Pressure-driven diffusive gas flows in micro-channels: from the Knudsen to the continuum regimes

Despite the enormous scientific and technological importance of micro-channel gas flows, the understanding of these flows, by classical fluid mechanics, remains incomplete including the prediction of flow rates. In this paper, we revisit the problem of micro-channel compressible gas flows and show that the axial diffusion of mass engendered by the density (pressure) gradient becomes increasingly significant with increased Knudsen number compared to the pressure driven convection. The present theoretical treatment is based on a recently proposed modification (Durst et al. in Proceeding of the international symposium on turbulence, heat and mass transfer, Dubrovnik, 3–18 September, pp 25–29, 2006) of the Navier–Stokes equations that include the diffusion of mass caused by the density and temperature gradients. The theoretical predictions using the modified Navier–Stokes equations are found to be in good agreement with the available experimental data spanning the continuum, transition and free-molecular (Knudsen) flow regimes, without invoking the concept of Maxwellian wall-slip boundary condition. The simple theory also results in excellent agreement with the results of linearized Boltzmann equations and Direct Simulation Monte Carlo (DSMC) method. Finally, the theory explains the Knudsen minimum and suggests the design of future micro-channel flow experiments and their employment to complete the present days understanding of micro-channel flows.     

A study of flows in tangentially crossing micro-channels

In the present study, flow fields in the three-dimensional, tangentially crossing micro-channels were studied. The effect of the relevant geometrical parameters such as the aspect ratio, contact surface area, surface to volume factor, flow rate and cross angle on the flow turning was reported. When the geometries and the flow conditions of the two crossing channels were the same, the fraction of turning flow was found to be dependent on the aspect ratio of the channel as reported previously in the literature. However, if the configuration and flow conditions of the two channels were different, the results need to be clarified. A parameter of non-dimensionalized surface to volume ratio was devised to characterize the flow turning. And the parameter was tested against its validity using numerical simulation and the available experimental data. The experiments on the crossing angle were conducted to show that larger angle in general yielded higher turning flow ratio. The results are expected to be useful in the passive control of flow in micro-fluidic devices among others.     

Electronic properties and gas adsorption behaviour of pristine,silicon-, and boron-doped (8, 0) single-walled carbon nanotube: A first principles study

Carbon nanotubes (CNTs) have received enormous attention due to their fascinating properties to be used in various applications including electronics, sensing, energy storage and conversion. The first principles calculations within density functional theory (DFT) have been carried out in order to investigate the structural, electronic and optical properties of un-doped and doped CNT nanostructures. O₂, CO₂, and CH₃OH have been chosen as gas molecules to study the adsorption properties based on zigzag (8,0) SWCNTs. The results demonstrate that the adsorption of O₂, CO_2, and CH₃OH gas molecules on pristine, Si-doped and B-doped SWCNTs are either physisorption or chemisorption. Moreover, the electronic properties indicating SWCNT shows significant improvement toward gas adsorption which provides the impact of selecting the best gas sensor materials towards detecting gas molecule. Therefore, these pristine, Si-, and B-doped SWCNTs can be considered to be very good potential candidates for sensing application.     

Effects of trapped air bubbles on frequency responses of the piezo-driven inkjet printheads and visualization of the bubbles using synchrotron X-ray

The reliability of drop formation is one of the key factors for the successful commercialization of inkjet printing applications. However, when the air bubble is entrapped from the nozzle exit, it leads to stop jetting of the droplets immediately. It has been known that the trapped air bubbles inside the chamber prevent a printhead from stable jetting. In this study the synchrotron X-ray has been used to visualize the air bubbles in the flow field inside of the printhead undergoing the standard jetting with the firing frequency of 1–20 kHz. An air pocket of bubble was formed repeatedly and reproduced well through the tested printheads at a certain jetting condition. To see the effects of the bubbles on the dynamic characteristics of the piezoelectric printheads, the piezoelectric velocity on the top of the pressure chamber was measured with a laser vibrometer. When the bubble was trapped at the nozzle exit orifice, the piezo velocity signals showed significantly different frequency peaks appearing in the spectrum and the high frequency components were identified with the frequency response measurements.     

Leakage monitoring research and design for natural gas pipelines based on dynamic pressure waves

Many types of gases, such as natural gas, hydrogen, and so on, are transported via pipelines using a chemical process, though leakages in these pipelines create waste and pose hazards and risks to industries, the environment and people. To monitor gas pipelines, a new leak detection and location method based on the amplitude attenuation model of dynamic pressure waves was designed and researched by experiments, compared with traditional method based on the propagation velocity and time differences as determined by the waveforms of the upstream and downstream signals. Both methods are achieved based on the propagation law of the dynamic pressure waves in the fluid flow. First, the fundamentals of the newly proposed method are clarified by considering the influence of gas flow on the waves. The experiments are then conducted in gas pipelines with 42 mm internal diameters. Finally, the results of the experiments are discussed and analyzed. The results indicate that all leakages can be detected by both methods but that the largest location error of the traditional method is −0.780%, whereas the largest location errors with respect to the new method are 0.054% with the experimental attenuation coefficients and 2.055% with the theoretical attenuation coefficients. It is further determined that the influence of the gas flow effects cannot be ignored by either method. Accordingly, the conclusions drawn suggest that the proposed methods can be applied to monitor gas pipelines.     

Minimizing fuel cost in gas transmission networks by dynamic programming and adaptive discretization

In this paper, the problem of computing optimal transportation plans for natural gas by means of compressor stations in pipeline networks is addressed. The non-linear (non-convex) mathematical model considers two types of continuous decision variables: mass flow rate along each arc, and gas pressure level at each node. The problem arises due to the presence of costs incurred when running compressors in order to keep the gas flowing through the system. Hence, the assignment of optimal values to flow and pressure variables such that the total fuel cost is minimized turns out to be essential to the gas industry. The first contribution from the paper is a solution method based on dynamic programming applied to a discretized version of the problem. By utilizing the concept of a tree decomposition, our approach can handle transmission networks of arbitrary structure, which makes it distinguished from previously suggested methods. The second contribution is a discretization scheme that keeps the computational effort low, even in instances where the running time is sensitive to the size of the mesh. Several computational experiments demonstrate that our methods are superior to a commercially available local optimizer.     

Static and dynamic minimum energy broadcast problem in wireless ad-hoc networks: A PSO-based approach and analysis

In this paper, we address the minimum energy broadcast (MEB) problem in wireless ad-hoc networks (WANETs). The researches in WANETs have attracted significant attentions, and one of the most critical issues in WSNs is minimization of energy consumption. In WANETs the packets have to be transported from a given source node to all other nodes in the network, and the objective of the MEB problem is to minimize the total transmission power consumption. A hybrid algorithm based on particle swarm optimization (PSO) and local search is presented to solve the MEB problem. A power degree encoding is proposed to reflect the extent of transmission power level and is used to define the particle position in PSO. We also analyze a well-known local search mechanism, r-shrink, and propose an improved version, the intensified r-shrink. In order to solve the dynamic MEB problem with node removal/insertion, this paper provides an effective simple heuristic, Conditional Incremental Power (CIP), to reconstruct the broadcast network efficiently. The promising results indicate the potential of the proposed methods for practical use.     

Static and dynamic preprocessing methods in neural networks

Preprocessing is recognized as an important tool in modeling, particularly when the data or underlying physical process involves complex nonlinear dynamical interactions. This paper will give a review of preprocessing methods used in linear and nonlinear models. The problem of static preprocessing will be considered first, where no dependence on time between the input vectors is assumed. Then, dynamic preprocessing methods which involve the modification of time-dependent input values before they are used in the linear or nonlinear models will be considered. Furthermore, the problem of an insufficient number of input vectors is considered. It is shown that one way in which this problem can be overcome is by expanding the weight vector in terms of the available input vectors. Finally, a new problem which involves both cases of: (1) transformation of input vectors; and (2) insufficient number of input vectors is considered. It is shown how a combination of the techniques used to solve the individual problems can be combined to solve this composite problem. Some open issues in this type of preprocessing methods are discussed.     

安全栅和避雷器在煤气柜中的应用

煤气柜及煤气输配系统是一个复杂又容易受到外界因素影响的系统,是作为工业与民用储备气体燃料的重要设备。本文叙述了煤气柜的安全防护措施,重点介绍了安全栅和避雷器的作用以及它们在煤气柜仪表系统中的应用。     

气体涡轮流量计安装使用

气体涡轮流量计是用于管道气体计量的精密仪表,若安装使用不当,不但影响流量计的计量精度,甚至会对流量计造成人为损坏。本文着重介绍气体涡轮流量计正确的安装使用方法。     

Experimental and numerical modeling of the gas atomization nozzle for gas flow behavior

Gas atomization is a widely used process for manufacturing of fine metal- and alloy-powder. To ensure a stable process with high yields of metal powder, the negative pressure at the melt delivery tube tip base and no flow separation conditions are necessary for a good atomization process. An important feature of these jets is that flow separation may occur over the outer surface of the liquid delivery tube for some conditions. Flow separation cause solidification and accumulation of metal, leading to a shape alteration of the liquid delivery tube in gas atomization process. Using computational fluid dynamics (CFD) software, a parametric study was conducted to determine the effects of atomizing gas pressure on the melt delivery tube tip base pressure and flow separation. Atomization gas pressures of 1.0, 1.3, 1.7, 2.2, and 2.7 MPa were used in the CFD model to initialize the pressure in gas inlet. CFD simulations were performed and the modeling results were compared with experimental data. These results showed that the CFD modeling can be used for the estimation of the melt tip base pressure of the nozzle. It is found that the flow separation formation is strongly dependent on the atomizing gas pressure.     

超声波流量计在煤气计量中的应用

流量测量在工业测量中是一个重要的参数。它不仅作为过程控制仪表，更重要的是它作为计量仪表，为企业的成本核算和市场运营提供可靠的数据。本文主要介绍了煤气流量测量中仪表的选型，并对超声波流量计的原理、特点及常见故障作了重点阐述。     

A study of heat distribution and dissipation in a micromachined chemoresistive gas sensor

A micromachined chemoresistive gas sensor was studied from the point of view of heat distribution and thermal dissipation: this innovative device for environmental pollutant gas monitoring, is based on a sensitive film of semiconductor metal oxides, kept in temperature by a platinum resistor. In order to avoid electrical interactions between the film heater and the contacts for the film reading, the heater is driven by a square wave, and the film is read when no voltage is provided. Since the working temperature of the film is extremely important for its operation, it is crucial to maintain the temperature fluctuations within few degrees; to this end, in this work we study the heat distribution and dissipation of such a device, aiming to set a proper heating frequency, which will assure a right stability of the working temperature.     

Rarefaction and compressibility effects on steady and transient gas flows in microchannels

The main theoretical and experimental results from the literature about steady pressure-driven gas microflows are summarized. Among the different gas flow regimes in microchannels, the slip flow regime is the most frequently encountered. For this reason, the slip flow regime is particularly detailed and the question of appropriate choice of boundary conditions is discussed. It is shown that using second-order boundary conditions allows us to extend the applicability of the slip flow regime to higher Knudsen numbers that are usually relevant to the transition regime.The review of pulsed flows is also presented, as this kind of flow is frequently encountered in micropumps. The influence of slip on the frequency behavior (pressure gain and phase) of microchannels is illustrated. When subjected to sinusoidal pressure fluctuations, microdiffusers reveal a diode effect which depends on the frequency. This diode effect may be reversed when the depth is shrunk from a few hundred to a few m.Thermally driven flows in microchannels are also described. They are particularly interesting for vacuum generation using microsystems without moving parts.     

用静压控制法实现小颗粒固体粉尘的料位检测控制

分析了SZ-25型散装饥的料位计容易被损坏和检测失准的原因．引出采用自行设计的静压式料位检测装置，并对该装置进行设计的理论依据分析．叙述该装置的工作原理，运行参数设置方法及其各部分设备的必要性．并对改造前后进行对比，最后对该装置的使用推广范围进行简述．     

Comparative gas sensing in copper porphyrin and copper phthalocyanine spin-coating films

Spin-coating films of tetra-(tert-butyl)-5,10,15,20-tetraazaporphyrin copper (CuTAP(t-Bu)₄) and tetra-(tert-butyl) phthalocyanine copper (CuPc(t-Bu)₄) was obtained and characterized by absorption spectra and atomic force microscopy. The gas sensing behaviors of the films were investigated with respect to NH₃ and NO₂ at room temperature. In addition, the interactions between complexes and gas analytes were also discussed. It was found that the devices exhibited better response and slow response-recovery times with increase of interactions between the complexes and analytes related with conjugated ability of complexes. This suggests that the response may imparted by use of strong interactions between the complexes and the analyte of interest at the expense of reversibility.     

Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.     

一体化阿牛巴流量计在煤气流量测量中的应用

主要介绍了一种新颖的在线安装式一体化阿牛巴流量计,论述了该流量计的工作原理、基本结构和性能特点,并结合实际使用情况,指出了其在杂质多、水分大的煤气流量测量中使用的效果。     

Dynamic modeling and linear model predictive control of gas pipeline networks

A linear model predictive control (LMPC) strategy is developed for large-scale gas pipeline networks. A nonlinear dynamic model of a representative pipeline is derived from mass balances and the Virial equation of state. Because the full-order model is ill-conditioned, reduced-order models are constructed using time-scale decomposition arguments. The first reduced-order model is used to represent the plant in closed-loop simulations. The dimension of this model is reduced further to obtain the linear model used for LMPC design. The LMPC controller is formulated to regulate certain pipeline pressures by manipulating production setpoints of cryogenic air separation plants. Both input and output variables are subject to operational constraints. Three methods for handling output constraint infeasibilities are investigated.     

Static and dynamic characteristics of the trunk and history of low back pain

A pilot research was undertaken to study the relationship between static and dynamic characteristics of the trunk and the existence of low back pain. Isoinertial dynamic tests were performed using an ISOSTATION B200 dynamometer on 33 non LBP workers and 11 LBP workers. Ranges of motion and isometric strengths were not systematically different between LBP and non LBP, while these differences were significant for the average absolute velocities during dynamics tests against 25 or 50% of the isometric torques. For the mean quadratic velocity during tests against 50% of the isometric torques, a value of 75 degrees per second can be proposed as a threshold for discriminating between LBP and non LBP. The sensitivity and specificity of this test increased to 92 and 88% respectively when the past LBP history was also taken into account. This confirms that trunk velocity during isoinertial movements is very sensitive to low back condition and research should be pursued to identify the biomechanical reasons for this and possibly differentiate between the different symptoms of LBP.