Ring Flame Research Articles

Hypergolic ignition induced by head-on collision of bi-propellant droplets: Monoethanolamine-based fuel and hydrogen peroxide

Hypergolic ignition by the binary collision of H2O2 and Monoethanolamine (MEA)-NaBH4 droplets has been experimentally studied with an emphasis on droplet inter-mixing, heat transfer, droplet swelling, and flame propagation. The entire ignition is phenomenally categorized into five stages. The H2O2 droplet penetrates the MEA- NaBH4 droplet after the collision and coalesces with it. The H2O2 fluid rapidly restores its sphericity inside the coalesced droplet, which is theoretically proved to be driven by interfacial tension. The average measured restoration time is 14.0 ms. Large-scaled bubbles are generated at the interfacial structure, leading to swelling of the coalesced droplet. The existence of apparent interface tension tends to limit the rate of droplet inflation at the beginning. As more bubbles are produced, the interface tension wanes, and the droplet swells rapidly. The droplet surface expansion rate is identified via the gray-level information in recorded images. A theoretical model quantifying the swelling rate was established based on the underlying mechanism for the swelling of the reacting droplet, which can be attributed to the flash boiling of the superheated H2O2 fluid. As bubbles accumulate inside the coalesced droplet, a dual-bell-shaped droplet configuration is formed. A flame ring structure is observed when the ignition occurs. Luminant flame sustains more than 10 ms until the fuel vapor is depleted.

Dynamics of flame rings in a thermally-conductive narrow channel: a numerical experiment

A numerical experiment is conducted to examine the ignition of a near-flammability-limit low-Lewis-number mixture and the evolution processes of the resulting premixed flames in a circular thermally-conductive narrow channel. A diffusive-thermal model is adopted to describe the fuel mass conservation in the gas phase and energy conservation in both the gas and the channel wall. Spalding's ‘one-dimensional idealization’ approximation is introduced to simplify all these conservation equations to a two-dimensional form over the plane parallel to the wall surface of the channel. As a result, the half channel height h constitutes the primary parameter controlling the heat exchange rate between the gas and the wall, which serves as a conductive heat loss mechanism from the perspective of the gas phase. For relatively large h, following ignition at the centre of the channel by a hot ignition kernel, the resulting flame front suffers diffusive-thermal instability and quickly breaks into a number of discrete reaction cells, which propagate forward towards the boundary of the channel. When h becomes sufficiently small, the reaction cells close upon themselves and transform to ring-shaped flamelets, herein termed flame rings, a 2-D analogue of flame balls formed in 3-D unconfined space. The effects of the half channel height h on the formation and dynamics of the flame rings are examined, together with a demonstration of a possible means to manipulate the flame ring dynamics by exploiting the 2-D character of the narrow channel configuration. It is suggested that, as a simple ground-based buoyancy-suppression strategy, the presently considered narrow channel configuration can be conveniently employed to study the properties related to the flammability limit of combustible mixtures.

Numerical simulation research on the unique thermal deviation in a 1000 MW tower type boiler

In this paper, a comprehensive numerical simulation, which was verified by measurement and design data, was conducted to investigate the issues of thermal deviation in a 1000 MW tower type boiler. Based on this, the cause of thermal deviation was found: a large swirling momentum intensity in the furnace leads to a great deflection of SOFA and then unburned carbon in fly ash gathering in the furnace center is extremely hard to be burn out. At last, a high temperature flame ring occurs at the gas entrance of platen heater zone, resulting in the thermal load deviation of platen heaters. The influences of deflection angle of SOFA nozzles and the main burners in the primary combustion zone were also investigated. The results show that an optimal deflection angle of SOFA nozzles and the main burners in the primary combustion zone can reduce the thermal deviation by decreasing the swirling momentum intensity in the whole area of full furnace. It suggests that modifying the deflection angle of SOFA nozzles to −18° is the best strategy for realizing a minimum thermal deviation case. The results of test on a real boiler adopting this best strategy show that it is effective to reduce thermal deviation.

Experimental and numerical investigation of turbulent isothermal and reacting flows in a non-premixed swirl burner

This paper focuses on the experimental and numerical investigation of CH4/air non-premixed flame stabilized over a swirler burner with radial fuel injectors. The flame operates under a global equivalence ratio Φ = 0.8, a high swirl number Sn = 1.4 and at atmospheric pressure. Reynolds averaged Navier-Stokes (RANS) calculations, Delayed-Detached Eddy Simulation (DDES) and experimental measurements are performed for both cases, non-reacting and reacting swirling flows. Numerical flow fields are compared with detailed Stereoscopic Particle Image Velocimetry (Stereo-PIV) fields under non-reactive and reactive conditions. Temperature measurements are also performed and compared to the computed ones in the reacting flow. The analysis of averaged results reveals the presence of a central recirculation zone (CRZ), a swirling jet region (SJ) and shear layers (SL) for both flows. The instantaneous turbulent structures at the burner exit, visualized by the Q-criterion, display different instability modes. The main instabilities are the vortex rings due to the Kelvin–Helmholtz instability, and finger structures generated by the swirling instability. The presence of the flame leads to increase the jet angle compared to the non-reacting flow. The main flame front is found highly wrinkled and rolled up around the vortex ring structures. A small flame ring is present near the fuel injector; it is formed due to the presence of a recirculation bubble (RB) at this region.

Accelerative expansion and DDT of stoichiometric ethylene/oxygen flame rings in micro-gaps

Acceleration and transition to detonation of expanding flame rings ignited at the center of 260μm and 120μm gaps between parallel flat pates were experimentally studied. The micro-spacing was initially filled with stoichiometric ethylene/oxygen mixtures at ambient pressure and temperature. Visualizations showed that the outward propagating reaction wave was initially smooth and circular, but petal-like wrinkles quickly developed on the flame ring. Flame wrinkles appeared earlier and closer to the ignition point as the gap width became smaller. The flame underwent fast acceleration during the onset of flame wrinkling, but the acceleration was relatively mild as the wrinkled flame ring continued to expand. Time exponents for the accelerative growth of corrugated flame rings were identical in the two highly confined gaps. The flame ring underwent deflagration-to-detonation transition as the propagation velocities abruptly surged from 1000m/s to over 2000m/s. The arc-shaped detonation waves initiated from local explosion spots on the flame ring were propagating at near Chapman–Jouguet velocities. The induction distance and time for detonation transition were both shorter in the smaller gap. Detonation cell patterns and the initiation locations were also clearly recorded through soot film visualizations.

Transition to detonation of an expanding flame ring in a sub-millimeter gap

Deflagration-to-detonation transition of a flame ring circularly expanding in a 260 μm gap filled with stoichiometric ethylene/oxygen mixtures initially at atmospheric pressure and temperature has been experimentally visualized. The results show that DDT can occur under the influence of wall confinement even for an expanding flame. DDT could be observed at a distance as short as ∼70 mm from the ignition spot, which corresponds to ∼130 μs after the ignition spark voltage breakdown. Velocity overshoot of reaction front velocity exceeding Chapman–Jouguet velocity was characterized. Cell structures were observed on the reaction fronts after DDT occurred. The visualizations also showed that smooth circular flame developed right after ignition quickly evolved into wrinkled flame as the flame ring propagated outwards. Flame propagating velocity was accelerated from ∼600 m/s to ∼1000 m/s during the wrinkled flame stage. A series of local explosion on the flame ring was observed during the DDT process, and resulted in an abrupt surge on reaction front propagation velocity.

Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition ( Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition ( Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.

Thin nanostructured LiMn 2O 4 films by flame spray deposition and in situ annealing method

A new approach has been developed to rapidly synthesize nanostructured LiMn 2O 4 thin films by flame spray deposition (FSD) and in situ annealing. A precursor solution of lithium acetylacetonate and manganese acetylacetonate in an organic solution was supplied through a flame spray pyrolysis (FSP) reactor. The liquid solution spray was ignited and stabilized by a premixed methane/oxygen flame ring surrounding the FSP nozzle. Thus, LiMn 2O 4 nanoparticles were formed by combustion and deposited onto a current collector followed by in situ annealing. Two different types of current collectors, i.e. stainless steel and aluminum coated with carbon-based primer were tested. The prepared thin films were characterized by X-ray diffraction and field-emission scanning electron microscopy. The electrochemical properties of the thin films were evaluated by cyclic voltammetry and galvanostatic cycling. The LiMn 2O 4 films exhibited good cyclability. Films that underwent sintering and crystal growth during in situ annealing developed more robust film structures on the current collector surface and exhibited better electrochemical performance than poorly adhered films.

High efficiency heat-recirculating domestic gas burners

Existing designs of most conventional domestic burners (CB) have typically relied on open combustion flame, where a large amount of energy loss with the flue gas arises, resulting in relatively low thermal efficiency (<30%). Against this background, a novel semi-confined porous radiant recirculated burner (PRRB) concept based on heat-recirculating combustion using the porous medium technology was developed for energy savings in domestic use and in the small-scale food processing industry. Performance of the new burner using the same ring burners as those in the CB, i.e. the PRRB(CB) were evaluated by comparing thermal efficiencies and the combustion characteristics with those of the conventional one (CB). Operating parameters such as heat input, flow type of the ring burners (conventional radial flow (CB) or swirling central flow (SB)) were clarified. The proposed PRRB(CB) is very effective in establishing a heat-recirculation mechanism from the hot exhaust gas to the combustion air, resulting in efficient combustion air preheating with maximum combustion air temperature of 300 °C. Thermal efficiency of the proposed PRRB(CB) is increased to about 12% higher than that of the conventional one (CB). Further improvement in thermal efficiency of the burner can be realized by combining the PRRB with the swirling central flame ring burner (SB), i.e. the PRRB(SB), yielding a maximum thermal efficiency of about 60% and, thus, energy saving of about 50% in average over the operating range. The proposed PRRB(SB) provides not only high thermal efficiency and considerable improvement in energy saving but also environmentally compatible emissions. With the model proposed, the calculated thermal efficiencies of the PRRB(SB) can be predicted and agree well with the experimental ones.