Increasing Standoff Distance Research Articles

Suspension Plasma Spray and Performance Characterization of Half Cells with NiO/YSZ Anode and YSZ Electrolyte

The use of a liquid feedstock carrier in suspension plasma spray (SPS) permits injection of fine powders, providing the possibility of producing sprayed coatings that are both thin and dense and have fine microstructures. These characteristics make SPS an attractive process for depositing highly efficient electrodes and electrolytes for solid oxide fuel cell (SOFC) applications. In this study, NiO-yttria stabilized zirconia (YSZ) anode and YSZ electrolyte half cells were successfully deposited on porous Hastelloy X substrates by SPS. The NiO-YSZ anode deposition process was optimized by design of experiment. The YSZ electrolyte spray process was examined by changing one parameter at a time. The results from the design-of-experiment trials indicated that the porosity of the as-deposited coatings increased with an increase of suspension feed rate while it decreased with an increase of total plasma gas flow rate and standoff distance. The deposition rate increased with an increase of total plasma gas flow rate, suspension feed rate, and standoff distance. The microstructure examination by SEM showed that the NiO and YSZ phases were homogeneously distributed and that the YSZ phase had a lamellar structure. It was observed that the density of the YSZ electrolyte layer increased as input power of the plasma torch increased. Electrochemical characterization of the fabricated cells indicated that an open cell voltage of 0.989 V at 500 °C and a peak power of 0.610 W/cm2 at 750 °C were reached.

An Investigation on Temperature Distribution Within the Substrate and Nozzle Wall in Cold Spraying by Numerical and Experimental Methods

During cold spraying (CS), heat exchange between the hot driving gas and the solid bodies, e.g., spray nozzle and substrate, results in the temperature redistribution within the solid bodies. In this study, numerical and experimental investigations on the heating behavior of the substrate and nozzle wall were conducted to clarify the temperature distribution within the solid bodies in CS. The results show that after heating by the hot gas, the highest temperature presents at the center point of the substrate and decreases toward the substrate back surface and edge. With increasing standoff distance or decreasing inlet temperature, the substrate temperature decreases gradually, but the temperature gradient within the substrate changes little. The numerical results are consistent with the experimental measurements. Besides, it is also found that increasing the substrate size (diameter) can lead to the gradual increment in the substrate temperature. Moreover, the numerical study on the temperature distribution within the nozzle wall reveals that the highest temperature presents at the throat section of the nozzle and that the nozzle material significantly affects the temperature distribution within the nozzle wall.

Ooty Interplanetary Scintillation – Remote-Sensing Observations and Analysis of Coronal Mass Ejections in the Heliosphere

In this paper, I investigate the three-dimensional evolution of solar wind density and speed distributions associated with coronal mass ejections (CMEs). The primary solar wind data used in this study has been obtained from the interplanetary scintillation (IPS) measurements made at the Ooty Radio Telescope, which is capable of measuring scintillation of a large number of radio sources per day and solar wind estimates along different cuts of the heliosphere that allow the reconstruction of three-dimensional structures of propagating transients in the inner heliosphere. The results of this study are: i) three-dimensional IPS images possibly show evidence for the flux-rope structure associated with the CME and its radial size evolution; the overall size and features within the CME are largely determined by the magnetic energy carried by the CME. Such a magnetically energetic CME can cause an intense geomagnetic storm, even if the trailing part of the CME passes through the Earth; ii) IPS measurements along the radial direction of a CME at ∼ 120 R⊙ show density turbulence enhancements linked to the shock ahead of the CME and the core of the CME. The density of the core decreases with distance, suggesting the expansion of the CME. However, the density associated with the shock increases with distance from the Sun, indicating the development of a strong compression at the leading edge of the CME. The increase of stand-off distance between ∼ 120 R⊙ and 1 AU is consistent with the deceleration of the CME and the continued outward expansion of the shock. The key point in this study is that the magnetic energy possessed by the transient determines its radial evolution.

Nonthermal Plasma Mitigation of Shock Wave in Supersonic Airflow~!2009-04-01~!2009-05-11~!2009-07-16~!

A non-thermal technique using a plasma spike, generated by 60 Hz periodic electric discharge, for shock wave mitigation is presented. The experiments were conducted in a Mach 2.5 wind tunnel. A plasma spike was generated in front of a wind tunnel model; it led to a transformation of the shock from a well-defined attached shock into a highly curved shock structure, which had increased shock angle and also appeared in diffused form. As seen in a sequence with increasing discharge intensity, the shock in front of the model moved upstream to become detached with increasing standoff distance from the model and was eliminated near the peak of the discharge. The power measurements excluded the heating effect as a possible cause of the observed shock wave modification. A theory using a cone model as the shock wave generator is presented to explain the observed plasma effect on shock wave. The analysis has shown that the plasma generated in front of the model can effectively deflect the incoming flow; such a flow deflection modifies the structure of the shock wave generated by the cone model, as shown by the numerical results, from a conic shape to a curved one. The shock front moves upstream with a larger shock angle, matching well with that observed in the experiment.

Hole Machining of Glass by Micro Abrasive Suspension Jets

Abrasive Suspension Jets (ASJ) is a new micro processing technique developed for micro processing of hard and brittle materials based on the traditional Abrasive Water Jet (AWJ). Based on drilling experiments of glass using MASJ technology, the dependence of material removal, the depth and the diameter of the machined holes on the process parameters, such as working pressure, processing time, standoff distance, incidence angle and concentration of abrasives were investigated. Experimental results show that the material removal is approximately proportional to working pressure, processing time and concentration of abrasives, except the standoff distance. It is founded that the processing time is the most remarkable influence factor on the material removal and the depth of the holes. But the working pressure doesn’t show obvious effects to the material removal and the depth of hole with lower pressure in MASJ. The increase of standoff distance will decrease the material removal and depth of hole, and the concentration of abrasives can improve a few of drilling ability. Further, it is founded that longer processing time and smaller standoff distance will achieve higher MASJ drilling efficiency and better quality of hole, with 90 degree jet incidence angle.

Plasma mitigation of shock wave: experiments and theory

Two types of plasma spikes, generated by on-board 60 Hz periodic and pulsed dc electric discharges in front of two slightly different wind tunnel models, were used to demonstrate the non-thermal plasma techniques for shock wave mitigation. The experiments were conducted in a Mach 2.5 wind tunnel. (1) In the periodic discharge case, the results show a transformation of the shock from a well-defined attached shock into a highly curved shock structure, which has increased shock angle and also appears in diffused form. As shown in a sequence with increasing discharge intensity, the shock in front of the model moves upstream to become detached with increasing standoff distance from the model and is eliminated near the peak of the discharge. The power measurements exclude the heating effect as a possible cause of the observed shock wave modification. A theory using a cone model as the shock wave generator is presented to explain the observed plasma effect on shock wave. The analysis shows that the plasma generated in front of the model can effectively deflect the incoming flow; such a flow deflection modifies the structure of the shock wave generated by the cone model, as shown by the numerical results, from a conic shape to a curved one. The shock front moves upstream with a larger shock angle, matching well with that observed in the experiment. (2) In the pulsed dc discharge case, hollow cone-shaped plasma that envelops the physical spike of a truncated cone model is produced in the discharge; consequently, the original bow shock is modified to a conical shock, equivalent to reinstating the model into a perfect cone and to increase the body aspect ratio by 70%. A significant wave drag reduction in each discharge is inferred from the pressure measurements; at the discharge maximum, the pressure on the frontal surface of the body decreases by more than 30%, the pressure on the cone surface increases by about 5%, whereas the pressure on the cylinder surface remains unchanged. The energy saving from drag reduction is estimated to make up two-thirds of the energy consumed in the electric discharge for the plasma generation. The measurements also show that the plasma effect on the shock structure lasts much longer than the discharge period.

Effect of standoff distance on coating deposition characteristics in cold spraying

In this study, the effect of standoff distance on coating deposition characteristics in cold spraying in cold spraying was investigated by the experiment and numerical simulation of particle acceleration. Al, Ti and Cu powders of different sizes were used as feedstocks. It was found that the deposition efficiency was decreased with the increase of standoff distance from 10 mm to 110 mm for both Al and Ti powders used in this study. However, for Cu powders, the maximum deposition efficiency was obtained at the standoff distance of 30 mm, and then the deposition efficiency decreased with further increasing the standoff distance to 110 mm. The standoff distance had a little effect on coating microstructure and microhardness for these three powders. Both the stain-hardening effect of the deposited particles and the shot-peening effect of the rebounded particles take the roles in coating hardness. It was also found that the surface of substrate or previously deposited coating could be exposed to a relatively high gas temperature at a short standoff distance.

On plasma mitigation of shock waves

Experiments were conducted in a Mach 2.5 wind tunnel to explore the modification effect on the shock wave structure by a plasma spike generated by an on-board 60 Hz electric discharge in front of a 60° cone-shaped model, which was used as a shock wave generator. Modification of the shock by the plasma spike from an attached conic shock into a highly curved shock structure, which has increased shock angle and also appears in diffused form, is demonstrated. Due to the cyclic nature of the generated plasma an unsteady shock motion during one discharge period was observed. As shown in a sequence with increasing discharge intensity, the shock in front of the model moves upstream to become detached with increasing standoff distance from the model and with increasing dispersion in the spatial distribution of the shock front. Experimental results exclude the heating effect as a possible cause of the observed shock wave modification. A theory also using a cone model as the shock wave generator is presented to explain the observed plasma effect on the shock wave. Analysis shows that the plasma spike can effectively deflect the incoming flow before the flow reaches the cone model; such a flow deflection modifies the structure of the shock wave generated by the cone model from conic shape to a curved one. The shock front moves upstream with a larger shock angle, consistent with the experimental results.

Motion and deformation of viscous drop in stokes flow near rigid wall

A boundary integral method was developed for simulating the motion and deformation of a viscous drop in an axisymmetric ambient Stokes flow near a rigid wall and for direct calculating the stress on the wall. Numerical experiments by the method were performed for different initial stand-off distances of the drop to the wall, viscosity ratios, combined surface tension and buoyancy parameters and ambient flow parameters. Numerical results show that due to the action of ambient flow and buoyancy the drop is compressed and stretched respectively in axial and radial directions when time goes. When the ambient flow action is weaker than that of the buoyancy the drop raises and bends upward and the stress on the wall induced by drop motion decreases when time advances. When the ambient flow action is stronger than that of the buoyancy the drop descends and becomes flatter and flatter as time goes. In this case when the initial stand-off distance is large the stress on the wall increases as the drop evolutes but when the stand-off distance is small the stress on the wall decreases as a result of combined effects of ambient flow, buoyancy and the stronger wall action to the flow. The action of the stress on the wall induced by drop motion is restricted in an area near the symmetric axis, which increases when the initial stand-off distance increases. When the initial stand-off distance increases the stress induced by drop motion decreases substantially. The surface tension effects resist the deformation and smooth the profile of the drop surfaces. The drop viscosity will reduce the deformation and migration of the drop.

Laser Ignition of DAAF, DHT and DAATO3.5

CO2 laser ignition experimental results are reported for the high-nitrogen materials 3,6-dihydrazino-1,2,4,5-tetrazine (DHT), 3,3′-diamino-4,4′-azoxyfurazan (DAAF), and mixed N-oxides of 3,3′-azo-bis(6-amino-1,2,4,5-tetrazine) (DAATO3.5, where the “3.5” indicates the average oxide content) at a maximum irradiance level of approximately 140 W/cm2. Diagnostics include a photodiode, indium antimonide (InSb) IR detector, high speed (HS) video and a CO2 photodetector. “First light” is measured for DAATO3.5 and DAAF, however, due to the low visible light emission of the gas phase, thermal runaway, as measured by the InSb, is used as the ignition criterion for DHT. Ignition in the gas phase is captured by the high speed camera. It is observed that an increase in laser irradiance results in an increase in ignition and flame stand-off distance for DAATO3.5. The high-nitrogen material laser ignition results are compared to the common nitramine explosive, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). Laser ignition delays for the different high-nitrogen materials are also compared in the context of Differential Scanning Calorimetry (DSC) data. It is determined that DSC onset temperature, while a rough indicator of ignition delay trends, is not the equivalent of a direct measure of ignition temperature.

A Study on Abrasive Water Jet Machining of Aluminum with Garnet Abrasives

In this study the effect of some cutting variables on the quality of the surface pnxiuced during abrasive water jet machining of aluminum has been investigated. The type of abrasive used was gamet of mesh size 80. The machining was done on the abrasive water jet machine WJ4080. The cutting variables were stand-off distance of the nozzle from the work surface. work feed rate and jet pressure. The evaluating criteria of the surface produced were width of cut, taper of the cut slot and work surface roughness. It was found experimentally that in order to minimize the width of cut; the nozzle should be placed close to the work surface. Increase in jet pressure results in widening of the cut slot both at the top and the at exit of the jet from the work. However. the width of cut at the bottom (exit) was always found to be larger than that at the top (at a stand-off distance of 3 mm and the work feed rate of 15 mm min-I). It was found that the taper of cut gradually reduces with increase in stand-off distance and was close to zero at the stand-off distance of 4 nun (at ajetpressure of 30 ksi and a work feed rate of 15 mm min-I). The feed rate of the work should be kept within 40 rum min- I (at the jet pressure of 30 h i and the stand-off distance of3 mm). because a feed rate beyond 40 mm min-I results in sharp increase in taper angle. The jet pressure does not show significant influence on the taper angle within the range of work feed and the stand-off distance considered. Both stand-off distance and the work feed rate show strong influence on the roughness of the machined surface. It was concluded that stand-off distance should be kept within 3 mm (at aiet pressure oDD ksi and a work feed rate ofl5 mm min-') and the work feed rate should be kept within 30 rum min-I (at ajetpressure of 30 hi and a stand-off distance of 3 nun) in order to have a good surface finish, since beyond those values of the parametern the roughness of the machined surface rises sharply. Increase in jet pressure shows positive effect in tenus of smoothness of the machined surface. With increase in jet pressure, the surface roughness decreases (at a stand-off distance of3mm and work feed of 15 mm min-I). This is due to fragmentation of the abrasive particles into smaller sizes at a higher pressure and due to the fact that smaller particles produce smoother surface. It was also found that within the jet pressure considered, the work surface is smoother near the top surface and gradually it becomes rougher at higher depths.

Examination of copper/stainless steel joints formed by explosive welding

In this study, bonding ability of copper and steel with explosion welding was investigated using different ratios of explosive and different stand-off distance. Experimental studies showed out that, copper and stainless steel could be bonded with a good quality of bonding properties with explosion welding. In the bonding interface, intermetallics were not formed. It was observed that, when explosive ratio and stand-off distance were increased smooth bonding interface was transformed to a wavy bonding interface. As the ratio of explosive and stand-off distance increase, the amplitude and wavelength of wave were increased. It was found that, hardness of bonding interface and outer face of plates were increased because of deformation that was originating from impact the effect. Total interface area increased as a result of wavy interface, which was caused by increased explosive ratio and stand-off distance. In addition, wavy interfaces did not separate after tensile-shearing test. Bending tests applied on bonded samples had different diameters indicated that interfaces of the bonded samples have not any defect. EDS analyses in SEM showed that diffusion did not take place between bonding plates, however, diffusion was observed after annealing of the bonded samples for different times.

A two-dimensional heat and fluid-flow model of single-roll continuous-sheet casting process

Single-roll continuous-sheet casting process has been simulated using a mathematical model based on considerations of fluid flow, heat transfer, and solidification. The principal model equations include momentum and energy balances which are written for various zones comprising the process. The flow of liquid metal in the pool is taken to be a two-dimensional recirculatory flow. The concepts of vorticity and stream function are used to reduce the number of equations and number of unknowns, respectively. Model equations and boundary conditions are written in terms of dimensionless variables and are solved, using an implicit finite difference technique, to give stream functions and velocity fields in the metal pool, temperature fields in the metal pool, sheet, and caster drum, and the final sheet thickness for various operating parameters. The parameters examined are: (1) rotational speed of the caster drum, (2) liquid metal head in the tundish, (3) superheat of the melt, (4) caster drum material, and (5) cooling conditions prevailing at the inner surface of the caster drum. The final sheet thickness decreases with increasing rotational speed of the caster drum and melt superheat, but it increases with increasing standoff distance and metal head in the tundish.

Magnetic field effects on the computed flow over a Mars return aerobrake

A numerical algorithm is developed to calculate the electromagnetic phenomena simultaneously with the fluid flow in the shock layer over an axisymmetric blunt body in a thermal equilibrium, chemical nonequilibrium environment. The flowfield is solved using an explicit time-marching, first-order spatially accurate scheme. The electromagnetic phenomena are coupled to the real-gas flow solver through an iterative procedure. The electromagnetic terms introduce a strong stiffness, which was overcome by using significantly smaller time steps for the electromagnetic conservation equation. The technique is applied in calculating the flow over a Mars return aerobrake vehicle entering the Earth's atmosphere. For the case where no external field is applied, the electromagnetic effects have little impact on the flowfield. With the application of an external magnetic field of 0.05 to 0.1 T, the solution indicates an increase in stagnation line shock standoff distance, wall pressure, and radiative heat transfer and a decrease in convective heat transfer.

A study of flat flames on porous plug burners: Structure, standoff distance, and oscillation

The stable structure and the time-varying behavior of stoichiometric methaneair flames on a porous metal burner at atmospheric pressure were studied using a numerical model which includes detailed chemical reactions. It has been confirmed that the model gives quantitative agreement with experiment for stable flame structures such as flame temperature, radical concentrations, and standoff distance, and also flame oscillation. It was shown that the overall activation energy and the standoff distance increase increase rapidly in the region of flame temperature < 1550K. Furthermore, it was also confirmed that the flame intrinsically oscillates due to the propagating time lag of the temperature disturbance from the burner surface to the reaction zone, and its instability is enhanced by a large standoff distance. On the basis of these results, it is asserted that the critical flame temperature for the stability of stoichiometric methaneair flames on porous plug burner is near 1550K.

The quenching of a deflagration wave held in front of a bluff body

This paper describes the analysis of two-dimensional premixed flames located in a nonuniform straining flow with particular attention to the case of flame held in front of a circular cylinder in a potential flow. Using activation energy asymptotics and a slow flame approximation, the problem of solving the aerothermomechanical equations is reduced to a generalized Stefan problem. In the neighborhood of the front stagnation point appropriate similarity equations are solved in closed form, and downstream of that point a numerical description is obtained. The behavior at the stagnation point depends in a complicated fashion on the straining rate β and the Lewis Number L, and for some values of these parameters the speed of the stretched flame is greater than the adiabatic flame speed, for others it is less. Quenching occurs if β is large enough, at a nonvanishing flame speed for some values of L, at zero flame speed for others. The full nature of the quenching phenomenon is revealed by a stability analysis. Numerical integration downstream of the stagnation point is carried out for a variety of cases. For a flame attached to a cylinder both the flame speed and the standoff distance increase significantly. For other flows it is possible to quench the flame downstream of the stagnation point.