Development of a novel flamelet-based model to include preferential diffusion effects in autoignition of CH4/H2 flames

This study reports on the development of a flamelet-based reduction method for autoignition of hydrogen enriched methane-based fuels. The main focus is on the inclusion of preferential diffusion effects in the Flamelet Generated Manifolds (FGM) technique for autoigniting flames. Such a development of the FGM methodology is inevitable since investigations with detailed chemistry indicate that preferential diffusion strongly affects autoignition of these mixtures. First, a novel flamelet configuration based on Igniting Mixing Layer (IML) flamelets is proposed to accommodate preferential diffusion in a flamelet database. At the next stage, transport equations for controlling variables are derived with additional terms to account for preferential diffusion effects. The extended FGM model has been evaluated by comparing its predictions with those of detailed chemistry in both laminar and turbulent situations. In laminar situations, it is revealed that the model is able to predict accurately autoignition time scales of one-dimensional hydrogen enriched flames. The turbulent situations are studied by performing Direct Numerical Simulations (DNS) of a two-dimensional unsteady mixing layer. In this configuration, the proposed model yields a precise prediction of autoignition time scales as well. The model has also been assessed using the widely used Igniting Counter-Flow (ICF) flamelets instead of IML flamelets which leads to less accurate predictions especially at high hydrogen contents. The predictive power of the proposed model combined with simplicity of its implementation introduces an attractive reduced model for the computation of turbulent flames

Analytical Study of Rotor Eccentricity Effects on Brushless Doubly Fed Machines Vibration

The Brushless Doubly Fed Machine (BDFM) with high reliability and robust structure demonstrates commercial and technical advantages both as a generator and motor for variable speed applications. As a generator it is particularly attractive to be used in offshore wind turbines where reliability improvement and maintenance cost reduction are the key factors in market growth. As a motor it may be utilized for adjustable speed drives. In this work, an analytical study has been performed on the BDFM’s vibration due to the interaction of its fundamental magnetic fields, exerting bending forces in the back iron. The effects of rotor eccentricity on exacerbating the machine’s vibration have been considered by assessing the stator back iron displacement function in the presence of rotor eccentricity. A prototype 250 kW BDFM built in frame size D400 was tested at different operating conditions in order to examine its noise and vibration levels. A set of measurements was conducted to assess the main vibration component frequencies developed by the machine at different rotor speeds. It is shown that the main vibration components are created by bending set-up in the back iron, rotor eccentricity, and the components with time and space harmonic natures. The results obtained from experimental tests agree with the analytical theory of BDFM vibration

LES of Delft Jet-in-Hot Coflow burner to investigate the effect of preferential diffusion on autoignition of CH4/H2 flames

This paper reports on numerical investigations of preferential diffusion effects in Large Eddy Simulation (LES) of turbulent lifted CH4/H2 flames. For this purpose, a combined LES and Flamelet Generated Manifolds (FGM) model is developed to simulate the Delft Jet-in-Hot Coflow (DJHC) burner. A novel type of flamelets, entitled “IML Flamelets”, has been used to tabulate the chemistry. IML flamelets are capable to incorporate preferential diffusion effects in autoigniting flames. The IML technique is coupled with LES to simulate the DJHC burner with CH4/H2 fuels where CH4 has been enriched with H2 ranging from 0% to 25% of the fuel volume. The significance of this study is to illustrate complex interactions of molecular diffusion, chemistry and turbulent transport. A good agreement has been found between LES and measurements for the velocity and OH fields. It turns out that preferential diffusion has a significant influence on the lift-off height and stabilization mechanism of the lifted H2-enriched turbulent flames. Predictions of the 0% H2 case indicate that inclusion of preferential diffusion in the combustion model modestly affects lift-off heights. However, for 5% H2, 10% H2 and 25% H2 cases, inclusion of preferential diffusion in the model affects strongly lift-off heights yielding much improved predictions compared to the unity Lewis number model. Predictions of lift-off heights and formation of ignition kernels agree very well with the measured instantaneous snapshots of OH chemiluminescence. It turns out that the combined FGM-IML approach can successfully capture main features of turbulent lifted flames such as formation of ignition kernels and stabilization mechanisms

Large eddy simulation of turbulent diffusion jet flames based on novel modifications of flamelet generated manifolds

A novel mathematical definition is introduced to achieve the inherently monotonic progress, namely Absolute Cumulative Variation (ACV). The classical progress variable is defined as a weighted summation of species mass fraction while weight factors are determined in the ad-hoc procedure. The ACV definition presents the systematic method to generate a fully bijective look-up table appropriate for the vast combustion applications. This method utilizing the preferential diffusion effects and has the potential to predict the autoignition delay time as well as pollutants, like YCO and YNO. The flamelet-generated manifold is coupled with ACV to make the ACV-FGM method. Furthermore, the Variable Ignition Mixing Layer (VIML) is presented as a modified method to generate a 2-D look-up table for the multi-inflow streams as well as varying composition reactants at the domain boundaries. This model helps to reduce the size of the look-up table for complex inflow boundary conditions and computational cost as well. The validation process for the ACV-FGM and VIML methods includes a one-dimensional laminar flame along with large eddy simulation (LES) of the Sandia piloted flames D, E, and F, and Delft Jet-Hot Coflow (DJHC) burner as lifted turbulent jet flame. The results indicate the ACV-FGM method successfully predicts the autoignition delay time, lift-off height, temperature rise as well as spices mass fractions and pollutants. Moreover, The VIML method appropriately reproduces the variation of chemical compositions and temperature at the domain boundary using the 2-D look-up table