In order to strengthen the internal flow and heat transfer and increase the stability of the flame, various methods to improve combustion efficiency were proposed. In terms of spatial structure, the ...optimization methods are divided into two categories: insertion geometry and internal molding. Both of them change the combustion efficiency by affecting the flow and heat transfer, but the degree is different; for catalytic combustion, heat and mass transfer, heterogeneous/homogeneous reaction and other derivative processes are the key to improve the combustion efficiency; as for fuels, changing the type and mixing mode are effective means to improve combustion efficiency. In addition, the research on improvement of the energy conversion and emission performance of micro thermophotovoltaic(MTPV) system, micro thermoelectric(MTE) system and micro engine(ME) was reviewed. Compared with the existing research, it is found that the single optimization method has limitations, it cannot guarantee the positive effect under all operating conditions, the future research direction is to combine the advantages of a variety of optimization methods, besides, with the development and maturity of technology, the number of studies on diffusion combustion is increasing, and its practicability has been improved. Meanwhile, the further research and development of micro-energy mechanical system(MEMS) technology can be effectively promoted by the application of field synergy principle and Molecular Dynamics in numerical simulation. Based on this work, the research status of flow, heat transfer and performance improvement in combustion-based MEMS can be presented to relevant researchers more clearly.
•Process in heat transfer performance improvement are summarized.•Process in micro combustion performance improvement are summarized.•Process in energy conversion performance improvement are summarized.•Future works for the performance improvement are introduced.
•Influencing parameters of the flame optimization schemes are summarized.•Influence parameters of the micro-thermal photoelectric system are analyzed.•Influence parameters of the thermoelectric ...conversion efficiency are investigated.•Technical limitations and development of the micro power equipment are summarized.
Hydrocarbon fuel and hydrogen are used as fuels for micro power equipment to replace traditional batteries and provide energy for micro-electromechanical systems, which has become a hot research direction. However, due to the reduction in the size of micro power equipment, not only the flame in the micro combustor is affected, but the overall energy conversion efficiency of micro power equipment is also affected by the size effect. Therefore, it is significance to analyze the energy loss and design optimization methods pertinently from the point of energy conversion. In the past ten years, many researchers have done research about the direction, which has been reviewed in this article. For the flame optimization, the influencing parameters of optimization methods such as exhaust gas recirculation, cavity combustor, bluff body combustor, porous media combustion, and hydrocarbon fuels mixed with hydrogen are summarized. For the micro-thermal photoelectric system, the influence parameters of combustion efficiency, radiant efficiency, spectral efficiency, view factor efficiency and photovoltaic cell efficiency are analyzed. In the micro-thermoelectric system, the influence parameters of combustion efficiency, heat conduction efficiency and thermoelectric conversion efficiency are analyzed. For the micro internal combustion engine, the influencing parameters of combustion efficiency and conversion efficiency are analyzed. Finally, the technical limitations and development of each micro power equipment are summarized in this article.
•The porous medium micro-combustor and thermoelectric system is developed.•Effects of the heat exchange and Seebeck converse effect on temperature field are analyzed.•Thermoelectric conversion ...performance of the micro thermoelectric system is analyzed.•Optimization directions of micro-thermoelectric system are proposed.
The energy conversion of micro combustion system has become the main research direction at present. However, due to the small size of micro-combustors, they face challenges such as unstable combustion and difficulty in effectively converting generated thermal energy into usable energy. This work mainly studies the thermoelectric conversion system based on porous medium micro-combustor, which mainly uses the high energy density of micro-combustor to replace the traditional battery energy and improve the working time of small equipment. And this work also establishes the coupling model of porous medium micro combustor and thermoelectric module. By analyzing the performance parameters of the micro thermoelectric system in different flow rate, the results show that at low flow rates, semiconductor materials with better thermoelectric performance, such as Bi2Te3, can be used to improve the energy conversion efficiency of the system. However, the low heat release from combustion leads to lower maximum output power. At high flow rates, there is a significant improvement in the thermoelectric conversion performance of the system, but it is limited by the temperature of the thermoelectric materials. To optimize the impact of these factors on micro-thermoelectric systems, the optimization methods were analyzed as: (1) The temperature uniformity of micro-combustor wall can be improved by using mixture with low equivalence ratio and high flow rate; (2) enhancing the thermal conductivity of the micro-combustor and porous medium to improve the thermal conductivity of micro thermoelectric system; (3) Using CH4-H2 mixed combustion to reduce combustion temperature and ensure that thermoelectric materials work within the normal temperature range; (4) The segmented thermocouple is used to improve the thermoelectric performance of thermoelectric module. The results show that using a mixture with low equivalence ratio and high flow rate can achieve a maximum conversion efficiency of 5.51%. Using mixed combustion to reduce combustion temperature can increase the maximum output power of the system to 57.8 W.
•Combustion stabilization technologies for micro/meso-scale combustors are reviewed.•Flammable range is expanded by applying combustion stabilization technology.•Combustion stabilization technology ...improves outer wall temperature distribution.•Combustion stabilization technology effectively reduces pollutant emissions.•Combustion stabilization technology improves energy conversion efficiency.
Owing to its high energy density, the combustion-based micro thermophotovoltaic system has promising prospects and becomes the most potential substitute for conventional power generation systems. As one of the most critical components in micro thermophotovoltaic systems, however, micro/meso-scale combustors are still confronted with many challenges due to the scale effect. Therefore, it is of great significance to develop combustion stabilization technologies for micro/meso-scale combustors. During the past two decades, many researchers have proposed various combustion stabilization technologies, which have been reviewed in this work. According to the working principle, different micro/meso-scale combustion stabilization technologies can be classified into three categories: heat recirculation, flow recirculation, and combustion optimization. The results indicate that these technologies can effectively expand the flammable range and have a positive effect on the stable combustion in micro/meso-scale combustors. Furthermore, the working performance of micro/meso-scale combustors, including thermal performance, emission performance, and energy conversion performance, has been evaluated. The results demonstrate that the application of different combustion stabilization technologies to micro/meso-scale combustors effectively increases the outer wall temperature distribution, reduces CO/NOX emissions, and improves the combustion/radiation efficiency. Finally, the challenges and prospects of future work are outlined. This work expounds on the research progress of micro/meso-scale combustors for micro thermophotovoltaic systems, which provides references for subsequent research and applications.
•A backward facing step and unique inlet shape are used in a micro combustors.•The non-premixed hydrogen/air combustors with catalyst segment are compared.•Key parameters affecting the performance of ...a non-premixed hydrogen/air combustors are summarized.
In this work, a type of non-premixed hydrogen/air micro combustor is designed for the application of micro-thermophotovoltaic (MTPV) systems. The micro combustor is presented with a backward-facing step and unique inlet shape to enhance the mixing performance and flame stabilization. The combustion, flow, and heat transfer characteristics of non-premixed hydrogen/air combustion in micro combustors with/without catalyst segment are numerically investigated. The results indicate that the homogeneous reaction is obviously weakened in the catalytic combustor, but a higher and more uniform outer wall temperature is obtained, and the outer wall temperature difference can be decreased by up to 28%. The flow characteristics of gaseous mixture in the catalytic combustor are better than those in the non-catalytic combustor. When the inlet velocity is 10 m/s, the average flow velocity and pressure loss of the catalytic combustor are decreased by up to 5.6% and 250 Pa, respectively. Furthermore, the heat of reaction and total heat flux at the gas–solid interface of the catalytic combustor are obviously higher and lower than those of the non-catalytic combustor, and the outer wall heat loss ratio is increased by up to 0.59% when the inlet velocity is 10 m/s. All in all, it can be concluded that adopting catalytic combustion to the micro combustor is extremely suitable for the application of MTPV systems.
In the context of international carbon neutrality, ammonia as a carbon-free energy has attracted extensive attention and research, and its renewability is the most prominent advantage. However, ...ammonia combustion is unstable, and harmful nitrogen oxides will be generated at the same time. In order to enhance the combustion stability of ammonia and control the emission of nitrogen oxides, the premixed combustion of NH3/H2/O2 in a segmented nozzle micro-combustor is studied in this paper. The mean wall temperature of segmented micro-combustor is 11.5 K higher than that of non-segmented micro combustor, and NO emission is reduced by 7.91 % at most. When mass fraction of hydrogen in the gas mixture XH2 increases from 10 % to 30 %, radiation efficiency increases by 13.5 %, but NO emission also increases. Considering that fuel-rich combustion can reduce NO emission, when hydrogen equivalence ratio ΦH2 = 1.0 and ammonia equivalence ratio ΦNH3 = 1.2, NO emission is reduced by 13.3 %, while radiation efficiency remains above 60 %. In addition, the radiation efficiency of the combustor increases by up to 4.15 % when the dimensionless length of segmented channel L6 is 7.5 mm and the dimensionless width of segmented channel D4 is 2.5 mm, and NO emission is reduced by 5.14 %.
The main part of a micro-thermophotovoltaic (MTPV) system is the micro combustor (MC). In this work, the performance of the traditional micro combustor (TMC) and the micro combustor with twisted tape ...(MCTT) is investigated numerically under various inlet velocities (9 m/s-12 m/s), H2/air equivalence ratios (0.7–1.0), lengths of the twisted tape (4 mm–16mm), and pitches (4 mm, 5.33 mm and 8 mm). The results show that compared with the TMC, the MCTT with a 16 mm long twisted tape has a 114.7 K, 12.2 W and 15.0% increase in the mean outer wall temperature T¯w, radiation power Pemitter and emitter efficiency ηmc, respectively, at vin = 9 m/s. The outer wall temperature Tow is more uniform when the length of the twisted tape is 4 mm or 8 mm, the maximum drop in temperature standard deviation is 38.9 K when vin = 9 m/s. When the equivalence ratio φha is less than 0.9, the ηmc is not significantly improved with the increase of the length of the twisted tape. Overall, the MCTT offers better promise for applications in MTPV systems than the TMC.
•A micro-cylindrical combustor with twisted tape is developed.•How to enhance combustion performance of the developed combustor is investigated.•The developed combustor has better potentiality for applications in MTPV system.
With the development of microfabrication technology and micro devices, the demand for Power Micro Electro Mechanical System (Power MEMS) is ever-increasing. However, traditional chemical batteries ...are not suitable for Power MEMS due to their low energy density. The combustion of hydrogen and hydrocarbon fuels offers a more promising alternative to conventional batteries. However, micro combustion faces the problems of flame instability and low combustion efficiency. Therefore, improving flame stabilization and combustion efficiency in micro combustions is necessary. Studies have made considerable progress in these aspects over the last decade. This paper summarized these studies and classified the optimization schemes according to flame stabilization and combustion efficiency. Besides, research on the Field Synergy Principle was discussed. The synergy between the flow field and temperature gradient field in the micro-scale domain will become a key research area in the future. It is proposed to insert porous media in MTES and MTPVS and adopt catalytic combustion. Adding hydrogen to the mixed gas was recommended. The equivalence ratio of the mixed gas in the range of 0.9–1.1 would be best. The equivalence ratio is the ratio of the theoretical requirement of air with complete combustion to the actual supply of air.
•Factors affecting the flame stability in micro combustors are summarized.•Factors affecting the residence time in micro combustors are summarized.•Factors affecting the global efficiency of micro combustors are summarized.•Future works to improve the performance of micro combustors are introduced.
The low thermal performance of conventional micro-combustor (CMC) is one of the major obstacles hindering the development of micro-thermophotovoltaic (MTPV) system. In the present research, ...micro-combustor with conical ring (MCCR) is proposed to enhance the overall performance. A comparative study of the temperature field, pressure field, velocity field and combustion efficiency of the CMC and MCCR is conducted. The numerical results show that when the inlet velocity is 8 m/s, compared with the CMC, the mean outer wall temperature, combustion efficiency and pressure loss of the MCCR-6 increased by 32.3 K (2.6%), 0.05% and 718.6 Pa (273.4%), respectively, while the standard deviation of outer wall temperature decreased by 31.4 K (49.5%). Additionally, the MC made using SiC provides a better overall performance, the conical ring (CR) is installed at 6 mm from the step with better overall performance.
•A conical ring insert combustor with premixed hydrogen/air combustion is developed.•The effect of conical ring mounting position and structural parameters on performance is investigated.•The velocity field, temperature field and pressure field are compared for different operating conditions.•How to enhance heat transfer performance of the MTPV system is investigated.•Developed combustor is of better potentiality for applications in MTPV system.
Under the background of the international energy crisis, it is urgent to develop a micro-thermophotovoltaic system using hydrogen as combustion energy. In order to optimize the micro-combustor in the ...system, the nozzle micro-combustors with five different channels are designed. All nozzle micro-combustors are numerically studied by using the mechanism of 9 components and 19 elementary reactions within the ANSYS Fluent 20.0, and their advantages and disadvantages in thermal performance, flame performance and chemical reaction are compared. It is concluded that the nozzle micro-combustor with constriction-expansion channel has the best performance among the five micro-combustors because of its reasonable segmented structure. Then, a new type of nozzle micro-combustor with segmented channel is designed, and the numerical study of segmented micro-burners and non-segmented micro-combustors with different inlet velocities and hydrogen/air equivalence ratios shows that the thermal performance of segmented micro-combustors is much better than that of non-segmented micro-burners. Therefore, compared with non-segmented nozzle micro-combustors, segmented nozzle micro-combustors have better application potential in micro-thermophotovoltaic applications.
•The nozzle micro-combustors with five different channels are investigated.•Thermal, flame and chemical reaction in nozzle micro-combustors are compared.•A new type of nozzle micro-combustor with segmented channel is designed.•Some parameters for influencing the performance enhancement are obtained.
