Nucleation of incipient soot or carbon black nanoparticles has significant scientific and industrial importance because it influences the particle size distribution, morphology and composition, hence its health and environmental impact as well as functional properties. Reversible Polycyclic Aromatic Hydrocarbon (PAH) Clustering (RPC) with van der Waals forces (vdW) was proposed recently as opposed to Irreversible PAH Clustering (IPC) for soot nucleation (Eaves et al., Proc. Combust. Inst, 35 (2015) 1787) to relax the assumption of stable dimer formation with physical bonds at flame temperatures. Here, the necessity of considering chemical bond formation between PAHs in a dimer for reducing soot nucleation reversibility in ethylene coflow diffusion flames with a wide range of nitrogen dilution ratios is demonstrated. An RPC model with Chemical Bond Formation (RPC-CBF) is developed and its performance is compared to those of RPC and IPC models. Only the RPC-CBF model with low reversibility for PAH addition on the surface of soot primary particles can predict soot volume fraction, average primary particle diameter, primary particle number density and the bimodal size distribution of soot agglomerates on the flame centerline within experimental uncertainty. While the IPC model overpredicts soot concentrations by a factor of five with increased dilution, the RPC model underpredicts it by more than two orders of magnitude for the flame with 68% nitrogen dilution. The RPC model fails to predict the observed bimodality of agglomerate particle size distribution in flames with low dilution because its predicted nucleation rate is much weaker compared to that of growth. Accounting for chemical bond formation for dimers is essential to form enough nuclei with increasing dilution and temperature. Also, low reversibility for PAH addition is required to predict the balance between nucleation and surface growth, hence the average size of soot primary particles. Crown Copyright (C) 2018 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.

A numerical study is conducted of methane-air coflow diffusion flames at microgravity (μg) and normal gravity (lg), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centerline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centerline of the flame to the wings in microgravity. Keywords: laminar diffusion flame,methane-air,microgravity, soot formation, numerical modelling

This study investigates computationally the influence of CO2 addition on the sooting behavior in premixed laminar ethylene/ oxygen/ argon burner stabilized stagnation (BSS) flames at the atmospheric pressure. The discrete sectional aerosol dynamics method combined with a reversible nucleation model and a novel model of reversible polycyclic aromatic hydrocarbon (PAH) condensation were employed to predict the size evolution of the particle size distribution (PSD) function. The predicted temperature profiles and PSD functions are in reasonably good agreement with the experimental data for nascent soot measured in the BSS configuration. The evolution of the PSD functions shows that CO2 addition reduces the soot nucleation and mass growth rates, consequently lowering the soot yield. The addition of CO2 reduces the concentrations of H, C2H2, C6H6, and large PAHs (e.g., pyrene) which all suppress the soot formation process through a chemical effect; the thermal effect of CO2 is not as strong as the chemical effect.

A novel model called Surface Shell Formation (SSF) is developed which predicts soot maturity based on the equilibrium nanostructure of poly aromatic hydrocarbons (PAHs) inside soot primary particles. The characteristic isotropic core-graphitic shell internal nanostructure of soot primary particles is used to distinguish nascent from mature soot primary particles. A new sectional soot model is developed to track particle Hydrogen to Carbon (H/C) ratio and the growth in the molecular weight of PAHs inside soot primary particles. An independent Arrhenius term describes particle dehydrogenation/carbonization. It is shown that soot maturity depends on both particle size and H/C ratio. Graphitic shell formation in mature soot particles is related to the surface PAHs that change configuration from edge on surface for nascent soot to face on surface for mature soot particles. The new model is validated against experimental data for laminar premixed, partially premixed and diffusion flames. The SSF model addresses the two major limitations of the current soot modeling approaches. First, it predicts H/C ratio of soot particles by considering soot carbonization. Second, it distinguishes nascent soot from mature soot primary particles based on the internal nanostructure of soot primary particles and the presence of the graphitic shell.

Two coalescence models based on different merging mechanisms are introduced. The effects of the soot coalescence process on soot particle diameter predictions are studied using a detailed sectional aerosol dynamic model. The models are applied to a laminar ethylene/air diffusion flame, and comparisons are made with experimental data to validate the models. The implementation of coalescence models significantly improves the agreement of prediction of particle diameters with the experimental data. Sensitivity of the soot prediction to the coalescence parameters is analysed. Finally, an update to the coalescence model based on experimental observations of soot particles in the flame oxidation regions has been introduced to improve its predicting capabilities.

A new model for condensation of polycyclic aromatic hydrocarbons (PAHs) on the surface of soot particles is developed based on the reversibility of the process. Statistical mechanics has been integrated with a novel PAH condensation concept to estimate the PAH evaporation rate. This estimation has been used to develop a condensation efficiency model which is easy to implement, computationally efficient, and relies on physical parameters which can be evaluated. The new condensation model, combined with a reversible nucleation model (Eaves et al., 2015), is validated for predicting soot formation in both premixed and nonpremixed flames. The validations are followed by an extensive sensitivity analysis to evaluate the role of dominant soot processes as well as PAH chemistry on soot formation and particle size distribution (PSD). Simulations include a set of burner stabilized stagnation premixed flames and a coflow ethylene/air diffusion flame. This work is the first study which successfully utilizes a single soot model to quantitatively predict soot formation in both flame configurations.

The effect of soot surface reactivity, in terms of the evolution of sites on the soot particles’ surface available for reaction with gas phase species, is investigated via modeling numerous ethylene/air flames, using a detailed combustion and sectional soot particle dynamics model. A new definition of a particles’ age is introduced. A methodology has been developed to study soot particle surface reactivity. Subsequently, it is investigated if the surface reactivity can be correlated with the particle age. An exponential function giving a smooth transition of surface activity with particle age is employed to model a variety of ethylene/air flames, which differ in fuel stream dilution levels, fuel stream premixing, and burner configurations. Excellent agreement with measured soot volume fractions of a variety of flames, burners, and datasets could be obtained with this approach. The newly developed function based on particle age eliminates the need to fit soot surface growth parameters to each experimental condition. Finally, the applicability and limitation of the new surface reactivity function for use in detailed soot formation models is discussed.

A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer–Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.

A numerical study is conducted of ethane–air coflow diffusion flames at pressures from 2 to 15atm. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a detailed sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with increasing pressure without any tuning of the model for different pressures. The model shows good agreement with the experimental data on both the flame wings and centerline regions. Peak wing and centerline soot volume fractions are found to scale with P2.49 and P2.02 respectively. This scaling compares well to that observed experimentally for methane–air and ethylene–air flames. As pressure is increased, the flame cross-sectional area decreases according to P−1.0 due to a constant mass flux and a thinning of the flame, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation dominates centerline soot formation. Surface growth and PAH condensation increase with increasing pressure primarily due to both of these processes being a function of surface area. This causes increases in soot volume fraction to further accelerate surface growth and PAH condensation, acting in a positive feedback manner. This positive feedback mechanism is initiated by increases in reaction rates caused by increases in gas phase density. Additionally, the significance of surface growth decreases with increasing pressure, while the role of PAH condensation increases.

In this paper, the performance of a plate finned tube heat exchanger, operating under frosting conditions is investigated. Frost accumulation on the fins and tubes of a heat exchanger, a direct result of combined heat and mass transfer of the moist air flowing across a cold surface, causes heat transfer performance degradation due to the insulating effect of frost layer and the coil blockage as the frost grows. A quasi-steady control-volume model for modeling a plain-fin round-tube heat exchanger under frosting condition considering the thermal contact resistance between the fin collar and tube was developed and the governing equations are solved numerically. In order to make the model more realistic, the variation of frost thickness along the fin has been taken also in consideration. Also the model is capable to predict the behaviour of an air cooler under no frosting condition. Comparisons were made based on the frost mass accumulation, pressure drop across the coil and energy transfer coefficient and results were found to agree well with reported experimental results. Also the effects of face velocity, relative humidity and working fluid inlet temperature on the air side pressure drop and overall heat transfer coefficient have been investigated.