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.

The passive suppression of combustion instability by quarter wavelength tube was hereby studied to absorb the oscillation pressure with large amplitudes caused by combustion instability. The suppression effects of quarter wavelength tube on combustion instability were systematically analyzed by combining the acoustic numerical simulation and the experimental research methods. Firstly, the influence of quarter wavelength tube on the acoustic characteristics of the system was analyzed using acoustic numerical simulation, and then, the acoustic absorption characteristics to external acoustic disturbance and the suppression effects on the self-excited combustion instability were experimentally studied. The results show that the quarter wavelength tube can effectively absorb the acoustic pressure when the dominant frequency of acoustic pressure is close to the resonance frequency of the system, and can effectively suppress the combustion instability under acoustic resonance. However, given that the quarter wavelength tube adds adjoint dominant frequencies after eliminating the original resonant frequency of the system, and the combustion instability is stabilized on the adjoint dominant frequencies, combustion instability suppression is different from noise suppression. In addition, the diameter of wavelength tube exercises obvious effects on the above characteristics. All these make it necessary to determine the best parameters and the maximum suppression efficiency by combining numerical simulation and experiments. The research results of this paper provide theoretical and technical supports for the suppression of combustion instability by the quarter wavelength tube.

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.

When combustion instability occurs, fluctuation in the release of heat couples with oscillating pressure, while the sensitivity of flame to acoustic disturbance restricts the oscillation intensity. This paper investigates the efficacy of helium in suppressing combustion instability. The flame structure, its sensitivity to acoustic disturbance and the inhibition of oscillating pressure with the addition of helium were studied by means of open tests, external-excited and self-excited combustion instability experiments. First of all, the addition of helium made larger flame surface area, which shaped the distributed flame, and the heat was such released over a broader space. Then, the external-exited combustion instability experiments confirmed that adding helium to fuel could decrease the sensitivity of flame to acoustic disturbance. Finally, Helium was used in the case of self-excited combustion instability to further investigate its effectiveness on the oscillation suppression. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) methods were used to study flame fluctuation intensity. The results showed that the amplitudes of oscillating pressure were greatly reduced by the added helium. For 250Hz mode, adding helium with 20% of fuel flow could significantly reduce the flame pulsation and reduce the pulsation pressure by more than half. However, for the 160hz mode, more helium should be added to achieve better results. When the helium flow exceeded 80% of fuel flow, the combustion instability could be converted to stable combustion.

The low frequency instability in LRE is modeled and analyzed using the double time lag model in the present work. To this end, the interaction between the fluctuations of injectors representing feed system oscillations on the one hand, and fluctuations of chamber pressure representing combustion oscillations on the other hand have been considered. The dynamics of chamber pressure is modeled through the double time lag or evaporization and mixing time lag model. This way, the low frequency instability boundary in liquid-gas and liquid-liquid engines is analyzed and their corresponding frequencies are determined. It is shown that increasing the pressure drop of injectors and gas residence time can stabilize the engine. Decreasing the mixing time in liquid-gas engines has a stabilizing effect, but this is not true for liquid-liquid engines. In these engines, the mixing time has different effects on the stability boundaries depending on the evaporization time of fuel and oxidizer. It is also concluded that low frequency instability can be initiated at special domains of frequency. The results are in agreement with the available theoretical and experimental results.

Sound is one of the forms of mechanical energy and the two main characteristics of sound are intensity or power and frequency or wavelength of sound.their performance against the incoming sound wave, industrial silencers can be divided into two general groups of resonating and absorption silencers, the main difference between these silencers is the release of sound energy from the channeling system, which is one of the common examples of the use of resonating type silencers, their use in It is the internal combustion engines that distinguish absorption silencers from the resonator type based on the fact that the main and visible part of the act of muting the sound is achieved by changing sound energy to heat energy.The goal of this article is to design a muffler based on the breaking of sound frequencies resulting from the movement of fluid in the exhaust output of vehicles, which leads to a reduction of at least 50 db of sound and gives the operator enough peace and concentration. In this article, after examining three types of mufflers, absorbent mufflers that use the properties of porous absorbent material to absorb passing sound and are the simplest form of mufflers, have been selected, analyzed and reviewed and are suitable for the OM457 engine of Idem Industrial Company. It designed for maximum inlet exhaust temperature is 520 and for the maximum kW power is 315 with the maximum discharge relative pressure of 185 mbar for homogenization with the standard atmosphere.

Premixed combustion is widely used for simulation of combustion chambers of gas turbines, utilized for low NOx emission applications. However, this category of gas turbine is susceptible to combustion instability. The main aim of this investigation is to focus on thermo-acoustic instability modes in gas turbine combustion chambers. Both analytical and experimental methods are applied for this study. For this purpose, an experimental combustion chamber is designed and fabricated and various experiments are planned and performed in order to achieve the behavior of combustion chamber during stable and unstable conditions. 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 1.2. To distinguish the frequency of combustion instability, through various operating conditions, probability density functions, spectral diagrams and the discretized fast Fourier transform of pressure wave oscillations are employed. Moreover, an analytical method is applied and a computer code is written and elaborated for this purpose. Instability frequencies are derived experimentally and analytically and the results are compared with each other. Accordingly, good agreements are observed between the results.