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The effect of differentiating the thermal conductivity between inner and outer walls on the stability of a U-bend catalytic heat-recirculating micro-combustor was investigated. To this end, a two-dimensional computational fluid dynamics (CFD) model was developed using the commercial code ANSYS Fluent (release 2020 R1) and, for different combinations of values for the inner and outer thermal conductivities, simulations of lean pre-mixed propane/air combustion were performed by varying the inlet gas velocity. Numerical results have shown that extinction is mainly ruled by the inner wall, whereas the outer wall controls blowout. Differentiating the thermal conductivity has been found to be an effective strategy to jointly exploit the better extinction resistance of low-conductive (i.e., insulating) materials, required by the inner wall, and better blowout resistance of highly conductive materials, required by the outer wall, thus enlarging the stable operating window of the catalytic micro-combustor compared to the use of the same material for both walls.

The influence of different shapes of pin fin arrays in micro combustors on combustion performance is investigated numerically. All the micro combustors show a high combustion efficiency and the one with triangle-B pin fin arrays exhibits superiority in combustion efficiency. Additionally, the combustor efficiency of all micro combustors is relatively low and reduces as the velocity increases. This works provides a great reference and guidance for the development of the micro burner in MTPV system.

This research presents a novel bluff-body and swirl-stabilized micro-combustor fueled by an ammonia/hydrogen mixture, aimed at enhancing flame stabilization for zero-carbon micro-combustion-based power generators. Employing numerical simulations, the study examines the effects of bluff-body geometry, inlet mass flow rate, vane angle, and combustor material on combustion and thermal efficiencies. Key findings demonstrate that the shape of the bluff-body significantly influences the combustion outcomes, with cone-shaped designs showing the lowest radiation efficiency among the tested geometries. The study identifies an optimal inlet mass flow rate of 9×10−6 kg/s, which achieves a combustion efficiency of 99% and superior uniformity in the mean outer wall temperature. While variations in flow rate primarily affect NO emissions and outer wall temperatures, they have minimal impact on combustion efficiency. Further analysis reveals that adjusting the vane angle from 15 to 60 degrees significantly improves mean outer wall temperatures, temperature uniformity, and combustion and radiation efficiencies, while also reducing NO emissions. The 60-degree angle is particularly effective, achieving approximately 44% radiation efficiency. Additionally, material selection plays a pivotal role, with silicon carbide outperforming others by delivering an optimized mean outer wall temperature (approximately 910 K), radiation efficiency (38.5%), and achieving the most uniform outer wall temperature. Conversely, quartz exhibits significantly lower thermal performance metrics.

With the improvement and development of micro-mechanical manufacturing technology, people can produce an increasing variety of micro-electromechanical systems in recent years, such as micro-satellite thrusters, micro-sensors, micro-aircrafts, micro-medical devices, micro-pumps, and micro-motors. At present, these micro-mechatronic systems are driven by traditional energy power systems, but these traditional energy power systems have such disadvantages as short endurance time, large size, and low energy density. Therefore, efforts were made to study micro-energy dynamical systems with small size, light gravity, high density and energy, and long duration so as to provide continuous and reliable power for these systems. In general, the micro-thermal photoelectric system not only has a simple structure, but also no moving parts. The micro-thermal photoelectric system is a micro-energy power system with good application prospects at present. However, as one of the most important structural components of micro-thermal photoelectric systems, the microburner, is the key to realize the conversion of fuel chemical energy to electric energy in micro-thermal photoelectric system. The studies of how to improve the flame stability and combustion efficiency are very necessary and interesting. Thus, some methods to improve the performance of micro-burners were introduced and summarized systematically, hoping to bring some convenience to researchers in the field.

The micro-thermophotovoltaic system is a prospective power generation system. The radiation performance of exterior walls is a critical component of MTPV, as it effectively determines the overall energy output. At a mixture intake speed of 10 m/s, the MCPFs-Diamond exhibits the most favourable thermal performance, as evidenced by the thermal distribution and the highest average temperature of the exterior walls, which measures 1255.3 K. Its available radiation energy can reach up to 43.6 W. In the case of micro combustors that incorporate alternative shapes of pin fins, the highest values for both the average external wall temperature and the available radiational energy are achieved when the gas intake speed is set to 8 m/s.

This work numerically examines the premixed combustion of partially cracked ammonia/air in a cavity-stabilized micro-combustor. Effects of the equivalence ratio (Φ) and inlet temperature (Tin) on the combustion features, flame–wall heat transfer and nitrogen-containing emissions are investigated quantitatively at a cracking ratio of 0.6. Results show that increasing Φ from 0.8 to 1.2 shifts the high-temperature region downstream and causes it to elongate axially. This spatial expansion decreases peak temperatures and distributes heat release over a longer distance. Mean wall temperature and overall heat loss are thus decreased due to weakened near-wall thermal interaction. NO formation closely follows the high-temperature and OH-rich zones. However, at Φ = 1.2, oxygen limitation suppresses NO production and redirects fuel-bound nitrogen towards N2O, enhancing its outlet emissions. As Tin increases from 300 K to 500 K, the improved reactivity of the mixture promotes an upstream shift of the main reaction zone. The reaction zone becomes more concentrated within the cavity. Such structural changes intensify NO formation but simultaneously compress the high-temperature zone, which reduces the wall-averaged temperature and overall heat loss. In the extended downstream post-flame region, lower temperatures and limited radical activity suppress NO2 formation and N2O decomposition. As a result, NO2 emissions decrease monotonically, while N2O emissions exhibit a gradual increase. These findings provide useful insights into the effects of operating parameters on combustion stability, heat transfer and nitrogenous pollutant evolution in microscale partially cracked ammonia flames.

Hydrogen is a promising zero-carbon fuel, and its application in the micro-combustor can promote carbon reduction. The structural design of micro-combustors is crucial for combustion characteristics and thermal performance improvement. This study investigates the premixed combustion characteristics of hydrogen/air in a micro-cylindrical combustor with double ribs, using an orthogonal design method to assess the impact of various geometric parameters on thermal performance. The results indicate that the impact of rib height, rib position, and inclined angle is greater than rib width and their interactions, while their influence decreases in that order. Increased rib height improves mean wall temperature and exergy efficiency due to an expanded recirculation region and increased flame–wall contact, but negatively affects temperature uniformity and combustion efficiency. Although double ribs enhance performance, placing them too close may reduce heat transfer due to the low-temperature region between the ribs. When the double ribs are positioned at the axial third equinoxes of the micro-combustor, the highest mean wall temperature is achieved. Meanwhile, with a rib height of 0.3 and an inclined angle of 45°, the micro-combustor achieves optimal thermal performance, with the mean wall temperature increasing by 61.32 K.

Unmanned aerial vehicles (UAV)s have unique requirements that demand engines with high power-to-weight ratios, fuel efficiency, and reliability. As such, combustion engines used in UAVs are specialized to meet these requirements. There are several types of combustion engines used in UAVs, including reciprocating engines, turbine engines, and Wankel engines. Recent advancements in engine design, such as the use of ceramic materials and microscale combustion, have the potential to enhance engine performance and durability. This article explores the potential use of combustion-based engines, particularly microjet engines, as an alternative to electrically powered unmanned aerial vehicle (UAV) systems. It provides a review of recent developments in UAV engines and micro combustors, as well as studies on flame stabilization techniques aimed at enhancing engine performance. Heat recirculation methods have been proposed to minimize heat loss to the combustor walls. It has been demonstrated that employing both bluff-body stabilization and heat recirculation methods in narrow channels can significantly improve combustion efficiency. The combination of flame stabilization and heat recirculation methods has been observed to significantly improve the performance of micro and mesoscale combustors. As a result, these technologies hold great promise for enhancing the performance of UAV engines.

With the rapid development of micro-energy power systems, the performance of micro-combustors as key components is in urgent need of further improvement. Aimed at enhancing combustion performance, a hollow hemispherical bluff body was used to analyze the methane combustion process. In this paper, we exploited the detailed reaction mechanism of methane/air with a laminar finite-rate model; the numerical analysis of methane combustion in the micro-combustor was carried out by Ansys Fluent software. The combustion, flow and thermal characteristics of the micro-combustor embedded with a hollow hemisphere bluff body (MCEHB) and the micro combustor embedded with a slotted hollow hemisphere bluff body (MCESHB) are compared, and the effect of slot width ratio on the combustion characteristics and thermal performance is discussed in detail. The results showed that the bluff body slotting treatment is not only beneficial to improving the velocity and temperature distribution behind the bluff body but also can improve the conversion rate of methane, especially at high inlet velocities. However, the conversion rate of methane is also affected by the slot width. When the slot width ratio below 0.5, the slot width corresponding to the peak methane conversion increased with the inlet velocity. Moreover, the bluff body slotting treatment can improve the wall temperature distribution, meanwhile expanding the high temperature area at the inner wall, thereby reducing the wall temperature fluctuation in the rear part of the micro-combustor. In addition, the optimal slot width ratio B increases with the inlet velocity. Since the inlet velocity is lower than 0.5 m/s, the optimal slot width ratio B is in the range of 0.3–0.375. However, as the inlet velocity exceeds 0.5 m/s, the optimal slot width ratio B moves to the range of 0.375–0.553. Furthermore, both large and small slot widths bring obvious temperature fluctuations to the micro combustor; the uneven wall temperature distribution phenomenon is detrimental to working performance. Therefore, the slot width ratio B of 0.375 only brings slight temperature fluctuations, indicating this is an optimal slot width ratio that should be chosen. This work has reference value for optimizing the design of the bluff body structure and improving the combustion performance of methane in the micro combustor.

Improving the flame stability and thermal behavior of the micro-combustor (MC) are major challenges in microscale combustion. In this paper, the micro combustions of an H2/air premixed flame in a swirl MC with various channel diameters (Din = 2, 3, 4 mm) were analyzed based on an established three-dimensional numerical model. The effects of hydrogen mass flow rate, thermal conductivity of walls, and the preferential transport of species were investigated. The results indicated that the flame type was characterized by the presence of two recirculation zones. The flame was anchored by the recirculation zones, and the anchoring location of the flame root was the starting position of the recirculation zones. The recirculation zones had a larger distribution of local equivalence ratio, especially in the proximity of the flame root, indicating the formation of a radical pool. The combustion efficiency increased with an increasing Din due to the longer residence time of the reactants. Furthermore, the MC with Din = 2 mm obtained the highest outer wall temperature distribution. However, the MC with Din = 4 mm had a better uniformity of outer wall temperature and large emitter efficiency due to the larger radiation surface. An increase in thermal conductivity boosts the thermal performance of combustion efficiency, emitter efficiency, and wall temperature uniformity. But there is a critical point of thermal conductivity that can increase the thermal performance. The above results can offer us significant guidance for designing MC with high thermal performance.