Stirred Reactor Approach Research Articles

A priori test of perfectly stirred reactor approach to evaluating mean fuel consumption and heat release rates in highly turbulent premixed flames

Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by Dave et al. ( J Fluid Mech 2020; 884: A46) from a statistically planar and one-dimensional, highly turbulent, moderately lean hydrogen-air flame propagating in a box are processed to perform a priori test of perfectly stirred reactor model. The test aims in particularly at estimating mesh resolution (or filter width within large eddy simulation framework) required to neglect variations in the temperature and mixture composition within a computational cell when evaluating mean (or filtered) fuel consumption and heat release rates. For this purpose, fuel consumption and heat release rates sampled directly from the DNS data and averaged over a cube of width [Formula: see text] are compared with fuel consumption and heat release rates calculated using the temperature and species concentrations averaged over the same cube. Moreover, turbulent burning velocities computed by integrating the former and latter rates are compared with one another. A ratio of [Formula: see text] to a laminar flame thickness [Formula: see text] is varied from 0.44 to 1.8. The obtained results indicate that the tested simple approach performs reasonably well (poor) if [Formula: see text] ([Formula: see text], respectively). This result is further supported by directly filtering fuel consumption rate in a laminar premixed flame. The values of the thickness [Formula: see text], calculated using detail chemical mechanisms for different fuels under elevated temperatures and pressures associated with combustion in piston engines, indicate that it is difficult to satisfy the constraint of [Formula: see text] in contemporary unsteady multidimensional numerical simulations of turbulent burning in such engines.

Modelling VOCs Emissions in Coal Fired Power Stations using Perfectly Stirred Reactor Approach

The main objective of this study is to evaluate the emissions of volatile organic compounds (VOCs) from coal-fired power stations. A quantitative understanding of the chemistry controlling the formation and destruction of these intermediate species is a prerequisite for the realistic modelling of pollutant formation in flames. Therefore, well investigated skeletal reaction mechanism has been built and introduced into perfectly stirred reactor model in order to accomplish prediction of some of these hazardous important intermediate species. This can be great help in considering lack of experimental VOCs data from fossil fuel fired power stations. Predicted results have been validated against ICSTM Furnace data where possible. The performance of the model against the large laboratory scale experimental data has resulted in considerable confidence in the use of this model for a full-scale boiler configuration. But, more confidence will be forthcoming from an increase in the amount of validation data, which is unluckily lacking at the moment. Furthermore, the model can be used in order to provide deeper insight into the formation processes of VOC species emitted from coal-fired power stations. However, this can be accomplished far better by including more elementary reactions into the skeletal reaction mechanism.

Large-eddy simulation of MILD combustion using partially stirred reactor approach

Subgrid-scale (SGS) parameterization and method for calculating filtered reaction rate are critical components of an accurate large-eddy simulation (LES) of turbulent flames. In this study, we integrate gradient-type structural SGS models with a partially stirred reactor approach by using detailed chemical kinetics to simulate a turbulent methane/hydrogen jet flame under moderate or intense low-oxygen dilution (MILD) conditions. The study examines two oxygen dilution levels. The framework is assessed through a systematic and comprehensive comparison of temperature, and mass fractions of major and minor species with experimental data and other reference simulation results. Overall, the statistics of the combustion field show excellent agreement with measurements at different axial locations, and a significant improvement compared to some previous simulations. It suggests that the proposed nonlinear LES framework is able to accurately model MILD combustion with reasonable computational cost.

On the role of mixing models in the simulation of MILD combustion using finite-rate chemistry combustion models

The present work shows an in-depth analysis about the role of mixing models on the simulation of MILD combustion using a finite-rate combustion model, the Partially Stirred Reactor approach (PaSR). Different approaches of increasing complexity are compared: a simple model based on a fraction of the integral time scale, a fractal-based mixing model and a dynamic mixing model based on the resolution of transport equations for scalar variance and dissipation rate. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet-in-Hot Co-flow (JHC) burner at different fuel-jet Reynolds numbers (5k, 10k and 20k) and different co-flow oxygen dilution levels (3%, 6% and 9%). The results indicate the major role of mixing models to correctly handle turbulence/chemistry interactions and clearly indicate the superior performances of the dynamic mixing model over the other tested approaches.

The Computational Singular Perturbation/Perfectly Stirred Reactor Approach in Reduced Chemistry of Premixed Ethanol Combustion

Ethanol is a bio-fuel widely used in engines as a fuel or fuel additive. It is, in particular, attractive because it can be easily produced in high quality from renewable resources. Its properties are of interest in many fields, such as gas turbines applications as well as fuel cells. In the past decades, research in chemistry and engineering has put a lot of effort into a better understanding of its gas-phase chemical kinetic properties during combustion processes. This work describes a methodology to define an optimal expression of the reaction progress variable in the context of tabulated chemistry in laminar premixed combustion. The choice of the reaction progress variable is based on the investigation of the wide range of consumption rates of the species involved in the reaction. Two methods are used: the computational singular perturbation method and a sensitivity analysis of the time scales evaluated with a perfectly stirred reactor. The thermochemical databases computed with these techniques are compared in the cases of a freely propagating flame and a Bunsen flame, in the laminar premixed regime and under stoichiometric conditions. The influence of the chemical kinetics on the laminar flame speed is estimated from the results of the freely propagating flame. The case where the differences in the performance between the databases become more pronounced is the Bunsen flame, where some databases lead to a premature ignition prediction of the flame.

A Comparative Study of Eight Finite-Rate Chemistry Kinetics for Co/H2 Combustion

We compare the performance and computational cost of 8 kinetic models (3 global and 5 elementary) that describe the finite-rate chemistry of syngas combustion. We apply them in simulating a turbulent jet flame with syngas diluted by 30% nitrogen. We model the turbulence by a modified k-epsilon model and the turbulence-chemistry interaction by the partially stirred reactor approach. To integrate the chemistry equations, we nominally use explicit fifth-order embedded Runge-Kutta ODE solver. But semi-implicit Bulirsch-Stoer and implicit Euler were also used. The computational time depends on the number of reaction steps and the ODE solver. Five models overpredict the maximum flame temperature (by 200 K-320 K). Two models underpredict it by 240 K and 580 K. The global model that is based on the Westbrook-Dryer (1981) model for hydrocarbon fuels gives the best agreement with measurements, and also has low computational demand. Therefore, it is recommended for modeling turbulent syngas flames.

Particle in Box: A Cell for in-situ Measurements of Porosity and Effective Diffusion Coefficient during Thermophysical Transformations

The aim of this work is to develop an experimental device for the measurement of porous media textural properties during physical or chemical transformation. The method is based on gas tracer experiments. The system investigated consists of a spherical reactor containing a spherical porous media. To avoid surface stresses, there is no contact between the sample and the reactor. We called the cell a particle in box, or ``PIB''. The porosity of the solid and the gas effective diffusivity were determined using a parametric identification method. The model is based on the perfectly stirred reactor approach. The PIB methodology was first checked at room temperature to determine the solid porosity and the effective diffusion coefficient. Experiments were then performed during transformation of a sample composed of a mixture of hydroxylapatite particles, glycerol, and water. In situ determination of the porosity and diffusivity from room temperature to 573 K (300 degrees C) was demonstrated.

Flame stabilization in high-velocity heterogeneous fuel-air mixtures

Weak-extinction data obtained from an experimental apparatus designed to simulate the characteristics of practical afterburner combustion systems are presented. The apparatus supplies heterogeneous fuel-air mixtures of varied temperature, velocity, and degree of prevaporization to V-gutter flameholders of various widths and shapes similar to those found in jet engine systems. The fuel employed is a hydrocarbon whose chemical composition and physical properties correspond to those of aviation kerosene JP 5. An analysis that accounts for heterogeneous physical transport processes is described. This analysis extends a previously derived homogeneous stability analysis, based on stirred-reactor theory, to include the characterization of bluff-body systems supplied with nonuniform, heterogeneous, fuel-air mixtures. The ability of the analysis to capture observed experimental trends indicates that a stirred-reactor approach, coupled with appropriate heterogeneous transport models, provides a framework for predicting the lean-blowout limits of practical flameholder systems.

Weak Extinction Limits of Large-Scale Flameholders

Weak extinction data obtained from an experimental apparatus designed to simulate the characteristics of practical afterburner combustion systems are presented. The apparatus supplies mixtures of varied composition (equivalence ratio and degree of vitiation), temperature and velocity to Vee-gutter flame holders of various widths and shapes similar to those found in jet engine systems. The fuel employed is a liquid hydrocarbon whose chemical composition and physical properties correspond to those of aviation kerosine, JP5. An equation for predicting weak extinction limits which accounts for upstream vitiation and the chemical characteristics of the fuel is derived from stirred reactor theory. The correlation between the predictions and experimental results indicates that the stirred reactor approach can provide a framework for predicting the lean blowout limits of practical flameholders over wide ranges of engine operating conditions.