The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Wei Wu - One of the best experts on this subject based on the ideXlab platform.
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Roles of the state-resolved OH(A) and OH(X) radicals in microwave plasma assisted combustion of premixed methane/air: An exploratory study
Combustion and Flame, 2014Co-Authors: Chuji Wang, Wei WuAbstract:Abstract We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio ( ϕ ), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, Ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The Ignition curves of plasma power versus ϕ FL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on Ignition and flame stabilization, i.e. the fuel lean flammability limit ( ϕ LFL ) was extended to ϕ = 0.2, as compared to ϕ = 0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis ( x direction) with a spatial resolution of 0.5 mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I ∼ x ) in the hybrid zone where plasma-assisted Ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450 K to 2400 K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308 nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the Ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the Ignition enhancement.
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roles of the state resolved oh a and oh x radicals in microwave plasma assisted combustion of premixed methane air an exploratory study
Combustion and Flame, 2014Co-Authors: Chuji Wang, Wei WuAbstract:Abstract We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio ( ϕ ), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, Ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The Ignition curves of plasma power versus ϕ FL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on Ignition and flame stabilization, i.e. the fuel lean flammability limit ( ϕ LFL ) was extended to ϕ = 0.2, as compared to ϕ = 0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis ( x direction) with a spatial resolution of 0.5 mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I ∼ x ) in the hybrid zone where plasma-assisted Ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450 K to 2400 K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308 nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the Ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the Ignition enhancement.
Chuji Wang - One of the best experts on this subject based on the ideXlab platform.
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Roles of the state-resolved OH(A) and OH(X) radicals in microwave plasma assisted combustion of premixed methane/air: An exploratory study
Combustion and Flame, 2014Co-Authors: Chuji Wang, Wei WuAbstract:Abstract We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio ( ϕ ), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, Ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The Ignition curves of plasma power versus ϕ FL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on Ignition and flame stabilization, i.e. the fuel lean flammability limit ( ϕ LFL ) was extended to ϕ = 0.2, as compared to ϕ = 0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis ( x direction) with a spatial resolution of 0.5 mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I ∼ x ) in the hybrid zone where plasma-assisted Ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450 K to 2400 K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308 nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the Ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the Ignition enhancement.
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roles of the state resolved oh a and oh x radicals in microwave plasma assisted combustion of premixed methane air an exploratory study
Combustion and Flame, 2014Co-Authors: Chuji Wang, Wei WuAbstract:Abstract We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio ( ϕ ), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, Ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The Ignition curves of plasma power versus ϕ FL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on Ignition and flame stabilization, i.e. the fuel lean flammability limit ( ϕ LFL ) was extended to ϕ = 0.2, as compared to ϕ = 0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis ( x direction) with a spatial resolution of 0.5 mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I ∼ x ) in the hybrid zone where plasma-assisted Ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450 K to 2400 K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308 nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the Ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the Ignition enhancement.
Yiguang Ju - One of the best experts on this subject based on the ideXlab platform.
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in situ plasma activated low temperature chemistry and the s curve transition in dme oxygen helium mixture
Combustion and Flame, 2014Co-Authors: Yiguang JuAbstract:Abstract The effect of non-equilibrium plasma activated low temperature chemistry (PA-LTC) on the Ignition and extinction of Dimethyl Ether (DME)/O2/He diffusion flames has been investigated experimentally in a counterflow burner with in situ nanosecond pulsed discharge at 72 Torr. A uniform discharge is generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The Ignition and extinction characteristics of DME/O2/He are studied by employing OH and CH2O Planar Laser Induced Fluorescence (PLIF) techniques at constant strain rates and O2 mole fractions on the oxidizer side with varying the DME mole fractions. Contrary to the conventional understanding, strong low temperature reactivity during Ignition process is observed for DME with non-equilibrium plasma activation even at 72 Torr and flow residence time of a few milliseconds. The OH PLIF shows strong OH signal at and after Ignition, whereas extremely low OH signal before Ignition. However, the CH2O PLIF experiments demonstrate that, with the increase of DME mole fraction on the fuel side, the CH2O PLIF signal intensity increases significantly before Ignition and decreased rapidly after Ignition. The low OH number density and high CH2O number density before DME Ignition clearly demonstrates the existence of PA-LTC at low pressure. Moreover, at higher O2 mole fraction and discharge repetition frequency, the in situ discharge significantly modifies the characteristics of Ignition and extinction, thus creating a new monotonically and fully stretched Ignition S-curve without an extinction limit. Compared to our previous study of methane, the existence of strong low temperature reactivity in DME oxidation makes Ignitions occur at much lower fuel mole fractions, thus accelerating the transition of Ignition curve from conventional S-curve to the fully stretched S-curve. The transition from the conventional S-curve to the new stretched Ignition curve at high plasma repetition rate indicates that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry even at low pressure. The chemical kinetic model for the plasma–flame interaction has been also developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling results reveal that the PA-LTC has a much shorter timescale comparing with that of thermally activated low temperature chemistry owing to the rapid radical production by plasma. The reaction pathways analysis shows that atomic O generated by the discharge is critical to controlling the population of radical pool.
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direct numerical simulations of nox effect on multistage autoIgnition of dme air mixture in the negative temperature coefficient regime for stratified hcci engine conditions
Combustion and Flame, 2014Co-Authors: Hossam A Elasrag, Yiguang JuAbstract:Abstract Direct numerical simulations (DNSs), for a stratified flow in HCCI engine-like conditions, are performed to investigate the effects of exhaust gas recirculation (EGR) by NO x and temperature/mixture stratification on autoIgnition of dimethyl ether (DME) in the negative temperature coefficient (NTC) region. Detailed chemistry for a DME/air mixture with NO x addition is employed and solved by a hybrid multi-time scale (HMTS) algorithm. Three Ignition stages are observed. The results show that adding (1000 ppm) NO enhances both low and intermediate temperature Ignition delay times by the rapid OH radical pool formation (one to two orders of magnitude higher OH radicals concentrations are observed). In addition, NO from EGR was found to change the heat release rates differently at each Ignition stage, where it mainly increases the low temperature Ignition heat release rate with minimal effect on the Ignition heat release rates at the second and third Ignition stages. Sensitivity analysis is performed and the important reactions pathways for low temperature chemistry and Ignition enhancement by NO addition are specified. The DNSs for stratified turbulent Ignition show that the scales introduced by the mixture and thermal stratifications have a stronger effect on the second and third stage Ignitions. Compared to homogenous Ignition, stratified Ignition shows a similar first autoIgnition delay time, but about 19% reduction in the second and third Ignition delay times. Stratification, however, results in a lower averaged LTC Ignition heat release rate and a higher averaged hot Ignition heat release rate compared to homogenous Ignition. The results also show that molecular transport plays an important role in stratified low temperature Ignition, and that the scalar mixing time scale is strongly affected by local Ignition. Two Ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode (the D-mode) and a spontaneous, high-speed, kinetically driven Ignition mode (the S-mode). Three criteria are introduced to distinguish the two modes by different characteristic time scales and Damkholer (Da) number using a progress variable conditioned by a proper Ignition kernel indicator (IKI). The results show that the spontaneous Ignition S-mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkholer number, while the D-mode is characterized by high scalar dissipation rate, low displacement speeds in the order of the laminar flame speed and a lower than unity Da number. The proposed criteria are applied at the different Ignition stages.
S J Hawksworth - One of the best experts on this subject based on the ideXlab platform.
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spontaneous Ignition of hydrogen leaks a review of postulated mechanisms
International Journal of Hydrogen Energy, 2007Co-Authors: G R Astbury, S J HawksworthAbstract:Abstract Over the last century, there have been reports of high pressure hydrogen leaks igniting for no apparent reason, and several Ignition mechanisms have been proposed. Although many leaks have ignited, there are also reported leaks where no Ignition has occurred. Investigations of Ignitions where no apparent Ignition source was present have often been superficial, with a mechanism postulated which, whilst appearing to satisfy the conditions prevailing at the time of the release, simply does not stand up to rigorous scientific analysis. Some of these proposed mechanisms have been simulated in a laboratory under superficially identical conditions and appear to be rigorous and scientific, but the simulated conditions often do not have the same large release rates or quantities, mainly because of physical constraints of a laboratory. Also, some of the release scenarios carried out or simulated in laboratories are totally divorced from the realistic situation of most actual leaks. Clearly there are gaps in the knowledge of the exact Ignition mechanism for releases of hydrogen, particularly at the high pressures likely to be involved in future storage and use. Mechanisms which have been proposed in the past are the reverse Joule–Thomson effect, electrostatic charge generation, diffusion Ignition, sudden adiabatic compression, and hot surface Ignition. Of these, some have been characterised by means of computer simulation rather than by actual experiment, and hence are not validated. Consequently there are discrepancies between the theories, releases known to have ignited, and releases which are known to have not ignited. From this, postulated Ignition mechanisms which are worthy of further study have been identified, and the gaps in information have been highlighted. As a result, the direction for future research into the potential for Ignition of hydrogen escapes has been identified.
Matt P Plucinski - One of the best experts on this subject based on the ideXlab platform.
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laboratory determination of factors influencing successful point Ignition in the litter layer of shrubland vegetation
International Journal of Wildland Fire, 2008Co-Authors: Matt P Plucinski, Wendy R AndersonAbstract:Factors affecting Ignition thresholds of the litter layer of shrubland vegetation were investigated using reconstructed litter beds in a laboratory. The factors investigated were fuel moisture content (FMC), litter type (primarily species), pilot Ignition source, and wind. Litter beds made from 11 different litter types were ignited with point Ignition sources. Litter from Allocasuarina nana (Sieber ex Spreng.) L.A.S. Johnson was used as the standard type across all experiments. Successful Ignition was defined as fire spreading a fixed distance from the Ignition point. Ignition success was modelled as a logistic function of FMC. Litter type had a major effect on ignitibility. The bulk density of the litter bed and the surface area of litter per volume of litter bed provided reasonably good predictors of the effect of litter type on Ignition success. Low-density litter beds ignited at higher FMCs than dense litter beds. The two densest litter beds failed to ignite with the procedures used here. The Ignition sources tested had significantly different effects on Ignition success. Larger Ignition sources were able to ignite wetter fuels than smaller sources. The presence of wind was found to have a different effect on Ignition success depending on the location of the Ignition source with respect to the litter bed. Wind decreased Ignition success when the Ignition source was located on top of the litter bed, but aided Ignition when the Ignition source was located within the litter bed.
