Showing papers on "Total pressure published in 2000"

Molecular dynamics results on the pressure tensor of polymer films

Abstract: Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular, PN(z), and parallel components, PT(z), of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ in the expression for PT(z) (z denotes the distance from the wall), but yield the same formula for the normal component PN(z). When evaluated by molecular dynamics (or Monte Carlo)-simulations PN(z) is constant, as required by mechanical stability. Contrary to that, the IK1-method leads to strong oscillations of PN(z). However, all methods give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, w...

Xenon fluoride laser excitation by transverse electric discharge

Abstract: Stimulated emission has been produced in mixtures of He, NF3, and Xe at total pressures between 300 and 1000 Torr. Laser emission was on lines at 3511 and 3531 A which have been associated with the excited XeF molecule. Excitation of the gas mixture was by a transverse electric discharge which produced pulses with peak currents of approximately 104 A and rise times of 20 ns. A maximum laser energy of 7 mJ was obtained from a gas mixture with a ratio of He:Xe:NF3 of 98.0:1.5:0.5 at a total pressure of 300 Torr.

Tip clearance effects in a turbine rotor : Part I : Pressure field and loss

Abstract: This paper presents an experimental investigation of the effects of the tip clearance flow in an axial turbine rotor. The effects investigated include the distribution and the development of the pressure, the loss, the velocity, and the turbulence fields. These flow fields were measured using the techniques of static pressure taps, rapid response pressure probes, rotating five-hole probes, and Laser Doppler Velocimeter. Part I of this paper covers the loss development through the passage, and the pressure distribution within the passage, on the blade surfaces, on the blade tip, and on the casing wall. Regions with both the lowest pressure and the highest loss indicate the inception and the trace of the tip leakage vortex. The suction effect of the vortex slightly increases the blade loading near the tip clearance region. The relative motion between the turbine blades and the casing wall results in a complicated pressure field in the tip region. The fluid near the casing wall experiences a considerable pressure difference across the tip. The highest total pressure drop and the highest total pressure loss were both observed in the region of the tip leakage vortex, where the loss is nearly twice as high as that near the passage vortex region. However, the passage vortex produces more losses than the tip leakage vortex in total. The development of the loss in turbine rotor is similar to that observed in cascades. Part II of this paper covers the velocity and the turbulence fields.Copyright © 2000 by ASME

Gas Jet–Workpiece Interactions in Laser Machining

Abstract: Laser machining efficiency and quality are closely related to gas pressure, nozzle geometry, and standoff distance. Modeling studies of laser machining rarely incorporate gas effects in part because of the complex structure and turbulent nature of jet flow. In this paper, the interaction of a supersonic, turbulent axisymmetric jet with the workpiece is studied. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The model is able to make quantitative predictions of the pressure, mass flow rate as well as shear force at the machining front. Effect of gas pressure and nozzle standoff distance on structure of the supersonic shock pattern is studied. Experiments are carried out to study the effect of processing parameters such as gas pressure and standoff distance. The measured results are found to match and hence validate the simulations. The interaction of the oblique incident shock with the normal standoff shock is found to contribute to a large reduction in the total pressure at the machining front and when the nozzle pressure is increased beyond a certain point. The associated reduction in flow rate, fluctuations of pressure gradient and shear force at the machining front could lower the material removal capability of the gas jet and possibly result in a poorer surface finish. The laser cutting experiments show that the variation of cut quality are affected by shock structures and can be represented by the mass flow rate. @S1087-1357~00!01702-0#

Automated Design of a Three-Dimensional Subsonic Diffuser

Abstract: A novel methodology is developed to integrate state-of-the-art computational e uid dynamics analysis, NURBS, and optimization theory to reduce total pressure distortion and sustain total pressure recovery within a curved three-dimensional subsonic S-duct diffuser by automated redesign of the diffuser shape. Two independent design variables are used. The change of the surface shape is assumed to be Gaussian. GASP with the modie ed Baldwin ‐ Lomax turbulence model (Baldwin, B. S., and Lomax, H., “ Thin Layer Approximation and Algebraic Model for Separated Turbulent Flows,” AIAA Paper78-257, 1978 )is employed for the e owe eld prediction and proved to give good agreement with the experimental surface pressure for the baseline S-duct diffuser geometry. The automated design optimization is performed with a gradient-based method to minimize the total pressure distortion based on the two design variables. The best cone guration obtained reduced distortion by typically 70% while keeping the total recovery essentially the same. The results indicate that the mechanism responsible for improved diffuser performance is the suppression of detrimental secondary e ows by changing the surface shape to redirect the e ow.

Computational Investigation of the Aerodynamic Effects on Fluidic Thrust Vectoring

Abstract: A computational investigation of the aerodynamic effects on fluidic thrust vectoring has been conducted. Three-dimensional simulation of a two-dimensional, convergent-divergent (2DCD) nozzle with fluidic injection for pitch vector control were run with the computational fluid dynamics code PAB using turbulence closure and linear Reynolds stress modeling. Simulations were computed with static freestream conditions (M=0.05) and at Mach numbers from M=0.3 to 1.2, with scheduled nozzle pressure ratios from 3.6 to 7.2 and secondary to primary total pressure ratios of p sub t,s/p sub t,p =0.6 and 1.0. Results indicate that the freestream flow decrease vectoring performance and thrust efficiency compared with static (wind-off) conditions. The aerodynamic penalty to thrust vector angle ranged from 1.5 degrees at a nozzle pressure ratio of 6 with M=0.9 freestream conditions to 2.9 degrees at nozzle pressure ratio of 5.2 with M=0.7 freestream conditions, compared to the same nozzle pressure ratios with static freestream conditions. The aerodynamic penalty to thrust ratio decreased from 4 precent to 0.8 percent as nozzle pressure ratio increased from 3.6 to 7.2. As expected, the freestream flow had little influence on discharge coefficient.

Numerical Modeling of Gas‐Phase Nucleation and Particle Growth during Chemical Vapor Deposition of Silicon

Abstract: A numerical model was developed to predict gas-phase nucleation of particles during silane pyrolysis. The model includes a detailed clustering mechanism for the formation of hydrogenated silicon clusters containing up to ten silicon atoms. This mecha nism was coupled to an aerosol dynamics moment model to predict particle growth, coagulation, and transport. Both zero-dimensional transient simulations, at 1-2 atm pressure, and one-dimensional steady-state stagnation-point flow simulations, at 1-2 Torr pressure, were conducted. The effects of carrier gas, temperature, pressure, silane concentration, and flow rate were examined. The results predict that hydrogen as carrier gas, compared to helium, suppresses nucleation, and that particle formation for the case of hydrogen carrier gas increases strongly with increasing initial silane-to-hydrogen ratio. For the conditions examined, predicted particle nucleation rates increase dramatically with increasing temperature. The effect of total pressure depends on the pressure regime: at 1-2 atm pressure particle formation is predicted to be insensitive to pressure, whereas at 1-2 Torr particle formation is predicted to increase strongly with increasing pressure. The predicted effects on particle formation of temperature, pressure, ca rrier gas, and silane concentration are all qualitatively consistent with published experimental results. In the stagnation-point f low simulations the flow rate is found to affect particle dynamics because of the opposed effects of convective transport toward th e heated water and thermophoretic transport away from the wafer.

Effects of Inlet Flow Field Conditions on the Performance of Centrifugal Compressor Diffusers: Part 1—Discrete-Passage Diffuser

Abstract: This is Part 1 of a two-part paper considering the performance of radial diffusers for use in a high-performance centrifugal compressor. Part 1 reports on discrete-passage diffusers (shown in Fig. 1) while Part 2 describes a test of a straight-channel diffuser designed for equivalent duty. Two builds of discrete-passage diffuser were tested, with 30 and 38 separate passages. Both the 30 and 38 passage diffusers investigated showed comparable range of unstalled operation and similar level of overall diffuser pressure recovery. The paper concentrates on the influence of inlet flow conditions on the pressure recovery and operating range of radial diffusers for centrifugal compressor stages. The flow conditions examined include diffuser inlet Mach number, flow angle, blockage, and axial flow nonuniformity. The investigation was carried out in a specially built test facility, designed to provide a controlled inlet flow field to the test diffusers. The facility can provide a wide range of diffuser inlet velocity profile distortion and skew with Mach numbers up to unity and flow angles of 63 to 75 deg from the radial direction. The consequences of different averaging methods for the inlet total pressure distributions, which are needed in the definition of diffuser pressure recovery coefficient for nonuniform diffuser inlet conditions, were also assessed. The overall diffuser pressure recovery coefficient, based on suitably averaged inlet total pressure, was found to correlate well with the momentum-averaged flow angle into the diffuser. Furthermore, the pressure recovery coefficient was found to be essentially independent of the axial distortion at diffuser inlet, and the Mach number, over the wide flow range (from maximum flow to the beginning of flow instabilities) investigated. It is thus shown that the generally accepted sensitivity of diffuser pressure recovery performance to inlet flow distortion and boundary layer blockage can be largely attributed to inappropriate quantification of the average dynamic pressure at diffuser inlet. Use of an inlet dynamic pressure based on availability or mass-averaging in combination with definition of inlet flow angle based on mass average of the radial and tangential velocity at diffuser inlet removes this sensitivity.

Kinetics of high-pressure removal of hydrogen sulfide using calcium oxide powder

Abstract: Sulfidation reaction of CaO at high pressure (up to 2 MPa) and high temperature (up to 900°C) to remove H2S in a coal-fired gasifier was studied in a high-pressure and temperature differential-bed flow-through reactor. Experimental conditions selected are typical for pressurized gasifiers. Effects of total pressure, H2S partial pressure, reaction temperature, fuel gas composition, and CaO surface area on the extent of sulfur capture and sorbent conversions were determined. The gasifier pressure affected the in-situ calcination of calcium carbonate particles through reduction in available surface area and pore volume of CaO formed, thus limiting the sulfidation conversion. Time-resolved conversion data of CaO sulfidation were analyzed using a modified grain model. The model incorporates external and internal diffusion, surface reaction, product layer diffusion, and the structural changes of the sulfiding CaO particle. The activation energy for the reaction was 37 kcal/mol. The estimated product layer diffusivity was 8×10−15 m2/s at 800°C with an associated activation energy of 38.4 kcal/mol—typical of solid state diffusion of ionic species through the product layer. The extent of conversion increased with increasing initial surface area and porosity of CaO particles. The high-pressure sulfidation reaction data for CaO will be useful in understanding and optimizing the in-gasifier H2S capture using calcium-based sorbents.

The Influence of Technical Surface Roughness Caused by Precision Forging on the Flow Around a Highly Loaded Compressor Cascade

Abstract: A highly loaded compressor cascade, which features a chord length ten times larger than in real turbomachinery, is used to perform an investigation of the influence of technical surface roughness. The surface structure of a precision forged blade was engraved in two 0.3-mm-thick sheets of copper with the above-mentioned enlarging factor (Leipold and Fottner, 1996). To avoid additional effects due to thickening of the blade contour, the sheets of copper are applied as inlays to the pressure and suction side. At the high-speed cascade wind tunnel, the profile pressure distribution and the total pressure distribution at the exit measurement plane were measured for the rough and the smooth blade for a variation of inlet flow angle and inlet Reynolds number. For some interesting flow conditions, the boundary layer development was investigated with laser-two-focus anemometry and one-dimensional hot-wire anemometry. At low Reynolds numbers and small inlet angles, a separation bubble is only slightly reduced due to surface roughness. The positive effect of a reduced separation bubble is overcompensated by a negative influence of surface roughness on the turbulent boundary layer downstream of the separation bubble. At high Reynolds numbers, the flow over the rough blade shows a turbulent separation leading to high total pressure loss coefficients. The laser-two-focus measurements indicate a velocity deficit close to the trailing edge, even at flow conditions where positive effects due to a reduction of the suction side separation have been expected. The turbulence intensity is reduced close downstream of the separation bubble but increased further downstream due to surface roughness. Thus the rear part of the blade but not the front part reacts sensitively on surface roughness.

Aerodynamic Design and Analysis of a High Pressure Ratio Aspirated Compressor Stage

Abstract: The pressure ratio of axial compressor stages can be significantly increased by controlling the development of blade and endwall boundary layers in regions of adverse pressure gradient by means of boundary layer suction. This concept is validated and demonstrated through the design and analysis of a unique aspirated compressor stage which achieves a total pressure ratio of 3.5 at a tip speed of 1500 ft/s. The aspirated stage was designed using an axisymmetric through-flow code coupled with a quasi three-dimensional cascade plane code with inverse design capability. Validation of the completed design was carried out with three-dimensional Navier-Stokes calculations. Spanwise slots were used on the rotor and stator suction surfaces to bleed the boundary layer with a total suction requirement of 4% of the inlet mass flow. Additional bleed of 3% was also required on the hub and shroud near shock impingement locations. A three-dimensional viscous evaluation of the design showed good agreement with the quasi three-dimensional design intent, except in the endwall regions. The three-dimensional viscous analysis predicted a mass averaged total pressure ratio of 3.7 at an isentropic efficiency of 93% for the rotor, and a mass averaged total pressure ratio of 3.4 at an isentropic efficiency of 86% for the stage.Copyright © 2000 by ASME

Modeling high-pressure char oxidation using langmuir kinetics with an effectiveness factor

Abstract: The global nth order rate equation has been criticized for lack of theoretical basis and has been shown to be inadequate for modeling char oxidation rates as a function of total gas pressure. The simple Langmuir rate equation is believed to have more potential for modeling high pressure char oxidation. The intrinsic Langmuir rate equation is applied to graphite flake oxidation data and agrees well with reaction rates at three temperatures over the entire range of oxygen pressure (1–64 atm). It also explains the change of reaction order with temperature. In this work, the intrinsic Langmuir rate equation is combined with (1) an effectiveness factor to account for pore diffusion effects and (2) a random pore structure model to calculate effective diffusivity. The resulting model is able to predict the reaction rates of large (ca. 8 mm) coal char particles as a function of gas velocity, total pressure, oxygen partial pressure, oxygen mole fraction, initial particle size, and gas temperature. This approach is also able to correlate the particle burnouts of pulverized (70 lm) coal char particles in a drop tube reactor as a function of total pressure, oxygen mole fraction, gas and wall temperatures, and residence time. The ability of the model to correlate data over wide range of temperature and pressure is promising.

A Theoretical Analysis of Mixed Lubrication by Macro Micro Approach: Part I—Results in a Gear Surface Contact

Abstract: Mixed lubrication is a key to bring the performance analysis to the failure analysis in most tribological components. A macro-micro approach to mixed lubrication has been developed in the present model. The relation between the average contact pressure and the average gap for a typical rough contact patch is first determined numerically in micro scale. Using this relation, the average gap, average oil-film pressure, and average contact pressure in a mixed-lubricated elastohydrodynamic contact can be solved simultaneously in macro scale by treating the contact to be smooth. The total pressure is assumed a superposition of average asperity contact pressure and lubricant pressure. The new approach is simple, efficient and robust, and covers entire range of the load ratio, from unity (dry contact) to zero (full-film EHL). In addition, it can be used for a wide range of operating conditions and on a much larger contact area with a much less computing time than deterministic simulation of mixed lubrication. Imp...

Can vortices in the flow across mechanical heart valves contribute to cavitation

Abstract: Cavitation in mechanical heart valves is traditionally attributed to the hammer effect and to squeeze and clearance flow occurring at the moment of valve closure. In the present study, an additional factor is considered—the contribution of vortex flow. Using a computational fluid dynamics analysis of a 2D model of a tilting disk mitral valve, we demonstrate that vortices may form in the vicinity of the inflow side of the valve. These vortices roll up from shear layers emanating from the valve tips during regurgitation. A significant decrease in the pressure at the centre of the vortices is found. The contribution of the vortex to the total pressure drop at the instant of closure is of the order of 70 mmHg. Adding this figure to the other pressure drop sources that reach 670 mmHg, it might be that this is the deciding factor that causes the drop in blood pressure below vapour pressure. The total pressure drop near the upper tip (750 mmHg) is larger than near the lower tip (670 mmHg), indicating a preferential location for cavitation inception, in agreement with existing experimental findings.

On finite β stabilization of the toroidal ion temperature gradient mode

Abstract: It is shown that finite beta stabilization of the toroidal ion temperature gradient (ITG) mode is attributable to the electron ballooning parameter (normalized electron pressure gradient) αe. A modest αe, much smaller than that required from the drift reversal α (normalized total pressure gradient) ≳2q2 where q is the safety factor, can effectively stabilize the ITG mode.

Fuel delivery system for combusting fuel mixtures

Abstract: Apparatus for reducing the total pressure of a compressible fluid fuel. The apparatus includes at least two closely spaced apart constant enthalpy expansion sections (10, 20), each section having at least one orifice (82, 92-96, 112-116), the orifices in adjacent sections being noncoaxial. The pressure reduction lowers flow velocity when mixed with the air to below the flame speed to promote ignition and stable combustion.

Total pressure determination with multifunction probe for aircraft

Abstract: A multifunction probe for aircraft to determine in particular the static pressure and the total pressure of an airflow located in the vicinity of the aircraft and the angle of incidence of the aircraft with respect to the airflow. The probe includes a movable blade configured to orient itself along the axis of the airflow. The movable blade includes at least one static pressure tap orifice located on one side of the blade and a pressure tap configured to measure the angle of incidence of the blade with respect to the airflow. The probe further includes a pressure measurement device associated with each pressure tap and a device to calculate the total pressure of the airflow as a function of the measurements carried out by the various measurement devices of the probe.

Effect of pressure and temperature on the competition between nondissociative and dissociative electron attachment to POCl3

Abstract: Branching between the nondissociative, POCl3−, and dissociative, POCl2−, products of the POCl3 electron attachment reaction has been examined as a function of buffer gas pressure and temperature. A strong positive pressure dependence is observed for the nondissociative channel, where at 303 K, the %POCl3− increases from 31% at 1 Torr to 94% at 675 Torr total pressure. Conversely, the dissociative channel displays a strong positive temperature dependence. The effect of pressure and temperature on the relative amounts of POCl3− and POCl2− observed is discussed in terms of the competition between the collisional stabilization and dissociation rates of the POCl3−* excited intermediate. The decomposition of POCl3−* is modeled with Rice–Ramsperger–Kassel decomposition kinetics weighted by the vibrational energy distribution of POCl3 neutrals. This model provides an excellent simulation of the experimental pressure and temperature dependencies of the electron attachment process.

Thermodynamics of the Casimir effect

Abstract: A complete thermodynamic treatment of the Casimir effect is presented. Explicit expressions for the free and the internal energy, the entropy and the pressure are discussed. As an example we consider the Casimir effect with different temperatures between the plates (T) resp. outside of them (T'). For T'