Search Results (8)

In this study, the generality and prediction accuracy of a generalised series model for the large eddy simulation of premixed and non-premixed turbulent combustion is explored. The model is based on the Taylor series expansion of the chemical source term in scalar space and implemented into OpenFOAM. The mathematical model does not depend on combustion regimes and has the correct limiting behaviour. The numerical error sources are also outlined and analysed. The model is first applied to a piloted methane/air non-premixed jet flame (Sandia Flame D). The statistical (time-averaged and RMS) results agree well with the experimental measurements, particularly with regard to the mixture fraction, velocity, temperature, and concentrations of major species CH₄, CO₂, H₂O, and O₂. However, the concentrations of the intermediates CO and H₂ are over-predicted, due to the limitations of the reduced reaction mechanism employed. Then, a Bunsen-piloted flame is simulated. Most of the statistical properties of both the reactive species and progress variables are well reproduced. The only major discrepancy evident is in the temperature, which is probably attributed to the experimental uncertainties of temperature fields in the pilot stream. These findings demonstrate the model’s generality for both a premixed and non-premixed combustion simulation, as well as the accuracy of prediction of reactive species distribution. Full article

This paper describes a new, simple, and highly selective analytical technique for the detection of sulfur dioxide in wine, as a real sample with a relatively complicated matrix. The detection of the above analyte was based on the electrogeneration of iodine from iodide on a boron-doped diamond electrode, without modifications, in the presence of 0.1 mol dm⁻³ HClO₄ as a supporting electrolyte. The electrogenerated iodine reacted with sulfur dioxide, forming iodide ions and sulfuric acid (i.e., a Bunsen reaction). The product of this reaction, the iodide ion, diffused back to the surface of the boron-doped diamond electrode and oxidized itself again. This chemical redox cycling enhanced the voltammetric response of the boron-doped diamond electrode. The selectivity of the determination was assured using NaOH and formaldehyde during sample preparation, and a blank was also measured and taken into account. The detection limit was estimated to be 10⁻⁶–10⁻⁷ mol dm⁻³. However, the content of sulfur dioxide in wine is significantly higher, which can lead to more accurate and reliable results. Full article

The predictions of turbulent burning velocity parameterizations for non-unity Lewis number flames have been assessed based on a single-step chemistry Direct Numerical Simulation (DNS) database of premixed Bunsen flames for different values of characteristic Lewis numbers ranging from 0.34 to 1.2. It has been found that the definition of the turbulent burning velocity is strongly dependent on the choice of projected flame brush area in the Bunsen burner configuration. The highest values of normalized turbulent burning velocity are obtained when the projected flame brush area is evaluated using the area of the isosurface of the Reynolds averaged reaction progress variable of 0.1 out of different options, namely the Favre averaged and Reynolds averaged isosurfaces of reaction progress variable of 0.5 and integral of the gradient of Favre and Reynolds averaged reaction progress variable. Because of the axisymmetric nature of the mean flame brush, the normalized turbulent burning velocity has been found to decrease as the burned gas side is approached, due to an increase in flame brush area with increasing radius. Most models for turbulent burning velocity provide comparable, reasonably accurate predictions for the unity Lewis number case when the projected flame brush area is evaluated using the isosurface of the Reynolds averaged reaction progress variable of 0.1. However, most of these parameterizations underpredict turbulent burning velocity values for Lewis numbers smaller than unity. A scaling relation has been utilized to extend these parameterizations for non-unity Lewis numbers. These revised parameterizations have been shown to be more successful than the original model expressions. These modified expressions also exhibit small values of L₂-norm of the relative error with respect to experimental data from literature for different Lewis numbers, higher turbulence intensity and thermodynamic pressure levels. Full article

A laminar diffusion flame was measured by X-ray Compton scattering. The temperature distribution was measured from an analysis of Compton scattered X-ray intensity. The chemical state distribution was obtained from a Compton scattered X-ray spectrum analysis (s-parameter analysis). The analysis of intensity and s-parameter of Compton scattered X-ray spectra indicate that the propane molecule emitted from the cylindrical Bunsen burner collapse immediately coincides with soot generation. Furthermore, the temperature increases up to 1500 K and a large amount of CO₂ was generated at the combustion field. Our results show that the Compton scattered X-ray analysis can be a novel nondestructive measurement for temperature and chemical states in a combustion reaction. Full article

In this article, the application of the FRAME (Frequency Recognition Algorithm for Multiple Exposures) technique is presented for multi-species measurements in symmetric and asymmetric ethylene/air diffusion flames. Laminar Bunsen-type and swirled diffusion flames are investigated to gain a better understanding of sooting combustion. For this purpose, simultaneous imaging is conducted in terms of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) and Laser-Induced Incandescence (LII) of soot particles. Subsequently, the approach is utilized for simultaneous imaging of hydroxyl (OH)-LIF and soot-LII. Here, the modulated LIF- and LII-signals are acquired together as a single sub-image—with a single exposure utilizing the full sensor size of a single camera. By employing the frequency-recognition algorithm on the single image, the LIF- and LII-signals are spectrally isolated—generating two individual LIF- and LII-images. The flame luminosity and out-of-focus light such as reflected surrounding laser light are detected as non-modulated signals in the unprocessed image. These unwanted signals are suppressed using the image post-processing, and, therefore, the image contrast of the two resulting images is improved. It is found that PAHs mainly exist in the inner region near the burner and are surrounded by soot. The majority of the OH is distributed on the outer edges of the flame—representing the reaction zone and soot-oxidation region of the flame. Full article

The efficiency of HI concentration/separation from a HI_x solution, (mixture of HI/H₂O/I₂) represents a crucial factor in the sulfur-iodine thermochemical water splitting process for hydrogen production. In this paper, an experimental study on HI cathodic concentration in HI_x solution using stacked electro-electrodialysis (EED) cells was carried out under the conditions of 1 atm and at three different temperature (25, 55 and 85 °C) and using a current density of 0.10 A/cm². Results showed that an increase in HI concentration can be obtained under certain conditions. The apparent transport number (t₊) in all the experiments was very close to 1, and the electro-osmosis coefficient (β) changed in a range of 1.08–1.16. The tests showed a detectable, though slow, increase in both the anodic iodine and cathodic hydriodic acid concentrations. Full article

Based on directed relation graph with error propagation methods, 39 species and 231 reactions skeletal mechanism were obtained from Mech_56.54 (113 species and 710 reactions) mechanism of methane. The ignition delay times, laminar flame propagation speed, and important species were calculated using the simplified mechanism at different pressures and equivalence ratios. The simulation results were in good agreement with that of detailed mechanisms and experimental data. The numerical simulation of the Bunsen burner jet flame was carried out using the simplified methane mechanism, and the simulation results well reproduced the temperature, flow fields and distribution of important species at flame zone. The compact methane reduced mechanism can not only correctly respond to its dynamic characteristics, but also can be well used for numerical simulation, which is of great significance in engineering applications. Full article

Reduction of nitric oxides (NOx) in aircraft engines and in gas turbines by lean combustion is of great interest in the design of novel combustion systems. However, the stabilization of the flame under lean conditions is a main issue. In this context, the present work investigates the effects of sinusoidal dielectric barrier discharge (DBD) on a lean inverse diffusive methane/air flame in a Bunsen-type burner under different actuation conditions. The flame appearance was investigated with fixed methane loading (mass flux), but with varying inner airflow rate. High-speed flame imaging was done by using an intensified (charge-coupled device) CCD camera equipped with different optical filters in order to selectively record signals from the chemiluminescent species OH*, CH*, or CO2* to evaluate the flame behavior in presence of plasma actuation. The electrical power consumption was less than 33 W. It was evident that the plasma flame enhancement was significantly influenced by the plasma discharges, particularly at high inner airflow rates. The flame structure changes drastically when the dissipated plasma power increases. The flame area decreases due to the enhancement of mixing and chemical reactions that lead to a more anchored flame on the quartz exit with a reduction of the flame length. Full article