A Numerical Study on Radiation-Induced Oscillatory Instability in CH$_4$/Air Diffusion Flames

Investigation of oscillatory stretch effects on the structure and extinction of counterflow diffusion flames

Extinction and ignition characteristics of -air counterflow diffusion flame were numerically investigated using a Flame-Controlling Method(FCM). A skeletal reaction mechanism, which adopts 17 species and 58 reactions, was used in the simulation. The extinction and ignition conditions of the -air diffusion flames were investigated with varying the global strain rate. Upper and middle branches of S-curve for the peak temperature in the inverse of the global strain rate space were obtained with the FCM. The structures of diffusion flames in the upper and middle branches of S-curve were compared. It was found that the global strain rate was not correlated with the local strain rate well in the low global strain rate region. It is expected that the FCM is very useful to obtaining the extinction and ignition condition of diffusion flame, such as fires.

Experiments on corresponding jet flames with stagnant point diffusion flames have been carried out in initial injection periods. A compensated measurement of maximum flame temperature, which is based on the ion signal, has been employed to inspect flame responses to time-varying strain rates. The flame responses are obtained at two conditions for the slowly time-varying strain rate and the case of flame extinction, and analyzed to confirm similarity between a stagnant point diffusion flame and an evolving jet diffusion flame. Nonsteady effects are addressed via the comparison between several time scales. The time variation with low strain rates, in which illustrates the flame behavior of the upper branch far from extinction in the well-known S-curve, is confirmed to produce a quasi-steady flame response through the nonsteady experiments. The time variation with strain rates in the case of flame extinction indicates an unsteady effect of flame response. It is therefore found that the flame responses near jet tip depend on time histories of characterized strain rates in the developing process.

Spray combustion under turbulent conditions occurs in many technical devices. Therefore, the proper prediction of the characteristics of turbulent spray flames is of vital importance for the design of new combustion technologies in view of efficiency and pollutant reduction, where the latter requires consideration of detailed chemical reaction mechanisms. Unfortunately, a direct inclusion of detailed chemical reactions dramatically increases the computational cost of the numerical simulations of technical combustion processes, and it is prohibitive in practical situations. Models based on the assumption that turbulent ames can be seen as an ensemble of laminar stretched flame structures, the so-called flamelet models, represent a very promising approach for the cost effective inclusion of detailed chemical reaction mechanisms in the simulation of turbulent spray flames. Several flamelet models are currently available in the literature for the simulation of pure non-premixed and pure premixed gas flames. Additionally, some two-regime flamelet formulations have been proposed in the last years for situations where nonpremixed and premixed gas combustion coexist and interact. These models, however, are not adequate for the simulation of turbulent spray combustion, since they do not take into account spray evaporation, which strongly affects the flame structure. Although a spray flamelet model has been proposed for the simulation of flames where non-premixed and evaporation-dominated combustion regimes coexist, most studies of turbulent spray flames use gas flamelet models, neglecting the effects of evaporation on the flame structure. In the present thesis, a common framework is developed in which the several single and two-regime flamelet models existing in the literature can be described and combined in order to advance the development of a comprehensive multi-regime spray flamelet model for turbulent spray flames. For this purpose, a set of multi-regime spray flamelet equations in terms of the mixture fraction and a reaction progress variable is derived, which describes all combustion regimes appearing in spray flames. The flamelet equations available in the literature for single and two-regime flames are retrieved from these multi-regime spray flamelet equations as special cases. Additionally, exact transport equations of the mixture fraction and its scalar dissipation rate are derived, which are then used to evaluate the validity of several assumptions commonly made in the literature during their derivation, such as the use of unity Lewis number and the negligence of spatial variations of the mean molecular weight of the mixture. These assumptions had not yet been tested for the calculation of the scalar dissipation rate of the mixture fraction in spray flames, and their validation is of vital importance for the formulation of any spray flamelet model. Numerical simulations of axi-symmetric laminar mono-disperse ethanol/air counterflow spray flames are carried out to analyze the influence of spray evaporation on the flame structure. Parametric studies of the influence of the initial droplet radius and strain rate are presented, which clearly illustrate the major importance of evaporation in the determination of the flame structure. Additionally, the relative importance of non-premixed and premixed combustion regimes in the previously analyzed counterflow spray flames is studied by means of the derived multi-regime spray flamelet equations. The results show that premixed effects can be neglected in this kind of flame with all fuel injected in liquid phase. Moreover, the derived transport equations of mixture fraction and its scalar dissipation rate are solved for the counterflow spray flames considered in this work considering and without considering the assumptions of unity Lewis number and spatially uniform mean molecular weight of the mixture. The results are compared, and it is found that the assumption of unity Lewis number may lead to non-physical values of the scalar dissipation rate of the mixture fraction, whereas the use of a mass-averaged diffusion coefficient of the mixture is an acceptable approximation. Effects associated with the spatial variation of the mean molecular weight of the mixture are found to be small at low strain rate and negligible at high strain rates. These results confirm the validity of the use of Fick's diffusion law in highly strained flames. Finally, a set of non-premixed spray amelet equation is obtained by neglecting premixed effects in the previously derived multi-regime spray flamelet equations. This set of equations, which is valid in situations where non-premixed and evaporation-dominated combustion regime coexist, is similar to the classical non-premixed gas flamelet equations, but it contains two additional terms for the description of evaporation effects. These equations are then used to evaluate the relative importance of the effects attributable to evaporation. The results show that they are always relevant and they should be always considered.

Combustion characteristics of hydrogen-rich alternative fuels in counter-flow diffusion flame configuration

Buoyancy effect on extinction limits in low strain rate counterflow diffusion flames of methane

Nonlinear dynamic behavior of diffusive-thermal instability in diluted CH₄/O₂ diffusion flames is numerically investigated by adopting detailed chemistry and transport. Counterflow diffusion flame is adopted as a model flame let. Particular attention is focused on the pulsating-instability regime, which arises for Lewis numbers greater than unity, and the instability occurs at high strain rate near extinction condition in this flame configuration. Once a steady flame structure is obtained for a prescribed value of initial strain rate, transient solution of the flame is calculated after a finite amount of strain-rate perturbation is imposed on the steady flame. Transient evolution of the flame depends on the initial strain rate and the amount of perturbed strain rate. Basically, the dynamic behaviors can be classified into two types, namely non-oscillatory decaying solution and diverging solution leading to extinction. The peculiar oscillatory solution, which has been found in the previous study adopting one-step chemistry and constant Lewis numbers, is not observed in this study, which is attributed to both convective flow and preferential diffusion effects.

Extinction characteristics and acoustic response of hydrogen-air diffusion flames at various pressures are numerically studied by employing counterflow diffusion flame as a model flamelet in turbulent flames in combustion chambers. The numerical results show that extinction strain rate increases linearly with pressure and then decreases, and increases again at high pressures. Thus, flames are classified into three pressure regimes. Such nonmonotonic behavior is caused by the change in chemical kinetic behavior as pressure rises. The investigation of acoustic-pressure response in each regime, for better understanding of combustion instability, shows different characteristics depending on pressure. At low pressures, pressure-rise causes the increase in flame temperature and chain branching/recombination reaction rates, resulting in increased heat release. Therefore, amplification in pressure oscillation is predicted. Similar phenomena are predicted at high pressures. At moderate pressures, weak amplification is predicted since flame temperature and chain branching reaction rate decreases as pressure rises. This acoustic response can be predicted properly only with detailed chemistry or proper reduced chemistry.

In this thesis, thermal-diffusive instabilities are studied experimentally in diffusion flames. The novel species injector of a recently developed research burner, consisting of an array of hypodermic needles, which allows to produce quasi one-dimensional unstrained diffusion flames has been improved. It is used in a new symmetric design with fuel and oxidizer injected through needle arrays which allows to independently choose both the magnitude and direction of the bulk flow through the flame. A simplified theoretical model for the flame position, the temperature and the species concentration profiles with variable bulk flow is presented which accounts for the transport properties of both reactants. The model results are compared to experiments with a CO2-diluted H2-O2 flame using variable bulk flow and inert mixture composition. The mixture composition throughout the burning chamber is monitored by mass spectrometry. An elaborate calibration procedure has been implemented to account for the variation of the mass spectrometer sensitivity as a function of the mixture composition. The calibrated results allow the effective mixture strength of the diffusion flames to be measured with a relative uncertainty of about 5 %. In order to properly characterize the flame produced, the velocity and temperature distribution inside the burning chamber are measured. The resulting species concentration and temperature profiles are compared to the simplified theory and demonstrate that the new burner configuration produces a good approximation of the 1-D chambered diffusion flame, which has been used extensively for the stability analysis of diffusion flames. The velocity profiles are also used to quantify the residual stretch experienced by the flame which is extremely low, below 0.15 s-1. Hence, this new research burner opens up new possibilities for the experimental validation of theoretical models developed in the idealized unstrained 1-D chambered flame configuration. The thermal-diffusive instabilities observed close to extinction are investigated experimentally and mapped as a function of the Lewis numbers of the reactants. The use of a mixture of two inerts (helium and CO2) allows for the effect of a wide range of Lewis numbers to be studied. A cellular flame structure is observed in hydrogen flames when the Lewis numbers is relatively low with a typical cell size between 7 and 15 mm. The cell size is found to scale linearly with the diffusion length, in good agreement with theoretical predictions. When the Lewis number is increased by using a higher helium content in the dilution mixture, the instabilities observed are planar intensity pulsation. The use of methane allowed pulsating flames to be generated for a wide range of bulk velocities and transport properties. The pulsating frequencies measured are in the 0.7 to 11 Hz range and were found to scale linearly with a diffusion frequency defined as U2/Dth multiplied by the square root of the Damkohler number. The experimental results presented here are the first observations of thermal-diffusive instabilities in such a low-strain flame. They constitute a unique dataset that can be used to quantitatively validate theoretical models on diffusion flame stability developed in the simplified one-dimensional configuration.

Chemical and radiation effects on flame extinction and NOx formation in oxy-methane combustion diluted with CO2

Tabulated chemical kinetics for efficient and detailed simulations of diesel engine combustion

Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames

How to attack complex gas phase combustion systems

Effect of strain rate on nanoparticles and soot in counterflow flames of ethylene/ethanol and ethylene/OME3