Tanvir Farouk

Simulation of dc atmospheric pressure argon micro glow-discharge

A hybrid model was used to simulate a dc argon micro glow-discharge at atmospheric pressure. The simulations were carried out for a pin-plate electrode configuration with inter-electrode gap spacing of 200??m together with an external circuit. The predicted voltage?current characteristics and current density profiles identify the discharge to be a normal glow-discharge. The neutral gas temperature predictions indicate that the discharge forms a non-thermal, non-equilibrium plasma. Experimental studies were conducted to validate the numerical model. Predictions from the numerical model compare favourably with the experimental measurements.

Microgravity droplet combustion: effect of tethering fiber on burning rate and flame structure

Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with published experimental data. The simulations were conducted using a transient one-dimensional spherosymmetric droplet combustion model, where the effect of the supporting fiber was implicitly taken into account. The model applied staggered convective flux finite volume method combined with high-order implicit time integration. Thermal radiation was evaluated using a statistical narrow band radiation model. Chemical kinetics and thermophysical properties were represented in rigorous detail. Tether fiber diameter, droplet diameter, ambient pressure and oxygen concentration were varied over a range for n-decane droplets in the simulations. The results of the simulations were compared to previously published experiments conducted in the Japan Microgravity Center (JAMIC) 10 second drop tower and the NASA Glenn Research Center (GRC) 5.2 second drop tower. The model reproduces closely nearly all aspects of tethered n-decane droplet burning phenomena, which included droplet burning history, transient and average burning rate, and flame standoff ratio. The predictions show that the presence of the tethering fiber significantly influences the observed burning rate, standoff ratio, and extinction.

Atmospheric pressure radio frequency glow discharges in argon : effects of external matching circuit parameters

Numerical simulations of radio frequency atmospheric pressure argon glow discharges were performed using a one-dimensional hybrid model. The discharge simulations were carried out for a parallel plate electrode configuration with an inter-electrode gap of 1.0 mm together with an external matching circuit. The external matching circuit parameters were found to have significant effect on the discharge characteristics. The results indicate that the discharge can operate at either the α or γ mode depending on the matching circuit parameters. The two modes of operation were found to be distinctly different. The predicted Ar* density was considered to provide qualitatively the visual appearance of the α or γ mode discharge. The α mode was found to have a luminous region in the center of the discharge. On the other hand, the γ mode had luminous regions very close to the electrodes which were followed by alternating dark and bright regions. The appearance of the simulated γ mode was found to resemble that of an atmospheric pressure direct current glow discharge. The predicted gas temperature indicated the γ mode to have higher gas temperature compared with the α mode.

Self-rotating dc atmospheric-pressure discharge over a water-surface electrode: regimes of operation

A dc atmospheric-pressure glow discharge produced between a metallic electrode and a water electrode is studied in this experiment. The discharge is characterized by means of visualization, high-speed imaging, voltage–current measurements, mass spectrometry and temperature measurements. Under certain conditions, the discharge exhibits a distinctive rotating motion in which the cathode spot remains stationary and the anode spot traces a circular pattern. Regimes of rotation occur in general at lower currents, at larger discharge gap lengths and when the water surface is the anode. Temperature measurements made in the rotating and stationary regimes show similar trends. Various metallic electrode materials, electrode geometries and discharge gases are investigated to determine the conditions under which rotation occurs. Rotation is only observed with a smooth cathode and a non-oxidizing anode material, such as water (or gold surface) that is either flat or otherwise provides no hindrances to the movement of the anode spot. Rotation is observed to occur in air and N2–H2 mixtures but not in pure N2, H2 or He; this suggests chemical mechanisms resulting in the formation of electronegative species as a possible cause for the rotation. Finally, measurements of the frequency of rotation of the discharge with respect to discharge length and current are made. These qualitative and quantitative results are used to evaluate various types of interactions as potential causes of this behavior.

Atmospheric pressure methane–hydrogen dc micro-glow discharge for thin film deposition

Atmospheric pressure methane–hydrogen dc micro-plasmas were studied for thin film deposition. Numerical simulations were performed using a hybrid model. The model contained detailed reaction mechanisms for the gas-phase discharge and the surface reactions to predict the species densities in the discharge and the deposition characteristics and its growth rate. Twenty-three species and an eighty-one step reaction mechanism were considered for the gas-phase methane–hydrogen discharge. The surface chemistry consisted of eighteen species and eighty-four reaction steps. The simulations were carried out for a dc parallel plate electrode configuration with an inter-electrode gap of 200 µm. An external circuit was also considered along with the discharge model and surface reactions. Basic plasma properties such as electron and species density, electric field, electron temperature and gas temperature were studied. Special attention was devoted to study the influence of discharge current and methane mass fraction on the plasma characteristics and the deposition characteristics and its rate. The , and concentrations were found to be the dominant hydrogen and hydrocarbon ions, respectively. It was found that in atmospheric pressure methane–hydrogen micro-plasmas, C2H6, C3H8, C2H4 and C2H2 are also present at high densities. CH2 and CH3 were found to be the main radicals, which are prominent growth species for diamond-like-carbon deposition. The simulations indicated significant gas heating in the entire regime of operation. Ion Joule heating was found to be dominant in the sheath whereas Franck–Condon heating and heavy particle reaction induced heating was dominant in the volume. A strong dependence of soot formation on gas temperature was observed. At higher discharge current and methane mass fraction the deposited film was predicted to have higher soot content. Mass spectrometry experiments were conducted to measure the methane conversion factor (cf) and the C2H2 density for different discharge currents. Both the methane cf and C2H2 density showed an increase with increasing discharge current. Predictions from simulations agreed favourably with the mass spectrometry measurements.

Modeling of direct current micro-plasma discharges in atmospheric pressure hydrogen

Numerical simulations and experimental studies were conducted to characterize direct current (dc) hydrogen discharge for a pin plate electrode configuration having an inter-electrode separation distance of 400 µm. A self-consistent two-dimensional hybrid model was developed to simulate the atmospheric pressure dc hydrogen micro-discharges. The discharge simulation model considered consists of momentum and energy conservation equations for a multi-component gas mixture, conservation equations for each component of the mixture (electrons, ions, excited species and neutrals) and state relations. The model uses a drift–diffusion approximation for the electron and the ion fluxes. The species considered include H, H2, H+, , , , H2 v=1 and the electrons. The electric field is obtained from the solution of Poissons equation. Numerical simulations and experimental measurements indicated some of the key features of a normal glow discharge: flat voltage–current characteristics and constant cathode current density. Basic plasma properties such as electron number density, gas temperature, electric field and electron temperature were studied. The model predicted a constant current density of ~22 A cm−2 in the normal glow regime. The normal current density was found to be a temperature scaled value of a low pressure normal current density. The ion Joule heating and Frank–Condon heating were found to be the dominant gas heating mechanisms. The peak gas temperature of ~500 K indicated the discharge to be a non-thermal non-equilibrium discharge. Predictions from the model compares favorably well with the experimental measurements.

Development of Reduced Kinetic Models for Petroleum-Derived and Alternative Jet Fuels

The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fuels derived from coal and natural gas, and also to the pressure and equivalence ratio dependences of the combustion responses of conventional Jet–A fuel. The formulation of surrogate fuels for Syntroleum S-8, Shell SPK and Sasol IPK, is described. Assuming these compositions, a detailed chemical kinetic model construction previously elaborated upon is extended and tested against reference data sets of shock tube ignition delay and laminar burning velocity. Calculations with the detailed kinetic model, containing 3147 species correctly represent the experimentally measured reactivity of the target fuels for shock tube ignition delay. The model also captures trends in the ignition delay for a reference Jet-A as a function of pressure and equivalence ratio. The earlier reported detailed model is expanded to encompass a range of n-alkane carbon numbers up to C16 and iso-cetane. The expanded model is validated against available shock tube ignition delay in detailed form and against laminar burning velocity datasets using a series of numerically reduced models of decreasing dimension for n-hexadecane, iso-cetane, and their mixtures. Though the detailed model reproduces the general kinetic behavior for the ignition delays of each jet fuel, the predicted values are generally longer than experimental results. A series of reduced models of the order of 100 species in size, are produced for simulation of flame environments. Calculations for laminar premixed flames for each jet fuel are similar with burning velocities for IPK flames marginally lower than those for the conventional Jet-A which in turn are marginally lower than those for S-8. The requirement for severely reduced, but high fidelity chemical kinetic numerical schemes that retain predictive capacities for the combustion behaviors of real liquid transportation fuels is addressed through the introduction of a strategy to produce “compact” models of the order of 35 species. The strategy utilizes calculations of the detailed model construct as a fundamental and scientific standard, to which engineering approximations achieved through adjusting reaction rates and omitting or diverting the fate of select reaction pathways at high carbon numbers are applied. The strategy is tested for the exemplar real fuel test case of the S-8 ignition delay and laminar

Reduced Kinetic Models for the Combustion of Jet Propulsion Fuels

Reduced chemical kinetic models to predict the combustion characteristics of jet propulsion fuels are produced and tested. The parent detailed kinetic model has been developed on the basis of a surrogate fuel formulation methodology that utilizes combustion property targets measured for a particular real fuel to formulate a chemical mixture of nalkanes, iso-alkanes and aromatic functionalities to emulate the combustion behavior of specific target jet aviation fuels. Detailed model predictions are compared against reflected shock ignition delays of both pure components and surrogate fuel mixtures. Systematically reduced models for each individual fuel component are produced and used to test the parent model performance against laminar burning velocity. Finally, a range of systematically reduced kinetic models for two, substantially different, validated surrogate fuels for a particular jet aviation fuel are produce and tested to allow the user a choice in computational cost versus reduced model fidelity. A reduced model of 233 species is produced that closely shares the predictability of the detailed model over the tested conditions. Analysis of the models provides a basis for further refinements in describing the chemical kinetic behavior of all conventional and alternative jet fuels. The limitations of the presented approach are discussed and needs for further refinements are identified.

Computational Studies of Atmospheric-Pressure Methane–Hydrogen DC Micro Glow Discharges

Atmospheric-pressure methane-hydrogen micro glow discharges were computationally investigated using a 2-D hybrid model. The plasma model was solved simultaneously with a model for the external circuit. Simulations were conducted for a pin-to-plate electrode configuration with an interelectrode separation of 400 ¿m. The spatiotemporal evolutions of electrons, species densities, electric field, and electron and gas temperatures were studied. A total of 81 reactions were considered, which included electron-neutral, electron-ion, ion-neutral, and neutral-neutral reactions. An 84-step reaction mechanism consisting of 15 surface species and four deposited bulk species was considered. A time-stepping technique was employed to address the time scales of plasma transport (in microseconds) and neutral and fluid transport (in milliseconds) in 2-D simulations with detailed volume and surface chemistry. The simulations indicated H3 + and CH5 + ions to be the most prominent hydrogen and hydrocarbon ions. The gas temperature predictions suggested the discharge to be operating as a nonthermal glow discharge. The effect of discharge current on both plasma and deposition characteristics was studied. The simulations predicted a flat voltage-current characteristic, indicating the discharge to be operating in normal glow mode. The predicted voltage-current characteristic was found to be in favorable agreement with the experimental measurements. With an increase in discharge current, the deposition rate profile expanded in the lateral direction, suggesting that deposition occurred at the cathode spot.

Effectiveness of Xenon as a Fire Suppressant Under Microgravity Combustion Environment

ABSTRACT The ‘FLame EXtinguishment’ (FLEX) program conducted by NASA on board the International Space Station (ISS) has been assisting in developing fire-safety protocols for low gravity applications through microgravity droplet combustion experiments. A wide range of fuels, including alcohols and alkanes, have been studied in different ambient conditions that also encompass the use of various diluent species and concentrations. A prime focus of the work has been to observe the relative effectiveness of atmospheric composition and pressure changes on fire suppression under ‘reduced’ gravity conditions. Here, detailed numerical simulations are performed to investigate the combustion and extinction characteristics of isolated sphero-symmetric 1.0–2.0-mm diameter methanol droplets burning in xenon (Xe)-enriched environments. Comparisons of diluent behaviors under identical conditions using argon (Ar), carbon dioxide (CO2), and helium (He) as the alternative diluent to nitrogen are also reported. The predictions are compared against ISS experiments with good agreement and with less satisfactory agreement with the results published earlier by Shaw and Wei (2012). Xenon as diluent rather than nitrogen results in reduced burning rate, larger extinction diameter and counter intuitively, and prolonged burning time. The limiting oxygen index (LOI) for xenon is found to decrease significantly from that found with argon, carbon dioxide, or helium. The numerical analyses indicate that the lower thermal diffusivity of xenon is the principal factor responsible for the remarkably lower LOI. Water accumulation within the methanol droplet and its relevance to the extinction process is also discussed. It is concluded that the combined observation of elevated peak gas temperature, its slow decay due to minimal diffusive heat loss, and the exceptionally lower LOI value associated with xenon as a diluent all detract from its utility for suppressing fire concerns in reduced gravity applications.