Numerical Modeling Of Combustion Research Articles

Development of a chemical-kinetic database for the laminar flame speed under GDI and water injection engine conditions

The use of direct injection, supercharging, stoichiometric operation and reduction of the engine displacement, necessary to limit the specific consumption without reducing the power, makes the current spark ignition engines sensible to both the detonation and the increase of the inlet turbine temperature. The current research has therefore focused on the study of strategies aimed at reducing the risk of detonation using traditional and innovative solutions such as water injection. The application and optimization of these strategies can not ignore the knowledge of physical quantities characterizing the combustion such as the laminar flame speed. The laminar burning speed is an intrinsic property of the fuel and it is function of the mixture composition (mixture fraction and dilution) and of the thermodynamic conditions. The experimental measurements of the laminar flame speed available in the literature, besides not being representative of the pressure and temperature conditions characteristic of GDI engines, rarely report the effects of dilution by EGR or water vapor. To overcome the limitations of the experimental campaign it is possible to predict the value of the laminar flame speed resorting to numerical combustion models based on chemical kinetics. The increased performance of computing systems makes affordable the use of chemical schemes with a high number of species and reactions without facing an excessive temporal cost. In this work it is presented a methodology for the construction of a laminar flame speed database based on a non-reduced kinetic scheme and an open source solver (Cantera) for a commercial gasoline surrogate under the typical conditions of GDI engines with the addition of the effects of dilution with water and EGR.

Numerical modeling of homogeneous gas and heterogeneous char combustion for a wood-fired hydronic heater

Abstract The pyrolysis of woody fuels produces two main products - pyrolysis gases and solid residue char which undergo homogeneous and heterogeneous reactions, respectively. Recent experimental measurements using a two-stage hydronic heater indicate the oxidization of these two products occur at distinctly different time scales with the pyrolysis gases burning immediately, and the majority of the char oxidization occurring later. In this study, these two oxidation pathways are explicitly accounted for in a numerical model that considers a non-homogeneous mixture of product flue gases. The model is based on a three-zone description of the heater which accounts for combustion and heat transfer using well-stirred reactor theory. The first zone describes the gasification of the wood fuel and burning of both pyrolysis gases and char. The second zone represents an after-burning stage. The last stage accounts for the transport of gases out the flue. Model predictions of O 2 , C O 2 , C O , H 2 O and temperature are compared to experimental measurements showing good overall agreement. Furthermore, the dual oxidation pathway description of combustion is shown to be critical to account for the dual-peak C O time signature. The first peak is associated with the burning of pyrolysis gases and the second corresponds to char oxidation.

Numerical modeling of oxy-methane combustion in a model gas turbine combustor

There is a renewed interest in oxy-fuel combustion of natural gas for reduction of greenhouse gas emissions. This has necessitated various experimental and numerical studies of oxy-fuel combustion. In the numerical combustion study, the radiation model and combustion chemistry are critical for accurate numerical predictions of oxy-fuel combustion characteristics. In this study, three global reaction mechanisms: Westbrook-Dryer (3 equations), Jones-Lindstedt (5 equations) and Jones-Lindstedt (7 equations) for oxy-methane combustion were combined with different weighted sum of gray gas radiation models (WSGGM) available in the literature to determine the most accurate combination for oxy-methane combustion modeling and simulation. Experiments were conducted in a non-premixed swirl stabilized model gas turbine combustor at a firing rate of 4 MW/m3-bar while varying the percentage of CO2 in the oxidizer (O2/CO2) mixture. Numerical model developed using ANSYS FLUENT 17 code was validated against the experimental results. The combinations of Jones-Lindstedt (5 equations) reaction mechanism and WSGGM model proposed by Bordbar gave the closest approximation of the flame temperature with an average deviation of 5.52%. The model combination also predicted the flame attachment to the fuel nozzle and flame lift-off at a high CO2 percentage in the oxidizer mixture. The results of the parametric study on the effect of CO2 percentage in the oxidizer mixture, combustor energy level and equivalence ratio on the combustion characteristics and CO emissions were also reported. The CO emissions monotonically increases with increasing percentage of CO2 in the oxidizer due to decreased residence time and the reduction in the flame temperature. While the CO emission increases with the energy level in the combustor up to 3.5 MW/m3 and decreases thereafter. The optimum equivalence ratio for minimum CO emission is 0.98 with approximately 2 PPM at 40% CO2 in the oxidizer.

Comparison of Numerical Combustion Models for Hydrogen and Hydrogen-Rich Syngas Applied for Dry-Low-Nox-Micromix-Combustion

The Dry-Low-NOx (DLN) Micromix combustion technology has been developed as low emission combustion principle for industrial gas turbines fueled with hydrogen or syngas. The combustion process is based on the phenomenon of jet-in-crossflow-mixing (JICF). Fuel is injected perpendicular into the air-cross-flow and burned in a multitude of miniaturized, diffusion-like flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. In the Micromix research approach, computational fluid dynamics (CFD) analyses are validated toward experimental results. The combination of numerical and experimental methods allows an efficient design and optimization of DLN Micromix combustors concerning combustion stability and low NOx emissions. The paper presents a comparison of several numerical combustion models for hydrogen and hydrogen-rich syngas. They differ in the complexity of the underlying reaction mechanism and the associated computational effort. The performance of a hybrid eddy-break-up (EBU) model with a one-step global reaction is compared to a complex chemistry model and a flamelet generated manifolds (FGM) model, both using detailed reaction schemes for hydrogen or syngas combustion. Validation of numerical results is based on exhaust gas compositions available from experimental investigation on DLN Micromix combustors. The conducted evaluation confirms that the applied detailed combustion mechanisms are able to predict the general physics of the DLN-Micromix combustion process accurately. The FGM method proved to be generally suitable to reduce the computational effort while maintaining the accuracy of detailed chemistry.

Compression ratio influence on combustion and emissions characteristic of hydrogen diesel dual fuel CI engine: Numerical Study

The effect of compression ratios variation on maximum potential hydrogen energy share in a dual fuel engine is studied by creating a numerical model for engine combustion and emission characteristics. Hydrogen is induction through the intake port, using port fuel injector (PFI). Compression ratio is varied in steps of 14.5, 16.5 and 19.5 in all the investigations. Hydrogen energy share variation ranged from 0% to 55%. The results show a clear trade-off between maximum hydrogen energy share and compression ratio. Knock limited maximum hydrogen energy share increased from 20% to 45% as compression (CR) is decreased from 19.5 to 14.5. At all compression ratios as the energy proportion of hydrogen is increased, emissions other than NOx show an apparent decline.

Numerical simulation of porous media combustion for high temperature heat exchanger

The purpose of this work is developing the numerical 1D model of porous media combustion for investigating porous media burner systems. The software is used to solve energy, mass transfer and chemical reaction equation of the combustion. The operating condition and property parameters, which mainly affect the functions and quality of the industrial burner design, such as the inlet velocity of the reactants, the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, will be investigated and validated with experimental data. For developing the procedure of experiment, three diameter sizes of porous media materials (5 mm, 10 mm, and 15 mm.) were used. As a result, the developed model will be used as a tool to explore temperature distribution of heat exchange to improve thermal performance and overall efficiency system. Moreover, this knowledge can be applied to design porous media burner systems for uniform temperature distribution operation.

Numerical investigation of high temperature synthesis gas premixed combustion via ANSYS Fluent

A numerical model of the synthesis gas pre-mixed combustion is developed. The research was carried out via ANSYS Fluent software. Verification of the numerical results was carried out using experimental data. A visual comparison of the flame contours that obtained by the synthesis gas combustion for Re = 600; 800; 1000 was performed. A comparison of the wall temperature of the combustion chamber, obtained with the help of the developed model, with the results of a physical experiment was also presented. For all cases, good convergence of the results is observed. It is established that a change in the temperature of the syngas/air mixture at the inlet to the combustion chamber does not significantly affect the temperature of the combustion products due to the dissipation of the H2O and CO2 molecules. The obtained results are of practical importance for the design of heat engineering plants with thermochemical heat recovery.

Cyclone furnace as a way for mercury removal from lignite

The power plants will have to meet the requirements regarding permissible Hg emission levels by 2021. A particularly unfavourable situation is observed for front of lignite, which contains more mercury than hard coal, and in addition its combustion also causes more CO2 emissions. The paper presents the proposal to use the cyclone furnace in the process of heat treatment for the process of the release of mercury from lignites. The structure and methods of operation of the cyclone furnace were presented. The paper also discusses the methodology of the process of numerical modelling of combustion and gasification of coal dust. The results showed that changing a fuel stream fed to the cyclone furnace allows for controlling the temperature and degree of fuel devolatilization. This leads to heating of the fuel to the desired temperature due to the expected level of mercury removal from the fuel. This is very important especially in the case of fuels containing significant amounts of moisture, such as lignite. The experiments performed in the study confirmed the results of numerical calculations for heating fuel and showed the possibility of over 90% removal of mercury from the fuel. The results confirmed the laboratory workplace rotary kiln.

Results of Numerical Modeling of Combustion Processes in a Vortex Chamber

The results of numerical modelling of combustion in a vortex chamber are presented. The calculations are performed using k-ε and k-ε with curvature correction of streamlines turbulence models. Upon combustion calculation of the well mixed mixture the Burning Velocity Model (BVM) was applied. The usage of the streamlines curvature correction allows to increase the accuracy of the prediction of the circumferential velocity component and the temperature field in the vortex chamber.

Numerical model of two-dimensional heterogeneous combustion in porous media under natural convection or forced filtration

A novel mathematical model and original numerical method for investigating the two-dimensional waves of heterogeneous combustion in porous media are proposed and described in detail. The mathematical model is constructed within the framework of the model of interacting interpenetrating continua and includes equations of state, continuity, momentum conservation and energy for solid and gas phases. Combustion, considered in the paper, is due to the exothermic reaction between fuel in the porous solid medium and oxidiser contained in the gas flowing through the porous object. The original numerical method is based on a combination of explicit and implicit finite-difference schemes. A distinctive feature of the proposed model is that the gas velocity at the open boundaries (inlet and outlet) of the porous object is unknown and has to be found from the solution of the problem, i.e. the flow rate of the gas regulates itself. This approach allows processes to be modelled not only under forced filtration, but also under free convection, when there is no forced gas input in porous objects, which is typical for many natural or anthropogenic disasters (burning of peatlands, coal dumps, landfills, grain elevators). Some two-dimensional time-dependent problems of heterogeneous combustion in porous objects have been solved using the proposed numerical method. It is shown that two-dimensional waves of heterogeneous combustion in porous media can propagate in two modes with different characteristics, as in the case of one-dimensional combustion, but the combustion front can move in a complex manner, and gas dynamics within the porous objects can be complicated. When natural convection takes place, self-sustaining combustion waves can go through the all parts of the object regardless of where an ignition zone was located, so the all combustible material in each part of the object is burned out, in contrast to forced filtration.

Numerical Modeling of Oxy-Fuel Combustion in a Model Gas Turbine Combustor: Effect of Combustion Chemistry and Radiation Model

Radiation modeling and combustion chemistry are critical to accurate numerical predictions of oxy-combustion characteristics. The presence of carbon dioxide (CO2) in oxy-fuel combustion acts as diluents and significantly changes the radiative properties of the combustion gases in comparison to air combustion. In this regard, three global reaction mechanisms: Westbrook-Dryer (3 equations), Jones-Lindstedt (5 equations) and Jones-Lindstedt (7 equations) for oxy-fuel combustion of methane were combined with different weighted sum of gray gas radiation models (WSGGM) available in literature to determine the most accurate combination for methane-oxy-fuel combustion modeling and simulation. This study was carried out in a non-premixed swirl stabilized model gas turbine combustor at a firing rate of 4MW/m3-bar. The modified Westbrook-Dryer (WD-oxy) mechanism could not predict the flame attachment to the fuel nozzle at 35% CO2 addition. The combinations of Jones-Lindstedt (5 equations) reaction mechanism and WSGGM model proposed by Bordbar gave the closest approximation to our experimental observations and predicted the flame attachment to the fuel nozzle.

Assessment of a novel numerical model for combustion and in-flight heating of particles in an industrial furnace

The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2 MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178 K higher compared to a particle, which trajectory led through zones with lower gas temperatures.

Numerical study of the ignition front propagation of different pelletised biomass in a packed bed furnace

In the current study, a 3D numerical model for biomass combustion in a packed bed furnace has been developed to study combustion of four types of biomass fuels (wood pellet, poplar pellet, olive stone and pine shavings) with different chemical composition and physical properties. A number of sub models representing various processes of the fuel thermal conversion were included through user defined subroutines coupled to commercial CFD software AVL Fire. Ignition front propagation velocity, maximum flow temperature, effect of excess air and moisture content have been studied with the change of air flow rate and compared with an experimental analysis of biomass fixed bed combustor. It is found that, air mass flow greatly affects ignition front velocity and maximum values are achieved under sub-stoichiometric condition. Highest ignition front velocity is achieved by pine shavings due to its smaller particle size and bed solid fraction. The numerical model reasonably predicted ignition front velocity in the oxygen-limited regime. In the reaction limited regime, simulation slightly over estimates the ignition front velocities except for the fuel pine shavings, but it followed the measured trend. Beyond the reaction limited regime, further increase in air mass flow forces combustion to extinguish due to increased convection cooling and a downward trend of the ignition front velocity is observed. Reason for the over predictions in reaction limited and cooling by convection regimes for the larger diameter fuels is explained.

Simplified numerical modelling of oxy-fuel combustion of pulverized coal in a swirl burner

In this work, a computational fluid dynamics (CFD) modelling has been performed to analyze pulverized coal combustion in a vertical pilot-scale furnace. The furnace with swirl burner is located at the Institute of Heat and Mass Transfer at RWTH Aachen University, where O2/CO2 combustion environment was adopted to study pulverized coal combustion experimentally (Toporov et al., 2008). A two-dimensional axisymmetric domain has been used in this work with a Lagrangian method to track coal particles. Performances of different Reynolds-Averaged-Navier-Stokes (RANS) turbulence models have been investigated. The radiative heat transfer using discrete ordinate (DO) model coupled with variants of weighted-sum-of-grey-gases (WSGGM) methods has been examined. The results obtained by these models have been compared with the experimental data. The DO radiation model with domain based WSGGM and SST k-omega turbulence model showed a very good match with the experimental data among other tested models. The accuracy of predicted results was comparable to LES results available in the literature. It is observed that to get preliminary results the current simplified model is good enough with accuracy comparable to LES modelling. By using this validated model, the influence of combustion environments such as air, oxy-steam (O2/H2O) and oxy-RFG (O2/CO2) on temperature and NO concentration distribution has been investigated. NOx produced was least in the oxy-steam environment.

Numerical Modelling of Solid Biomass Combustion: Difficulties in Initiating the Fixed Bed Combustion

Abstract Bio mass is a renewable energy which has become an important fuel to produce thermal energy or electricity. It has no adverse effect on environment as it reduces carbon dioxide emissions. Biomass combustion is a difficult phenomenon due to its properties with less carbon, more volatile matters and lower calorific value. Fixed bed combustion is a useful technique for thermal conversion of solid biomass fuels as this can fire up a broad range of fuels with different properties and requires less fuel preparation. But due to complex solid combustion mechanism and inadequate knowledge on process the development of such combustion system is limited. Numerical modelling of this combustion system has some advantages over experimental analysis. But developing a complete model for this type of combustion system is a challenge. Due to its characteristic properties, modelling of biomass combustion has to overcome many difficulties. One such problem is initiating the combustion in numerical modelling. The usual way to model biomass combustion is to divide the system in to two phases namely solid phase and gas phase. Modelling the solid phase of this combustion system requires the major effort. In this work, a detailed analysis of difficulties occurred in initiating the biomass combustion has been presented. A complete three-dimensional numerical model is used for transient analysis of fixed bed biomass combustion. The bed is considered as a porous medium and solid phase of the combustion is characterised by using sub models with several variables in a commercial CFD code.

Numerical prediction of the chemical composition of gas products at biomass combustion and co-combustion in a domestic boiler

In recent years the numerical modelling of biomass combustion has been successfully applied to determine the combustion mechanism and predict its products. In this study the influence of the addition of waste glycerin in biomass wood pellets on the chemical composition of exhaust gases has been investigated. The pellets have been prepared from spruceand pine wood sawdust without and with addition of waste glycerin. The waste glycerol is a undesirable by-product of biodiesel transesterification at oil manufacturing. The produced pellets were being burned in the 10 kW domestic boiler adapted to wood pellets combustion. The possibilities of pollutants generation (CO2 , CO, NOx SOx and compounds containing chlorine) in the exhaust gases coming from the boiler were numerically calculated using the latest version of CHEMKIN-PRO software, introduced by the American company Reaction Design. The results of the calculations correspond to the data obtained on a real object, in particular: combustion temperature, gas pressure, residence time of fuel in the burner, air flow, fuel consumption, as well as elementary composition of fuel supplied into the boiler. The proposed method of predicting the chemical composition of exhaust gases allows proper control of the combustion process and can be considered as an important step in reducing the pollutants (lower emission of NOx , SOx and CO2 neutral) and thus to contribute to the improvement of the environmental quality. In addition, knowledge of the amounts of Clbased compounds produced in combustion process (under given conditions), can serve as an important hint in terms of corrosion prevention of boiler- and chimney steels.

Experimental Test, Model Validation, and Viability Assessment of a Wave-Rotor Constant-Volume Combustor

Design and testing of a wave-rotor constant-volume combustor achieved stable combustion at near-atmospheric inlet conditions and demonstrated the potential of pressure-gain combustion using a wave rotor. An experiment rig with a motor-driven, room-temperature rotor with large thermal mass operated for short durations within heating limits of extensive in-passage rotating instrumentation. Over 30 successful tests were completed, including a 3 s run amounting to about 2000 individual firing events. Fast deflagrative combustion was observed with varied ethylene fuel distribution in the passages, showing good combustor operability, insensitive to leakage. Remarkably high flame speeds and a net pressure gain were indirectly indicated from measurements. A time-marching, spatially one-dimensional numerical model of gas dynamics and combustion was used for aerothermodynamic design, applying loss models previously calibrated with pressure-exchange nonreacting wave-rotor experiments. Major features and trends of the measured gas dynamic and combustion processes showed good agreement with predictions and validated current design methods. Different fuel distributions were tested to better calibrate ignition and combustion submodels. Simulations illustrate the likely explanations for cases with and without observed ignition, spillage during the filling process, and mixture requirements for consistent torch ignition. The viability of wave rotors for realizing a pressure-gain combustor is discussed.

Modelling of the aerodynamic processes of water-tube boiler DKVR(DE) -10/14

The results of numerical modeling of flow and combustion of methane with oxygen in the air in a swirling furnace plume are presented. Calculation model of oil-gas burner device GMG-7 and two-drum steam water-tube boiler DE-10-14 is built. Modelling іs carried out by сontrol volume method. The flame structure, the distribution of gas velocity in the furnace volume and aerodynamic resistance of furnace are defined. Calculations are made for heating boiler with a secondary emitter and without secondary emmiter. Figs.: 3. Table. 1. Refs.: 11 titles.Keywords: numerical modeling; flow process; steam water-tube boiler; aerodynamic resistance.

Numerical Simulation of a Vortex Combustor Based on Aluminum and Steam

In this paper we report a new development in the numerical model for aluminum-steam combustion. This model is based on the diffusion flame of the continuum regime and the thermal equilibrium between the particle and the flow field, which can be used to calculate the aluminum particle combustion model for two phase calculation conditions. The model prediction is in agreement with the experimental data. A new type of vortex combustor is proposed to increase the efficiency of the combustion of aluminum and steam, and the mathematical model of the two phase reacting flow in this combustor is established. The turbulence effects are modeled using the Reynolds Stress Model (RSM) with Linear Pressure-Strain approach, and the Eddy-Dissipation model is used to simulate the gas phase combustion. Aluminum particles are injected into the vortex combustor, forming a swirling flow around the chamber, whose trajectories are traced using the Discrete Phase Model (DPM). The simulation results show that the vortex combustor can achieve highly efficient combustion of aluminum and steam. The influencing factors, such as the eccentric distance of the inlet of aluminum particles, particle size and steam inlet diameter, etc., are studied.

3D numerical modelling of turbulent biogas combustion in a newly generated 10 KW burner

This study concentrates on the 3D numerical modelling of combustion of different biogases in a generated burner and combustor. The main goal of this study is to investigate the combustion characteristics (such as temperature and emissions) of biogases through a combustor due to depletion of natural gas. Moreover, the effect of the preheated air on flame temperatures of biogases have been studied in the present study. Finally, the effect of H2S amount in biogas on SO2 emissions has been investigated within these predictions. The numerical modelling of turbulent diffusion flames has been performed by using the standard k–ε model of turbulent flow, the PDF/Mixture Fraction combustion model and P-1 radiation model in the combustor. A CFD code has been used for all predictions. Temperature gradients have been determined on axial and radial directions for better understanding combustion characteristics of biogases. Modelling has been studied for thermal power of 10 kW and excess air ratio of λ = 1.2 for each biogas combustion. The first finding is that combustion of biogases is possible via the newly generated burner. Moreover, the results show that the one of biogas is very close to methane in terms of temperature distributions in the combustor due to including high amount of methane compared to other biogases. It is also concluded that the flame temperatures of biogases increase with preheating the combustion air as expected. It is finally revealed that SO2 emissions increase as amount of H2S in biogas is increased through the combustor.