Large Condenser Research Articles

Numerical investigation of fan performance in a forced draft air-cooled steam condenser

Inlet air flow distortions in a large air-cooled steam condenser (ACSC), caused by structures, wind and other fans may result in a significant reduction in fan performance, or volumetric effectiveness, as well as fan blade vibration. This phenomenon has an adverse effect on the heat rejection capacity of an ACSC due to a decrease in air mass flow rate. In this study the effect of inlet flow distortions on the flow rate through the fans in an ACSC is numerically investigated by modelling the flow field in a section of such a system using the computational fluid dynamics code, FLUENT. The effect of platform height on the volumetric effectiveness of two different types of axial flow fans is considered. The two fans have the same diameter, number of blades and rotational speed, but feature different blade designs, and hub-tip-ratios of respectively 0.153 and 0.4. Numerical simulations show all-round superior performance in terms of volume flow rate for the fan with a hub-tip-ratio of 0.4. It is furthermore confirmed that the addition of a walkway can significantly increase the flow rate through the fans located near the edge of the fan platform.

Magnetic condensation, Abelian dominance, and instability of Savvidy vacuum in Yang-Mills theory

We propose a novel type of color magnetic condensation originating from magnetic monopoles so that it provides the mass of off-diagonal gluons in the Yang-Mills theory. This dynamical mass generation enables us to explain the infrared Abelian dominance and monopole dominance by way of a non-Abelian Stokes theorem, which supports the dual superconductivity picture of quark confinement. Moreover, we show that the instability of Savvidy vacuum disappears by sufficiently large color magnetic condensation.

A comparative study of two large radiative dew water condensers

In order to improve the yield of dew condensation from atmospheric vapor, two large (30 m 2 in area) insulated plane radiative condensers, inclined at 30°, were installed in Ajaccio (Corsica island, France; latitude 41°55′N, longitude 8°48′E). Prototype P1 was elevated such that the underside was open and exposed. Prototype P2, however, was enclosed on all sides and closer to the ground. Both used a special radiative foil that enhances dew formation. The period of observation for P1 was July 22, 2000–November 11, 2001, and for P2 was December 10, 2001–December 10, 2003. All data were compared with respect to the same horizontal calibration plate of polymethylmethacrylate (Plexiglas) placed at 1 m above the ground on a sensitive recording balance. Water yield of both prototypes were compared and correlated against meteorological data (cloud cover, relative humidity, wind speed, condenser temperature and air temperature). Both prototypes exhibit improved performances when compared with the calibration plate: more dew days (+16% and +15% for P1 and P2, respectively); decrease of the humidity threshold (−3% and −4.4% for P1 and P2); increase of dew yields for wind speeds up to 3 m s −1. A model of the mass and thermal exchanges with the ambient air was used. Two adjustable parameters (heat and mass transfer coefficients) are used in the model. The values of these parameters were found larger than the values obtained in continental sites where dew forms with weak wind, thus emphasizing the peculiarities of dew formation in windy islands. When data are reduced with the calibration PMMA data, prototype P1 provided average water yields slightly larger than the enclosed prototype P2, a result that can be attributed to the influence of surface thermal radiation.

Parametric excitation of a Bose-Einstein condensate in a one-dimensional optical lattice

We study the response of a Bose-Einstein condensate to a periodic modulation of the depth of an optical lattice. Using Gross-Pitaevskii theory, we show that a modulation at frequency $\ensuremath{\Omega}$ drives the parametric excitation of Bogoliubov modes with frequency $\ensuremath{\Omega}∕2$. The ensuing nonlinear dynamics leads to a rapid broadening of the momentum distribution and a consequent large increase of the condensate size after free expansion. We show that this process does not require the presence of a large condensate depletion. Our results reproduce the main features of the spectrum measured in the superfluid phase by St\"oferle et al. [Phys. Rev. Lett. 92, 130403 (2004)].

A new path to ultracold hydrogen

Ultracold hydrogen offers unique possibilities for precision spectroscopy, studies of atomic interactions, and the creation of quantum fluids containing mixtures of hydrogen and deuterium. Current techniques for trapping and cooling hydrogen have produced large condensates with N ∼ 109 atoms, but suffer from a variety of experimental limitations. Among these are the slow evaporative cooling rate due to the small H–H elastic-scattering cross section, the need for a superfluid helium film in the initial thermalization process, a geometry that severely limits detection efficiency, and the inability to trap deuterium. We are constructing a new apparatus based on buffer-gas cooling that will overcome these problems. To accelerate evaporative cooling, the thermalization rate is increased by simultaneously loading lithium and hydrogen into a 4.2 T anti-Helmholtz trap. Lithium accelerates evaporative cooling because the Li–H elastic-scattering cross section is ∼1200 times larger than that of H–H. Hydrogen and lithium will be produced by laser ablation of solid LiH in a 3He buffer gas held at temperatures of ∼350 mK. Because no wall collisions are needed for initial thermalization, ablating a solid sample of LiD should enable studies of deuterium. The apparatus is in the final stages of construction. We report on results of initial tests and discuss the new scientific opportunities made possible by this approach.PACS No.: 32.80.Pj

Efficient rapid production of a Bose-Einstein condensate by overcoming serious three-body loss

We report the efficient production of a large Bose-Einstein condensate in $^{87}\mathrm{Rb}$ atoms. This is achieved by quickly reducing the radio frequency of the magnetic field at a rate of $\ensuremath{-}96.8\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}∕\mathrm{s}$ during the final stage of evaporative cooling, and we have produced a condensate with $(2.2\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms against a serious three-body recombination loss. We observed the dependence of the cooling efficiency on the rate at which the truncated energy changes by measuring the condensate growth for three kinds of radio-frequency sweep. The experimental results quantitatively agree with calculations based on the quantum kinetic theory of a Bose gas.

THE MODE-DEVIATION EFFECT OF TRAPPED SPINOR BOSE GAS BEYOND MEAN FIELD THEORY

The deviation effect of spinor mode from the single-mode for a spin-1 Bose gas of trapped atoms is studied beyond the mean field theory. Based on the effective Hamiltonian with nondegenerated level of the collective spin states, the splitting level of the system energy due to the deviation effect has been calculated. For the large condensates of 87 Rb and 23 Na with atom number N>105, the splitting fraction of the energy, arising from the magnetization exhibited by the trapped Bose gas, is found to have a typical order of (10-4~10-8), decreasing as N-2 for 87 Rb and increasing as -N-2 for 23 Na , respectively.

Condensation of pairs of fermionic atoms near a Feshbach resonance.

We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822+/-3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.

Triggered massive-star formation at the border of the HII region Sh 104

We present millimetre-line imaging of the Galactic H ii region Sh 104. We show that it is surrounded by a ring of molecular gas and dust. Four large molecular condensations are regularly spaced around the ring. These condensations are themselves fragmented and contain several massive dense cores. A deeply embedded cluster is observed in the near IR towards the largest condensation. It contains at least one massive star ionizing an ultra-compact H ii region. The Sh 104 region is a good illustration of the “collect and collapse” model for star formation triggered by the expansion of an H ii region.

Question of oil-gas prospects in northern regions of the Barents-Kara shelf (Franz-Viktor Trough)

The significant hydrocarbon potential of the shelf areas of the Barents and Kara Seas has been demonstrated by the discovery of very large gas and gas condensate fields in their southern areas. Against this background the increase in oil reserves has not been so significant. Nevertheless there is a geologic basis for predicting large zones of oil accumulation in the north of the Barents-Kara basin, which is an area of border highs. First of all, the depth to source beds is not great, and this provides for the generation of mostly liquid hydrocarbons. The neotectonic environment was also more favourable for formation and preservation of oil pools. Areas of this type are characterised by large zones of oil and bitumen accumulation related to lateral migration of hydrocarbon fluids from more subsided parts of oil-gas basins. All this is substantiated by much geochemical data derived from study of bitumen shows on Franz Josef Land and bottom samples from the surrounding offshore areas. The bitumen shows are largely viscous and solid natural bitumens. Their content in host rock is comparatively insignificant at fractions of a percent. All this indicates migration of predominantly liquid hydrocarbons from deeper horizons of the sedimentary cover as well as absence of significant surface precipitation accompanying destruction of oil pools. In this light the region may be regarded as favourable for oil. It is therefore important that geochemical anomalies be recognised in areas of local positive structures in the north of the Barents-Kara shelf. Information has been gathered in recent years in the Franz-Viktor Trough that indicates a large swell-like structure of north-south trend in its southern part (see Figures 1 & 2). It is designated as the Ladin High in connection with recovery of sediments of the Ladin Stage at station 52 (Figure 2). Reflection and common depth point seismic profiling show the Ladin High as clearly expressed in the relief of the Precambrian (Vendian) basement. Its dimensions are 110 km x 20-40 km. Its base is outlined by the 3 km structure contour, and the crest is at a depth of 1.5-1.7 km. Ladin High retains its inherited form in the Phanerozoic cover, although more gentle, particularly in its upper part. Small cupolas that measure 10-20 km in cross section and with amplitudes of 0.4-0.6 km have been mapped on the reflection profiles. These coincide in direction with the trend of the Ladin High (See Figure 3).

Amplification of local instabilities in a Bose-Einstein condensate with attractive interactions.

We study the collapse of large homogeneous Bose-Einstein condensates due to intrinsic attractive interactions. We observe the amplification of a local instability by seeding a momentum state p and suddenly switching the scattering length negative via a Feshbach resonance. We also observe the appearance of atoms in the conjugate momentum state as required by momentum conservation. For large condensates, the time scale for this depletion process becomes faster than that for global collapse.

Vortices near surfaces of Bose-Einstein condensates

The theory of vortex motion in a dilute superfluid of inhomogeneous density demands a boundary layer approach, in which different approximation schemes are employed close to and far from the vortex, and their results matched smoothly together. The most difficult part of this procedure is the hydrodynamic problem of the velocity field, many healing lengths away from the vortex core. This paper derives and exploits an exact solution of this problem in the two-dimensional case of a linear trapping potential, which is an idealization of the surface region of a large condensate. It thereby shows that vortices in inhomogeneous clouds are effectively ``dressed'' by a nontrivial distortion of their flow fields, that image vortices are not relevant to Thomas-Fermi surfaces, and that for condensates large compared to their surface depths, the energetic barrier to vortex penetration disappears at the Landau critical velocity for surface modes.

Bose-Einstein Condensates in Dilute Trapped Atomic Gases

The static and dynamic behavior of dilute trapped Bose-Einstein condensates at low temperature follows from the Gross-Pitaevskii equation for the condensate and the Bogoliubov equations for the linearized small-amplitude normal modes. The uniform system serves to illustrate the theoretical methods and much of the basic physics. The principal new effect of the confining trap is to introduce an additional length scale (the size of the single-particle ground state) and energy scale (the single-particle ground-state energy). Most recent experiments use large condensates, when the repulsive interactions expand the condensate considerably and thus reduce the kinetic energy associated with the nonuniform density. In this regime (known as the "Thomas-Fermi" limit), the system can be treated as locally uniform, which greatly simplifies the analysis. When the condensate contains one or more vortex lines, the nonuniform trap potential and local line curvature drive the resulting vortex motion. Experiments have confirmed various predicted precessional motions in considerable detail. Mixtures of two distinct bosonic species allow for new coupled dynamical motions that alter the topology of the original single complex order parameter. In particular, application of near-resonant electromagnetic fields yields a coupled system that no longer has quantized circulation. Such experimental techniques created the first vortex line by spinning up one of the components. The introduction of optical traps has allowed the study of what are called "spinor" condensates. In this case, all hyperfine states are trapped, in contrast to the more common magnetic traps that confine only a subset of the various hyperfine states. The rotational invariance of the interparticle interactions significantly restricts the allowed states of these spinor condensates.

Local Vortex Generation and the Surface Mode Spectrum of Large Bose-Einstein Condensates

A combination of analytical and numerical approaches obtains the complete dispersion curve for surface excitations in a condensate held in a plane linear potential. This improvement on previous approximate results yields an accurate formula for the local Landau critical velocity for vortex formation at the surface of a sufficiently large condensate, which agrees very well with recent experiments.

Scaling issues in a submillimeter MEMS microphone

A silicon based microphone with a 0.5 mm diam diaphragm has been developed using a combination of surface and bulk micromachining or MicroElectro Mechanical Systems (MEMS) techniques. This device is intended for high volume general purpose markets such as telephony, where cost is a key factor. In batch fabricated MEMS devices, cost is critically determined by the device size. Yet the performance standards for this microphone are set by relatively large electret condenser microphones (ECM) built in competing technologies. This paper will discuss the scaling issues involved in meeting high performance goals in a very small MEMS device. In particular, tradeoffs made to achieve high sensitivity, low noise, and unconditional stability will be discussed.

Avalanches in a Bose-Einstein condensate.

Collisional avalanches are identified to be responsible for an 8-fold increase of the initial loss rate of a large (87)Rb condensate. We show that the collisional opacity of an ultracold gas exhibits a critical value. When exceeded, losses due to inelastic collisions are substantially enhanced. Under these circumstances, reaching the hydrodynamic regime in conventional Bose-Einstein condensation experiments is highly questionable.

Dense cores in the L1630 molecular cloud: discovering new protostars with SCUBA

Maps of the 450 micron and 850 micron dust continuum emission from three star-forming condensations within the Lynds 1630 molecular cloud, made with the SCUBA bolometer array, reveal the presence of four new submillimetre sources, each of a few solar masses, two of which are probably Class I, two Class 0, as well as several sources whose existence was previously known. The sources are located in filaments and appear elongated when observed at 450 microns. They likely have dust temperatures in the range 10 to 20 K, in good agreement with previous ammonia temperature estimates. Attempts to fit their structures with power-law and Gaussian density distributions suggest that the central distribution is flatter than expected for a simple singular isothermal sphere. Although the statistics are poor, our results suggest that the ratio of `protostellar core' mass to total virial mass may be similar for both large and small condensations.

Quasiperiodic versus irreversible dynamics of an optically confined Bose-Einstein condensate

We consider the evolution of a dilute Bose-Einstein condensate in an optical trap formed by a doughnut laser mode. By solving a one dimensional Gross-Pitaevskii equation and looking at the variance and the statistical entropy associated with the position of the system we can study the dynamical behavior of the system. It is shown that for small condensates nonlinear revivals of the macroscopic wave function are expected. For sufficiently large and dense condensates irreversible dynamics takes place for which revivals of regular dynamics appear as predicted in [9]. These results are confirmed by a two dimensional simulation in which the scales of energy associated with the two different directions mimic the experimental situation.

Eigenvalue Reanalysis and Condensation with General Masters

AbstractIn design optimization very often several changes of a structure are necessary to reach predetermined demands, and in each step very large eigenvalue problems have to be solved which are small modifications of each other. For large eigenvalue problems condensation methods are used to reduce the number of degrees of freedom to manageable size. In this note we take advantage of preceding computations by implementing eigenvectors of previous models as general masters into condensation.

Enumerating Large Orbits and Direct Condensation

We describe a new algorithm for direct condensation, which is a tool in computational representation theory. The crucial point for this is the enumeration of very large orbits for a group acting on some set. We present a variation of the standard orbit enumeration algorithm that reduces the amount of storage needed and behaves well under parallelization. For the special case of matrices acting on a finite vector space an efficient implementation is described. This allows us to use condensation methods for considerably larger permutation representations than could be handled before.