High-intensity Combustion Research Articles

Effect of Different Nozzle Arrangements on Combustion Flow Characteristics in a Swirl Coupling Multiple Direct Injection Combustor

ABSTRACT The influence of multiple direct injection nozzle arrangements with hexagonal, rhombus, and circular shapes on the combustion flow in gas turbines was numerically studied, and the combustion flow characteristics of swirl combustion combined with multiple direct injection combustion were analyzed, such as velocity and temperature distribution, vortex structure, NOX (nitrogen oxides) emission, flame stretch rate, etc. The result shows that for these different nozzle arrangements, the difference in velocity and temperature distribution is not obvious, but the difference in vortex structure is large. The hexagonal arrangement can form several counter-rotating vortex pairs (CVP) near the side wall, the rhombus arrangement can form several deflection vortex structures outside the center swirl, and the circular arrangement forms the least vortex structure. Furthermore, the circular arrangement is most effective in reducing the non-uniformity of the outlet temperature, OTDF is 0.00548, and it has a higher flame stretch rate, which means that it has a higher combustion intensity. However, the hexagon arrangement has the higher NOX emission, reaching 3.16 ppm.

Retrofit and application of pulverized coal burners with LNG and oxygen ignition in utility boiler under ultra-low load operation

An oxygen-rich and low NOx burner integrated with liquefied natural gas (LNG) was proposed to address unstable combustion and high NOx emissions from a 330 MW subcritical boiler under ultra-low load operation in China. To assess the effectiveness of the retrofit, Chemkin and Fluent softwares were utilized to construct a new NOx model and calculate NOx generation, based on the combustion of pulverized coal gas and LNG. Further, an eddy dissipation concept (EDC) model, which can reflect detailed chemical reactions, was applied to calculate gas-phase reactions in the furnace. The results showed that when performing the deep peak shaving after the retrofit, the combustion in the furnace was stable under 50% or more load, and NOx emission level at the furnace outlet was lower than 350 mg/m3 (6% O2 content, dry basis). Under 25% load, the oxygen-rich burner integrated with LNG was applied, and the pulverized coal flow entered the furnace in a state of high-intensity combustion, which effectively promoted the stability of combustion in the furnace. The reductive combustion state with reductive free radicals generated by LNG decomposition inhibited NOx formation. Consequently, NOx emissions from the furnace outlet decreased from 380 mg/m3 to 316 mg/m3.

Burning and plume flow behaviors of annular pool fires: with and without air entrainment through the pool center.

Annular pool fires, frequently happened in chemical industries, have a significant influence on environmental pollution. Air pollution, greenhouse gas emissions, water pollution, and soil contamination are general ways of environmental hazards caused by the annular pool fires. This study built upon our previous study (Environ. Sci. Pollut. Res., 2023, 30(21): 59781-59792.), and extended to investigate the combustion and fire plume flow behaviors of annular pool fires, both with and without air entrainment through the hollow center of the annular pool. Results show that when there is no air entrainment through the hollow center, the low combustion intensity area at the plume's central axis gradually extends while the high combustion intensity area concentrates at higher places and the flame height increased by nearly 40% from a solid pool (Din/Dout = 0) to the annular pool (Din/Dout = 0.80). Additionally, the area with high combustion intensity is more concentrated at a higher position. The combustion of annular pool fires was found to be dominated by non-premixed diffusion combustion. The center of the annular pool fires is dominated by air prior to flame merging and by fuel vapor after the merging occurs. For annular pool fires with air entrainment through the center of the pool, the combustion intensity increases as Din/Dout at the plume base increases. And, the flame height decreased by nearly 25% as Din/Dout increases. Flame burning occurs both on the outside and inside of the plume, exhibiting a "double layer" combustion characteristic. It reveals that the combustion of the fire plume transitions to premixed diffusion combustion. The center of the annular pool fire is predominantly composed of air. Understanding and controlling annular pool fires can lead to new methods for remediating fuel spills, reducing pollution from combustion, and advancing research in fluid mechanics.

Oxidation behavior and kinetics of shale oil under different oxygen concentrations

The oxidation behavior and kinetics of crude oil within tight shale differ from those of traditional light and heavy oils. In this study, the static oxidation tests on tight shale samples were conducted under different oxygen concentrations (11%, 16%, 21%, and 26%), followed by analysis of effluent gas and oxidized oil. Specifically, the compositional changes in shale oil at the molecular level were studied. Additionally, the self-ignition potential of shale oil under different oxygen concentrations was evaluated using accelerating rate calorimetry, and the corresponding kinetic parameters were determined. The results indicated that as the oxygen concentration increased from 16% to 26%, the relative abundance of O1 species decreased from 10.96% to 8.66%, whereas that of O3 species increased from 6.16% to 8.46%. Also, the relative abundance of chain saturated aliphatic acids and tricyclic naphthenic acids was observed to decrease and increase, respectively, in the shale oil oxidized at 26% oxygen concentration compared to the other shale oils oxidized at oxygen concentration lower than 26%. The shale oil displayed significant oxidation activity under different oxygen concentrations (16%, 21%, and 26%), and an increased oxygen partial pressure resulted in a shorter induction period of oxidation and higher combustion intensity. The kinetics results showed that the combustion activity of coke was enhanced while increasing oxygen concentration from 16% to 26%. The shale oil encountered fewer concentration-dependent oxidation reactions under higher oxygen concentration. Overall, this study provides valuable insights into the oxidation behavior of crude oil within tight shale.

Combustion Intensification Mechanism in a Vortex-Tube Reactive Flow

The combustion intensification mechanism was revealed in a stratified vortex-tube combustor. The results show that a triple-flame structure is formed in this combustor and the peak heat release is located exactly at the triple-point position. The peak heat release rates in the five selected representative cases studied in this paper are all over . The stability limit is wide, and the lean stability limit is always lower than 0.2. The decreased mixing length and decreased velocity difference between the fuel and oxidant streams delay the mixing process and then bring about a stratified distribution of species. This is crucial for the generation of a triple-flame structure and the corresponding high species concentration in the vicinity of the reaction zone under lean conditions. The Damköhler numbers on the lines across the triple points were calculated with values above 1.0 in each of the cases, indicating that the species’ transport time is longer than the chemical reaction time. Therefore, the transport flux of species determines the final combustion strength. The synergistic coupling of the flowfield and species field gives rise to a large transport flux of the key species in the vicinity of the reaction zone. The results show that the corresponding fuel transport flux is large, with values all over . This is the principal reason for the high combustion intensity in this combustor. The different transport fluxes of the critical species are responsible for the different chemical reaction strengths on the triple point in each of the different cases.

Introducing metal oxide as solid oxygen carrier for energy release enhancement of boron suspension fuel

Boron (B) suspension fuel is a novel high-density fuel for aerospace propulsion. However, it suffers from significant incomplete energy release of B during combustion, which seriously restricts its practical application. This study focuses on the key causes of this problem, insufficient oxygen supply, and proposes to introduce metal oxides (MOx) as solid oxygen carriers to form B-MOx and replace pure B to enhance the energy release characteristics of B suspension fuel. TG-DSC analysis is conducted to sort out CuO as the optimal additive for B oxidation enhancement and its enhancement mechanism is analyzed with thermal equilibrium calculation. It was found that CuO could generate active O atoms through two-step reduction, thus achieving efficient oxygen supply for B and reduce its initial reaction temperature from 810 °C to 661.7 °C with an optimal B-CuO ratio of 1:1. CO2 laser ignition experiment and combustion residue analysis are conducted to analyze the combustion characteristics of B-CuO/JP-10 suspension fuel. Compared with original B/JP-10 suspension fuel, B-CuO/JP-10 suspension fuel presents much higher combustion intensity with local explosion after ignition, and a second stage of intensive energy release is realized with the reignition and explosion of solid agglomerate. The oxidation of JP-10 and B are enhanced with no incomplete oxidation product found in the residue. The enhancement mechanism of CuO on B suspension fuel is proposed including the generation of active radical, efficient internal oxygen supply and enhanced external oxygen supply.

Ignition enhancement of liquid kerosene by a novel high-energy spark igniter in scramjet combustor at Mach 4 flight condition

Spark ignition system is the most widely used method in the combustor. However, it is generally considered unsuitable for scramjet engines due to the shortage of ignition energy. This study firstly presents a novel long electrode distance high-energy spark igniter (LHSI), which significantly increases the energy deposition of spark by simultaneously improving the energy storage and discharge efficiency. The discharge and ignition characteristics of LHSI were investigated based on the discharge waveforms and high-speed photography. The results show that the discharge efficiency of LHSI can reach 62.4% with the single-pulse stored energy of 100 J while that of conventional high-energy spark igniter (HSI) is only 28.2%. The ignition process produced by such a high-energy spark is firstly reported in a scramjet combustor. An initial flame kernel with a larger size and higher combustion intensity can be produced, which is capable of igniting the cavity with proper local equivalence ratio rapidly and reliably. Compared with HSI, the ignition process of LHSI is more active and the ignition limit can be obviously extended. A novel tail flame ignition mode is observed. It can be concluded that the spark ignition with greater energy deposition is worthy to be further studied in scramjet.

Heat transfer characteristics of tubular heat exchanger using reverse air injection flameless combustion

The heat transfer characteristics of a co-axial triple tube reformer using flameless combustion were studied herein. Methane steam reformers for hydrogen production have been developed with a long history, but as hydrogen has recently come into the limelight as a future energy, research on the development of more efficient reformers is being pursued. Flameless combustion has the advantage of being applicable for the high performance reformers, but studies to date have focused on the combustion phenomenon itself, and research on the heat transfer phenomenon is lacking. In this study, RAI (reverse air injection) flameless combustion, which generates high intensity combustion and heat transfer, was applied to the reformer and its heat transfer effect was analyzed in parallel with experiments and CFD analysis. In the experiment, it was confirmed that the RAI flameless combustion increased the heat transfer by up to 58% compared to the conventional flame type combustion, and the case of two air nozzles was found to have a better heat transfer enhancement effect than the case of four. However, the case using four air nozzles showed a more axisymmetric flow pattern. The heat transfer enhancement effect of RAI flameless combustion is caused by the high-speed air jet creating a highly recirculating flue gas to generate uniform temperature distribution and high convective heat transfer on the surface. As RAI flameless combustion is characterized by being mixed with fuel at the rear end of a strong air jet, which is a completely different configuration from these existing combustors, it is expected that the limitations of the EDC model, which has been widely used in previous studies, will become more prominent. The equilibrium-PDF model based on chemical equilibrium and tabulated chemistry gave a closer result to the experimental results.

Interaction of fuel spray and air swirl on the combustion of residue oils

Effervescent atomization technology was applied in Waste Lubricant Oils (WLO) combustion. The tests were conducted in a 150 kW prototype boiler with a horizontally oriented cylindrical shape combustion chamber in which the combustion process was studied in terms of the interaction between a swirling air flow and fuel spray pattern. The combustion performance was assessed by continuously monitoring gaseous emissions, particularly carbon monoxide (CO) emission. Two different air feeding strategies were tested, varying in swirl number and six different configurations of the injection nozzle orifice. Three nozzles had a single orifice varying in diameter and discharge end geometry, and the others had three divergent orifice arrangement varying in cone angle and diameter. At the same time, the gas-to-liquid ratio (GLR) of the effervescent atomizer was also investigated, allowing to draw important illations about its influence on the combustion efficiency. The results show that the use of effervescent atomizers allows to burn of WLO, with CO emissions as low as 15 ppm at 8% of oxygen (O2) when using a three orifice of 1 mm with 15° of cone angle nozzle. This result was achieved by applying high-intensity combustion air flow swirl. However, the one orifice nozzles with penetrant spray pattern combined with low swirl intensity air flow also produced satisfactory results. With CO emissions of 27 ppm at 8% of O2 with a 1.2 mm single orifice nozzle. Furthermore, it was also demonstrated that the increment of the GLR in a range between 0.25 and 0.35 can be responsible for the reduction of 40 to 50% of the CO emissions when burning WLO.

Mixing and combustion characteristics in a scramjet combustor with different distances between cavity and backward-facing step

The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection. Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance. The cavity shear-layer mode, the lifted cavity shear-layer mode, and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5, 4.5, and 7.5. In both non-reacting and reacting flow fields, the numerical results are essentially in agreement with the schlieren photography, flame chemiluminescence images, and wall pressure, which verify the reliability of the numerical method. The results of non-reacting flow fields show that the Backward-Facing Step (BFS) can promote the flow separation downstream at a fixed distance. The more forward the separation position is, the larger the separation zone is in the non-reacting flow field. Furthermore, the larger the separation zone is, the higher the intensity of combustion in the reacting flow field is. A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor. The flame presents remarkable three-dimensional characteristics in the reacting flow fields. When dimensionless distance equals to 4.5, there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor, while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5. In the cavity flameholding combustors with a backward-facing step, the higher combustion intensity may bring much total pressure loss to the combustor. Thus, it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.

Hydrogen-enriched methane combustion in a swirl vortex-tube combustor

The hydrogen-enrichment combustion performance was investigated numerically by employing methane mixed with different volume proportions of hydrogen, wherein a vortex-tube combustor was employed to achieve a steady combustion process. Results show that this combustion technique indicates good adaptability to hydrogen-enrichment combustion together with an ultra-steady combustion process. The lean stability limit is always within 0.15, which decreases further with the increase of hydrogen content, whilst the amplitude of pressure fluctuation △p is always within 500 Pa with a uniform flame front. The generated non-premixed property flame structure can guarantee a high concentration of components in the reaction zone, which improves the local concentration of species and yields a large stability limit. The strong vortex flow can decrease the local flow velocity to inhibit flame blow-out and yield a large tangential velocity component. The high combustion intensity generates a large density gradient. The large density gradient and large tangential velocity promote the laminarization of the flow, resulting in a large Richardson number Ri* that is always much greater than 1.0. The laminarization of the flow field provides good aero-dynamic and thermo-dynamic stability. Further, the thermo-acoustic coupling is ultra-weak as well, and therein the Ra(x) is always smaller than 0.0024, indicating good flame-dynamic stability. In summary, the resultant good aero-dynamic, thermo-dynamic, and flame-dynamic stabilities of this vortex-tube combustor are the principal reasons for the super-steady hydrogen-enrichment combustion.

Effect of Swirl on Temperature Decay Function in Straight Blade Liquid Fuel Swirl Burner

The need to shield the surroundings from fire or combustion generated pollutants has led to substantial demand for perk up burner design with better performance. In this study, the geometry of a liquid fuel burner has been modified by introduction of straight edge blade swirler and thereafter, measurement using chromium-zinc thermocouple of radial and axial temperature distributions. The straight blade swirl generator consists of 6 number of blades at angles 20°, 30°, 40°, 50° and 60°, respectively was fired by conventional diesel in an experimental model liquid fuel swirl burner. The highest axial temperature of combustion without and with swirler was 930℃ and 1121℃, respectively, while the highest radial temperature without and with swirler was 942℃ and 1130℃, respectively. The achieved 21% performance improvement suggests that burners with swirl have varied higher flame temperature and combustion intensity than their counterpart without swirl. The outcome of this study has revealed that higher agitation of the swirl significantly affected the thermal profile and this subsequently led to high combustion intensity. The changes can be attributed to turbulence being provoked at selected angles of swirl blade. The effect is enhanced as the angle of swirl blade increases, howbeit, until a threshold is reached.

Study of turbulent flame characteristics of water vapor diluted hydrogen-air micro-mixing combustion

Hydrogen has been regarded as an important alternative fuel, especially in gas turbine applications. However, the premixed combustion of pure hydrogen has problems such as easily flashback and high NOx emissions. A more reasonable method of combustion is to use micro-mixing technology combined with water vapor dilution. This work experimentally investigated the influences of dilution rate (D) and equivalence ratio (φ) on the flame characteristics of hydrogen-air micro-mixing combustion diluted with water vapor. Results show that increasing D reduces the flame area by reducing the downstream OH radical signal, which is contrary to the effect of CO2 dilution. The flame local curvature radius shows that the micro-mixing combustion is a high turbulence intensity combustion, and the convex structure is more frequent. Moreover, increasing φ or decreasing D leads to a decrease in the instantaneous average curvature radius (Rave), indicating more small-scale wrinkle structures are generated. D and φ have a similar influence on the fluctuations of Rave and flame area in the time domain, while the fluctuation of the flame area is more evident near the lean-burn limit. When D = 25%, the frequency characteristics of the flame structure and chemical reactions are synchronized.

Modifying the ignition, combustion and agglomeration characteristics of composite propellants via Al-Mg alloy additives

The performance of propellants, explosives and pyrotechnics is known to benefit from the use of metallic alloys as fuel additives. Previous studies have shown that switching from pure Al to Al-Mg alloy can improve the agglomeration and combustion features of composite propellants. However, the optimal ratio of Al and Mg has yet to be explored. We experimentally examine the effects on ignition, combustion and agglomeration arising from substituting pure Al particles with four different Al-Mg alloys whose Mg content ranges from 10 to 60%. Using thermogravimetric−differential scanning calorimetry, we find that switching to Al-Mg alloy can enable oxidation to occur earlier and can increase the oxidation rate and the enthalpy of thermal reaction, which is found to be positively correlated with the Mg content. Using laser ignition measurements, we find that the addition of Mg can promote the ignition and combustion of Al, with (Al/Mg)0.5 exhibiting the most balanced combination of a high combustion intensity, a short ignition delay, and a high mass burning rate. Through high-pressure measurements taken at 1 to 7 MPa, we find that switching to Al-Mg alloy can increase the propellant burning rate (by nearly a factor of two) and the pressure exponent (from 0.23 to 0.43). We also find that the burning rate and pressure exponent are both highest for the propellant containing (Al/Mg)0.1, but that both quantities decrease with further increases in Mg content. High-speed microscopic surface imaging at 1 MPa reveals that increasing the Mg content can increase the overall combustion intensity of the propellant and decrease the diameter of spherical agglomerates, consistent with the observed size distribution of the condensed combustion products. From these measurements, it can be concluded that (Al/Mg)0.5 provides the optimal balance of ignition, combustion and agglomeration characteristics in the formulation of composite propellants containing Al-Mg alloy.

A numerical study of the influence of pilot fuel injection timing on combustion and emission formation under two-stroke dual-fuel marine engine-like conditions

Stricter regulations imposed on emissions are motivating the scientific community to consider studying alternative fuels to achieve low emission, high efficient dual-fuel (DF) marine engines. In this context, three dimensional computational fluid dynamic (CFD) simulations are performed to study the combustion and emission formation under two-stroke, dual-fuel marine engine-like conditions. The DF engine configuration consists of a pilot diesel fuel and a high-pressure, direct injection (HPDI) of natural gas (NG). The simulation results are validated under both high load (high charge density) and low load (low charge density) operating conditions. Detailed analysis of the flame development and emission formation are performed. The interaction between the pilot diesel jets and the methane flame jets is studied. Based on the results, the further methane jets penetration in the low load case leads to better air–fuel mixing and a higher combustion intensity than that in the high load. Effects of the pilot fuel injection timing on combustion and emission formation and the governing mechanisms are also investigated in detail. Results indicate that the intense combustion of the accumulated methane expands the methane flame towards the piston when the pilot injection timing is retarded. The NO formation is lower in the high load case with higher charge density due to the lower combustion intensity. Also, retarding the pilot injection timing decreases the NO formation.

Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.

Experimental study on head combustion characteristics of SiC foam ceramic burner

Foam ceramic burner has significant advantages of high combustion intensity, low pollutant emission and high thermal efficiency. There is relatively little experimental research on SiC foam ceramic burners, especially the gradually-varied SiC foam ceramic. A combustion experiment system is designed to study the combustion characteristics of SiC foam ceramic burners. The influence of aperture structure on combustion chamber temperature distribution and pollutant emission under a stable combustion state is obtained. The results show that the influence of aperture structure on combustion characteristics is not linear when the aperture is 20 PPI. Compared with the uniform structure, the gradually-varied foam ceramics can improve the combustion chamber temperature while maintaining a low pollutant emission, which has a good application prospect.

A novel model of liquid film flow and evaporation for thermal protection to a chamber with high temperature and high shear force

Thin liquid film evaporation is a prevailing cooling technique for thermal protection to the inner wall of a chamber with high-intensity combustion. A novel model is developed to investigate the flow and evaporation of liquid film in a rocket combustion chamber with high temperature and high shear force, in which shear stress at the interface between liquid film and high temperature combustion gas with high velocity, capillary pressure of liquid and disjoining pressure associated with liquid film evaporation are taken into account. A numerical algorithm of the backward solution is deliberately adopted to solve this model so as to eliminate the entrance effect caused by different inlet conditions of liquid film. For given temperature and hot gas rate in a chamber, a stably evolution regime of liquid film with the thickness from thick to thin is depicted corresponding to the total process of being sheared into film and being evaporated to dryness. Results indicate that the three forces mentioned above play critical parts in flowing and evaporating of liquid film during its different evolution regimes. It was shown that shear stress and capillary pressure have a positive effect on the flow and evaporation of film, as well as a negative effect from the disjoining pressure. The effects of these forces are different dependent on different initial average thickness of film, and the distribution profile of these forces are totally illustrated in detailed. The prediction of liquid film length, from being injected to being evaporated, by the proposed model is in a good agreement with available experimental data. All findings are expected to provide a fundamental guideline for the design and optimization of efficient thermal protect to the combustion chamber of space engine.

Characterization of Miombo species used by rural communities as fuelwood in Northern Mozambique

ABSTRACT The rural population of Mozambique has a strong preference for using native tree species for fuelwoods instead of Eucalyptus wood. The reforestation projects failed to respond to the wishes of consumers who considered eucalyptus wood to be low quality compared to the native species traditionally used by the population. The present study aimed to determine the wood characteristics of Miombo woodland species which are often used as an energy source in Sanga district, northern Mozambique, and then compare the results with those found in the literature on Eucalyptus species used for the same purpose. Chemical, physical, energetic, and thermal properties were analyzed. Among all studied species, U. kirkiana and J. globiflora presented high quality of wood for energy purposes, in Northern Mozambique. These species showed high basic density and extractive contents, which are the characteristics responsible for providing high energy density, longer wood combustion duration, and higher combustion intensity. These last three characteristics are considered of greater importance for the choice of fuelwood by the communities when compared to the Eucalyptus. The Eucalyptus are fast-growing species and are recommended for selecting genotypes with high energy density and favorable combustion parameters in order to introduce new energy planting programs in Mozambique to guarantee the wood supply in a short time and the conservation of natural resources.

5-2-1 高負荷型アンモニアバーナーの開発

5-2-1 高負荷型アンモニアバーナーの開発