Experimental investigation on flame pattern formations of DME-air mixtures in a radial microchannel

Flame pattern formations of premixed DME-air mixture in a heated radial channel with a gap distance of 2.5 mm were experimentally investigated. The DME-air mixture was introduced into the radial channel through a delivery tube which connected with the center of the top disk. With an image-intensified high-speed video camera, rich flame pattern formations were identified in this configuration. Regime diagram of all these flame patterns was drawn based on the experimental findings in the equivalence ratio range of 0.6-2.0 and inlet velocity range of 1.0-5.0 m/s. Compared with our previous study on premixed methane-air flames, there are several distinct characteristics for the present study. First, Pelton-wheel-like rotary flames and traveling flames with kink-like structures were observed for the first time. Second, in most cases, flames can be stabilized near the inlet port of the channel, exhibiting a conical or cup-like shape, while the conventional circular flame was only observed under limited conditions. Thirdly, an oscillating flame phenomenon occurred under certain conditions. During the oscillation process, a target appearance was seen at some instance. These pattern formation characteristics are considered to be associated with the low-temperature oxidation of DME.     

Numerical investigation on the self-ignition and combustion characteristics of H2/air in catalytic micro channel

The self-ignition of hydrogen/air is an important process in the micro thermophotovoltaic system. The transient numerical models of gas-phase reaction and catalytic reaction in the various catalytic micro combustors were built and verified. The self-ignition process of gas-phase reaction caused by catalytic reaction in the catalytic micro channel with conventional heat dissipation was studied. The self-ignition process could be divided into four stages, fuel diffusion stage - pure catalytic reaction stage - flame front moving stage - stable combustion stage. The ignition time and temperature limit at different inlet temperatures, inlet velocities and channel heights were analyzed. The results showed that the wall quenching effect, thermal effect and flame propagation effect are dominant at low temperature, medium temperature and high temperature respectively. The catalyst length and the mixture internal energy were the main factor at low inlet velocity and high inlet velocity respectively. The steady-state time was also studied in the various operation conditions. Finally, the catalytic combustion characteristics in the stable combustion stage were analyzed. The influence of inert section length, inlet temperature and inlet velocity on the maximum temperature and fuel conversion ratio were investigated.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Evolution of the ignition and combustion process at high velocity at the channel entrance

The article deals with the non-stationary self-ignition of the non-prepared in hydrogen-air mixture in supersonic flow for understanding the influence of the inlet flow velocity on the ignition mode. A series of experiments was carried out at significant change in the inlet velocity into the model channel at the identical flow parameter and equivalence ratio. At the velocity increase, the ignition region shifts downstream due to the change in the wave structure of the flow. Simultaneously, the significant increase of the propagation time of the flame front over the channel occurs that results in the growth of time to reach the stationary combustion regime. It is shown that a flame flashback is a necessary condition for the implementation of combustion at the supersonic flow velocity in the channel, regardless of the inlet velocity. Under identical inlet condition the increase of the Mach number leads to an acceleration of the transition to the subsonic combustion and channel choking.     

用光学纹影摄影术观察分析定容燃烧室内氢燃料的燃烧过程

论述了采用纹影摄影术和高速摄影法观察分析氢气和空气预混合燃料在定容燃烧室内的火花点火燃烧过程,定性地分析了预混合氢气燃料的火焰形态和变化过程,以及燃烧室内的初始压力和空燃比对火焰传播速度及其燃烧压力的影响,通过采用纹影摄影术方法,初步揭示了预混合氢气燃料在定容燃烧室内燃烧时火焰初期紊流产生的机理,以及由开始的层流状火焰发展到湍流状火焰的过程,研究结果表明,预混合氢气燃料燃烧的火焰传播速度及燃烧压力明显地受燃烧室内的初始压力和空燃比的影响。     

Experimental investigation of combustion characteristics and NOx formation of coal particles using laser induced breakdown spectroscopy

To study the mechanism of coal combustion and NO_x formation, the combustion of coal particles in different atmospheres (O₂/N₂, O₂/CO₂) with different O₂ concentrations was investigated using the CO₂ laser as a heat source. The spatial distribution of atoms and groups (e.g., H at 656.2 nm, O at 777.3 nm, CN at 388.3 nm) relating to the combustion flame were measured simultaneously using laser induced breakdown spectroscopy (LIBS). The residual energy was measured during the collection of LIBS spectra in the combustion process, which could be characterized the temperature profiles of combustion flame due to the positive correlation with temperature. The combustion stage could be clearly discriminated by the emission of H and CN, along with the flame temperature. The residual energy obtained in different atmospheres indicated that the impact of combustion atmosphere on flame temperature is greater in the char combustion stage rather than volatile combustion stage. It was determined from the temporal and spatial distribution of residual energy and CN intensity that a higher flame temperature leads to a higher concentration of CN. The correlation between the generation of CN and the NO_x formation was also investigated to show that the formation approaches of NO_x are similar in the O₂/CO₂ and O₂/N₂ atmospheres, while the fuel-N conversion paths are different between volatile combustion and char combustion stages. The measurement of temporal and spatial distributions of LIBS spectra with varying flame temperatures is significant in revealing the mechanism of coal-particle combustion and NO_x formation.     

High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air

In the present paper, direct numerical simulation (DNS) is performed to analyze the characteristic structures of a supersonic jet lifted hydrogen-air flame with Reynolds number of 22, 000, and Mach number of 1.2. The fuel consisting of 85% H₂ and 15% N₂ by volume is injected into hot co-flow air from a round orifice. Overall 975 million grids are used to compute the complex multi-scales phenomena. A Damköhler number and a flame index are defined to analyze combustion modes and the mixedness of the flame. Complicated characteristic elements of the supersonic jet lifted flame are observed, i.e. a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both fuel-lean and fuel-rich mixtures, and a flame region consisting of outer diffusion combustion and inner weaker premixed combustion in the far field. At the leading edge of the fame base, auto-ignition takes place primarily in the fuel-lean mixture where the mixedness mode is opposed. Downstream of the laminar flame base, the combustion becomes turbulent due to the intensified mixing of fuel and air, which results in the subequilibrium values of temperature and OH concentration. Detonation occurs near the sonic layer, and then sustains the combustion in higher dissipative mixture. The flame near the stochiometric condition keeps non-premixed, and the other non-premixed flame elements could be observed in the very fuel-rich region. Through the reacting field the premixed flame appears near the shear layer. The combustion intensity decreases in the far field where the inner non-premixed flame disappears gradually.     

The heat flux research in hydrogen supersonic combustor at Mach number of 4

The work is devoted to the study of the intensity of heat transfer in a supersonic combustion chamber at a Mach number of 4 under conditions of ignition and transition to intense combustion, including the transition to choking the channel. The experiments were carried out on a combustion chamber model in the connected pipeline mode with flow parameters in the channel close to flight conditions at Mach numbers 6–8. The experimental model is a rectangular channel with a flame holder in the form of backward facing step (BFS). Fuel injection was carried out in front of BFS on the top and bottom walls of the model through 8 circular holes, which were situated under the angles of 45° or 90°. It has been revealed that the choice of the fuel injection scheme leads to an increase in the level and a change in the distribution of the heat flux along the length of the combustion chamber. A decrease in the angle of hydrogen injection makes it possible to significantly reduce the heat flux into the wall of the combustion chamber, while choking the channel is accompanied by a twofold increase in the heat flux.     

The effect of a perforated plate on the propagation of laminar hydrogen flames in a channel – A numerical study

Laminar hydrogen flame propagation in a channel with a perforated plate is investigated using 2D reactive Navies-Stokes simulations. The effect of the perforated plate on flame propagation is treated with a porous media model. A one step chemistry model is used for the combustion of the stoichiometric H₂–air mixture. Numerical simulations show that the perforated plate has considerable effect on the flame propagation in the region downstream from the perforated plate and marginal effect on the upstream region. It is found to squeeze the flame front and result in a ring of unburned gas pocket around the flame neck. The resulting abrupt change in flow directions leads to the formation of some vortices. Downstream of the perforated plate, a wrinkled “M”-shape flame is observed with “W” shape flame speed evolution, which lastly turns back to a convex curved flame front. Parametric studies have also been carried out on the inertial resistance factor, porosity, perforated plate length and its location to investigate their effects on flame evolution. Overall, for parameter range studied, the perforated plate has an effect of reducing the flame speed downstream of it.     

Propagation of hydrogen–oxygen flames in Hele-Shaw cells

Flame propagation in Hele-Shaw cells with a micro-sized gap was experimentally investigated. The evolution of flame front morphology was recorded via Schlieren photographs as the hydrogen-oxygen (H₂–O₂) mixture was ignited at ambient temperature and pressure. By varying gap size, two different regimes of flame propagation are identified: 1) the non-accelerating flame in narrow gaps; 2) the self-accelerating flame in relatively wide gaps. For the former, the initial flame front is globally circular, and subsequently evolves into branches separated from the surface, exhibiting dendritic-growth and fingering shapes. In the latter regimes, the flame front exhibits a cellular structure and accelerates nearly sonic speed due to hydrodynamic instabilities. It is found that the flame acceleration depends non-monotonically on the gap size due to the competing mechanisms of viscosity friction and heat loss through the walls. The effect of equivalence ratio on the non-accelerating flame is studied to identify the mechanism controlling the local extinction flame.     

Hysteresis in flame stabilization in a hydrogen fueled supersonic combustor

Hysteresis in flame stabilization mode transitions in a hydrogen-fueled strut-stabilized supersonic combustion test rig was experimentally observed and studied. Air was vitiated using H₂–O₂ combustion products to stagnation conditions of 8.65 bar and 1350 K and was expanded through a rectangular nozzle to Mach number 2.5. H₂ fuel was injected transversely using a strut positioned at the center of the combustor. The equivalence ratio (ER) was changed in time to study its effects on flame stabilization modes. Shadowgraph and wall pressure measurements were used to study the shock system generated by the strut in the supersonic combustor. High-speed OH1 chemiluminescence and high-speed flame imaging were used to study the heat release zones and flame structure of different combustion modes and transitions between them. Three different combustion modes were observed, namely: divergent section flame (CM1), strut wake stabilized flame (CM2), and jet stabilized flame (CM3). CM1 was observed at a very low ER, where the H₂ was ignited by the normal shock positioned in the divergent section. At this point, the weak shock system at the strut is unable to ignite the fuel. At higher ER, CM2 was observed, as a stronger shock system ignites the richer mixture at the wake of the strut. It was observed that the mixture auto-ignites in the strut wake and doesn't flashback from the divergent section. When the ER is further increased, the stronger injection shock reduces the local velocity and increases the static temperature, enhancing the flame speed of the richer mixture. Thus, the flame flashes back to the fuel jet. Two hysteresis were observed in the supersonic combustor based on ER as a time-varying input. The flame stabilization mode has two solutions based on the history of the change in ER, hence indicating hysteresis. The hysteresis between CM1 and CM2 is because of the retention of the temperature and radicals in the recirculation zone at the wake of the strut. The hysteresis between CM3 and CM2 is because of the retention of the temperature and radicals in the horseshoe vortices around the fuel jets. Understanding hysteresis will help design scramjets with wider operability.     

Study of the combustion efficiency of polymers using a pyrolysis–combustion flow calorimeter

The combustion efficiency of various polymeric materials was studied using a pyrolysis–combustion flow calorimeter (PCFC). Decreasing the combustion temperature in a PCFC leads to partial combustion and lower heat release rates. Combustion efficiency versus combustion temperature was modeled using a phenomenological equation and model parameters were related to the chemical structures of eight pure polymers. The flame inhibition effect was evaluated for two classical approaches in flame retardancy by plotting the combustion efficiency versus the combustion temperature. In the first one (the reactive approach), polystyrenes with different chemical groups substituted on the aromatic ring were studied. In the second one (the additive approach), three well-known flame retardants were incorporated into an ABS matrix: ammonium polyphosphate, tetrabromobisphenol A (TBBA), and a TBBA/antimony trioxide system. Results confirm the flame inhibition effect of halogenated compounds in both approaches. Finally, a correlation between peaks of heat release rate (pHRR) in a cone calorimeter and in a PCFC was attempted. Predicting pHRR in a cone calorimeter using a PCFC appears possible when no barrier effect is expected, if PCFC tests are carried out at a precise combustion temperature, for which the combustion efficiencies in both tests are the same.     

Simulations of combustion oscillation and flame dynamics in a strut-based supersonic combustor

This paper numerically investigated the dynamic characteristics of combustion in a model scramjet. Three-dimensional compressible large eddy simulation was performed on a hydrogen fueled combustor and pressure fluctuations were recorded. The analysis of pressure data showed that the combustion processes are intrinsically unstable under supersonic air inflow conditions. Flame dynamics were convinced by the fluctuations in flame lift-off distance away from the strut base. Combined with the corresponding time interval, instantaneous flame speed was calculated. Results indicated that pressure oscillations at different locations show difference in amplitude, frequency, and the underlying control mechanism. Flame front oscillation analysis showed that the flame–shock interaction in the strut recirculation zone was responsible for the combustion instability. Flame dynamics were compared with low-speed turbulent lifted flames. The transition between flame propagation just after the strut and shock-induced combustion in the subsonic bubble at the intersection of two wall-reflected oblique shocks made for the flame stabilization.     

Effect of hydrogen addition on the dynamics of premixed C1–C4 alkane-air flames in a microchannel with a wall temperature gradient

The effect of hydrogen (H₂) addition on the flame dynamics of premixed C₁–C₄ alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H₂ mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH₄/air mixture shows a higher HRR than C₃–C₄ alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH₄/air mixture, whereas it decreases monotonically for C₃H₈/air and C₄H₁₀/air mixtures with H₂ addition. The amount of HRR and flame propagation velocity decreases with increase in H₂ addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH₄/air mixture, which disappears due to H₂ addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H₂ in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel.     

The role of shock-flame interactions on flame acceleration in an obstacle laden channel

Flame acceleration was investigated in an obstructed, square-cross-section channel. Flame acceleration was promoted by an array of top and bottom surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. This work is based on a previous investigation of the effects of blockage ratio on the early stage of flame acceleration. This study is focused on the later stage of flame acceleration when compression waves, and eventually a shock wave, form ahead of the flame. The objective of the study is to investigate the effect of obstacle blockage on the rate of flame acceleration and on the final quasi-steady flame-tip velocity. Schlieren photography was used to track the development of the shock-flame complex. It was determined that the interaction between the flame front and the reflected shock waves produced from contact of the lead shock wave with the channel top, channel bottom, and obstacle surfaces govern the late stage of flame acceleration process. The shock-flame interactions produce oscillations in the flame-tip velocity similar to that observed in the early stage of flame acceleration, but only much larger in magnitude. Eventually the flame achieves a globally quasi-steady velocity. For the lowest blockage obstacles, the velocity approaches the speed of sound of the combustion products. The final quasi-steady flame velocity was lower in tests with the higher obstacle blockage. In the quasi-steady propagation regime with the lowest blockage obstacles, burning pockets of gas extended only a few obstacles back from the flame-tip, whereas burning pockets were observed further back in tests with the higher obstacle blockage.     

Unstretched unburned flame speed and burned gas Markstein length of diluted hydrogen/air mixtures

Fundamental combustion characteristics of H₂/air flames with the addition of actual H₂/air combustion residuals (a mixture of 65% N₂ + 35% H₂O by mole) are examined experimentally and numerically at 1–2 bar, 373–473 K, equivalence ratio of 0.7, and dilution ratios of 0–40%. Spherically expanding flame measurements at constant pressure show that flame speed and adiabatic flame temperature drop almost linearly with increasing diluent level. Detailed numerical simulations and analyses of sensitivity coefficients reveal that this is because of the low chemical reactivity of the dilution mixture. On the other hand, the change in burned gas Markstein length with the dilution mixture addition is found more complex and cannot be represented with a linear trend. Experimental flame speed data are compared with results of chemical kinetic analyses obtained by several chemical mechanisms in order to assess the accuracy of these models.     

Experimental studies on dynamics of methane–air premixed flame in meso-scale diverging channels

The propagation characteristics of a laminar premixed flame front in meso-scale straight and diverging channels of 5°, 10° and 15° with inlet dimension of 25 mm × 2 mm are reported in this paper. The downstream part of the channels was heated with an external heat source, to maintain a positive wall temperature gradient along the direction of fluid flow. These investigations show that planar flames are observed near flash back limits. Negatively stretched flames were observed for moderate flow rates and rich mixtures and for high flow rates, flames were positively stretched. These flames were either symmetric or asymmetric in nature. Partially stable flames were observed at high velocities for rich mixtures, whereas for lean mixture partially stable flames were observed for all flow rates. All the divergent channels showed an improvement in high velocity limits compared with the straight channel for the same mixture. Planar flames observed in the experiments helped in determining the laminar burning velocities for these mixtures at different preheat temperatures. A co-relation of laminar burning velocity with mixture preheat temperature is also obtained for a stoichiometric methane–air mixture. This co-relation S_u/S_u,o = (T_u/T_u,o)^1.558 is in good agreement with the earlier co-relations.     

Experimental study on induced accelerated combustion of premixed hydrogen-air in a confined environment

The study on induced accelerated combustion of premixed hydrogen-air in a confined environment is of great significance for the efficient utilization of hydrogen energy in internal combustion engines. The accelerated flame induced by the orifice plate is more stable and easy to control, which is beneficial to achieve controlled and rapid turbulent combustion. In this work, the accelerated combustion process induced by the orifice plate, and the influence of the orifice structure and initial conditions on the flame propagation and combustion characteristics were investigated by constant volume combustion bomb and schlieren method. The results show that the combustion process induced by the orifice plate consists of three stages: the initial stage of propagation, the accelerated stage of the orifice plate, and the end combustion stage. The reduction in aperture induces greater turbulence intensity and increases the perturbation of the orifice plate to the flame, resulting in a substantial increase in flame propagation speed through the orifice plate. As the initial pressure and the equivalence ratio increase, the velocity of turbulent flame induced by the orifice plate and the change rate of the velocity before and after the orifice plate increase. As the initial temperature increases, the turbulent flame propagation velocity does not change much, and the velocity change rate before and after the orifice plate decreases. The effect of the initial conditions on flame acceleration induced by the orifice plate is essentially the influence of flame propagation speed and instability. The greater the flame propagation speed and the stronger the flame instability, the stronger the induced turbulence and the greater the influence of the turbulent flow disturbance, and the greater the velocity of the turbulent flame induced by the orifice plate. There exists an optimum aperture for the shortest combustion duration at any initial conditions, but the optimal diameter is not sensitive to changes in initial conditions. The effect of orifice-induced combustion acceleration is remarkable, and the combustion durations induced by each orifice plate are shortened by more than 50%.     

Effect of hydrogen-air mixture diluted with argon/nitrogen/carbon dioxide on combustion processes in confined space

The effects of the dilution with inert gas on different combustion processes in confined space are investigated by utilizing a newly designed constant volume combustion bomb (CVCB) equipped with a perforated plate. Hydrogen-air mixture diluted with argon, nitrogen and carbon oxide of different proportions is employed in the present work. Combustion phenomena were all captured by high-speed Schlieren photography including flame propagation, compression wave formation as well as pressure oscillation. The results show that the dilution of inert gas slows down flame propagation in the combustion chamber. The velocity deficit increases in the order of Ar/N₂/CO₂, which indicates that CO₂ is a better inhibitor of flame propagation than Ar and N₂. The evaluated jet flow accelerates continuously driven by the forward spreading laminar flame and the velocities at different inert gas conditions decrease in the sequence of Ar/N₂/CO₂. No shock wave occurs during the combustion process when inert gases are introduced into the chamber. The amplitude of pressure oscillations decreases with diluted mixture due to the absence of flame-shock interactions. Besides, the peak pressure shows difference among different inert gases.     

Large-scale flame structures in ultra-lean hydrogen-air mixtures

The paper discusses the peculiarities of flame propagation in the ultra-lean hydrogen-air mixture. Numerical analysis of the problem shows the possibility of the stable self-sustained flame ball existence in unconfined space on sufficiently large spatial scales. The structure of the flame ball is determined by the convection processes related to the hot products rising in the terrestrial gravity field. It is shown that the structure of the flame ball corresponds to the axisymmetric structures of the gaseous bubble in the liquid. In addition to the stable flame core, there are satellite burning kernels separated from the original flameball and developing inside the thermal wake behind the propagating flame ball. The effective area of burning expands with time due to flame ball and satellite kernels development. Both stable flame ball existence in the ultra-lean mixture and increase in the burning area indicate the possibility of transition to rapid deflagrative combustion as soon as the flame ball enters the region filled with hydrogen-air mixture of the richer composition. Such a scenario is intrinsic to the natural spatial distribution of hydrogen in the conditions of terrestrial gravity and therefore it is crucial to take it into account in elaborating risk assessments techniques and prevention measures.