Conical Diffuser Research Articles

The effects of momentum diffusers and flow guides on the efficiency of stratified hot water seasonal heat stores

Hot water seasonal heat stores (HWSHS) carry the solar thermal energy from energy-rich seasons of the year over to energy-poor seasons so as to ensure the availability of solar energy throughout the year. Momentum diffusers and flow guides are designed to charge and discharge the harvested solar thermal energy within HWSHS in a stratified manner to enhance the efficiency of the solar systems. To evaluate the efficiency of an HWSHS, a characterization scheme developed for general stratified thermal energy stores (TES) (Sol Energy 2007; 81:1043–1054) is used. It addresses the First Law and Second Law concerns over a TES simultaneously. This study is confined to systems that use the same nozzles at fixed positions in both charging and discharging cycles. Different parameters related to axial, conical and radial diffusers as well as a variety of flow-guide designs are studied. The results suggest that a nozzle that brings about better diffuser action by minimizing entropy generation may not necessarily improve the energy response and guarantee better overall efficiency of the HWSHS. Of all, the different nozzle designs experimented with the conical diffusers with smaller angles of diffusion produced the best overall efficiency. Copyright © 2008 John Wiley & Sons, Ltd.

Analysis of the Cavitating Draft Tube Vortex in a Francis Turbine Using Particle Image Velocimetry Measurements in Two-Phase Flow

Partial flow rate operation of hydroturbines with constant pitch blades causes complex unstable cavitating flow in the diffuser cone. A particle image velocimetry (PIV) system allows investigating the flow velocity field in the case of a developing cavitation vortex, the so-called vortex rope, at the outlet of a Francis turbine runner. The synchronization of the PIV flow survey with the rope precession allows applying the ensemble averaging by phase technique to extract both the periodic velocity components and the rope shape. The influence of the turbine setting level on the volume of the cavity rope and its centerline is investigated, providing a physical knowledge about the hydrodynamic complex phenomena involved in the development of the cavitation rope in Francis turbine operating regimes.

216 円すいディフューザの内壁面上の旋回流による圧力回復への効果(GS5-3 流体(管路内流れ))

216 円すいディフューザの内壁面上の旋回流による圧力回復への効果(GS5-3 流体(管路内流れ))

The fluid dynamics of a miniature dilution tunnel for internal-combustion engine aerosol measurement

This paper investigates the fluid dynamics of a particular mini-dilution tunnel using LDV, flow visualization, a tracer sample technique and CFD. The mini-dilution tunnel studied had a 3.175 mm inside diameter tube discharging on the centerline of the tunnel where the diameter increases in a single step to 7.62 cm. The large diameter portion of the tunnel was 75 cm long. Most of the testing was performed at a flow rate of 15 l/min. The experimental investigation indicates that the flow field in the particular dilution tunnel tested has a persistent jet throughout its length, and this confined jet creates eddy recirculation zones which may cause the temperature and dilution histories of particles trapped in these eddies to be significantly different than particles which remain in the jet until extracted by the sample probe. Similarly, the location of the sample probe could also affect measured size distribution profiles, particularly if it were moved in or out of the path of the persistent jet. In addition to the simple tunnel geometry with a single abrupt expansion, a conical diffuser and a perforated plate were separately tested to investigate their effects on the tunnel fluid dynamics. The particular diffuser tested appeared to cause the jet to stall and therefore led to an even more unpredictable path for the fluid within. Limited testing with the perforated plate indicated that it increased the jet deceleration and laminarization, and therefore could lead to a more predictable flow path for aerosol sampled from the tunnel.

Experimental investigations of flow through conical diffusers with and without wake type velocity distortions at inlet

Diffusers play a vital role in many fluid machines to convert kinetic energy into pressure energy. The field of application of diffusers is very wide and diffusers are used in gas turbines, pumps, fans, wind tunnels, water tunnels and many other fluid flow systems. This paper discusses the results of flow through straight conical diffusers of half-cone angles 5° and 7° with steady uniform velocity of flow and wake type distorted flow at inlet. The wake type distortion at inlet was produced by a streamlined body and a bluff body. A low speed wind tunnel was used for the experiments and the diffusers were fabricated from cast aluminium blocks. The mean velocity and the turbulence parameters were measured using a constant temperature hot-wire anemometer.

Design and Analysis of A 3-D Conical Diffuser

Design and Analysis of A 3-D Conical Diffuser

Microcircuitry for two types of achromatic ganglion cell in primate fovea.

Synaptic circuits in primate fovea have been quantified for midget/parvocellular ganglion cells. Here, based on partial reconstructions from serial electron micrographs, we quantify synaptic circuits for two other types of ganglion cell: the familiar parasol/magnocellular cell and a smaller type, termed "garland." The excitatory circuits both derive from two types of OFF diffuse cone bipolar cell, DB3 and DB2, which collected unselectively from at least 6 +/- 1 cones, including the S type. Cone contacts to DB3 dendrites were usually located between neighboring triads, whereas half of the cone contacts to DB2 were triad associated. Ribbon outputs were as follows: DB3, 69 +/- 5; DB2, 48 +/- 4. A complete parasol cell (30 microm dendritic field diameter) would collect from approximately 50 cones via approximately 120 bipolar and approximately 85 amacrine contacts; a complete garland cell (25 microm dendritic field) would collect from approximately 40 cones via approximately 75 bipolar and approximately 145 amacrine contacts. The bipolar types contributed differently: the parasol cell received most contacts (60%) from DB3, whereas the garland cell received most contacts (67%) from DB2. We hypothesize that DB3 is a transient bipolar cell and that DB2 is sustained. This would be consistent with their relative inputs to the brisk-transient (parasol) ganglion cell. The garland cell, with its high proportion of DB2 inputs plus its high proportion of amacrine synapses (70%) and dense mosaic, might correspond to the local-edge cell in nonprimate retinas, which serves finer acuity at low temporal frequencies. The convergence of S cones onto both types could contribute S-cone input for cortical areas primary visual cortex and the middle temporal area.

Numerical study of flow in conical diffuser with vortex generator jets

To develop vortex generator jet (VGJ) method for flow control, the turbulence flow in a 144°conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with κ -e turbulence model. The diffuser performance, based on different velocity ratio (ratio of the jet speed to the mainstream velocity), is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics (CFD) method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.

C26 円すい広がり管内旋回流にみられる不安定性の制御(C2 流体工学2)

C26 円すい広がり管内旋回流にみられる不安定性の制御(C2 流体工学2)

Quasistatic nonlinear characteristics of double-reed instruments

This article proposes a characterization of the double reed in quasistatic regimes. The nonlinear relation between the pressure drop, deltap, in the double reed and the volume flow crossing it, q, is measured for slow variations of these variables. The volume flow is determined from the pressure drop in a diaphragm replacing the instrument's bore. Measurements are compared to other experimental results on reed instrument exciters and to physical models, revealing that clarinet, oboe, and bassoon quasistatic behavior relies on similar working principles. Differences in the experimental results are interpreted in terms of pressure recovery due to the conical diffuser role of the downstream part of double-reed mouthpieces (the staple).

NUMERICAL ANALYSIS OF TURBULENT FLOW IN A CONICAL DIFFUSER

A commercial CFD code, CFX-TASCflow, is used to predict turbulent flow in a conical diffuser. The computation was performed using a low-Reynolds number k–e model, low-Reynolds number k–ω model, a low-Reynolds number k–ω based non-linear algebraic Reynolds stress model, and a second moment closure with a wall-function. The experimental datasets of Singh [1] and Kassab [2] are used to validate the numerical results. The results show that all the turbulence models reproduce the static pressure coefficient distribution reasonably well. The low Reynolds number k–ω models give better prediction of the friction velocity than the second moment closure. The models also predict the Reynolds shear stress reasonably well but fail to reproduce the correct level of the kinetic energy.

Burning velocity of premixed turbulent flames in the weakly wrinkled regime

Turbulent burning velocities have been measured for methane/air and ethylene/air planar flames stabilized in a wide-angled conical diffuser in which the flow is decelerated axially in a controlled manner. Novel instrumentation, involving a rotating drum device, has been developed to measure the instantaneous flame height, utilizing the UV emission from the excited OH radical in the flame. Secondary flow in a high-speed wall jet is used to generate a uniform primary flow across the diffuser. Turbulence generators at the inlet provide varying turbulent fields. The flames are stabilized at different heights depending upon the equivalence ratio, the turbulence, and the inlet velocity. The turbulent burning velocity is assumed to be equal to the unburnt reactant velocity at the average height of the flame measured using the drum, and this velocity is deduced from measurements with a hot-wire anemometer in the absence of combustion. The turbulent burning velocities thus measured are compared with theoretical correlations. They are affected by the Darrieus–Landau instability in the region u ′ / S l < 1 , and they maintain values of S t / S l ∼ 3 – 4 down to u ′ / S l ∼ 0.2 . This is in accordance with a theory developed by Bychkov and co-workers, which allows for the nonlinear interaction of the turbulence with the flame instability.

Autoimmune Retinal Dysfunction in Sarcoid Chorioretinopathy

Visual loss, photophobia and night-blindness in a middle-aged patient with a previous history of cancer are highly suggestive of cancer-associated retinopathy. A 63-year-old man complained of slowly progressive bilateral visual loss, photophobia and night-blindness. His past medical history was remarkable for surgical and medical treatment of a carcinoma of the base of his tongue nine years earlier. Visual acuity was 0.3 RE and 0.15 LE, with constricted visual field in both eyes. Ocular examination and fluorescein angiography were non-relevant. Electroretinography revealed diffuse cone and rod dysfunction. ICG angiography showed multiple small widespread hyperfluorescent spots. A chest CT scan revealed multiple focal lesions which were biopsied. Histological diagnosis was sarcoidosis. Upon systemic steroid therapy, visual function gradually improved, and ICG anomalies disappeared. Vision recovered to 1.0 RE and 0.8 LE with normal visual field in both eyes. Sarcoidosis may present as an autoimmune retinal dysfunction with photophobia and night blindness as the presenting symptoms.

English

Turbulent flow in a diffuser with swirl occurs in many commonly used fluid mechanicaldevices,eg, diffusers located downstream of a gas turbine, and in certain types of combustionchambers. Diffusers are widely used for converting kinetic energy to pressure, and a reliableprediction method of such flows with the required flow conditions would lead to the design offluid machinery with improved efficiency. As a first step, turbulent swirling flow through a 12oincluded angle conical diffuser for a swirl parameter, m = 0.18 was numerically investigated usingvarious turbulence models like standard k- , RNG-based k- , shear-stress transport (SST) kandReynolds stress model (RSM). Though the comparison between the experimental and thepredicted mean velocity profile by RSM is superior to that by RNG kandSST models, the lattertwo models give closer comparison with the experimental pressure distribution. Subsequently,computation of flow inside a complex duct involving axisymmetric annular diffuser, transitionfrom rectangular to circular cross section, and exit pipe have been carried out using RNG kandSST k models.The comparison of computed and experimental results indicates that theSST k modelgives predictions with reasonable accuracy.

The peculiar optic, dielectric and X‐ray diffraction properties of a fluorinated de Vries asymmetric diffuse cone‐model ferroelectric liquid crystal

A new semi‐fluorinated chiral smectic liquid crystal, W504, is investigated by electro‐optic, dielectric and X‐ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle θ; typical characteristics of a SmA*–SmC* transition following the de Vries asymmetric diffuse cone (ADC) model in which the non‐zero director tilt in SmC* arises through an ordering of tilting directions rather than an actual increase in average molecule tilt ⟨θmol⟩. In W504 a small increase in ⟨θmol⟩ of about 4° is however detected in the SmC* phase. Although the increase in molecule inclination is much less than the increase in director tilt θ, saturating close to 30°, it leads to a shrinkage of the smectic layers by about 1 Å, a result of the large initial molecule tilt in the SmA* phase, ⟨θmol⟩SmA*≈30°. The tilting transition in W504 is thus mainly an ADC model disorder–order transition, but it also has a component of a structural transition. The semi‐fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (0 0 1) X‐ray scattering peak. In thin films the (0 0 1) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head–tail symmetry of the liquid crystal phase at the LC–air interface.

Interaction between struts and swirl flow in gas turbine exhaust diffusers

The increasing use of gas turbines in combined cycle power plants together with the high amount of kinetic energy in modern gas turbine exhaust flows focuses attention on the design of gas turbine diffusers as the connecting part between the Brayton/Joule and the Rankine parts of the combined cycle. A scale model of a typical gas turbine exhaust diffuser is investigated experimentally. The test rig consists of a radial type, variable swirl generator which provides the exhaust flow corresponding to different gas turbine operating conditions. Static pressure measurements are carried out along the outer diffuser walls and along the hub of the annular part and along the centerline of the conical diffuser. Velocity distributions at several axial positions in the annular and conical diffuser have been measured using a Laser Doppler Velocimeter (LDV). Pressure recovery coefficients and velocity profiles are depicted as a function of diffuser length for several combinations of swirl strength, tip flow and strut geometries. The diffuser without struts achieved a higher pressure recovery than the diffuser with struts at all swirl angle settings. The diffuser with cylindrical struts achieved a higher pressure recovery than the diffuser with profiled struts at all swirl angle settings. Inlet flows with swirl angles over 18° affected the pressure recovery negatively for all strut configurations.

Dielectric spectroscopy of de Vries-type smectic-A*–smectic-C*transitions

We report results of dielectric investigations on a number of ferroelectric liquid crystalline (FLC) compounds with different degrees of layer shrinkage in the smectic-A(*) -smectic-C(*) phase transitions. With a decreasing extent of layer shrinkage the investigated FLCs exhibit a significantly increasing soft-mode absorption, decreasing leading Landau coefficient alpha, and a considerably broader mean-field regime. We explain these tendencies by the fact that the low layer shrinkage materials come closer to the diffuse cone model of de Vries than to the common model of rigid rods which maintain their orientational order during tilting. In the case of the diffuse cone model the tilt-angle fluctuations are decoupled from variations of the smectic layer spacing, which explains the observed behavior.

348 管路型円すいディフューザの脈動流れ特性 : 広がり角度や振動数の影響(G05-6 流体機械(1),G05 流体工学)

Pulsating turbulent flow in conical diffusers having total divergence angles of 12, 16 and 24°were investigated experimentally. Distributions of the pressure and axial velocity and the power loss were obtained for the pulsating flows at the Womersley numbers α of 10 to 40, the mean Reynolds numbers of Reta =15000〜30000 and the oscillatory Reynolds number of Reos =10000. The power loss of the pulsating flow in the diffusers is larger than that of the steady flow and is highly dependent on the flow rate ratio. A discussion, furthermore, is given for the influences of the divergence angle and the Womersley number on the time-dependent profiles of the phase-averaged velocity and the turbulence intensity.

On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal mixture

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic Ca* (SmCa*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmCa* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmCa* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.

A Study on the Control of Surface Condensation on a Multi-Cone Diffuser in Cold Air Distribution Applications

Multi-cone diffusers have good air distribution characteristics in cooling applications. However, surface condensation has been a problem impeding the use of multi-cone diffusers in cold air applications. An experimental study and a computer analysis are presented to show the causes of surface condensation on a multi-cone diffuser when distributing cold air. This study also attempts to alter the flow field characteristics near the diffuser cone surfaces by using an open inner cone with an insert that has a porous structure. Computer analysis has shown that the resultant airflow from the inner cone will repel the entrained room air away from the diffuser, thus preventing contact of the more humid air with the cold cone surfaces. The analysis has also shown that the altered flow field will cause the air dew point temperature in the vicinity of the diffuser to be lower than the temperature of the cone surfaces. Inserts with different porosities that were effective in preventing surface condensation were tested and the results are presented. These results indicate that the proposed airflow control method is effective in preventing surface condensation.