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Here we propose a generalized procedure for a two-parameter assessment of the Combustion stability (CS) of industrial gas turbines. In evaluating the CS, this procedure employs two parameters of measured dynamic pressure data: the Root-mean-square (RMS) pressure as the primary parameter and the damping ratio as a secondary parameter. The former tells the time-averaged level of the dynamic pressure, and, the latter, the degree of acoustic energy loss. A data point pairing the two parameters, which are evaluated at a specific instance of a combustion process, identifies Instantaneous combustion stability (ICS) by its location on a 2-D domain of both parameters. Collective representation of the ICS points on the domain produces a CS map of the combustion process. The locus of the ICS point on the map represents the temporal variation of CS during the combustion process. The biparametric assessment procedure divides the CS map into three regimes (i.e., stable, transitional and unstable) by utilizing two threshold values for the RMS pressure and one for the damping ratio. The feasibility of the proposed procedure was tested with the dynamic pressure data from a model gas turbine combustor burning synthetic natural gas. Then the technique was applied experimental data obtained from a laboratory-scale lean premixed combustor to identify the three regimes of the combustion process of a reported case. We found that the procedure is able to provide gas turbine operators with valuable information on CS during a combustion process, especially on the transitional regime.

Combustion instabilities can excite various thermoacoustic modes. Once the mode is identified, it becomes possible to estimate the overall pressure field or its maximum magnitude. However, determining the specific mode of being excited remains a challenging problem. Methods that estimate the thermoacoustic modes solely based on experimental measurements are essential. This paper focuses on azimuthal modes in annular combustors and proposes a method for estimating the mode order by minimizing residuals using the least squares approach with fewer sensors. The effectiveness and accuracy of this method are verified through simulations under various disturbances. When the estimated mode order aligns with the actual dominant mode order, the residual is minimized. Even under disturbances such as random noise and non-dominant modes, the dominant mode order can still be accurately estimated, with the residual indicating the dominance of the primary mode over the disturbance terms.

This paper addresses the issue of high-frequency combustion instability in liquid rocket engines and investigates the suppression of unstable combustion using Helmholtz acoustic cavities. A gaseous oxygen-methane model rocket engine thrust chamber equipped with six Helmholtz acoustic cavities was designed. Based on this configuration, hot-firing tests were conducted on the model engine thrust chamber to examine the effects of Helmholtz acoustic cavities on reducing combustion instability amplitudes. Additionally, by varying the cavity lengths, the study explored how different cavity dimensions influenced the suppression of combustion instabilities.

Combustion instabilities in annular systems raise fundamental issues that are also of practical importance to aircraft engines and ground-based gas turbine combustors. Recent studies indicate that the injector plays a significant role in the stability of combustors by defining the flame dynamical response and setting the inlet impedance of the system. The present investigation examines the effects of combinations of injectors of two different types ($U$ and $S$) on thermoacoustic instabilities in a laboratory-scale annular combustor and compares different circumferential staging strategies. The combustor operates in a stable fashion when all injection units belong to the $S$-family, but exhibits large amplitude pressure oscillations when all these units are of the $U$-type. When the system comprises a mix of $U$- and $S$-injectors, it is possible to determine the number of $S$-injectors leading to stable operation. For a fixed proportion of $U$- and $S$-injectors, some arrangements give rise to stable operation while others do not. Results also show that introducing symmetry-breaking elements affects the system's modal dynamics. These experimental observations are interpreted in an acoustic energy balance framework used to derive an expression for the growth rate as a function of the describing functions of the flames formed by the different injectors and their respective azimuthal locations. Growth rates are determined for the different configurations and used to explain the various observations, estimate the system damping rate and predict the location of the nodal line when the standing mode prevails.

A detailed study on the plasma-assisted combustion (PAC) characteristics of premixed propane/air mixture is presented. The PAC is measured electrically, as well as optically with a multichannel spectrometer. The characteristics are demonstrated by stable combustion temperature and combustion stability limits, and the results are compared with conventional combustion (CC). Stable combustion temperature measurements show that the introduction of PAC into combustion system can increase the stable combustion temperature, and the increment is more notable with an increase of discharge voltage. Besides, the rich and weak limits of combustion stability are both enlarged when plasma is applied into the combustion process and the increase of discharge voltage results in the expansion of combustion stability limits as well. The measurements of temperature head and emission spectrum illustrate that the kinetic enhancement caused by reactive species in plasma is the main enhancement pathway for current combustion system.

To reduce the carbon emissions and improve the combustion stability of pulverized coal furnaces under low-load conditions, this paper carries out a numerical investigation on hydrogen-blended pulverized coal combustion. The combustion characteristics with different hydrogen co-firing ratios are analyzed. The results indicate that the high-temperature zone near the burner outlet expands significantly as the hydrogen co-firing ratio increases, providing a stable high-temperature environment for the ignition of pulverized coal. At a hydrogen co-firing ratio of 5%, the combustion system exhibits a relatively high burnout rate.

Micro-combustion stability is critically influenced by wall material properties. This study investigates the impact of wall density (ρw) and specific heat capacity (cw) on the ignition and combustion characteristics of a propane/air mixture within a micro-channel via transient numerical simulation. The microreactor, with an initial temperature of 300 K, is fed with a preheated mixture at 569 K. The results demonstrate that increasing either wall density or specific heat capacity significantly delays ignition, exhibiting a pronounced linear trend. The ignition time increased from 17 s to 42 s as ρw escalates from 4000 to 10,000 kg/m3, and from 17 s to 132 s as cw rises from 250 to 2000 J/(kg·K). However, these properties are found to have a negligible effect on the maximum combustion temperature and the HTR (heterogeneous reaction) contribution. Furthermore, when the volumetric heat capacity (ρwcw) is held constant, variations in ρw and cw show no influence on ignition time, peak temperature, and HTR contribution. The transient evolution of the wall temperature distribution confirms that ignition consistently originates near the channel inlet. This work underscores the pivotal role of wall material properties in governing ignition dynamics and provides essential insights for the thermal design and material selection of micro-combustors.

This study focuses on a 660 MW supercritical W-shaped flame boiler. Utilizing real-time DCS data and field tests, a comprehensive diagnosis was conducted on its operational characteristics at 35% and 25% (with oil-support combustion) rated loads. The research results show that oil-assisted combustion reduced the amplitude of furnace pressure fluctuations by 39.6%, significantly enhancing combustion stability; however, it also caused a marked decline in both the economic and environmental performance of boiler operation. In terms of economics, a sharp increase in exhaust gas heat loss was the primary cause of the efficiency drop. At 25% load, boiler efficiency decreased to 88.04%, an absolute reduction of over 4 percentage points from the design value. Simultaneously, the attemperation water ratio surged to 4.70%, resulting in substantial energy waste. Regarding environmental performance, a “pollution transfer” phenomenon was observed. SO 2 emissions decreased by over 60%; however, the concentration of particulate matter (PM) emissions doubled, and NOx emissions remained high, indicating a heavy reliance on end-of-pipe treatment.

Polyoxymethylene dimethyl ether (PODE) and methanol are important low-carbon substitutable fuels for reducing carbon emissions in internal combustion engines. In the research, the impacts of methanol ratio, injection timing, and intake temperature on HCHO generation and emission were investigated using both engine tests and numerical simulations. Results suggest that an increase in methanol ratio suppresses auto-ignition tendency of PODE, leading to the increase of ignition delay period, pressure peak, and heat release rate peak inside the cylinder. The decrease in in-cylinder combustion temperature contributes to an increase in HCHO emission due to partial oxidation of methanol in the cylinder and exhaust pipe. While the injection timing is gradually postponed from −10 °CA ATDC to 2 °CA ATDC, in-cylinder high-temperature area decreases, the quantity of unburned methanol increases, but part of HCHO is converted to HCO due to H radical influence, resulting in 72% increased HCHO emission. With the increment of intake temperature, the oxidation and decomposition of in-cylinder methanol accelerate, leading to an improvement in combustion stability, more uniform temperature distribution, and a decrease in unburned methanol, which results in lower HCHO emission. When the intake temperature is rose from 30 to 60 °C, HCHO emission decreases by 11.2%.

Combustion instability analysis in annular systems often relies on reduced-order models that represent the complexity of combustion dynamics in a framework in which the flame is represented by a ‘flame describing function’ (FDF), portraying its heat release rate response to acoustic disturbances. However, in most cases, FDFs are only available for a limited range of disturbance amplitudes, complicating the description of the saturation process at high oscillation levels leading to the establishment of a limit cycle. This article shows that this difficulty may be overcome using a novel experimental scheme, relying on injector staging and in which the oscillation amplitude at limit cycle can be controlled, enabling us to measure FDFs from simultaneous pressure and heat release rate recordings. These data are then exploited to replace the standard modelling, in which the heat release rate is expressed as a third-order polynomial of pressure fluctuations, by a function of the modulation amplitude, allowing an easier adaptation to experimental data. The FDF is then used in a dynamical framework to analyse a set of staging configurations in an annular combustor, where two families of injectors are mixed and form different patterns. The limit-cycle amplitudes and the coupling modes observed experimentally are suitably retrieved. Finally, an expression for the growth rate is derived from the slow-flow variable equations defining the modal amplitudes and phase functions, which is shown to exactly agree with that obtained previously by using acoustic energy principles, providing a theoretical link between growth rates and limit-cycle amplitudes.