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An experimental investigation of the stability limits of biogas in a swirling non-premixed burner has been carried out. A mixture of 60% methane and 40% carbon dioxide was used to reach the typical biogas composition. Vane swirlers with 30°, 45°, and 60° angles were used to make the swirling air. The biogas stability limits and flame behavior under swirling conditions were tested. Besides, effects of air dilution with nitrogen and carbon dioxide on biogas stability limits were investigated. The results show that using swirl can enhance the flame stability limits approximately four or five times comparing to non-swirling air stream. Adding N2/CO2 to the air had negative effects on the flame stability but no changes were observed in the flame structure. The maximum air dilution was also obtained when 27% and 15% N2 was added to the swirling air under strong and weak swirl, respectively. [ABSTRACT FROM AUTHOR]

In this paper, turbulent non-premixed CH4 +H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in the moderate or intense low-oxygen dilution regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flame regimes of stable asymmetric and RERI have been observed in meso scale reactor. [•] By increasing the inlet velocity, the equivalence ratio is decreased to near unity. [•] The increase of velocity more than a level reduces the flame presence in the reactor. [•] Increasing the dilution percent restricts the asymmetric stable flame region. [•] In such a meso scale reactor just asymmetric flame as a stable flame can be formed. [ABSTRACT FROM AUTHOR]

Abstract: Reduction of air pollutants formation from hydrocarbon combustion process requires improvements in combustion systems. The moderate and intense low oxygen dilution (MILD) combustion technique is an opportunity to achieve such a goal. MILD combustion is a combustion regime which can be attained by high temperature preheating and high level dilution. In this paper, the mechanism of CO and CO2 formation for a CH4/H2 fuel mixture is studied under MILD combustion condition of a jet in hot coflow (JHC) burner. This investigation is done using the computational fluid dynamics (CFD) and also zero dimensional well-stirred reactor (WSR) analysis. The RANS equations with modified k–ε equations are solved in an axisymmetric 2D computational domain. The DRM-22 reduced mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration and fuel hydrogen content are studied on methane oxidation pathways. Results show that the higher hydrocarbon oxidation pathways are effective on CO and CO2 formation under MILD condition. In the methane oxidation mechanism, the ratio between the main route and ethane route is the main reason of CO increment at higher O2 level under MILD condition in JHC laboratory burner. The WSR analysis illustrates that a decrease of O2 concentration in oxidizer does not necessarily lead to lower production of CO and CO2. [Copyright &y& Elsevier]

Abstract: Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H2/CH4 flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH2O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH2O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front. [Copyright &y& Elsevier]

The stability behavior of a jet diffusion flame developing in a co-flowing stream is studied experimentally, using natural gas and propane as fuel gases. Effects of oxidant and fuel stream velocities and oxidant stream dilution have been studied. The results of experiments showed that with increasing fuel jet Reynolds number, there appears along the flame a point that is accompanied by reaction zone sudden expansion. Flame becomes turbulent downstream from this point. This point is called transition point. More increment of fuel jet Reynolds number moves the transition point to the upstream. Furthermore, two types of stability limits are observed. Blow-off of the rim-stabilized flame is the first stability limit. The second one is the break-off or extinction of the turbulent portion of the flame at the transition point from laminar to turbulent flow. The oxidant and fuel streams are in environmental temperature. In dilution experiments, the oxidant primary stream is oxygen that is diluted with nitrogen or carbon dioxide. In the other experiments oxidant is environmental air. [ABSTRACT FROM AUTHOR]

In this paper, the importance of fluctuations in flow field parameters is studied under MILD combustion conditions. In this way, a turbulent non-premixed CH4+H2 jet flame issuing into a hot and deficient co-flow air is modeled using the RANS Axisymmetric equations. The modeling is carried out using the EDC model to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. Results illustrate that although the fluctuations in temperature field are small and the reaction zone volume are large in the MILD regime, the fluctuations in temperature and species concentrations are still effective on the flow field. Also, inappropriate dealing with the turbulence effect on chemistry leads to errors in prediction of temperature up to 15% in the present flame. By decreasing of O2 concentration of hot co-flow air, the effect of fluctuations in flow field parameters on flame characteristics are still significant and its effect on species reaction rates does not decrease. On the other hand, although decreasing of jet inlet Reynolds number at constant inlet turbulence intensity addresses to smaller fluctuations in flow filed, it does not lead to lower the effect of turbulence on species distribution and temperature field under MILD combustion conditions. [ABSTRACT FROM PUBLISHER]

Abstract: Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H2/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H2+95%CH4, 10%H2+90%CH4 and 20% H2+80%CH4(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release. [Copyright &y& Elsevier]

In this paper, the importance of molecular diffusion versus turbulent transport in the moderate or intense low-oxygen dilution (Mild) combustion mode has been numerically studied. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147-1154] were used for modelling. The EDC model was used to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of an H2/methane jet flame. The importance of molecular diffusion for various O2 levels, jet Reynolds numbers and H2 fuel contents was investigated. Results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number, decreasing the oxygen concentration in the airflow or increasing H2 in the fuel mixture, the influence of molecular diffusion on Mild combustion increases. [ABSTRACT FROM AUTHOR]