A Review of Micro Power System and Micro Combustion: Present Situation, Techniques and Prospects

Abstract

Micro burner is the fundamental element of a micro energy power system. The performance, output power, and efficiency of the system are directly involved by the combustion stability, efficiency, and temperature distribution of the exterior wall. Owing to the small combustion space of the micro burner and the resident short time of the premixed fuel/air, the fuel is difficult to burn completely, resulting in poor burning efficiency and flame stability. Therefore, the study of micro burner technology is the focus of current research to improve combustion performance. This article introduces the micro power system, micro combustion technology, and combustion status and characteristics, focusing on four kinds of micro combustion technology. The purpose is tantamount to fully understand the current status of micro combustion technology and compare the characteristics of different combustion technologies. For improving output power and efficiency of the power system, the combustion stability and performance are enhanced, which provides theoretical support for the effective realization of micro scale combustion and application.

1. Introduction

With the rapid development of micro machining technology and the demand for portable electronic devices, various micro electro mechanical systems (MEMS) are widely adopted [1,2,3]. Epstein and Senturia et al. [4] firstly defined the concept of micro combustion, where the characteristic length scale of confined spaces is on the order of 100–1000 µm. Then, this concept is applied to micro scale devices, and the micro scale equipment has a profound and significant impact on people’s lifestyles and society development [5]. However, these micro power devices are always driven by batteries, while the small/microscale devices are limited by the size of batteries, that is, the power supply is restrained, i.e., durability and power output [6,7]. For instance, 90% of the volume of a microsensor communication device is occupied by batteries [8]. Therefore, these devices urgently need micro energy or a power plant with high energy density [9], so that they can give full play to their advantages and further miniaturization and multifunction [10]. However, the batteries commonly employed in micro electro mechanical systems have some problems, such as long charging time, large size, weight, and low energy density. As shown in

Table 1. Power density values of various energy sources based on complete combustion of CO₂/H₂O at 298 K and 1 atm [11].

Energy Source	Power Density (MJ/kg)	Energy Source	Power Density (MJ/kg)
Lead-acid cells	0.0792	N-butane	49.6
Nickel-cadmium cells	0.158	Propane	50.3
Li-ion cells	0.468	Methane	55.5
Lithium-sulfur cells	0.792	Hydrogen	142
Methanol	22.7	Diesel oil	45.3
Alcohol	30.5	Gasoline	45.8
N-octane	48.2

, the energy densities of common batteries and fuels are compared, in which hydrocarbons and hydrogen are several orders of magnitude higher than lithium batteries [11]. In terms of energy conversion efficiency and energy density, micro energy power equipment and micro combustion have a wider application prospect [12]; this is because, compared with the traditional lithium battery power supply system, the system has a higher energy density lighter weight and longer operating time [13].

With the concern of energy depletion and environmental damage and the strong demand for portable electronic equipment, hydrocarbon-fueled micro burner has become an important research field and has broad application prospects [14]. As illustrated in Figure 1, the chemical energy released by fuel can be utilized in four ways [11,14]: ① Self-heat source; ② Thermal energy converts electrical energy, such as micro thermophotovoltaic system (Micro-TPV)/micro thermoelectric generator (Micro-TEG); ③ Conversion of thermal energy into chemical energy, serving as a heat source in multifunctional reactors (e.g., methane catalytic reforming/ammonia to hydrogen); ④ Converting thermal energy into mechanical energy, such as Micro Propeller, Micro Rotary Engine, and Micro Gas Turbine Engine. The micro combustors, as the core components of micro energy power systems (micromotors and power electrical systems), have been put forward higher requirements for working stability and sustainability: stable and continuous supply of high-temperature heat sources, high energy density, low noise exhaust pollution, high safety [3,15,16], etc. It should be noted that the operation of a micro burner also brings great challenges because of the narrow combustion space, large surface volume ratio, and heat dissipation ratio [17]. Besides, research on micro combustion integrates the scientific and technological knowledge and achievements of micro machinery, fluid, aerodynamics, heat transfer, and thermodynamics in micro devices [18,19]. Its in-depth understanding and development will also drive the progress of these related disciplines.

The burning stability and energy efficiency immediately impact the performance of the system [20]. However, the resident time of combustion in the micro cavity is shortened sharply due to the small combustion chamber [21]. So, it is difficult to ensure the complete heat release of fuel, and the enhanced surface-to-volume ratio and serious heat loss reduce the flame stability, combustion efficiency, and energy utilization efficiency in the micro power systems [22,23]. These reasons will lead to a reduction in limited stable combustion, incomplete burning, and decrease in efficiency, thus reducing the adjustable range of the output power and efficiency of the micro power system, increasing the emission of pollutants [24,25]. This brings new challenges to the micro energy power system.

Micro combustion is widely employed, while the unstable flame, inadequate combustion, and high heat loss ratio challenge the application. To improve the output power and efficiency of the micro system, approaches to enhance combustion stability and thermal performance are adopted. In this work, the micro power system and combustion are overviewed, where the main application is introduced. Then, the methods of enhanced combustion stability are compared and summarized, such as blended fuels, catalytic, and porous media combustion and burner optimization. Finally, the prospects for micro combustion are concluded and expected.

2. Micro Power System and Combustion

2.1. Micro Power Generation

The main application of the micro energy system is to employ hydrocarbon-fueled combustion to release heat in micro devices, and then output high-density power via energy conversion [26,27]. Research efforts have been made regarding a micro power system; i.e., burner structure design and system optimization, and various forms of micro heat engine systems, are designed and proposed [11,28]. Epstein et al. [29,30] firstly devised a three-layer, silicon-based micro gas turbine engine in 1997. Then, Waitz et al. [31] manufactured a micro gas turbine engine with six layers of silicon in 2000. Besides, a miniature burner (silicon-based) is developed with hydrogen, ethylene, propane, and other gases fueled [18,32]. Furthermore, the power output of these gas turbines is 6 to 8 orders of magnitude less than that of conventional macroscopic scale turbines [33]. It usually produces hundreds of megawatts of power, while the volume of the combustion chamber is less than 200 mm³. However, power density exceeds 1000 MW/m³, which possesses high utilization potentiality [34,35,36]. Besides, Zhang et al. [37] tested a micro power generation system powered by a circumferential pulse micro turbine.

A micro thermoelectric system, the main application of micro power generation, adopts PV/TEG cells to convert thermoelectric energy to electrical power [38], as depicted in Figure 2. The system is mainly composed of a micro burner and thermoelectric conversion module without moving parts. Sitikz et al. [39] designed a “Mier-FIRE” miniature thermoelectric system with a Swiss roll combustor. Vican et al. [40] made a micro power device by using Swiss roll structure and alumina ceramics, which can convert heat into electric energy. When the fuel chemical energy output is 6.8~9.1 W, the electric energy output of 30~500 MW can be obtained. Schaevita et al. [41] obtained a stable voltage of 7 V by using platinum to catalyze the burning of hydrogen/air in a micro combustion chamber. Yoshida et al. [42] proposed a micro thermoelectric system composed of a catalytic burner with butane-fueled combustion and a thermal generator. When the fuel input is 6.6 W, the maximum output power and total efficiency are 184 mW and 2.8%, respectively. In particularly, the micro-TPV system is a typical micro thermoopticalelectric device, which is widely employed and studied in micro power sources [43]. Chou and Yang et al. [44,45,46] have conducted hydrogen-fueled combustion under micro conditions and developed a micro-TPV system, which consists of a micro burner, filter, PV cell, and heat fins, obtaining a power output of 1–2 W. Besides, Meng et al. [47] optimized a novel micro power generation system by coupling micro-TPV and micro-TEG, where two GaSb PV cells are applied to convert high radiation temperature and two Bi-Te modules are employed to utilize the medium wall temperature. The results showed that a system efficiency of 2.5% can be achieved. For the conversion of thermal to photoelectric, the high radiation temperature and the area–volume ratio of the micro burner can achieve high radiation power [48,49].

2.2. Micro Combustion Characteristics

Compared with conventional combustors, the structural size of a micro burner at the millimeter or submillimeter level brings challenges to the design and manufacture [50]. It also challenges combustion in micro conditions. For instance, the viscous effects are enhanced at small size channels, the physical residence time of mixed gas is shortened, and the ratio of area–volume of the combustion chamber increases sharply [51,52]. These factors directly/indirectly affect the flame and heat release of chemical energy, which makes combustion characteristics in micro/conventional combustors greatly different.

(1)

Short residence time

Micro combustion is determined by diffusion time and reaction time due to the small volume of the chamber; that is, the residence time is short [53]. It is proportional to the volume and pressure of the combustion chamber. For example, the reaction time of hydrogen or hydrocarbon with air/O₂ is about 0.01~0.001 s, which is placed on the same order of magnitude under the condition of small scale [54]. Hence, the Damköhler number D_a, the ratio of the residence time τ_r of the mixture to the combustion time τ_c, is employed. It indicates whether the fuel is completely combusted in the combustor, and can be defined as [55,56]:

$$D_{\mathrm{a}}=\frac{{\tau }_{\mathrm{r}}}{{\tau }_{\mathrm{c}}}$$

(1)

when D_a < 1, the reaction time is longer than the residence time of the mixture and the incomplete fuel is blown out, which reduces the combustion efficiency and results in the low efficiency of the power generation [57]. So, the reduction of the burner scale has more restrictions on the reaction time and residence time.

(2)

High heat loss ratio

The heat loss ratio of the combustor can be given [58]:

$$q\&=\frac{Q}{m}=\frac{{\int }_{s}q\&dS}{{\int }_V\rho dV}=\frac{\overline{q{\&}^{″}}}{\overline{\rho }}=\left(\frac{S}{V}\right)$$

(2)

where, q& is heat loss density, $\overline{q{\&}^{″}}$ is the average heat loss density, S is the inner surface area of the combustor, and V is the chamber volume.

Besides, the hydraulic diameter of a burner can also be employed to compare the combustion heat release rate and the heat dissipation rate:

$$\frac{E^{″}}{E^{\prime }}\propto \frac{1}{d_{\mathrm{h}}^{1.2}}$$

(3)

where, E′ is the mixture combustion heat release rate, E′′ is the heat dissipation rate of the inner wall, and d_h is the interior wall heat dissipation rate.

With the decrease of combustion chamber size under micro scale, the surface-to-volume ratio S/V increases rapidly. According to Equations (2) and (3), the heat loss ratio of combustor is significantly increased with the reduction of burner size, i.e., the smaller of d_h, which also accelerates the heat dissipation of the inner wall. For a conventional burner, the surface-to-volume ratio is approximately 3~5 m⁻¹, while it reaches 500 m⁻¹ under the micro condition of the combustor [59,60]. Hence, the surface heat loss of the micro combustor is a key problem for the application.

(3)

Weak flame stability

The stability of micro combustion is mainly determined by the reactions process, heat transfer, and the interaction of burned/unburned gas in the chamber [61,62]. Factors, i.e., fields of flow, temperature, and species, affect the flame stability, which is more susceptible in the microchannel, resulting in combustion instability: flame blowout, flame oscillations, and apparently asymmetrical distribution [63,64,65]. Glassman et al. [66] defined the extinction distance as the maximum inner diameter of the burner without flame flashback. The quenching distances of premixed combustion of hydrogen/hydrocarbon are presented in

Table 2. Quenching distance of premixed combustion of hydrogen/hydrocarbon.

Fuel/Air	Flameout Distance (mm)	Fuel/O2	Flameout Distance (mm)
H2/air	0.6	H2/O2	0.2
CH4/air	2.5	CH4/O2	0.3
C2H2/air	0.5	C2H2/O2	0.2
C3H8/air	2.1	C3H8/O2	0.25

, which can be measured in a constant volume combustor [67]. In addition, the high heat loss ratio of combustor walls can also easily lead to non-self-sustaining combustion: thermal extinction or radical extinction in a small chamber [68]. In particular, radical extinction is mainly caused by the inner wall, which absorbs the active energy of radicals and makes it difficult to react. So, external wall heating is proposed to enhance flame stability [69]. Furthermore, the larger surface-to-volume ratio increases the wall heat loss and collision probability of radicals to the inner wall in unit volume; so, the mentioned two mechanisms of flame extinction both affect the micro power system [70,71]. This is because the quenching distance of micro combustion is very small and close to the burner scale, as exhibited in Table 2.

2.3. Summary

The micro-combustion-based generator, which gains a higher energy density and longer duration with simple structure, light weight, and small size [72], is considered to be one of the most promising candidates for the power sources of micro devices [73]. However, the applications of the micro burner are limited by shortcomings, i.e., high heat loss ratio and weak flame stability, etc. The micro chamber volume and the dramatic augment of the surface-to-volume ratio affect the burning process and flame stability in three respects [74,75,76]. So, approaches should be proposed to enhance flame stability, heat transmission, and extend the application of micro combustion.

3. Research Actuality of Micro Combustion

3.1. Fuel Selection and Blended Combustion

Research on micro combustion is mainly focused on optimizing the fuel/air mixing process, strengthening flame stability, and improving the radiation temperature of micro burner [77,78,79]. As mentioned in Section 2, fuel properties strongly affect flame stability in micro combustors. Yilmaz et al. [80] and Chen et al. [81,82] found optimizing the fuel/air mixing process can prolong the residence time of gas and increase the flame temperature, which can improve the burning efficiency. Hydrogen is the most valuable fuel in micro burning because of the combustible wide range, high energy density, and lack of NO_x emissions [83,84]. However, hydrogen-fueled combustion is close to the combustor inlet, resulting in the high radiation temperature locates at the burner upstream and the temperature gradient is also remarkable, which is not conducive to device power output and the reliability of the system [85,86]. An effective method is the addition of other hydrocarbon fuel to alter the reaction processes and flame propagation [87]. Law and Kwon et al. [88] pointed out that adding an appropriate amount of propane to hydrogen-fueled burning can significantly inhibit the hydrodynamic instability of the flame and thermal diffusion. Furthermore, the addition of hydrogen to hydrocarbon also improves the stability of combustion and adjusts the flame anchoring position in the combustion chamber [89]. For example, adding a 10~20% molar concentration of hydrogen to the mixture of methane/air [90], propane/air [91] and carbon monoxide/air [92] can improve the energy conversion efficiency of premixed combustion. Obviously, changing the combustion characteristics, reducing the limit of flame quenching, and improving the burning stability are effective approaches to enhance micro combustion [52,93,94].

A small amount of hydrogen/hydrocarbon addition promotes the combustion in the micro burner via the additional radicals [95,96]. Pan et al. [95] found that hydrogen can expand the flammability of propane. When propane-fueled combustion is blended with 20% hydrogen, the blow-off limit can reach 1.4 m/s. As exhibited in Figure 3, Peng et al. [96] compared the combustion performance of methane/propane/hydrogen-mixed fuel, and found that the premixed combustion with 7.5% methane obtained the best thermal performance, while the 5% propane blended combustion gained the highest radiation temperature. As shown in Figure 4, Wang et al. [97] researched the effect of hydrogenation on the burning of methane/air mixture, and found that the addition of a small amount of hydrogen to methane and propane promotes the formation of OH radicals and effectively inhibits the deflagration. It also improves flame stability and reduces the formation of NO_x, reducing environmental pollution [98,99]. Moreover, blended burning in micro combustors enhances flame stabilization and improves emitter heat performance without the optimization of chamber setting [100].

3.2. Catalytic Combustion

As mentioned in Section 2, the adsorption of free radicals on the inner wall and the high heat loss rate challenge the ignition and stable propagation of micro burning [101,102]. Research shows that catalytic combustion is an effective means of enhancing the combustion in a micro burner [103,104]. Figure 5 depicts the reaction process of catalytic combustion. Catalytic combustion can effectively reduce the ignition temperature (290–450 °C [105]), improve energy conversion efficiency, and broaden the combustion limit, which also reduces the generation of NO_x [106,107]. Furthermore, the heat conductivity and specific thermal capacity of the catalyst are much higher than that of gas, which is conducive to the thermal transmission of the gas wall and can effectively preheat the mixture and strengthen the flame stability [108,109]. For catalytic combustion, the catalyst adsorbs the gas, which is diffused to the catalytic surface, affecting the homogeneous reactions [83,110]. To explore the effects of surface catalytic (heterogeneous reaction [111]) on gas reaction in detail, Karagiannidis et al. [112] studied the combustion limit of methane/air with Pt catalyst under various pressures. The results showed that the limit of combustion stability is extended and the intensities of heterogeneous/homogeneous reactions are both enhanced, when pressure increases from 1 bar to 5 bar. Chen et al. [113] compared the catalytic combustion of methane/air in a micro combustor with single/multi channels, as shown in Figure 6. The results show that thermal recycling can lead to a substantial improvement in stability, but this excess enthalpy effect only occurs in high insulating materials [114]. Thermal recycling strongly affects the blowout, but has little effect on the extinction.

Harmful gases are produced in micro combustion with some new or hybrid fuels, such as NH₃, where CO, NO_x, and HCs can be formed under some operating conditions [115]. For example, NO_x is sensitive to operating temperature, and the high temperature is conducive to the formation of NO_x [116]. Figure 7 depicts the formation routes of NO_x, which can be formed via the process of fuel consumption, thermal process, and instantaneous process. For the reduction of NO_x, methods of setting catalysts [84], a perforated plate [25], and increasing the hydrogen content [117] can be employed. For instance, the application of a catalyst can reduce the residence time of burning reactions and form a low concentration of NO_x. The perforated plate in the combustor is mainly employed to form a hot recirculation area in the combustor, where a low concentration of NO_x gas is produced in the low temperature flame region near the perforated plate [118]. Besides, adding hydrogen is mainly related to the large amount of H and O that promotes the formation of NO, and the high H₂ content leads to the reduction of NH₃ and N radical, which is beneficial to reduce the production of NO [64]. Moreover, the mixing of hydrogen and ammonia to reduce NOx emissions has great application potential in practical applications.

Catalytic combustion can achieve efficient lean burning at a low temperature by reducing reaction time and increasing residence time [42,120]. For the lower requirement of activation energy of catalytic combustion, fuels can be ignited with a lower initial temperature even under micro conditions [121]. Zhang et al. and Yan et al. [122,123] found that the employment of platinum as a catalyst can effectively expand the combustion limit at lean combustion. As shown in Figure 8, the catalyst structure and the application of catalytic methane combustion are presented, which is a hot spot for research on combustion. Reinke et al. [124] verified the homogeneous reaction mechanism of methane/air with catalyst Pt at 1–16 bar, and evaluated the effectiveness of various basic gas-phase reaction mechanisms. Besides, the importance of the gas-phase reaction mechanism to catalytic combustion is provided. Li et al. [125] confirmed the feasibility of asymmetric catalytic of micro burner for non-premixed combustion, and the peak surface radiation efficiency reached 25.9%. Ran et al. [126] studied the catalytic combustion of CH₄/O₂ in a micro channel with a height of 1 mm and a length of 1 mm. Besides, Namazi et al. [127] proposed a pore-scale simulation method to study the catalytic methane combustion in the fiber porous, and found that it can predict the temperature distribution and the methane conversion rate with a relative error of 6.4%.

Catalytic combustion can reduce the initial temperature of burning and improve the combustion limits and efficiency [104,128]. However, a catalyst will lose its functions due to the poisoning and cannot guarantee a long-term effect because of the catalytic shedding or sintering [129]. Furthermore, it may also cause material failure due to uneven thermal stress between various parts of the combustion device. So, one of the key points of catalytic combustion is to develop efficient catalysts, find efficient loading materials, and design reasonable burners.

3.3. Combustor Optimization

For micro-TPV systems and other micro power generators, the temperature distribution of the burner surface affects the power output and system efficiency [130]. Furthermore, heat loss is also a key issue in a micro combustor, affecting the combustion stability and system working performance. Weinberg et al. [131] firstly proposed the excessive enthalpy combustion, where heat from reactions preheats the unburnt gas via the burner wall, improving the enthalpy value of reactants and expanding the flammability limit. Hua et al. [132] also depicted that heat conduction in the combustion chamber contributes to the flame stability of hydrogen/air in micro combustors and reduces the heat loss of the system. Leach et al. [133] investigated the effects of the axial thermal conduction of burner wall and heat loss caused by the environment on hydrogen/air burning in micro tubes. It is found that the gradually enhanced heat conduction of the wall is beneficial to the stability of combustion and the improvement of energy density. Veeraragavan et al. [134] analyzed the effect of axial heat conduction on the temperature distribution of micro combustor and flame velocity of methane/air, and illuminated that the flame regime is relocated and flame velocity is changed due to the heat conduction between the wall and preheating zone in the combustion chamber. Pan et al. [135] researched the effects of the ratio of wall thickness to burner diameter on H₂/O₂ combustion, and found that the thin wall is beneficial to the reduction of axial heat loss. Besides, Wang et al. [136] demonstrated that the continuous preheating of the combustor outer wall via the blowing of warm air leads to stabilizing the flame and increasing the flammable limits.

The selection of appropriate material and thickness of the burner wall is another important aspect of micro burning [137]. Obviously, the appropriate thickness of the burner’s outer wall directly impacts the flame stability and the utilization of the heat generated by reactions in a micro combustor, affecting the radiation temperature and system power output [138,139]. The wall properties, i.e., heat conductivity and emissivity, also affect the heat transfer through the burner wall [140,141,142]. High thermal conductivity can increase the radiation temperature of the burner while reducing the stability of the flame [143]. This is because the heat transfer from the inner wall to unburned completely gas is conducive to preheating the mixture, maintaining stable combustion, and improving the reaction intensity [144,145]. Therefore, it is necessary to select suitable heat conductivity of the burner to gain higher radiation efficiency and large power output [87,88].

For improving combustion stabilization and energy efficiency of a micro combustor, efforts and investigations are conducted, where the combustor with various chamber settings is proposed, as indicated in

Table 3. Summary of different burners.

Author	Model	Pattern	Remarks
Gao et al. [157]		Inserting guide vanes, bluff body	Higher uniformity and average temperature.
Peng et al. [158]		Installing front cavity	Improving flame stability and energy conversion efficiency.
Akhtar et al. [159]		Channels with varied configuration	Increasing vorticity, enhancing heat transfer.
Baigmohammadi et al. [160]		Wire insertion method	Improving combustion process, increasing wall temperature.
Akhtar et al. [63]		Changing curvature	Outer wall temperature and energy conversion efficiency increased by 110 K and 7.84%.
Alipoor et al. [161]		U-tube	Better preheating, increasing flammability limits.
Ansari et al. [10]		Coupling baffle with bluff wall	Temperature uniformity increased 87.5%.
Aravind et al. [6]		Three-step with recirculation path	Favorable temperature uniformity, enhancing flame stability
Fan et al. [162]		Micro Swiss roll	Reducing the flame stretching effect, increasing the blow-out limit.
Su et al. [163]		Multichannel	Lessening temperature difference, improving temperature uniformity.

. Obviously, modification of the combustion chamber alters the flow field of gas, affecting the preheating of unburnt reactants, the flame anchoring, distribution of radicals, and temperature; the chamber setting includes a cavity, step, and multichannels [146,147,148]. Setting ribs, baffles, and bluff body with an appropriate position can change the internal flow field of a micro combustor, producing a backflow zone and a low-speed region near the wall [149]. It effectively prolongs the local residence time and enhances the mixing performance of radicals, improving flame stabilization and burning efficiency [150]. Lee et al. [151] studied the flame dynamics of lean hydrogen/air burning in a narrow channel with a bluff body, and found that the bluff body can effectively maintain flame stability under the condition of subcritical velocity. Niu et al. [152] investigated the methane/air burning limit in a micro channel with five trapezoidal bluff body at an equivalence ratio of 0.9. The results illustrated that four types of the flow field successively appeared with the increase of blockage ratio and inlet velocity in the combustion chamber. Besides, Wan et al. [153] found that the bluff body can extend the combustion limit of premixed hydrogen/air by 3–5 times in a micro planar. Yan et al. [154] showed that the blocking ratio between bluff body and chamber size also strongly affects flame stability. Bagheri et al. [62] also simulated the combustion characteristics and flame stability of premixed H₂/air in the micro burner with bluff body (shapes of round, oval, diamond, semicircular, semi-oval, triangle, crescent, arrow, and wall blades), and found that the blowout limit, combustion efficiency, wall temperature, and emission temperature of the combustion chambers are dramatically changed [155]. As depicted in Figure 9, the tensile effect in the shear layer of the micro combustor with triangular and semicircular bluff bodies causes the flame blowout, and the triangular bluff body has stronger dynamics effects than that of the semicircular bluff body on flame stretch [156].

The optimization of a fluid field in a combustor usually provides a thermal recirculation zone, which is considered to be the most critical factor affecting the burning process [164,165,166], as shown in Figure 10. It reduces the quenching diameter and increases the combustion temperature because of thermal recycling; moreover, the reaction time can be extended and the flammability limit can be increased [148,167]. Yang et al. [168] discovered that setting a backward-facing step in a miniature stainless steel tube is a simple and effective approach to enhancing combustion stability. It creates a low-velocity zone and provides a region for flame anchors [169], thereby prolonging the residence time of reactants, expanding the flow range and operating condition, so a high and uniform temperature distribution of the external wall can be achieved. Li et al. [170,171] demonstrated that the expansion section of micro tube can effectively stabilize the flame position, and the appropriate length of the expansion section or the step position is conducive to increasing the burner wall [172]. Besides, Yang et al. [73] learned the average outer wall temperature of a micro burner with heat recovery is 123 K better than that of a micro combustor without thermal recuperation, as shown in Figure 11. Hosseini and Taywade [62,114] found that internal thermal recovery can enhance the heat quenching limit, while external thermal recirculation obtains a better burner radiation efficiency; the expansion of the recirculation zone also increases burning efficiency. More structural forms are also employed to form reflux zone in a micro combustor, such as Swiss roll [173,174] and U-Bend [175,176]. Zhong et al. [177] conducted a methane/air combustion test on a miniature Swiss roll, and found that the thermal recirculation in the combustor center can effectively preheat reactants and enhance the combustion. Besides, the equivalence ratio and the initial temperature of reactants also significantly affect the thermal recirculation in the combustion chamber [170,178].

3.4. Porous Media Combustion

Combustion in porous media is an efficient regenerative burning technology without a heat exchanger and energy auxiliary peripherals [179]. The heat capacity, heat storage, thermal conductivity, and emissivity of the solid skeleton of porous media are higher than that of gas [180,181], which can strongly boost the heat transmission of gas–gas and gas–wall in combustor [182]. As exhibited in Figure 12, the released heat of fuel is effectively transferred to the porous media zone, where the solid skeleton of porous media can be rapidly heated up, preheating the unburned gas at upstream [183,184]. Besides, the high heat storage and heat capacity can also contribute to the thermal transmission of burning gas to unburned gas through gas–solid [185]. So, as shown in Figure 13, the burning intensity, combustion limit, and output power can be improved by adding porous media in the micro combustor [186].

There exists a significant difference between the burning with/without porous media because of the heat recirculation [187], as depicted in Figure 14. The setting of porous media in a micro burner can effectively improve flame stability and thermal transmission by utilizing the high radiation and thermal conductivity of the solid skeleton of porous media [188,189]. Li et al. [142] found that porous media can strengthen the heat transmission between the gas and the wall surface, where it can also be regarded as a solid matrix for burning in a micro combustor. Besides, the burning efficiency can also be enhanced in a micro burner. Liu et al. [184] demonstrated that the combustion efficiency in the porous media is close to 100%, owing to the strong thermal recycling effect via solid skeleton. Besides, the porous medium significantly boosts the thermal recirculation and transfer in a combustor, which contributes to obtaining a stable flame near the combustion limit [190], thereby extending the lean burn limit of hydrocarbon [184,191]. This is because the combustion characteristics and flame propagation are also strongly modified. Voss et al. [192] learned that the flame thickness of burning in porous media is thicker than that of free flame. Shi et al. [193] found that the diffusion of the reaction component in porous media leads to an increase in flame width and a decrease in flame height. Liu et al. [194] indicated that the temperature distribution of gas and porous media are significantly changed with the increase of porous media in emissivity. Burning in porous media can transfer the reaction heat to other regions of the combustion chamber, reducing the flame temperature and increasing the upstream and downstream temperatures of the burner, thereby improving the radiation temperature and energy efficiency of the micro burner [72,195,196]. Pan et al. [197] improved the flame stability, burning efficiency, and system efficiency of micro-TPV by adding porous media to a micro combustor. Research from Li et al. [198] and Peng et al. [199] demonstrated that the width, filling position, porosity, and particle size of porous media strongly affect flame stability, combustion characteristics, and combustion efficiency, thereby impacting the heat and working performance of micro power system, as shown in Figure 15.

To sum up, porous media with a high capacity of heat storage and regenerative performance can preheat unburned gas and improve thermal transmission in a combustor. So, flame stability, the flammable limit of hydrocarbon, and combustion efficiency can be prominently improved, which contributes to the miniaturization of burners and micro devices.

3.5. Summary

Methods to enhance micro combustion stabilization are introduced and compared, which gains various characteristics for combustion under different operating conditions. Premixed combustion utilizes free radicals to promote combustion and improve flame stability, while the probably-toxic gases in the exhaust gas should be reduced, such as mixed ammonia, hydrogen-oxygen, etc. Catalytic combustion is mainly adopted to reduce the reaction time and maintain a stable flame. Besides, the optimization of chamber structure forms the regions of backflow, and low velocity enhances the heat transfer of gas-gas and preheating, promoting flame anchoring and improving combustion efficiency. Table 3 summarizes the optimization of the burner structure, inserting blunt body and block, setting cavity, changing channel shape, etc. For the approaches of burner optimization, a higher wall temperature and energy conversion efficiency can be obtained. Meanwhile, the effects of chamber setting on combustion stability and flame stretch are varied, so the combination of these means is also employed to improve the micro combustion and system performance. Furthermore, the combustion of porous media improves the heat transmission of gas–gas and gas–wall, enhancing flame stability and boosting the radiation temperature of burner. Besides, the emerging methods of fire stabilization are adopted, such as hyperenthalpy combustion. So, the development of new (economic, safe, efficient) combustion technology is still needed to extend the application of micro combustion.

4. Conclusions and Prospects

A micro energy power generator is urgently needed because of the development of micro machining technology and the demand for portable devices. As the core element of the micro power resource, micro burning has the advantages of lightweight, high energy density, and long working duration. However, the narrow combustion chamber, large surface-to-volume ratio, and high heat dissipation ratio have brought great challenges to combustion stability, limit, operating range, thermal efficiency, and power output of the micro generator. In view of the above problems and challenges, research has been carried out to reduce the limitation of the characteristic dimension of flame quenching and improve combustion stability and efficiency, where the burner structural optimization, porous medium, catalytic, blended combustion, etc., are proposed. To provide strong support for efficiency improvement, burner optimization and processing design of micro energy power devices are employed. It can be further investigated from the following aspects:

(1)

Low–zero carbon fuels and liquid fuels can be adopted to meet the requirements of the emissions target and the miniaturization of equipment, where the combustion with blended fuels, such as NH₃/H₂, is recommended.

(2)

New combustion technology can be employed to solve the problem of incomplete combustion and short residence time, such as porous media catalytic combustion, and an efficient catalyst should be employed.

(3)

Combustor optimization still plays an important role in flame stability and combustion efficiency, where new materials with appropriate thermal properties and novel designs of the chamber structure are proposed, such as multi-inlets and combined burners.

(4)

For high energy density and power output, a combination of power generation technologies should be employed, such as the coupling of TPV and TEG. The new simple and efficient stable burning in micro combustors is the advantage of wide application of micro power systems.