Recent research and development on ramjet and supersonic combustion ramjet (scramjet) engines is concerned with producing greater thrust, higher speed, or lower emission. This is most likely driven by the fact that supersonic/hypersonic propulsion systems have a broad range of applications in military sectors. The performances of such supersonic/hypersonic propulsion systems depend on a series of physical and thermodynamic parameters, such as the fuel types, flight conditions, geometries and sizes of the engines, engine inlet pressure/velocity. As a propulsion system, a stable and efficient combustion is desirable. However, self-excited large-amplitude combustion oscillations (also known as combustion instabilities) have been observed in liquid- and solid-propellant ramjet and scramjet engines, which may be due to acoustic resonance between inlet and nozzle, vortex kinematics (large coherent structures), and acoustic-convective wave coupling mechanisms due to combustion. Such intensified pressure oscillations are undesirable, since they can lead to violent structural vibration, and overheating. How to enhance and predict the engines’ stability behaviors is another challenge for engine manufacturers. The present work surveys the research and development in ramjet combustion and combustion instabilities in ramjet engines. Typical active and passive controls of ramjet combustion instabilities are then reviewed. To support this review, a case study of combustion instability in solid-fueled ramjet is provided. The popular mode decomposition algorithms such as DMD (dynamic mode decomposition) and POD (proper orthogonal decomposition) are discussed and applied to shed lights on the ramjet combustion instability in the present case study.

In this study, we perform a numerical investigation of a steady laminar stagnation flow flame stabilized at a wall with the consideration of heat transport, focusing on a lean hydrogen/air mixture with a fuel/air equivalence ratio 0.6. We discuss the NO emissions and their formation rates under various conditions, such as flow velocity and combustion pressure. It is found that the predominant reaction pathway for NO formation involves NNH radicals, though this changes near the wall surface. Beyond examining the wall’s influence on flame structures, the present work focuses on the impact of combustion process on materials. Specifically, the accumulation of atomic hydrogen at the wall surface is explored, which is significant for the consequent modeling of potential hydrogen embrittlement. Additionally, the growth rate of oxide layers on the material surface increases significantly if the combustion pressure and consequently the combustion temperatures are enhanced. These investigations offer valuable insights into how combustion processes affect material, which is useful for designing engineering components under high-temperature environments.

The inferior flammability of coal gasification fine slag (CGFS) from entrained-flow gasifiers hampers its resourceful utilization. However, the reasons behind its poor flammability still need to be investigated. This paper conducted a comparative study on the combustion characteristics of three CGFS samples: CGFS_GSP, CGFS_SN, and CGFS_OMB (subscripts GSP, SN, and OMB representing different gasification processes), using experimental techniques such as TG/DTG and combustion kinetic model fitting methods. Additionally, a comprehensive investigation into the physicochemical properties of CGFS was conducted. The objective was to elucidate the causes behind the poor flammability of CGFS. The results revealed that CGFS exhibits lower volatile matter content and higher activation energy than their corresponding raw coal (RC), leading to a significantly higher ignition temperature. The ignition temperatures of RC1, RC2, and RC3 are 361.82°C, 378.66°C, and 404.99°C, respectively. In contrast, the ignition temperatures of CGFS_GSP, CGFS_SN, and CGFS_OMB are 549.08°C, 566.58°C, and 532.67°C, respectively. During the combustion reaction, the temperature (T_max) at which CGFS reaches its maximum weight loss rate is significantly higher than the temperature (T_maxIII) at which fixed carbon in raw coal reaches its maximum weight loss rate. The T_maxIII of RC1, RC2, and RC3 are 450.90°C, 457.19°C, and 452.77°C, respectively. In contrast, the T_max of CGFS_GSP, CGFS_SN, and CGFS_OMB are 583.55°C, 608.20°C, and 582.18°C, respectively. The maximum weight loss rate of different types of CGFS is also significantly lower than the fixed carbon combustion maximum weight loss rate of their respective raw coal samples. The physicochemical characterization results of CGFS demonstrate that, compared to the corresponding raw coal, there is a significant reduction in the proportion of active sites in CGFS. Simultaneously, the proportion of C-C/C-H on the surface of residual carbon in CGFS decreases. In contrast, the proportion of O=C-O significantly increases, suggesting a shift toward a more stable state of carbon-containing functional groups. This study is expected to offer essential theoretical support for the efficient combustion utilization of CGFS.

Digital twin is a cutting-edge technology in the energy industry, capable of predicting real-time operation data for equipment performance monitoring and operational optimization. However, methods for calibrating and fusing digital twin prediction with limited in-situ measured data are still lacking, especially for equipment involving complicated multiphase flow and chemical reactions like coal-fired boilers. In this work, using coal-fired boiler water wall temperature monitoring as an example, we propose a digital twin approach that reconstructs the water wall temperature distribution with high spatial resolution in real time and calibrates the reconstruction using in-situ water wall temperature data. The digital twin is established using the gappy proper orthogonal decomposition (POD) reduced-order model by fusing CFD solutions and measured data. The reconstruction accuracy of the digital twin was initially validated. And then, the minimum number of measured data sampling points required for precise reconstruction was investigated. An improved uniform data collection method was subsequently developed. After that, the computational time required for the digital twin and the traditional CFD was compared. Finally, the reconstruction method was further validated by in-situ measured temperature from the in-service boiler. Results indicate that the established digital twin can precisely reconstruct the water wall temperature in real time. Thirty-nine sampling points are sufficient to reconstruct the temperature distribution with the original data collection method. The proposed uniform data collection method further reduces the mean relative errors to less than 0.4% across four test cases, and with the constrained technique, the errors decrease to 0.374% and 0.345% for Cases 1 and 3, which had poor reconstructions using the original sampling point arrangement. In addition, the reconstruction time of the digital twin is also considerably reduced compared to CFD. Engineering application indicates that the reconstructed temperatures are highly consistent with in-situ measured data. The established water wall temperature digital twin is beneficial for water wall tube overheating detection and operation optimization.

This study delves into the theoretical exploration of the effects of injector and orifice arrangement, spray angle, and orifice size on combustion and emission characteristics of horizontal opposed two-stroke engines. By employing numerical simulations, the research systematically investigates how variations in these parameters influence engine performance and emissions. The findings underscore the significance of injector and orifice configuration in optimizing fuel spatial mixing and atomization, resulting in improved indicated thermal efficiency and indicated mean effective pressure. However, it is noted that while emissions of HC, Soot, and CO can be maintained at low levels by injector and orifice configuration, NO_x emissions tend to be relatively higher. Moreover, the study highlights the impact of spray angle on combustion dynamics, where an optimum spray angle is identified for achieving peak thermal efficiency and effective pressure due to the improvement between spray distribution and impingement. Additionally, the study reveals the critical role of nozzle diameter in combustion and emissions control, with an optimal diameter leading to enhanced thermal efficiency and reduced emissions of Soot, HC, CO, and CO₂ to some extent. Overall, these findings offer valuable insights into optimizing engine performance and emissions control strategies in horizontal opposed two-stroke engines, guiding future research and development efforts in the field.