Ignition Enhancement Research Articles

Investigation on ignition enhancement characteristics by high-energy spark in a scramjet combustor under low flight Mach conditions

Ignition characteristics of the long electrode distance high-energy spark igniter (LHSI) in a cavity-stabilized scramjet combustor at low flight Mach has been numerically and experimentally investigated and compared with those of conventional lower energy spark igniter (SI). The effects of ignition and injection schemes on ignition performance were analyzed and the enhancement of greater spark energy were verified. The results show that the widest ignition boundary of SI is observed when the kerosene is injected in far field due to the sufficient evaporation of kerosene droplets and the disturbance caused by upstream cavity. Due to the greater energy deposition by LHSI, the ignition performance is markedly improved and the ignition boundary of combustor is extended from 0.131–0.148 to 0.091–0.224 under the scheme of I1+L2. The increased energy deposition of spark plasma ignites more combustible mixture and enhances the initial combustion, leading to a larger high-temperature zone. This is crucial for ensuring the survival of the flame kernel against dissipation of airflow in the scramjet combustor operating at low fight Mach numbers due to the short duration of spark discharge.

Experimental study on the ignition delay characteristics of pyrolysis gas from kerosene with hydrogen peroxide addition

The ignition issue of the hypersonic aircraft remains a challenging task due to the extremely short residual time of milliseconds. To study the potential candidate of hydrogen peroxide for ignition enhancement, the ignition delay times of pyrolysis gas with the addition of 0–5000 ppm hydrogen peroxide were experimentally measured and quantitatively analyzed at temperatures of 833.8–962.1 K, equivalence ratios of 0.5–1.5, pressures of 31.9–48.8 bar, and the dilution of 45 vol% Ar. A novel injection system based on a Laval nozzle was designed to well atomize and evaporate the liquid hydrogen peroxide. The experimental results showed that the enhancing effect of hydrogen peroxide was proceeded by three stages, in which the decreasing efficiency gradually weakened with the increase of additive fraction. The ignition delay times of pyrolysis gas dramatically shortened by 66.1 % with 1000 ppm of hydrogen peroxide. Only a decrement of 19.1 % was obtained as the additive fraction increased from 1000 to 4000 ppm. The decline at 4000–5000 ppm even reduced to 2.0 %. It is suggested to add no more than 1000 ppm of hydrogen peroxide from the perspective of the highest acceleration efficiency on the ignition delay times of pyrolysis gas.

Investigation on the exploding foil initiators ignition enhanced by explosion-electricity coupling

Explosion-electricity coupling (EEC) is a technical method to induce electric energy into the plasma material produced by explosion to improve the output of explosion. Exploding foil initiator (EFI) which could produce plasma during electric explosion can serve as a good carrier for studying the EEC. To investigate the enhancement ability and mechanism of EEC in EFI ignition performance, a kind of EFI chips which could realize the EEC effect was designed and fabricated to observe the characteristics of current and voltage, flyer and plasma temperature during Boron Potassium Nitrate (BPN) ignition of the EFI. It was found that the EEC could enhance EFI ignition in terms of energy utilization, ignition contact surface, and high-temperature sustainability of plasma: firstly, the EEC prolonged the late time discharge (LTD) phase of the electric explosion, making the energy of capacitor effectively utilized; secondly, the EEC could create a larger area of ignition contact surface; last of all, the EEC effect enhanced its high-temperature sustainability by sustaining continuous energy input to plasma. It also was found that the ignition voltage of BPN could be reduced by nearly 600 V under the condition of 0.4 μF capacitance. The research has successfully combined EEC with EFI, revealing the behavioral characteristics of EEC and demonstrating its effective enhancement of EFI ignition. It introduces a new approach to improving EFI output, which is conducive to low-energy ignition of EFI, and expected to take the ignition technology of EFI to a new level.

Enhancement of plasma assisted ignition by multi-voltage pulse discharges: A theoretical study

Currently, compression ignition engine technology is becoming less competitive compared to spark ignition engine technology due to emission standards. Lean combustion strategies are preferable for spark ignition engines, but they require enhanced ignition strategies compared to current stoichiometric spark ignition engines. In this paper, we lay physical foundations for such enhanced ignition strategies that can support lean combustion strategies in spark ignition engines. In this theoretical study, we conduct a preliminary foray into the possibility of controlling the species composition of a cold-plasma kernel as a precursor for an ignition process. We consider a multi-pulse electric source system coupled to a DBD (Dielectric Barrier Discharge) electrode immersed in a dry air medium. The coupled system is designed such that the medium in the electrodes’ gap is subjected to a pulse structure which is composed of a peak voltage followed by a secondary low-voltage pulse. Use is made of a comprehensive mathematical model that includes the electric field, potential and current, with the relevant conservation equations for 24 species and 168 kinetic reactions, while considering different energy modes (rotational, vibrational, electronic excitation, dissociation, and ionization) within the electrodes’ gap. The model was validated against independent published results, and was used to understand the mechanism of energy conversion to species that play a central role in terms of the ultimate ignition dynamics. Various pulse repetition frequencies and different pulse constructions were considered. Special attention was given to the amount of deposited energy, and to the energy channeled to known ignition supportive modes and species. The present results show that the energy deposition can be divided into two main stages which are characterized by high and low voltage levels, respectively. We propose that for more successful ignition, the amount of energy deposition during the second (low voltage) stage should be higher than that during the first (high voltage) stage. During the second stage, the deposition of energy into specific modes can be fine-tuned by setting appropriate voltage levels. Based on these findings, we demonstrate how control of the sequence of voltage pulses can further increase enhancement of ignition and combustion promoting processes.

Optimization and impact of modified operating parameters of a diesel engine emissions characteristic utilizing waste fat biodiesel/di-tert-butyl peroxide blend

This study comprehensively investigated the transesterification process of converting waste pork fat oil into biodiesel as a fuel for the common rail direct injection (CRDI) diesel engine. In addition, Di-tert-butyl peroxide (DTBP) is used as an ignition enhancer to minimize exhaust gas emissions. The test fuels are diesel and diesel-waste pork fat biodiesel (WPFB)-DTBP blend (B20DTBP10 by volume), were used to analyze the engine outcome parameters with modified engines operating conditions like various compression ratios (CRs 16, 17.5, and 19) and injection pressures (IPs 400, 500 and 600 bar) at maximum load. The experiment results revealed that higher CRs and IPs, the heat release rate (HRR) and cylinder pressure increased. The brake thermal efficiency (BTE) is enhanced by increasing CRs and IPs but lowered than neat diesel. Smoke opacity decreased while increasing CRs and IPs. At CR 19 and IP 600 bar, smoke opacity decreased by 18.29%, related to baseline diesel. Increasing CRs and IPs lowered HC and CO emissions, but increasing IPs increased both emissions. HC and CO emissions were lowered in CR 19 and IP 500 bar by 21.73% and 28.49% more than neat diesel. The lower CRs and IPs, the oxides of nitrogen (NOx) emissions decreased, but further increasing CRs and IPs, the NOx emissions increased. Compared to baseline diesel, NOx emissions decreased in CR 16 and IP 400 bar by 15.49%. It was discovered that with increasing CRs and IPs, the emissions decreased with a minor rise in NOx emissions. Furthermore, the experimental results matched the predictions of an artificial neural network (ANN) model. The research concluded that the blend B20DTBP10 with modified engine operating conditions CRs and IPs suits diesel engine operation.

LPG-Diesel Dual Fuel Emission Characteristics: A Review

The global shift away from fossil fuels for energy, driven by concerns about emissions and energy security, has sparked significant interest in alternative fuels. One promising approach involves combining alternative fuels, like Liquefied Petroleum Gas (LPG), with traditional diesel engines. This strategy, known as the dual-fuel concept, aims to harness the strengths of each fuel type while reducing emissions and improving engine performance. This review focuses primarily on LPG as a dual fuel for diesel engines. LPG, a gaseous fuel primarily composed of propane and butane, offers substantial environmental benefits, including a 15% reduction in CO2 emissions, a 30% reduction in CO emissions, and a 50% reduction in NOx emissions compared to diesel. However, using pure LPG in diesel engines faces a challenge related to its ignition behavior. This challenge can be overcome by blending LPG with substances like Hydrotreated Vegetable Oil (HVO) or diesel fuel, which act as ignition enhancers. The main goal of this review is to provide a comprehensive overview of the potential advantages and applicability of the dual-fuel system combining diesel and LPG. It will explore various experimental setups, designs, and injection techniques, presenting diverse findings related to both engine performance and emissions. Additionally, the review will discuss additives that can further improve the combustion process. Overall, the research highlights the promising role of LPG as a dual fuel in reducing emissions and enhancing the efficiency of diesel engines, contributing to a more sustainable energy future.

Three-stage hybrid NSD/DC plasma assisted n-C5H12/O2/N2 ignition: Improved energy efficiency and reduced NOx/N2O emissions

This work presents a three-stage hybrid nanosecond discharge (NSD) and direct current (DC) discharge assisted n-C5H12/O2/N2 ignition with lower discharge energy and higher energy efficiency. The reaction rate coefficients between O2(a1Δg) and n-C5H12 are updated by mass/Master equation simulation. The promotive ignition enhancement via reaction channel O2(a1Δg) + n-C5H12 → C5H10 + H2O2 in the hybrid NSD/DC discharge is demonstrated. The results show that the ignition delay time of the three-stage hybrid plasma assisted ignition is 1–2 orders of magnitude shorter than that of the two-stage hybrid discharge when the reduced electric field strength in the DC discharge stage is over 10 Td. Meanwhile, the plasma-enhanced energy efficiency of three-stage hybrid discharge increases. This improved efficiency on ignition enhancement is due to the kinetic enhancement by the electronically excited species and radicals (O2(a1Δg), O(1D), N2(B) and O). The kinetic and thermal effects by plasma are effectively intergated to enhance ignition. However, more discharge energy deposited in the vibrationally excited states N2(v) in the two-stage hybrid discharge is less efficient on ignition enhancement due to the thermal contribution via vibrational-translational relaxation. In the aspect of NOx/N2O emissions, reduction efficiency of the three-stage hybrid discharge is two times higher than that of the two-stage hybrid discharge. By controlling the N/N(2D) production pathway e + N2 → e + N + N(2D), the three-stage hybrid discharge reduces NOx and N2O production by 61.1 % and 90 %, respectively. The results show that the ignition delay time is non-monotonically dependent on the discharge pulse number. There exist optimal pulse number, reduced electric field and effective threshold power density of DC discharge to achieve maximum ignition enhancement and low NOx/N2O emissions. This study provides valuable insights into the use of NSD/DC hybrid discharge technology in advanced engines to achieve energy-efficient ignition enhancement and reduce NOx/N2O emissions.

Accelerated ignition-shock coupling and deflagration to detonation transition by ozone kinetic enhancement of dimethyl ether mixture

This study aims to understand the effect of kinetic enhancement by ozone addition on the Deflagration to Detonation Transition (DDT) and ignition-shock coupling in a microchannel using dimethyl ether (DME). Simultaneous high-speed chemiluminescence and shadowgraph imaging are carried out to analyze the flame front propagation, shock wave structures, autoignition, and ignition-shock coupling during the DDT process. The experimental results show that for all cases, with and without ozone kinetic enhancement, autoignition and DDT always occur in a reactivity gradient region with oblique shock cluster between the flame front and the flame acceleration-induced precursor shock. The observed DDT occurs via the Zeldovich hot spot gradient mechanism through the following stages: (1) Oblique shock cluster formation and acoustic compression between the flame front and the precursor shock, (2) autoignition initiation in the oblique shock cluster region, (3) spontaneous ignition wave propagation, and (4) ignition-shock coupling. It is found that autoignition location, oblique shock strength, induction zone length, ignition-shock coupling, and DDT onset flame velocity are very different from ozone kinetic enhancement. As the ozone addition is higher, the autoignition is advanced and the ignition location moves farther ahead of the flame front at a lower flame front propagation speed and weaker oblique shock clusters. Two different ignition-shock coupling modes are identified depending on ozone concentration. Without ozone kinetic enhancement, DDT occurs via ignition wave coupling with the precursor shock. However, ozone addition can accelerate the autoignition and initiate a direct ignition-shock coupling for DDT without interaction with the precursor shock. The present results show that kinetic enhancement in autoignition using an ignition enhancer such as ozone or plasma is critical and effective in accelerating and controlling DDT.

Towards sustainable transportation and integrating ANN, RSM, and exergy analysis for biofuel-diesel blend optimisation of biodiesel ignition enhancer blends

Towards sustainable transportation and integrating ANN, RSM, and exergy analysis for biofuel-diesel blend optimisation of biodiesel ignition enhancer blends

Towards sustainable transportation and integrating ANN, RSM, and exergy analysis for biofuel-diesel blend optimisation of biodiesel ignition enhancer blends

Towards sustainable transportation and integrating ANN, RSM, and exergy analysis for biofuel-diesel blend optimisation of biodiesel ignition enhancer blends

Ignition enhancement and NOx formation of NH3/air mixtures by non-equilibrium plasma discharge

This work computationally studies plasma assisted low temperature NH3/air ignition and NOx formation in a repetitively-pulsed nanosecond discharge at atmospheric pressure by using an experimentally-validated plasma-combustion kinetic model. The results show that plasma discharge significantly enhances low temperature NH3 ignition. Compared with thermal ignition, the ignition delay time is shortened by 2–5 orders of magnitude due to the kinetic enhancement of excited species and radicals. The radicals (NH2, NH, H and O) produced through electron impact dissociation and quenching of electronically excited N2*, O(1D) and N(2D) promote OH production and further accelerate NH3 oxidation. The results show that there exists a non-monotonic dependence of ignition delay time on the reduced electric field strength. The optimum ignition enhancement is achieved at 250 Td at which the production of electronically excited species and radicals is most efficient. The vibrationally excited species produced at lower electric fields (<100 Td) are less effective in enhancing ignition because they only induce gas heating through the fast vibrational-translational relaxation by NH3 and H2O. At a higher electric field, although the efficient production of NH2, NH, H, O and OH by plasma creates new low temperature reaction pathways in enhancing low temperature NH3 ignition, the ignition is inhibited through NH + NO = N2O + H and chain-termination reaction NH2 + HO2 = NH3 + O2. The ignition delay times at different equivalence ratios show that the ignition enhancement by plasma is more effective at fuel-lean conditions due to the faster generation of N2(B), O(1D) and O from air, leading to accelerated NH3 oxidation via O(1D) + NH3→ NH2 + OH, NH3 + O = NH2 + OH and NH2 + O = NH + OH. The sensitivity analysis shows that the reactions involving O and O(1D) production are more effective on NH3 ignition enhancement than the fuel dissociation by electrons. Moreover, the ignition is also enhanced by NOx formation in plasma via reactions NH2 + NO = NNH + OH and NO + HO2 = NO2 + OH. This work advances the understanding of non-equilibrium excitation and NOx formation by plasma discharge on low temperature NH3 ignition.

Modeling study of the ignition characteristics of directly injected 1,3-Cyclohexadiene in surrogate Gasoline/Air under RCCI combustion conditions

In this work a validated chemical kinetic model of the combustion of multicomponent gasoline surrogate fuels containing 680 reactions of 152 species has been used to simulate conditions reminiscent of those of Reactivity Controlled Compression Ignition (RCCI) in order to evaluate 1,3-cyclohexadiene (CYC6H8) as the high reactivity directly injected fuel and a gasoline surrogate fuel as the low reactivity premixed fuel. A single-zone modeling approach was adopted using a modified Cantera example describing the simulation of a gaseous Diesel-type internal combustion engine. The adaptable chemical structure of CYC6H8 to cis-1,3,5-hexatriene (lC6H8), load, start of injection and premixed fuel mass fraction were varied in order to analyze the potential of CYC6H8 as an ignition enhancer. Results have been compared to n-heptane and show that CYC6H8 and its isomer lC6H8 have the potential to be used as the high reactivity directly injected fuel in dual-fuel RCCI combustion, especially at high load conditions. For increased load, the ignition promoting effect is more pronounced at lower premixed fuel mass fractions and less affected by a delayed start of injection. The ignition promoting effect of lC6H8 is more pronounced than CYC6H8 for a decreasing premixed fuel mass fraction. Results from the kinetic analysis indicate that at increased concentrations of lC6H8, the thermal effects are becoming more significant.

Identifying the enhancement mechanism of Al/MoO3 reactive multilayered films on the ignition ability of semiconductor bridge using a one-dimensional gas-solid two-phase flow model

Energetic Semiconductor bridge (ESCB) based on reactive multilayered films (RMFs) has a promising application in the miniature and intelligence of initiator and pyrotechnics device. Understanding the ignition enhancement mechanism of RMFs on semiconductor bridge (SCB) during the ignition process is crucial for the engineering and practical application of advanced initiator and pyrotechnics devices. In this study, a one-dimensional (1D) gas-solid two-phase flow ignition model was established to study the ignition process of ESCB to charge particles based on the reactivity of Al/MoO3 RMFs. In order to fully consider the coupled exothermic between the RMFs and the SCB plasma during the ignition process, the heat release of chemical reaction in RMFs was used as an internal heat source in this model. It is found that the exothermal reaction in RMFs improved the ignition performance of SCB. In the process of plasma rapid condensation with heat release, the product of RMFs enhanced the heat transfer process between the gas phase and the solid charge particle, which accelerated the expansion of hot plasma, and heated the solid charge particle as well as gas phase region with low temperature. In addition, it made up for pressure loss in the gas phase. During the plasma dissipation process, the exothermal chemical reaction in RMFs acted as the main heating source to heat the charge particle, making the surface temperature of the charge particle, gas pressure, and gas temperature rise continuously. This result may yield significant advantages in providing a universal ignition model for miniaturized ignition devices.

Prediction and performance optimisation of a DI CI engine fuelled diesel–Bael biodiesel blends with DMC additive using RSM and ANN: Energy and exergy analysis

Synthesis of biofuel from bioresources represents one of the greatest propitious options for achieving a cleaner production of energy and the global circular bio-economy. The superior cetane rating of biodiesel makes it a fitting fuel for CI diesel engines. The present study aims to valorise Bael seeds for biodiesel production. Bael is a species of treeindigenoustoIndia. The diesel–Bael biodiesel was blended with dimethyl carbonate (DMC) as an ignition enhancer and investigated in a selected diesel engine. This work reports that the Bael biodiesel blend gets a tolerable level of BTE against neat fossil diesel. An utmost BTE of 30.68% was achieved with B15DMC5 which is very close to diesel (31.8%) at peak load. Certainly, BSFC was suppressed and emissions lessened by DMC inclusion. For instance, the CO and HC emissions substantially reduced to a minimum of 0.19% and 179 ppm with the B17.5DMC2.5 blend at peak load. NOx emission is directly proportional to Bael biodiesel concentration, Notably, NOx emission is well controlled at 50% and 75% load even though with the increasing biodiesel concentration in the blend. Though, control of emissions at peak load is still a crucial issue. Nevertheless, the DMC additives controlled the NOx emission at peak load. B15DMC5 blend exhibits progress in combustion through the optimized value of HRR, CGP, and CGT against B20. The presence of NOx and smoke opacity without additives is suppressed up to 7% and 10% with additives. Availability (exergy) and exergy efficiency at different loads of both neat diesel and blended fuels, mainly shaft, cooling water, and exhaust availability were calculated. The results of the study illustrated that the input availability increased at peak loads, as well as gross work output, was maximum at peak load due to higher fuel exergy present in the combustion chamber. A maximum exergy efficiency of 68% was recorded by B15DMC5 at peak load. Finally, the process optimization by RSM has been validated by experimental results and further authenticated with ANN. The results of the RSM model promote it as statistically significant. Generate the regression equation for all output responses. The optimized operating parameters of engine load of 50%, 10% of biodiesel blend, and additive concentration of 3.59% achieved the most optimum output response. The results of the predicted optimized condition of BTE, BSFC, CO, HC, NOx, and Smoke opacity showed 24.051%, 0.441861 kg/kW hr, 0.2028%, 113.607 ppm, 316.099 ppm, and 14.4286% respectively. The R2 values from RSM and ANN DoE models were 0.992834 and 0.99984. This study concludes that a superior precision certainty from both RSM and ANN models is beneficial for engine overall performance estimation. This phenomenon provides a good sign to implement the ANN to forecast engine operational behavior in the future. The novel approach of process optimization by RSM and ANN for Bael biodiesel with DMC blended fuel operate DI CI engine supports the SDGs of the UN such as Good health and Well-being (SDG: 3), Affordable and Clean Energy (SDG: 7), Responsible Consumption and Production (SDG: 12), Climate Action (SDG: 13) and Life on Land (15).

Effect of pilot fuel injection strategies and EGR on a CRDI diesel engine powered by simmondsia chinensis seed biodiesel-methyl acetate blend

The primary goal of this research is to produce energy from discarded Simmondsia chinensis seeds. In a CRDI diesel engine, methyl acetate (MA), a novel and renewable component, is blended with Simmondsia chinensis biodiesel (SCB), and various fuel injection techniques and exhaust gas recirculation (EGR) are employed. After that, the fuel was compared to diesel fuel to improve ignition patterns and reduce exhaust emissions. Non-edible biodiesel has a higher viscosity than diesel, resulting in higher emissions. So, the novel oxygenated additive MA was used as an ignition enhancer. Initially, the test used diesel at 10% EGR and DBMA20 (50% Diesel, 30% SCB + 20% MA by vol.) at 10% EGR. Results found that the brake thermal efficiency (BTE) decreased, and emissions increased. Further, work carried out modified operating parameters pilot injection timing (PIT), pilot fuel injection quantity (PFIQ), and EGR at maximum load conditions. The experiments were done with an optimal blend ratio of DBMA20 at 10% EGR with PIT (35°bTDC, 40°bTDC, and 45°bTDC) and PFIQ (10, 15, and 20% by vol.). The results revealed that at peak load, PIT 45° at PFIQ 20%, the BTE increased by 1.71%, BSFC decreased by 21.87%, SO reduced by 26.61%, HC reduced by 47.69%, CO reduced by 56.41%, and NOx reduced by 11.34 % when contrasted with the DBMA20 outcomes with 10% EGR at standard injection settings. So, the PIT 45° at PFIQ 20% with 10% EGR was better than the other operating conditions. As a result, using biodiesel along with a pilot fuel injection technique was viable in operating conditions to improve efficiency and reduce emissions in CRDI diesel engine applications.

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.

Long-lasting XUV activation of helium nanodroplets for avalanche ionization

We study the dynamics of avalanche ionization of pure helium nanodroplets activated by a weak extreme-ultraviolet (XUV) pulse and driven by an intense near-infrared (NIR) pulse. In addition to a transient enhancement of ignition of a nanoplasma at short delay times fs, long-term activation of the nanodroplets lasting up to a few nanoseconds is observed. Molecular dynamics simulations suggest that the short-term activation is caused by the injection of seed electrons into the droplets by XUV photoemission. Long-term activation appears due to electrons remaining loosely bound to photoions which form stable ‘snowball’ structures in the droplets. Thus, we show that XUV irradiation can induce long-lasting changes of the strong-field optical properties of nanoparticles, potentially opening new routes to controlling avalanche-ionization phenomena in nanostructures and condensed-phase systems.

Two enhancements in microwave-assisted spark ignition and their causes

As a prospective ignition technique, microwave-assisted spark ignition (MAI) efficiently enhances ignition with plasma absorbing microwave energy. This paper achieves MAI through radiating a single 0.2 ms microwave pulse after the initial spark ignition, and investigates the ignition performance and the precise absorbed energy of MAI in different absorption patterns. The absorption pattern is tailored by the microwave pulse power. The results show that the enhancement of MAI can be divided into two categories based on ignition performance and absorbed energy: the glow enhancement and the incandescence enhancement. Compared to the glow enhancement, the incandescence enhancement has a greater energy absorption efficiency and thus induces more significant ignition enhancement. Additionally, the effect of gas parameters, including gas pressure and components, on these two enhancement categories is studied. Increasing pressure or decreasing methane concentration facilitates the generation of incandescence enhancement. Ultimately, the causes of both enhancements are discussed. The incandescence enhancement may be originated from the ionization of metal particles in the microwave field, whereas the glow enhancement is attributed to gas ionization. This work highlights the potential application of metal-microwave discharge on MAI, which can play a crucial role in suiting different conditions and further understanding the mechanisms of MAI.

Nitrogen Jet Array for Ignition Enhancement in a Rocket-Based Combined Cycle Combustor

In a rocket-based combined cycle combustor operating at scramjet mode, the size of the cavity recirculation flow might be reduced when the cavity is subjected to an expansion fan, which is detrimental to ignition. To address this challenge, a method based on a nitrogen jet array is proposed and evaluated in a combustor with a Mach 2.92 inflow. Nitrogen is supplied through injectors in the cavity. In the case without nitrogen injection, the cavity recirculation flow is so small that the spark plug cannot ignite the combustor. In stark contrast, the interaction between the supersonic inflow and the nitrogen jets on the cavity ramp significantly elevates the pressure in the cavity, and it enlarges the cavity recirculation flow. During the ignition process, the enlarged cavity recirculation flow is essential to holding the weak self-sustained flame. The heat, hot products, and radicals generated by the weak self-sustained flame elevate the pressure in the cavity and further enlarge the cavity recirculation flow, which paves the way for a successful ignition. Once a successful ignition is achieved, turning off the nitrogen jet array does not lead to flame extinction. Compared to the mass flow rate of the inflow, the nitrogen consumption for ignition enhancement is minor.

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.