This paper describes an investigation of combustion instability mechanism in premixed gas turbines. First, the preliminary concept of combustion instability is reviewed and then all types of frequency modes of combustion instability are studied. The results show that combustion instability is excited at first longitudinal mode; therefore the first mode or probably the second mode of longitudinal wave is the most important modes. Consequently, in this investigation only first longitudinal mode is considered. This simulation is based on equivalence ratio oscillations, which is an important mechanism for gas turbine combustion instability. The results are validated according to previons analytical and experimental results and show good agreements.

Lean premixed combustion is widely used in recent years as a method to achieve the environmental standards with regard to NOx emission. In spite of the advantages, premixed combustion systems with equivalence ratios less than one are susceptible to combustion instability. To study the lean combustion instability experimentally, one premixed combustion setup, equipped with reactant supplying system, is designed and manufactured. In this research, gaseous propane is introduced as fuel and experiments are performed at nearly atmospheric pressure with equivalence ratios within the range of 0.7 to 1.5 and fuel mass flow rates of 2 to 4 gr/s. It has been observed that the combustion chamber of the gas turbine becomes unstable when equivalence ratio is less than one. To distinguish the combustion instability, through various operating conditions, probability density functions, spectral diagrams, space distribution of pressure oscillations, along with Rayleigh criteria are employed. Accordingly, equivalence ratio effects in stabilizing the unstable combustion system are investigated experimentally. Moreover, convective delay time is calculated for all experiments and the results are compared with Rayleigh Criteria, showing relatively good agreements. Finally, stability limits are identified, based on inlet mass flow rate and equivalence ratio.

This paper reports a CFD modeling study to investigate the hydrogen-air mixture combustion in a micro scale chamber. Nine species with nineteen reversible reactions were considered in the premixed combustion model. The effect of operational and geometrical conditions including; combustor size, wall conductivity, reactant flow rates and hydrogen feed splitting on combustion stability and outlet gas temperature were investigated. The results show that the wall thermal conductivity has a significant effect on the combustion especially at smaller chamber size with high ratio of chamber surface area to its volume. In addition, the results reveal that high heat loss from chamber wall, small chamber and high input rate may cause flame quenching. Moreover, the modeling results indicate that a stable combustion in a micro combustor can be achieved at an optimum operational condition.

This research describes unsteady two-dimensional reacting flow around a circular cylinder. The numerical solution combines random vortex method for incompressible two dimensional viscous fluid flow with a Simple Line Interface Calculation (SLIC) algorithm for propagation of flame interface. To simplify the governing equations, two fundamental assumptions namely Low Mach Number and Thin Flame Thickness are used. Numerical and graphical representation of vorticity field, velocity variation on the wake axis, the effect of combustion on stream line pattern and location of vortex element at Reynolds numbers of 3000 and 9500 are discussed. The numerical results for the non-reacting flows fall within the range of the experimental measurements while the results of the reacting case are qualitatively following the physics.

A methane-air turbulent premixed flame is simulated via probability density function (PDF) and Reynolds averaged Navier-Stokes (RANS) methods. In the PDF approach, molecular mixing is modelled through the modified Curl’s model. A Monte Carlo method is used to solve the PDF transport equation. Also, the run time averaging and local time stepping procedures are incorporated to increase the accuracy and reduce the computational time of the PDF simulation. In the RANS approach, the averaged chemical reaction rate term is modeled by the eddy breakup-finite rate model. A finite difference discretization on a staggered grid is utilized to obtain the numerical solution for the RANS equations. The characteristics and differences of the two above mentioned methods, including computational time and predicted mean fields, are investigated in detail for premixed flames. It is observed that the discrepancy of the predicted mean fields between the two methods is large especially in regions near the flame. In addition, the predicted flame length by the PDF method is approximately half the flame length predicted by the RANS method.

The mechanism of air pollutant generation during combustion of gaseous fuels is simulated with ASPEN Plus process simulator release 9.1 -3 (1994). Based on the concept of minimization of total Gibbs free energy of the system, adiabatic flame temperatures for several stoichiometric fuel/air mixtures are calculated.Analysis of combustion products including air pollutants with concentrations as low as 1 ppb is carried out and compared with previous works. Sensitivity analysis to study the effect of changing reaction temperature and fuel/air ratio on pollutant concentrations are also carried out and the results are discussed.Detailed tables and various graphical resul ts are presented, demonstrating the powerful capabilities of ASPEN Plus in combustion modeling and air pollution studies.

In the present study, numerical modeling of the combustion of a fully laminar stoichiometric premixed reactive flow of methane-air inside a 2D micro-combustion chamber was investigated. The aim of this study is the investigation of occurance, phenomenology and effective parameters such as flame stability on combustion process in MEMS devices for energy or propulsion generation on small scales for space exploration applications. The results show that flame stability in a micro- combustion chamber strongly depends on the combustion chamber wall thickness Lw, the combustion chamber wall thermal conductivity coefficient Kw, the combustion chamber width L, the outer wall convective heat transfer coefficient hout and reactive mixture velocity Vin.

This study investigates the combustion of hydrogen-methane mixtures in the annular combustion chamber of a C30 microturbine. The primary objective is to evaluate the impact of premixed methane-hydrogen combustion on pollutant emissions and outlet temperature in an annular combustion chamber. Simulations were performed using a partially premixed combustion model and the k-ε turbulence model, employing the Probability Density Function (PDF) approach for chemical reaction modeling. To ensure a detailed analysis of pollutant emissions, comparisons were conducted at a constant turbine inlet temperature. The results indicate that adding hydrogen to methane increases NOx emissions due to the higher flame temperature compared to pure methane, even at constant turbine inlet temperatures. However, this blend can reduce fuel consumption by up to 35%. Additionally, a fuel mixture of 60% methane and 40% hydrogen results in a 61% reduction in CO2 emissions. The study further revealed that, owing to the premixed nature of the fuel-air mixture, the annular geometry, and the swirling flow pattern within the combustion chamber, a fuel blend containing 30% hydrogen can lower NOx emissions to 16.1 ppm—significantly less than the 46 ppm reported in previous studies. Moreover, increasing the hydrogen fraction in the fuel reduced CO emissions by 16%. These findings demonstrate that annular combustion chambers with premixed flows and hydrogen-methane fuel blends have considerable potential for reducing pollutant emissions and optimizing fuel consumption