Planar Micro-combustor Research Articles

Experimental and numerical investigation on flame pattern formations of non-premixed CH4 and air in a planar micro-combustor

Non-premixed CH4/air flames in a rectangular micro-combustor of 2.0 mm (width) × 10 mm (height) × 31 mm (length) were experimentally investigated. The results demonstrated that flame cannot occur within the micro-combustor if nominal equivalence ratio ϕ ≤ 1.0 for any average velocity (V). Five flame patterns appeared and were termed as “single internal C-shaped flame”, “dual internal C-shaped flames”, “single ε-shaped flame”, “dual internal and external flames” and “single external flame”. Cold-state numerical simulations unraveled that fuel stratification occurred in the height direction due to the buoyancy effect, which is especially pronounced at ϕ = 1.3–1.5. Consequently, “dual internal C-shaped flames” is formed, consisting of an upper flame cell and a lower flame cell. At ϕ = 1.1–1.3, fuel stratification phenomenon grew weakened, which led to “single ε-shaped flame” at ϕ = 1.2–1.3 and “single internal C-shaped flame” at ϕ = 1.1–1.2. At sufficient high average velocities, one or both flame cells will be pushed out of the combustor. In conclusion, the present work verified the possibilities of formation of stable flames inside the micro-combustor with a width less than the quenching distance of stoichiometric mixture. Moreover, this study revealed the important role of buoyancy effect in flame pattern formations within micro-combustors with a large enough channel height.

Combustion and emission performance evaluation in ammonia/hydrogen-powered system with baffle and secondary injection applied

Ammonia has recently gained significant attention due to its potential for zero carbon emissions during combustion, coupled with ease of storage and transportation. However, challenges related to poor combustion stability and high NOx emissions have limited the widespread use of NH3. To address these issues, this paper introduces a baffle structure and a secondary fuel injection strategy, and their optimal positions, widths, and injection ratios are determined through three-dimensional numerical calculations. The results demonstrate that the baffle effectively anchors the flame, enhancing both flame stability and wall temperature. Additionally, the disturbed flow near the baffle enhances the reaction between NO and NH3, resulting in reduced NO emission. The baffle structure with l = 1/10 and w = 4/5 achieves the optimal overall thermal characteristics, with approximately a 5.2 % increase in average wall temperature compared to the conventional straight-channel micro-planar combustor. Moreover, while increasing the secondary combustion injection ratio (R) significantly reduces NO emission, it also exacerbates ammonia leakage. The minimum achievable NO emission of 531 ppm is achieved at R = 0.1, where NH3 leakage is minimal. This study validates the feasibility of introducing a baffle and a secondary fuel injection strategy to enhance flame stability, improve thermal characteristics, and reduce NOx emissions in ammonia-hydrogen combustion.

Flame visualization and spectral analysis of combustion instability in a premixed methane/air-fueled micro-combustor

The present study experimentally explores the combustion instability characteristics of premixed methane/air flames in a planar micro-combustor by utilizing the high-speed camera and the noise data acquisition system. It aims to identify the sound pressure fluctuation of unstable flames and shed light on the relationship between flame structure and thermoacoustic instability under micro-scale conditions. Four types of flame structures, i.e., flame with repetitive extinction and ignition (FREI), weak flame, steady flame, and oscillating flame, are experimentally observed by varying operating parameters. Among them, the weak flame at low flow velocities and the oscillating flame at high flow velocities form thermoacoustic oscillation. For weak flames, the collision with the wall and the local deformation of the flame result in high-frequency harmonics. Varying the inlet velocity from 0.14 m/s to 0.15 m/s leads to the pressure fluctuation of the weak flame being reduced by 10 Pa, and the dominant frequency being changed from 125 Hz to 167 Hz. As for oscillating flame, the variation of the inlet velocity plays a decisive role in the sound pressure fluctuation. At the equivalence ratio of 0.7, an increase of 0.01 m/s in the inlet velocity reduces the pressure fluctuation by 4 Pa.

Impact of H2 Blending of Methane on Micro-Diffusion Combustion in a Planar Micro-Combustor with Splitter

An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions.

Experimental and kinetic analyses on the flame dynamics and stabilization of ammonia/hydrogen-air mixtures in a micro-planar combustor

With the combination of experimental and numerical techniques, flame dynamics and stability of ammonia/hydrogen co-firing combustion are unraveled. Five types of fundamental micro-flame structures are, for the first time, experimentally confirmed with varying blending ratios (ε) and equivalence ratios (ϕ), along with a flame stability diagram delineated. It is shown that elevating ε from 30% to 50% extends the blowoff limit by a factor of 2.77 by suppressing or delaying the occurrence of certain flames. This thus leads to a turndown ratio of 15 in ammonia/hydrogen mixtures comparable to those of hydrocarbon fuel-based industrial combustors. To gain insights into the flame stabilization enhancement, further kinetics analyses are conducted with the validated reaction mechanism. The laminar burning velocity of the binary mixture is found to be increased remarkably with increasing ε, which facilitates flame propagation. Kinetics analyses reveal that varying hydrogen content not only enables the production/destruction pathways of major steps to be changed but also leads to the different sensitivity responses of the laminar burning velocity to reaction steps. This is also accompanied by the increase in the reaction rate of the major chain-branching step, thus signifying the importance of the chemical effect for enhanced flame stability. This work sheds light on the fundamental flame dynamic and enhancement mechanisms of ammonia/hydrogen co-firing combustion from macro and molecular perspectives.

Effect of dimethyl ether blending on methane flame morphology transformation and stabilization mechanism in a micro-planar combustor

This paper presented a novel blended combustion solution for mixing methane/dimethyl ether/air in a micro combustion chamber. Simulations were used to compare the pure methane combustion with the blended combustion, focusing on the OH species distribution, y-axis velocity, and strain rate to reveal the flame dynamics differences in different combustion conditions. The results showed that the blowout limit was enhanced from 0.53 m/s to 1.18 m/s at a blending ratio of 50 %. The blending of DME was also beneficial in weakening the flame asymmetry, delaying the generation of inclined flames in the original combustion, and replacing the inclined flames by a double-peaked U-shaped flame and an inverted U-shaped flame at an equivalence ratio of 0.9. Overall, this study not only provides a reliable fuel mixture for enhancing the combustion process under microscale conditions, but also reveals the flame transition characteristics.

Investigation of CH4 and porous media addition on thermal and working performance in premixed H2/air combustion for micro thermophotovoltaic

Primary obstacles to the development of micro-thermoelectric generators are the gigantic heat loss ratio and flame instability. Premixed H2/air combustion in the micro-planar combustor with porous media (PM) and CH4 blended is experimentally and numerically conducted to boost the generator working performance. Effects of porosities, PM length and CH4 blending on combustion characteristics and thermal performance are detailedly studied at various operating conditions. Results indicate that flame stability is enhanced in PM, and the burner radiation temperature is also significantly boosted because of heat recirculation and heat transmission from flame. PM porosity ε = 0.9 balances the effects of solid skeleton on flame propagation and thermal improvement, which is conducive to flame stabilization. The coupling of PM setting and 25 % CH4 blending strongly improves the combustor thermal performance via altering the flame location and enhancing heat transmission. It also ameliorates the uniformity of radiation wall. Moreover, prolonging porous matrix has a positive effect on the improvement of thermal performance under a high chemical energy input. Particularly, combustor #C-Ls13 obtains an electrical output of 2.5 W and a system efficiency of 2.1 % at 25 %CH4-75 %H2 fueled combustion.

Investigation of thermal performance and energy conversion in a novel planar micro-combustor with four-corner entrances for thermo-photovoltaic power generators

A novel planar micro-combustor with four-corner entrances is proposed by considering high and uniform outer wall temperature distribution used in micro thermo-photovoltaic generator. For this, a three-dimensional numerical model with detailed chemical mechanism, transport, and heat transfer and a thermo-photovoltaic energy conversion model are developed to examine its performance. Results show that the proposed micro-combustor has excellent wall temperature uniformity because a uniform distribution of high-temperature zones of four individual flames is formed in the combustion chamber. Thermal performance and energy efficiency of the proposed micro-combustor are better than conventional micro-combustor at a low flow rate ( m ) and large equivalence ratio ( φ ). Additionally, the implemented plate-insertion strategy effectively overcome the deterioration of combustor performance under higher m or lower φ . Proposed micro-combustor with plates insertion has the largest wall temperature and energy efficiency, no matter what φ and m are adopted. Plates extend the fluid ravel path and produce some recirculation zones, which promotes heat transfer, preferential diffusion, and flame anchoring, while its pressure loss is larger. Of all the tested cases, the system efficiency reaches a maximum of 6.39%, corresponding to 12.1 W electricity output. This work provides effective combustor design and research insights to develop high-efficiency micro-power system. • A novel micro-combustor with four entrances is proposed for micro-TPV system. • Excellent performance of wall temperature uniformity is obtained. • Inserting plates boosts thermal performance and energy conversion efficiency. • A maximum system efficiency of 6.39% is achieved with 12.1 W electric output.

Numerical study on flame shape transition and structure characteristic of premixed CH4/H2-air in the micro-planar combustor

The three-dimensional steady-state numerical simulation with a detailed chemical reaction mechanism is used to study the flame shape transition and flame structure formation mechanism of CH4/H2/air premixed combustion in a micro-planar combustor. The results indicate that hydrogen addition has a great influence on the flame shape transition and flame location. Regarding the flame structure, the addition of hydrogen leads to the formation of a U-shaped flame with double peaks, which enhances the combustion stability to a certain extent. The formation mechanism of the bimodal U-shaped flame is revealed through OH radical, Y component velocity and strain rate analysis. The main influencing factors are the coupling of flow field and flame, and flame-wall thermal coupling. Furthermore, the propagation characteristics of the bimodal U-shaped flame location with varied inlet velocity are elaborated. Meanwhile, it is found that flame asymmetry degree enlarges with the increasing of inlet velocity, which effectively transfers the influence of tensile force on the flame shape, and therefore the transition from U-shaped flame with double peaks to the inclined flame is suppressed. The findings presented in this work can be of value to understand the enhanced mechanism of fuel-blended combustion in micro-combustor.

Role of hydrogen addition in propane/air flame characteristic and stability in a micro-planar combustor

For combustion occurring under micro-scale conditions, how to enhance the flame stability is of significance. For this, a micro-planar quartz combustor with transparent main walls is designed to experimentally shed lights on the flame stabilization mechanism of premixed propane flame with hydrogen added in terms of flame dynamics. Experimental results show that as far as the pure propane combustion is concerned, there are six types of flame structures with varying the gaseous inlet velocity, i.e., flame repetitive extinction and ignition (FREI flame), cellular flame, planar flame, U-shaped flame, inclined flame and spinning flame. Furthermore, blending hydrogen is found to significantly extend the flame stability. This is due to the fact that the addition of hydrogen can overcome the inlet velocity inertia and restrain flame instability caused by hysteresis. For the inlet velocity considered in this work, hydrogen addition can inhibit transformation of flame type from U-shaped flame to inclined flame. Finally, the flame position, thickness and curvature of U-shaped flame are shown to be significantly affected by hydrogen addition ratio. Specifically, the larger the hydrogen addition ratio, the smaller the flame thickness, indicating that hydrogen addition is conducive to maintaining symmetry of the flame back and forth. In general, this work has an implication of designing a high stability micro-power system by reasonably considering the inlet parameter.

Thermal performance and NOx emission characteristics studies on a premixed methane-ammonia-fueled micro-planar combustor

In the present study, thermal and emission performances of a micro-planar combustor fuelled with methane/ammonia mixture are numerically investigated. For this, a 3D low-Mach number solver is developed based on OpenFOAM. The effects of equivalence ratios Φ, inlet volume flow rates Vi, and ammonia molar fractions in the fuel XNH3 are considered. It is found that the outer wall mean temperature T-w and the non-uniformity R-Tw vary non-monotonically. There is a contradictory variation between CO and NO emissions with Φ. A higher N2O emission occurs under the fuel-rich conditions, while it can be reduced by increasing Vi. Vi plays a critical role in affecting thermal performance. Increasing Vi could lead to an increase of T-w. However, an appropriate Vi should be selected to avoid a higher R-Tw and a lower radiation efficiencyηr. The variation of NO and CO emission are insensitive to the variation of Vi. The variation of XNH3 almost has negligible effect on T-w but it can lead to a decrease of R-Tw, especially under fuel-rich conditions. Increasing XNH3 improves ηr at Φ = 0.8 and 1.0 while it has little effect at Φ = 1.2. The addition of ammonia can reduce the CO2 emission but lead to the increased NO emission. Increasing XNH3 results in the increase of CO emissions at stoichiometric conditions, but the decrease at rich conditions. N2O emission is observed to be increased first and then decreased with increased XNH3. This work provides a preliminary study on the thermal and emission performances of a methane-ammonia fuelled micro-thermo-photo-voltaic system.

A numerical study on non-premixed H2/air flame stability in a micro-combustor with a slotted bluff-body

A planar micro-combustor with a slotted bluff-body was developed for non-premixed H2/air combustion. Numerical simulation was conducted and the flame blow-off limit under blockage ratio of ξ = 0.45, 0.55 and 0.64 was obtained (corresponding half height of the bluff-body is 0.25, 0.30 and 0.35 mm, total height of the channel is 1.1 mm). It was found that the blow-off limit peaks at ξ = 0.55. The simulation results of flame blow-off dynamics demonstrate that when the blockage ratio is small, the flame root cannot be anchored by the recirculation zone and the flame is entirely blown off the bluff-body. However, if the blockage ratio is relatively large, flame splitting occurs due to excess strain rate before flame being blown out. Analysis reveals that increasing the blockage ratio of the bluff-body has both positive and negative impacts. On the one hand, a large blockage ratio can increase the turbulence kinetic energy in the recirculation zone, which promotes fuel-air mixing and flame stability. On the other hand, the augmented stretch effect leads to local extinction and flame splitting. As a result, the combustor with a medium blockage ratio of the bluff-body can gain a tradeoff between the positive and negative aspects and achieve a largest blow-off limit.

Novel flame dynamics in rich mixture of premixed propane–air in a planar microcombustor

This paper reports the first experimental observations of various novel unsteady flame propagation modes in a two-dimensional, high-aspect ratio rectangular quartz channel with a positive wall temperature gradient for rich premixed propane–air mixtures. Various flame propagation modes are observed on progressively increasing the mixture velocity, while keeping the equivalence ratio fixed: FREI (Flame with Repetitive Extinction and Ignition) mode, oscillating FREI mode, oscillating flame mode, and wavy flame mode. The FREI mode resembles the classical FREI flame propagation reported earlier in the literature. In the oscillating FREI mode, the flame front oscillates in the transverse direction between the upper and lower walls of the channel, while propagating upstream as in the classical FREI mode. A sudden peak in flame intensity is observed in this mode before its extinction at an upstream location. In the oscillating flame propagation mode, the flame front anchors itself at an axial location and exhibits periodic oscillations in the transverse direction without extinction. In the wavy flame mode the flame anchoring happens at a location close to the downstream end of the channel. The flame front exhibits visibly irregular fluctuations, while anchored at this axial location. A Fast Fourier transform analysis of the flame intensity data shows that FREI and oscillating FREI modes consist of a single dominant frequency of ∼100 Hz, whereas multiple dominant frequencies are present for oscillating and wavy flame modes. The appearance of these multiple oscillating and propagating flame modes is attributed to flame bifurcation behavior due to thermal-wall coupling in the channel.

Non-premixed combustion characteristics and thermal performance of a catalytic combustor for micro-thermophotovoltaic systems

Micro-combustors with reliable flame stability and high surface temperature are crucial for micro-thermophotovoltaic devices. Recently, we verified the feasibility of catalytic combustion for non-premixed CH4/air in an asymmetrical planar micro-combustor. The upper limit of operational average velocity is 1.2 m/s and the maximum combustion efficiency is 52.4%. In the present work, a symmetrical configuration was proposed to improve the performance of the prototype combustor. Numerically investigation shows that the symmetrical combustor can be operated at an average velocity above 9 m/s, and the maximum combustion efficiency and surface radiation efficiency are 98.5% and 25.9%, respectively. In addition, as the average velocity is increased, the combustion efficiency decreases monotonically while the radiation efficiency and radiant energy output increase first and then decrease. Moreover, the combustion efficiency achieves its peak value under stoichiometric ratio. The accumulation of O2 near the Pt surface under fuel lean condition causes a sharp increase in O(s) coverage and a decrease in heat release rate. Furthermore, a larger wall thermal conductivity leads to more uniform wall temperature distribution and better heat recirculation effect on the incoming gases. However, a medium thermal conductivity of 1–10 W/(m·K) favors a larger radiant energy output.

NOx emission and thermal performances studies on premixed ammonia-oxygen combustion in a CO2-free micro-planar combustor

Unlike hydrocarbon fuel, ammonia is a carbon-free and renewable energy source. It is also regarded as one of the potential energy carriers. However, ammonia combustion for power generation is not well studied under micro-scale conditions, especially concerning nitrogen oxides (NOx) emission. For this, thermal performances and NO emission characteristics of premixed ammonia/oxygen combustion are numerically investigated on a micro-planar combustor. The effects of 1) the equivalence ratio ϕ, 2) inlet temperature Tin and 3) inlet pressure Pin are examined. The outer wall mean temperature (OWMT) is found to vary non-monotonically, as the mixture varies from lean to rich conditions, with the peak occurring at ϕ = 0.9 mainly due to the optimal heat transfer performance. However, a low ϕ could lead to the high nitric oxides (NO) concentrations because of the high flame temperature as well as O atom concentrations. Up to 75.5% of NO reduction could be achieved, as ϕ is optimized. Furthermore, increasing Tin is shown to be associated with a low OWMT and NO concentration. In addition, varying Pin is shown to lead to not only OWMT being changed but also NO formation being mitigated. The decrease in NO concentration for a high Pin is mostly attributable to the short residence time of a high flame temperature in the channel and low OH concentrations. This work reveals that optimizing the operating thermodynamic parameters is an effective means to reduce NO emissions and to improve thermal performances.

Bluff-body effect on thermal and NOx emission characteristics in a micro-planar combustor fueled with premixed ammonia-oxygen

In this work, we examine the bluff body effect on thermal and NOx emission performance in a micro-planar combustor fueled with premixed ammonia/oxygen using three-dimensional numerical simulations. For this, a three-dimensional model is developed with a detailed chemical kinetic mechanism. The effects of 1) the presence/absence of the bluff-body, 2) its dimensionless height h and 3) its dimensionless axial location l are examined. It is found that the implementation of a bluff-body can significantly affects the thermal and NO emission characteristics. Up to 39.1 % reduction of NO formation and 41.6 K increase in the outer wall temperature (OWT) can be achieved as the bluff-body is optimized, when compared to the case in the absence of the bluff-body. Furthermore, increasing h leads to OWT being increased, but it can suppress the NO generation in some cases. In addition, increasing l is shown to be involved with a high OWT and a low NO concentration, mainly due to the variation in the flow field and thus heat transfer characteristics. Finally, the dimensionless pressure loss is strongly affected by the NH3 flow rate, and the dimensionless parameters h and l. This work reveals that implementing a proper designed bluff-body is an effective way to enhance thermal performance and reduce NOx emission.

Effect of hydrogen addition on conjugate heat transfer in a planar micro-combustor with the detailed reaction mechanism: An analytical approach

In this study, a steady state analytical investigation of conjugate heat transfer in a planar micro-combustor is presented by considering the detailed reaction mechanisms for a methane/air mixture with 10% and 20% hydrogen addition. The primary objective is studying the effects of hydrogen addition on the wall and gas temperature distribution in order to propose a practical solution to manage the significant heat transfer in micro-combustors. The reactive mixture is divided into the pre-flame, reaction, and post-flame zones. Then, the conservation equations are analytically solved in each zone using the matching conditions. Moreover, to present a general analysis, appropriate non-dimensional thermal parameters are recommended considering the thermal interaction between the reactive mixture, solid structure and ambient. As a result, appropriate correlations for the normalized wall temperature profile are presented for different situations that can be used as a prescribed wall temperature distribution in numerical simulations. Moreover, it is found that for the cases with solid-fluid thermal diffusion ratio greater than 50, the thermal properties can negate the effect of hydrogen addition on the wall temperature distribution.

Developing mathematical modeling of the heat and mass transfer in a planar micro-combustor with detailed reaction mechanisms

Developing mathematical modeling of the heat and mass transfer in a planar micro-combustor with detailed reaction mechanisms

Effects of wall thickness and material on flame stability in a planar micro-combustor

Flame is prone to lose its stability in micro-combustors due to the large amount of heat loss from the external walls. On the other hand, heat recirculation through the upstream combustor walls can enhance flame stability. These two aspects depend on the structural heat transfer, which is associated with the thickness and thermal conductivity of the combustor walls. In the present study, the effects of wall thickness and material on flame stability were numerically investigated by selecting two thicknesses (δ=0.2 and 0.4 mm) and two materials (quartz and SiC). The results show that when δ=0.2 mm, flame inclination occurs at a certain inlet velocity in both combustors, but it happens later in SiC combustor. For δ=0.4 mm, flame inclination still occurs in quartz combustor from a larger inlet velocity compared to the case of δ=0.2 mm. However, flame inclination in SiC combustor with δ=0.4 mm does not happen and it has a much larger blowout limit. Analysis reveals that a thicker wall can enhance heat recirculation and reduce heat loss simultaneously. Moreover, SiC combustor has larger heat recirculation ratio and smaller heat loss ratio. In summary, the micro-combustor with thicker and more conductive walls can harvest large flame stability limit.

A numerical investigation on non-premixed catalytic combustion of CH4/(O2 + N2) in a planar micro-combustor

Non-premixed catalytic combustion in micro-combustors has not been reported so far. The present study demonstrates the feasibility of non-premixed CH4/(O2 + N2) catalytic combustion in a planar micro-combustor with a height less than the quenching distance. The impacts of inlet velocity, combustor height, nominal equivalence ratio and O2 concentration on the combustion characteristics were numerically investigated. First, it is found that with the increase of inlet velocity, the catalytic reaction zone moves downstream and the reaction intensity increases first and then decreases. Second, as the combustor height is increased, the catalytic reaction zone broadens and the largest heat release rate decreases, while the maximum temperature and combustion efficiency show non-monotonic variations. Thirdly, a slightly fuel-rich condition can increase Pt(s) coverage, enhance catalytic reaction on the oxidant side, and improve combustion efficiency. Finally, oxygen enrichment can significantly enhance both heterogeneous and homogeneous reactions, leading to a dramatic improvement in combustion efficiency.