investigate the effect of oxygen enrichment on thermal radiation when introducing oxygen into the center of a normal coflow diffusion flame, while maintaining an outer coflow of air. Two fuels are investigated: methane, being the primary constituent of natural gas and having a low propensity for soot formation, and ethylene, which acts as a reference fuel with a much higher propensity for soot formation. The flow rates of air in the outer annulus and fuel in the inner annulus are held constant, while the flow rate of oxygen in the central tube is systematically increased. Total radiation, which is converted to radiation loss fraction, is measured using a wide-angle radiometer. Soot volume fraction and soot temperature are measured using two-color ratio pyrometry with a digital camera.For low flow rates of central oxygen when an inner oxygen-fuel flame is apparent, the thermal radiation is increased due to an increase in soot concentration. At these low flow rates, thermal radiation is increased by 20% for methane and 26% for ethylene, as compared to a normal coflow flame without central oxygen. The increase in radiation from these flames results in a decreasing, or relatively constant soot temperature in the soot laden region as the increased soot concentration radiates heat from the flame, even though gas temperatures upstream near the central oxygen flame are quite high. For relatively high flow rates of central oxygen, radiation from the tri-coflow methane flame increases by up to 31% compared to a normal coflow flame, while radiation from the ethylene flame monotonically decreases. For both fuels, when oxygen flow rates are high, soot concentration decreases (due to a change in flame shape and enhanced oxidation rates) and soot temperature increases. Product gases are the dominant emitters of radiation in the methane flames studied. Soot is the dominant radiant emitter in the ethylene flames until high flow rates of central oxygen reduce the soot concentration, such that high temperature product gases take over as the dominant radiant emitters. Overall, low flow rates of central oxygen can increase thermal radiation from the coflow methane flame by 20% due to increased soot concentration, and high flow rates of central oxygen can increase thermal radiation by 31% due to increased gas temperatures. However, the methane flames with central oxygen still emit at least 41% less radiation than the coflow ethylene flame. These results indicate that when considering the effect of oxygen enrichment on thermal radiation, the change in overall flame (gas and soot) emissivity can be just as important as the change in temperature.

Synthesising copper-carbon nanotube composites through methane diffusion flame

Flame synthesis of carbon nanotubes (CNT) on copper substrate is a novel approach for generating copper-carbon nanotube composites for improving thermal and electrical properties of copper material. The copper substrates were first cleaned and etched with concentrated sulphuric acid. Methane fuel was used in two different laminar flame configurations namely Normal Diffusion Flame (NDF) and Inverse Diffusion Flame (IDF). Under NDF, copper substrate melted due to high temperature blue flame shielding carbon-rich yellow flame in the structure of NDF which was further validated through numerical simulation. Unlike NDF, there is a clear separation between decomposed carbon-rich yellow flame from the oxidising blue flame in IDF that creates two highly differentiated temperature zones. This realises the suitable conditions for CNT synthesis on a copper substrate, generating copper-CNT composites with tubular diameters of 20 nm to 30 nm. This highlights the opportunity of using flame synthesis as a low-costs approach in fabrication of copper-CNT composites.

Modeling soot formation in laminar coflow ethylene inverse diffusion flames

A numerical study is conducted using the CoFlame code to investigate the sooting characteristics of ethylene-fueled laminar coflow inverse diffusion flames (IDFs), aiming to gain insights into the different flame structure and soot formation features between an IDF and a normal diffusion flame (NDF). The effects of oxygen mole fraction in the oxidizer stream, denoted as Oxygen Index (OI), on flame structure and soot production are studied over the OI range of 21% to 33%. The soot model parameters were chosen to produce overall reasonably good agreement between the predicted and measured in terms of soot volume fraction, mean primary particle size, and the mean number of primary particles per aggregate in both the normal and inverse configurations. A sensitivity study showed that soot nucleation and soot surface growth by PAH adsorption modeling should be considered simultaneously as a strongly coupled process through competing for polycyclic aromatic hydrocarbons (PAHs). The IDF configuration displays distinct characteristics from the NDF one in terms of flame structure as well as soot production. In IDFs soot is formed along the outer edge of reaction front, and does not cross the reaction front to the oxidizer, decoupling completely soot oxidation which leads to soot emission from the open flame tip. Modeling results indicate that in the IDF configuration the hydrogen abstraction acetylene addition (HACA) process is severely suppressed, while the adsorption of PAH molecules is dominant to soot surface growth at relatively low temperatures. Nucleation of new particles is significant all along the flame height, causing emission of young soot. IDF represents an ideal configuration to study soot surface growth by PAH adsorption. Increasing OI enhances all the soot processes, especially surface growth by PAH adsorption. Although soot oxidation rate by OH radicals is also strongly enhanced, it remains about two orders of magnitude lower than soot surface growth rate by PAH adsorption.

Structure of CH4–Air Inverse Diffusion Flames in a Multi-slit Burner

Inverse diffusion flame (IDF) configurations have shown to be robust to changes in fuel feed and combining desired characteristics of both diffusion and premixed flames. In this work, methane IDFs are characterized in a multi-slit burner. To this end, a multi-technique experiment with photography, particle image velocimetry, and flame chemiluminescence through spectroscopy is conducted. The flames studied encompass a range of conditions (0 ≤ Vr ≤ 100) with laminar flow at two different power outputs (238 and 476 W). Four characteristic IDF structures were identified and denominated as types A, B, C, and D. Type A do not exhibit IDF-like characteristics and behave like normal diffusion flames. Types B, C, and D were found to exhibit a dual nature, with regions varying from a diffusion-like regime to a premixed-like regime. The velocity ratio Vr was found to be the most important parameter when characterizing IDF, whereas the use of a global equivalence ratio ϕg can be misleading because it does not offer insight in actual reaction stoichiometry. Type C flames displayed ideal characteristics for the application in appliances as a result of its stability and absence of undesired yellow tips.

Observations of long duration microgravity spherical diffusion flames aboard the International Space Station

Spherical diffusion flames of gaseous fuels supported on a porous spherical burner are investigated experimentally in microgravity for long durations aboard the International Space Station (ISS). Experimental results are compared with a transient numerical model. The experiments involve normal spherical diffusion flames (i.e., fuel flowing into oxidizer) burning ethylene in an oxygen/nitrogen mixture at atmospheric pressure. The stoichiometric mixture fraction and adiabatic flame temperature are varied by diluting the fuel with nitrogen and varying the oxygen concentration in the ambient. The diagnostics consist of color video, thin-filament pyrometry for gas temperature, thermocouple measurements of burner temperature, and photomultiplier tube measurements of broad and narrow-band flame emission. Burn times range from 28 – 336 s. All flames grow monotonically after ignition. Peak gas temperature generally decreases with time. Radiative extinction is observed when, at constant reactant flow rate, the flame expands until radiation heat loss approaches the chemical heat release. Before a flame experiences complete extinction, it may enter an oscillatory mode in which part of the flame partially extinguishes and then reforms. The oscillations grow in magnitude until complete extinction. Sufficiently small flames with low radiation losses exhibit quasi-steady behavior, but burner heating is significant, and the experimental duration is dictated by the temperature limit of the porous sphere.

Comparative Study on the Dimethyl Ether Combustion Characteristics in Normal and Inverse Diffusion Spherical Flame Geometries.

This paper makes a comparative study on the normal diffusion flame (NDF) and inverse diffusion flame (IDF) characteristics of dimethyl ether (DME) in microgravitational spherical diffusion flame geometry by simulations with detailed fuel chemistry and a transport model. It is found that there always existed two combustion modes (i.e., hot flame and cool flame) in either NDF or IDF condition. The combustion progress of hot flames was controlled by diffusive mixing, while that of cool flames was controlled by low-temperature competing kinetics. The cool-flame structure dynamics were far away from the chemical equilibrium. The low-temperature branching rate of DME was positively dependent on the oxygen level, while its termination rate was enhanced with the increasing temperature. Being rather distinct from the NDF counterpart, DME IDFs had the oxygen-enriched combustion feature in either hot- or cool-flame condition. Furthermore, DME hot-flame extinction was induced by thermal radiative loss, while the cool-flame extinction was induced especially by the decrease of the low-temperature branching rate. Compared with hot NDFs, it would be of less effectiveness to control the hot IDF combustion process by positive measures. However, combustion in the latter configuration was much more stable than the former. In either NDF or IDF geometry, the cool-flame chemistry could help to extend the fuel flammability range considerably, and the two-reaction-zone structure of cool flame was responsible for cool-flame stability. In addition, the IDFs had much better ignition performance than the NDF counterpart.

Transient dynamics of radiative extinction in low-momentum microgravity diffusion flames

Transient dynamics of growing diffusion flames produced by a low-momentum fuel flow emanating at the flat surface has not yet been properly addressed. The flames of interest resemble properties of laminar jet flames and the candle or droplet flames produced by a point fuel source. However, due to the considerable size of the fuel source, these flames appear to be dissimilar to either of the two cases. The practical importance of studying the stability and dynamics of these flames stems from the need to scrutinize the flammability of condensed fuels in normal and zero gravity. This paper presents comprehensive 3D simulations of the low-momentum zero- and normal gravity diffusion flames generated by the BRE (Burning Rate Emulator) burner. The scenarios of interest represent the subset of the ongoing orbital experiments in which flame development in the normal atmosphere is considered. The experimental observations have shown that these flames are prone to the local extinction followed by fluctuating instability and complete quenching. Such behavior was numerically predicted in the earlier “blind” simulations and is now replicated for the orbital test conditions. Time to flame extinction predicted in this work agrees with the experimental observations. Extinction occurs due to the excessive radiative losses when the instantaneous radiative fraction grows up to 60–70% of the current heat release rate. The excessively high level of the radiative losses is facilitated by the long residence time in the low-momentum microgravity flame. The transient dynamics of flame disintegration after the local extinction onset is analyzed. The hook-like (bibrachial) structure of flame edge is predicted after the extinction onset. When the re-ignition occurs, a classical triple (tribrachial) flame develops and propagates along the stoichiometric surface.

In-Situ Measurement of Soot Volume Fraction and Temperature in Axisymmetric Soot-Laden Flames Using TR-GSVD Algorithm

To investigate soot formation and oxidation mechanism in combustion processes, determination of soot volume fraction (SVF) and temperature distributions in laminar co-flow axisymmetric soot-laden flames are still crucially important. This study provides a convenient and low-cost solution with a new inversion algorithm for in-situ measurements of SVF and temperature in axisymmetric soot-laden flames. The measurement instrumentation combines an industrial color camera and a tablet computer for in-situ measurements of SVF and temperature in three kinds of experimental flames. Based on the flame image processing, the Tikhonov regularization coupled with a generalized singular value decomposition (TR-GSVD) algorithm is proposed to do the inversion process. Comparing with common inversion algorithms, the TR-GSVD algorithm provides more accurate results. The measurement results show that the reduction in fuel flow of normal diffusion flame would result in higher temperature and lower SVF through the combined effect of radiant heat loss and fuel pyrolysis. The decrease of the equivalent ratio of partially premixed flame leads SVF to increase first and then decrease, and temperature to rise continuously, which is caused by the effect of fuel pyrolysis and the competition of soot formation and oxidation. For inverse diffusion flame, an increase in oxidant flow and a decrease in fuel flow lead to higher SVF and temperature, which is believed to be caused by the reduction of overall equivalence ratio. The detailed explanation is that the enhanced exothermic reactions lead to the temperature rise, and both the temperature and fuel pyrolysis affect soot formation.

Double blue zones in inverse and normal laminar jet diffusion flames

Past experimental and computational works indicate that double blue zones are possible in hydrocarbon diffusion flames. This study examines these zones in inverse (IDF) and normal (NDF) laminar gas jet diffusion flames. Over 100 diffusion flames were observed with various fuels, diluents, dilution levels, and flow rates. Upon close examination, the IDFs and NDFs are seen to have double blue zones, separated at the flame tip by up to 1.6 and 0.9 mm, respectively, with a relatively dark region in between. By partially premixing the fuel and oxidizer streams, it was observed that, for both IDFs and NDFs, the zone toward the fuel side is rich, while that toward the oxidizer side is stoichiometric. Chemiluminescence was investigated using cameras and bandpass filters emphasizing emissions from OH* (310 nm), CH* (430 nm), CO2* (455 nm), and C2* (515 nm). The images were deconvolved using onion peeling. Broadband CO2* emissions were subtracted from the images of OH*, CH*, and C2* emissions to find profiles of spectral emissive power for these species. The thin, blue-green zone on the fuel side coincides with the peaks in CH* and C2* emissions in the fuel-consumption zone, while the broader, blue stoichiometric zone coincides with the peaks in OH* and CO2* emissions in the oxygen-consumption zone. The spectral emissive power peaks for C2*, CO2*, CH*, and OH* are typically 3 times as high in the IDF as in the NDF, which is attributed to the higher scalar dissipation rates.

Comparative study on characteristics of soot from n-decane and RP-3 kerosene normal/inverse diffusion flames

RP-3 kerosene is the most important hydrocarbon fuel used in civilian airplanes in China. Up to date, there are few experimental literatures on soot formation and oxidation of RP-3 kerosene available. In this experimental study, the evolution, nanostructure, and oxidation reactivity of the soot particles for RP-3 kerosene in normal and inverse diffusion flames, were investigated. The n-decane was selected as a base fuel for the comparison. Thermophoretic techniques followed by transmission electron microscopy were used to study the evolution of the soot particles in the different locations at the centerline and radial stream of the flames. Nanostructure, lattice parameters, and oxidation reactivity of the soot particles were examined by high-resolution transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis. The results showed that the size of RP-3 kerosene flame soot particles was bigger than n-decane flame at 20 mm and 30 mm height above the burner surface for normal diffusion flame. In inverse diffusion flame, RP-3 kerosene flame had bigger soot aggregates than n-decane flame. Moreover, RP-3 kerosene flame nanostructure had larger fringe length and shorter fringe tortuosity than n-decane flame in normal and inverse diffusion flames, which reflect lesser oxidation reactivity of the RP-3 kerosene flame. X-ray diffraction spectrum and thermogravimetric curves displayed good agreements with the high-resolution transmission electron microscopy images. Finally, the nanostructure of soot particles had a strong relation to their oxidation reactivity.