High Equivalence Ratio Research Articles

Soot Size Determination In A Swirl Stratified Premixed Ethylene-Air Flame By A Sampling Probe And Elastic Light Scattering

An experimental study was performed to measure the size and number density distributions of soot particles produced in turbulent, stratified (high spatial equivalence ratio gradient), swirled (rotating flow) premixed ethylene-air flames at atmospheric pressure. Soot particle size and number density measurements are initially performed using a Scanning Mobility Particle Sizer (SMPS). For this purpose, two-stage sampling and dilution device designed for hot gas analysis was used. The dilution rate of the sampling apparatus was evaluated to ensure a most representative probing. In a second step, the ex-situ measurements were replaced by single-shot measurements recorded with a high cadency planar multi-angle light scattering (2D-MALS) laser diagnostic technique, based on the relationship between aggregate size and light scattering angle. In a third step, results obtained by the SMPS as well as by the optical diagnostic were compared. Because a direct confrontation of the obtained results obtained is not feasible due to difference in nature of both techniques, a two-step post-processing methodology as developed. For this purpose, the median electric mobility diameters measured by SMPS were converted into diameters of gyration by means of a semi-empirical conversion tool. Moreover, a time average data processing was applied to the soot size distributions recorded by 2D-MALS, considering that SMPS measurements performed over a relatively large acquisition time converge to a single value. Results obtained in stratified swirled premixed flames by SMPS exhibit an overall good agreement with the optical measurements. This good similarity between the results provides a remarkably high degree of confidence in the quality of the measurements for both measurement systems investigated. Furthermore, the results suggest that a comparison of soot aggregate size and number density measurements between a sampling probe and a light scattering technique is still possible in turbulent flames at atmospheric pressure. While the current work has focused on the performances of SMPS and the 2D-MALS approach on a one-to-one basis, future work will also address the application of these diagnostics to high-pressure two-phase flames for which measurements are made in confined environments.

Flow field of a rotating detonation engine fueled by carbon

Solid–gas rotating detonation engines have been widely studied, but experimental limitations have prevented the full information of the flow field from being revealed. This paper describes a numerical investigation of the effect of the equivalence ratio on the two-phase flow field of a rotating detonation engine fueled by carbon and air. The discrete phase model and multiple surface reaction model are employed to determine the flow and combustion of carbon particles. The Reynolds-averaged Navier–Stokes equations are solved for the gas flow. The results show that a low-temperature air gap appears in place of deflagration in the two-phase flow field, and the gap extends into the products. Before the detonation wave, this air gap is the difference between the air and fuel layers. At higher equivalence ratios, two rotating detonation waves are formed by the contact between the high-temperature products and the fuel–air mixture.

Development of an Air/Fuel Premixer for a Gas-Fueled Two-Layer Porous Media Burner

ABSTRACT Air-fuel premixers are integrated parts of the premixed burner systems. The present paper describes the modeling, design optimization, and testing of an air-fuel premixer for a 5 kW natural gas porous media burner. The design was aimed at decreasing NOx and CO emissions by forming a homogeneous mixture of air and fuel. The design parameters were optimized employing CFD analysis coupled with a design of experiments (DOE) method. Then, a test model, based on the optimized design, was built and installed in a porous media burner testing apparatus. The test results have shown that the new premixer successfully reduced NOx emissions. It also improved flame stability by preventing flashback in several cases which revealed a new flashback mechanism in such burners. This improvement is achieved by preventing locally near-stoichiometric zones with high flame speeds from being formed. The effect of the new premixer on CO emissions was shown to depend on the firing rate and equivalence ratio. While reduced CO emissions are achieved at high firing rates and equivalence ratios, the level of CO emissions is increased for low firing rate and equivalence ratio magnitudes.

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 OH∗ 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.

Experimental Investigation of a Cylindrical Air-Breathing Continuous Rotating Detonation Engine with Different Nozzle Throat Diameters

A continuous detonation engine with various exhaust nozzles, analogous to typical scramjet cavity combustors with variable rear-wall heights, was adopted to perform a succession of cylindrical air-breathing continuous rotating detonation experiments fueled by a non-premixed ethylene/air mixture. The results show that the detonation combustion was observed to self-sustain in the combustor through simultaneous high-speed imaging covering the combustor and isolator. A long test, lasting more than three seconds, was performed in this unique configuration, indicating that the cylindrical isolator–combustor engine exhibits potential for practical applications. Three distinct combustion modes were revealed with varied equivalent ratios (hybrid mode, sawtooth wave mode, and deflagration mode). The diameter of the nozzle throat was critical in the formation of rotating detonation waves. When the nozzle throat diameter was larger than the specific value, the detonation wave could not form and self-sustain. The upstream boundary of the shock train was supposed to be close to the isolator entrance in conditions of a high equivalence ratio and small nozzle throat diameter. In addition, it was verified that periodic high-frequency pressure oscillation could cause substantial impacts on the incoming flow as compared with the steady deflagration with the same combustor pressure.

Multi-channel gliding arc plasma-assisted ignition in a kerosene-fueled model scramjet engine

A multi-channel gliding arc (MCGA) plasma was utilized to ignite a kerosene-fueled model scramjet engine with an inflow of Ma2.92. High-speed photography from the top and side view of the scramjet combustor, high-speed CH* chemiluminescence imaging coupled with the electrical measurements were used to investigate the ignition process. A three-dimensional RANS simulation was used to show the distributions of velocity, temperature, and local equivalence ratio of the supersonic cold flow. In the ignition process, the formation of the flame kernel mainly depends on the discharge characteristics of the MCGA plasma, whereas the flame propagation process is mainly determined by the cavity flow. The MCGA plasma is more likely to produce a large area of the flame kernel in the spark-type discharge with the long arc column and overlaps of each channel gliding arc. The larger flame can be generated subsequently by the merging of the flame kernels ignited by the MCGA plasma, which develops into the resident flame. The resident flame is unable to be formed in a relatively high local equivalence ratio, and it is hard to spread to the back of the cavity along the shear layer due to the large velocity gradient with a relatively low temperature. The resident flame is formed several times by the MCGA plasma ignition, which increases the possibility of the flame spreading to the back of the cavity to form the mainstream flame. The local equivalence ratio can significantly affect the cavity ignition of the kerosene-fueled scramjet combustor.

Effects of flame-flame interaction on emission characteristics in gas turbine combustors

AbstractIn real gas turbines, multiple nozzles are used instead of a single-nozzle; therefore, interactions between flames are inevitable. In this study, the effects of flame-flame interaction on the emission characteristics and lean blowout limit were analysed in a CH4-fueled single- and dual-nozzle combustor. OH* chemiluminescence imaging showed that a flame-interacting region, where the two flames from the nozzles were merged, was present in the dual-nozzle combustor, unlike the single-nozzle combustor. Flow-field measurements using particle image velocimetry confirmed that a faster velocity region was formed at the flame merging region, thereby hindering flame stabilisation. In addition, we compared the emission indices of NOx and CO between the two combustors. The emission indices of CO were not significantly different; however, a distinct effect of flame-flame interaction was indicated in NOx. To understand the effect of flame-flame interaction on NOx emissions, we measured temperature distribution using a multi-point thermocouple. Results showed that a wider high-temperature region was formed in the dual-nozzle combustor compared to the single-nozzle combustor; this was attributable to the high OH* chemiluminescence intensity in the flame-interacting region. Furthermore, it was confirmed that the size of this interacting region caused the deformation of the temperature distribution in the combustor, which can induce a difference in the increase ratio of NOx emission between high and low equivalence ratio ranges. In conclusion, we confirmed that flame-flame interaction significantly affected temperature distribution in the downstream of the flame, and the change in temperature distribution contributed primarily to the varying concentration of the emission gas.

Combustion features of CH4/NH3/H2 ternary blends

The use of so-called “green” hydrogen for decarbonisation of the energy and propulsion sectors has attracted considerable attention over the last couple of decades. Although advancements are achieved, hydrogen still presents some constraints when used directly in power systems such as gas turbines. Therefore, another vector such as ammonia can serve as a chemical to transport and distribute green hydrogen whilst its use in gas turbines can limit combustion reactivity compared to hydrogen for better operability. However, pure ammonia on its own shows slow, complex reaction kinetics which requires its doping by more reactive molecules, thus ensuring greater flame stability. It is expected that in forthcoming years, ammonia will replace natural gas (with ∼90% methane in volume) in power and heat production units, thus making the co-firing of ammonia/methane a clear path towards replacement of CH4 as fossil fuel. Hydrogen can be obtained from the pre-cracking of ammonia, thus denoting a clear path towards decarbonisation by the use of ammonia/hydrogen blends. Therefore, ammonia/methane/hydrogen might be co-fired at some stage in current combustion units, hence requiring a more intrinsic analysis of the stability, emissions and flame features that these ternary blends produce. In return, this will ensure that transition from natural gas to renewable energy generated e-fuels such as so-called “green” hydrogen and ammonia is accomplished with minor detrimentals towards equipment and processes. For this reason, this work presents the analysis of combustion properties of ammonia/methane/hydrogen blends at different concentrations. A generic tangential swirl burner was employed at constant power and various equivalence ratios. Emissions, OH∗/NH∗/NH2∗/CH∗ chemiluminescence, operability maps and spectral signatures were obtained and are discussed. The extinction behaviour has also been investigated for strained laminar premixed flames. Overall, the change from fossils to e-fuels is led by the shift in reactivity of radicals such as OH, CH, CN and NH2, with an increase of emissions under low and high ammonia content. Simultaneously, hydrogen addition improves operability when injected up to 30% (vol), an amount at which the hydrogen starts governing the reactivity of the blends. Extinction strain rates confirm phenomena found in the experiments, with high ammonia blends showing large discrepancies between values at different hydrogen contents. Finally, a 20/55/25% (vol) methane/ammonia/hydrogen blend seems to be the most promising at high equivalence ratios (1.2), with no apparent flashback, low emissions and moderate formation of NH2/OH radicals for good operability.

Experimental investigation on effects of equivalence ratio on combustion with knock, performance, and emission characteristics of dimethyl ether fueled CRDI compression ignition engine under homogeneous charge compression ignition mode

This study deals with the effects of different equivalence ratios on combustion with a knock, performance, emissions of a DME fueled automotive CI engine under HCCI mode. The conventional common rail direct injection (CRDI) CI engine was modified to run with DME fuel (manifold injection) under HCCI mode. The experiments were conducted on the engine at different equivalence ratios at a constant speed of 1400 rpm. Two stages of combustion (low-temperature reaction (LTR) region and high-temperature reaction (HTR) region) were observed under HCCI mode. The LTR region occurs at the temperature range from 700 to 750 K. In contrast, the HTR region was at a temperature beyond 1000 K. The cyclic variations with the coefficient of variation (COV) of maximum in-cylinder pressure, indicated mean effective pressure with knock peak pressure and knocking intensity (KI) were analyzed for assessment of combustion stability. Based on the knocking intensity, the optimum equivalence ratio with controlled auto-ignition (CAI) was found as 0.55, with peak in-cylinder pressure of 70.87 bar and peak in-cylinder temperature of 1815 K. The COV for all the parameters and KI found below the limit value (COVlimit − 5% and knocklimit − 5 MW/m2) till 0.55 equivalence ratio and beyond the equivalence ratio, the knock occurred resulting in uncontrolled auto-ignition (UAI). Ultra-Low NOx (10–15 ppm) and zero smoke emissions with a considerable reduction in CO and HC emissions were observed at a higher equivalence ratio from DME fueled HCCI mode. However, CO and HC emissions were higher at lower equivalence ratio. This study shows that the simultaneous reduction of NOx and smoke emissions could be achieved in a DME fueled HCCI engine.

Application of delay embedding and recurrence analysis to a noisy nonlinear map for cyclic combustion dynamics of SI engines

A noisy nonlinear dynamical (NND) model available for cyclic variations in SI engines is examined using nonlinear time series methods. Heat release time series computed from this model is analyzed by employing bifurcation diagrams, return maps, recurrence plots, and recurrence quantitative analysis techniques. The analysis methodology is at first applied to a noisy Logistic map to optimize the necessary parameters, and later the same is used for the NND map. The presence of noise hides the local patterns of the dynamics and higher periodic orbits, making the bifurcation diagrams look fuzzy for both maps. An increase in the noise level and uncombusted residuals in the NND map advance the onset point of bifurcations toward a higher equivalence ratio. The application of the NND model is extended to represent the SI engine combustion dynamics for a higher amount of internal EGR as it is favored for cycle-resolved control due to its feedback period of one combustion cycle. The combined effect of lean charge and internal EGR leads to a more complex dynamical nature. The addition of a higher amount of internal EGR adds some complex chaotic-like regions in the bifurcation diagrams. For a low level of fluctuations in the NND map, more than four noisy periodic orbits are observed for such operating conditions. The geometry of phase space trajectories for the low-level noisy model data with a high level of internal EGR resembles a chaotic system. Bifurcation diagrams, recurrence plots, and quantitative recurrence analysis indicate an intermittent route to chaos at a highly lean and diluted charge. The presence of deterministic chaotic characteristics in combustion dynamics, along with effort-saving techniques of data acquisition and estimating combustion phasing, depicts the possibilities of practical application in active onboard control of engine systems.

Effect of high-pressure detonation products on fuel injection and propagation characteristics of detonation wave

The effect of high-pressure detonation products on fuel injection and propagation characteristics of detonation wave has been investigated in the form of ion voltage by varying the equivalence ratio (ER), air mass flux, and operation duration with hydrogen-air mixtures. It has been shown experimentally that the ion voltage decays gradually during the initial stage of rotating detonation wave (RDW). The attenuation of ion voltage is a general phenomenon, and the decay rate of ion voltage and its peak value of the trough state are related to the equivalence ratio and air mass flux. The analysis of interaction between the combustor and hydrogen plenum indicates that the feedback of high-pressure detonation products leads to the attenuation of ion voltage. In addition, the long-duration tests show that the ion voltage will recover to a steady state with the extension of reaction time, when the purgation (products leaving plenums) of detonation products is greater than feedback (products entering plenums) of detonation products in the hydrogen plenum. The recovery of ion voltage starts earlier at the higher equivalence ratio and air mass flux, and the peak value of ion voltage in the steady state also increases with the increase of equivalence ratio and air mass flux. A low frequency oscillation about 10–12 Hz occurs in the RDW at some operation conditions. This low frequency oscillation is related to the interaction between the combustor and hydrogen plenum, and can be eliminated by either increasing the equivalence ratio or decreasing the air mass flux.

Numerical study on the wall-impinging diesel spray mixture formation, ignition, and combustion characteristics in the cylinder under cold-start conditions of a diesel engine

The combustion of wall-impinging diesel spray is a typical phenomenon in a diesel engine due to the limited in-cylinder space, especially for small and medium-sized diesel engines under the cold-start condition. To investigate the wall-impinging diesel spray combustion process at low-temperature and low-speed conditions, the mixture formation, ignition, and combustion characteristics of the fuel were quantitatively analyzed using a computational fluid dynamics model. It was discovered that the diesel spray was guided by the piston surface and the limited in-cylinder space, resulting in more vapor-phase fuel flowing into the region between the two spray paths, Thus, high-temperature ignition (HTI) occurred in this region. During a cold-start, in-cylinder HTI also occurred in the region between the two spay paths with different injection timings. The flame then propagated first to the region with the higher equivalence ratio near the piston surface and finally to the region with the lower equivalence ratio in the center of the combustion chamber. Because an earlier injection timing resulted in both a leaner mixture in the cylinder at top dead center and the transition period from the low-temperature reactions to the high-temperature reactions, there was a limited ability to adjust the HTI and thermal power conversion by controlling the injection timing. The optimal control of in-cylinder combustion during the cold start of a diesel engine depends on the HTI, the center of combustion, and the deposited fuel mass. The optimal of injection timing during a cold start of the diesel engine in this study was −13 °CA ATDC.

Experimental investigation of synthesis gas production in fuel-rich oxy-fuel methane flames

Combustion processes with pure oxygen (oxy-fuel) instead of air as an oxidant are attractive for e.g. high temperature thermal or thermochemical and gasification processes. The absence of nitrogen as an inert gas leads in such combustion processes to an increase in temperature and species concentrations. The scope of this study is the determination of axial profiles of major educts (CH4, O2) and major combustion products such as H2, H2O, CO and CO2 in flat fuel-rich premixed methane-oxygen flames (2.5 ≤ ϕ <3.0). A Heat-Flux-burner was used to stabilize a quasi-adiabatic one-dimensional flame at a preheating temperature of TP = 300 K and atmospheric pressure. Gas samples were taken at different HAB using a quartz probe and analyzed by a GC/MSD. The influence of the probe on determined species concentrations was investigated using CFD simulations. Additionally, one-dimensional calculations with detailed chemistry were performed using the GRI3.0 and CalTec2.3 mechanism. Their differences in regards to H2, H2O and CO concentrations were thoroughly investigated and could be explained by the detailed C2-chemistry of the CalTech2.3. The results of the analyzed gas samples show a rapid reduction and increase in the flame zone, of the educts, major products and higher hydrocarbons (C2H2, C2H4 and C6H6), respectively. Both mechanisms show a sharper gradient of the synthesis gas production in comparison to the experimental results, nevertheless a tendency towards the CalTech2.3 scheme is observed. In the post flame zone the determined species concentrations of H2 and CO2 correspond to the results of the GRI3.0, whereas in the case of H2O and CO the CalTech2.3 showed better performance. The calculations of higher hydrocarbons are in better agreement with higher equivalence ratio.

NOx emission reduction in ammonia-powered micro-combustors by partially inserting porous medium under fuel-rich condition

Based on the fact that NOx generation will be inhibited under the condition of high equivalence ratio, the micro-combustors with double-rib structure partially inserted with porous medium (PM) are selected to examine the NOx emission performances in fuel-rich combustion of ammonia. The effects of: 1) the inlet velocity, 2) the porous material and 3) the porosity are evaluated on NOx emission reduction. Besides, the kinetic rate of key reactions contributing NOx production are analyzed. It is found that, the PM can effectively reduce NO generation and emission under a larger inlet velocity. With the increased thermal conductivity of porous material, the emission of NO is shown to be decreased by 99.98% from 2.70 × 104 ppm to 5.17 ppm, while N2O emission is increased by 10000% from 24.8 ppm to 2.52 × 103 ppm. In addition, the porous material with a higher thermal conductivity is beneficial to improving the uniformity of the combustor outer wall temperature. When the PM’s porosity is increased from 0.5 to 0.9, NO emission is increased by 4120% from 3.53 × 102 ppm to 1.49 × 104 ppm, while N2O emission is decreased by 71.5% from 3.29 × 103 ppm to 9.38 × 102 ppm. The present work reveals that the NOx emission can be effectively reduced, while increasing the thermal performance of micro-combustors by adopting appropriate porous medium.

Effects of pulsed injection on ignition delay and combustion performance in a hydrogen-fuel scramjet combustor

This paper describes the pulsed injection of hydrogen fuel with an incoming Mach number of 2.5 in a directly connected pulse combustion wind tunnel. The hydrogen pulse injection test with 33 Hz–62hz frequencies was carried out by using a mechanical pulse injector.The effects of different injection frequencies on the ignition delay and combustion performance of the hydrogen fuel are analyzed by means of flame self-luminescence photography and wall pressure measurements, and the results are compared with those of steady injection. The results shows that there is an optimal pulsed injection frequency--39 Hz in the frequency range studied in this paper, at this frequency, the ignition delay of hydrogen is much shorter than that of steady injection.In addition, the study also found that pulsed injection improves the combustion performance of hydrogen, so that the energy released by hydrogen combustion at lower equivalence ratios is equal to that released by steady hydrogen injection at a higher equivalence ratio. For both pulsed and steady injection, the development of the hydrogen flame can be attributed to the propagation theory of diffusion flame. The pulse frequency is not observed to have any significant effects on hydrogen combustion performance.

Experimental Study on the Influence of Gas-Solid Heat Transfer in a Mesoscale Counterflow Combustor

ABSTRACT Combustion at small scales is inhibited by the large amount of external surface area leading to excessive heat losses and extinguishment. This limitation is diminished by transferring heat from the products to the reactants through solid surfaces leading to the development of heat recirculating reactors. A promising design is the counterflow configuration in which reactants in one channel are preheated by energy transferred through the wall from the products in the adjacent channel. In the lean equivalence ratio limit of the reactor, this arrangement encourages the stabilization of the flame. In the high equivalence ratio limit, the enhanced heat transfer may encourage blow-off. Therefore, it is important to understand the relationship between heat transfer and the operating range of the reactor. In this paper, the importance of the channel surface area-to-volume ratio is investigated analytically and experimentally in a mesoscale combustor. Reactors with different channel shapes were fabricated via additive manufacturing and studied. The change in heat transfer area affected the stable operating range, maximum temperature, and location of flame stabilization. The emission measurements showed low CO and NOx emissions. For all reactors, the stable range was limited by flashback when the burning rate exceeded the flow velocity and by blow-off when the burning rate was insufficient. This work quantifies the importance of heat transfer surface area to the operation of the counterflow reactor and guides the optimization of reactor design.

Development of a reduced chemical kinetic mechanism for biodiesel/natural gas mixture

Biodiesel as an alternative fuel for diesel in compression ignition engines cab be used directly to this type of engine without any significant changes in geometry. Some negative points of biodiesel in compression ignition (CI) engines such as high viscosity, injector carbonize and high NOx emissions cause to use this fuel with other fuels such as alcohol fuels and natural gas based on desired properties of second fuel. Numerical study of fuels such as biodiesel needs to have a chemical kinetic mechanism that can predict combustion characteristics and output emissions. In this study, a reduced detailed mechanism of biodiesel introduced by Zhang et al. is chosen to merged with natural gas GRI-Mech 3.0 mechanism to obtain a chemical kinetic mechanism of biodiesel/natural gas. Direct relation graph with error propagation (DRGP) method and consequently sensitivity analysis was employed to 525 reactions and 126 species that modified with highly sensitive reactions. At first, reduced mechanism of biodiesel/natural gas compared with merged mechanism of these fuels’ mixture with 0-Dimension simulation of ignition delay and 1-Dimension simulation of flame speed. Obtained results show error less than 1% for all intake temperatures and equivalence ratios. In the following, the reduced mechanism with zero percent of natural gas compared with original biodiesel mechanism. Yet again, results show errors almost near zero in lower equivalence ratio and acceptable error, in maximum condition about 10%, in moderate and high equivalence ratios (ERs) during predicting ignition delay with 0-Dimension simulation. Also, flame speed in this evaluation shows less than 2% error in 1-Dimension simulation.

A comparative study on the autoignition characteristics of cyclopropane and propane at high temperatures

An understanding of the autoignition characteristics of cyclopropane, the smallest cycloalkane, would have potential implications for constructing combustion and pyrolysis models for polycyoalkane fuels. However, there are limited studies on the combustion chemistry of cyclopropane in the literature, and the ignition difference between cyclopropane and propane is also unclear. In this work, we explored the reaction pathways of cyclopropane and propane at high temperatures by quantum-chemical calculation and kinetic modeling, and a combustion mechanism for cyclopropane was constructed based on the public core mechanism, UCSD. The computed results show that cyclopropane has a ring-opening reaction, H-abstraction and addition-ring-opening reactions with active radicals and oxygen molecules in the chain initiation process, while propane only has the usual H-abstraction and bond dissociation reactions. Moreover, the kinetic modeling results show that cyclopropane (air mixture) has a shorter ignition delay time (IDT) than propane at a high equivalence ratio (ϕ = 3, p = 5.5 atm), but a longer IDT than that of propane when the equivalence ratio decreases to 0.333, in accordance with available experimental data. Sensitivity analyses demonstrate that reactions of H and CH3 radicals with oxygen molecules accelerate both the ignition of cyclopropane and propane under the conditions considered in this study. Reaction pathway analyses indicate that cyclopropane is consumed through conversion to propylene at high temperatures, whereas oxidation of propane is mainly initiated by H-abstraction reactions. In contrast with propane, the IDTs of cyclopropane increase with decreasing equivalence ratio. The reason is that ring strain favors the ring-opening reaction of cyclopropane to propylene both kinetically and thermodynamically. At high equivalence ratios, more energy is released, leading to an increase in the system temperature which promotes the ignition of cyclopropane.

Flammability enhancement of swirling ammonia/air combustion using AC powered gliding arc discharges

Aiming at the potential development of ammonia as a carbon-free renewable fuel, this work investigates the flammability and NOx emission of swirling ammonia/air flames and utilizes plasma to enhance ammonia combustion. First, the well-designed rapidly-mixed swirl burner can anchor compact ammonia/air flames under a wide range of flow conditions, with the current maximum heat release of approximately 4.7 kW. The flame is progressively detached from the quartz confinement tube and goes blow-off when the equivalence ratio drops below approximately 0.7–0.8. Then, to alleviate the problem of low flammability, gliding arc discharges driven by a 12.5 kHz alternating current (AC) power supply are facilitated to extend the lean blow-off margin to approximately 0.3–0.4. The localized flame kernels surrounding the discharge column are detected by high-speed photography. The planar laser-induced fluorescence (PLIF) imaging of OH radicals, optical emission spectroscopy, and NH2* chemiluminescence measurement are performed to interpret the intermediate chemistry. Finally, the NOx emission of the swirling ammonia/air flame is measured by a flue gas analyzer. Results show that although the AC-powered gliding arc exhibits weak global effects on the NOx reduction of burner-stabilized ammonia/air flames prevailing at higher equivalence ratios (e.g., φ > 0.75), leaner flames stabilized by discharges can achieve NOx emissions below 100 ppm due to the thermal DeNOx mechanism.

Characterization of energy dissipative cushions made of Ni–Ti shape memory alloy

Earthquake-resistant design of structures requires dissipating seismic energy by deformations of structural members or additional fuse elements. Owing to its easy-to-produce, plug-and-play, high equivalent damping ratio, and large displacement capacity characteristics, energy dissipative steel cushions (SCs) were found to be an efficient candidate for this purpose. However, similar to other conventional metallic dampers, residual displacement after a strong shaking is the most notable drawback of the SCs. In this work, cushions produced from Ni–Ti shape memory alloy (SMA) are evaluated numerically by experimentally verified finite element models to assess their impact on the performance of earthquake-resistant structures. Furthermore, a reinforced concrete testing frame is retrofitted with energy dissipative steel and Ni–Ti cushions. Performance of the frames (e.g. dissipated energy by the cushions, hysteretic energy to input energy ratio, maximum drift, and residual drift) with different types of cushions are evaluated by nonlinear response history analyses. The numerical results showed that the SCs are effective to reduce peak responses, while Ni–Ti cushions are more favorable to reduce residual drifts and deformations. Hence, a hybrid system, employing the steel and SMA cushions together, is proposed to reach optimal seismic performance.