Finite-rate Model Research Articles

Simulation of Opposed-Jets Configuration H<sub>2</sub>/Air, CH<sub>4</sub>/Air

The opposed-jets configuration is very used in industrial systems. The actual practical applications use clean fuels which in stead of classical hydrocarbons. The present work is a numerical simulation of opposed diffusion jets using FLUENT6.3.26. We have compared different turbulence models and combustion models and mechanisms to find which gives the best predictions for this type of flows. We have used methane and hydrogen fuels because they are considered as clean fuels. The comparison between k-ε, k-omega and RSM turbulent models shows that both of k-ε and RSM gives good results. The use of k-ε is more practical because it requires less long time to be implied. The comparison between the combustion models shows that EDC gives more realistic results than eddy dissipation and Finite rate models. In addition, the detailed chemical mechanisms are more adequate to this model. For both methane and hydrogen flames, the detailed mechanisms gives good results and temperatures.

Simulation of Opposed-Jets Configuration H<sub>2</sub>/Air, CH<sub>4</sub>/Air

The opposed-jets configuration is very used in industrial systems. The actual practical applications use clean fuels which in stead of classical hydrocarbons. The present work is a numerical simulation of opposed diffusion jets using FLUENT6.3.26. We have compared different turbulence models and combustion models and mechanisms to find which gives the best predictions for this type of flows. We have used methane and hydrogen fuels because they are considered as clean fuels. The comparison between k-ε, k-omega and RSM turbulent models shows that both of k-ε and RSM gives good results. The use of k-ε is more practical because it requires less long time to be implied. The comparison between the combustion models shows that EDC gives more realistic results than eddy dissipation and Finite rate models. In addition, the detailed chemical mechanisms are more adequate to this model. For both methane and hydrogen flames, the detailed mechanisms gives good results and temperatures.

Study of combustion process with jet-ignition of propane-air mixtures

Abstract The paper presents the study of combustion process of a homogenous lean propane-air mixture in the cylindrical combustion chamber ignited by a hot gas jet from the pre-ignition chamber. A rich propane-air mixture in the pre-chamber is ignited by the spark plug and the exhaust gasses flow from the chamber trough the holes in the wall. The mathematical model of gas exchange and energy balance in chambers with a laminar finite-rate model taking into account the two-step Arrhenius chemical kinetics is presented. The work presents results of thermodynamic parameters of the charge obtained in CFD simulations in Fluent and Kiva3v for three configurations: with one hole in the wall of the ignition chamber, with three holes and without an ignition chamber. Modelling and simulation have shown faster burning of the mixture for jet ignition with three holes of the pre-chamber. The results of simulations were verified by experimental studies in the combustion chamber of the same geometry by the Schlieren method. The work presents flame front propagation, pressure traces and pressure increment speed for two mixtures with a different equivalence fuel-air ratio. Experimental results proved the simulation observation of faster flame propagation in the main chamber with three holes

High temperature energy storage performances of methane reforming with carbon dioxide in a tubular packed reactor

High temperature heat transfer and energy storage performances of methane reforming with carbon dioxide in tubular packed reactor are investigated under different operating conditions. Experimental results show that the methane reforming in tubular packed reactor can efficiently store high temperature thermal energy, and the sensible heat and heat loss besides thermochemical energy storage play important role in the total energy storage process. When the operating temperature is increased, the thermochemical storage efficiency first increases for methane conversion rising and then decreases for heat loss rising. As the operating temperate is 800°C, the methane conversion is 79.6%, and the thermochemical storage efficiency and total energy efficiency can be higher than 47% and 70%. According to the experimental system, the flow and reaction model of methane reforming is established using the laminar finite-rate model and Arrhenius expression, and the simulated methane conversion and energy storage efficiency fit with experimental data. Along the flow direction, the fluid temperature in the catalyst bed first decreases because of the endothermic reaction and then increases for the heat transfer from reactor wall. As a conclusion, the maximum thermochemical storage efficiency will be obtained under optimal operating temperature and optimal flow rate, and the total energy efficiency can be increased by the increase of bed conductivity and decrease of heat loss coefficient.

Predicting Radiative Heat Transfer in Oxy-Methane Flame Simulations: An Examination of Its Sensitivities to Chemistry and Radiative Property Models

Measurements from confined, laminar oxy-methane flames at different O2/CO2dilution ratios in the oxidizer are first reported with measurements from methane-air flames included for comparison. Simulations of these flames employing appropriate chemistry and radiative property modeling options were performed to garner insights into the experimental trends and assess prediction sensitivities to the choice of modeling options. The chemistry was modeled employing a mixture-fraction based approach, Eddy dissipation concept (EDC), and refined global finite rate (FR) models. Radiative properties were estimated employing four weighted-sum-of-gray-gases (WSGG) models formulated from different spectroscopic/model databases. The mixture fraction and EDC models correctly predicted the trends in flame length and OH concentration variations, and the O2, CO2, and temperature measurements outside the flames. The refined FR chemistry model predictions of CO2and O2deviated from their measured values in the flame with 50% O2in the oxidizer. Flame radiant power estimates varied by less than 10% between the mixture fraction and EDC models but more than 60% between the different WSGG models. The largest variations were attributed to the postcombustion gases in the temperature range 500 K–800 K in the upper sections of the furnace which also contributed significantly to the overall radiative transfer.

Towards a hybrid Eulerian–Lagrangian CFD modeling of coal gasification in a circulating fluidized bed reactor

Numerical model of coal gasification in circulating fluidized bed (CFB) using Eulerian–Lagrangian approach is presented in this paper. The Dense Discrete Phase Model (DDPM) model of ANSYS FLUENT is used to simulate the flow of the particulate phase in the coal gasifier. The coal particles, with a size distribution, are tracked in the fluid velocity field including coupling between the phases. Kinetic Theory of Granular Flow is utilized to model the particles’ interactions. The analyzed CFB comprises a small scale experimental facility in which coal is gasified in air and air/steam mixture. The reactor is composed of a barrel like bottom part with developed internal recirculation of the solid phase and 3.74m high riser section. The homogenous gas phase reactions are modeled using the finite rate and eddy dissipation models. The heterogeneous reactions on coal particles surface are modeled using the finite rate chemistry. A total number of 14 reactions are considered. Results of the simulations were compared with experimental data.

Application and theoretical analysis of the flamelet model for supersonic turbulent combustion flows in the scramjet engine

In the framework of Reynolds-averaged Navier–Stokes simulation, supersonic turbulent combustion flows at the German Aerospace Centre (DLR) combustor and Japan Aerospace Exploration Agency (JAXA) integrated scramjet engine are numerically simulated using the flamelet model. Based on the DLR combustor case, theoretical analysis and numerical experiments conclude that: the finite rate model only implicitly considers the large-scale turbulent effect and, due to the lack of the small-scale non-equilibrium effect, it would overshoot the peak temperature compared to the flamelet model in general. Furthermore, high-Mach-number compressibility affects the flamelet model mainly through two ways: the spatial pressure variation and the static enthalpy variation due to the kinetic energy. In the flamelet library, the mass fractions of the intermediate species, e.g. OH, are more sensible to the above two effects than the main species such as H2O. Additionally, in the combustion flowfield where the pressure is larger than the value adopted in the generation of the flamelet library or the conversion from the static enthalpy to the kinetic energy occurs, the temperature obtained by the flamelet model without taking compressibility effects into account would be undershot, and vice versa. The static enthalpy variation effect has only little influence on the temperature simulation of the flamelet model, while the effect of the spatial pressure variation may cause relatively large errors. From the JAXA case, it is found that the flamelet model cannot in general be used for an integrated scramjet engine. The existence of the inlet together with the transverse injection scheme could cause large spatial variations of pressure, so the pressure value adopted for the generation of a flamelet library should be fine-tuned according to a pre-simulation of pure mixing.

Validating Thermal Response Models Using Bench-Scale and Intermediate-Scale Fire Experiment Data

The thermal response of a fiber reinforced polymer composite was measured by the bench-scale Cone Calorimeter and Intermediate-scale Calorimeter (ICAL) fire experiments. Finite-rate and infinite-rate pyrolysis models were used to predict the response of the composite panels under the same thermal boundary conditions as in the fire tests. It was shown that both models can give acceptable temperature, mass loss, and effective char thickness predictions. The effect of internal gas convection on thermal response predictions was determined insignificant at low heat flux levels. The thermal insulation at the back surface of the composite panel significantly increased both temperature and mass loss predictions.

Four Zones Modeling of the Downdraft Biomass Gasification Process: Effects of Moisture Content and Air to Fuel Ratio

A mathematical model of a biomass downdraft gasification process has been developed. The model was considered to be four modules that are drying, pyrolysis, oxidation, and reduction which the products of upper module will be the reactants of the next. The one-dimensional kinetic finite rate models were applied to drying and reduction modules. The pyrolysis model took place when drying temperature reached 473K. The equilibrium sequence model was used to describe the oxidation process by considering the sequence of order of reaction rates. For the model validation the comparison showed a good agreement. In the parametric study the moisture content increasing affected the height of drying, and pyrolysis zones increasing while the critical char bed length of reduction zone decreased from 0.53 to 0.25 m. The height of these zones decreased along A/F increasing. The results of this study can be used in the reactor design evaluation.

CFD modelling for atmospheric pollutants/aerosols studies within the complex terrains of urban areas and industrial sites

Computational fluid dynamic (CFD) modelling of pollution dispersion and chemical conversion to aerosol particles in the atmospheric boundary layer (ABL) has been studied. The investigation focused on the numerical modelling above complex orographic terrains of urban areas and industrial sites including the dispersion of toxic substances in the air as a result of accidents. A finite-rate model of chemical reactions, including the turbulence chemistry for modelling the reaction between nitric acid and ammonia, has been applied. As supporting experiments, online monitoring of the spatial distribution of pollutants and aerosols has been performed above real complex areas. Minimal detectable concentrations 8 μg m–3 (SO2), 20 μg m–3 (NO2), 2 μg m–3 (O3) and minimal detectable absorptivity 5 × 10–7 cm–1 (aerosols) have been reached.

Numerical Simulation of Chemical Non-Equilibrium Flow in Hydrocarbon Fuel Scramjet Nozzle

Based on 8-specice, 12-step finite-rate chemical reaction models, the chemical non-equilibrium flows of hydrocarbon fuel scramjet single expansion ramp nozzle (SERN) were numerical simulated. The chemical dynamic characteristics of flow in SERN were analyzed in detail. The performances of SERN were compared at various inlet static temperatures. The numerical results show that the chemical non-equilibrium effect had great influence on the performance of the nozzle at the relatively high inlet static temperature. However, influence induced by chemical non-equilibrium effect can be neglected when nozzle inlet static temperature is less than 2500K.

Modeling of Pulverized Coal Combustion in Turbulent Flow with the Consideration of Intermediate Reactions of Volatile Matter

The eddy dissipation concept (EDC) model with the consideration of the intermediate reactions for the volatile matter (VM) combustion was applied to simulate turbulent pulverized coal/air jet combustion adjacent to a bluff-boy type model burner. The VM of the coal was assumed to consist of CO, H2, and CH4, and its chemistry was described by a 16-speices and 41-step skeletal mechanism for CH4/air combustion. The model was compared with some other conventionally used ones, including the eddy dissipation (ED) model, eddy dissipation or finite rate (ED-FR) model, mixture fraction probability density function (MF-PDF) model, and the EDC model with a global reaction mechanism for gas phase combustion (EDC_G), in predicting temperature profiles, maximum flame temperature, flame shape, and ignition position and in CPU cost. The predicted temperature profiles and flame positions were further compared with reported experimental data. It was found that the EDC model with the consideration of the intermediate reactions for VM combustion improved prediction in the temperature field and thus ignition position. With the adoption of the full in situ adaptive tabulation (ISAT) method, two-third of the overall CPU time could be saved, making the EDC model more acceptable in comparison with the ED-type models and MF-PDF model. On the basis of the simulation results, it is suggested that intermediate reactions of the VM should be considered when high accuracy of flame temperature and ignition position prediction is desired in simulation of pulverized coal combustion.

Numerical analysis of gasification performance via finite-rate model in a cross-type two-stage gasifier

The gasification process of a pressurized, oxygen-blown, entrained-flow E-Gas like gasifier through numerical modeling is investigated by solving the 3-D, steady-state Navier–Stokes equations with the Eulerian–Lagrangian method. Eight chemical reactions are solved via the Finite-Rate/Eddy-Dissipation Model. The preliminary gasification process is successfully modeled and the global chemical reactions are proved to be strongly affected by the finite rates. The results of parametric study show that the increasing O2/Coal ratio results in a decrease of CO, but an increase of CO2 and exit temperature. With a modified water–gas-shift reaction rate, a more reasonable trend is obtained that as the coal slurry concentration decreases, the mass flow rate of H2, CO2, and H2O increase while that of CO decreases. As the amount of coal slurry mass flow in the first stage increases, the exit temperature and the mole fraction of H2 and CO2 increase, while that of CO decreases. However, different fuel distributions do not provide notable influence on gasification performance due to the large space inside the E-Gas gasifier allowing complete reaction. The overall results show that the present CFD model can adequately capture the gasification behavior and analyze gasification performance inside the gasifier.

Computational Simulation of the Influence of Inert Particles on Incomplete Combustion of Methane at a Low Air Factor

It is well known that the gas–solid system plays a significant role in many industrial processes. It is a complex physical and chemical process, generally consisting of heat transfer, mass transfer, species diffusion, and chemical reactions. In this paper, the reaction of methane with air at a low air factor and the gas flow in a fluidized bed with 0.1 mm solid particles are computationally simulated to enable the study of the effect of the inert particles on the species diffusion and the chemical reactions. The reaction of methane and air is modeled by a two-step reaction mechanism that produces a continuous fluid phase composed of six gases (CH4, CO, O2, CO2, H2O, and N2) and discrete solid particles in the reactor. The simulation results are compared with experiment and show that the finite rate model and the eddy dissipation model can well describe the reactions of gases in high-density gas–solid systems. The distribution of each gas and the particle behaviors are analyzed for incomplete combustion at different concentrations of loaded solid particles. The inert particles change the reactions by enhancing both the chemical kinetics and the species diffusion dynamics.

Uncertainty Analysis of Reaction Rates in a Finite-Rate Surface-Catalysis Model

Uncertainty Analysis of Reaction Rates in a Finite-Rate Surface-Catalysis Model

Studies of the Effect of Spray Inlet Conditions on the Flow and Flame Structures of Ethanol-Spray Combustion by Large-Eddy Simulation

Large-eddy simulation (LES) of ethanol spray-air combustion with a poly-dispersed initial droplet size distribution is presented here by using an Eulerian-Lagrangian approach, a sub-grid-scale kinetic energy stress model, and a filtered finite-rate combustion model with a sub-grid scale reaction rate called the second-order moment (SOM) combustion model, proposed by our research group. The simulation results are validated in detail by experiments. Furthermore, the flow and flame structures of spray combustion with different spray cone angles and cone angle thickness are studied. The results show that for the case of smaller spray cone angle thickness, the coherent structures in the high temperature zone tend to shed more clearly. High temperature develops around the coherent structures in the region of high vapor concentration, but not inside the large vortices. For the spray combustion with larger spray cone angle thickness, the vortex shedding at the outside of the flame zone is faster than that with smaller spray cone angle thickness. The instantaneous temperature maps of different spray flame structures with smaller cone angle thickness indicate the existence of small flame islands, expressing the droplet-group combustion, which is not observed in single-phase jet combustion and not obvious in the case of larger cone angle thickness.

A Numerical study of the Fire-extinguishing Performance of Water Mist in an Opening Machinery Space

Spray fire is a primary disaster in machinery space. Previous studies demonstrated that small spray fires were difficult to extinguish with water mist. A numerical and experimental study of the process of water mist interacting with the small size spray fire was conducted. Studies on suppression of vertical spray fire and horizontal spray fire were carried out, respectively. Three-dimensional physical model was developed based on the experiments. The adopted numerical model is FLUENT. The Large eddy simulation (LES) method, laminar finite-rate model, and Discrete Phase Model were selected to solve the turbulent flame, turbulence-chemistry interaction and particles, respectively. There are qualitative agreements of simulation with the experiment. It was confirmed that water mist can suppress spray fires rapidly, and that the extinguishing time for vertical spray fires were relatively shorter.

Simulation on thermoelectric device with hydrogen catalytic combustion

The micro-thermoelectric-generator based on catalytic combustion of hydrogen and oxygen was designed. With the application of general finite reaction rate model in CFD software of FLUENT, the effect of inlet parameters on the highest temperature difference between the hot and cold plate of the generator was studied. Results showed that, the temperature in the heating and cooling channel of the micro-thermoelectric-generator was uniform; with the increasing of inlet reactant temperature, the highest temperature difference increased, but the total efficiency of the generator decreased. Results can be used to the further design and optimization of micro-thermoelectric-generator based on hydrogen catalytic combustion.

Capturing Secondary Cavitation – A Step Towards Numerical Assessment of Cavitation Nuisance

Numerical simulations using incompressible Large-Eddy Simulation together with a mixture assumption and a finite-rate mass transfer model demonstrate the presence of several cavitation mechanisms important for cavitation nuisance of hydrodynamic machinery such as marine propellers and power turbines. One of such mechanisms is brought about by re-entrant jets, which can cut sheet cavities and thus lead to shedding and travelling cavities which in turn may cause nuisance as they collapse. However, severe erosion and noise generation also stem from the formation and development of small scale structures called secondary cavitation, formed e.g. in shear layers through vortex roll-up, which are also important for cavitation nuisance risk. The extent of the resolution of the secondary cavitation in the computational setting is discussed. Cavitation on a NACA0015 foil at 6° angle of attack and σ = 1.0 in a three-dimensional domain is studied.

Micro-Thermoelectric-Generator Design and Analysis Based on Catalytic Combustion of Hydrogen

The micro-thermoelectric-generator based on combustion of hydrogen and oxygen was designed. With the application of general finite reaction rate model in CFD software of FLUENT, the effect of inlet parameters on the highest temperature difference between the hot and cold plate of the generator was studied. Results show that, the temperature in the heating and cooling channel of the micro-thermoelectric-generator is uniform; With the increasing of inlet reactant temperature, the highest temperature difference increases, but the total efficiency of the generator decreases. Results can be used to the further design and optimization of micro-thermoelectric-generator based on hydrogen catalytic combustion.