Anders Brink

Micro Mixing Time in the Eddy Dissipation Concept

The turbulent micro mixing in the Eddy Dissipation Concept (EDC) has been investigated. The EDC is a model for turbulence-chemistry interaction modeling that allows for the use of multi-step reactions. The EDC was originally developed assuming that the reaction kinetics is fast compared to the turbulent mixing. With a more detailed description of the chemistry the interaction between turbulence and chemistry becomes more complex which may call for a revision of the model. Three ways to estimate the turbulent mixing times have been investigated. The first is the time scale proposed in the EDC, i.e., the lime scale related to the Kolinogorov time scale. The second is a time scale based on the geometrical mean of the Kolinogorov time scale and the time scale given by k/ϵ. The third time scale is the time scale given by 4k/epsiv;, The fuel chemistry was modeled with a three-step simplified mechanism A separate model based on calculations with a comprehensive reaction mechanism was used for estimating the chemical time scale at which extinction occurs. In the study a natural gas-fired sudden expansion reactor was modeled. The use of the Kolmogorov related time scale resulted in an overestimation of the extinction effects, whereas best result was obtained when the mixing time was modeled with the time scale given by 4kϵ.

Modeling of Oxy-Natural Gas Combustion Chemistry

Two turbulence ‐chemistry interaction models that can be used in numerical modeling of oxy-natural gas e ame combustion,wherethereactionkineticsarefastandthermaldissociationintheproductsisofimportance,havebeen compared and investigated. Detailed in-e ame measurements, carried out in a coaxial jet diffusion e ame of natural gas burning in pure oxygen, are presented and are used to validate the models. Both turbulent combustion models, namely, the presumed probability density function (PDF) model and the eddy dissipation concept (EDC), were combined with a chemical thermodynamic equilibrium procedure to describe the chemistry. The models differ in that,inthepresumedPDFmodel,astatisticalviewpointisutilizedwhencalculatingthelocalcomposition,whereas, in the EDC, the turbulent mixing rate plays a more dominant role. The calculations showed that, although the temperature e eld could be well predicted, the concentrations of intermediate species appeared too high. Similar predictions were obtained with both models, the largest differences were found in the e ame sheet in the vicinity of the burner inlet. The much smaller ine uence of the description of the chemistry found in the e ame calculations compared to that in the thermodynamic equilibrium calculations indicates that radiation has a strong smoothing effect on the results.

Sugarcane vinasse CO2 gasification and release of ash-forming matters in CO2 and N2 atmospheres.

Gasification of sugarcane vinasse in CO2 and the release of ash-forming matters in CO2 and N2 atmospheres were investigated using a differential scanning calorimetry and thermogravimetric analyzer (DSC-TGA) at temperatures between 600 and 800°C. The results showed that pyrolysis is the main mechanism for the release of the organics from vinasse. Release of ash-forming matters in the vinasse is the main cause for vinasse char weight losses in the TGA above 700°C. The losses are higher in N2 than in CO2, and increase considerably with temperature. CO2 gasification also consumes the carbon in the vinasse chars while suppressing alkali release. Alkali release was also significant due to volatilization of KCl and reduction of alkali sulfate and carbonate by carbon. The DSC measured thermal events during heating up in N2 atmosphere that correspond to predicted melting temperatures of alkali salts in the char.

THE JOINT PROBLEM OF MODEL STRUCTURE DETERMINATION AND PARAMETER ESTIMATION IN QUANTITATIVE IR SPECTROSCOPY

Abstract A method for automatically selecting the wave numbers best suited for quantitative analysis as well as for simultaneously estimating the parameters in the emerging model is presented. As an indicator of the goodness of the model, Akaikes information theoretic criterion (AIC) is used. Since this approach involves the maximum likelihood estimate of the parameters, the problem of how to scale the data prior to the calculations is eliminated. The method described in this paper is not restricted to Fourier transform infrared (FTIR) problems, but can be applied to other similar problems, where both the model structure and the parameters should be determined. During the calibration stage, a mixed-integer nonlinear programming problem must be solved. It is demonstrated that the use of such modern optimization techniques makes it possible to solve these types of problems without tremendous computational effort. During the prediction stage the obtained model is easy to use.

Analyzing Local Combustion Environment with a Flamelet Model and Detailed Chemistry

Measurements have been done in order to obtain information concerning the effect of EGR for the smoke and NOx emissions of a heavy-duty diesel engine. Measured smoke number and NOx emissions are explained using detailed chemical kinetic calculations and CFD simulations. The local conditions in the research engine are analyzed by creating equivalence ratio temperature (Phi-T) maps and analyzing the CFD results within these maps. The study uses different amount of EGR and the standard EN590 diesel fuel. The detailed chemical kinetic calculations take into account the different EGR rates. The CFD calculations are made with a flamelet based combustion model together with detailed chemistry. The results are compared to a previous study where a hybrid local flame area evolution model combined with an eddy breakup type model was used in the CFD simulations. It was observed that NOx emission trends can be well captured with the Phi-T maps but the situation is more difficult with the engine soot. Hence, the conclusion is the same as in the previous study with the hybrid combustion model. However, the local reaction zone is qualitatively very different with the flamelet model as compared to the hybrid model. Phi-T maps were also constructed for the total fixed nitrogen, using a detailed description of the nitrogen chemistry. EGR is typically used as an NOx abatement technique, having the purpose to lower the temperature and thus the formation of thermal-NO. However, these maps revealed a new functionality of EGR as a NOx abatement method.

CFD investigation of deposition in a heat recovery boiler: Part I - a dual-layer particle conversion model

Deposits of unconverted particles in the heat recovery boilers used in the copper flash smelting process may cause operational problems. In this work, a dual-product layer shrinking-core-like particle model is presented. In this model the conversion can be solved from a single non-linear equation. Computational Fluid Dynamics (CFD) can provide detailed insight into the processes in the heat recovery boiler. Used with CFD, this particle conversion model can be used to study the effect of boiler design and process parameters on the depositing particles.

CFD investigation of deposition in a heat recovery boiler: Part II – deposit growth modelling

In the copper flash smelting process, deposition of dust particles can diminish the capacity of the heat recovery boiler. A deposit growth model was developed and implemented in a commercial CFD code to simulate deposition in a heat recovery boiler. In the model, a force balance of the particle landing on the wall is used as the deposition criterion. The thickness increase of the deposit layer is calculated from the mass flow of sticking particles. Simulations are used to demonstrate the applicability of the model. They also help in providing insight into the process.

Nitric oxide yield from combustion of a low calorific gasification product gas : numerical and experimental study

The nitrogen oxide yield in a combustor burning an ammonia rich product gas from a bubbling bed gasifier has been investigated. The measurements showed a surprisingly small effect of the excess air in the combustor on the NOx emissions. To understand the reason for this behaviour the nitrogen chemistry in the combustor has been studied using CFD combined with a detailed description of the fuel-N chemistry. With higher oxygen excess a NOx yield of 22% is calculated compared to a measured NOx yield of 21%. In the case with lower oxygen excess the corresponding numbers are 20% and 18%, respectively.

Potential for thermochemical conversion of biomass residues from the integrated sugar-ethanol process - Fate of ash and ash-forming elements

In this work, potential for thermochemical conversion of biomass residues from an integrated sugar-ethanol process and the fate of ash and ash-forming elements in the process are presented. Ash, ash-forming elements, and energy flows in the process were determined using mass balances and analyses of eight different biomass samples for ash contents, elemental compositions, and heating values. The results show that the ash content increases from the sugarcane to the final residue, vinasse. The cane straw, which is left in the field, contains one-third of the energy and 25% of the K and Cl while the vinasse contains 2% of the energy and 40% of the K and Cl in the cane. K and Cl in biomass fuels cause corrosion and fouling problems in boilers and gasifiers. Over 85% of these elements in the straw are water soluble indicating that water leaching would improve it for utilization in thermochemical conversion.

Analysis of combustion processes using computational fluid dynamics—A tool and its application

Abstract Numerical simulation by means of Computational Fluid Dynamics (CFD) has developed over recent years to a valuable design tool in engineering science. In the beginning mainly applied to address fluid dynamic questions it is nowadays capable to predict in detail conditions in various complex technical processes. State of the art commercial CFD codes are almost always set up as multi-purpose tools suitable for a wide variety of applications from automotive industry to chemical processes and power generation. However, since not highly specialized in all possible fields of application, CFD codes should be rather seen as a collection of basic models that can be compiled and extended to individual tools for special investigations than as readily applicable tools. In power generation CFD is extensively used for simulation of combustion processes in systems like utility boilers, industrial furnaces and gas turbines. The purpose of these simulations is to analyze the processes, to optimize them with regard to efficiency and safety and to develop novel techniques. Since combustion processes have been the target for CFD software for long, also standard models available in the codes are of high quality as long as modelling of conventional combustion systems is concerned. However, as soon as characteristics of novel combustion systems or fuels or detailed effects within a certain process are of interest, the limits of these standard models are reached easily. At this point extension of standard models by process specific knowledge is required. This paper presents some of the opportunities CFD offers when applied to analyse different combustion systems. Practical examples presented are ash deposition predictions on heat exchanger surfaces and walls in a bubbling fluidised bed furnace and detailed nitrogen oxide emission predictions for the same furnace type. Furthermore, the extension of a standard model using process specific data is presented for the fuel conversion process in a black liquor recovery furnace.