Plasma Flame Research Articles

Microwave-induced plasma on spherical flame dynamics of spark-ignited hydrogen-air under water dilution

Microwave-assisted spark ignition (MAI) offers a commercially feasible way to enhance conventional spark ignition performance under extreme conditions. While previous studies suggested microwave pulses had no obvious effect on self-sustained flame from the perspective of energy deposition, the hydrodynamic effect of microwave plasma remained under-explored. This study experimentally investigated the effect of the microwave-induced plasma on lean hydrogen flame dynamics (Lewis number ∼ 0.35), examining the influence of water content (rH2O) in this process under different pulse repetition frequency (PRF) and pressures. Using a linear high-speed shadow imaging system coupled with electrical diagnostics, we synchronously recorded the flame and plasma morphology evolution while monitoring spark and microwave energy. Results revealed that microwave pulses during spark ignition generated wrinkles and perturbances accelerating the cracking and cellularization of the subsequent self-sustained flame. This effect intensified with increasing PRF from 1 to 10 kHz, particularly at high rH2O. The overall microwave impact on early flame development was more pronounced at high rH2O, attributed to stronger interactions between the microwave plasma jet and flame front due to reduced distance. Interestingly, electrical diagnostics showed an inversed relationship between total absorbed microwave energy and rH2O, highlighting the crucial role of the first microwave pulse at 1 kHz PRF. This study demonstrates that early microwave pulses can influence hydrogen flame dynamics even in the self-sustained stage, offering new insights into MAI mechanisms. These findings open avenues for research into active control of self-sustained flame dynamics, potentially leading to improved ignition strategies in challenging combustion environments.

Effects of Zr/Ti ratios on the structure and ablation characteristics of (Zr,Ti)C–SiC composites

Dense (Zr,Ti)C–SiC composites containing a dual-phase solid solution were fabricated by spark plasma sintering using homemade Mg-modified multiphase ceramic powders as raw materials. The ablation behaviour suggested that the composite with a molar ratio of Zr/Ti = 3:1 exhibited the best ablation resistance, with low mass and linear ablation rates of −2.56 ± 0.89 mg/s and 1.26 ± 0.40 μm/s, respectively, after ablation for 90 s under a plasma flame test at around 2300 °C. It is asserted that the formation of an oxide film consisting of a self-sintered ZrO2 skeleton and a high-viscosity Ti–Mg–Si–O liquid phase was responsible for the excellent ablation resistance, which has remarkable efficacy in resisting the scouring effect of ablative flames and the erosion induced by oxidizing gases on the composite matrix.

A new 915 MHz coaxial-line-based microwave plasma source

Microwave plasma is known for its versatility in providing tailored operating conditions (pressure, working gas composition and residence time of reagents) for specific applications. Microwave plasma sources (MPSs) are vital in modern applications, demanding continuous improvement. This work introduces a coaxial-line-based nozzleless MPS that operates at atmospheric pressure at an unique frequency of 915 MHz. The measured electrodynamic characteristics in nitrogen of the MPS highlighted the need for improved energy efficiency of the device. The main novelty of this work lies in improving an energy efficiency of the presented MPS, which led to an advanced new version of the device. To achieve this, a dual strategy is employed. Firstly, numerical simulations are used to design a construction modifications to the MPS, which should increase the efficiency of transferring microwave energy from the microwave source to the generated plasma. In this step, a standard model for homogeneous plasma and a two-port equivalent method were used. Then, the theoretical results were experimentally validated by manufacturing a new energy improved version of the MPS. In the new MPS the achieved reflected microwave power (losses) was less than 3% of incident microwave power in the tested range of nitrogen flow rate (50–100 Nl/min). Compared to the MPS before improvement, this means a two-fold decreasing the reflected microwave power. To test the new MPS, the electrodynamic characteristics of the new device version and properties of the microwave plasma generated in nitrogen, using optical emission spectroscopy (OES), were investigated. The OES was used to determine the vibrational Tvib and rotational Trot temperatures of nitrogen molecules and molecular ions. In this work, the estimated Tvib and Trot temperatures for nitrogen molecules ranged from 4000 to 5300 K, depending on discharge conditions, while for nitrogen molecular ions, the temperatures changed between 4700 and 6100 K, respectively. Both the Tvib and Trot temperatures decrease linearly along the plasma flame.

Flow characteristic investigation of localized arc filament plasma assisted turbulent jet and Bunsen flame

Localized arc filament plasma actuators (LAFPAs) have shown the capability to alter the entrainment of freestream air into a jet. This paper presents an investigation into the effects of LAFPAs on the air jet and methane/air premixed Bunsen flames at different Reynolds numbers. The flow disturbance covering the laminar to turbulent transient conditions was generated by a high voltage plasma discharge system of LAFPAs. The high speed Z-type Schlieren technique was applied to visualize the instantaneous flow and flame structures, and an optical flow algorithm was used to estimate the velocity distribution and further analyze the turbulent effect induced by the plasma. The results illustrate that, in the presence of LAFPA operation, the turbulence jet was accelerated and the jet mixing enhanced. Meanwhile, with the help of LAFPAs, the global consumption burning velocity increased significantly by the area enhancement via plasma–flame interaction. The flame front response to the flow disturbance consists of more surface stretch and flame separation. Furthermore, turbulence spectra analysis of the images reveals a considerable increase in turbulent fluctuations in both cold flow and reacting flow.

Improving the Power Efficiency of a Microwave Plasma Source by Using the Principle of a Variable-Impedance Waveguide

Microwave plasma sources are used in various applications, but inefficiency can limit their usefulness. Researchers have investigated a new approach with which to improve efficiency by using a waveguide with a special tuner. A standard waveguide was compared to one equipped with a stub tuner, which allows for the fine-tuning of the electrical properties. The experiment used a frequency of 2.45 GHz and tested input powers of 1 kW and 1.5 kW. Experiments showed that waveguides with tuners transferred power more efficiently, with fewer reflected and more transmitted microwaves. While peak temperatures remained similar, the tuned waveguide produced a hotter plasma flame (30–60 °C) at the same distance. Notably, increasing the input power further boosted the flame temperature by 300 °C. These findings suggest that stub tuners can significantly improve the efficiency of microwave plasma sources. This paves the way for more efficient plasma generation technologies and potentially allows for applications requiring even higher power.

High-temperature ablation behavior and failure mechanism of (Hf0.25Zr0.25Ti0.25Ta0.25)C high-entropy carbide in wide temperature range

The ablation behavior of (Hf0.25Zr0.25Ti0.25Ta0.25)C was investigated using a plasma flame gun over a wide temperature range of 2070–2600℃. The structural transformation during ablation and the effect of temperature on the failure mechanism were discussed. Results showed a sharp increase in mass and linear ablation rates from 0.23 mg·cm−2·s−1 and 4.6 μm·s−1 at 2070℃ to 4.6 mg·cm−2·s−1 and 37 μm·s−1 at 2600℃. This variation was attributed to the temperature's impact on on the microstructure and composition of the oxide layer. During ablation, Hf/Zr-rich oxycarbides preferentially nucleated within the amorphous Hf-Zr-Ti-Ta-C-O layer, leading to phase separation of the oxide layer. With increasing ablation temperature, the filler and binder oxides with low melting point (e.g., (Hf,Zr)TiO4, TaZr2.75O8) were rapidly consumed. The skeleton oxides (e.g., (Hf,Zr)O2) failed to bond and peeled off. The transiently formed (Hf,Zr,Ti,Ta)Ox layer was repeatedly destroyed and reconstructed, resulting in the continuous damage and eventual failure of the matrix.

Hydrogeochemical characterization and water quality evaluation for drinking and irrigation purposes of coastal aquifers of Middle Andaman

A comprehensive study to characterize the hydrogeochemistry of the region and the impact of saltwater intrusion on groundwater was conducted along the eastern coast of Middle Andaman of Andaman and Nicobar (A&N) Islands. The escalating population growth and intensified tourism activities have resulted in the over-extraction of groundwater. Seismic activities led to the opening of lineaments to the sea and the dissolution of limestone in the influence of seawater. 24 groundwater samples and 1 reference sample from sea were taken from various locations of middle Andaman. The analysis involved the determination of major cations, anions, and heavy metals using Inductively Coupled Plasma Optical Emission spectroscopy (ICP), spectrophotometry, and flame photometry. Furthermore, X-ray diffraction analysis, binary diagrams, Chloro-alkaline indices (CAI), Gibbs Plot, correlation matrix, Piper plot, Chadha’s plot and Principal Component Analysis (PCA) to the major ions data indicated rock-water interactions, strong correlations among alkali and alkaline earth metals, and interactions between seawater and carbonate minerals respectively. The water quality index indicated “very poor to unsuitable for drinking purposes” in 24% of the samples. Additionally, indices for irrigation suitability; total hardness (TH), residual sodium carbonate (RSC), and Magnesium adsorption ratio (MAR) were found to be detrimental for irrigation in 80%, 08%, and 12% of the water samples, respectively. These results highlight the importance of implementing effective water resource management techniques, such as groundwater extraction rates, adopting appropriate water treatment technologies to mitigate the adverse impacts of heavy metals, saltwater intrusion and maintain water quality for human consumption and agricultural use in the region.

Ablation behavior of ZrC-SiHfOC-MoSi2 coating for carbon/carbon composites under Ar-O2 plasma flame

To improve ablation resistance of ZrC-SiMOC-based (M = transition metal) coatings, a ZrC-SiHfOC-MoSi2 coating was investigated. The results showed that the MoSi2 shell limits the decomposition of SiC under SAPS and decreases pores of the coating. ZrC-SiHfOC coating showed large-area peeling after an Ar-O2 plasma flame ablation for 90 s. In contrast, ZrC-SiHfOC-MoSi2 coating revealed low ablation rates after ablation for 360 s, with the mass and linear ablation rates of −0.05 mg/s and 0.14 μm/s, respectively. MoSi2 provides rich SiO2 to form ZrSiO4 to reduce the damage by plasma flame, thereby improving the ablation resistance of ZrC-based coatings.

Microstructure and ablation behaviour of C/C-SiC-ZrC-Cu composites prepared by reactive melt infiltration

To satisfy the requirements for the new generation solid rocket motors on high ablation-resistant throat materials, an idea that merges passive and active thermal protection together was applied to modify carbon/carbon composites. In this study, SiC and ZrC were used as passive thermal protection materials along with the active thermal protection material Cu to modify C/C composites. C/C-SiC-ZrC-Cu composites were prepared by two-step reactive melt infiltration, which involved the infiltration of Si/Zr followed by Cu/Ti. The Cu content of the C/C-SiC-ZrC composites was controlled by controlling the open porosity of the porous C/C-SiC-ZrC composites. Microstructure, composition and ablation resistance of the C/C-SiC-ZrC-Cu composites were investigated. The results showed that the composites with high Cu content and density exhibited excellent short-term ablation resistance under the 42 kW plasma flame, with a negative linear ablation rate after 20 s of ablation. The active heat dissipation of Cu during the ablation process results in a significant reduction of the thermal response temperature on the composite surface. Moreover, the generated Si-Ti-Zr-O layer could form an effective barrier to protect the matrix.

Modulation of particle residence time in powder size-dependent ZrC coating on carbon-carbon composite with vacuum plasma spray

Recently, carbon-carbon (C/C) composites have been widely employed in various application fields due to their exceptional properties, but their oxidation at relatively low temperature around 500 °C poses a challenge to be used in high-temperature conditions. To address the drawback, the coating of ultra-high temperature ceramic (UHTC) layer on the C/C composites has been suggested as one of promising solutions. However, the quality of UHTC coating layers obtained through vacuum plasma spraying (VPS) is heavily influenced by the size of the introducing raw powders to coat the layers. Therefore, we investigated the size effects of ZrC powders in the range of (10–50) μm, one of UHTCs, to optimize the coating quality. By modulating process conditions, the coating quality with thick ZrC layer having lower porosity was obtained when using the powder around 34 μm. From the results, we found the gas residence time related to the particle velocity in the plasma flame is one of key parameters to be modulated by VPS operational parameters, which could improve the quality of the coating layer even for large powder cases.

Excellent cyclic ablation resistance of MoAlB ceramics under nitrogen plasma flame at 2000 °C

Thermal protection materials (TPMs) are crucial in advancing aerospace technology. In order to explore the applicability limits and reusability of MoAlB ceramics used as TPMs, the cyclic ablation performance and mechanisms were studied in a nitrogen plasma flame at about 2000 °C. After 1 to 5 ablation cycles (5 min per cycle), MoAlB ceramics retained their structural integrity. The linear and mass ablation rates were measured as 4.73 µm/s and 2.98 mg/s, respectively, after five ablation cycles (total of 25 min). The ablation resistance of MoAlB ceramics was primarily ascribed to the oxidation layer formed through their decomposition and oxidation. As the cycles extended, the plasma flame gradually damaged the oxide layer and led to the erosion and failure of the MoAlB matrix. Nevertheless, MoAlB ceramics exhibit exceptional resistance against cyclic and long-duration ablation, rendering it a highly promising application for TPMs.

Microstructure evolution and ablation mechanisms of Csf/ZrB2-SiC composites at different heat fluxes under air plasma flame

In this study, short carbon fibre (Csf) reinforced ZrB2-SiC composites were fabricated via direct ink writing followed by precursor infiltration and pyrolysis. The ablation behaviour of the composites was investigated at heat fluxes of 4–6 MW/m2 under an air plasma flame, which revealed excellent ablation resistance with negative linear ablation rates of − 0.1 to − 0.5 µm/s. The results showed that ZrO2-SiO2 oxide layer was formed on the ablation surface after ablation, which provided effective protection to the internal materials. With an increase in heat flux, ZrO2 precipitated from the SiO2 melt first in the form of nano-tetragonal ZrO2 (t-ZrO2), owing to the vaporisation of SiO2. During cooling, the nano t-ZrO2 particles were retained, and some of them reacted with SiO2 to form ZrSiO4, thus improving the thermal stability of the SiO2-ZrO2 oxide layer. In contrast, t-ZrO2 agglomerated and grew owing to capillary force and finally transformed into monoclinic ZrO2 (m-ZrO2). Moreover, a higher heat flux led to the size of precipitation ZrO2 increases, meanwhile increasing the content of t-ZrO2 and ZrSiO4 in the oxide layer.

Ablation behavior and mechanisms of high-entropy rare earth titanates (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)2Ti2O7 coating deposited by plasma spraying technology

High-entropy rare earth titanates (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)2Ti2O7 (YHT) have great application potential in the thermal protection coating field. In this work, the YHT coating was successfully prepared by atmospheric plasma spraying. The influence of different spraying powers and coating ablation behavior by plasma flame on the phase composition and microstructure morphology of the coating surface, cross-section and fracture surface were investigated using X-ray diffraction, scanning electron microscopy and atomic force microscope. Results show that the YHT powder has a sufficiently melted state before impacting the metal substrate and the coating can be effectively deposited through optimizing the plasma spraying power. And the YHT coatings undergo slight sintering, leading to the formation and growth of a spherical structure at 1200 °C, while the grain preferential growth, mechanical failure and water-like structure occurred due to strong thermal shock at 1400 °C. In addition, when the coating surface temperatures are 1200 °C and 1400 °C, the corresponding heat insulation temperatures are 590 °C and 620 °C, respectively, indicating that YHT coating has good thermal insulation ability. This study provides a powerful theoretical basis and technical support for the application of high-entropy rare earth titanates YHT as a thermal protection material for high temperature devices.

Microstructure, wear and corrosion properties of Cu–SiC/WCCo composite coatings on the Cu substrate surface by plasma spray method

In this study, the microstructure, wear and corrosion properties of Cu–SiC/WCCo composite coatings produced on Cu substrates by plasma spray method were investigated. In order to achieve this aim, SiC and WCCo powders were added to copper at different rates such as 5 %, 10 % and 20 % by weight. In order to determine the microstructure properties and phase distribution of the produced coatings, optical microscope, SEM-EDS and XRD analysis were investigated. The reciprocating wear method was used in order to determine the wear properties and the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods was used to determine the corrosion properties. The electrical conductivities of the coating layers were performed according to the eddy current principle. XRD analysis results showed that in both coating types, ceramic particles decomposed in the plasma flame and as a result, binary and ternary metallographic phases were formed. There were significant increases in hardness by adding ceramic particles to Cu, especially with 20 % WCCo additive, the hardness obtained was 82 % higher than the Cu substrate. When the wear properties were examined, both abrasive/adhesive wear types were observed in the copper substrate and coating layers. In all samples, material loss increased as the load increased. Both EIS measurements and Tafel curve results showed that the Cu-5% WCCo coating had the best corrosion resistance among the coatings. When the electrical conductivity results of all samples were compared, the electrical conductivity decreased with increasing rate of carbide in the coating.

Ablation performance and mechanism of fibrous monolithic ZrB2-SiC ceramics prepared by wet-spinning co-extrusion method under plasma flame

The fibrous monolithic structure exhibits extensive graded fracture as well as load redistribution upon fracture due to the unique layered microstructure, providing an effective way to toughen ceramic materials. In this work, fibrous monolithic ZrB2-SiC/SiCw (FZSw) and fibrous monolithic ZrB2-SiC/BN ceramics (FZB) were prepared by wet-spinning and co-extrusion method using SiCw and BN as the interfacial layer. The strengths, toughness, and the work of rupture of FZSw were 117 MPa, 3.9 MPa m1/2, 104 J/m2, while that of FZB were 103 MPa, 2.6 MPa m1/2, 98 J/m2, respectively. FZSw and FZB were subjected to 200 s plasma ablation tests under 12.84 MW/m2 and plasma flame above 3000 °C. The mass and linear ablation rates of FZSw after testing were -0.34 mg/s and -0.2 μm/s, respectively, and remain intact, showing non-ablation characteristics, while FZB underwent delamination and destruction. The introduction of SiCw interface layer improved the thermal conductivity of FZSw and reduced the surface temperature of FZSw. Higher oxidation temperature and higher SiC content of the SiCw interface layer result in a great quantity of SiO2, which effectively isolated oxygen from further erosion.

Deposition behavior and mechanism of thermal sprayed SiC

AbstractIt is widely known that silicon carbide (SiC) is challenging to thermal spray. However, the nature of deposition and the underlying mechanisms are still not clearly understood. In the current work, the process of atmospheric plasma‐sprayed SiC was investigated in detail. SiC exhibits sublimation severely under the plasma spraying (PS). More than 65 mol.% SiC particles lost by sublimation during PS estimated by calculation. Due to the nonuniformity of temperature in plasma flame, a small amount of SiC particles was retained. Moreover, a small amount of Si and SiO2 was experimentally observed after deposition process from the single‐splat characterization. According to the first‐principles calculations, the detachment energy of C atom from the lattice (0.591 eV) is lower than that of Si atom (1.257 eV). The adsorption energies of an O atom on the Si‐ and C‐terminated surfaces are −9.549 and −7.631 eV, respectively. Therefore, C is more likely to form gaseous products after decomposition, and Si is more likely to be retained to form Si and SiO2. Mullite was found on the grain boundaries of molten Si, which was attributed to the rapid diffusion of the Al2O3 substrate along the grain boundaries of Si, promoting the spreading of molten splats. It is difficult to stack particles through self‐diffusion among SiC particles to form coatings. The discovery on thermal deposition mechanism of SiC in PS is helpful to promote the practical application of SiC coatings on preparation optimization and interfacial bonding improvement.

Mechanical and plasma ablation properties of double-interface fibrous ZrB2-SiC ceramics for ultra-high-temperature application

Double-interface fibrous ZrB2-SiC ceramics (DZSw) were prepared by wet-spinning and hot-pressing. The introduction of double-interface layers improves the interface bonding and crack tolerance making the bending strength and toughness of DZSw increased to 607 MPa and 6.3 MPa m1/2, respectively. After ablation testing in 9.34 MW/m2 plasma flame, DZSw remained intact, with linear and mass ablation rates of − 0.3 µm s-1 and − 0.11 mg s-1, respectively. However, single-interface fibrous ZrB2-SiC ceramic (SZSw) produced penetrating cracks, so DZSw presented high ablation resistance than SZSw. The introduction of a dense ZrB2-SiC outer interface layer in DZSw interrupts the three-dimensional connected structure of the porous SiCw interface layer in SZSw and consequently reduces the oxygen diffusion rate of the ceramics. Moreover, the high emissivity and thermal conductivity enable DZSw to serve in the high heat flux ablation environment caused by faster flight speed.

Enhanced ablative performance of ZrB2 bioinspired fiber-aggregated ceramics with a design of dual-interface structure

ZrB2–SiC/BN/ZrB2–SiC dual-interface fiber-aggregated ceramic (DFC) was fabricated by wet-extruding and hot-pressing, then the ablation resistance of the ceramics was investigated by plasma flame with heat flux of 8.2 MW/m2 for 120 s. After ablation, the DFC remained an intact structure, and had the mass and linear ablation rates of −0.263 mg s−1 and -0.171 μm s−1, respectively. Compared with the catastrophic failure in mono-interface fiber-aggregated ceramics, DFC presented a typical non-ablative behavior during ablation. The enhanced ablation resistance of DFC was attributed to the fast heat transfer channel and continuous dense barrier of SiO2–ZrO2 with low oxygen permeability brought by dual-interface structure.

Kinetic Model of Vacuum Plasma Expansion in a Cylindrical Gap

Results of a theoretical description of collisionless kinetics of radial expansion of two-component (electron–ion) plasma in the one-dimensional cylindrical formulation of the problem are presented. The electric-field mechanism of supersonic expansion of the plasma flame due to the motion of the electron–ion ensemble and self-consistent electric field in the diode with the potential difference applied to it is demonstrated. The spatiotemporal evolution of the ion energy distribution function, electric potential, and rate of expansion of the emission boundary of the plasma flame is shown. The calculated rates of flame expansion at the copper cathode (~1.5 × 106 cm/s) well agree with the experimental data.

Kinetic Model of Vacuum Plasma Expansion in a Cylindrical Gap

Results of a theoretical description of collisionless kinetics of radial expansion of two-component (electron–ion) plasma in the one-dimensional cylindrical formulation of the problem are presented. The electric-field mechanism of supersonic expansion of the plasma flame due to the motion of the electron–ion ensemble and self-consistent electric field in the diode with the potential difference applied to it is demonstrated. The spatiotemporal evolution of the ion energy distribution function, electric potential, and rate of expansion of the emission boundary of the plasma flame is shown. The calculated rates of flame expansion at the copper cathode (~1.5 × 106 cm/s) well agree with the experimental data.