FUEL AND COMBUSTION
Year:2016 | Volume:8 | Issue:2|Papers Count:7

In this study, the combustion of a fuel jet with supersonic air flow has been studied numerically. The injected gas was a preburned mixture of hydrogen and air with equivalence ratio of 4.5. The flow field was investigated with and without incident oblique shock wave. The simulations have been performed using open Foam software in a three-dimensional field. A two-equation turbulence model, SST-kw, and the PaSR combustion model were used. The aim of this study was to examine the impact of the shock wave on flame stabilization. According to the results, the reaction occurs at low rates without the oblique shock. However, when the incident shock hits the bottom surface downstream of the injection slot, reaction rate increases, indicating the flame stabilization in this case. These findings are in good agreement with experimental results.

The objective of the present work is to investigate the influence of varying the percentage of distributed air flow rate via swirler, primary jets and dilution jets on reactive flow characteristics and NOX and CO emissions in a gas turbine model combustor. A Finite Volume staggered grid approach is employed to solve the governing equations that are linearized implicitly and also discretized by a second order method. The central difference discretization and second-order upwind schemes are applied respectively for the space derivatives of the diffusion and the advection terms in all transport equations. In the numerical simulation of reactive two-phase flow of this combustion chamber, the realizable k-e turbulence model, steady flamelet combustion model and discrete ordinates radiation model have been used. The spray and atomization of liquid fuel droplet is modeled by an Eulerian–Lagrangian method. The present study is performed for four different cases of air injection and in the first case, boundary conditions are based on the laboratory conditions. After validation of the numerical results for the first case using experimental data, the subsequent cases are studied. Among the outcomes of the present work, the followings can be mentioned: comparison of velocity and temperature distributions, mass fraction percentage of the carbon dioxide, carbon monoxide and nitrogen oxide concentration at exit plane, as well as the formation method of flow structure in longitudinal cross-section of combustion chamber for the above-mentioned four reactive flow cases. Results show that the air distribution of the first case in laboratory conditions is not optimal and in the case where the air flow rate is reduced from the beginning to the end of the combustion chamber, the mass fraction of carbon monoxide and nitrogen oxide are minimum and the amount of carbon dioxide is maximum.

In the present study, the effects of inlet velocity on characteristics of Repetitive Extinction-Ignition Dynamics are numerically investigated. Hydrogen-air mixture (with equivalence ratio=0.5) enters into a heated micro channel with a prescribed wall temperature. Low Mach number approach is considered for governing equations in numerical simulation and also detailed chemistry, and different mass diffusivity of species is utilized. The dynamic behavior is studied by two parameter, amplitude and frequency. The results show that the amplitude of repetitive extinction-ignition dynamics increases with increasing the inlet velocity, while the frequency has a descending-ascending behavior. For detailed study of this phenomenon the chemical reaction approach is used by considering the reaction rate parameter. The results illustrate that for high inlet velocitis the reactions tend to produce light species such as O, H and OH. The effects of inlet velocity on flame propagation velocity are also studied. For lower inlet velocities, flame stays longer in an extinction-ignition period in the channel, while increasing the inlet velocity causes the flame to extinguish faster.

In this paper, cavitation flow inside the injector nozzle has been numerically simulated using ANSYS Fluent v15. The validations were performed with experimental results of Winkhofler et al. (2001). The calculated results from the three dimensional numerical simulation of cavitating flow in the nozzle with mixture multi-phase cavitating flow model have good agreement with the experimental data. Several important parameters such as mass flow rate and velocity profiles were used for the validations. The cavitation model used in the simulations is Schnerr and Sauer cavitation model. Due to high Reynolds numbers, turbulence effects have been taken into account by RANS methods using RNG k-e model. PRESTO discretization method is used for pressure equation and second upwind discretization method is used for momentum equation. The discharge coefficient is computed for several cavitation numbers and needle lift. The results show that the needle lift and seat shape of the nozzle have a strong influence on the volume fraction of diesel vapor and the discharge coefficient in the nozzle.

Reduction of environmental pollutants caused by combustion in power plant systems is one of the main challenges for the researchers. To pinpoint the mechanisms of formation and transport of combustion pollutants, it is necessary to have an accurate prediction of temperature field and combustion products. For this reason, simulation of turbulent combustion flows has attracted much attention in recent years. An appropriate combustion model is required for simulation of these flows. Flamelet model is the most favorite combustion model, due to inherent separation of the turbulent flow field and the chemical reactions. Moreover, the consideration of unsteady flamelet in modeling complex physical phenomena such as radiation heat transfer and slow chemical processes (of pollutants) leads to better results than the steady flamelet assumption. The purpose of this study is to investigate the application of steady and unsteady flamelet models in the simulation of turbulent diffusion bluff body flame. Predictions of temprature and mean mixture fraction using steady flamelet model have shown very good agreement with experiment data. NO mass fraction in steady-state simulations using two different chemical mechanisms GRI3.0 and GRI2.11 is over predicted. While NO mass fraction in the unsteady flamelet modeling using mechanism GRI2.11 have shown good agreement with the experimental data. Thus, unsteady effects are important in slow processes such as the formation of NO.

Low exhaust emissions and fuel consumption, made diesel engines suitable for passenger car applications today and various concepts have been presented for mixture formation improvement that has very important influence on the combustion efficiency and fuel consumption. One of these concepts which is used recently is micro-hole group nozzle. Using of injector with micro hole nozzles, the atomization of fuel spray is improved and this leads to have better air fuel mixture and lower engine emissions. In the present research, effect of using this kind of injector nozzle holes has been studied numerically using AVL fire Software. For numerical simulation, spray break up and atomization models and problem boundary conditions have been validated using experimental results of similar works, then the effect of micro hole configuration and angle between holes on spray penetration, droplet size and vaporized mass were investigated for two configuration of a (one plane type) and b (two planes type). Results showed that for micro hole nozzle, increasing the angle between two holes would increase spray penetration and evaporated mass and decrease droplets Sauter mean diameter.

In this paper, performance of a gas turbine cycle equipped with a solid oxide fuel cell and Stirling engine is discussed from thermodynamic viewpoint. A thermodynamic analysis is performed for all components of the cycle, and a separate electrochemical and thermal analysis is conducted for the utilized fuel cells. With parametric study of the hybrid system, the influences of compressor pressure ratio, turbine inlet gas temperature, the number of cells, the type of fluid used in the Stirling engine, and the angular velocity of the Stirling engine on efficiency and power of the hybrid system are investigated. Based on the presented comparison, generated power of the proposed system is about three times larger than the simple gas turbine cycle and 1.3 times larger than the power produced by a hybrid gas turbine and fuel cell system. The obtained results show that the electrical efficiency of the proposed system is about 82 percent, while the fuel cell and gas turbine hybrid system and simple gas turbine cycle efficiencies are about 50 percent and 30 percent, respectively.