Development Of Combustion Model Research Articles

A quantitative explanation for the apparent anomalous temperature dependence of OH + HO2 = H2O + O2 through multi-scale modeling

Kinetic models for complex chemical mechanisms are comprised of tens to thousands of reactions with rate constants informed by data from a wide variety of sources – rate constant measurements, global combustion experiments, and theoretical kinetics calculations. In order to integrate information from distinct data types in a self-consistent manner, a framework for combustion model development is presented that encapsulates behavior across a wide range of chemically relevant scales from fundamental molecular interactions to global combustion phenomena. The resulting kinetic model consists of a set of theoretical kinetics parameters (with constrained uncertainties), which are related through kinetics calculations to temperature/pressure/bath-gas-dependent rate constants (with propagated uncertainties), which in turn are related through physical models to combustion behavior (with propagated uncertainties). Direct incorporation of theory in combustion model development is expected to yield more reliable extrapolation of limited data to conditions outside the validation set, which is particularly useful for extrapolating to engine-relevant conditions where relatively limited data are available. Several key features of the approach are demonstrated for the H2O2 decomposition mechanism, where a number of its constituent reactions continue to have large uncertainties in their temperature and pressure dependence despite their relevance to high-pressure, low-temperature combustion of a variety of fuels. Here, we use the approach to provide a quantitative explanation for the apparent anomalous temperature dependence of OH+HO2=H2O+O2 – in a manner consistent with experimental data from the entire temperature range and ab initio transition-state theory within their associated uncertainties. Interestingly, we do find a rate minimum near 1200K, although the temperature dependence is substantially less pronounced than previously suggested.

A Physics and Tabulated Chemistry Based Compression Ignition Combustion Model: from Chemistry Limited to Mixing Limited Combustion Modes

This paper presents a new 0D phenomenological approach to predict the combustion process in multi injection Diesel engines operated under a large range of running conditions. The aim of this work is to develop a physical approach in order to improve the prediction of in-cylinder pressure and heat release. Main contributions of this study are the modeling of the premixed part of the Diesel combustion with a further extension of the model for multi-injection strategies. In the present model, the rate of heat release due to the combustion for the premixed phase is related to the mean reaction rate of fuel which is evaluated by an approach based on tabulated local reaction rate of fuel and on the determination of the Probability Density Function (PDF) of the mixture fraction (Z), in order to take into consideration the local variations of the fuel-air ratio. The shape of the PDF is presumed as a standardized <i>β<i/>-function. Mixture fraction fluctuations are described by using a transport equation for the variance of Z. The standard mixture fraction concept established in the case of diffusion flames is here adapted to premixed combustion to describe inhomogeneity of the fuel-air ratio in the control volume. The detailed chemistry is described using a tabulated database for reaction rates and cool flame ignition delay as a function of the progress variable c. The mixing-controlled combustion model is based on the calculation of a characteristic mixing frequency which is a function of the turbulence density, and on the evolution of the available fuel vapor mass in the control volume. The developed combustion model is one sub-model of a thermodynamic model based on the mathematical formulation of the conventional two-zone approach. In addition, an extended sub-model for multi injection is developed to take into account interactions between each spray by describing their impact on the mixture formation. Numerical results from simulations are compared with experimental measurements carried out on a 2 liter Renault Diesel engine and good agreements are found.

Coal Particles Combustion Model and Micro Feeding Technology in Drop Tube Furnace

For investigation of wide use of drop tube furnace (DTF), the design and characterization of DTF and shaftless screw microfeeder are presented. With a developed group combustion model, a introduced non-dimensional number is to characterize the combustion zone of coal particles in DTF. Calculated relation is proposed to be operation criteria of DTF and design principle of feeder. It is found that the test results of coal feeder show good performance for micro feeding.

Comparison of gas-phase mechanisms applied to RDX combustion model

Two detailed gas-phase chemical mechanisms for RDX – Yetter and coworkers, herein ‘Y2’ [K. Prasad, R.A. Yetter, M.D. Smooke, Combust. Sci. Technol. 124 (1997) p. 35.]; Cal. Tech. group, herein ‘CTM’ [(a) A.D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, J. Phys. Chem. A 104 (2000) 2261. (b) D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, J. Comput. Aided Mater. Des. 8 (2001) 203. (c) D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, Available from: http://www.wag.caltech.edu/home/rpm/projects/hedm/] – have been tested using a recently developed combustion model. The results are compared with each other and experimental data. Burning rates predicted using CTM are about 15% higher than Y2, but both compare well with experimental data across a wide pressure range. Also, majority species profiles are in reasonable agreement with data from a 0.5 atm pressure experiment. However, comparison of predicted trace species profiles to experiments indicates neither mechanism reproduces all measured trace species well; furthermore, most of these trace species occur along main reaction pathways. Detailed chemical analysis indicates the main initial RDX reaction is surprisingly very different for the two mechanisms. NO 2 scission dominates using Y2, but HONO elimination dominates using CTM, in spite of the NO 2 scission reaction having by far the largest RDX decomposition rate coefficient in each mechanism. Analysis shows the unexpected result using CTM is due to a curious global kinetics phenomenon arising in the product pathway: the ring-opening reaction, RDXR → RDXRO, where RDXR is the cyclic radical formed upon NO 2 scission, has a much smaller rate coefficient in CTM compared to Y2. This causes the reaction to be a bottleneck, and so the NO 2 scission reaction goes into partial equilibrium rather than being forwards. Tests were performed to see how the predicted burning rates would be affected by changes in some of the most sensitive rate parameters. Some of the key parameters leading to the differing predictions have been identified. These results will help guide future efforts to understand and develop an accurate representation of the actual RDX combustion chemistry.

Quantification of transient stretch effects on kernel–vortex interactions in premixed methane–air flames

Relative flame speeds of time-dependent highly curved premixed methane–air flames (spark-ignited flame kernels) interacting with a laminar vortex have been quantified using high-speed chemiluminescence imaging, particle image velocimetry, and piezoelectric pressure measurements. The goals of this study are to improve fundamental understanding of transient stretch effects on highly curved premixed flames, to provide practical insight into the turbulent growth of spark-ignited flame kernels in internal combustion (IC) engines burning light hydrocarbon fuels, and to provide data for IC engine ignition and combustion model development. Lean and rich CH 4–O 2–N 2 flames were tested ( ϕ = 0.64 , 0.90, and 1.13, with nitrogen dilution to equalize the flame speeds ( S b ) in the absence of vortex interaction). Transient stretch rates were varied using three different vortex strengths, and the size of the flame kernel at the start of the vortex interaction controlled by time delay between ignition and vortex generation. Vortex interactions with small (∼5 mm radius) flame kernels were found to increase burning rates for lean ( ϕ = 0.64 ) flame kernels substantially. Burning rates for rich ( ϕ = 1.13 ) flames were decreased, with total flame kernel extinction occurring in extreme cases. These small flame kernel–vortex interactions are dominated by transient stretch effects and thermodiffusive stability, in agreement with premixed flame theory. However, vortex interactions with larger methane–air flame kernels (∼30 mm radius) led to slight flame speed enhancements for both lean and rich flame kernels, with the flame–vortex process dominated by increased flamefront area generated by vortex-induced flame wrinkling.

Prediction of the Autoignition Delay Time of Producer Gas from Biomass Gasification

The greenhouse effect and the increasing demand for energy have encouraged the use of alternative fuels, with agricultural and forestry biomass waste being two of the main renewable sources that European and Spanish policies have been promoting during the last several years. In this article, theoretical results for the chemical autoignition delay time of producer gas obtained from the gasification of lignocellulosic biomass are presented. These results, together with those obtained in a previous work related to the laminar flame speed of the gas, are of great interest both for understanding the chemical kinetic mechanisms that control the fuel oxidation process and for the development of combustion models that provide significant information to be used as a tool for the optimization and design of internal combustion engines. The CHEMKIN software, in conjunction with the GRI-Mech chemical reaction mechanism, has been used to compute the autoignition delay time for different producer gas compositions, diffe...

Estimation of the Laminar Flame Speed of Producer Gas from Biomass Gasification

Because of the importance that the energy use of agricultural and forestry wastes has acquired over the last years, results for the laminar flame speed of producer gas coming from the gasification of lignocellulosic biomass are presented in this work. These results have great interest for the development of combustion models that provide significant information to be used as a tool for the optimization and design of specific internal combustion engines. The CHEMKIN software, together with the GRI-Mech chemical reaction mechanism, has been used to compute the laminar flame speed for different producer gas compositions, different values of pressure and temperature, and different producer gas/air equivalence ratios. The results have been compared with those obtained in an experimental combustion bomb, as well as with the laminar flame speed obtained for conventional fuels, showing that the flame speed of the producer gas is less than that of isooctane but greater than that of methane. A sensitivity analysis ...

Development of Combustion Model Incorporating Intermediate Species and Numerical Simulation of Mixing and Combustion in Transient Spray Flames.

Numerical simulation and analysis of transient sprays were performed based on an Eulerian gas and Lagrangian drop formulation coupled with a newly developed combustion model which takes into account intermediate species of H2 and CO. The formulation consists of the conservation equations of mass, momentum, energy and species. All equations are solved numerically in fully coupled form. Numerical results are presented paying attention to the local mixing in the transient spray flame and parametric effects of injection velocity and ambient temperature on the combustion.

96/01081 In situ combustion model development and its applications for laboratory studies

96/01081 In situ combustion model development and its applications for laboratory studies

In situ combustion model development and its applications for laboratory studies

A one-dimensional in situ combustion model was developed for oil and gas prduction calculations using a frontal-advance theory combined with mass and energy balance equations and simplified by boundary conditions, limitations and assumptions. The model is run for data from laboratory combustion tube experiments. Model runs for such experiments show that the results are in good agreement with the experimental values for the dry combustion mode, but not for the wet combustion mode where the water injection is high.

An Integrated Approach to Spray Combustion Model Development

Abstract The major elements of an approach to the development of a more powerful spray combustion model is discussed. A model is sought that treats, in a numerically-efficient manner, unsteady, multidimensional, turbulent reacting flows with polydisperse. multicomponenl sprays and properly accounts for transient droplet heating and vaporization, liquid-phase mass diffusion, and droplet drag and trajectory. Major advances are reviewed and important issues remaining for resolution are discussed.

Modeling of coal-combustion processes

Modeling of coal-reaction processes has received significant emphasis over the past decade. Yet, model development has not reached the point where significant use is made in process development for coal utilization. One of the key recommendations from a panel of industrial and university professionals to the U.S. Department of Energy on research needs for coal utilization was that “Development of combustion models is needed to the point where they will find application in the management and control of practical systems.” The purpose of this publication is to present a review and evaluation of the development of coal-reaction models. Coal-combustion models are classified and potential uses identified. Then, the state of development of models for fixed (or slowly moving) beds, fluidized beds and suspended (or entrained) beds is presented. Six fixed-bed models, ten fluidized-bed models and thirteen suspended-bed models are reviewed and compared. Selected comparisons of model predictions with measurements are shown and research needs are noted. Emphasis is placed on suspended-bed models.

Temperature and Velocity Measurements in Premixed Turbulent Flames

Turbulent flame speed data for premixed flames of methane-air, propane-air and ethylene-air mixtures stabilized in grid turbulence are reported and discussed. It is shown that turbulence effects on flame speed cannot be fully correlated by the turbulence length scale and r.m.s. velocity in the cold flow. Rather there appear to be significant flame-flow-turbulence interactions affecting both turbulence level in the reaction zone and measured flame speeds. Results of detailed velocity measurements, including autocorrelations, by laser velocimetry are used to elucidate the nature of these interactions. It is concluded that flame speed experiments must be designed and conducted to provide sufficient information (e.g., boundary conditions) to allow for reconstruction of the flow field and these interactions by modelers if the data are to be of value in turbulent combustion model development and evaluation.