Experimental investigation on the scale effects on diffusion H2/air flames in Y-shaped micro-combustors

In the present study, characteristics of diffusion H₂-air flame in three Y-shaped cylindrical micro-combustors with a diameter of d = 1, 2 and 3 mm were investigated experimentally. The mixture was ignited near the exit of a 200-mm long horizontal channel. First, it is found that more flame propagation modes appear in the combustor with d = 2 mm. Moreover, noise emission commences earlier in larger combustors. Two stages of noise emission are detected in the combustors with d = 2 and 3 mm, whereas only one stage appears under d = 1 mm. In addition, the mean flame propagation speed is the minimum in the combustor with d = 2 mm. Furthermore, the results show that the length of edge flame becomes longer in the combustor with a larger diameter. Interestingly, traveling isolated flame cells, which is also termed “traveling flame street”, is observed in the micro-combustors with d = 1 mm. Finally, the regime diagrams of flame propagation modes are drawn for the three micro-combustors. In conclusion, the present study not only reveals the relationship between flame characteristics and physical and geometrical parameters, but also provides useful guidance for the design of micro-combustors.     

Dependence of the blowout limit on flow structure,heat transfer,and pressure loss in a bluff-body micro-combustor

In this study, we numerically investigated the main factors affecting the flame blowout limit of bluff-body micro-combustors with a blockage ratio (δ) varying from 0.3 to 0.8; the combustion performance of these combustors is investigated as well. Measurements from the literature (Fan et al., 2012, Int J Hydrogen Energy, 37, 19,190–19,197) were used to validate the present numerical models. The predicted results are in good agreement with the experimental data, with a relative error of less than 10%. The results show that the blowout limit increases with δ in a non-monotonic manner. The establishment of a low-velocity recirculation behind the bluff body and recirculation of heat and key radicals help increase the flame blowout limit, whereas the stretching of reaction zones has an unfavorable effect. In contrast, heat loss contributes negligibly to the difference between the blowout limits in the micro-combustors for different values of δ. An extra pressure loss or initial power input is required to improve the blowout limits. An available size of the in-built bluff body should be carefully selected to maximize its efficiency and to considerably improve the blowout limit; however, this should be realized while ensuring a low cost of pressure loss when designing a micro-combustor. In general, δ = 0.5 is relatively optimal and recommended for the straight combustion channel discussed herein.     

Effects of different channels on performance enhancement of a nozzle hydrogen-fueled micro combustor for micro-thermophotovoltaic applications

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.     

Experiments on combustion regimes for hydrogen/air mixtures in a thin layer geometry

A series of experiments in a thin layer geometry performed at the HYKA test site of the KIT. Experiments on different combustion regimes for lean and stoichiometric H₂/air mixtures were performed in a rectangular chamber with dimensions of 200 × 900 x h mm³, where h is the thickness of the layer (h = 1, 2, 4, 6, 8, 10 mm). To model a gap between a fuel cell assembly and a metal housing, three different layer geometries were investigated: (1) a smooth channel without obstructions; (2) a channel with a metal grid filled 25% of chamber length and (3) a metal grid filled 100% of chamber length. The blockage ratio of metal grid has changed from 10 to 60% of cross-section. Detail measurements of H₂/air combustion behavior including flame acceleration (FA) and DDT in closed rectangular channel have been done. Five categories of flame propagation regimes were classified. Special attention was paid to analysis of critical condition for different regimes of flame propagation as function of layer thickness and roughness of the channel. It was found that thinner layer suppresses the detonation onset and even with a roughness, the flame may quench or, in thicker layer, is available to accelerate to speed of sound. The detonation may occur only in a channel thicker than 4 mm.     

Numerical investigation on mixing performance and diffusion combustion characteristics of H2 and air in planar micro-combustor

In the present work, the effects of inlet velocity and channel height (H₀ = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H₂ and air in a 2D planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H₀ = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H₀ = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.     

Effects of ultra-high injection pressure and micro-hole nozzle on flame structure and soot formation of impinging diesel spray

The effects of ultra-high injection pressure (P_inj = 300 MPa) and micro-hole nozzle (d = 0.08 mm) on flame structure and soot formation of impinging diesel spray were studied with a high speed video camera in a constant volume combustion vessel. Two-color pyrometry was used to measure the line-of-sight soot temperature and concentration with two wavelengths of 650 and 800 nm. A flat wall vertical to the injector axis is located 30 mm away from the injector nozzle tip to generate impinging spray flame. Three injection pressures of 100, 200 and 300 MPa and two injector nozzles with diameters of 0.16 and 0.08 mm were used. With the conventional injector nozzle (0.16 mm), ultra-high injection pressure generates appreciably lower soot formation. With the micro-hole nozzle (0.08 mm), impinging spray flame shows much smaller size and lower soot formation at the injection pressure of 100 MPa. The soot formation is too weak to be detected with the micro-hole nozzle at injection pressures of 200 and 300 MPa. With eliminating the impact of injection rate on soot level, both ultra-high injection pressure and micro-hole nozzle have an obvious effect on soot reduction. Soot formation characteristics of impinging spray flame were compared with those of free spray flame using both the conventional and micro-hole nozzles. With the conventional nozzle, flat wall impingement deteriorates soot formation significantly. While soot formation characteristics of free spray flame with the micro-hole nozzle are not altered obviously by flat wall. Liquid length of the 0.16 mm nozzle is longer than the impingement distance and liquid length of the 0.08 mm nozzle is shorter than the impingement distance. Liquid impingement upon the wall is responsible for the deteriorated soot level of impinging flame compared to that of free flame with the conventional nozzle.     

Fabrication and performance test of catalytic micro-combustors as a heat source of methanol steam reformer

Two different types of H₂ catalytic micro-combustors were fabricated and evaluated as a heat source of methanol steam reformer through MEMS fabrication technology with photosensitive glass wafers. In a packed-bed micro-combustor design, ceramic foam coated with Pt was the catalyst bed. In the thin-film-coated combustor, Al₂O₃ was used as catalyst supports and coated on the combustion chamber wall. Pt was coated on the Al₂O₃ thin-film, which was constructed on the wall. The preparation of Al₂O₃ coating solution and coating process was set up based on sol–gel method. Both combustors had a combustion chamber whose height was 1 mm and the external volume of combustors was 1.8 cm³. Catalytic combustion of H₂ was stable with both combustors. H₂ conversions were over 90% for packed-bed micro-combustor and over 99% for Pt/Al₂O₃ coated micro-combustor. Both combustors burned 80 ml/min of H₂. The catalytic micro-combustors fabricated were applicable to the methanol steam reforming system for 20 W level PEMFC.     

Dynamics of premixed hydrogen-air flame propagation in the duct with pellets bed

Hydrogen, as the promising clean alternative energy in the future, is in the spotlight now all over the world. However, its flammable and explosive hazards should be highly considered during its practical application. In this study, the experiments are performed to study premixed hydrogen-air flame propagation in the duct with pellets bed, especially for fuel-rich condition. High-speed schlieren photography is employed to capture flame front development during the experiments. As well as the pressure transducer, is used to track the pressure buildup in the flame propagation process. Different diameters of pellets and different concentrations of gas mixture are considered in this experimental study. The typical evolutions about the tulip flame are similar in all cases, although the tulip flame formation time caused by the laminar flame speed are different. The flame propagation velocity is pretty enhanced in fuel-lean mixture under the effect of large diameter pellets bed, but it is significantly suppressed in fuel-rich conditions. While for the small diameter pellets (d = 3 mm), the suppression effect on flame propagation and pressure is obtained over a wider range of equivalence ratios, especially a better suppression effect is generated near the stoichiometric condition.     

The effect of gravity and thermal expansion on the propagation of a triple flame in a horizontal channel

We investigate the effect of thermal expansion and gravity on the propagation of a triple flame in a horizontal channel with porous walls, where the fuel and oxidiser concentrations are prescribed. The triple flame therefore propagates in a direction perpendicular to the direction of gravity, a configuration that does not seem to have received any dedicated investigation in the literature. In particular, we examine the effect of the non-dimensional flame-front thickness ? on the propagation speed of the triple flame for different values of the thermal expansion coefficient α and the Rayleigh number Ra. When gravity is not accounted for (Ra = 0), and for small values of ?, the numerically calculated propagation speed is found to agree with predictions made in previous studies based on scaling laws [1]. We show that the well known monotonic relationship between U and ?, which exists in the constant density case when the Lewis numbers are of order unity or larger, persists for triple flames undergoing thermal expansion. Under strong enough gravitational effects (Ra ? 1), however, the relationship is no longer found to be monotonic. For a fixed value of ?, the relationship between the Rayleigh number and the propagation speed is shown to vary qualitatively depending on the value of ? chosen, exhibiting hysteresis if ? is small enough and displaying local maxima, local minima or monotonic behaviour for other values of ?. All of the steady solutions presented in the paper have been found to be stable, except for those on the middle branches of the hysteresis curves.     

A novel Swiss-roll micro-combustor with double combustion chambers: A numerical investigation on effect of solid material on premixed CH4/air flame blow-off limit

A novel Swiss-roll micro-combustor with double combustion chambers is proposed to improve flame stability and extend blow-off limits. This study is aimed to numerically investigate the effect of solid material (i.e., SiC, stainless steel and copper) on premixed CH₄/air flame blow-off limit and reveal the flame stability mechanism. The simulated results show that this developed novel Swiss-roll micro-combustor not only can significantly anchor the flame owing to the flow recirculation behind the flame holders and the backward-facing steps, but also can further extend CH₄ blow-off limits owing to heat recirculation in the long Swiss-roll preheating channels. The three solid material micro-combustors present the relatively slight difference in the recirculation-zone size but the remarkably difference in heat recirculation and heat loss. Good heat recirculation and low heat loss rate are the dominant reason that is responsible for the differences of the blow-off limits in this micro-combustor. The stainless steel micro-combustor achieves the highest blow-off limits while the copper micro-combustor achieves the lowest blow-off limit. These deep insights can give some useful information to design a similar Swiss-roll micro-combustor.     

Parametric study of a laser ignited hydrogen–air mixture in a constant volume combustion chamber

Experiments were carried out in a constant volume combustion chamber (CVCC) to investigate flame kernel development and flame speed of hydrogen–air mixtures having different fuel–air ratios. A Q-switched Nd: YAG laser with 1064 nm wavelength and pulse duration of 6–9 ns was used for ignition by generating laser induced plasma inside the CVCC. In this study, laser induced ignition of hydrogen–air mixtures was investigated using different initial chamber filling pressures (P = 2.5 bar–10 bar) at different initial temperatures (373 K–523 K). A variable optical setup with converging lenses having different focal lengths (f = 100–250 mm) were used to position the plasma at various locations inside the CVCC. A high speed camera recorded the flame kernel development and a piezoelectric pressure transducer recorded the pressure–time history for all the experiments. The main objective of this study was to determine the dependence of combustion properties of laser ignited hydrogen–air mixtures on lasers, optical configurations and initial conditions prevailing in the CVCC.     

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.     

Influence of longitudinal obstacle spacing on the deflagration-to-detonation transition through a bank of obstacles in a hydrogen-air mixture

Flame acceleration and deflagration-to-detonation transition (DDT) in a channel containing an array of staggered cylindrical obstacles and a stoichiometric hydrogen-air mixture were studied by solving the fully-compressible reactive Navier-Stokes equations using a high-order numerical algorithm and adaptive mesh refinement. Four different longitudinal spacings (ls) of the neighboring obstacle rows (i.e., ls = 15.28, 19.1, 25.4, and 38.2 mm, corresponding to 1.2, 1.5, 2 and 3 times of obstacle diameter, respectively) were used to examine the effect of obstacle spacing on flame acceleration and DDT. The results show that the main mechanisms of flame acceleration and transition to detonation in all the cases studied are consistent. While the flame acceleration is caused by the growth of flame surface area in the initial stage, it is governed by shock-flame interactions in the later stage when shock waves are generated. The focusing of strong shocks at flame front is responsible for the initiation of detonation. It was found that the flame propagation speed and the DDT run-up distance and time are highly dependent on ls. Specifically, the flame acceleration declines as ls increases, since a larger ls leads to less disturbance of flow by obstacles per unit channel length. For detonation initiation, both the run-up distance and time increase with the increase of ls. It is interesting to note that the DDT distance and time increase significantly as ls increases from 19.1 mm to 25.4 mm. This is related to the slowdown of the increase rate of energy release over a period before DDT occurs under large ls condition, because every time the flame passes over an obstacle row the shock-flame interaction is delayed and numerous isolated pockets of unburned gas material are formed.     

Effect of bluff body shape on the blow-off limit of hydrogen/air flame in a planar micro-combustor

We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of bluff body shape on the blow-off limit was investigated with a detailed H₂/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.     

Turbulent flame structure characteristics of hydrogen enriched natural gas with CO2 dilution

This paper investigated the hydrogen enriched methane/air flames diluted with CO₂. The turbulent premixed flame was stabilized on a Bunsen type burner and the two dimensional instantaneous OH profile was measured by Planar Laser Induced Fluorescence (PLIF). The flame front structure characteristics were obtained by extracting the flame front from OH-PLIF images. And the turbulence-flame interaction was analyzed through the statistic parameters. The role of hydrogen addition as well as CO₂ dilution on the features of turbulent flame were revealed by those parameters. In this work, hydrogen fractions of 0, 0.2 and CO₂ dilution ratios of 0, 0.05 and 0.1 were studied. Results showed that hydrogen addition can enhance turbulent burning velocity ST/S_L through decreasing the scale of the finer structure of the wrinkled flame front, caused by the smaller flame instability scale. In contrast, CO₂ dilution decreased turbulent burning velocity S_T/S_L due to its inactive response to turbulence perturbation and larger flame wrinkles. For all flames, the probability density function (PDF) profile of the local curvature radius R shows a bias to positive value, resulted from the flame intrinsic instability. The PDF profile of R decreases with CO₂ dilution, while the value of local curvature radius corresponding to the peak PDF is larger. This indicates that larger wrinkles structure was generated due to CO₂ dilution, which leads to the decrease in S_T/S_L as a consequence. Hydrogen addition increases the flame volume and results in more intense combustion. CO₂ dilution has a decrease effect on flame volume for both X_H2 = 0 and X_H2 = 0.2 while the decrease is obvious at X_H2 = 0.2, Z_CO2 = 0.1. In all, hydrogen enrichment improves the combustion while CO₂ can moderate combustion. Therefore, adding hydrogen and CO₂ in natural gas can be a potential method for adjusting the combustion intensity in combustion chamber during the combustor design.     

Numerical investigation of the effect of slit-width on the combustion characteristics of a micro-combustor with a centrally slotted bluff body

The effect of slit-width on the combustion characteristics of a micro-combustor with a centrally slotted bluff body is numerically studied. The non-dimensional fractional slit-width (d) is varied in the range of 0.1–0.8. Simulations are performed for inlet velocities ranging from 6 m/s to 35 m/s. The effect of slit-width on the combustion efficiency is observed to be a function of the inlet velocity. At smaller inlet velocities, the combustion efficiency increases upon increasing the slit-width initially (d ≤ 0.4), decreases (d = 0.4 to 0.5) and monotonously increases from thereon. On the contrary, at higher values of inlet velocity, the combustion efficiency monotonously increases with the increasing slit-width. It is observed that the average exhaust gas temperature increases with the increasing slit-width, reaches a maximum at moderate slit-widths (d = 0.6 and 0.7) and then decreases. It is observed that the local exhaust gas temperature at moderate slit-widths for higher values of inlet velocities follows a bi-modal distribution. It is also observed that the operating range of the micro-combustor with moderate slit-width (d = 0.6 and 0.7) is limited by the phenomenon of longitudinal flame splitting.     

Influence of CH4/air injection location on non-premixed flame blow-off limits in a novel micro-combustor

Stable flame is still one of the challenging issues in the micro-combustors. Besides, the premixed flame may flash back under the large equivalence ratio, and the non-premixed combustion is an effective way to avoid the flame flash-back issue. In this work, a novel non-premixed CH₄/air micro-combustor with the flame holder, the backward-facing steps and the separated preheating channels is developed to avoid the flame flash-back issue and extend the flame blow-off limits. This paper investigates the influence of two different CH₄/air injection locations on the flame blow-off limits numerically and profoundly reveals the mechanisms in this micro-combustor. Results show that this novel configuration can considerably anchor the flame without the flame flash-back issue and significantly extend flammability owning to the flow & heat recirculations. Compared with the “Air_(out)/CH_4(in)” injection location, the “Air_(in)/CH_4(out)” injection location exhibits a comparatively larger size of recirculation, causing a more vital anchorage ability. In addition, after the preheating in the preheating channels, the CH₄/air mixture of the “Air_(in)/CH_4(out)” injection location presents a higher temperature level than that of the “Air_(out)/CH_4(in)” injection location. Consequently, compared to the “Air_(out)/CH_4(in)” injection location, the larger blow-off limits are attained for the “Air_(in)/CH_4(out)” injection location. These results provide new insights into the stable non-premixed flame and give valuable guidance for designing the similar non-premixed micro-combustor.     

Three-dimensional flame displacement speed and flame front curvature measurements using quad-plane PIV

In this work we demonstrate the use of a quad-plane particle image velocimetry technique in order to investigate the three-dimensional behavior of several important quantities for combustion research such as the flame displacement speed and the flame front curvature. In results from a premixed methane flame stabilized in a diffuser burner, a comparison of three-dimensional and two-dimensional data is made in order to critically analyze the error of the usually performed planar measurements. It is shown that two-dimensional measurements can only give an estimate of the real situation under certain circumstances, such as with mainly spherical structures in the flame and the perfect alignment of both the flame propagation and flow direction to the measurement plane. However, in turbulent flames, this alignment can never be achieved due to fluctuations stemming from turbulence. The application of two crossed planes leads to significant improvements and good agreement with the three-dimensional quantities can be observed, although no perfect match is achieved. Flame displacement speeds ranging from ?0.4 s_L to 4.5 s_L with a mean of 1.1 s_L were recorded, but were not correlated with the flame curvature or strain rate.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Experimental and numerical investigations of hydrogen–air premixed combustion in a converging–diverging micro tube

Experimental and numerical studies of hydrogen–air premixed combustion in a converging–diverging micro tube with inner diameters of the inlet, throat, and outlet of 2, 1, and 2 mm, respectively, have been performed to study the combustion and flame characteristics. The influences of the equivalence ratio (Φ) and inlet velocity (v_in) are investigated. The experiments reveal that the v_in range for stable combustion—between 3.4 and 41.4 m/s—was significantly expanded, particularly when Φ = 1.4. This effect can primarily be attributed to the converging–diverging structure. As Φ increased, both the wall and the flame temperatures exhibited an increasing–decreasing trend; the largest heat loss ratio occurred at Φ = 1.0. The ignition position initially moved upstream and then moved downstream. The flame thickness increased and then decreased, reaching its peak value at Φ = 1.2. The flame length decreased monotonously. As v_in increased, the wall temperature increased, the flame temperature decreased, and the flame moved downstream to grow thicker and longer.