O2 Mass Fraction Research Articles

Exploring Vortex–Flame Interactions and Combustion Dynamics in Bluff Body-Stabilized Diffusion Flames: Effects of Incoming Flow Velocity and Oxygen Content

An afterburner encounters two primary features: high incoming flow velocity and low oxygen concentration in the incoming airflow, which pose substantial challenges and contribute significantly to the deterioration of combustion performance. In order to research the influence of oxygen content on the dynamic combustion characteristics of the afterburner under various inlet velocities, the effect of oxygen content (14–23%) on the field structure of reacting bluff body flow, flame morphology, temperature pulsation, and pressure pulsation of the afterburner at different incoming flow velocities (0.1–0.2 Ma) was investigated in this study by using a large eddy simulation method. The results show that two different instability features, BVK instability and KH instability, are observed in the separated shear layer and wake, and are influenced by changes in the O2 mass fraction and Mach number. The oxygen content and velocity affected the oscillation amplitude of the downstream flow. As the O2 mass fraction decreases, the flame oscillation amplitude increases, the OH concentration in the combustion chamber decreases, and the flame temperature decreases. Additionally, the amplitude of the temperature pulsation in the bluff body flame was primarily influenced by the temperature intensity of the flame and BVK instability. Moreover, the pressure pulsation is predominantly affected by the dynamic characteristics of the flow field behind the bluff body. When the BVK instability dominated, the primary frequency of the pressure pulsation aligned with that of the temperature pulsation. Conversely, under the dominance of the KH instability, the temperature pulsation did not exhibit a distinct main frequency. At present, the influence of oxygen content and incoming flow rate on the combustion performance of the combustion chamber is not clear. The study of the effect of oxygen content on the combustion characteristics of the combustion chamber at different incoming flow rates provides a reference for improving the performance of the combustion chamber and enhancing the combustion stability.

Simulation of underground coal gasification ignition in deep coal seam based on transitional diffusion mechanism: Influence of inlet temperature and O2

Underground coal gasification (UCG) in deep coal seam was attracting more and more attentions all over the world due to its characteristics of safety, clean and low carbon. In the study, the ignition of UCG in deep coal seam was simulated by employing computational fluid dynamics (CFD). A new method setting gas diffusion in coal seam as transitional diffusion (TD) was put forward, and the TD and Fickian diffusion (FD) mechanism were compared. It was elucidated that TD mechanism was more accurate for UCG model. The default FD mechanism in coal seam overestimated the reaction rate of coal seam. With TD mechanism, coal seam in turn went through external heating controlled (EHC) region and hybrid heating controlled (HHC) region during the ignition of UCG. The demarcation temperature between EHC and HHC regions was 720 K. Then, various inlet temperatures and mass fractions of O2 were attempted during the simulation of UCG ignition. The time spent by the ignition was shortest with 1100 K inlet temperature and it was ∼180 s. The less the inlet mass fraction of O2 was used, the slower combustion rate was. CO2 was the most potential indicator to get access to the underground condition.

Multi-stage oxidation of iron particles in a flame-generated hot laminar flow

The paper presents multi-parameter high-speed optical diagnostics of single iron particle oxidation in a hot laminar flow consisting of O2, H2O, CO2, and N2. Prior to luminous combustion, micron-sized particles are visualized using diffuse-backlight-illumination with high temporal and spatial resolutions, enabling in situ particle sizing. The incandescent burning particles are then monitored by direct high-speed imaging. This approach allows for a successful determination of the multiple oxidation stages of individual iron particles, including the solid phase, melting, and liquid phase. Two characteristic timescales are quantified as a function of the resolved particle diameter at various O2 volume fractions, namely the solid-phase oxidation time (SOT, defined as the time between the onset of rapid particle heating and melting) and the liquid-phase combustion time (LCT, defined as the duration between melting initiation and peak luminosity). SOT shows a linear dependence on the inverse O2 mass fractions at relatively high O2 levels but it tends to plateau when the O2 content is below 20vol.%. LCT is inversely proportional to the O2 mass fraction, suggesting that liquid-phase oxidation of iron particles is limited by external oxygen diffusion. The experimental LCT quantitatively agrees with a theoretical mode developed for nonvolatile particle combustion in the diffusion-limited regime, considering O2 as the only oxidant. This also implies that at the presence of O2, the oxidizing effect of H2O and CO2 on diffusion-limited iron particle combustion is negligible.

Consequence analysis of heptane multiple pool fire in a dike

Multiple Pool Fire (MPF) reasonably enhances the flame height, the rate of fuel combustion and irradiation due to cascading effect and flame merging of multiple pools. The distribution of temperature and radiative heat flux from the source is the key parameter in predicting safety distance. The current study’s major objective is to develop a Computational Fluid Dynamics (CFD) model to calculate the safety distance of MPF and validate the predicted results (flame temperature, radiative heat flux, CO2 and O2 mass fraction) with the experimental data. The flame structure and its thermal properties have been evaluated using the Reynolds Averaged Navier Stokes (RANS) and the Large Eddy Simulation (LES) model. The CFD results are found to be in close agreement with the experimental findings. The LES turbulence model predicts more accurately the flame structures and the fluctuations with an error of less than 3%, while the Re-Normalization Group k-ε model predicts the average characteristics. This authenticates the accuracy of computational methodology and robustness of the LES turbulence model to predict MPF flame characteristics. Furthermore, this computational methodology can be utilised by the industries for quantitative risk assessment and the worst-case scenario of various pool fires to save the human and material of the workplace beforehand.

Coupling an oxygen generation cycle with an oxy-fuel combustion spark ignition engine for zero NOx emissions and carbon capture: A feasibility study

An oxycombustion spark-ignition engine coupled to an O2 production cycle based on a mixed electronic-ionic ceramic-type membrane is studied in this paper. The exhaust gases drive oxygen production through a heat exchanger network and a set of turbochargers. Initially, a study at medium speed is carried out to determine the influence of the oxygen concentration and the engine compression ratio on the system performance. A 16.3% of fuel consumption reduction is reached when 30% of O2 mass fraction and an engine compression ratio of 20 are implemented when compared with conventional engine operation. Then, a full load study in a wide range of engine speeds is performed, where a maximum fuel consumption reduction of 21% is obtained at high speed (5000rpm) comparing with conventional engine operation. The study shows the whole cycle has demonstrated promising performance parameters at different working conditions (O2 mass fraction, compression ratio, engine load and speed).

Combustion optimization in consolidated porous media for thermo-photovoltaic system application Using Response Surface Methodology

Motivated by the need to optimize the operation of an MTPV system, three consolidated porous media (PM) geometries namely spherical, conical and cylindrical and a non-PM combustor are numerically simulated. The geometry with the best methane conversion rate is identified and optimized with the right porous media and combustor diameter ratio. Using Response Surface Methodology, a design of experiment (DOE) approach is adopted to examine the effects of three independent parameters namely: wall thermal conductivity, chamber radius and O2 mass fraction on output parameters critical to the operation of the MTPV system specifically radiation heat transfer rate and emitter efficiency. Results show that PM combustion produces higher and uniform wall temperatures compared to the non-PM combustor. The spherical shape consolidated PM produces the highest average methane conversion rate of 96.8% and highest mean wall temperature of 1032 K even though it produces the least flame temperature. The highest radiation heat transfer rate predicted by DOE and CFD simulations are 3.25 W and 3.17 W respectively. The best conditions to realize the optimal MTPV performance is by employing the spherical PM combustor with diameter ratio of 0.7, porosity of 0.5, equivalence ratio of 0.8, wall thermal conductivity of 22.99 W/(m∙K).

HMB-CFD study of oxygen-enriched pulverised coal combustion in an ironmaking blast furnace

Pulverised coal injection (PCI) into blast furnace (BF) is a dominant technology for partial coke replacement and lowering production cost, and high oxygen enrichment is a recent promising technology for further increasing coal combustion and thus increasing pulverised coal rate (PCR) and lowering emissions. In this paper, a heat and mass balance (HMB) model and a three-dimensional computational fluid dynamics (CFD) model of pulverised coal flow and combustion in the lower part of the BF are combined to investigate the effect of oxygen enrichment on coal combustion in terms of operational feasibility and coal combustion efficiency, where the injection schemes such as blast volume, oxygen enrichment, PCR are designed using the HMB model; and the local detailed coal combustion is studied using the CFD model, including the effect of oxygen enrichment and PCR on in-furnace phenomena including gas-solid flow, temperature field and particularly, coal burnout. The CFD simulations reveal that the combustion efficiency is the result of the combined effects of oxygen enrichment and PCR. The high oxygen enrichment in the blast enhances gaseous combustion and coal combustion. The high PCR needs more time and distance for preheating and devolatilisation, which was detrimental to coal combustion near tuyere; on the other hand, more particles in combustion release more heat which is beneficial to the continued combustion of unburned particles. As a result of the combined effects, the inflection points could be found in some key phenomena, including gas temperature, particle temperature, O2 mass fraction, etc. This HMB-CFD study implies that it is highly necessary to consider both feasible operation windows and the detailed impacts of the oxygen enrichment on coal combustion under industrial BF conditions; and thus should be used in future PCI investigations.

Study of increasing thermodynamic efficiency and hydrogen amount in the process of solid fuels and biomass gasification

The effect of fuel composition such as hard coal and biomass in fluidized bed on the thermodynamic efficiency of three gasifier types, low heating value (LHV) and syngas composition was studied by simulation in CFD program. The paper presents various small gasification systems with fluidized bed of water-coal mixture and water-biomass mixture both with oxygen (air) and without oxygen. The work presents the extended mathematical model of gasification process taking into account also 13 kinetic chemical reactions enabled to apply them in CFD program. The chemical model is based on surface reactions given in the model of Equivalent Reactor Network (ERN) in Ansys Energetico. Simulation of gasification of different reactor designs enabled to obtain the mass concentration of chemical compounds in the syngas which influenced on thermodynamic efficiency and LHV. Additionally, heating of the gasifier walls by electrical system without air gives higher mass ratio of hydrogen (15%) and carbon monoxide (75%) in the syngas than in the oxidized gasification system. The simulation tests showed high thermodynamic efficiency, taking into account the exergy above 80%, both for the oxygen gasification of hard coal-slurry and biomass as well as for the externally heated gasifier without the supply of oxygen. The highest thermodynamic efficiency 90.5% was obtained during gasification of energy willow in the oxidation reactor due to the large difference in LHV of syngas and fuel. Obtaining the maximum possible mass ratio of H2 in syngas and a high thermal efficiency for the OXGR system from the gasification of subbituminous coal dust occurs at a ratio C/O2 = 0.314 and at almost stoichiometric ratio of C/H2O = 0.63. In the case of gasification of biomass in the form of finely ground energy willow in the OXGR system, these ratios were respectively: O2/C = 1.82 and C/H2O = 0.47. While maintaining the almost stoichiometric ratio C/H2O = 0.75, resulting from the water-gas shift reaction, low heating value of the syngas for the analysed gasification systems was over 25 MJ/kg. The paper shows comparison of mass fractions of CO, H2, H2O, H2S, CO2, O2, CH4, tar and other chemical compounds in the syngas. Application of the gasification reactor with external heated walls and without oxygen inflow produces the syngas without tar and volatile species in comparison to the systems with additional oxygen supply. In oxidized reactors the syngas contains a large amount of CO2 about 25–30% of total mass.

Transient study during clay bricks cooking in the traditional kiln; CFD numerical study

Traditional technologies for clay brick burning are often confronted with firing problems. The produced brick is of poor quality due to poor combustion inside the kiln. So that, cooking plays an important role because it determines the quality of the final product. Therefore, this work presents a transient computational fluid dynamic (CFD) study of firing clay bricks in the traditional oven. The heat transfer modeling in the furnace mainly is made from hydrodynamic equations and combustion reactions. Therefore, numerical simulation is done using ANSYS-Fluent software, using the standard k-ε turbulence model and the Eddy Dissipation turbulent combustion model. The results focused on the flow field, evolution of temperature, and O2 mass fraction to verify the combustion process inside the kiln. The results show that the kiln reaches the maximum temperature is in order of 900 °C and therefore verification process occurs. Moreover, the results revealed that the O2 mass fraction has the highest values at the inlet and it decreases gradually inside the kiln which gives good evidence about combustion process propagation.

Assessment of a flamelet approach to evaluating mean species mass fractions in moderately and highly turbulent premixed flames

Complex-chemistry direct numerical simulation (DNS) data obtained from lean methane-air turbulent flames are analyzed to perform a priori assessment of predictive capabilities of the flamelet approach to evaluating mean concentrations of various species in turbulent flames characterized by Karlovitz numbers Ka=6.0, 74.0, and 540. Six definitions of a combustion progress variable c are probed and two types of probability density functions (PDFs) are adapted: (i) actual PDFs extracted directly from the DNS data or (ii) presumed β-function PDFs obtained using the DNS data on the first two moments of the c-field. Results show that the mean density, the mean temperature, and the mean mass fractions of CH4, O2, H2O, CO2, CO, CH2O, CH3, and HCO are very well predicted using the temperature-based combustion progress variable cT and the actual PDF. For other considered species, the quantitative predictions are worse but still appear to be encouraging (with the exception of CH3O at Ka=540). The use of the flamelet library obtained from the equidiffusive laminar flame improves results for H2, HO2, and H2O2 at the highest Karlovitz number. Alternative definitions of the combustion progress variable perform worse and the reasons for this are explored. The use of the β-function PDF yields worse results for intermediate species such as OH, O, H, CH3, and HCO, with this PDF being significantly different from the actual PDF. Application of the flamelet approach to rates of production/consumption of various species is also addressed and implications of obtained results for modeling are discussed.

Numerical Investigation Into the Effect of Structural Parameters of Parallel Flow Field With Cooling Channels on Fuel Cell Performance

Abstract The electrochemical reaction inside a high-power fuel cell generates a lot of heat. Excessive heat affects the performance of the membrane, so it is necessary to introduce coolant. The main objective of coolant is regulating the temperature of relatively high-power proton exchange membrane fuel cell (PEMFC) stacks efficiently. The coolant channel has a great influence on the performance of PEMFC. In this work, a multiphase, 3D PEMFC model with serpentine flow channel is developed. In order to rank structural parameters according to the degree of influence on fuel cell performance, this study analyzed the current density, O2 mass fraction, and the distributions of temperature based on an orthogonal test scheme with three factors and three levels. The results show that rib width between the reactant flow channel and the cooling channel has the greatest influence on the current density, and gas flow channel width has the least influence.

Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell—Part I: Model development and base case study

In this study, the internal transport phenomena and mechanism inside an air-cooled proton exchange membrane fuel cell (PEMFC) are investigated. It helps to understand the factors that affect the performance of an air-cooled PEMFC and optimize the design of Membrane Electrode Assembly (MEA) and the flow field. This series article contains two parts. In this paper, i.e., Part Ⅰ of this series, a three-dimensional, two-phase flow, non-isothermal, steady-state Computational Fluid Dynamics (CFD) model is established to investigate the liquid water generation mechanism and the species distributions inside an air-cooled PEMFC single cell with a Base Case flow field design. Dry hydrogen and ambient air (the relative humidity and the stoichiometry are 60% and 150 separately) are considered for the simulation and validation. It is found that the liquid water appears mostly inside the cathode electrode underneath the cathode rib. Inside the anode gas diffusion layer (GDL), the mass fraction of H2 underneath the cathode ribs is lower than that underneath the cathode channels, while the mass fraction of H2O shows the opposite. The distributions of O2 mass fraction and H2O mass fraction inside the cathode GDL have the same trend as those of H2 mass fraction and H2O mass fraction inside the anode GDL. The membrane water content is periodically distributed from channel to channel and its value underneath the cathode rib is much larger than that underneath the cathode channel. The current density distribution is affected by the distribution of water content, i.e., the part underneath the cathode rib shows a larger current density than that underneath the cathode channel.

Numerical investigation on effects of alkali metals release during combustion of char particle groups

Alkali metals, especially Na, are generally accepted as the primary cause of serious ash-related problems in the industrial applications of pulverized coal. This paper investigates the effects of different variables, including particle size, ambient temperature, O2 mass fraction, and atmospheres: O2/N2 and O2/CO2 at particle level. The fictitious domain method (FDM) provides a true direct numerical simulation (TDNS) for the investigation. Computational results show the correlation between alkali metals release and carbon consumption during char particle combustion. Here, high ambient temperature is more positive to alkali metals release than carbon consumption. In an oxygen-dominated atmosphere, a small amount of CO2 tends to form a higher temperature field than N2, which promotes a greater release of alkali metals. This paper also investigates the specific effects of volume fraction and non-uniform distribution of particles for the combustion of particle groups. The computational results show that both temperature field and alkali metals release are significantly affected by the volume fraction of particles. The effects of the non-uniform distribution of particles, e.g., uniform distribution, dense distribution on the windward side, dense distribution on the leeward side, and asymmetric distribution, on temperature field and alkali metals release vary with different volume fractions of particles. As such, the non-uniform distribution of particles plays a role in changing the mass transfer of the oxidant and the heat transfer.

Surface density function evolution and the influence of strain rates during turbulent boundary layer flashback of hydrogen-rich premixed combustion

The statistical behavior of the magnitude of the reaction progress variable gradient [alternatively known as the surface density function (SDF)] and the strain rates, which govern the evolution of the SDF, have been analyzed for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow. The non-reacting part of the channel flow is representative of the friction velocity based Reynolds number Reτ = 120. A skeletal chemical mechanism with nine chemical species and twenty reactions is employed to represent hydrogen-air combustion. Three definitions of the reaction progress variable (RPV) based on the mass fractions of H2, O2, and H2O have been considered to analyze the SDF statistics. It is found that the mean variations of the SDF and the displacement speed Sd depend on the choice of the RPV and the distance away from the wall. The preferential alignment of the RPV gradient with the most extensive principal strain rate strengthens with an increase in distance from the cold wall, which leads to changes in the behaviors of normal and tangential strain rates from the vicinity of the wall toward the middle of the channel. The differences in displacement speed statistics for different choices of the RPV and the wall distance affect the behaviors of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of the effective normal strain rate experienced by the progress variable isosurfaces for different wall distances and choices of RPVs.

Combined convection–radiation heat transfer from n-octane combusted gas passing through different geometrical ducts

The combined radiation–convection heat transfer occurs in many industrial devices; in many occasions, the significant role of radiation is neglected in engineering modeling. The current study is an attempt to address the importance of radiation heat transfer involvement in numerical computations. The investigation takes place in two heat transfer modes of with radiation (case_rad) and without radiation consideration (case_norad) for different duct geometries of base cylinder, small cylinder, divergent frustum cone, and convergent frustum cone. To account the radiation effect in the ducts, discrete transfer radiation method is applied, while a weighted sum of gray gases model for absorption coefficient modeling in non-homogenous participating media is appropriate for combustion of n-octane gas in ducts. The convergent duct structure shows the most radiative flux of − 1514.4 W m−2, whereas small cylinder demonstrates the lowest radiative flux of − 1033.14 W m−2. The overall heat flux of convergent configuration outperforms due to homogenous distribution of O2 and high oxygen mass fraction for combustion, while increasing the surface area in the flow direction aids the heat transfer improvement. The small cylinder is characterized with better flow dynamics (Vmax = 25.72 m s−1, TKEmax = 9.45 m2 s−2); however, its deficiency in heat transfer leads to high temperature along the duct (Tmax = 710 K).

Effect of hydrogen addition on NOx formation mechanism and pathways in MILD combustion of H2-rich low calorific value fuels

Hydrogen is a major component of low heating value (LHV) fuels like syngas, coke oven gas, and blast furnace gas. These fuels can potentially replace methane in the industrial process under MILD combustion conditions. NOx formation mechanism in MILD combustion regime is different from conventional air combustion due to uniform temperature distribution and low oxygen concentration. Hence, it is important to investigate the effect of hydrogen addition inmethane (CH4) on different NOx formation mechanism and pathways under MILD combustion conditions. A well-stirred reactor (WSR) model with atmospheric pressure and long residence time was employed to simulate the ideal MILD combustion conditions with Chemkin Pro software and GRI Mech 2.11. H2 and CH4 blends ranging from 0% H2 (100% CH4) to 60% H2 (B60 = 40%CH4/60%H2) were selected as fuels to emulate MILD combustion of LHV fuels. The oxygen mass fraction was kept from 3% to 9%, and the fuel blends were altered. The results showed that hydrogen addition enhanced NNH route while decreasing prompt route and N2O intermediate route whereas thermal NOx route was insignificant for lower O2 mass fraction of 3% and 6%. It was observed that H2 addition reduced NO-reburning which led to more NOx emission. Moreover, the sensitivity analysis was conducted to analyze the pathway of NOx formation and to quantitively separate the relative contribution of the individual route on the total NOx formation, which was beneficial to control the NOx emission of LHV fuels during MILD combustion.

Effect of Simulated-EGR (N2) on the distribution characteristics of equivalence ratio and the formation of exhaust emissions in a CI engine under early injection conditions

The objective of this study is to numerically investigate the effect of simulated-EGR by changing of N2 gas mass fraction on the fuel mass fraction distributions and the exhaust gas formation in a cylinder of a CI engine under early injection timing conditions. This work provides the optimal operating conditions for a CI engine affected by simulated-EGR (N2).In order to obtain experimental results to validate the numerical analysis, an experiment was conducted using a single-cylinder diesel engine which consisted of an intake system, a test engine control system, and exhaust emissions analyzers. The results of the numerical analysis were compared and validated with the obtained experimental results. The ECFM-3Z model and Mundo Tropea Sommerfeld model were applied as sub-models for the combustion and wall interaction analyses, respectively. The Mundo Tropea Sommerfeld model has two regimes as deposition and splash, which is determined by the splashing parameter (K). In order to simulate the EGR effect, the O2 and N2 mass fractions, which change the overall equivalence ratio from 0.54 to 1.01 in the cylinder, were varied under the same fuel injection mass. The start of energizing timing was swept from BTDC 03deg to BTDC 29deg.It was revealed that the peak cylinder pressure and IMEP value decreased when the overall equivalence ratio increased, because the combustion performance deteriorated by the low O2 mass fraction in the cylinder. When the start of energizing timing was advanced, the peak cylinder pressure increased, because the enough air and fuel mixing time were secured. This allowed a homogeneous mixture to form in the cylinder, which tends to increase the proportion of premixed combustion. Specially, when the start of energizing timing was near BTDC 20deg, the ISNO and ISSoot simultaneously decreased. The reason is that a lot of fuel was injected to the upper end of the piston bowl, which formed a liquid wall film at the center area of the cylinder. The liquid wall film evaporated and suppressed the increase in cylinder temperature due to the effect of the latent heat of vaporization. Furthermore, it created a widely distributed high fuel mass fraction. Finally, the optimal operating conditions to improve engine performance was determined to be φ = 0.68 and teng = BTDC 14deg because this condition had both low ISNO and ISSoot values, and high IMEP value. From these results, the found optimal operating conditions were expected to improve both exhaust emissions and combustion performance for the CI engine.

Computational Analysis of Reacting Flows in Afterburner

Gas turbine afterburner is used during take-off, combat, maneuvers, and emergencies when the aircraft engine needs more thrust than normal. A 60° sector full-scaled afterburner, with extended domain of three times the nozzle diameter in the axial direction and two times the nozzle diameter in the radial direction, is modeled. The numerical calculations are performed using SIMPLE algorithm and k–ε model has been used for turbulence. Kerosene (C12H23) is taken as fuel and virtual injectors are specified for fuel injection. Energy equation and species transport with the Discrete Phase model is selected for computations. Maximum density of 1.25 kg/m3 is observed and the density of the fluid reduced to 0.2 kg/m3 at the exit of nozzle after combustion. The desired Mach number of 1.1 could be observed at the exit of the nozzle. The CO2 mass fraction increased from 0 to 0.075 whereas the O2 mass fraction decreased from 0.23 to 0.145 from the inlet to the exit of afterburner. The maximum temperature of 2500 K is observed radially at 0.2 m, from the center of the afterburner and axially at a distance of 0.9 m of afterburner. The obtained results are validated with published experimental and computational fluid dynamics results.

The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band

Abstract. Photoacoustic spectroscopy is a sensitive in situ technique for measuring the absorption coefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS) instruments require accurate calibration by comparing the measured photoacoustic response with a known level of absorption for a calibrant. Ozone is a common calibrant of PAS instruments, yet recent work by Bluvshtein et al. (2017) has cast uncertainty on the validity of ozone as a calibrant at a wavelength of 405 nm. Moreover, Fischer and Smith (2018) demonstrate that a low O2 mass fraction in the bath gas can bias the measured PAS calibration coefficient to lower values for wavelengths in the range 532–780 nm. In this contribution, we present PAS sensitivity measurements at wavelengths of 405, 514 and 658 nm using ozone-based calibrations with variation in the relative concentrations of O2 and N2 bath gases. We find excellent agreement with the results of Fischer and Smith at the 658 nm wavelength. However, the PAS sensitivity decreases significantly as the bath gas composition tends to pure oxygen for wavelengths of 405 and 514 nm, which cannot be rationalised using arguments presented in previous studies. To address this, we develop a model to describe the variation in PAS sensitivity with both wavelength and bath gas composition that considers Chappuis band photodynamics and recognises that the photoexcitation of O3 leads rapidly to the photodissociation products O(3P) and O2(X, v > 0). We show that the rates of two processes are required to model the PAS sensitivity correctly. The first process involves the formation of vibrationally excited O3(X̃) through the reaction of the nascent O(3P) with bath gas O2. The second process involves the quenching of vibrational energy from the nascent O2(X, v > 0) to translational modes of the bath gas. Both of these processes proceed at different rates in collisions with N2 or O2 bath gas species. Importantly, we show that the PAS sensitivity is optimised for our PAS instruments when the ozone-based calibration is performed in a bath gas with a similar composition to ambient air and conclude that our methods for measuring aerosol absorption using an ozone-calibrated PAS are accurate and without detectable bias. We emphasise that the dependence of PAS sensitivity on bath gas composition is wavelength-dependent, and we recommend strongly that researchers characterise the optimal bath gas composition for their particular instrument.

Surface Density Function statistics in hydrogen-air flames for different turbulent premixed combustion regimes

ABSTRACTThe choice of reaction progress variable (RPV) on the statistical behavior of the surface density function (SDF) and the strain rates, which govern the evolution of SDF, have been analyzed using a detailed chemistry Direct Numerical Simulation (DNS) database of freely propagating statistically planar -air flames with an equivalence ratio of 0.7. The DNS database consists of three cases spanning the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) regimes of premixed turbulent combustion. For this analysis, the RPV is defined based on the mass fractions of H2, O2 and H2O. The mean variations of the SDF and the flame displacement speed, , have also been found to be dependent on the choice of the RPV. The progressive weakening (strengthening) of the preferential alignment of the RPV gradient with the most extensive (compressive) principal strain rate with increasing Karlovitz number leads to changes in the behaviors of normal and tangential strain rates from one combustion regime to another. The differences in displacement speed statistics for different choices of RPV affect the behaviors of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of effective normal strain rate experienced by the -isosurfaces for different choices of RPVs.