Gas Flow Profile Research Articles

Quenching Homogeneity and Intensity Improvement in Batch Mode High Pressure Gas Quenching

Abstract Modern high pressure gas quenching processes are favorable for certain cases compared to conventional liquid media quenching, because of their advantages like, e.g., pure convective heat transfer and a lower impact of the process on the environment. But, typical gas quenching facilities may exhibit inhomogeneous flow conditions through the quench load and the parts, resulting in a scattering of the quenching result. The upstream gas flow profile of the load has been identified as one key factor that determines the local flow conditions and the heat transfer distribution from the specimen. This incoming gas velocity distribution results from the interaction between upstream placed entities, as global flow guidance, baffles, and heat exchangers. The global intensity of the quenching process is determined on one side by the pressure drop that results from the flow resistance of the quench load inside the chamber and the total volumetric flow rate in the system. On the other side, a significant part of the gas flow goes in the gap between the load and the chamber walls and does not contribute to the quenching process of the load. The first part of the investigation addresses the modeling and simulation of the flow structure inside a commercial high pressure gas quenching chamber. A multi-scale simulation model on different detailed levels is performed leading to fast convergence of the simulation. For validation of the simulation, the second part describes an experimental analysis of the gas flow inside a model quenching chamber. Here, gas velocity measurements and flow visualizations are performed. Finally, a quenching run with cylindrical parts in a double-chamber vacuum furnace is described in order to confirm the model results. Different upstream velocity profiles to the load are adjusted to demonstrate their influence on the quenching result.

Neutral gas flow and ring-shaped emission profile in non-thermal RF-excited plasma needle discharge at atmospheric pressure

We demonstrate that a balance between convective helium flow and ambient air diffusion creates a unique ring-shaped light emission profile by means of finite element analysis of the atmospheric pressure RF-excited plasma needle. The plasma needle has a point-to-plane geometry with a radius of 30 µm at the tip and an inter-electrode gap of 1 mm. We employ a coupled model between time-dependent plasma dynamics based on a fluid model and steady state neutral gas flow in two-dimensional cylindrical coordinates. When the mean inlet gas velocity is 1.5 m s−1 and the discharge is in high-power glow mode at 200 mW, the concentration of air drastically increases near a treated surface being away from the needle tip. As a result, Penning ionization by helium metastables and air (nitrogen) peaks at an off-axis position, corresponding to the ring-shaped emission profile in cylindrical coordinates. The off-axis ionization peak leads to an off-axis flux peak of nitrogen ions onto the treated surface. The ‘ion wind’ and gas heating have only minor effects on the discharge structure under the conditions considered here.

Study of gas cavity size hysteresis in a packed bed using DEM

When a high velocity gas jet is introduced into a packed bed a cavity is formed. The size of the cavity shows hysteresis on increasing and decreasing gas flow rates. This hysteresis leads to different cavity sizes at same gas flow rate depending on the bed history. The size of cavity affects the gas flow profiles in the packed bed. In this study the cavity size hysteresis phenomenon has been modeled using discrete element method along with turbulent gas flow. A reasonable agreement has been found between computed and experimental results on cavity size hysteresis. The effect of various parameters, such as nozzle height from the bed bottom and packing height, on the cavity size hysteresis has been studied. It is found that inter-particle interaction forces along with gas drag and bed porosity play an important role in describing the cavity size hysteresis. The injection of gas flow allows the particles to go to an unconstrained state than they were previously in, and their ability to remain in that state, even under decreased gas drag force, leads to the phenomenon of cavity size hysteresis.

Analysis of a three-dimensional particle velocity sensor for design optimization

The omni-directional sensitivity pattern of an integrated, three-dimensional acoustic sensor consisting of four hot-wire particle velocity sensors is analyzed theoretically and experimentally. The influence of the probe surface on the measured direction of particle velocity is investigated and a description of the gas flow profile around the integrated sensor is presented. A theoretical model describing the fluid flow in the vicinity of a simplified geometry, a plate of finite width, is shown to give a relatively adequate description for the gas flow around the probe. The analysis results in a number of design rules to optimize the device performance.

Dual-Heater Reactor Design for Hybrid Physical-Chemical Vapor Deposition of ${\rm MgB}_{2}$ Thin Films

A new reactor configuration for the hybrid physical-chemical vapor deposition (HPCVD) of MgB2 thin films was developed. In this new design, the substrate is resistively heated and inversely mounted in the gas stream. The Mg supply is placed downstream of the substrate and is inductively heated. The dual-heater reactor is designed to enable the growth of MgB2 thin films at lower substrate temperatures, permit the growth of thick films and is amenable to scale-up for film deposition over larger substrate areas than the original HPCVD design. Computational fluid dynamics (CFD) based reactor modeling was used to simulate the gas flow, temperature and gas phase concentration profiles in the reactor in order to identify conditions suitable for film deposition. A study of boron thin film deposition from B2H6 in the dual-heater configuration was carried out and the experimental results were compared to CFD simulations which utilized a previously developed gas-phase and surface chemistry model to predict the growth rate. The boron film growth rate slightly decreased as the substrate temperature was increased from 650degC to 750deg C in both the experiments and model simulation. The modeling results indicate that boron deposition is limited by the mass transfer of BH3 to the substrate under these conditions and the temperature dependency arises due to the upstream depletion of the boron precursor. MgB2 films were grown at a substrate temperature of 710degC and Mg temperature of 720degC using the dual-heater configuration. The film properties were consistent with previously published results of MgB2 on sapphire in the original HPCVD configuration with a superconducting transition temperature, Tc, of 40.3 K and residual resistivity ratio (RRR) of 13.

FAIMS Operation for Realistic Gas Flow Profile and Asymmetric Waveforms Including Electronic Noise and Ripple

The use of field asymmetric waveform ion mobility spectrometry (FAIMS) has rapidly grown with the advent of commercial FAIMS systems coupled to mass spectrometry. However, many fundamental aspects of FAIMS remain obscure, hindering its technological improvement and expansion of analytical utility. Recently, we developed a comprehensive numerical simulation approach to FAIMS that can handle any device geometry and operating conditions. The formalism was originally set up in one dimension for a uniform gas flow and limited to ideal asymmetric voltage waveforms. Here we extend the model to account for a realistic gas flow velocity distribution in the analytical gap, axial ion diffusion, and waveform imperfections (e.g., noise and ripple). The nonuniformity of the gas flow velocity profile has only a minor effect, slightly improving resolution. Waveform perturbations are significant even at very low levels, in some cases approximately 0.01% of the nominal voltage. These perturbations always improve resolution and decrease sensitivity, a trade-off controllable by variation of noise or ripple amplitude. This trade-off is physically inferior to that obtained by adjusting the gap width and/or asymmetric waveform frequency. However, the disadvantage is negligible when the perturbation period is much shorter than the residence time in FAIMS, and ripple adjustment appears to offer a practical method for modifying FAIMS resolution.

Two-Dimensional Axi-Symmetric Model of a Solar-Thermal Fluid-Wall Aerosol Flow Reactor

A solar-thermal fluid-wall reactor consisting of three concentric vertical tubes is constructed to dissociate methane to hydrogen and carbon black using concentrated solar power. Several aspects of the design are modeled for scaling the system up: the heat transfer and its effect on the integrity of the materials, and the fluid flow of all gas streams within the reactor. It is determined that the inlet gas temperatures, mass flow rates, and permeability of the porous wall affect the gas flow profile through the porous tube wall. By increasing the inlet gas temperature and/or the tube permeability in the hot zone section of the reactor, a more uniform flow profile can be obtained along the length of the tube.

System for Increasing Exhaust Gas Temperature for Catalyst Activation at Light Load Condition in Diesel Engines

Although in-cylinder combustion improvements have steadily led to reduced exhaust emissions in diesel engines, effective after treatment technology is increasingly important for meeting future extremely low emissions targets. Oxidation catalysts have already been fitted to some passenger cars and trucks. However, at low load operation the exhaust gas temperature is only about 373 K, too low for catalyst activation. In this study, to increase catalyst activation at low load, throttling of intake and exhaust gas flow, cylinder deactivation, and exhaust cam profile modifications were investigated for increasing the exhaust gas temperature. With an optimized combination of these methods, the exhaust gas temperature could be increased to above 573 K and achieve catalyst activation even for idling conditions.

Epitaxial Growth of 2 inch 3C-SiC on Si Substrates by Atmospheric Hot Wall CVD

AbstractAs a high mobility, wide bandgap semiconductor, 3C-SiC has great promise. In this paper, we examined to obtain 3C-SiC epilayer on Si substrates using hot-wall CVD furnace and report the use of hexamethyledisilane (HMDS) and propane as reaction gases to grow uniform thickness on 2 inch (100), (111), (110) and (211) orientation of Si substrates. A horizontal atmospheric pressure CVD reactor was used. A reaction zone was specially designed. To obtain uniform thickness of the epilayer, inside of the suscceptor hole was intentionally tapered along flow direction as follows; inlet of the square hole is 13 mm × 60 mm and outlet of the hole is 7 mm × 60 mm, and laminar channel for changing the gas flow profile was managed. The susceptor was surrounded by graphite foam. Temperature of the suscepotor was measured at inside wall of the susceptor by optical pyrometer. H2 flow rate for etching was 3 slm. An initial carbonization procedure was performed using 0.9 sccm propane at 1250 oC for 2-3 minutes. During the growth of SiC at 1300 °C, the flow rate of HMDS was 0.75-1.2 sccm and the flow rate of propane was 0.1 – 0.5 sccm. The hydrogen carrier gas flow rate was 3-10 slm. Typical growth rate was 4.5 micron /h. Uniform thick 3C-SiC was obtained. The samples were examined using ultra violet light spectrometer and RHEED.

Two-dimensional transient numerical simulation of solids and gas flow in the riser section of a circulating fluidized bed

A computational fluid dynamics software (CFX) was modified for gas/particle flow systems and used to predict the flow parameters in the riser section of a circulating fluidized bed (CFB). Fluid Catalytic Cracking (FCC) particles and air were used as the solids and gas phases, respectively. Two-dimensional, transient, isothermal flows were simulated for the continuous phase (air) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. Two-dimensional gas and particle flow profiles were obtained for the velocity, volume fraction, and pressure drop for each phase. Calculations showed that the inlet and exit conditions play a significant role in the overall mixing of the gas and particulate phases and in the establishment of the flow regime. The flow behavior was analyzed based on the different frequency of oscillations in the riser. Comparison of the calculated solids mass flux, solids density and pressure drop with the measured pilot-scale PSRI data (reported in this paper) showed a good agreement.

A Study of Cyclohexane Diffusion in Hybrid (Organic/Inorganic) Gels Monitored by an Oscillating Balance Reactor

Hybrid (organic/inorganic) gels are studied in terms of cyclohexane diffusion transients with an oscillating balance reactor (OBR) which, unlike conventional microbalances, has the well-defined gas flow profile characteristics of packed beds (PB's). The OBR-based flow-through adsorption system, when properly designed, has very good time resolution and is particularly useful in modeling adsorption processes under more realistic PB geometries. With the test sample adopted in this study, the “semirigid” nature of hybrid gels becomes apparent when simple relaxation models are chosen to fit experimental data.

Simulation of particles and gas flow behavior in the riser section of a circulating fluidized bed using the kinetic theory approach for the particulate phase

Gas/particle flow behavior in the riser section of a circulating fluidized bed (CFB) was simulated using a computational fluid dynamics (CFD) package by Fluent. Fluid catalytic cracking (FCC) particles and air were used as the solid and gas phases, respectively. A two-dimensional, transient and isothermal flow was simulated for the continuous phase (air) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. This approach treats each phase separately, and the link between the gas and particle phases is through drag, turbulence, or energy dissipation due to particle fluctuation. Gas and particle flow profiles were obtained for velocity, volume fraction, pressure, and turbulence parameters for each phase. The computational values agreed reasonably well with the available experimental results. Our computational results showed that the inlet and outlet design have significant effects on the overall gas and solid flow patterns and cluster formations in the riser. However, the effect of the initial condition tended to disappear after some time. The main frequencies of oscillations of the system were obtained in different regions of the riser. These frequencies are important in comparing the computational results with the available time-averaged experimental data.

Gas dynamics of the ICP-MS interface: impact pressure probe measurements of gas flow profiles

T. N. Olney, W. Chen and D. J. Douglas, J. Anal. At. Spectrom., 1999, 14, 9 DOI: 10.1039/A806205F

A NEW DEVICE FOR OXYGENATION AND DRUG ADMINISTRATION BY TRANSTRACHEAL JET VENTILATION

S203 Introduction: The incidence of the potentially life-threatening "cannot intubate, cannot ventilate" situation is approximately 1/10.000 anesthetics [1]. Percutaneous transtracheal jet ventilation (TTJV) is a widely accepted, effective treatment to restore adequate arterial oxygenation. Unfortunately, the equipment required for "conventional" TTJV is often not immediately available in emergency situations and demands regular maintenance. These systems bear a high risk of barotrauma in cases of treating emergency respiratory distress and complete upper airway obstruction. Alternative, mostly self-assembled transtracheal systems (e.g. oxygen insufflation by squeezing a bag connected to a transtracheal needle (or iv-catheter) by a cutted barrel of a 2-ml-syringe and a tubus connector [2]) fail to achieve effective oxygenation albeit with a lower risk for barotrauma. A new device ("flow-modulator") is introduced allowing oxygenation and drug administration into the patient's lungs by manually controlled, jet-ventilation-like, pulsed gas flow. Methods: The device [3] consists of a short, non-compliant tube (1) with several openings (2) located at opposite sites in front of a syringe connector (3). The device is connected between a transtracheal needle (or iv-catheter) and an oxygen supply delivering oxygen at a rate of at least 15 liters/minute. Intermittent closing and releasing leads to a jet-ventilation-like, pulsed flow. The ratio of the transtracheal needle's (or iv-catheter's) cross-section and the total cross-section of the non-closed openings of the device determines the effective gas flow to the patient (e.g. with a ratio of 1 to 3, 25% of the total gas flow reaches the patient's lungs). Successive closing of openings with equal diameters induces an exponential gas flow increase to the patient's lungs, whereas a linear gas flow profile is achievable with different (decreasing) diameters. Besides, effective jet-nebulizing by injecting a liquid drug through the syringe connector is possible. Currently, the efficacy of the device is examined in an animal model approved by the Institutional Animal Investigation Committee. (Figure 1)Figure 1Results: The closure of all openings and subsequent high gas flow to the patient is only achievable by an intentional digital maneuver. Total release of the openings ensures prompt, quantitative gas discharge to the atmosphere thus minimizing the risk of barotrauma even in case of upper airway obstruction. The construction of the device is characterized by the absence of any mechanical parts. Maintenance is not required. This includes a maximum of system-immanent safety. Malfunction is excluded - a barotrauma can only occur following a physician's fault. First data suggest sufficient oxygenation with concomitant moderate hypercapnia and respiratory acidosis. In no case barotrauma or severe hemodynamic deterioration were observed. Discussion: The device ("flow-modulator") allows safe, variable, manually controlled oxygen-flow into the patient's lungs assuring sufficient oxygenation with moderately impaired ventilation. In addition, drugs (e.g. epinephrine) can be nebulized, reaching even more peripheral tracheobronchial areas. This small device, easy to understand and safely to handle, may become a beneficial tool in the management of the difficult airway - since it fits the smallest pocket even "on the road".

A Study on the Swirling Flow Field in the Freeboard of a Vortexing Fluidized Bed Combustor.

A computational fluid dynamics model, based on χ-ε turbulence model for the closure of steady turbulent flow, was used for the evaluation of the swirling flow field. Also a vortexing fluidized bed combustor(VFBC)cold model testing facility was designed, fabricated, and tested. Computations and experimental data permitted the investigation of the effects of various operating parameters on the gas flow profiles and vortex numbers. The results show that a stable suspension layer in the freeboard was produced by the swirling gas motion. The vortex number is found to increase with the secondary air velocity, increase with the diameter of the imaginary circle, and decrease with the ratio of volumetric flow rates of the primary air to the secondary air. In this paper, a correlation for evaluating the vortex number was proposed which was shown to be one of the most useful parameters in the design of a VFBC.

Three-dimensional equipment modeling for chemical vapor deposition

Three-dimensional (3D) numerical equipment modeling using the simulator PHOENICS-CVD was used to investigate a low pressure chemical vapor deposition process for boron and phosphorus doped silicon glass deposition from TEOS, PH3, O2, and TEB gas sources. The simulation allows the 3D visualization of gas flow, temperature, and chemical concentration profiles in the reactor, as well as the calculation of the deposition rate as a function of position. Mixing of gases from different inlets and deposition uniformity due to reactive gas depletion were studied as a function of pressure, gas flow velocity, and reactor geometry. Comparison with experimental deposition rate data shows that published chemical models can well describe the undoped deposition, while additional chemical reactions seem to become important when PH3 is introduced in the reactor for phosphorus doped glass deposition.

Effect of solid particles on gas holdup in flotation columns—II. Investigation of mechanisms of gas holdup reduction in presence of solids

Part I showed that the presence of solids decreased gas holdup under conditions typical of flotation in columns. Four mechanisms which might explain this effect are evaluated here: bubble coalescence, slurry density and viscosity changes, changes in radial gas holdup and flow profiles, and bubble wake effects. The first two cannot explain the holdup decrease but the second two can. It was shown experimentally that solids do not cause bubbles to coalesce in flotation columns. Using the drift flux model, changes in density and viscosity of the slurry due to the presence of solids could not account for the observed reduction in gas holdup. A drift flux analysis for the gas-slurry system suggested both that the holdup and flow were not distributed uniformly and that the bubbles rose more rapidly in the presence of solids. Two gas holdup and flow profiles were postulated which could account for the lower gas holdup. These profiles had an upward flow in the center of the collumn and an annular downward flow near the wall. The bubble wake mechanism postulates that wake stability is increased in the presence of solids due to increased viscosity. This, in turn, increases the probability of in line bubble-bubble interaction where the wake velocity of the leading bubble increases the rise velocity of the trailing bubble. The resulting higher rise velocity reduces the gas holdup. It is proposed that the effect of solids on reducing gas holdup is a combination of an increase in the rise velocity of bubbles due to stabilization of the bubble wake and a change in the holdup and flow profiles from flat to non-uniform.

Numerical Simulation of Gas-Solid Two-Phase Flow in Horizontal Pipe. Interaction between Two Phases and Change of Energy Due to Collision of Particle.

A numerical simulation of gas-solid two-phase flow is carried out for mixed-size coarse particles and for uniform ones in a horizontal pipe of 40 mm inside diameter for mean air velocities of 12 and 16 m/s and solid loading ratio less than 1.14. Interaction between gas and particles is considered in the mean flow level of gas. The gas flow profile is modified by particles when the loading ratio is large, and the modification is significant in the acceleration region of particles. The main source of particle rotation is collision with the pipe wall and the particle-particle collision decelerates the rotation. The particles lose kinematic energy mostly by collision with the wall.

Investigation of the chemical vapor deposition of silicon carbide from tetramethylsilane by in situ temperature and gas composition measurements

The chemical vapor deposition (CVD) of silicon carbide (SiC) from tetramethylsilane Si(CH{sub 3}){sub 4} (TMS) on a graphite susceptor at 1200-1500 K is studied in a low pressure ({approx}100 Pa) cold-wall reactor under laminar flow conditions. In addition to material characterization (electron microscopy and chemical analysis), the gas-phase temperature distribution and composition are investigated by combining several in situ and ex situ diagnostics. Coherent anti-Stokes Raman spectroscopy (CARS) on TMS and H{sub 2} (produced from TMS decomposition) in the hot zone of the reactor gives the rotational temperature distribution of the molecules and their concentrations. Within a few mean free paths from the surface, the H{sub 2} gas temperature is lower than the surface temperature. This is due to nonunity accommodation coefficient {alpha} of H{sub 2} on SiC. A simple analytical model yields {alpha} = 0.05 for H{sub 2} on SiC. Using gas transport coefficients and the experimental value of {alpha} for H{sub 2}, a two-dimensional numerical code is used to compute the gas flow and temperature profiles in the reactor. The increase of the H{sub 2} concentration and the decrease of TMS concentration close to the surface reveals that gas-phase pyrolysis of TMS occurs within a few millimeters frommore » the hot surface. The gas composition at the outlet of the reactor is analyzed by mass spectrometry and IR absorption spectroscopy. The global gas conversion and material balance between deposited SiC, powders, and exhaust gases is obtained Si atoms of TMS molecules are mostly converted into solid SiC and powders. In the gaseous products a small fraction of trimethylsilane SiH(CH{sub 3}){sub 3} is detected. Other gases in decreasing order of importance are H{sub 2}, CH{sub 4}, C{sub 2}H{sub 4}, and C{sub 2}H{sub 2}. These results are compared with predictions of some thermodynamic models and chemical mechanisms reported in the literature. 42 refs., 15 figs., 5 tabs.« less

Low-temperature photoluminescence topography of MOCVD-grown InGaP, AlGaAs and AlGaAs/GaAs single quantum wells

Low-temperature photoluminescence topography has been used to monitor local variations in composition of In1-yGayP and AlxGa1-xAs layers which were grown lattice-matched on semi-insulating LEC GaAs substrates by MOCVD. Whereas the in content varies by less than +or-1% over the whole 2 inch water, the Al concentration changes by about +or-3%. In AlxGa1-xAs/GaAs quantum well (QW) structures the dependence of photoluminescence energy on QW thickness is used to estimate spatial variations in the thickness of the well material. The resulting thickness topogram is compared with numerical simulations of the gas flow profile in the MOCVD reactor.