Low Gas Flow Rates Research Articles

Imposed work of breathing during high-frequency oscillatory ventilation: a bench study

IntroductionThe ventilator and the endotracheal tube impose additional workload in mechanically ventilated patients breathing spontaneously. The total work of breathing (WOB) includes elastic and resistive work. In a bench test we assessed the imposed WOB using 3100 A/3100 B SensorMedics high-frequency oscillatory ventilators.MethodsA computer-controlled piston-driven test lung was used to simulate a spontaneously breathing patient. The test lung was connected to a high-frequency oscillatory ventilation (HFOV) ventilator by an endotracheal tube. The inspiratory and expiratory airway flows and pressures at various places were sampled. The spontaneous breath rate and volume, tube size and ventilator settings were simulated as representative of the newborn to adult range. The fresh gas flow rate was set at a low and a high level. The imposed WOB was calculated using the Campbell diagram.ResultsIn the simulations for newborns (assumed body weight 3.5 kg) and infants (assumed body weight 10 kg) the imposed WOB (mean ± standard deviation) was 0.22 ± 0.07 and 0.87 ± 0.25 J/l, respectively. Comparison of the imposed WOB in low and high fresh gas flow rate measurements yielded values of 1.63 ± 0.32 and 0.96 ± 0.24 J/l (P = 0.01) in small children (assumed body weight 25 kg), of 1.81 ± 0.30 and 1.10 ± 0.27 J/l (P < 0.001) in large children (assumed body weight 40 kg), and of 1.95 ± 0.31 and 1.12 ± 0.34 J/l (P < 0.01) in adults (assumed body weight 70 kg). High peak inspiratory flow and low fresh gas flow rate significantly increased the imposed WOB. Mean airway pressure in the breathing circuit decreased dramatically during spontaneous breathing, most markedly at the low fresh gas flow rate. This led to ventilator shut-off when the inspiratory flow exceeded the fresh gas flow.ConclusionSpontaneous breathing during HFOV resulted in considerable imposed WOB in pediatric and adult simulations, explaining the discomfort seen in those patients breathing spontaneously during HFOV. The level of imposed WOB was lower in the newborn and infant simulations, explaining why these patients tolerate spontaneous breathing during HFOV well. A high fresh gas flow rate reduced the imposed WOB. These findings suggest the need for a demand flow system based on patient need allowing spontaneous breathing during HFOV.

Ozone Transfer in a New Gas-Induced Contactor with Microbubbles

The process of ozonation is optimized when the gaseous ozone is efficiently dissolved into the liquid to be treated. The characteristics of microbubbles, such as high specific interfacial area, slow rising velocity and high inner pressure, suggest their desirability for the mass transfer of gas. In this study, a new microbubble generation system was developed and applied to an ozone contactor, and its characteristics were investigated in a semi-batch bubble column at different gas and water flow rates. The volumetric mass transfer coefficient increases with the increase of both induced-gas and water flow rates. High ozone transfer efficiency was obtained at low gas flow rates, which indicates that the present gas-induced ozone contactor using microbubbles is a desirable device for the dissolution of ozone at high concentrations.

Characteristics of Particulate Matter Collection in a Diesel Engine Using a Fluidized-bed DPF

A fluidized bed filter was applied to filtrate particulate matter (PM) in diesel engine exhaust gas. Experiment was carried out using a single-cylinder DI diesel engine to investigate the effects of gas velocity, bed particle diameter and bed height on PM and smoke filtration efficiencies and pressure drops. A theoretical work has been also conducted for further investigation of the effects of the mentioned parameters on PM filtration. The experimental results show that the fluidized-bed DPF provides high PM filtration efficiency and low-pressure drop especially under lower gas flow rates. Smaller diameter bed particles provide higher filtration efficiency under the low gas flow rate ; however, the filtration efficiency drastically decreases with the increase in gas flow rate due to excessive fluidization of the bed particles. The increase in bed height gives higher PM filtration efficiency while it causes an increase in pressure drop. The calculation results indicate that the diffusion collection is a dominant mechanism for PM filtration under the conditions of this study. The lower PM filtration ratios at high exhaust gas flow rates are caused by reduction of the filtration efficiency in diffusion collection.

Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Liquid water transport and removal from the gas diffusion layer (GDL) and gas channel of a polymer electrolyte fuel cell (PEFC) are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate (or stoichiometry) with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Mixing In A Gas/liquid FlowCountercurrent Bubble Column

This paper presents results of experimental and numerical investigations into the hydrodynamics of a bench scale bubble column reactor. Countercurrent bubble column reactors are the reactors most commonly used in water treatment for effecting the mass transfer of ozone to the aqueous phase. The experimental reactor is a glass cylinder with an internal diameter of 17 cm and height of 1.8 m. Gas is introduced at the bottom of the column via a 2.5 cm spherical diffuser and water is introduced to the top of the column through a manifold packed with glass spheres. Residence time distribution (RTD) studies were conducted for a range of gas flow rates chosen to span the dispersed flow bubble regime. A computational fluid dynamics computer code was used to model flow in the bubble column. Numerical studies were performed to refine and validate the CFD model and to gain insights into the fluid dynamics of countercurrent flow bubble columns. Investigations identified a gas flow rate within the ideal bubbly flow regime at which large-scale hydrodynamics and phase distribution were significantly different from those encountered at lower gas flow rates. These results will be used in subsequent studies in which ozone mass transfer and chemistry are included.

Characteristics of the plasma parameters in the fabrication of microcrystalline silicon thin films using 915 MHz ECR plasma

Plasma parameters in the fabrication of μc-Si thin films using 915 MHz ECR plasma with SiH 4/H 2 mixtures were investigated experimentally. Plasma parameters such as electron temperature and ion density were measured by a heated Langmuir probe. The deposition rate and the volume fraction of μc-Si were measured by spectrophotometer and Raman spectroscopy, respectively. As a result, it was observed that the characteristics of plasma parameters in SiH 4/H 2 plasma were different with those in H 2 plasma although the concentration of SiH 4 was only about 0.1. In addition, it was found that relatively high deposition rate can be realized even under the condition of both low gas pressure and low gas flow rate.

Heat input properties of hollow cathode arc as a welding heat source

In order to clarify whether a hollow cathode arc (HCA) can be used as a welding heat source in space, investigations into the fundamental characteristics of HCA were experimentally performed under low pressure conditions. The HCA method enables an arc discharge to ignite and maintain under low pressure conditions; in contrast, low pressure conditions make it extremely difficult for the conventional gas tungsten arc method to form an arc discharge. In an earlier paper, it was shown that the melting process by HCA is very sensitive to process parameters such as the gas flow rate and arc length, and a deep penetration forms when the arc length is long and the gas flow rate is low. In this paper, the distribution of the arc current on the anode surface and the plasma properties of the HCA under low pressure conditions have been made clear and the total heat energy to the anode has been discussed in order to understand the heat input properties of the HCA. The result shows that the HCA in the case of a low gas flow rate is a high and concentrated energy source, and the high energy input to the anode contributes to the deep penetration.

Cold Modelling of Slag Splashing in LD Furnace by Oxygen Lance with Twisted Nozzle Tip

The effects of a conventional lance tip with 4 normal nozzles and a lance tip with 4 twisted nozzles on slag splashing were investigated. A cold model for LD converters, Jiuquan Iron & Steel Co., was established and the amount of slag splashed onto the model walls was measured in the cold modelling experiments. The results from the experiments show that at low top gas flow rate the amount of slag splashing for the tip with twisted nozzles is less than that for the conventional tip, whereas at high top gas flow rate the amount of slag splashing for the nozzle twisted lance is greater than that for the conventional lance. For the nozzle twisted lance tip, slag splashing rates increase with increase in slag amount, lance height and top gas flow rate and the maximum amount of slag splashing for the tip with twisted nozzles can be obtained under the process parameters of 47.6 Nm 3 /h gas flow rate, 12% slag amount and 247 mm lance height. More slag droplets can be directed onto the cone with a nozzle twisted lance tip than with a normal lance tip at high top gas flow rate.

The influence of growth conditions on the microstructure and critical currents of TFA-MODYBa2Cu3O7 films

The influence of three processing parameters, temperature, gas flow rate and water pressure, on theYBa2Cu3O7 filmgrowth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal filmprocessing conditions to achieve high critical currents have been determined. We have found thatthe growth conditions maximizing the critical current density are those where the nucleation ofa-axis oriented grains is minimized, as determined byμ-Raman spectroscopy. Under these conditions the normal state resistivity is very near tothat of single crystals because a vanishingly small film porosity is achieved. Transmissionelectron microscopy analysis of films quenched from the growth temperature gives somehints for understanding the mechanism linking the film porosity with the concentration ofa-axis grains. A cross-linked influence of different processing parameters,such as temperature and water pressure, or water pressure and gasflow, has been demonstrated. The optimal growth temperatures are790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gasflow rate and water pressure. The optimal processing ranges are a compromise between twodifferent competing phenomena influencing the quality of the films: inhomogeneous filmformation due to HF gas stagnancy at small nominal growth rates (low gas flow rate orwater pressure) and perturbed crystallinity at high gas flow rates or water pressures.

Fractionation of Alfalfa into Leaves and Stems using a Three Pass Rotary Drum Dryer

Fractionation/separation of chopped alfalfa into leaf and stem components was investigated using a small industrial three-pass rotary drum dryer. Both drying and fractionation of chopped alfalfa were successfully achieved in a single operational process. Generally, fractionation is carried out only as a separate process and, therefore, more costly overall. The success of fractionation was determined by the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process (i.e. separation efficiency) and by the purity (leaf or stem proportion) of the component in a product stream collected at the cyclone or drum exit. A series of experiments under different operating conditions were performed to establish the optimum process conditions leading to maximum separation efficiency and leaf purity. The operating variables used during experimentation were the outer drum end gate opening area, material feed rate, gas flow rate and gas temperatures at the drum inlet. The experimental results indicated that leaf purity decreased and separation efficiency increased with a decrease in dryer drum end gate opening area. A decrease in separation efficiency was observed with an increase in material feed rate. On the other hand, leaf purity increased and separation efficiency decreased with a decrease in gas flow rate. The decrease in separation efficiency was most significant at low gas flow rates. Separation efficiency decreased with a decrease in gas inlet temperature. Stem moisture content was observed to be consistently higher than the leaves. Lowering stem moisture levels to an acceptable level of 8% moisture content results in over drying of the leaves, leading to a loss in product quality. It was determined that the relative difference in moisture levels of stems and leaves at the drum and cyclone exit, respectively, can be averted by re-circulating in the order of 20% of the wet material (stems) from the drum exit back through the dryer inlet.

Hydrodynamic Measurements of Up- and Down-Pumping Pitched-Blade Turbines in Gassed, Agitated Vessels, Using Positron Emission Particle Tracking

The liquid-phase mixing in gassed vessels agitated by up- and downward-pumping 30° and 45° pitched-blade turbines has been investigated using the radioactive tracer technique called Positron Emission Particle Tracking (PEPT). Only the flow patterns of the 45° down-pumping pitched-blade turbine were significantly affected by the injection of gas at specific flow rates of up to 1.5 vvm. A fluid agitation index was used to quantify the effectiveness of each impeller, based on the ratio of the average fluid velocity and the impeller tip velocity. At a given agitation speed, the 45° pitched-blade turbines were the most efficient at transferring momentum to the fluid, as indicated by higher circulation indexes. The efficiency of the 45° down-pumping pitched-blade turbine was significantly reduced upon aeration of the vessel, even at low specific gas flow rates of 0.5 vvm.

Reoxidation on the Surface of Molten Low‐Carbon Aluminum‐Killed Steel

The unwanted formation of oxide on the surface of molten steel is a process that plagues virtually all grades of steel and often results in unacceptable defect distributions in the finished product. The kinetic rates of the reaction remain largely unknown due to the difficulties in experimental measurements at high temperature involving reactive molten melts. This paper presents recent experimental work and analysis of oxide evolution on the surface of Al‐killed steel melts at oxygen partial pressures of Po2= 1~5×10−5atm, by using a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. The effects of gas flow rate (170~300 cm3/min) and temperature (1580~1630°C) were investigated. The oxide phase formed on the melt surface was in all cases Al2O3 but not the thermodynamically stable FeAl2O4. It was found, under the range of experimental conditions in this study, that the rate controlling mechanism for oxide nucleation and growth was gas phase mass transfer of oxygen to the melt surface. The morphology of the oxide changed gradually from distinctly dendritic at low gas flow rates to aggregates as the flow rate was increased.

Investigation of nitrogen flow variation into a radio frequency plasma cell on plasma properties and GaInNAs grown by molecular beam epitaxy

The radio frequency (rf) plasma and its operating conditions play a large, if not dominant role, in the differences of characteristics of molecular beam epitaxy-grown dilute-nitride samples between various research groups. Optimization of the rf plasma itself has become more important due to recent discoveries that ions cause significant damage to the optical quality of the material. An investigation on the effects of different nitrogen gas flows into the rf plasma cell was conducted. GaInNAs quantum wells were grown with different flow rates and their optical quality was examined using photoluminescence. A Langmuir probe was used to qualitatively analyze the ion flux originating from the plasma. We find that lower gas flow rates lead to a “sparse” operating condition and to higher ion counts which degrade the optical quality of the material. Higher flow rates are necessary to minimize ion counts and obtain higher quality materials.

Visualization and void fraction measurement of gas-liquid two-phase flow in a commercial plate heat exchanger by thermal neutron radiography

Adiabatic air-water two-phase flows in a commercial plate heat exchanger were visualized by thermal neutron radiography method as a nondestructive test in order to clarify liquid distributions. Liquid distributions in a single channel and a multichannel plate heat exchanger were investigated. From the results of a single channel, it was shown that gas phase tended to flow at the center of the net-like conduit and liquid fraction at the both side was higher. However, in the case of higher gas volumetric flux above 19 m/s, liquid distributions seemed to be homogenous. Measured average void fractions were lower than those by the homogenous model. On the other hand, from the results of a multichannel, it was shown that liquid distribution into the each channel strongly depended on inlet conditions. In the case of higher liquid flow rate and lower gas flow rate, liquid fraction became higher with the deeper channel due to larger liquid momentum. However, in the case of lower liquid flow rate and higher gas flow rate, the opposite tendency of liquid distribution was observed, that is to say, liquid fraction was lower with the deeper channel.

Characterization of a very low power argon CCP

The very low power Ar rf capacitively coupled plasma (CCP) (10.9 MHz, 5–75 W) at atmospheric pressure could be sustained at low gas flow-rates (0.1–2 l min−1) on a sharp conducting electrode. The Ar plasma is characterized from four points of view: stability, plasma parameters, influence of the plasma on the waveform of the rf oscillations and analytical capabilities. The stability diagram was obtained by observation of the behaviour of the plasma as a function of Ar flow-rate and plasma power. Six different regions were identified, the most important of them being the region defined as stable plasma. This state offers the optimum conditions for its characterisation and for its use as a spectral source. The characteristic temperatures (rotational, excitation and ionization) were determined using as thermometric species the OH radical, Ar, Pb and Fe. The work proposes a new method to calculate the electric parameters (resistance, capacitance and absorbed power) of CCP. The method is based on the measurement of the frequency of generated oscillations, the current flowing through the plasma and the voltage on the plasma-sustaining electrode and on the electric model of the plasma. The presence of the plasma inside the resonant circuit of the radiofrequency oscillator determines the change in rf oscillations waveform. By Fourier analysis of the oscillations waveform in the absence and in the presence of the plasma, the high order harmonics absorbed by the plasma were identified. The plasma was tested as a spectral source for the analysis of pneumatically nebulized liquid samples. The detection limits for 16 elements are presented.

Effect of Flow Orientation on Characteristics of Gas-Liquid Interfacial Waves in Narrow Rectangular Channel

To clarify the effect of flow orientation on the gas-liquid interfacial structure and frictional pressure gradient in a narrow rectangular channel, simultaneous measurements of pressure gradients and spatio-temporal characteristics of gas-liquid interface are conducted for three different flow orientations ; vertical upward flow (V flow), horizontal flow between horizontal plates (H-H flow) and horizontal flow between vertical plates (H-V flow). The velocity field in the liquid flow with interfacial waves is also measured by the PIV and LIF methods. The interfacial structure at low gas and low liquid flow rates is greatly affected by the flow orientation, and the complicated interfacial structure is observed in V flow. To predict the frictional pressure gradient, the wave model which takes account of the wall shear stress for the wave propagation and base film flow is presented. From the comparison of experimental results with predictions by the present model, it is found that the difference in the frictional pressure gradient between the three flow orientations is closely related to the characteristics of interfacial waves as well as the average liquid velocity.

Wide-Range Two-Dimensional Imaging of NO Density Profiles by LIF Technique in a Corona Radical Shower Reactor

Planar laser-induced fluorescence was employed during the process of NO/sub x/ removal in a corona radical shower system. Using the wide-range imaging (image size: 240 mm in width and 160 mm in height), two-dimensional distributions of ground-state NO could be observed not only in the discharge zone but also both in the downstream and the upstream regions of the reactor. The obtained results showed that the density of NO molecules decreased not only in the plasma region formed by the corona streamers and the downstream region of the reactor but also in the upstream region of the reactor. The effect of the gas injection through the nozzles electrode on the NO profile in the reactor was negligible. The NO removal rate was almost the same for both cases with and without the injection gas once the streamer discharge was produced. In the present reactor at low main gas flow rate, it was considered that electrohydrodynamic flow became to be dominant, and the flow toward the upstream affected the decrease of NO in the upstream region. This fact is important for optimizing the performance of the nonthermal plasma reactor.

Modified Nukiyama–Tanasawa and Rizk–Lefebvre models to predict droplet size for microconcentric nebulizers with aqueous and organic solvents

Aerosols characteristic of several organic solvents (hexane, acetone, xylene, toluene, methanol, and ethanol) produced by a direct injection high efficiency nebulizer (DIHEN) are measured using a phase Doppler particle analyzer (PDPA) over a wide range of operating conditions (nebulizer gas = 0.2–1.0 L min−1, solvent flow rate = 10–500 μL min−1). The Sauter mean diameter, defined as the volume-to-surface area ratio (D3,2), and cumulative count percent of organic aerosol are measured and compared with those of aqueous droplets. These parameters are chosen because the performance of inductively coupled plasma (ICP) as an excitation and ionization source is affected by the size of the introduced droplets, particularly in the case of direct injection nebulizers where the primary aerosol is directly introduced to the plasma without being filtered by the spray chamber. The size distribution of the droplets demonstrates a notable shift toward smaller droplets, and is generally narrower when organic solvents are used instead of water. This effect is more pronounced for hexane and acetone, having a considerably lower surface tension and viscosity, respectively, compared with water. A D3,2 of 4.6 μm is obtained for hexane, compared with 7.2 μm for aqueous solutions at a nebulizer gas flow rate of 0.2 L min−1 and a solution uptake rate of 50 μL min−1. This decrease in droplet size is less significant for ethanol, methanol, toluene and xylene. Experimental results are also compared to D3,2 values calculated by the Nukiyama–Tanasawa (N–T) equation and Rizk–Lefebvre (R–L) model. While the cited models correctly predict the trend in size variation as a function of nebulizer gas flow rate, and to some extent solvent characteristics, an overestimation and an underestimation of D3,2 is observed for all tested solvents at low nebulizer gas flow rates for the N–T model and the R–L model, respectively. Modified equations are proposed which are capable of predicting D3,2 values for several solvents with greater accuracy.

Species specific isotope dilution with on line derivatisation for determination of gaseous mercury species

A species specific isotope dilution (SSID) method for determination of the volatile mercury (Hg) species Hg0, (CH3)2Hg, CH3HgX and HgX2 (where X is a counter ion) in gaseous samples was developed. The procedure involved on line derivatisation of ionic Hg species with sodium tetraethylborate during sampling, before collection of species on a Tenax TA tube in series with an Au–Pt tube. Collected species were thermally desorbed and determined by GC-ICP-MS. The procedure provided absolute detection limits of 0.5 pg for CH3HgX and (CH3)2Hg, 5 pg for HgX2 and 25 pg for Hg0. Recoveries and transformations of the investigated species were studied for the different procedural steps and the method was tested for microcosm experiments on brackish water sediment samples, as well as for laboratory air samples. As a result of methylation and reduction processes within the sediment, gaseous emission rates could be determined for all investigated species after addition of an isotope enriched aqueous inorganic 199Hg2+ standard to the sediment. Hg0 was the only specie detected in laboratory air samples (4.5 ng m−3). The procedure for derivatisation of ionic Hg species prior to collection on Tenax TA reduces memory effects and transformations of CH3HgX and HgX2, which is especially advantageous at low sample gas flow rates and for complex gaseous matrices, such as emissions from sediment samples. SSID provides simplified methodological development, as well as efficient correction for matrix induced losses and transformations of Hg species. The method could be useful for various biogeochemical studies of Hg, as well as for reliable risk assessments during, for example, waste management and soil remediation.

Ｈｏｌｌｏｗ Ｃａｔｈｏｄｅ Ａｒｃにおけるアーク柱の特性

Hollow Cathode Arc (HCA) was developed as a plasma source under the condition of low pressure in the 1960s, and various researches were performed to make clear the mechanism of the HCA. Since the HCA is a plasma source under low pressure environment, it attracts attention as a welding heat source in space. Moreover, the HCA is expected to be useful for the industrial application on the earth, because the melting process by HCA is very active and the penetration is huge in comparison with that by the conventional GTA.In the previous paper, the discharge and melting process by the HCA has been discussed, and it has been shown that a concentrated columnar arc and a huge penetration are obtained in the case of low gas flowrate.In the present paper, the current distribution on the anode surface and the floating potential of the HCA column have been made clear and the plasma structure of the HCA has been discussed to understand the heat input to the anode by the HCA. It is concluded that the huge penetration by the HCA in the case of low gas flowrate is closely related to concentrated current distribution on the anode surface and the strong flow of electrons in the HCA.