نشریه سوخت و احتراق
سال سیزدهم شماره 4 (پیاپی 33، زمستان 1399)

In this investigation, the combustion process in DLR scramjet is simulated and studied by considering multi-stage reaction mechanism. This study is conducted both three-dimensionally and two-dimensionally with the help of Fluent. The results indicate that the general performance of the scramjet is nearly the same for both the two-dimensional and three-dimensional cases, that is to say the difference between calculated thrust and combustion efficiency in these two cases is only 0.9% and 1.4%, respectively. To consider the effect of the chemical mechanism on the obtained results, a two-dimensional simulation of the scramjet is done with a detailed chemical mechanism, which includes 19 reactions. The results illustrate that the general performance of the scramjet is nearly the same. More accurately, the difference between thrust and combustion efficiency in the one-reaction and the multi-reaction cases is only 1.4% and 4.4%, respectively. In contrast, the subsonic region behind the strut in three-dimensional case and multi-stage reaction mechanism case are 48.9% and 17.7% longer than the base case, respectively.

Two-phase flow field upstream of the discharge orifice of effervescent atomizer has been numerically studied at 0.38 L/min liquid flow rate and different gas to liquid mass ratios of 0.32%, 0.62%, and 1.24%. The internal flow of the atomizer has been three-dimensionally simulated using the volume of fluid method. Aeration gas is considered as an ideal gas at all aeration levels due to the high velocity of it at the exits of the aerating tube. At any aeration level, the simulation results have been represented by the contours of nitrogen volume fraction at different times and the contours of velocity. At all considered aeration levels, according to experimental results, the contours of nitrogen volume fraction show the annular flow regime is dominant in the discharge passage of the simulated atomizer. Also, by increasing the aeration level, both the liquid film thickness in the discharge passage and changes of it along the passage will be decreased. Therefore, it can be expected the spray of the effervescent atomizer at high aeration levels be finer due to a reduction of the liquid film thickness in the discharge passage and more uniform due to fewer changes in the liquid film thickness in this passage.

Reducing fuel consumption and emissions, as well as increasing the power and efficiency, have always been important in IC engines researches. Due to the positive effects of adding water to the inlet fuel-air mixture through the manifold of a low-temperature engine, the effect of the temperature of water added to the low-reactivity fuel in an RCCI engine with gasoline-diesel dual-fuel has been investigated. The water temperature varies between [20-60]oC. Water is substituted for gasoline with different mass-ratios; so that the reduction of fuel will be equal to the amount of water added. For numerical simulation, the AVL-Fire was used as a couplet with the detailed chemical kinetics code. For validation, numerical results are compared with similar experimental data. The results show that by increasing the mass-ratio of the replaced water up to 10%, the in-cylinder pressure and temperature as well as the NOx increase significantly; but with an increase in mass-ratio to 15%, a decreasing trend is seen. With the increase in the mass-ratio to 20%, it will again lead to an increasing trend; however, at this mass-ratio, the in-cylinder temperature, and the NOx have not increased significantly, despite the increase in power characteristics. Then, considering the mass-ratio of 20%, the evaluation of water temperature increasing shows an improving trend; so that the maximum average in-cylinder pressure is increased and leads to a slight increase in the IMEP. Also, due to the increase in combustion quality, CO is significantly reduced, as well as the ISFC is reduced up to 2%.

Combustion of hypergolic propellants increases the specific impulse in the thrusters due to high temperature products. In this paper, the combustion process will be investigated through the axis of a bi-propellant thruster by an in-house code with chemical reaction mechanism. This code includes several models for injection, droplet evaporation, liquid film, combustion and heat transfer through liquid and gas films. The Astrium thruster with MMH as fuel and NTO as oxidizer has been simulated. By implementing a detail mechanism with 1619 steps, the thruster has been simulated at different total mass flow rates and results have been validated by experimental data. Then, injector dimension effects on the droplet evaporation and combustion have been investigated. Results show that by increasing the injector dimension, the droplet evaporation length increases, so the flame structure changes in the combustion chamber. Therefore, the combustion products enter the nozzle with higher temperature and as a result, the thruster specific impulse increases.

Combustion of a mixture of inhomogeneity reactants is a type of combustion in which the mixing of fuel and air is not complete. This study investigated the numerical effect of the pilot location on the turbulent jet flame lift off height with different levels of inhomogeneity in the combustion chamber. The effect of different equevalence ratios has also been investigated. In this numerical study, the configuration modeling of Navier Stokes equations using Reynolds averaging method, standard k-ε turbulence and reactive flow modeling of Eddy Dissipation Concept (EDC) method have been used. Observations show that the flame lift off height decreases with increasing inhomogeneity length and the flame attach during a certain inhomogeneity length, which depends on the equevalence ratio and pilot location. In all of the cases studied, a range of inhomogeneity lengths was found during which the flame was attached. The results show that the flame lift off height in nonpremixed mode is higher than in the case where the fuel-air mixture ignites almost premixed in the chamber. The higher the equevalence ratio, the lower the flame lift off height. Based on the three locations studied in the side wall for the pilot location and the results obtained, the pilot in the middle area of the side wall had a lower flame lift off height and a higher attached flame range. Also, the flame area inside the chamber changes as the inhomogeneity changes.

Optimizing and reducing the fuel consumption and thus preserving national economic resources is one of the important goals in the economy of any country. Using the natural gas instead of liquefied petroleum fuels and reducing energy consumption in industries such as cement can be a practical solution to meet the above-mentioned goals. In this study, numerical simulations are performed to enhance combustion and calcination processes in the Hormozgan cement plant calciner. To such aim, various optimizations are carried out to replace highly pollutive expensive, mazot fuel with natural gas and to improve calcination and combustion processes. Obtained results show that proposed modifications give the calciner the capability to be operated by 100% natural gas, while they reduce the energy consumption and pollutant emissions significantly. Finally, the best proposed case is implemented in the cement calciner to replace mazot by natural gas and reduce energy consumption and pollutant emissions. Site data clearly validate the proposed modifications to be effective enough to achieve all the objectives of the project.

Environmental issues and air pollution have attracted a lot of attention in recent years. Thermal power plants are among the air pollutants. One of the solutions used for better combustion and less pollution in other combustion systems, including gas turbines, is the idea of ​​water vapor injection. In this study, it was considered that with the idea of ​​adding water vapor to the gas turbine, a similar work would be done in the boilers, for which in this research, a 11 Megawatts single axisymmetric burner in the form of two dimension is modeled using the open source software OpenFoam and the ReactingFoam solver. For simulation, the equations for mass, momentum, energy, and species conservation were solved together as a coupling, and the pressure corrections in the momentum equations was performed according to the PIMPLE algorithm. The model used for the turbulence is the  model, the radiation model fvDOM, the combustion model used PaSR model and the reaction mechanism GRI-3, which includes 53 species and 325 reactions. To validate the solver and the models used, a standard problem was used which is a simple burner and its experimental results are available. After proving the correctness of the solution, the mentioned problem was simulated. It was observed that in the dry air combustion chamber, the temperature and, consequently, the NOx output are high. The flame temperature decreased after the steam injection in three proportions by 5, 10, and 15% of the total inlet air, so that the temperature in the axisymmetric boiler decreased by 370 Kelvin. Also, in the axisymmetric boiler, the amount of NO output decreased 81 % and the amount of output decreased 76%. However, carbon pollutants, specifically for CO which was important in this studyincreased by 58% to 17 ppm but was still much lower than national and European standards.