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Modelling internal combustion engines (ICEs) with multidimensional computational fluid dynamics (CFD) is challenging due to the fuel spray, combustion chemistry and moving piston involved. To accurately simulate ICEs as well as to maintain acceptable computational cost, dynamic mesh refinement techniques could be applied. Although modelling ICEs with staggered mesh is ubiquitous in the engine combustion community, the dynamic refinement of staggered mesh in the context of multidimensional engine simulations is not reported in the literatures due to the numerical difficulties in the momentum solution at the gird fine-coarse interface and to deal with moving boundaries. To address these difficulties, we propose a dynamic refinement scheme to cell-by-cell refine the staggered mesh on-the-fly based on the fuel evaporation and thermochemical state in the multidimensional dynamic staggered mesh and it is able to handle the complexity caused by the moving boundaries in engine simulations and momentum solution at the fine-coarse interface in the dynamic mesh. This mesh refinement scheme is then implemented in the widely-used ICEs simulation codes for engine simulations with fuel sprays. A wide variety of computational examples including shock wave, compression, constant volume fuel spray, constant volume fuel spray combustion are first employed to test the scheme and finally, compression ignition combustion engine simulations are conducted with the proposed methodology. [ABSTRACT FROM AUTHOR]

Highlights • A hybrid model based on the classical CTC model and CHEMKIN model was proposed. • The proposed hybrid model is able to model RCCI combustion with detailed chemistry. • The hybrid model is robust and efficient for RCCI combustion simulations. Abstract This study proposed a hybrid model consisting of a characteristic time combustion (CTC) model and a closed reactor model for the combustion modelling with detailed chemistry in RCCI engines. In the light of the basic idea of the CTC model of achieving chemical equilibrium in high temperature, this hybrid model uses the CTC model to solve the species conversion and heat release in the diffusion flame. Except for the diffusion flame, the auto-ignition in RCCI combustion is computed by a closed reactor model with the CHEMKIN library by assuming that the computational cells are closed reactors. The border of the transition between the CTC model and closed reactor model is determined by two criteria, a critical temperature and a critical Damköhler number. On the formulation of this hybrid model, emphasis is placed on coupling detailed chemistry into this hybrid model. A CEQ solver for species equilibrium calculations at certain temperature, pressure was embedded with CTC for detailed chemistry calculation. Then this combustion model was integrated with the CFD framework KIVA4 and the chemical library CHEMKIN-II and validated in a RCCI engine. The predicted in-cylinder pressure and heat release rate (HRR) show a good consistency with the data from the experiment and better accuracy than that computed from the sole closed reactor model. More importantly, it is observed that this model could save computational time compared with closed reactor model due to less stiff ordinary differential equations (ODEs) computation. A sensitivity analysis of the critical temperature and critical Damköhler number was conducted to demonstrate the effect of these two parameters in the current model. [ABSTRACT FROM AUTHOR]

In this study, numerical analysis of fuel structures on engine soot particles’ mass and size were done by CFD combustion modelling using diesel and different levels of unsaturated biodiesel fuels through the KIVA4-CHEMKIN platform. The proposed numerical approach, with a quad-component skeletal mechanism of biodiesel blend surrogates along with a multi-step phenomenological soot particle model, could capture the soot particle characteristics of test fuels with acceptable accuracy under engine combustion conditions. The reduction of exhaust soot from biodiesel combustion, compared to diesel fuel, was attributed to the suppressed soot precursors formation and lower number of particles in total. However, it was concluded that the biodiesel fuel with a higher fraction of unsaturated FAMEs (more double carbon bonds C C) contributed more to the formation of soot precursors, thus producing a higher amount of soot particles in mass and numbers as a consequence of accelerated soot particle nucleation and soot surface growth. [ABSTRACT FROM AUTHOR]

In the two limits of a well-mixed charge (e.g. homogenous charge compression ignition (HCCI)) engine, and a well separated charge (e.g. conventional diesel combustion (CDC)) engine, the CHEMKIN coupled computational fluid dynamics (CFD) codes approach has been proved to work well in predicting both chemistry controlled combustion and mixing controlled combustion. In this study, it is shown that for some certain cases in the new combustion mode reactivity controlled compression ignition (RCCI) engine where flame propagation exists, the CHEMKIN approach could fail to predict the combustion characteristics. To extend the capability of the existing CHEMKIN models in combustion, a flame propagation model (FPM) accounting for combustion in partially premixed mixtures was proposed and coupled into the CFD framework KIVA4. In this FPM model, the turbulent flame speed was calculated and the heat release and species conversion rate in the turbulent flame brush was solved with detailed chemical kinetics. A NOx sub-mechanism and a nine-step phenomenological soot model were also coupled into the current integrated model for emission prediction. This model was validated in 3 different dual-fuel experimental engines by comparing the in-cylinder pressure, heat release rate (HRR), NOx emissions and soot emissions. The CHEMKIN and CHEMKIN-FPM were also compared in terms of their capability of predicting the combustion in different combustion regimes. The results show that under certain operating conditions when CHEMKIN failed in the combustion prediction, the current CHEMKIN-FPM shows a superior ability in predicting combustion characteristics. [ABSTRACT FROM AUTHOR]

Modelling soot formation and oxidation in diesel engines has been a long-standing challenge. In this study, soot particle characteristics in terms of particle dynamics, particle size and number density were modelled by integrating a multi-step phenomenological soot model into the KIVA-CHEMKIN CFD code for compression ignition engine combustion simulations. This semi-detailed soot model is dedicated to solving rate equations using sub-models to account for precursor formation, soot particle inception and coagulation as well as soot surface growth and oxidation. Soot growth and oxidation is inherently derived from the concentration of certain species found in the chemical reaction mechanism used, namely H, O 2 , C 2 H 2 and the nucleating polycyclic aromatic hydrocarbon (PAH). Acetylene is taken as the core precursor species in nucleating PAH, soot inception as well as acetylene-assisted soot surface growth. The integrated multi-step phenomenological soot model has been validated under gasoline and diesel dual-fuel engine conditions. The predicted histograms of soot particle number along with size distribution contribute towards the understanding of dominant factors that affect soot formation. The factors affecting soot particle under varying engine loads and fuel conditions were extensively investigated. Generally, the predicted soot particle size was larger for heavy-sooting conditions as compared to low-sooting conditions. For port-injected gasoline-dominated combustion, there is less soot being discharged. This might be attributed to two reasons. First, a more homogenous mixture is realized with less diesel fuel, thus effectively reducing the formation of soot precursors. The other reason is the intensive heat release which enhances the depletion of soot precursors, thereby impeding the growth of soot particles in terms of size and mass. [ABSTRACT FROM AUTHOR]

In this work, the spray characteristics of ether fuels such as dimethyl ether (DME) and diethyl ether (DEE) have been numerically investigated using KIVA-4 CFD code. A new hybrid spray model developed by coupling the standard KHRT model to cavitation sub model was used. The detailed thermo-physical properties of ether fuels have been predicted and validated with experimental results available from literature. The cavitation inception inside the injector nozzle hole has been studied for ether fuels in comparison with diesel fuel. It was found that ether fuels cavitates higher compared to that of conventional diesel fuel because of its low viscosity. The spray tip penetration of diesel fuel was longer than that of ether fuels due to high viscosity and density of diesel fuel. Ether fuels characterized by low Ohnesorge number and high Reynolds number showed better atomization behavior compared to that of the diesel fuel. [ABSTRACT FROM AUTHOR]

Varying fuel injection strategies is one of the promising methods to reduce engine out emissions and improve its performance as injection characteristics have great influences on combustion process. Out of various injection strategies, injection rate shaping is potentially an effective technique to reduce emission from engines. Injection rate shaping helps in reducing NOx emissions and reduces combustion noise. This work investigates the effect of injection rate shaping on combustion and emission characteristics of a direct injection diesel engine fueled by biodiesel. The CFD simulations were performed using multi-dimensional KIVA-4 code coupled with CHEMKIN chemistry solver. A detailed chemical kinetics of methyl decanoate (MD) and methyl-9-decenoate (MD9D) with 112 species and 498 reactions were used as surrogate fuel for biodiesel. The injection rate shapes were varied in terms of boot length (long, medium and short boot length) and boot pressure (low, medium and high boot pressure) and it was found from the results that a trade-off between NOx and soot emissions were obtained for long boot length, and high boot pressure injection rate profiles. [ABSTRACT FROM AUTHOR]

A multi-dimensional computational fluid dynamics (CFD) modeling was conducted on a D.I. (Direct Injection) diesel engine fueled by bio-diesel based on KIVA4 code. Comprehensive chemistry prediction of bio-diesel fuel was taken into account by enhancing the combustion model of the default KIVA4 code. An advanced multi-component fuel combustion model was applied to accurately predict the oxidation of saturated and unsaturated agents of the bio-diesel fuel using a reduced chemical kinetics mechanism. In order to accurately model spray, atomization and evaporation of the bio-diesel fuel, detailed thermophysical properties of fuel components were predicted and tabulated in the fuel routine of the KIVA4 code. After the validation for cylinder pressure and heat release rate at engine mid load with experimental engine tests, further numerical studies were performed to investigate effects of injection strategies such as double and triple injection pulses and axial location of injector nozzle. It has been found that cylinder peak pressure was increased by applying double and triple injections and some enhancements on output power was also observed. Moreover, it was found that nozzle axial location has considerable effect on combustion of the bio-diesel fuel where wall impingement of the liquid bio-diesel fuel resulted in lower evaporation and higher unburnt hydro carbon (UHC) emission. [ABSTRACT FROM AUTHOR]

The performance of rotary engines is significantly influenced by the flow field. In this study, a detailed mathematical model was integrated into the simulation software FLUENT to investigate the gas flow field in a peripheral ported rotary engine by including a dynamic mesh model and a turbulent flow model. The models were also validated by experimental data. The basic flow mechanism in the combustion chamber was numerically studied. Meanwhile, the effects of the three major parameters on the flow field inside the combustion chamber, namely, rotating speed, intake shape, and intake angle, were also investigated. Results showed that a constantly changing swirl was formed in the combustion chamber during the intake and compression strokes as a result of the combined effects of the pocket of the rotor and the swirls in the combustion chamber. The swirl eventually broke into a unidirectional flow near the top dead center because of the significant decrease in combustion chamber volume. Furthermore, with the change in rotating speed, intake shape, and intake angle, significant differences in flow speed, inertia, and distribution were observed when the fluid entered the combustion chamber, which, in turn, led to obvious differences in the flow field, volume coefficient, and average turbulence kinetic energy in the combustion chamber. [ABSTRACT FROM AUTHOR]

In the present study, effects of hydrogen induction on combustion characteristics of a compression ignition diesel engine were investigated and a comprehensive model for identifying knocking combustion was developed. This was done by defining number of critical local regions within the CFD (computational fluid dynamics) computational domain for a hydrogen assisted compression ignition engine. Regional parameters such as local pressure rise rate, local heat release rate and local concentration change of specific chemical species were used for knock identification. Comprehensive chemical kinetics mechanisms of diesel and hydrogen fuels were used enabling detailed chemistry predictions. After validation of the model for extensive diesel operating conditions; 1%, 3%, 5% and 7% hydrogen induction in volume in intake air was considered for a single case to investigate knocking combustion. Using the developed knock prediction model, results showed knocking combustion for hydrogen-air premixed charges richer than 5% by volume. This was well captured by the regional pressure rise rate and heat release rate diagrams. Moreover, regional data showed that knock occurred in central parts of the piston bowl and above the piston crown, whereas location near to cylinder wall did not show the same trend. In former locations, very high rate of production and consumption for HO 2 as a free radical was resulted. This was coincided with higher hydrogen consumption and temperature rise. [ABSTRACT FROM AUTHOR]

Highlights: [•] Nozzle flow and spray characteristics of methyl esters have been studied. [•] A new hybrid spray model was implemented into KIVA4 CFD code. [•] Nozzle flow simulation shows methyl stearate has less cavitation. [•] Methyl linoleate atomization is comparable with diesel atomization. [•] Methyl stearate has poor atomization compared to other methyl esters. [ABSTRACT FROM AUTHOR]

• A new reduced reaction mechanism for toluene and decalin oxidation was proposed. • The current mechanism was comprehensively validated with experimental data. • Negative temperature coefficient regime of decalin ignition delay time was analyzed. • Sensitivity analysis was conducted for four typical polycyclic aromatic hydrocarbons. In this work, a reduced toluene-decalin reaction mechanism containing 108 species and 566 reactions was proposed for computational fluid dynamics (CFD) simulations and polycyclic aromatic hydrocarbon (PAH) formation predictions in combustion engines. The present mechanism was validated with the available literature data including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCM), species mole fractions measured in premixed flames and jet stirred reactors (JSR), and laminar flame speeds. Moreover, a sensitivity analysis was performed on toluene and decalin flames to further investigate the main formation pathways of four representative PAHs: benzene (A 1), naphthalene (A 2), phenanthrene (A 3) and pyrene (A 4). In general, the simulation results exhibited a reasonable agreement with the experimental data. The negative temperature coefficient (NTC) behaviors of decalin IDTs were accurately reproduced by the proposed mechanism. The PAH species concentrations were also well captured for ethylene and benzene flames. The sensitivity analysis results indicated that the main formation reactions for PAHs have a strong link with ring structures. The decompositions of toluene and decalin primarily contributed to the formation of A 1. For toluene, A 2 was formed by the reactions A 1 - + C 4 H 3 = A 2 and IC 4 H 5 + A 1 = A 2 + H 2 + H, while for decalin, the self-combination reaction of C 5 H 5 became the main pathway. In addition, the reactions of the aromatic molecules and radicals significantly promoted the formation of A 3 and A 4 for both toluene and decalin. [ABSTRACT FROM AUTHOR]

• A new bi-variate soot model is proposed for description of soot aggregate formation. • This model enables a detailed insight into soot particle dynamics at low CPU cost. • This model is able to predict engine-out soot emissions with high accuracy. • This model is superior to previous models in description of soot oxidation. In this work, a new pseudo bi-variate soot model is proposed for description of soot aggregate formation and oxidation in compression ignition engines. This soot model is a combination and modification of existing sub-models for soot nucleation, aggregation, surface growth and oxidation. The main advantage of this model over other bi-variate models is that it is able to provide a detailed insight into the soot aggregate formation processes with much less numerical difficulty. This new soot model has been implemented into a commercial computational fluid dynamics code and solved using an advanced method of moments. The resulting code is applied to investigate the formation of soot aggregates in a compression ignition engine operated under different conditions. Results suggest that this new pseudo bi-variate soot model is computationally cheap with little CPU cost induced by the treatment of the detailed soot particle dynamics. The model is able to correctly capture the emission trend for soot particles formed in compression ignition engines under different operating conditions. Detailed insight into the evolution of the soot aggregates in the engine cylinder can be provided. [ABSTRACT FROM AUTHOR]

Hetero-/homogeneous combustion of hydrogen–air mixture in a platinum-coated micro channel was studied by means of three-dimensional (3D) Computational Fluid Dynamics (CFD) model using detailed reaction mechanisms for homogeneous (gas phase) and heterogeneous (catalytic) reactions. The influence of heterogeneous reaction on homogeneous reaction was obtained by discussing the hetero-/homogeneous reaction characteristics, the process and competitiveness of heterogeneous reaction. Intense depletion of fuel and product formation near the inlet has shown that fuel absorption reaction was predominant whiles there was no homogeneous reaction. The free radicals from homogeneous reaction near the catalytic surface were drained, resulting in the interruption of chain reaction. The desorbed product as the third-body promoted the chain terminating reaction to deactivate free radical. The competitiveness value of heterogeneous reaction for capturing fuel was 14.85%, indicating a weak reaction intensity in the catalytic combustor. The heterogeneous reaction presented an inhibition effect on homogeneous reaction but greatly enhanced the combustion efficiency in the micro channel. [ABSTRACT FROM AUTHOR]

This study aims at investigating the syngas production and sulfur conversion mechanisms during the chemical looping gasification (CLG) process with the industrial by-product petroleum coke (petcoke) as fuel, which is beneficial to the waste-to-energy process. The chemical kinetics including petroleum coke decomposition, char gasification, oxygen carrier reduction and the sulfur species reaction model are incorporated into the dynamic model to simulate a CLG fluidized bed reactor with iron-based oxygen carriers. Results suggest that the model is able to well predict the time-varying concentrations of the syngas products and sulfur-containing gases. The near-zero COS emission from the reactor outlet confirms the previous experimental results and contributes additional evidence that the presence of steam enhances the conversion of COS to H 2 S. The effects of temperature, steam and N 2 flow rates on the petcoke CLG performance are also evaluated. The results indicate that higher temperature and steam flow rate lead to an improvement in the conversion of carbon and sulfur in petroleum coke. However, further increasing the gas flow rates may result in a more intense fluidization state, which might significantly facilitate the OC reduction reactions with flammable gases, thus increasing the CO 2 and SO 2 production in the petcoke CLG process. [Display omitted] • A CFD model is developed to investigate syngas production from petcoke CLG process. • The model considering sulfur species reactions is well validated against experiments. • The presence of steam facilitates H 2 S production but impedes COS generation. • Effects of temperature and steam flow rate on petcoke conversion are evaluated. • Further increasing the gas flow rates results in a more intense fluidization state. [ABSTRACT FROM AUTHOR]

• A CFD model is developed to simulate biomass CLG with continuous feedstock. • The model is well validated against experiments in terms of time-varying results. • The particle behaviors have strong impacts on the gas composition distribution. • Increase of temperature or biomass feeding rate benefits the syngas production. • Introducing steam as gasifying agent remarkably enhances the CLG performance. Integrating the gasification process with the chemical looping technology presents a promising route for biomass conversion with the objective to obtain high quality syngas without air separation. In this study, the biomass gasification with iron-based oxygen carrier and continuous feedstock in the bubbling fluidized bed (BFB) fuel reactor has been investigated based on the computational fluid dynamics (CFD). The solid phases including fuel and oxygen carriers are modeled based on the pseudo-fluid assumption. The numerical model integrates the multi-fluid model and the chemical reaction models involving the decomposition and gasification of biomass and the heterogeneous reactions between gases and metal oxides. The predicted time-varying outlet concentrations of five gas components agree well with the experimental data from the literature. The impacts of the mixing and segregation behaviors between two solid phases on the gas composition distribution are analyzed. The effects of operation temperature, fuel feeding rate and steam content on the chemical looping gasification (CLG) performance are also investigated. The concentrations of CO and H 2 as well as the gas yield and gasification efficiency increase while the concentrations of hydrocarbons and CO 2 decrease with the escalating temperature because of the facilitation of higher temperature on the endothermic reactions. Raising the feeding rate of biomass leads to a higher gasification efficiency with more valuable syngas but a lower carbon conversion efficiency due to the relatively lower OC-fuel ratio. The gasification atmosphere containing 10–50% of steam also brings remarkable enhancements on the H 2 concentration, gas yield and gasification efficiency. [ABSTRACT FROM AUTHOR]

• A new approach for soot modelling in engine simulation is proposed. • A moment projection method is adopted to handle the soot oxidation problem. • A detailed insight into soot particle dynamics can be achieved at low CPU cost. In this work, an efficient numerical approach for detailed soot particle dynamics modeling in three-dimensional engine simulation is proposed. The recently developed moment projection method is incorporated into engine computational fluid dynamics codes to handle the detailed soot particle dynamics. The main advantage of moment projection method over other methods of moments is its ability to handle the soot oxidation problem which plays a significant role in determining the evolution of soot particles in engines. The new approach has been applied to study the soot formation and oxidation processes in a dual-fueled compression ignition engine operated under different conditions. Results suggest that the new approach is highly efficient with little CPU cost induced by the treatment of the detailed soot model. The integrated engine-soot model is able to correctly capture the emission trend for soot particles under different fuel injection timings. With this new approach, a detailed physical and chemical insight into the soot formation and oxidation processes inside the engines can be achieved. The findings indicate that the proposed numerical approach is a promising framework for soot modeling in three dimensional engine simulations. [ABSTRACT FROM AUTHOR]

As a zero-carbon fuel and hydrogen carrier, ammonia has received much attention for its excellent carbon reduction potential. To explore the feasibility of zero-carbon ammonia as fuel for in small-scaled Wankel rotary engines, a computational fluid dynamics model coupled with a kinetic mechanism was established and validated. It is found that the fuel mixture cannot be ignited when the hydrogen substitution ratio (HSR) is less than 5%. Increasing HSR shortens flame development period and intensifies combustion. When HSR is greater than 12.5%, the fuel can be burned up, and the position of peak heat release rate remains close to 20°EA aTDC. Elevated HSR leads to higher NO emissions but lower NO 2 and N 2 O emissions. As expected, advancing ignition timing (IT) significantly enhances combustion efficiency and reduces emissions. Advancing the IT results in a slight increase in the unburned area at the rear of combustion chamber, coupled with a rapid decrease in the unburned area at the front, collectively reducing unburned fuel. When IT is advanced from −5 to −35°EA aTDC, emissions and performance increase rapidly, whereas when advanced to −45°EA aTDC, both are nearly unchanged and combustion efficiency decreases. • Using ammonia and hydrogen in small-scaled Wankel rotary engines • Flame propagation is highly dependent on ammonia mass fraction. • Optimizing ignition timing can reduce emissions while increasing efficiency. • Increasing the hydrogen mass fraction reduces NO 2 and N 2 O but increases NO. [ABSTRACT FROM AUTHOR]

• CO 2 dilution produces lowest temperature increase under MILD combustion mainly due to chemical effect. • Negative heat release region disappears under MILD combustion in regardless of diluents. • CO 2 is preferable than both N 2 and H 2 O for establishing MILD combustion regime. • CO formation increases in CO 2 -diluted MILD combustion from CO 2 + CH 2 (S) ↔ CH 2 O + CO. • C 2 H 2 formation is extremely low under MILD combustion, suggesting low sooting tendency. To deepen the understanding of combining moderate or intense low-oxygen dilution (MILD) combustion with oxy-fuel combustion for enhancing flame stability while realizing carbon capturing and storage, this paper presents a numerical study of methane MILD combustion in three atmospheres, i.e.: O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O, with both computational fluid dynamics (CFD) and kinetic calculation approaches. Firstly, CFD predictions for the three conditions were performed following a systematic validation of the numerical method against experimental measurement from methane/air MILD combustion in a laboratory-scale closed furnace. Subsequently, kinetic calculations with a well-stirred reactor model was used to quantitatively identify the operating ranges of MILD combustion in the three atmospheres for methane. Moreover, the kinetic calculation provided additional insight into the fuel oxidation pathway. The results reveal that replacing N 2 with either CO 2 or H 2 O would help to establish MILD combustion mode from the viewpoint of lower temperature increase, due to both physical and chemical property discrepancies among the diluents. Specifically, the chemical effect and physical effect are responsible to the lower temperature rise for CO 2 -diluted case and H 2 O-diluted case, respectively. Inside the MILD combustion furnace, the negative heat release region disappears in regardless of atmospheres, indicating the eliminated fuel pyrolysis process under MILD combustion mode. Detailed analysis of the flame structure suggests that the combustion regimes inside the furnace in the three atmospheres are all in well-stirred combustion regime, and CO 2 -diluted case has the most extended reaction zone. Kinetic calculation indicates that CO 2 or H 2 O dilution would result in a wider MILD combustion operating range compared to N 2 dilution, while it is more pronounced for CO 2. These observations all imply that MILD combustion will be more easily established with CO 2 dilution than N 2 or H 2 O dilution. However, higher CO formation is obtained in O 2 /CO 2 , forcing more attention to be paid on CO emission under CO 2 -diluted MILD combustion. Furthermore, the hydrocarbon recombination route is negligible under MILD combustion in spite of the atmospheres, implying lower sooting tendency as compared with conventional combustion. [ABSTRACT FROM AUTHOR]

Graphical abstract A schematic summary of the ash formation and deposition modelling processes including the input parameters, combustion codes, ash deposition sub-models and outputs. Highlights • A summary of ash deposition mechanisms used in numerical and experimental studies. • A comprehensive review of ash deposition modelling with particle sticking behavior. • A comprehensive review of ash deposition modelling with combustion codes. • A review of ash deposition modelling with particle rebound and removal behavior. • A comprehensive review of the modelling of ash deposit morphology. Abstract This paper presents a comprehensive review on the development of the modelling of ash deposition with particle combustion, sticking, rebound and removal behaviors. The modelling of ash deposit morphomology is also included. Ash deposition in coal and biomass fired boilers will induce many ash-related issues (such as slagging, fouling and corrosion) which will reduce the boiler efficiency and capacity. Some traditional prediction methods have been proposed to evaluate ash deposition. However, these methods are based on chemical compositions of ash deposits and the operating temperatures in boilers, which are unable to fully predict the complex ash deposition process. Great efforts have been made to develop mechanistic models to predict ash deposition processes in nature. The behavior of ash formation and deposition in the boilers plays a key role in the design of boilers and the selection of fuels. The ash formation process is primarily due to the fragmentation and coalescence of mineral matters in fuels and only a small portion of ash is formed. When ash particles impact the heat transfer surfaces, only a few particles will deposit on these surfaces and several ash deposition mechanisms have been identified to predict their behaviors, such as inertial impaction (for large particles), thermophoresis (for fine particles) and condensation (for vapors). The ash deposition mechanisms used in the experimental, numerical and mechanistic studies coupled with the fuels and investigated systems are summarized in this paper. Numerous attempts have been made to develop different models to overcome the shortcomings of traditional methods. As such, various numerical attempts have been made to predict the growth behavior of ash deposition in furnaces and boilers by employing comprehensive combustion models coupled with high fidelity computational fluid dynamics (CFD) modelling methods for different types of fuels. Furthermore, several combustion codes have been incorporated into the ash deposition models, including the fuel combustion process (the release of volatiles, devolatilization and char combustion), wall reaction and consumption sub-models as well as the packed bed and overbed combustion sub-models. Moreover, several ash deposition sub-models have been developed to predict the ash deposition growth behavior using computational fluid dynamics (CFD) methods in combustors of different scales. In order to better understand the impact behavior of ash particles, some analytical models such as dynamic models and kinetic models have been developed. For accurate prediction of impaction efficiency, an impaction correction factor has been proposed to reduce the effect of coarse meshes on the impaction efficiency. Also, the stickiness of ash particles which is determined by kinetic energy, viscosity and molten degree of ash particles plays a key role in the ash formation and deposition processes and determines whether ash particles stick on the surfaces or rebound from the surfaces. Briefly, three main types of particle adhesion theories are used to evaluate if an impacting particle bounces off or sticks to the surface, namely, the viscosity-based empirical model, the critical velocity model and the melt fraction model. When the particles impact the heat transfer surfaces, they may rebound from the surface or remove the ash deposit. The particle surface energy with its static contact angle is also important in determining the sticking efficiency. In addition, several rebound criteria have been proposed to predict the particle rebound behavior, which include the critical rebound velocity, energy balance, excess energy, bouncing potential and critical impact angle on a flat or oblique plate or on a heat transfer tube. Tremendous efforts have also been made to develop the theories and mechanisms of ash deposition as well as removal on the surfaces of heat exchangers, some of which were based on the Kern-Seaton theory. Furthermore, several removal sub-models have been proposed to predict the particle removal behavior, including the energy balance, moment conservation, energy dissipation, critical moment theory, critical shear velocity and critical impact angle. In the actual fouling process, the growth behavior of fouling on the tube surfaces changes the original tube shape continuously. The fly ash deposited on the surface alters the boundary of heat transfer and affects the distribution of the temperature, flow field as well as the deposition rates. Various theoretical methods have been proposed to predict the ash deposit morphology, including the lattice Boltzmann method (LBM), dynamic meshing method, and time and mass magnification factors. In addition, many investigations have focused on developing models for inter-particle thermal conductivity by studying the ash deposit microstructure to characterize the thermal and morphological changes through an ash deposit. Several ash deposition layer models have been experimentally and theorectically studied in details, including the two-layer, three-layer, four-layer and six-layer sub-models. [ABSTRACT FROM AUTHOR]

A decoupled numerical modelling method is developed in this study to simulate the whole combustion process of biomass in a grate firing boiler, which includes the thermochemical conversion of biomass in the fuel-bed and gaseous combustion in the freeboard. With the aid of this modelling method, the objective of this study is to explore the NO X reduction mechanism as well as to investigate the potential of internal flue gas recirculation technology (IFGRT) on the combustion process and emissions formation in a 40 t/h biomass-fired grate boiler. Computational fluid dynamics (CFD) modelling results show that IFGRT can be realized in the grate boiler by establishing intense flue gas recirculation within the boiler, which allows for lower peak combustion temperatures and smaller flame kernel sizes, while improving the overall average gas temperature. Consequently, NO X emission can be reduced mainly via the thermal formation route in comparison with the conventional combustion case. More specifically, the parallel over-fired air (OFA) burner configuration is suggested for implementation to produce even lower NO X emission compared to the staggered OFA burner configuration. To further understand the reasons behind the NO X reduction, NO X formation and destruction mechanisms are also examined through reduced order modelling (ROM) with the help of detailed reaction chemistry. It is revealed that flue gas recirculation inhibits NO X formation from thermal, NNH and N 2 O routes. Although NO X destruction rate through reburning is suppressed, the net NO X production rate is found to be decreased under the condition of IFGRT. Moreover, as the flue gas recirculation ratio increases, final NO X emission shows a decreasing trend. [ABSTRACT FROM AUTHOR]

Numerical simulation of stoichiometric hydrogen-air premixed flame in a platinum-coated micro channel was performed to investigate hetero-/homogeneous combustion characteristics. An elliptical two-dimensional (2D) Computational Fluid Dynamics (CFD) model with detailed reaction mechanisms for homogeneous (gas phase) and heterogeneous (catalytic) reactions was adopted. The hetero-/homogeneous reaction characteristics with different inlet flow velocities and flame speeds were used to evaluate the effect of heterogeneous reaction on homogeneous reaction in the combustor with 2 mm length of catalyst segmentation. The effect of heat generation of the heterogeneous reaction was also analyzed. The numerical results indicated that the highest temperature in the catalytic combustor was lower than that in the combustor without catalyst. In the presence of heterogeneous reaction, the species (OH) concentration near the catalytic surface decreased significantly due to the dominated heterogeneous reaction at that region. The homogeneous ignition distance increased with increasing inlet flow velocity. The heat transfer from the catalytic surface to the gaseous mixture in the catalytic zone exhibited a high intensity under the large flow velocity, and the preheated incoming flow improved the heterogeneous reaction to inhibit the homogeneous reaction. The decreased flame speed in the catalytic combustor further demonstrated that the homogeneous reaction was suppressed by the heterogeneous reaction. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of operating pressure (up to 8 atm) on the moderate or intense low-oxygen dilution (MILD) combustion of methane. Computational fluid dynamics (CFD) modeling was firstly conducted for a laboratory-scale combustor to obtain the distributions of temperature and minor species (CH 2 O and OH), and pollutants (CO and NO) emission under elevated operating pressures. To provide further insight into the heat release characteristics, flame structure, CO formation mechanism as well as extinction limit, a counterflow diffusion flame with hot oxidizer diluted oxidizer configuration was also modeled using detailed chemistry. The CFD modeling results show that increasing the operating pressure tends to increase both peak value and spatial gradient of the combustion temperature, and reduce the width of the reaction zone, indicating the departure of MILD combustion regime when using the identical burner. However, enhancing the internal flue gas recirculation by minimizing the air nozzle diameter can help sustain the MILD combustion regime. The kinetic modeling shows that MILD combustion features only positive heat release region, while traditional combustion exists both negative and positive heat release regions, where multiple peaks are observed. In addition, MILD combustion shows a dual oxygen consumption stage regardless of operating pressures. Simultaneously, MILD combustion exhibits a stretched S-curve of flame response without extinction and re-ignition processes, while traditional combustion presents obvious extinction and ignition points. These unique phenomena can be used for identifying MILD combustion regime in future work. • Impact of pressure on MILD combustion of methane was studied numerically. • Only the positive part is exhibited in the heat release profile. • A dual-stage oxygen consumption process is attained by MILD regime. • Flame response to stretch rate was compared between HTC and MILD modes. • Higher velocity helps to sustain MILD regime at elevated pressures. [ABSTRACT FROM AUTHOR]