Modeling Of Combustion Processes Research Articles

QUESTIONS AND PROBLEMS OF MATHEMATICAL MODELING QUA NONEQUILIBRIUM OF COMBUSTION PROCESSES

On the basis of thermodynamic analysis, new mathematical models of the combus- tion process (thermal theory) and vibrational combustion are constructed. A global inhomo- geneity of the system can be described as an inhomogeneous distribution of the enthalpy over a two-component mixture. In this case, for the combustion process in the phase space of the variables (%, P, T, n, S, E), an increase in the enthalpy is not a total differential. An increase in the enthalpy is a total differential on the local equilibrium manifold (a laminar combustion process). These two assertions, which allow one to single out in the phase space the corre- sponding adiabatic of the combustion process (the Hugoniot adiabatic) and the equation for the entropy, close the classical mathematical model of the combustion process. The above numerical experiments show that two regimes of the combustion process (deflagration and detonation) depend on the structure of the standard chemical potential Moreover, a control of the passive component velocity at the inlet results in (depending on the structure of the standard chemical potential) high-frequency oscillations, which are responsible for a blow-up.

Numerical Modelling of Gaseous Fuel Combustion Process with the Stepwise Redistribution of Enriched Combustion Air

The authors of this paper present the results of simulations for burner system design changes in the smelting aggregate. Based on the analysis of the existing burner system in the experimental aluminium smelting equipment, changes in the burner design were proposed. The obtained results are presented in tables and figures. The properties of the proposed changes were investigated using the simulation software ANSYS. The simulations confirmed the suitability of the proposed system for shortening the flame length and intensification of the mixing of gaseous media.

Numerical Study on NOx Emissions of Methane Re-Combustion in a 600 MWe Coal-Fired Boiler

The fuel staging combustion technology is a promising low NOx combustion technology for coal-fired boiler. In order to reduce NOx emissions, the burners of a 600 MWe coal-fired boiler are retrofitted in which methane gas is selected as a secondary fuel for re-combustion. The CFD models of combustion process are built to investigate effects of the methane gasratio on combustion process and NOx emissions. A total of 4 cases are numerically studied, including the pure coal combustion case, the coal combustion with 7.5%, 10%, 12.5% of methane gas re-combustion cases respectively. The results show that the re-combustion of methane can reduce the temperature at primary combustion zone, but increase the temperatures at the re-combustion area and the furnace outlet. The NOx concentration at the furnace outlet reduces with the increasing methane gas ratio. Methane re-combustion can greatly benefit to the NOx emissions reduction.

Degree centrality of combustion reaction networks for analysing and modelling combustion processes

Combustion research still needs more advanced fundamental understanding of combustion chemistry and dynamics from molecule scale to particle. The latter is also needed for soot and nanoparticles formation and combustion system control such as homogeneous charge compression ignition engine, and flame regimes and instability. The complex interactions between hundreds of species linked within thousands of reactions continue to be a challenge to analyse and model. The focus on this paper is to develop a method to facilitate the modelling and analysing of the detailed kinetics chemistry of fuels combustion. Through the use of combustion reaction networks (CRNs) analysis of degree centrality, principal species are identified during a combustion process by exploiting the introduced definition of principal species. A principal, central or the more active species of a combustion process, at a specific time step or cell of area/volume mesh, is the more tied up to other species in the CRN and so have the largest value of degree centrality. The accuracy of the dynamic identification of principal species, locally adapted to the thermochemical conditions at each time step/cell of the simulated combustion process, used by the employed directed relation graphs method of mechanisms reduction, is proved. The simulations were carried out using an adjusted dynamic adaptive chemistry approach of detailed chemistry implementing. It is demonstrated that an ‘active’ species in a combustion system would not necessary be considered as a part of important species set needed for its predictive simulations.

Predicting the Temperature and Composition – Dependent Density and Viscosity of Diesel Fuel – Ethanol Blends

Density and viscosity are very important fuel properties which have a major influence not only on the fuel production, transportation and distribution processes but also on the processes that take place in an internal combustion engine. Developing robust and high precision density and viscosity models for stabilized diesel fuel – ethanol blends helps the production of fuel to adhere to the quality requirements regarding density and viscosity and the modeling and simulation of injection and combustion processes. For modeling the density and the viscosity of diesel fuel – ethanol blends, five mixtures were prepared with ethanol content up to 15 % (v/v) and were stabilized by adding tetrahydrofuran as a surfactant at room temperature. The temperature-dependent density and viscosity of the blends were measured at four different temperatures (0, 15, 40 and 50 °C) using an SVM 3000 type apparatus. Based on experimental data, several mixing rules were fitted to them and three new models were developed, of which two need only one experimental value. These models yield very good accuracies, presenting average relative deviations of 0.0604 % in the case of density and 3.8931 % in the case of viscosity.

Effect of the residual water content in gels on solution combustion synthesis temperature

The composition of gels and xerogels, as well as their transformation during heating and dehydration, determine the thermochemistry of solution combustion synthesis reactions. An improved descriptive thermodynamic model of combustion processes was formulated on the basis of the investigated formation of complex compounds of metal ions with organic fuel (glycine, citric acid, urea, and PVA) in nitrate solutions. The intensity of SCS reactions was found to depend on the strength of Ni2+–ligand complexes. The effect of heat loss during combustion on the ΔTmax value was analyzed for the model system Ni(NO3)2·nH2O–Fuel–H2O. It was found the heat loss occurs due to the presence of various amounts of structurally-bound water in gels and xerogels before the combustion.

Multi-objective combustion optimization based on data-driven hybrid strategy

In order to reduce pollutant discharge and improve boiler efficiency, data-driven hybrid strategy is proposed to solve the problem of multi-objective combustion optimization. First, massive historical operation data of a coal-fired power station are preprocessed (including resampling, steady-state detection, data cleaning and cluster analysis), so as to divide the whole boiler working condition into different partitions. Next, combustion association rules based multi-objective optimal strategy is applied to extract a combustion optimal rule from every partition, and a combustion optimal rule-base is built up by merging all the rules, so that the preliminary combustion optimization can be quickly finished based on the combustion optimal rule-base. Meanwhile, combustion mathematical model based multi-objective optimal strategy is applied to develop the LSSVR (least square support vector regression) model of boiler combustion process for every partition, and a combustion optimal model-base, which contains all the LSSVR models, is built up. After that, an improved multi-objective particle swarm optimization algorithm is presented to calculate Pareto optimal solutions depend on the corresponding LSSVR model with the constraint of real-time boiler working condition. To achieve further combustion optimization, the method of multiple attribute decision making is used to determine the unique solution from all the Pareto optimal solutions. Data-driven hybrid strategy is to combine the above two strategies. Simulation experiments verified the validity and feasibility of data-driven hybrid strategy on multi-objective test function ZDT1. Taking advantage of data-driven hybrid strategy, the comprehensive average of NOx emissions dropped by 29.63% and the comprehensive average of boiler efficiency increased by 0.69% in the application experiments with historical operation data under some boiler working conditions. Data-driven hybrid strategy based multi-objective combustion optimization makes the integration of instantaneity and effectiveness, so that it is suitable for online application.

Numerical Predictions on the Influences of Inlet Temperature and Pressure of Feed Gas on Flow and Combustion Characteristics of Oxy-pulverized Coal Combustion

ABSTRACT Carbon capture and storage employing oxy-fuel combustion is one of the most promising options to reduce greenhouse gas emission. For the transition from conventional air-fired combustion to oxy-fuel combustion, there is a necessity of detailed investigation of various phenomena that occur during oxy-fuel combustion. Numerical modeling of oxy-fuel combustion process can serve the purpose effectively and efficiently. The current paper presents a thorough numerical investigation of the combustion process of pulverized coal under oxy-fuel conditions. The influence of inlet temperature and pressure of feed gas is investigated in the present study. Computational fluid dynamics simulation is performed for the test facility located at the Institute of Heat and Mass Transfer at Aachen University. The numerical results obtained employing various Reynolds Averaged Navier Stokes (RANS) models have been compared with measured data. The results showed that the strength and axial dispersion of the internal recirculation zone (IRZ) are enhanced at higher inlet temperature. The combustion of volatile matter present in coal shifts near the burner which results in shorter flames at higher inlet temperature. The flame temperature increased by ~9% with increase in inlet feed gas temperature from 313 to 800 K. Inlet feed gas pressure of 10.0 bar has ~38% increment and ~32% reduction in IRZ length and flame length, respectively, than the base case.

Predicting the Density and Viscosity of Biodiesels and Biodiesel Blends by the Regular-Solution Theory

Accurately predicting density and viscosity is required by the production of biodiesels and biodiesel blends that can meet the required quality standards, by the modeling and simulation of injection and combustion processes, and by the design of reactors, distillation units, and separation processes. Here, the regular-solution theory is used to develop novel predictive equations for the density of biodiesels and the density and viscosity of the (biodiesel + biodiesel) and (biodiesel + petrodiesel) blends. Systematic comparisons with experimental data and with the most widely used equations are made. The new density equation provides accurate predictions for the biodiesels and the above-mentioned blends, but Kay’s mixing rule cannot provide satisfactory predictions for the biodiesels. The new viscosity equation gives good predictions for the above-mentioned blends; particularly, its predictions are much better than those of the Grunberg–Nissan equation, even though the Grunberg–Nissan equation uses a blend-specific parameter determined from the blend viscosity.

Effects of structural materials’ chemical composition as considered in mathematical modeling built objects combustion processes

The article presents a symbolic (mathematical) model of combustion process alongside with its software implementation. The authors focus on effects of chemical composition of construction and finishing (decoration) materials involved in the process. The model was programmed before validation for adequacy and thus presents something more than description of chemical and physical moieties of combustion process.

Thermodynamic modeling of combustion process of the internal combustion engines – an overview

The mathematical description of combustion process in the internal combustion engines is a very difficult task, due to the variety of phenomena that occurring in the engine from the moment when the fuel-air mixture ignites up to the moment when intake and exhaust valves beginning open. Modeling of the combustion process plays an important role in the engine simulation, which allows to predict in-cylinder pressure during the combustion, engine performance and environmental impact with high accuracy. The toxic emissions, which appears as a result of fuels combustion, are one of the main environmental problem and as a result the air pollutant regulations are increasingly stringent, what makes the investigation of the combustion process to be a relevant task.

Pressure measurement in the cylinder of four-stroke marine engine – simulation analysis

The measurement of combustion pressure relies on connecting a pressure sensor to a four-stroke marine engine cylinder by a channel led out from the engine cylinder. The geometry of the channel distorts the results of combustion pressure measurements. The purpose of the work is to create a model of combustion processes in engine cylinders. The model uses a mathematical description of the indicator channel on one of the engine cylinders. The input data to the model and the data necessary for its verification came from the direct measurements on the research facility. The test object was a four-stroke Sulzer 3AL25/30 engine loaded by an electric power generator. During calculations, different diameters and lengths of measurement channel were simulated. The obtained results allowed to formulate the conclusion that the geometry of the measurement channel has a significant impact on the measurement results. The increase of the length of the measuring channel as well as the diameter of the channel causes disturbances in the measurement of the maximum pressure.

Analysis of Combustion of Coal-Water Fuel in Low-Power Hot-Water Boiler via Numerical Modeling and Experiments

The paper presents an experimental and computational study of various regimes of firing coal-water fuel in a low-power hot-water boiler that enable both dry and liquid slag removal. The experimental studies were carried out on a pilot industrial boiler adapted for firing coal-water fuel. The fuel was prepared using flotation products of beneficiation of K grade hard coal. The experiments were accompanied by numerical modeling of combustion processes. The mathematical model includes description of the carrier phase motion (based on the RANS approach with the two-parameter Menter SST k-ω turbulence model), radiation transfer (based on the P1 approximation of the spherical harmonics method for a two-phase two-temperature gray medium), particle motion (based on the Lagrange approach), and gas phase combustion (based on a combination of the kinetic model of combustion of gas components with the vortex break model). The physico-mathematical model was tested on the problem of combustion of coal-water fuel (CWF). One of the goals of the work was verification of a complex mathematical model. A comparative analysis of the results of the numerical modeling and experimental data showed that the model reliably described the process of burning in a combustion chamber. An advanced design of hot-water boiler enabling regimes of both dry and liquid slag removal was investigated. The chamber was shown to provide the necessary conditions for firing CWF in terms of reliability and economy.

Research of combustion process of construction polymeric materials in zero-gravity

In order to provide fire safety of inhabited pressurized compartments of manned spacecraft (hereinafter InPC SC), a research of combustion process of construction polymeric materials in zero-gravity conditions was carried out. The research was practiced in a flying bench IL-76, in a free-falling container, in an experimental unit (hereinafter – EU) “Ploskiy kanal” (two-dimensional duct), where combustion conditions of polymeric materials were close to combustion conditions in zero-gravity. Such conditions were modeled via decrease of intensity of natural convection on the long-term orbital station “Mir” on EU “Skorost” in an orbital flight. On the basis of research results of the physical model of combustion process of polymeric materials in zero-gravity were formed. Previously unknown material combustion parameters, introduced as fire danger indexes to practice of design of new means and methods of fire safety provision in InPC SC, were discovered. A fundamentally modern technology of fire suppression in InPC SC is presented. Inhabited pressurized compartments of the Russian segment of the International space station are equipped with automatic fire extinguishing system designed on the basis of results of the research.

Predicting NOx emissions in diesel engines via sigmoid NARX models using a new experiment design for combustion identification

Diesel engines are still widely used in heavy-duty engine industry because of their high energy conversion efficiency. In recent decades, governmental institutions limit the maximum acceptable hazardous emissions of diesel engines by stringent international regulations, which enforces engine manufacturers to find a solution for reducing the emissions while keeping the power requirements. A reliable model of the diesel engine combustion process can be quite useful to search for the best engine operating conditions. In this study, nonlinear modeling of a heavy-duty diesel engine NOx emission formation is presented. As a new experiment design, air-path and fuel-path input channels were excited by chirp signals where the frequency profile of each channel is different in terms of the number and the direction of the sweeps. This method is proposed as an alternative to the steady-state experiment design based modeling approach to substantially reduce testing time and improve modeling accuracy in transient operating conditions. Sigmoid based nonlinear autoregressive with exogenous input (NARX) model is employed to predict NOx emissions with given input set under both steady-state and transient cycles. Models for different values of parameters are generated to analyze the sensitivity to parameter changes and a parameter selection method using an easy-to-interpret map is proposed to find the best modeling parameters. Experimental results show that the steady-state and the transient validation accuracies for the majority of the obtained models are higher than 80% and 70%, respectively.

Application of different <i>k</i>-<i>ε</i> turbulence models on combustion process modelling in small-scale pellet stoves for household heating

Constant demand for development and construction improvement of small scale pellet stoves and boilers (nominal power up to 50 kWth) for household heating leads to conclusion that appropriate upgrading should be done on that field. Optimisation of stove construction should be performed based on results of experimental tests, as well as results of numerical analysis and mathematical modelling of combustion process. This is compulsory, because stove construction shall comply with demands for energy and environmental characteristics defined in appropriate quality standards. Besides, stove should operate with pellets of different quality, produced from various biomass raw materials. Considering that numerical analysis is less time consuming process than experimental tests, nowadays it is more often used for development and improvement of combustion appliance construction. As the turbulence modelling is one of the key parts in the eligible numerical analysis, the aim of the research presented in this paper is to define the influence of different k-ε turbulence models and their application on defined CFD mathematical model of combustion process in small-scale pellet stove for household heating.

Operational experience and modelling of biomass combustion process in cogeneration systems with ORC units

Organic Rankine Cycle (ORC) technology is nowadays considered as interesting option for utilization of local biomass resources. The number of plants worldwide is constantly growing within typical range of electric power output form 0.2 to 3.0 MW. In such systems biomass combustion chambers with step or vibrating grates are used. Heat is transferred to the thermal oil in a separate spiral heat exchanger. The biggest problems in such system are variable and inaccurately predictable properties of biomass that directly influences operation of combustion equipment, and thus the performance of entire cogeneration plant. In this paper experiences from two plants are presented. The plants are located in Krosno (Poland) and in Ostfildern near Stuttgart, Germany. In the first one high quality wood chips are used whereas the second one is powered by low quality biomass with the mass ash content approaching 18%. Although similar, both plants use different strategies to control the combustion process. Long term measurements are used to develop biomass combustion models in the plants. The paper also presents results of implementation of the online optic measurement system for monitoring of biomass properties such as LHV, ash and water content.

Lab-scale evaluation of possible mercury speciation in flue gas and mercury emission from combustion of pulverised solid fuels

In this work a new lab-scale method dedicated to the evaluation of both concentration and oxidation level of mercury in flue gases from pulverised fuel fired boiler was proposed. To detect the abovementioned parameters, 2 main steps need to be evaluated. Firstly, a calorimeter bomb is utilised - by a proper implementation of mass balance of mercury within substrates and products, the quantity of oxidised mercury in gaseous products can be evaluated. Then, to simulate solid fuel fired power unit and to calculate mercury concentrations in flue gases, one of the stoichiometric mathematical models of combustion process must be applied. Early validation of the method showed considerable differences between solid fuels in mercury oxidation efficiencies and concentrations in flue gasses. Four examined fuels (lignite, hard coal and 2 types of solid biomass) was investigated. Calculated mercury concentrations in raw flue gas (>700°C) varied between 4 and 75 µg/m3ref. The lowest quantity of oxidised forms ofHg in flue gases were identified in the case of investigated lignite (27% of total Hg), while significantly higher – for selected hard coal (72%) and one type of biomass (with high chlorine concentration; up to 98%).

Application of different <i>k</i>-<i>ε</i> turbulence models on combustion process modelling in small-scale pellet stoves for household heating

Constant demand for development and construction improvement of small scale pellet stoves and boilers (nominal power up to 50 kWth) for household heating leads to conclusion that appropriate upgrading should be done on that field. Optimisation of stove construction should be performed based on results of experimental tests, as well as results of numerical analysis and mathematical modelling of combustion process. This is compulsory, because stove construction shall comply with demands for energy and environmental characteristics defined in appropriate quality standards. Besides, stove should operate with pellets of different quality, produced from various biomass raw materials. Considering that numerical analysis is less time consuming process than experimental tests, nowadays it is more often used for development and improvement of combustion appliance construction. As the turbulence modelling is one of the key parts in the eligible numerical analysis, the aim of the research presented in this paper is to define the influence of different k-e turbulence models and their application on defined CFD mathematical model of combustion process in small-scale pellet stove for household heating.

Parameterisations of slow invariant manifolds: application to a spray ignition and combustion model

A wide range of dynamic models, including those of heating, evaporation and ignition processes in fuel sprays, is characterised by large differences in the rates of change of variables. Invariant manifold theory is an effective technique for investigation of these systems. In constructing the asymptotic expansions of slow invariant manifolds, it is commonly assumed that a limiting algebraic equation allows one to find a slow surface explicitly. This is not always possible due to the fact that the degenerate equation for this surface (small parameter equal to zero) is either a high degree polynomial or transcendental. In many problems, however, the slow surface can be described in a parametric form. In this case, the slow invariant manifold can be found in parametric form using asymptotic expansions. If this is not possible, it is necessary to use an implicit presentation of the slow surface and obtain asymptotic representations for the slow invariant manifold in an implicit form. The results of development of the mathematical theory of these approaches and the applications of this theory to some examples related to modelling combustion processes, including those in sprays, are presented.