Response of microscopic bubbles to sudden decrease of ambient pressure in viscous compressible liquid

Intpoduction'Sudden Changes Of liquid flow velocity in pipingsystems are accompanied by high amplitode Pressareoscillations. In some phases of osdllations the Pressarecan reach negative Values (tensile sacsses). la thesecases the microscopic bubbles dispersed in the liquidgrow tO large cavihes. ~ some delay, when theliquid pressure inereases, they collapse again.The dynndcs Of a single gas or gas-vapour bubblein v~ ambient Pressal has been a subject ofnumerous stUdies over the years. Main …     

Aerodynamic and mass transfer characteristics of an annular bistable impinging jet with a fluidic flip-flop control

An annular nozzle has been designed on the basis of fluidic principles. The nozzle forms actively controlled air jet. Numerical and experimental investigations were performed in several subsequent steps, namely numerical simulation (using a commercial CFD code FLUENT), geometry adaptation, models manufacturing, flow visualization, hot-wire measurement, and mass transfer (naphthalene sublimation) experiment. A “jet switching” possibility has been discussed, an undesirable hysteresis effect has been suppressed. Present collaborative numerical and experimental investigations have resulted in a better understanding of mechanisms involved in controlled impinging jets, as well as in a further improvement of the particular nozzle geometry.     

Dynamic behaviour of non-isothermal compressible natural gases mixed with hydrogen in pipelines

This paper focuses on non-isothermal transient flow in mixed hydrogen–natural gas pipelines. The effect of hydrogen injection into natural gas pipelines has been investigated in particular the pressure and temperature conditions, Joule–Thomson effect, linepack and energy consumption of the compressor station. The gas flow is described by a set of partial differential equations resulting from the conservation of mass, momentum and energy. Real gas effects are determined by the predictive Soave–Redlich–Kwong group contribution method. The Yamal-Europe gas pipeline on Polish territory has been selected as case study.     

Recursive formulation on thermal analysis of an annular fin with variable thermal properties

This study introduces a discrete model which is pertinent for calculating thermal performance of singular annular fin with variable thermal properties. The singular annular fin can be divided into several circular sections, and each section can take its variable thermal properties, such as heat transfer coefficient and thermal conductivity, into account. The result from each section can be combined and calculated together by a recursive formula. Then, the solutions of temperature distribution and heat transfer rate on singular annular fin can be quickly obtained. For this model, the recursive formulae for both conditions with and without heat transfer on fin tip are demonstrated. Finally, some examples including composite materials for an annular fin have been successfully simulated through the present approach.     

Forced convection heat transfer inside an anisotropic porous channel with oblique principal axes: Effect of viscous dissipation

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A¹ is defined to combine both the effect of the dimensionless permeability ratio parameter K¹=(K₁/K₂) and orientation angle φ into one parameter. The effects of the parameter A¹, the Darcy number Da and the modified Brinkman number Br¹ on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A¹, Da and Br¹ have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A¹, called as critical value A_cr¹, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values A_cr¹.     

Numerical simulation on the behavior of a liquid jet into an air flow

A numerical simulation has been performed to clarify the effects of turbulence in a liquid on the deformation of the liquid jet surface into an air flow. The turbulences in the liquid jet were simulated by the Rankin vortices, and the liquid jet surface was tracked numerically by the volume of fluid method. By numerical simulations, the onset of the protrusions on the liquid jet surface is caused by the vortices in the liquid, and the surrounding air flow plays an important role in the amplification of the protrusions. The amplification rate of the trough displacement is proportional to the air‐to‐liquid velocity ratio. At large imposed vortex intensities, the trough displacement increases with the vortex intensity. On the other hand, at small imposed vortex intensities, the amplification of the trough displacement is also affected by factors other than vortex intensity. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 473–484, 2001     

Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume

Experiments were carried out inside a 25 m³ vented combustion test facility (CVE) with a fixed vent area sealed by a plastic sheet vent. Inside the CVE, a 0.64 m³ open vent box, called RED-CVE was placed. The vent of the RED-CVE was left open and three different vent area were tested. Two different mixing fans, one for each compartment, were used to establish homogeneous H₂ concentrations. This study examined H₂ concentrations in the range between 8.5% vol. to 12.5% vol. and three different ignition locations, (1) far vent ignition, (2) inside the RED-CVE box ignition and (3) near vent ignition (the vent refers to the CVE vent). Peak overpressures generated inside the test facility and the smaller compartment were measured. The results indicate that the near vent ignition generates negligible peak overpressures inside the test facility as compared to those originated by far vent ignition and ignition inside the RED-CVE box. The experiments with far vent ignition showed a pressure increase with increasing hydrogen concentration which reached a peak value at 11% vol. concentration and then decreased showing a non-monotonic behaviour. The overpressure measured inside the RED-CVE was higher when the ignition was outside the box whereas the flame entered the box through the small vent.     

Combined effect of variable fluid properties on nonisothermal flow of an inelastic fluid with viscous heating

This study investigates the effect of variable thermal conductivity and variable viscosity on the flow characteristics and flow rates of an inelastic fluid, including viscous heating terms in the analysis. The exponential dependence of viscosity on temperature is modeled through Arrhenius law and shear rate dependence is modeled through Carreau rheological equation while it is assumed that the thermal conductivity varies linearly with temperature. Flow governing motion and energy balance equations are coupled and the solution is found iteratively facilitating a pseudospectral method based on the Chebyshev polynomial expansions. Effect of variable thermal conductivity and viscosity as well as the effect of power law index, material time constant, Brinkman and Peclet numbers on the velocity and temperature profiles are presented. Influence of the above parameters on the flow rate is also calculated.     

瑞士dimaco CTB--170立式内外圆仿形机床改造

1引言CTB-170立式活塞环内外圆仿形车铣床。是瑞士dimaco公司于上世纪80、90年代研发的。专用于活塞环生产加工的设备,问世后经过几代改良。该设备集活塞环内外圆同时仿形加工、开口于一体化的机加工机床。克服了过去车内圆、车外圆、开口三道工序必须由三台机床分开加工,而产生同心度差、物流中间过程时间长、效率低、产品质量不稳定等缺点。由于性能优异,被许多国内活塞环厂家引进,     

Numerical study of compressible convective heat transfer with variations in all fluid properties

The effects of property variations in single-phase laminar forced micro-convection with constant wall heat flux boundary condition are investigated in this work. The fully-developed flow through micro-sized circular (axisymmetric) geometry is numerically studied using two-dimensional continuum-based conservation equations. The non-dimensional governing equations show significance of momentum transport in radial direction due to μ(T) variation and energy transport by fluid conduction due to k(T) variation. For the case of heated air, variation in C_p(T) and k(T) causes increase in Nu. This is owing to: (i) reduction in T_w, (T_w ? T_m), and (?T/?r)_w and (ii) change in ?T_m/?z results in axial conduction along the flow. The effects of ρ(p,T) and μ(T) variation on convective-flow are indirect and lead to: (i) induce radial velocity which alters u(r) profile significantly and (ii) change in (?u/?r)_w along the flow. It is proposed that the deviation in convection with C_p(T), k(T) variation is significant through temperature field than ρ(p,T), μ(T) variation on velocity field. It is noted that Nu due to variation in properties differ from invariant properties (Nu = 48/11) for low subsonic flow.     

An experimental and theoretical study of solidification in a free-convection flow inside a vertical annular enclosure

The natural convection and solidification in an annular enclosure has been studied experimentally and theoretically. Here the inner cylinder of the annular enclosure was cooled below the solidification temperature of the water, while the outer cylinder was kept on a uniform temperature well above 0 °C. The problem of the unsteady growth of the ice-layer on the inner cold cylindrical surface is studied theoretically and an approximate solution has been obtained for the quasi-steady development of the ice-layer thickness. In addition, it has been found that the influence of the contact layer between the frozen layer and the cold surface is of significant importance for the solidification process. Results are presented and compared between the experimental and the analytical investigation.     

Experimental study on the characteristics of liquid layer and disturbance waves in horizontal annular flow

INTRODUCTIONGas-liquid two-phase annular flow is encountered ina wide range of industrial applications, including nuclear reactors, evaporators and condensers. The studyof annular flow not only because of the need to understand the heat transfer characteristics of such aflow, but also because annular flow may be precursory to liquid dryout in the challnel, which is significant in nuclear reactor safety and in corrosion pre-vention. An important feature of horizontal annularflow is the effe…     

Numerical modelling of isothermal release and distribution of helium and hydrogen gases inside the AIHMS cylindrical enclosure

Hydrogen is a highly flammable gas and accidental release in confined space can pose serious combustion hazards. Numerical studies are required to assess the formation of flammable hydrogen cloud within confined spaces. In the present study, numerical investigations on the release of helium and hydrogen gases as high-velocity jets and their subsequent distribution inside an unventilated cylindrical enclosure (AIHMS facility) has been carried out as a first step towards numerical studies on hydrogen distribution in confined spaces for safety assessments. Experimental data for jet release of helium at volume Richardson number 0.1 and subsequent distribution has been used as benchmark data. Sensitivity studies on the influence of grid sizes, time-steps and turbulence models are performed. The performance of the validated numerical model is evaluated using statistical performance parameters. Similarity relations are used to determine input parameters for hydrogen jet for corresponding experimental data with helium jets. Finally, the mixing and flammability aspects of hydrogen distribution inside the enclosure are studied using four numerical indices that quantify mixing and deflagration potential of a distribution. It is concluded that the helium experiments can be used for validation of numerical models for hydrogen safety studies and any one of the similarity relationships, viz., equal buoyancy, equal volumetric flow, or equal concentration can be used for assessing the behaviour of hydrogen release and distribution within confined spaces.     

Effects of fuel properties on the combustion behavior of different types of porous beds soaked with combustible liquid

Experimental studies were conducted to investigate the behaviors of non-spread diffusion flames of liquid fuel-soaked porous beds. The effects of the properties of the porous bed and the fuel on combustion behaviors, including flame temperature profiles, combustion duration time, fuel consumption and the amount of fuel residue in the porous beds, were studied in the experiments. Additionally, a heat transfer model was used to predict the amount of fuel consumption in the porous beds. Consistency between the predicted and experimental results confirmed that heat conduction is the controlling mechanism in the combustion behavior of liquid fuel-soaked porous beds.     

Unsteady MHD flow of a dusty viscous liquid in a rotating channel with hall currents

The unsteady magnetohydrodynamic flow of a dusty, viscous, incompressible liquid in a rotating channel, in the presence of a transverse magnetic field with Hall currents, is considered. Analytical expressions for the velocities of the liquid and the dust particles are obtained. The effects of Hall current m, and the rotation parameter K on the velocity are shown graphically, and are discussed.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Photoelectrical and charge transfer properties of hydrogen-evolving TiO2 nanotube arrays electrodes annealed in different gases

TiO₂ nanotube arrays were fabricated by sonoelectrochemical anodic oxidation and calcined in nitrogen, air, or 5% hydrogen/nitrogen which was denoted as TNT-A, TNT-N, and TNT-H, respectively. All annealed TiO₂ nanotube arrays samples exhibited similar surface morphology. With UV illumination (365 ± 15 nm), the photocurrent density of the TNT-A, TNT-N and TNT-H was about 0.27 mA/cm², 0.45 mA/cm² and 0.60 mA/cm², respectively. The trapped electron at the Ti⁴⁺ center of TiO₂ nanotube arrays shows absorption at around 500-700 nm. From the XPS measurement, it was found that annealing in 5% hydrogen/nitrogen helped the sample obtain a greater defect density. Because of the reduction of Ti⁴⁺ and the formation of oxygen vacancies, the charge transfer resistance appeared in this order: TNT-A > TNT-N > TNT-H. Thus TNT-H harvested the greatest charge carrier density of 9.86 × 10²⁰ cm⁻³, TNT-N and TNT-A obtained a charge carrier density of 1.38 × 10²⁰ cm⁻³ and 1.06 × 10²⁰ cm⁻³, respectively. Accordingly, the hydrogen production rate by water splitting over TNT-A, TNT-N and TNT-H (320-780 nm irradiation, 3 h) was about 120 μL/h cm², 159 μL/h cm² and 231 μL/h cm², respectively.     

Convective heat transfer and pressure drop of annular tubes with three different internal longitudinal fins

Pressure drop and heat transfer characteristics of air in three annular tubes with different internal longitudinal fins were investigated experimentally at uniform wall heat flux. The tested tubes have a double‐pipe structure with the inner blocked tube as an insertion. Three different kinds of fins, plain rectangle fin, plain rectangle fin with periodical ridges and wave‐like fin, were located peripherally in the annulus. The friction factor and Nusselt number can be corrected by a power‐law correction in the Reynolds number range tested. It was found that the tube with periodical ridges on the plain fin or with wave‐like fin could augment heat transfer; however, the pressure drop was increased simultaneously. In order to evaluate the comprehensive heat transfer characteristics of the tested tubes, two criteria for evaluating the comprehensive thermal performance of tested tubes were adopted. They are: 1) evaluating the comprehensive heat transfer performance under three conditions: identical mass flow, identical pumping power, and identical pressure drop; 2) the second law of thermodynamics, i.e., the entropy generation. According to the two different evaluating methods, it was found that the tube with wave‐like fins provided the most excellent comprehensive heat transfer performance among the three tubes, especially when it was used under higher Reynolds number conditions. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 29–40, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20186     

The effects of the number and angle of microgrooves on the liquid film in horizontal annular two-phase flow

Measurements of the liquid film profile and pressure drop were made in tubes with grooves of varying angle (0°, 9° and 18°) as well as tubes with varying numbers of grooves at a common angle (18°). These data indicate that the grooves act to redistribute the liquid film on the inner wall perimeter and that only a relatively small number of grooves (12) is needed to do this. The grooves also affect the stability of the vapor-liquid interface and can generate lower average liquid film thickness. The pressure loss is directly related to the number of grooves for the tubes studied.