Ignition Time Research Articles

Effects of the PMMA Molecular Weight on the Thermal and Thermo-Oxidative Decomposition as the First Chemical Stage of Flaming Ignition

The piloted and the spontaneous ignition of low and high molecular weight (LMW and HMW) polymethyl methacrylate are simulated using a one-dimensional condensed-gas phase model for constant heat fluxes in the range of 25–150 kW/m2. Purely thermal (nitrogen) and thermo-oxidative (air) decomposition is considered, described by a single and four-step kinetics for the low and high molecular weight polymer, respectively. Different optical properties are also examined. The same trends of the ignition time and other ignition parameters are always observed. Due to a more significant role of the chemical kinetics, the effects of the sample molecular weight and reaction atmosphere are higher at low heat fluxes. Times are shorter for the black HMW samples and thermo-oxidative kinetics. For piloted ignition, factors are around 2.8–1.6, whereas for thermal decomposition, they are 1.3–1.2. The corresponding figures are 1.8–1.3 and 1.3–1.1, in the same order, for the spontaneous ignition. Overall, the effects of the molecular weight are more important than those related to the reaction kinetics environment. These differences are confirmed by the comparison between predictions and measurements.

Preparation and properties of halogen-free flame retardant PE based wood-plastic composite materials

ABSTRACT The experiment carried out halogen-free flame retardant modification treatment on polyethylene-based wood-plastic composite materials. The results showed that after adding flame retardant, the ignition time of the material increased to 76 seconds, an increase of 22 seconds. At combustion temperatures above 530°C, only a small amount of the material’s residue continues to evaporate. When the temperature rises to 800°C, the difference in the residual carbon content of the samples gradually becomes larger. The residual carbon content of sample 1# is only 10.1%. At this time, the residual carbon content of samples 7#, 8# and 9# are respectively Reaching 27.12%, 27.09% and 25.60%. The above results show that the thermal stability of composite materials has been greatly improved after adding nano-flame retardants, providing new ideas and methods for the application of flame retardant and halogen-free flame retardant materials of wood-plastic composites.

Glowing and piloted flaming ignition of vertical wood exposed to power-law thermal radiation

Fire hazards are dangerous to human life and property safety. Fire accidents occur frequently in wood-frame villages all over the world. In this paper, glowing and flaming ignition of vertical wood heated by five power-law heat fluxes (HF) is experimentally studied using a self-built apparatus. Comparative ignition tests with and without piloted ignitor were performed, and some crucial variables were collected, encompassing surface and in-depth temperatures, mass loss rate, and ignition time. A 1D numerical model was developed to simulate experimental results and determine temperature-dependent thermodynamics of wood by inverse modelling. The results showed that ignitor induced flaming ignition, whereas absence of ignitor enabled only glowing ignition. Flaming ignition was preceded by flash ignition. Surface temperatures of flaming ignition were well predicted by numerical model. However, the predictive accuracy declined when modelling in-depth temperatures due to cracks in char layer. Three stages were identified in both ignition modes: a preheating stage, a water evaporation stage, and a pyrolysis stage. The numerical model underestimated mass loss rates due to additional heating contributed by flash ignition and char cracks. Critical temperature and critical mass flux were comparatively analyzed, and the former was used in assessing ignition time. Measured ignition times were well estimated by an analytical correlation. The numerical model accurately predicted flaming ignition times but overpredicted glowing ignition times due to char cracks. The proposed model can contribute to the fire safety retrofitting of historic buildings and minimize human casualties and property damage.

Numerical investigation on gaseous fuel injection strategies on combustion characteristics and NO emission performance in a pure hydrogen engine

With the proposal of “dual-carbon” goals and increasingly stringent emission regulations, hydrogen as a zero-carbon alternative fuel has great potential in engine applications. In the appliance of pure hydrogen engines, direct injection plus lean burn technology can effectively improve engine performance and reduce emissions. Therefore, this work establishes a three-dimensional (3D) simulation model of the pure hydrogen spark-ignition internal combustion engine (ICE) coupled with chemical reaction kinetics. Then, the applicability of different commonly used hydrogen simplified mechanisms to characterize the combustion process based on experimental data is evaluated, and the feasibility of the transient calculation model is verified. Then, the engine combustion and emission performance are compared by adopting hydrogen port injection (HPI) and hydrogen direct injection (HDI) strategies. Meanwhile, the effects of hydrogen injection timing (HIT) on flow, combustion and nitric oxide (NO) emission processes under direct injection plus lean burn mode are analyzed. The results show that: 1. For mixture distribution at ignition timing (IT), the hydrogen should be distributed near the spark plug and avoid its concentration being too rich since the richer mixture will lead to intense combustion and rapid heat release. 2. When using HDI mode, the engine combustion performance and power performance are better than that of the HPI mode. Delaying HIT, the power performance improves under HDI modes, while both the indicated thermal efficiency (ITE) and indicated mean effective pressure (IMEP) reached the maximum at Case6-HDI-100 with the values are 40.52 % and 0.498 MPa, respectively. 3. Considering the engine combustion, power and emission performance, Case6-HDI-100 (HIT at 100 °CA BTDC) is the preferred scheme. Its ITE and IMEP are 8.49 % and 37.33 % higher than that of Case1-HPI (HPI mode scheme) and 6.50 % and 14.95 % higher than that of Case4-HDI-220 (the lowest scheme under HDI mode), respectively, while nitrogen oxide (NOx) emissions meet the European Stage V non-road emission standards. This study provides some reference values for the combustion design of pure hydrogen ICEs.

Burning Properties of Combined Glued Laminated Timber

This study delved into the combustion properties of combined glulam bonded using polyurethane (PUR) and resorcinol-phenol-formaldehyde (RPF) adhesives. The experiment involved three distinct wood species, namely, spruce, alder, and beech, which were combined in homogeneous, non-homogeneous symmetrical, and non-homogeneous asymmetrical arrangements. These species were selected to represent a spectrum, namely, softwood (spruce), low-density hardwood (alder), and high-density hardwood (beech). The varying combinations of wood species illustrate potential compositions within structural elements, aiming to optimize mechanical bending resistance. Various parameters were measured during combustion, namely, the heat release rate (HRR), peak heat release rate (pHRR), mass loss rate (MLR), average rate of heat emission (ARHE), peak average rate of heat emission (MARHE), time to ignition (TTI), and effective heat of combustion (EHC). The findings indicate that incorporating beech wood into the composite glulam resulted in an increase in heat release, significantly altering the burning characteristics, which was particularly evident at the second peak. Conversely, the use of spruce wood exhibited the lowest heat release rate. Alder wood, when subjected to heat flux at the glued joint, displayed the highest heat emission, aligning with the results for EHC and MARHE. This observation suggests that wood species prone to early thermal decomposition emit more heat within a shorter duration. The time to ignition (TTI) was consistent, occurring between the first and second minute across all tested wood species and combinations. Notably, when subjected to heat flux, the glulam samples bonded with PUR adhesive experienced complete delamination of the initial two glued joints, whereas those bonded with RPF adhesive exhibited only partial delamination.

Fabrication of briquettes from charcoal fines using tannin formaldehyde resin as a binder

Charcoal fines, a waste emanating from charcoal transportation and handling, were utilized in the fabrication of briquettes using tannin-formaldehyde resin as a binder to meet ever expanding energy demand. A collection of four briquette samples were fabricated with binder proportions of 25%, 30%, 35%, and 40%. These briquettes were characterized using Fourier transform infra-red and thermogravimetric analyses techniques. Furthermore, the briquettes were subjected to physical parameters namely bulk density, impact resistance index (IRI), water resistance index (WRI), and water boiling test. The bulk density of the briquettes was 1.153-1.495 g/cm<sup>3</sup>, IRI was 6.79-73.33, and WRI was 99.24-99.29. The briquettes exhibited an ignition time of 5.38-6.21 minutes, boiling time of 19.50-37.20 minutes, burning rate of 3.20-8.70 g/minute, and a specific fuel consumption of 54.70-64.30 g/L. Higher heating value range for the briquettes was 19.76-23.23 MJ/kg and the briquettes with 40% binder showed the best physical qualities with great fuel potential. Therefore, the fabricated briquettes have demonstrated great potential as a source of cleaner and sustainable energy.

Natural halloysite nanotubes decorated with organophosphorous for highly flame‐resistant epoxy composites

AbstractHighly flame‐resistant epoxy composites have important application value in many industrial fields. Unfortunately, epoxy resins (EPs) are inherently flammable materials and exhibit great fire hazard. In this study, the commercially available 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) was chemically immobilized on the surface of natural halloysite nanotubes (HNTs) via the linkage of vinyltrimethoxysilane to obtain a novel flame‐retardant VD@HNTs containing P/Si/Al elements. It was found that the flame‐retardant VD@HNTs can effectively improve the thermal stability and flame resistance of epoxy composites. Typically, when 10% of VD@HNTs is added in EP, the limiting oxygen index of as‐prepared VD@HNTs/EP composites reaches 30.3%, and it also successfully reaches V‐0 classification in the UL‐94 test. Besides, the ignition time of VD@HNTs/EP composites was enhanced from 42 s for pure EP to 60 s, and the maximum heat release rate was reduced from 1221 to 758 kW∙m−2. These great findings promise an innovative and efficient strategy for improving the fire safety of EP products and may also offer valuable insight toward fabricating various novel organic–inorganic flame retardants as well as highly flame‐resistant polymer composites.Highlights A novel flame‐retardant VD@HNTs containing P/Si/Al elements was prepared by the chemical decoration of HNTs with organophosphorous. The VD@HNT can effectively improve the thermal stability and flame resistance of epoxy composites. Synergistic flame‐retardant mechanism of phosphorus‐based flame retardants and natural HNTs was classified.

Synthesis of MXene-Based functional coatings on rigid polyurethane foam surfaces: A comparative study of layer-by-layer self-assembly and hydrothermal methods

A flame retardant for rigid polyurethane (RPUF) was designed and synthesized to expand the application field of RPUF. The flame retardants, namely MXene/PEI/PA-L and MXene/PEI/PA-H, were synthesized using phytic acid (PA), polyethyleneimine (PEI), and MXene through layer-by-layer (LBL) self-assembly method and hydrothermal method, respectively. In terms of mechanical performance, MXene/PEI/PA-L imparted brittleness to RPUF while MXene/PEI/PA-H enhanced its toughness, enabling RPUF to adapt to diverse application scenarios. The flame retardant properties were evaluated via vertical combustion test, cone calorimetry test, and thermogravimetric test, and the gas phase flame retardant mechanism was analyzed by TG-FTIR. MXene/PEI/PA constituted a N-P-Ti synergistic flame retardant system. During combustion, MXene/PEI/PA formed a layered dense carbon layer to cut off the external oxygen transport channel and absorbed many oxygen-containing free radicals. MXene/PEI/PA-L demonstrated efficient smoke suppression capability, while MXene/PEI/PA-H showed superior thermal insulation performance. The ignition time (TTI) of the composite foam reached 47 s, the total smoke release (TSR) value decreased by 92 %, and the flame retardant grade reached 5VA. Coated RPUF had excellent flame retardant ability.

Glowing and flaming autoignition of wood exposed to coupled convective and radiative heating

Autoignition of solid fuels driven by coupled convective and radiative heating is frequently encountered in fires, whereas its controlling mechanism is rarely revealed. To challenge this issue, autoignition tests of 5 and 15 mm thick cylindrical wood samples were conducted using a newly developed apparatus enabling coupled convective and radiative heating. The total heat flux imposed on wood was maintained at 25 kW/m2, while the fractions of the two heating components changed as the airflow velocity and temperature varied in the ranges of 0–1.51 m/s and 298–473.3 K. Glowing ignition occurred in all scenarios except for one case due to enhanced heat loss and declined radiative heating. Flaming ignition was observed in all conditions except for 298 K airflow temperature. Four stages were identified during flaming combustion: a preheating and deformation stage, a glowing combustion stage, a coupled glowing and flaming combustion stage, and an extinction stage. Compared to 5 mm samples, the surface temperature rise of 15 mm wood was significantly delayed due to its thermally thick nature. By fitting experimental data, a new formula was proposed to correlate glowing ignition temperature with radiative heat flux, airflow velocity, and airflow temperature. Both glowing and flaming ignition times were affected by airflow and radiative heating in a complex manner, and their values to the power of −0.5 approximately linearly depended on radiative heat flux. Damköhler number and nondimensional heat loss ratio were employed to quantitively analyze the flaming ignition propensity. Meanwhile, an ignitability trend map identifying the flaming and non-flaming ignition regimes was drawn.

Influence of operating parameters on hydrogen DISI engine at injection pressure-drop by experimental investigation and Taguchi method

Hydrogen, as a low-density, zero-carbon energy source, often faces injection pressure drop when used as an alternative fuel for multi-parameter regulation in direct injection spark ignition (DISI) engines. In this paper, the effects of operating parameters including torque, excess air coefficient (λ), ignition timing, and injection timing on combustion, emissions, and energy distribution of a hydrogen DISI engine were first investigated at injection pressures of 100 bar and 30 bar, respectively, followed by Taguchi methodology and analysis of variance (ANOVA) to determine the optimal parameter combinations and their contribution to typical indicators of engine performance. The results showed that injection pressure-drop deteriorated the coefficient of variation (COVIMEP), nitrogen oxides (NOx) emissions, and brake specific fuel consumption (BSFC) in the set of cases. And the deteriorating BSFC is mainly due to the increase in heat loss power (PHL) or the exhaust power (PEx). Taguchi method and ANOVA showed that injection pressure drop changed COVIMEP and BSFC regarding optimal ignition timing selection and PEx regarding optimal injection timing selection and it also increased the torque contribution to COVIMEP by 26.54 %. PHL and PEx should be simultaneously considered because their choice of optimal operating parameters is a “Trade-off” relationship in most cases.

A relative comparison of HCCI, PCCI, and RCCI combustion strategies: an alternative fuels perspective

Low temperature combustion (LTC) strategies have the potential for simultaneous reduction in oxides of nitrogen (NOx) and soot emissions while achieving higher thermal efficiency. Commercial widespread implementation of LTC strategies demands addressing several challenges, including narrow operating load range, lack of ignition timing control, and reducing high unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. These challenges could be because the conventional engine design and fuels cannot adapt well to LTC modes. Thus, replacing conventional diesel fuel with suitable alternative fuels for LTC strategies is essential. In the present work, three LTC strategies, Homogenous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI), and Reactivity Controlled Compression Ignition (RCCI), are compared with conventional diesel combustion in a production, light-duty diesel engine from an alternative fuel perspective to come up with a befitting strategy and fuel to achieve wider operating range and lower emissions. The fuel selection strategy based on the fundamental fuel property requirements of the three LTC strategies has been discussed in detail. The baseline reference data is fixed by comparing three LTC strategies with conventional diesel combustion using diesel and gasoline as reference fuel at 40% load, the maximum common achievable load among the three LTC strategies. This is followed by an investigation of the effect of alternative fuels across three LTC strategies to address the shortcomings of the LTC strategies. The results show that the engine operating load range could be extended, and HC and CO emissions are reduced significantly with alternative fuels in the three LTC strategies.

Geographically divergent trends in snow disappearance timing and fire ignitions across boreal North America

Abstract. The snow cover extent across the Northern Hemisphere has diminished, while the number of lightning ignitions and amount of burned area have increased over the last 5 decades with accelerated warming. However, the effects of earlier snow disappearance on fire are largely unknown. Here, we assessed the influence of snow disappearance timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snow disappearance, which led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest–southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over northwestern (southeastern) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snow disappearance induced earlier ignitions of between 0.22 and 1.43 d earlier per day of earlier snow disappearance in all ecoregions between 2001 and 2019. Early-season ignitions (defined by the 20 % earliest fire ignitions per year) developed into significantly larger fires in 8 out of 16 ecoregions, being on average 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snow disappearance is a good direct proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that lead to earlier ignitions. This indicates that snow disappearance timing is an important trigger of land–atmosphere dynamics. Future warming and consequent changes in snow disappearance timing may contribute to further increases in western boreal fires, while it remains unclear how the number and timing of fire ignitions in eastern boreal North America may change with climate change.

Ignition Delay Times and Chemical Kinetic Model Validation for Hydrogen and Ammonia Blending With Natural Gas at Gas Turbine Relevant Conditions

Abstract Ignition delay times from undiluted mixtures of natural gas (NG)/H2/Air and NG/NH3/Air were measured using a high-pressure shock tube at the University of Central Florida. The combustion temperatures were experimentally tested between 1000 and 1500 K near a constant pressure of 25 bar. As mentioned, mixtures were kept undiluted to replicate the same chemistry pathways seen in gas turbine combustion chambers. Recorded combustion pressures exceeded 200 bar due to the large energy release, hence why these were performed at the high-pressure shock tube facility. The data are compared to the predictions of the NUIGMech 1.1 mechanism for chemical kinetic model validation and refinement. An exceptional agreement was shown for stoichiometric conditions in all cases but strayed at lean and rich equivalence ratios, especially in the lower temperature regime of H2 addition and all temperature ranges of the baseline NG mixture. Hydrogen addition also decreased ignition delay times by nearly 90%, while NH3 fuel addition made no noticeable difference in ignition time. NG/NH3 exhibited similar chemistry to pure NG under the same conditions, which is shown in a sensitivity analysis. The reaction CH3 + O2 = CH3O + O is identified and suggested as a possible modification target to improve model performance. Increasing the robustness of chemical kinetic models via experimental validation will directly aid in designing next-generation combustion chambers for use in gas turbines, which in turn will greatly lower global emissions and reduce greenhouse effects.

Numerical evaluation of ignition timing influences on performance of a stratified-charge H2/methanol dual-injection automobile engine under lean-burn condition

Influences of ignition timing (IT) on combustion, carbon monoxide (CO) and nitric oxide (NO) regulated emissions and formaldehyde (HCHO) and unburned methanol (CH3OH) unregulated emissions in a hydrogen (H2)/methanol dual-injection automobile engine under lean-burn condition were studied numerically. The results demonstrate that at lean-burn (excess air ratio = 2.0) condition, the peak cylinder pressure (PCP), peak heat release rate (PHRR) and peak cylinder temperature (PCT) all decrease with delayed IT, and their corresponding crank angles are also late; ignition delay (ID) and combustion duration (CD) decrease with delayed IT; CO, HCHO and CH3OH emissions increase with delayed IT, while NO emission decreases with delayed IT. At a fixed IT, PCP, PHRR and PCT for with adding 5 % H2 (RH2 = 5 %) are higher than those for without adding H2 (RH2 = 0 %), and their corresponding crank angles for RH2 = 5 % are earlier than those for RH2 = 0 %; ID and CD for RH2 = 5 % are lower than those for RH2 = 0 %. CO, HCHO and unburned CH3OH emissions for RH2 = 5 % are also much lower than those for RH2 = 0 %, while NO emission for RH2 = 5 % is also higher than that for RH2 = 0 %. The influence order of adding H2 to reduce emissions of engine is: CO > HCHO > unburned CH3OH.

Uticaj vremena paljenja i trajanja sagorevanja na karakteristike performansi dizel motora koji koristi vibe 2-zonski model

Efficient energy exploitation is a necessary issue because it helps reduce fuel consumption and environmental pollution. Finding the optimal ignition timing (IT) for diesel engines to create high power and efficiency deserves attention. This study utilizes AVL BOOST simulation software with the Vibe 2-Zone combustion model to investigate the effect of IT and combustion duration on engine characteristics such as power, torque, and brake-specific fuel consumption (BSFC) at different engine loads and speeds. Then, the prediction models of the optimal ITs versus combustion durations for maximum power and minimum BSPFC were computed. The results show that ITs strongly affect engine performance characteristics. The optimal ITs that the engine produces maximum power at different combustion durations are unaffected by engine load. In contrast, they are considerably influenced by engine load when considering the engine-generated BSFC. The correlations of optimal IT versus combustion duration are linear functions. The prediction models can be utilized to predict the optimal ignition timings of the engine since the experimental time can be reduced when applied to the actual engine.

Mechanically induced self-propagating reactions (MSRs) to instantly prepare binary metal chalcogenides: assessing the influence of particle size, bulk modulus, reagents melting temperature difference and thermodynamic constants on the ignition time

Mechanically induced self-propagating reactions (MSRs) of metal chalcogenides can be completed within few minutes and seem to be partly governed by particle size distribution and bulk modulus of the reagents.

Self-evolutionary recycling of flame-retardant polyurethane foam enabled by controllable catalytic cleavage.

Polyurethane (PU) foams, pivotal in modern life, face challenges suh as fire hazards and environmental waste burdens. The current reliance of PU on potentially ecotoxic halogen-/phosphorus-based flame retardants impedes large-scale material recycling. Here, our demonstrated controllable catalytic cracking strategy, using cesium salts, enables self-evolving recycling of flame-retardant PU. The incorporation of cesium citrates facilitates efficient urethane bond cleavage at low temperatures (160 °C), promoting effective recycling, while encouraging pyrolytic rearrangement of isocyanates into char at high temperatures (300 °C) for enhanced PU fire safety. Even in the absence of halogen/phosphorus components, this foam exhibits a substantial increase in ignition time (+258.8%) and a significant reduction in total smoke release (-79%). This flame-retardant foam can be easily recycled into high-quality polyol under mild conditions, 60 °C lower than that for the pure foam. Notably, the trace amounts of cesium gathered in recycled polyols stimulate the regenerated PU to undergo self-evolution, improving both flame-retardancy and mechanical properties. Our controllable catalytic cracking strategy paves the way for the self-evolutionary recycling of high-performance firefighting materials.

Experimental study on the factors influencing performance and emissions of hydrogen internal combustion engines

Hydrogen internal combustion engines (H2-ICEs) have advantages such as clean combustion and zero carbon emissions, and have become one of the important technical routes for decarbonization in the internal combustion engine industry. In this paper, several key factors affecting the performance and emissions of hydrogen internal combustion engines, such as ignition timing, excess air coefficient, and hydrogen injection timing, were systematically studied on a spark ignition multi-point injection (MPI) hydrogen internal combustion engine bench. The experimental results indicate that the ignition timing controls the combustion phase of hydrogen. Moderate early ignition can improve the brake thermal efficiency (BTE) while having little impact on the NOX emissions. Excess air coefficient(λ) can significantly affect the performance and emissions of H2-ICE. Along with the increase of the λ, the NOX emissions first increases and then continues to decline. When the λ reaching 2.1 or above, near zero emissions of NOX can be achieved. The advance of hydrogen injection timing will slightly increase the peak of cylinder pressure and instantaneous heat release rate. However, overall, the impact of hydrogen injection timing on BTE and NOX emissions is not significant on MPI H2-ICE.

The construction of an alternative fuel skeleton with a simplified n-heptane / 1-butene mechanism

Diesel is an important energy in the current world development. The promotion of industrial development has also brought serious drawbacks, energy depletion, and environmental pollution. Hence, the construction of new alternative diesel fuel has become an indispensable problem. Based on the detailed mechanism model of n-heptane and 1-butene, the simplified mechanism model of n-heptane and 1-butene was constructed by using oxidation path and temperature sensitivity analysis and various mechanism simplification methods. The experimental results show that the ignition delay of the simplified mechanism and the concentration of main components meet the experimental requirements. Coupled with the two simplified mechanisms, a diesel simplified framework containing 136 components and 746 primitive reactions was constructed. The HCCI ignition time was calculated for the diesel simplified framework. The simplified framework of diesel oil can predict the ignition time of HCCI with high precision under different temperature conditions. It can accurately describe the fuel combustion process.

Carrier-phase DNS study of particle size distribution effects on iron particle ignition in a turbulent mixing layer

The ignition and combustion of iron particles in a turbulent mixing layer is studied by means of three-dimensional carrier-phase direct numerical simulations (CP-DNS). A particular focus is set on particle size distribution (PSD) effects on the ignition behaviour by comparing CP-DNS results from using a realistic experimental PSD to DNS data based on a monodisperse (MD) particle cloud with the same equivalence ratio. The CP-DNS solves the Eulerian transport equations of the reacting gas phase and resolves all turbulent scales, while the particle boundary layers are modelled in the Lagrangian point-particle framework. A previously validated sub-model for the oxidation of iron to Wüstite (FeO) that accounts for both diffusion- and kinetically-limited combustion is employed. The mixing layer is initialised with an upper stream of air carrying cold iron particles and an opposed lower stream of hot air. Simulation results show distinct differences in the ignition behaviour between the MD and PSD cases. The ignition of the PSD case is delayed compared to the MD case and does not show any significant particle clustering prior to ignition. Further investigations indicate that the particle size has a crucial effect on the mixing process and ignition time. Small particles start their oxidation process early and already consume some of the available oxygen, while not crucially affecting the gas temperature due to their limited iron mass contribution. Conversely, a slower entrainment into the lower stream combined with higher thermal inertia and the prior oxygen depletion by the small particles leads to a delayed oxidation of the larger particles. As a net result, the PSD case shows a wide spread of individual particle ignition delay times and overall delayed bulk ignition compared to the MD case, where the majority of the particles ignites over a shorter period of time.