نشریه سوخت و احتراق
سال نهم شماره 2 (پیاپی 18، پاییز و زمستان 1395)

Spar ignition in turbulent methane-air jet is studied by a compressible 3D Large Eddy Simulation in OpenFOAM code. The thicened flame model with the one-step chemical mechanism is used for methane combustion. The spar is modeled by artificial enthalpy source in energy equation at the spar location. The validation of the calculations is performed using the experimental results of the turbulent jet of airflow and non reacting mixing of methane jet. The ignition phenomenon and flame propagation are investigated in details for different conditions of initial jet temperature. The results show that the flame propagation speed, the flame lift-off and the flame distance to the axis change with changing the initial temperature.

In the present study, the flame response and the effect of equivalence ratio and inlet temperature on the flame response are numerically investigated using Weller flamelet combustion and LES turbulent models. The results show that with increasing excitation amplitude at a constant frequency, theheat release ratio increases; the increment is smaller at higher frequencies. Due to the combustor geometry, two recirculating zones are formed. Any change in the amplitude and frequency can affect these recirculation zones, especially the central recirculation zone. At the low frequencies (below 50Hz), increasing the excitation amplitude affects flame transfer functioninconsiderably, because of no influence of the recirculation zoneson the heat release. At higher frequencies, an increase in the amplitude has a more influence on value of flame transfer function. It is shown that by increasing the amplitude, up to frequency of 140 Hz, the phase of flame transfer function slightly reduced, while this is intensified with increasing the equivalence ratio or flame inlet temperature. Furthermore, by increasing the equivalence ratio or inlet temperature of the flame, heat release ratio and the flame transfer function are reduced.

In this work, numerical investigation of a double-swirled gas turbine model combustor (GTMC) was carried out using RANS approach with three different turbulence models of RNG k-, Realizable k- and RSM, and two different turbulence-chemistry interaction models of EDC (Eddy Dissipation Concept) and TPDF (Transported Probability Density Function). A detailed reduced mechanism of DRM22 (with 22 species and 104 reactions) was used to represent the chemical reactions. GTMC with a good optical access for laser measurements provided a useful database for swirling diffusion flames at atmospheric pressure. Comprehensive comparisons were done for the predictions and measurements of velocity, mixture fraction, temperature, and chemical species concentrations of , , , , , , and . Results showed an acceptable accuracy of predictions. This means that the simplified 2D-axisymmetric simulation has the ability to capture the important features and structure of combustion field in a double highly swirled chamber, like GTMC, with much lower CPU time in comparison with the costly 3D simulations. This study illustrated that using RSM turbulence model presents acceptable results for the flow field, while the other turbulence models were not capable of capturing quantitively acceptable results. In terms of comparison between the turbulencechemistry interaction models, TPDF led to a good prediction for major species and flame structure near the inlets, while the EDC predicted more accurately downstream of the flow field. Morever, the analysis of flame structure showed that mixing of fuel and oxidizer under double-swirl configuration happens fast and in high levels. In addition, using this type of mixing led to stabilization of main reaction zone in the center of combustion chamber near the injection plane. As a result, under double-swirl injection configuration clean and high quality combustion with reduced size of combustion chamber can be achieved simultaneously.

The purpose of the present paper is to investigate the influence of different chemical mechanisms on non-premixed combustion of Kerosene liquid fuel in a gas turbine model combustor. Numerical simulations of two-phase reacting flow in this combustor is performed by Realizable k-ε turbulence model, Steady Laminar flamelet combustion model, and Discrete Ordinates radiation model in a structured finite volume mesh. An Eulerian-Lagrangian approach is applied to model droplet spray of liquid fuel. The results in the present paper are obtained using three different chemical reaction mechanisms for Kerosene and the boundary conditions in the three cases are based on the experimental conditions. Validation of the results are performed by comparing the velocity and temperature distributions with the avalible experimental data. Accordingly, the results obtained by the first chemical reaction mechanism, which consists of 17 species and 26 reactions, are more accurate and closer to the experimental data. Comparisons of the mean velocity and temperature distributions with available experimental data are one of the outputs of the present paper. Also, the quantity of scalar dissipation rate, mixture fraction, mass fraction and the rate of formation of carbon dioxide, water vapor, Kerosene and nitric oxide are compared for the three different chemical reaction mechanisms. The results show that because of differences in the decomposition rate of C12H23 and consumption rate of O2, average mixture fraction in the chemical mechanisms are different. In addition, the scalar dissipation rate of the flame is affected by the turbulent flow in the three mechanisms. Further, the concentration of produced NO and CO2 in the flame region using the three mechanisms are different due to the flame temperature differences.

In this work, numerical investigation of a double-swirled gas turbine model combustor (GTMC) was carried out using RANS approach with three different turbulence models of RNG k-, Realizable k- and RSM, and two different turbulence-chemistry interaction models of EDC (Eddy Dissipation Concept) and TPDF (Transported Probability Density Function). A detailed reduced mechanism of DRM22 (with 22 species and 104 reactions) was used to represent the chemical reactions. GTMC with a good optical access for laser measurements provided a useful database for swirling diffusion flames at atmospheric pressure. Comprehensive comparisons were done for the predictions and measurements of velocity, mixture fraction, temperature, and chemical species concentrations of , , , , , , and . Results showed an acceptable accuracy of predictions. This means that the simplified 2D-axisymmetric simulation has the ability to capture the important features and structure of combustion field in a double highly swirled chamber, like GTMC, with much lower CPU time in comparison with the costly 3D simulations. This study illustrated that using RSM turbulence model presents acceptable results for the flow field, while the other turbulence models were not capable of capturing quantitively acceptable results. In terms of comparison between the turbulencechemistry interaction models, TPDF led to a good prediction for major species and flame structure near the inlets, while the EDC predicted more accurately downstream of the flow field. Morever, the analysis of flame structure showed that mixing of fuel and oxidizer under double-swirl configuration happens fast and in high levels. In addition, using this type of mixing led to stabilization of main reaction zone in the center of combustion chamber near the injection plane. As a result, under double-swirl injection configuration clean and high quality combustion with reduced size of combustion chamber can be achieved simultaneously.

Biodiesel fuels generally contain five types of fatty acid ethyl esters (Palmitate, Stearate, Oleate, Linoleate and Linolenate), which affect the thermo-physical properties of biodiesel fuels. The effects of each of the mono ester of fatty acids in biodiesel on the thermo-physical properties (density, viscosity, heating value and cetane number) were examined. Biodiesels were produced by transesterification method using ethanol and sodium hydroxide catalyst and seven types of vegetable oils (sunflower, soybean, canola, olive, corn and grapeseed). Thermo-physical properties of each sample were measured according to ASTM standards. For each of the properties, a nonlinear regression model based on five independent variables of the fatty acids ethyl esters (FAEE) were presented. The correlation coefficient regression models for density, viscosity, heating value and cetane number were obtained equal to 0.9457, 0.9169, 0.9731 and 0.9029, respectively. The results showed that of the ethyl stearate (C18=0) impact factor on the density is equal to 0.803, which is lower than the standard (0.86), and Methyl Linolenat (C18=3) impact factor on the density is 0.913, which is more than the standard (9.0). Ethyl Stearate(C18=0) impact factor on the viscosity is equal to 8.41, which is higher than the standard (6). Also, Ethyl stearate has the greatest impact on heating value of biodiesel, and Ethyl Palmitate (C16=0) and Ethyl Linolenate (C18=3) have the smalles impact. Impact factor of Methyl Linolenate (C18=3) on the cetane number of biodiesel is 19.652, which is less than the standard (47). So, by increasing the amount of saturated fatty acids (especially ethyl stearate), viscosity, heating value and cetane number of biodiesel fuel increase, but the density decreases. If the heating value and cetane number of fuel increase, the engine power is increased, but if the viscosity increases and density dicreases, the atomization of the fuel is incomplete. Therefore, the production of biodiesel from vegetable oils with high saturated fatty acids increases the engine power.