Temperature oscillations in a slug tube flow

The special feature of Mathematica in directly manipulating complex numbers is used to find the temperature oscillations in a slug flow inside a circular tube, subjected to a periodically varying inlet temperature. Mathematica inputs used are as simple as possible. Examples show how to obtain the temperature oscillations in any desired location of the channel.     

Directional separation of hydrogen-containing gas mixture by hydrate-membrane coupling method

Recovery of hydrogen (H₂) from H₂-containing gas mixtures has great significance for energy conservation, cost reduction and benefit increase. However, the common separation methods have the ubiquitous problem due to phase equilibrium principle and results in the conflict between H₂ concentration and H₂ recovery rate in the product gas. Consequently, an innovative conception of hydrate-membrane coupling approach is proposed in this work. In the separation process, hydration and membrane permeation two separation driving forces coexist to achieve the aim of strengthening mass transfer kinetics. H₂ and non-H₂ components (hydrocarbons) are synchronously and directionally selected by membrane and hydrate to improve different phase compositions. Therefore, the gas in feed side could keep relatively high two separation driving forces (H₂ fugacity and hydrocarbons fugacity). The results show that the coupling method could synchronously increase both the concentration and the recovery rate of H₂ in the product gas. At the same time, the volume and concentration of the hydrocarbons in hydrate both increases effectively. It indicates that hydrate and membrane separation methods support each other in the separation process. The hydrate-membrane coupling method fundamentally solves the issue of the decreasing driving force resulting from single separation method and phase equilibrium relationship.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Experimental investigation on heat transfer enhancement of a tube with coiled-wire inserts installed with a separation from the tube wall

An experimental investigation was conducted for the thermo-hydraulic performance of a circular tube with coiled-wire inserts which were installed with a small separation from the inner wall of the tube. The wire inserts had an equilateral triangular cross-sections with a constant side length of e = 6 mm and they were coiled with three different pitch-to-diameter ratios: P/D = 1, P/D = 2, P/D = 3. A specific method was employed to coil the wires so that an edge of the triangle was oriented to face the incoming air flow. The coiled-wire inserts were installed with 1 and 2 mm separation from the inner tube wall so that the heat transfer enhancement due to the laminar boundary layer disturbance could be investigated. Experiments were performed for Reynolds numbers from 3429 to 26,663. The coiled-wire inserts led to a significant increase in both the heat transfer rate and friction factor over the smooth tube based on coil pitches and clearance. The maximum thermal performance was observed around 1.82 for the P/D = 1, s = 1 type at a Reynolds number of 3429. In conclusion, the laminar boundary layer disturbance can be effectively enhanced by using these types of coiled-wire inserts.     

Numerical simulation of flow field and temperature separation in a vortex tube

A numerical analysis of flow field and temperature separation in a uni-flow vortex tube type is described. Effects of the turbulence modeling (k–ε model and ASM), numerical scheme (hybrid, upwind and second-order upwind) and grid density on calculation of energy separation in the vortex tube are also conducted. It is found that the calculated results are in reasonably good agreement with the experimental data for both the static and total temperatures; the use of the ASM improves slightly the accuracy of the predictions than that the k–ε model. It is also observed that larger temperature gradients appear in the outer regions close to the tube wall for the static temperatures and the separation effect or the difference of the total temperature is high in the core region near the inlet nozzle. The maximum total temperature in the field is visible at the axis location of x/D_o between 0.5 and 1.0 for the ASM.     

Numerical study of acoustic coupling in spinning detonation propagating in a circular tube

Spinning detonations propagating in a circular tube were numerically investigated with a two-step reaction model by Korobeinikov et al. The time evolutions of the simulation results were utilized to reveal the propagation behavior of single-headed spinning detonation. Three distinct propagation modes, steady, unstable, and pulsating modes, are observed in a circular tube. The track angles on a wall were numerically reproduced with various initial pressures and diameters, and the simulated track angles of steady and unstable modes showed good agreement with those of the previous reports. In the case of steady mode, transverse detonation always couples with an acoustic wave at the contact surface of burned and unburned gas and maintains stable rotation without changing the detonation front structure. The detonation velocity maintains almost a CJ value. We analyze the effect of acoustic coupling in the radial direction using the acoustic theory and the extent of Mach leg. Acoustic theory states that in the radial direction transverse wave and Mach leg can rotate in the circumferential direction when Mach number of unburned gas behind the incident shock wave in the transverse detonation attached coordinate is larger than 1.841. Unstable mode shows periodical change in the shock front structure and repeats decoupling and coupling with transverse detonation and acoustic wave. Spinning detonation maintains its propagation with periodic generation of sub-transverse detonation (new reaction front at transverse wave). Corresponding to its cycle, whisker is periodically generated, and complex Mach interaction periodically appears at shock front. Its velocity history shows the fluctuation whose behavior agrees well with that of rapid fluctuation mode by Lee et al. In the case of pulsating mode, as acoustic coupling between transverse detonation and acoustic wave is not satisfied, shock structure of spinning detonation is disturbed, which causes failure of spinning detonation.     

Numerical investigation of the thermal separation in a Ranque–Hilsch vortex tube

The application of a mathematical model for the simulation of thermal separation in a Ranque–Hilsch vortex tube is presented in this paper. The modelling of turbulence for compressible, swirling flows used in the simulation is discussed. The work has been carried out in order to provide an understanding of the physical behaviors of the flow, pressure, temperature in a vortex tube. A staggered finite volume approach with the standard k–ε turbulence model and an algebraic stress model (ASM) is used to carry out all the computations. To investigate the effects of numerical diffusion on the predicted results, the second-order upwind (SOU) and the QUICK numerical schemes are used and compared with the first-order upwind and the hybrid schemes. The computations show that the differences of results obtained from using the various schemes are marginal. In addition, results predicted by both turbulence models generally are in good agreement with measurements but the ASM performs better agreement between the numerical results and experimental data. The computations with selective source terms of the energy equation suppressed show that the diffusive transport of mean kinetic energy has a substantial influence on the maximum temperature separation occurring near the inlet region. In the downstream region far from the inlet, expansion effects and the stress generation with its gradient transport are also significant.     

Numerical study of energy separation in a vortex tube with different RANS models

The aim of the present paper is to investigate numerically the energy separation mechanism and flow phenomena within a vortex tube. A 3D computational domain has been generated considering the quarter of the geometry and assuming periodicity in the Azimuthal direction which was found to exhibit correctly the general behaviour expected from a vortex tube. Air is selected as the working fluid. The flow predictions reported here are based upon four turbulence models, namely, the k–?, k–ω and SST k–ω two-equations models and the second moment closure model (RSM). The models results are compared to experimental data obtained from the literature. Four cases have been considered by changing the inlet pressure from 200 up to 380 kPa. It has been observed that all the above mentioned models are capable of predicting fairly well the general flow features but only the advanced RSM model is capable of matching correctly the measured cold and hot outlet temperatures. All the other models over predict the mean temperature difference by values up to twice the measured data.     

Internal thermal coupling in direct-flow coaxial vacuum tube collectors

This investigation covers the impact of low flow rates on the efficiency of coaxial vacuum tube collectors. Measurements show an efficiency reduction of 10% if reducing the flow rate from 78 kg/m² h to 31 kg/m² h for a collector group with 60 parallel vacuum tubes with a coaxial flow conduit at one-sided connection. For a more profound understanding a model of the coaxial tube was developed which defines the main energy fluxes including the internal thermal coupling. The tube simulations show a non-linear temperature profile along the tube with the maximum temperature in the outer pipe. Due to heat transfer to the entering flow this maximum is not located at the fluid outlet. The non-linearity increases with decreasing flow rates. The experimentally determined flow distribution allows simulating the measured collector array. The simulation results confirm the efficiency decrease at low flow rates. The flow distribution has a further impact on efficiency reduction, but even at an ideal uniform flow, a considerable efficiency reduction at low flow rates is to be expected. As a consequence, low flow rates should be prevented for coaxial tube collectors, thus restricting the possible operation conditions. The effect of constructional modifications like diameter or material variations is presented. Finally the additional impact of a coaxial manifold design is discussed.     

Quantification of energy produced from an evacuated tube water heater in a real setting

For the first time, actual data regarding an evacuated tube SWH has been collected from an implemented system in Lebanon. The data collected was based on a number of sensors installed at various points in the system. The results indicated real hot water use of the household and operational temperatures throughout the year. The system provided more than 98% of the household needs indicating a clear over sizing of the system. Other findings include excessive heat generated in the summer, necessitating forced shadowing of the collectors. The system provided 3049 kWh/y and saved the family around $195. More significant savings are obtained by the electricity company since electricity in Lebanon is subsidized. This case study provides a reference point for the observed savings rather than estimated savings.     

Influences of geometry and flow pattern on hydrogen separation in a Pd-based membrane tube

The abatement of concentration polarization in a membrane tube is of the utmost importance for improving the efficiency of hydrogen separation. In order to enhance the performance of hydrogen separation, the characteristics of hydrogen permeation in a Pd-based membrane system under various operating conditions and geometric designs are studied numerically. The effects of Reynolds numbers, shell size, baffle, and pressure difference on hydrogen mass transfer across the membrane are evaluated. The predictions suggest that a larger shell deteriorates concentration polarization, stemming from a larger H₂ concentration boundary layer. Baffles equipped in the shell are conducive to disturbing H₂ concentration boundary layer and reducing concentration polarization at the retentate side, thereby intensifying H₂ permeation. The more the number of baffles, the less the increment of improvement in H₂ permeation is. The installation of one baffle is recommended for enhancing H₂ separation and it is especially obvious under the environments of high pressure difference. Within the investigated ranges of Reynolds number at the permeate side and the retentate side, the feasible operating conditions are suggested in this study.     

Hydrogen production from water gas shift reactions in association with separation using a palladium membrane tube

Hydrogen production from water gas shift reactions (WGSRs) of synthesis gas (syngas) followed by separation via a Pd membrane was studied experimentally. In the reactions, a variety of combinations of a high‐temperature shift reaction (HTSR), a low‐temperature shift reaction (LTSR) and a palladium (Pd) membrane tube were considered. The results indicated that the CO conversion from the LTSR was close to that of the HTSR and LTSR in series; however, the latter with the Pd membrane could provide a much low CO concentration at the permeate side. On the other hand, while the produced hydrogen diffused through the membrane, methane was also found at the both sides of the membrane due to the methanation reaction activated by the Pd membrane. In the present system, increasing the steam/CO ratio enhanced the forward reaction of the WGSRs and elongated the residence time of the reactants in the catalyst beds, resulting in the increases of CO conversion and hydrogen recovery. As a whole, the concentration of CO in the separated hydrogen was lower than 50 ppm from the combination of the HTSR and the LTSR with the membrane, whereby the produced hydrogen could be applied in proton exchange membrane fuel cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature distributions in rotary-blade coupling of a 4WD vehicle with longitudinal ribbed turbulators

The instability of the thermal characteristics of rotating-blade coupling (RBC), with longitudinal ribbed turbulators, in real-time four-wheel-drive (4WD) vehicles has been investigated. The key parameters approximate to the operating conditions of a real vehicle. The co-axial rotating channel flow exhibits a pattern of unstable Taylor vortices. Longitudinal rib-roughened walls were employed to prevent high temperatures in local regions. The local temperature-distributions of the RBC with longitudinal ribbed turbulators were measured and compared with those with smooth walls. Longitudinal ribbed turbulators can: (i) prevent local increases in temperature, (ii) increase the lives of the parts of a rotational blade connector and (iii) protect the motive sources of 4WD vehicles.     

Numerical investigations on flow characteristics and energy separation in a Ranque Hilsch vortex tube with hydrogen as working medium

In the last decades, the theory of energy separation in vortex tubes is debated broadly based on the heat transfer and work transfer between core and peripheral flow layers. Many parameters were considered in the literature. However, the present study involves the inlet energy considered collectively towards energy separation. In this paper, three-dimensional computational fluid dynamic simulations are discussed in vortex tube to analyze the energy separation phenomena in different cases by varying the working medium such as hydrogen and air having specific heat variation. The energy at the inlet is maintained same in both cases by adjusting the inlet mass flow rate. The results from this study are validated with recently published literature using hydrogen as a working medium. Vortex tube with hydrogen as working medium yields a temperature separation of 8 K lower than air as working medium. Further studies on vortex tube with hydrogen as a working fluid is explored at different inlet temperatures relative to the room temperature. Vortex tube with hydrogen at an inlet temperature of 400 K gives better temperature separation as compared to other inlet temperatures considered in this study.     

One-point fitting of the flux density produced by a heliostat

Accurate and simple models for the flux density reflected by an isolated heliostat should be one of the basic tools for the design and optimization of solar power tower systems. In this work, the ability and the accuracy of the Universidad de Zaragoza (UNIZAR) and the DLR (HFCAL) flux density models to fit actual energetic spots are checked against heliostat energetic images measured at Plataforma Solar de Almería (PSA). Both the fully analytic models are able to acceptably fit the spot with only one-point fitting, i.e., the measured maximum flux. As a practical validation of this one-point fitting, the intercept percentage of the measured images, i.e., the percentage of the energetic spot sent by the heliostat that gets the receiver surface, is compared with the intercept calculated through the UNIZAR and HFCAL models. As main conclusions, the UNIZAR and the HFCAL models could be quite appropriate tools for the design and optimization, provided the energetic images from the heliostats to be used in the collector field were previously analyzed. Also note that the HFCAL model is much simpler and slightly more accurate than the UNIZAR model.     

水平管外TFE/NMP部分膜状冷凝热、质耦合传递过程研究

通过对水平管外双组分(TFE／NMP为三氟乙醇／氮甲基吡咯烷酮)部分膜状冷凝过程特点的分析，建立起部分膜状冷凝过程中热质传递过程的物理模型。以双膜理论为基础，利用部分膜状冷凝的特点，通过对界面传质、液膜内质量平衡、界面相平衡、界面能量平衡和汽膜截面能量平衡的分析计算，得到汽相温度和界面温度分布、汽相及液相NMP质量分数分布，由此进一步计算出冷凝膜厚分布、液膜传热系数分布和热流密度的分布。计算的热流密度与相关实验作了比较，发现与实验能较好的吻合。     

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.     

Pulverized coal burnout in blast furnace simulated by a drop tube furnace

Reactions of pulverized coal injection (PCI) in a blast furnace were simulated using a drop tube furnace (DTF) to investigate the burnout behavior of a number of coals and coal blends. For the coals with the fuel ratio ranging from 1.36 to 6.22, the experimental results indicated that the burnout increased with decreasing the fuel ratio, except for certain coals departing from the general trend. One of the coals with the fuel ratio of 6.22 has shown its merit in combustion, implying that the blending ratio of the coal in PCI operation can be raised for a higher coke replacement ratio. The experiments also suggested that increasing blast temperature was an efficient countermeasure for promoting the combustibility of the injected coals. Higher fuel burnout could be achieved when the particle size of coal was reduced from 60–100 to 100–200 mesh. However, once the size of the tested coals was in the range of 200 and 325 mesh, the burnout could not be improved further, resulting from the agglomeration of fine particles. Considering coal blend reactions, the blending ratio of coals in PCI may be adjusted by the individual coal burnout rather than by the fuel ratio.