Technical note A simple shock tunnel driver gas detector

A device has been developed to detect the arrival of the driver gas in a shock tunnel. The detector is small enough to be used in conjunction with other experiments. It works by choking a duct when the specific heat ratio is increased past a critical value. Times are given for the onset of a 7.5% contamination level in flows with freestream enthalpies ranging from 3–9 MJ/kg. These results are compared with and are shown to be in agreement with measurements made with a mass spectrometer. Results displaying the rate at which the test gas is contaminated are also given.     

Mass spectrometric measurements of driver gas arrival in the T4 free-piston shock-tunnel

Available test time is an important issue for ground-based flow research, particularly for impulse facilities such as shock tunnels, where test times of the order of several ms are typical. The early contamination of the test flow by the driver gas in such tunnels restricts the test time. This paper reports measurements of the driver gas arrival time in the test section of the T4 free-piston shock-tunnel over the total enthalpy range 3–17 MJ/kg, using a time-of-flight mass spectrometer. The results confirm measurements made by previous investigators using a choked duct driver gas detector at these conditions, and extend the range of previous mass spectrometer measurements to that of 3–20 MJ/kg. Comparisons of the contamination behaviour of various piston-driven reflected shock tunnels are also made. PACS 07.75.th; 47.40.-x     

Experimental investigations of an air curtain device subjected to external perturbations

Although plane air jets are often used as dynamic barriers to separate two environments, only a few works have explored their sensitivity to perturbations. We investigated the influence of sharp changes of pressure on the flow field of a device designed to avoid air-borne contamination. Laser tomography and tracer gas experiments clearly indicate that the air curtain is strongly sensitive to perturbations such as draughts. The results highlight that the control of air curtains used in open protection devices should be further investigated.     

Analysis and application of solid-gas flow inside a venturi with particle interaction

A two-phase one-dimensional solid—gas flow model which describes the flow inside a variable area duct has been developed. The model includes multiparticle equations and considers particle—particle interaction. Predictions have been compared with experimental data for the pressure drop and pressure recovery through two venturis at different solid to gas loading ratios. Accurate knowledge of the particle-size distribution is extremely important for good comparison. No meaningful single particle-size diameter is found that yields predictions to agree with the measurements. The venturi may be used as a measuring device for solid—gas flow rates for systems if the particle-size distribution is accurately known. However, the venturi-diffuser section loses its effectiveness in recovering the pressure as the solid loading increases.     

Microbubble formations in MEMS-fabricated rectangular channels: A high-speed observation

A microfluidic device that consists of MEMS-fabricated rectangular channels is developed for stable and sequential production of monodispersed microbubbles. The central inlet for gas phase is located between two inlets for liquid phases, where the device works as a two-fluid atomizer. The interfacial flow mechanism of microbubble formation at the junction of the inlets in the device is investigated using a high-speed visualization technique and digital image processing. The periodic formation process is successfully realized by the consideration of the wettability between the microbubble and the channel wall. The produced microbubbles are relatively uniform in size, and the size is controlled from 113 to 153 μm by changing the flow rates of the liquid and gas phases. Furthermore, a simple theoretical model to predict the equivalent diameter of microbubbles is developed by considering the mass balance of the gas phase in the formation process, where the experimental and theoretical results are in reasonable agreement.     

Behavior of the unsteady jet of a mixture of a pressurized gas and dispersed particles discharged from a circular duct into the atmosphere

The unsteady axisymmetric jet produced by discharge of a mixture of a pressurized gas and dispersed particles from a circular duct into the atmosphere is studied within the framework of two-velocity, two-temperature gas dynamics. An attempt is undertaken to allow for the effective pressure due to random particle motion. The collision mechanism is found to be essential to radial expansion of the flow. Experimental data that support the results obtained are reported. Mozhaiskii Military Spacecraft Engineering Academy, St. Petersburg 197082. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 151–157, January–February, 1999.     

双凹摩擦摆隔震烟风道结构地震响应

传统烟风道板式滑动支座可减少道体热胀冷缩时的摩擦阻力,但抗震耗能能力不足,缺少变形后的复位能力,且会约束道体的转动而可能导致结构破坏.将摩擦摆隔震支座用于烟风道,可同时具有热滑移、隔震功能,允许道体在温度作用下自由转动.本文对烟风道采用双凹摩擦摆中间隔震的结构体系地震响应进行了研究,建立了横向地震作用下简化的三自由度地震响应分析模型,其中双凹摩擦摆采用三线性滞回模型,推导了一阶状态空间微分运动方程.该模型的分析结果与有限元实体模型分析结果非常接近.利用简化模型研究了不同场地类别、不同强度地震激励作用下双凹摩擦摆的恢复力特点及隔震效果,结果表明:与非隔震结构相比,双凹摩擦摆隔震的烟风道的道体反力、支架剪力均得到了控制.     

NUMERICAL STUDIES ON THE MIXING OF CH_4 AND KEROSENE INJECTED INTO A SUPERSONIC FLOW WITH H_2 PILOT INJECTION

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Driver gas contaminationin a detonation-driven reflected-shock tunnel

A computational study has been carried out to examine the effects of driver gas contamination in the NASA HYPULSE facility at GASL when operating with a detonation driver in reflected-shock tunnel mode. Unlike high-enthalpy shock tunnels which use helium as a driver gas, the driver gas in a detonation driver consists of a mixture of water vapour and argon, which has very different chemical and thermodynamic properties than those of helium. The purpose of the present work is to quantitatively evaluate the effects of driver gas contamination on the flow properties in the test section. Two computational analyses have been performed. The first analysis examined the nozzle flow under the influence of a prescribed level of driver gas contamination. In the second analysis, the transient development of the driver gas leakage in the reflected-shock region in the shock tube is studied. The unique flow features brought about by the detonation-driver gas and the method for detecting the contamination are discussed.Received: 6 August 2002, Revised: 22 September 2003, Published online: 10 February 2004Correspondence to: R.S.M. ChueAn abridged version of this paper was presented at the 23rd International Symposium on Shock Waves at Fort Worth, Texas, July 23 to 27, 2001     

Measurement of dynamic gas disengagement profile by using an analog output level gauge

 The dynamic gas disengagement profile was measured in a 0.14 m diameter and 3.66 m high plexiglas column by using an analog output gauge, which was connected to a data acquisition system. This analog output gauge is a high accuracy continuous measurement level gauge. It is made up of a wave guide, a float, a motion or stress sensing device and a probe housing. The fluid level at any gas velocity is obtained by using the data acquisition system. The dynamic gas disengagement profile produced one slope in the bubble flow and two slopes in the churn turbulent flow representing unimodal and bimodal distributions of bubbles. Received: 13 September 1995/Accepted: 26 July 1996     

Motion of a gas inclusion in a capillary under the action of vibration

The motion of gas inclusions in a liquid-filled duct under the action of vibration for comparable cross-sectional dimensions of the inclusion and the duct is studied. Two limiting cases of inclusion motion differing with respect to the drag mechanism are considered. For low velocities, it is assumed that the drag is mainly determined by the capillary forces and the friction in the liquid film separating the gas inclusion from the duct wall. As the inclusion velocity increases, the main contribution to the drag is made by such mechanisms as flow separation, the formation of a low-pressure region in the wake, etc. It is demonstrated that due to the vibration a gas inclusion traveling in a capillary under the action of steady forces is halted at certain points of the capillary. The capillary behaves like a filter, impermeable for inclusions smaller than a certain threshold size. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 85–92, May–June, 1998. The work received financial support from the Russian Foundation for Basic Research (project No.96-01-01813).     

不同参数下环形通道内湍流旋流流动的模拟

应用一种合理考虑湍流一旋流相互作用及湍流脉动各向异性的新的代数ＲｅｙｎｏｌｄＳ应力模型,对环形通道内的湍流旋流流动进行了数值模拟．研究了旋流数、进口轴向速度和内外半径比等参数对环形通道内湍流旋流流动的影响,以及由此产生的流场变化对强化环形通道内传热的作用．     

Shock waves in dense granular flows down a chute

Large scale, three dimensional computer simulations of monosized, viscoelastic, spherical glass particles flowing in an inclined duct were performed using a phenomenological model based on the modified Kelvin–Maxwell model. The particle flow rate in the model duct was regulated using a stationary wedge located in the middle of the duct. At low flow rates of glass particles, a continuous flow was observed similar to that excited by steadily and rapidly adding glass particles to the top of a heap. However, at high flow rates, a totally different situation arises where a flow with a different nature was established in the duct. The situation was found to be analogous to the case of a supersonic gas flow in a duct, where a curved-bow shock was observed to have formed on the upper edges of the duct adjacent to the wedge. In addition, in supersonic granular flows the viscous and conductance effects spread the shock changes over a finite shock layer.     

Prediction of turbulent gas–solids flow in curved ducts using the Eulerian–Lagrangian method

The flow of particulate two‐phase flow mixtures occur in several components of solid fuel combustion systems, such as the pressurised fluidised bed combustors (PFBC) and suspension‐fired coal boilers. A detailed understanding of the mixture characteristics in the conveying component can aid in refining and optimising its design. In this study, the flow of an isothermal, dilute two‐phase particulate mixture has been examined in a high curvature duct, which can be representative of that transporting the gas–solid mixture from the hot clean‐up section to the gas turbine combustor in a PFBC plant. The numerical study has been approached by utilising the Eulerian–Lagrangian methodology for describing the characteristics of the fluid and particulate phases. By assuming that the mixture is dilute and the particles are spherical, the governing particle momentum equations have been solved with appropriately prescribed boundary conditions. Turbulence effects on the particle dispersion were represented by a statistical model that accounts for both the turbulent eddy lifetime and the particle transit time scales. For the turbulent flow condition examined it was observed that mixtures with small particle diameters had low interphase slip velocities and low impaction probability with the pipe walls. Increasing the particle diameters (>50 μm) resulted in higher interphase slip velocities and, as expected, their impaction probability with the pipe walls was significantly increased. The particle dispersion is significant for the smaller sizes, whereas the larger particles are relatively insensitive to the gas turbulence. The main particle impaction region, and locations most prone to erosion damage, is estimated to be within an outer duct length of two to six times the duct diameter, when the duct radius of curvature to the duct diameter ratio is equal to unity. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical simulation of effects of flow maldistribution on heat and mass transfer in a PEM fuel cell stack

A 3D numerical study was carried out to analyze flow, heat and mass transfer first in a single half-cell cathode channel of proton exchange membrane (PEM) fuel cell. From practical point of view, it is necessary to put the appropriate number of cells in a stack. Hence, the above study on a single half-cell is extended to a stack of channels. Due to stacking, the assumption of uniform flow distribution would no longer hold true. Therefore, the channel flow-maldistribution is considered. The water formed at the active surface due to the electrochemical reaction diffuses through the porous layer and eventually enters the gas flow duct. The higher gas velocities in the duct result in faster water vapour removal which leads to a lower value of water vapour into the duct and hence a lower Nusselt number.     

Identification of the Impact Location in a Gas Duct System Based on Acoustic Wave Theory and the Time Frequency

This paper presents a new method for detecting the impact location in a buried gas duct. Gas leakage is often caused by the mechanical impact forces of construction equipment. In order to prevent gas leakage due to an impact force, it is necessary to detect the impact location at an early stage. For the detection of the impact location in a pipeline system, the correlation method has been used as the conventional method. For the application of the correlation method, the diameter of a duct should be small so that the acoustic wave inside the duct can propagate with non-dispersive characteristics, in the form of, for example, a plane wave. However, when the diameter of the duct is large, the acoustic waves inside the duct propagate with dispersive characteristics owing to the reflection of the acoustic wave off of the wall of the duct. This dispersive characteristic is related to the acoustic modes inside a duct. Therefore, the correlation method does not work correctly for the detection of the impact location. This paper proposes new methods of accurately measuring the arrival time delay between two sensors attached to duct line system. This method is based on the time-frequency analyses of the short time Fourier transform (STFT) and continuous wavelet transform (CWT). These methods can discriminate direct waves (non-dispersive waves) and reflective waves (dispersive waves) from the measured wave signals through the time-frequency analysis. The direct wave or the reflective wave is used to estimate the arrival time delay. This delay is used for the identification of the impact location. This systematic method can predict the impact location due to the impact forces of construction equipment with more accuracy than the correlation method.     

高速液体受限射流扩展形态研究

采用一种火药燃烧驱动液体喷射的新装置及其测试系统，研究受限空间中高速惰性液体射流的扩展结构。观察了环境反压、液体粘性、喷嘴结构等参量对射流扩展形态的影响，分析了射流雾化机理。研究结果对改进燃烧室设计及控制燃烧稳定性有一定的指导意义。     

Investigation of Submerged Jets with an Extended Initial Laminar Region

The method of producing laminar submerged jets using a device, whose length is comparable with the jet diameter, is described. A submerged air jet, 0.12 m in diameter, produced by means of this technique is experimentally investigated in the Reynolds number range from 2000 to 13 000. Hot-wire anemometer measurements of the flow parameters and laser visualization of the flow are performed. It is shown that the device developed makes it possible to produce submerged jets with the laminar regions as long as 5.5 jet diameters. The initial regions of such jets can be used to study the development of disturbances in submerged jets, as well as used in medicine and engineering in organizing various gasdynamic curtains which produce zones with given properties with respect to purity and composition inside another gas media.     

Particle image velocimetry investigation of a finite amplitude pressure wave

Particle image velocimetry is used to study the motion of gas within a duct subject to the passage of a finite amplitude pressure wave. The wave is representative of the pressure waves found in the exhaust systems of internal combustion engines. Gas particles are accelerated from stationary to 150 m/s and then back to stationary in 8 ms. It is demonstrated that gas particles at the head of the wave travel at the same velocity across the duct cross section at a given point in time. Towards the tail of the wave viscous effects are plainly evident causing the flow profile to tend towards parabolic. However, the instantaneous mean particle velocity across the section is shown to match well with the velocity calculated from a corresponding measured pressure history using 1D gas dynamic theory. The measured pressure history at a point in the duct was acquired using a high speed pressure transducer of the type typically used for engine research in intake and exhaust systems. It is demonstrated that these are unable to follow the rapid changes in pressure accurately and that they are prone to resonate under certain circumstances.     

Effects of cross ribs on heat/mass transfer in a two-pass rotating duct

Two-pass internal cooling passage with rib turbulators has been investigated for convective heat/mass transfer under rotating conditions. The flow and heat transfer characteristics in the cooling passage are very complicated so that it is required the detail analysis to design more efficient gas turbine blades. A naphthalene sublimation technique is employed to determine detailed local heat transfer coefficients using the heat and mass transfer analogy. The local heat/mass transfer and flow pattern in the cooling passage are changed significantly according to rib configurations, duct turning geometries and duct rotation speeds. Four different rib configurations are investigated to obtain the combined effects of the angled rib, duct turning and rotation. The results show that the duct rotation generates the heat/mass transfer discrepancy between the leading and trailing walls due to the secondary flows induced by the Coriolis force. The angled ribs generate a single rotating secondary flow with the cross-rib arrangement and the duct turning makes a strong Dean-type vortex. These vortices affect significantly the heat/mass transfer on the duct wall. The overall heat transfer pattern on the leading and trailing surfaces for the first and second passes are dependent on the duct rotation, but the local heat transfer trend is affected mainly by the rib arrangements. In addition, the present study observes the rotating effect in the two-pass smooth duct to obtain the baseline data in comparison with the ribbed duct for various rib arrangements.