Combustion behavior and stability map of hydrogen-enriched oxy-methane premixed flames in a model gas turbine combustor

The article describes an experimental study and comparison of the combustion behavior and determines the stability map of turbulent premixed H₂-enriched oxy-methane flames in a model gas turbine combustor. Static stability limits, in terms of flashback and blow-out limits, are recorded over a range of hydrogen fraction (HF) at a fixed oxygen fraction (OF) of 30% and a particular inlet bulk velocity, and the results are compared with the non-enriched case (HF = 0%). The static stability limits are also recorded for different inlet bulk velocity (4.4, 5.2, and 6 m/s) and the results are compared to explore the effect of flow dynamics on operability limits of H₂-enriched flames. The stability maps are presented as a function of equivalence ratio (0.3–1.0) and HF (0%–75%) plotted on the contours of adiabatic flame temperature (AFT), power density (PD), inlet Reynolds number (Re) and reacting mixture mass flow rate ($\dot{m}$) to understand the physics behind flashback and blow-out phenomena. The results indicated that both the flashback and blow-out limits tend to move towards the leaner side with increasing HF due to the improved chemical kinetics. The stability limits are observed to follow the Reynolds number indicating its key role in controlling flame static stability limits. The results showed that H₂ enrichment is effective in the zone from HF = 20% up to HF = 50%, and O₂ enrichment is also effective in a similar zone from OF = 20% up to 50%, with wider stability boundaries for H₂ enrichment. Axial and radial temperature profiles are presented to explore the effect of HF on the progress of chemical reactions within the combustor and to serve as the basis for validation of numerical models. Flame shapes are recorded using a high-speed camera and compared for different inlet velocities to explore the effects of H₂-enrichment and equivalence ratio on flame stability. The equivalence ratio at which a transition of flame stabilization from the inner shear layer (ISL) to the outer recirculation zone (ORZ) occurs is determined for different inlet bulk velocities. The value of the transition equivalence ratio is found to decrease while increasing the inlet bulk velocity. Flame shapes near flashback limit, as well as near blow-out limit, are compared to explore the mechanisms of flame extinctions. Flame shapes are compared at fixed adiabatic flame temperature, fixed inlet velocity and fixed flow swirl to isolate their effects and investigate the effect of kinetic rates on flame stability. The results showed that the adiabatic flame temperature does not govern the flame static stability limits.     

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.     

Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body

Combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body were numerically investigated. Results reveal that the blow-off limit of the micro-combustor with a regular triangular pyramid bluff body is 2.4 times of that in the micro-combustor without bluff body. With the increase of inlet velocity, the recirculation zone expands and preferential transport effect behind the bluff body is intensified. Therefore, the local equivalence ratio in the recirculation zone increases when Φ = 0.8, but the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds 10 m/s. When Φ < 1.0, adding small amount of hydrogen into gas mixture can speed up the significant elementary reaction, leading to an increase of methane conversion. It's found that both the methane conversion rate and the temperature behind the bluff body reaches the highest when blockage ratio increase to 0.22.     

Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a micro-combustor with a bluff body

Combustion characteristics of lean hydrogen/air mixture in a planar micro-channel with a bluff body were investigated experimentally and numerically. Effects of the inlet velocity and equivalence ratio on the blow-off limit, combustion efficiency and exhaust gas temperature were examined. The results show that the blow-off limit is greatly extended as compared with that of the micro-combustor without a bluff body. Moreover, the blow-off limit increases as the equivalence ratio is increased from 0.4 to 0.6. Furthermore, with the increase of inlet velocity, the flame front is prolonged and becomes narrower, and the high temperature segment of outer wall shifts downstream. In addition, the combustion efficiency and exhaust gas temperature increase first and then decrease with the increase of the inlet velocity. Finally, comparatively high combustion efficiency can be maintained over the whole combustible velocity range at a moderate equivalence ratio.     

The impact of channel gap distance on flame splitting limit of H2/air mixture in microchannels with wall cavities

We recently confirmed the “tip open phenomenon” of lean H₂/air flame in a microchannel with cavities. The critical inlet velocity when fuel conversion ratio drops to 80% was defined as “flame splitting limit”. In the present work, we numerically studied the impact of channel gap distance. Results showed that corresponding limits for 1.0-mm, 0.8-mm and 0.6-mm channels are 26 m/s, 33 m/s and 16 m/s respectively, exhibiting a non-monotonic dependence. The analysis reveals that when the gap distance is decreased from 1.0 mm to 0.8 mm, the proportion of fuel that involved into the cavities is increased, flame length is reduced simultaneously, and better preheating of the fresh mixture is attained. These positive effects lead to an increase in flame splitting limit. As the gap distance is further reduced to 0.6 mm, the excessive stretch effect results in complete extinction of downstream flame, causing a decrease of the splitting limit.     

Numerical study of methane and air combustion inside a small tube with an axial temperature gradient at the wall

A numerical study on CH₄ and air premixed combustion inside a small tube with a temperature gradient at the wall was undertaken to investigate the effects of inlet velocity, equivalence ratio and combustor size on combustion characteristics. The simulation results show that the inlet velocity has a significant influence on the reaction zone, and the flame front shifts downstream as the inlet velocity increases. The results also show that, the inlet velocity has no obvious effects on the flame temperature. The highest flame temperature is obtained if the equivalence ratio is set to 1. It is disclosed that the combustor size strongly influences the combustion characteristics. The smaller the combustor size is, the more difficult it is to maintain the steady combustion. The smallest combustor size that the stable flame can be sustained is determined mainly by the wall temperature of the micro-combustor under the given conditions. The higher the wall temperature is, the smaller the smallest combustor size. Therefore increasing wall temperature is an effective way to realize flame stabilization for a given combustor size.     

Combustion and heat release characteristics of hydrogen/air diffusion flame on a micro-jet array burner

Hydrogen (H₂) is considered as a carbon-free alternative fuel. The heat release characteristics of H₂ flame as a key parameter in its combustion process are unclear. In this study, the combustion and heat release characteristics of H₂/air diffusion flame on a micro-jet array burner were experimentally and numerically investigated. It is shown that the OH distribution and flame length based on Bilger mechanism are in good agreement with the experimental results. Furthermore, the intensity and distribution of OH and heat release rate can be adjusted by the thermal power and equivalence ratio. A uniform flame with intensive heat release rate can be achieved at a thermal power of 0.1 kW. R41: H + O₂ = OH + O and R43: H + O₂ + M = HO₂ + M are the main reactions with oxidizer consumption to form reactive radicals. R40: OH + H₂ = H₂O + H and R47: OH + OH = O + H₂O with OH consumption are the main heat release reactions at the upstream and downstream of the flame.     

Effect of multiple bluff bodies on hydrogen/air combustion characteristics and thermal properties in micro combustor

The bluff body is commonly used to improve micro combustion. The micro combustor with multiple rectangular bluff bodies in a single row was proposed. The effects of bluff bodies on H₂/air combustion characteristics were numerically studied. The temperature distributions, ignition position, combustion efficiency and blow-out limit were investigated via changing the total width and number of bluff bodies. The results show that the combined use of multiple bluff bodies can further expand the blow-out limit of H₂/Air. The effect of high temperature and viscous force on the flow velocity is main factors for the flame morphology. When the total width of bluff bodies is 2 mm, the blow-out limit decreases with the increase of bluff body number. When the total width of bluff bodies is 4 mm and 6 mm, the blow-out limit increases with the increase of the number of bluff bodies. With the increase of inlet velocity, the complete combustion efficiency decreases. The combustion efficiency in the combustor with wider blow-out limit decreases more slowly. It indicates that the combustor with multi-bluff bodies is more suitable for the operation conditions with high flow velocity.     

Numerical study on methane/air combustion characteristics in a heat-recirculating micro combustor embedded with porous media

Micro-combustor is a portable power device that can provide energy efficiently, heat recirculating is considered to be an important factor affecting the combustion process. For enhancing the heat recirculating and improving the combustion stability, we proposed a heat-recirculating micro-combustor embedded with porous media, and the numerical simulation was carried out by CFD software. In this paper, the effect of porous media materials, thickness and inlet conditions (equivalence ratio, inlet velocity) on the temperature distribution and exhaust species in the micro combustor are investigated. The results showed that compared with the micro combustor without embedded porous media (MCNPM), micro-combustor embedded with porous media (MCEPM) can improve the temperature uniformity distribution in the radial direction and strengthen the preheating capacity. However, it is found that the embedding thickness of porous media should be reasonably arranged. Setting the thickness of porous media to 15 mm, the combustor can obtain excellent comprehensive capacity of steady combustion and heat recirculating. Compared the thermal performance of Al₂O₃, SiC, and ZrO₂ porous media materials, indicating that SiC due to its strong thermal conductivity, its combustion stabilization and heat recirculating capacity are obviously better than that of Al₂O₃ and ZrO₂. With the porous media embedded in the micro combustor, the combustion has a tempering limit of more than 10 m/s, and the flame is blown out of the porous media area over 100 m/s. The reasonable equivalence ratio of CH₄/air combustion should be controlled within the range of 0.1–0.5, and “super-enthalpy combustion” can be realized.     

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.     

A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: Design and modelling

Currently, combustion-based micro power devices encounter the problem of low conversion efficiency. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor is proposed, because thermophotovoltaic (TPV) cells and thermoelectric (TE) modules work at different temperature levels. The system consists of a planar micro-combustor with a bended extension at the exit, two GaSb TPV modules to convert high temperature thermal radiation and two Bi–Te based TE modules attached to the bended extension to harness medium temperature thermal energy. The mathematical modelling approach to quantify the power output and conversion efficiency is systematically presented. The modelling results show that the integration of the TE modules could significantly improve the system efficiency. When burning the H₂/air mixture, the overall system efficiency could reach 2.5% under the flow condition of U₀ = 3 m/s and Φ = 1.0. Finally, measures for better thermal management to further enhance the conversion efficiency are discussed.     

Experimental investigation of ignition and combustion characteristics of single coal and biomass particles in O2/N2 and O2/H2O

Oxy-steam combustion is a potential new-generation option for CO₂ capture and storage. The ignition and combustion characteristics of single coal and biomass particles were investigated in a flow tube reactor in O₂/N₂ and O₂/H₂O at various oxygen concentrations. The ignition and combustion processes were recorded using a CCD camera, and the two-color pyrometry was used to estimate the volatile flame temperature and char combustion temperature. In O₂/N₂ and O₂/H₂O, coal ignites heterogeneously at <O₂> = 21–50%. In O₂/N₂, biomass ignites homogeneously at <O₂> = 21–30%, while it ignites heterogeneously at <O₂> = 40–50%. In O₂/H₂O, biomass ignites homogeneously at <O₂> = 21–50%. With increasing oxygen concentration, the ignition delay time, volatile burnout time and char burnout time are decreased, and the volatile flame temperature and char combustion temperature are increased. At a certain oxygen concentration in both atmospheres, the ignition delay time, volatile burnout time and char burnout time of biomass are shorter than those of coal. Moreover, biomass has a higher volatile flame temperature but a lower char combustion temperature than coal. The ignition delay time, volatile burnout time and char burnout time in O₂/H₂O are lower than those in O₂/N₂ for coal and biomass. The presence of H₂O can improve the combustion rates of coal and biomass. The volatile flame shows a lower temperature in O₂/H₂O than in O₂/N₂ at <O₂> = 21–50%. The char combustion shows a lower temperature in O₂/H₂O than in O₂/N₂ at <O₂> = 21–30%, while this behavior is switched at <O₂> = 40–50%. The results contribute to the understanding of the ignition and combustion characteristics of coal and biomass in oxy-steam combustion.     

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.     

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.     

Effects of inlet parameters on combustion characteristics of premixed CH4/Air in micro channels

In order to improve the stability of combustion and the working performance of the micro-combustor, this paper focuses on the effects of inlet parameters such as inlet temperature, mass flow rate and equivalence ratio. Three micro-combustors with different channel-heights are fabricated, and the three-dimensional calculation model is built to research on the combustion characteristics of premixed CH₄/Air mixture. It was found that with inlet gas mixture temperature increasing, the flammability limits of the combustion under micro-scale conditions were expanded, and the channel height under which the flame can exist in the combustor reduced from 3.0 mm to 2.0 mm after preheating. On the preheating basis, increasing the equivalence ratio of the gas mixture Ф improved the intensity of gas-phase reaction due to the simulation of the important free radical like OH. Furthermore, when the inlet mass flow rate of methane $\stackrel{?}{m_{c{h}_4}}$was increased and between $\stackrel{?}{m_{1c{h}_4}}$and $\stackrel{?}{m_{5c{h}_4}}$, it shows that the external wall temperature was higher in the micro-combustor of H = 2.5 mm compared with that of H = 3.0 mm. When in the micro-combustor of H = 3.0 mm, more fuel could be burned and $\stackrel{?}{m_{\max c{h}_4}}$ = 2.85 × 10^?6 kg/s.     

Experimental and numerical investigation of stability and emissions of hydrogen-assisted oxy-methane flames in a multi-hole model gas-turbine burner

This paper reports experimental and numerical study of stability and combustion characteristics of premixed oxy-methane flames with hydrogen-enrichment (CH₄–H₂/O₂–CO₂ flames) in a model multi-hole burner for clean energy production in gas turbines. The combustor lean blow-out (LBO) limit was presented on an equivalence ratio (Ø) - hydrogen fraction (HF: volumetric fraction of H₂ in a mixture of H₂+CH₄) map spanning over Ø-values of 0.1–1 and HF-values of 0–70% at fixed hole jet velocity and oxygen fraction (OF: volumetric fraction of O₂ in a mixture of O₂+CO₂) of 5.2 m/s and 30%, respectively. The condition of the combustion chamber is assumed to be depicted by the corrugated premixed flame regime. The premixed turbulent flame was modeled using the reaction progress variable flame front topology approach with the Large Eddy Simulation (LES) technique. The recorded combustor stability maps showed great resistance of the micromixer burner technology to flashback, recommending its use for stable gas turbine operation. The results show that H₂-enrichment widens the combustor operability limits (higher turndown ratio) by extending the LBO from Ø = 0.45 at HF = 0% down to Ø = 0.15 at HF = 70% with a slight reduction in the heat release factor by 0.1. The high reactivity and higher flame speed of H₂ ensures the sustenance of flame at lower equivalence ratios. At high equivalence ratios, H₂ addition enhances the reaction rates and makes both the primary and secondary reaction zones shorter and more intense. Increasing HF leads to increase in the Damköhler number (Da) and decrease in both the Karlovitz number (Ka) and flame thickness. The CO emission at the combustor outlet reduced significantly from 241 ppm at HF = 0% to 33.1 ppm at HF = 10%, then it increased back to 364 ppm at HF = 50%.     

A numerical investigation on combustion characteristics of H2/air mixture in a micro-combustor with wall cavities

Combustion characteristics of H₂/air mixture in a micro-combustor with wall cavities were investigated numerically. The effects of inlet velocity, equivalence ratio, and the length–depth ratio of the cavity were studied. The results show that at a high enough velocity the flame splits in the middle which leads to a large amount of fuel leakage and a sharp decrease in the conversion rate of hydrogen. Meanwhile, the flame splits at the inner wall which gives rise to two high temperature regions and double temperature peaks at outer wall. Moreover, the flame-splitting limit is extended at a higher equivalence ratio due to a more intensive reaction. Furthermore, the flame-splitting limit increases for a larger length–depth ratio of the cavity, whereas the wall temperature level decreases. Therefore, excessive large length–depth ratios are not beneficial for this type of micro-combustors if the combustor walls are used as heat sources of thermoelectric or thermal photovoltaic devices.     

Experimental study on superheated steam generation by the reaction of high humidity hydrogen and oxygen in a model internal combustion steam generator

Internal combustion steam cycle (ICSC) is a novel steam power cycle using hydrogen as an energy carrier to produce superheated steam. High humidity hydrogen produced during fast hydrogen production process is directly used to produce superheated steam by combusting with stoichiometric oxygen without hydrogen storage. The ICSC efficiency is greatly affected by the content of non-condensable gas in superheated steam. In the present study, superheated steam generation by high humidity hydrogen was investigated in a model internal combustion steam generator. Effects of H₂O/H₂ molar ratio of humid hydrogen and velocity ratio of humid hydrogen to oxygen on non-condensable gas content, combustion efficiency, and mixing rate were evaluated. The results showed that the critical H₂O/H₂ ratio for the humid hydrogen humidity limit was 2.8. With increasing velocity ratio, mixing rate and combustion efficiency increased under the same H₂O/H₂ ratio. The H₂O/H₂ reaction rate monotonously decreased as the H₂O/H₂ ratio increased from 1.0 to 2.5, while the mixing rate increased along with the velocity ratio. The combustion efficiency initially increased and subsequently decreased, and the peak value was reached at a H₂O/H₂ ratio of 1.75. This result indicated that the humid H₂-O₂ combustion was controlled by diffusion under H₂O/H₂ ratios of 1.0 to 1.75, but turned to be controlled by chemical kinetics when the H₂O/H₂ ratio ranged between 1.75 and 2.5.     

Large eddy simulation of reacting flow in a hydrogen jet into supersonic cross-flow combustor with an inlet compression ramp

Large eddy simulation (LES) has been performed to investigate transverse hydrogen jet mixing and combustion process in a scramjet combustor model with a compression ramp at inlet to generate shock train. Partially Stirred Reactor (PaSR) sub-grid combustion model with a skeleton of 19 reactions and 9 species hydrogen/air reaction mechanism was used. The numerical solver is implemented in an Open Source Field Operation and Manipulation (OpenFOAM) and validated against experimental data in terms of mean wall pressure. Effects of a shock train induced by the inlet compression ramp on the flame stabilization process are then studied. It can be observed that the interaction of the oblique shock and the jet mixing layer enhance the combustion and stabilize the flame. Symmetrical recirculation zone, which contributes to the flame anchoring of the supersonic transverse jet combustion, is observed in the near wall region of 10 < x/D < 20. The hydrogen fuel is transported from the center of jet plume to the near wall region on both sides of the central plane (z/D = 0) and thus intense combustion near the wall is observed due to the enhanced mixing and shock compression heating. Besides, the jet penetration in the reacting field is different from that in non-reacting case with the influence of the interaction between the reflected oblique shock and the jet shear layer on the windward side.     

Flame dynamics of hydrogen/air mixture in a wavy micro-channel

The focus of this work is the numerical study of stable and pulsatory flame burst in an undulating geometry, using premixed hydrogen and air (with an equivalence ratio of φ = 1.0). This work extends other works in the literature by considering a linear temperature profile along the wall. This allows an analysis of the flow dynamics without forcing the location of the flame (as is the case with hyperbolic temperature profiles). The interaction between the flow dynamics and the combustion reaction is then analysed, leading to a better understanding of the physics in more general flows.Simulations were performed in OpenFoam using very detailed chemical reactions and different molecular diffusivities for each species. The results obtained show that at low inlet velocity (4 m/s) the flame became stable, and, at higher inlet velocities, the flame showed pulsatory burst dynamics. The interaction between the fluid dynamics and the combustion response proved to be important, especially because of the vortices that are formed due to the nonlinear geometry of the burner. As the inlet velocity increases, the heat release rate transmitted through the vortices decreases and a delay in ignition occurs, as evidenced by a decrease in the pulsatory burst frequency and an increase in the maximum value of the heat release rate (although not sufficient to increase the maximum temperature amplitude).In addition, we also carried out an analyses of the axial velocity and of the H₂ and OH mass fractions of the flame dynamics.