Development of a porous burner for low calorific gaseous fuels offering a wide operating range [in press]

This work presents the development of a burner for the utilization of low calorific value waste gas, as it arises in the production of high purity hydrogen from biogas using an oxidative steam reforming process. Stable combustion of different fuel gases with fluctuating gas composition over a wide operating range is assured by the application of combustion in an inert porous medium (PIM) utilizing a kinematic flame stabilization mechanism. The development of the PIM-burner bases on calculated effective flame speeds within PIM derived from a 1-D numerical model including harsh operating conditions with preheating temperatures above 800 K and carbon dioxide concentration of 70 %-vol in the fuel gas. Experiments are conducted on a tailored test rig in order to validate numerical predictions by comparison of calculated effective flame speeds to eff ective flame speeds derived from temperature measurements in PIM

Implementation and Validation of a Computationally Efficient DNS Solver for Reacting Flows in OpenFOAM

To meet future climate goals, the efficiency of combustion devices has to be increased. This requires a better understanding of the underlying physics. The simulation of turbulent flames is a challenge because of the multi-scale nature of combustion processes: relevant length scales span over five orders of magnitude and time scales over more than ten. Because of this, the direct numerical simulation (DNS) of turbulent flames is only possible on large supercomputers. A DNS solver for chemically reacting flows implemented in the open-source framework OpenFOAM is presented. The thermo-chemical library Cantera is used to compute detailed transport coefficients based on kinetic gas theory. The multi-scale nature of time scales, which are much lower for the combustion chemistry than for the flow, are bridged by an operator splitting approach, which employs the open-source solver Sundials to integrate chemical reaction rates. Because the direct simulation of turbulent flames has to be performed on supercomputers, special care has been taken to improve the computational performance. A tool was developed which generates highly optimized C++ source code for the computation of chemical reaction rates. Additionally, a load balancing approach specifically made for the computation of chemical reaction rates is employed. In total, these optimizations can reduce total simulation times by up to 70 %. The accuracy of the new solver is assessed from different canonical testcases: 2D and 3D Taylor-Green vortex simulations show that the solver can reach up to fourth order convergence rates and that results differ by less than 1 % when compared to spectral DNS codes. Molecular diffusion and chemical reaction rates are compared to solutions of 1D flames from Cantera, showing perfect agreement. The solver is used to simulate the Sydney/Sandia burner. The simulation is performed on one of Germany\u27s largest supercomputer on 28 800 CPU cores, employing 150 million cells and a chemical reaction mechanism with 19 species and about 200 reactions. Comparison with experimental data shows excellent agreement for time averaged and RMS values

Determination of a correlation for predicting lean blow off limits of gaseous fueled, premixed turbulent jet flame arrays enclosed in a hexagonal dump combustor

Combustion of natural gas with air in gas turbines is a key technology for efficient provision of electric energy and heat. More stringent regulations regarding the emission of pollutants, such as NOx emissions, are necessitating research on technologies to reduce NOx formation during the combustion process. One technical approach onto the reduction of NOx-formation during combustion is fuel-lean premixed combustion. Current lean combustion concepts applied in stationary gas turbine combustors rely on flame stabilization through recirculation of hot flue gas using swirling flows. Swirl stabilized flames may be prone to combustion instabilities especially in lean premixed arrangements. Therefore, another approach is followed in the present study. In this concept, a matrix of turbulent lean premixed jet flames in a dump combustor is applied. The matrix burner consists of a nozzle with an array of circular channels in a hexagonal arrangement and a combustion chamber with a hexagonal cross section. In order to develop an appropriate burner design based on this concept, the experimental determination and theoretical evaluation of the lean blow out limit using different nozzles and operating conditions were conducted in this work in order to quantify the influence of different parameters on the flame stability. The varied geometric parameters are the diameter of the circular channels in the burner matrix as well as the ratio of the free cross section area of the nozzle to the cross section are of the combustion chamber, the combustor area dump ratio. The lean blow limit was determined at different preheating temperatures and flow velocities. The results show that the velocity at the LBO limit increases with increasing channel diameter, area combustor dump ratio and preheating temperature. The experimental results of three matrix burner are correlated in terms of a critical Damkoehler number and it is shown through experimental validation, that the Damkoehler number correlation derived is capable of predicting the LBO of a scaled matrix burner

Growth-related profiles of remanent flux in bulk melt-textured YBaCuO crystal magnetized by pulsed fields

We have studied the remanent magnetic flux distribution in bulk melt-textured YBa2Cu3O7 (YBCO) crystals after their magnetization in quasi-static and pulsed magnetic fields up to 6T. It has been shown that, provided that the magnetic pulse is sharp enough and its amplitude much exceeds the twice penetration magnetic field, the pulse magnetization technique becomes extremely sensitive to the sample inhomogeneities. Using this method with appropriate parameters of the magnetic pulse, we have particularly demonstrated that the growth of YBCO crystals in the growth sectors (GSs) responds for a macroscopic arrangement of weaks links -- they mostly appear inside of GSs, but not along the GS boundaries.Comment: 8 pages in LaTeX2e, 5 figures. Revised version, submitted to Supercond. Sci. Techno

Temperature dependency of the laminar burning velocity of fuel-rich methane oxygen measurements

First experiments to determine laminar burning velocities of methane-pure oxygen mixtures were carried out in 1932 by Jahn [1] for a wide range of equivalence ratios Φ (0.2 to 2.64) using a Bunsen burner. Since then, new and most important more accurate methods were developed to determine laminar burning velocities. One of these methods, namely the Heat Flux Method, which was introduced by de Goey et al. [2] in 1993, was used in the current work to validate the results for fuel-rich methane oxygen mixtures (Φ = 2.38 to 2.64) as published by Jahn. Regarding the current Heat Flux Bruner setup the range of velocities that can be determined are limited between 9 and 50 cm/s, which also limits the range of investigated equivalence ratios (Φ = 2.38 to 3.03), which is wider as the one investigated by Jahn [1]. Furthermore, the influence of the pre-heating temperature was also investigated by a variation of it from 263 up to 455 K. Based on these experimental data the temperature dependency of laminar burning velocities of fuel-rich methane oxygen mixtures was determined and as a result the coefficient α of the power law correlation SL = SL0 (T/T0)α was calculated. Due to the increase of the laminar burning velocity at higher pre-heating temperatures, the laminar burning velocities could also be determined at equivalence ratios up to a maximum value of Φ = 3.33 (TP = 455 K). The increase in accuracy of measurement methods to determine laminar burning velocities over the last decades [3] leads to an observed decrease in measured flame speeds. This tendency is confirmed in the current experiments, where the determined laminar burning velocities are lower than the ones measured by Jahn [1]. Regarding the temperature dependency of the laminar burning velocity, the results indicate that for the range of investigated equivalence ratios and temperatures (300 K to 455 K) the power law coefficient α was observed to be almost constant

Analysis of CH2_{2}O x OH as marker for fuel-rich air to pure oxy-fuel flames under higher preheat temperature and elevated pressure

The scope of the present work is a numerical and experimental investigation about the range of validity in terms of applicability of CH$_{2}$OxOH as a marker for the heat release rate (HRR) for fuel-rich air to pure oxy-fuel flames including preheating and elevated pressure. Therefore, laminar, freely propagating 1d CH$_{4}$ flames were calculated, where oxygen content in the oxidizer (from air to pure oxy-fuel combustion), inlet temperature and pressure were varied for a wide range of the equivalence ratios. The preheat temperature and pressure were parametrically changed from 300 K to 573 K and 1 bar to 5 bar, respectively. Different reaction mechanisms were used, namely GRI30, DLR, USC/II, Caltech2.3 and ABF. The performance of the CH$_{2}$OxOH as a marker for HRR is assessed in terms of correlation coefficients of their profiles in laminar flames. The comparison of the obtained correlations of CH$_{4}$/air and CH$_{4}$/O$_{2}$ flames shows that in case of air combustion, the HRR can be accurately estimated by the product of CH$_{2}$OxOH for slightly rich flames (Φ = 1.5), whereas the quality of the correlation degrades with increasing equivalence ratio. In contrary, the correlation coefficient increases with higher equivalence ratios in the fuel-rich domain for enhanced oxygen contents in the oxidizer. For pure oxyfuel combustion, the best correlation is found at an equivalence ratio of approximately Φ = 3.0. Elevated pressure leads in all flames to better correlations at lower equivalence ratios compared to standard inlet conditions, whereas preheating induces the opposite trend and expands the valid regime. A series of CH$_{4}$/air flames were also investigated experimentally in a range of the equivalence ratio between 1 < Φ < 2 at standard inlet conditions. The qualitative CH$_{2}$O (excitation at 355 nm) and OH (excitation at 283 nm) concentration were resolved applying two-dimensional LIF for flames stabilized at a McKenna burner. Comparisons show similar trends for measurements and numerical simulations