Pure Condensation Research Articles

A choice of pure steam vertical in-tube condensation model for simulating a passive residual heat removal system

The steam generator secondary passive residual heat removal system (SGSPRHRS) is designed with three natural circulation loops. Its thermal-hydraulic characteristics are often simulated using the RELAP5 code. A test facility is simulated with the same size as one type of SGSPRHRS using RELAP5/MOD3.2 code and it is found that in some conditions, due to the limitation of the default pure steam vertical in-tube condensation model (PSVCM) in RELAP5/MOD3.2 code, the simulation results signify cantly under-estimate the experimental data of heat exchanger (HX) water level while over-estimate the experimental data of HX outlet condensed water temperature and ascending pipe (AP) steam volumetric flow rate. To find out the PSVCM simulating condensation heat transfer for this type of SGSPRHRS more accurately, the solutions of three PSVCMs (modified UCB, Oh and Lee models) for laminar conditions and two PSVCMs (Shah and Kim models) for turbulent conditions are presented in detail and six different PSVCMs are formed from the combinations of selected PSVCMs for laminar and turbulent conditions. The six different PSVCMs are implemented into the RELAP5/MOD3.2 code and simulations of the test facility are performed using the modified RELAP5/MOD3.2 codes. By the comparisons of simulation results with the experimental data, a PSVCM (Oh model for laminar conditions and Kim model for turbulent conditions) is favored due to its most accuracy of simulating condensation heat transfer for the test facility of this type of SGSPRHRS in the six different PSVCMs.

Visualization study of condensation of ethanol–water mixtures in trapezoidal microchannels

A visualization experiment is carried out to investigate the ethanol–water vapor mixtures condensation flow patterns in an array of microchannels under a wide range of concentration (2–60%). The text microchannel is a trapezoidal silicon channel with a hydraulic diameter of 165.87μm and a length of 50mm. Along the flow direction, annular, annular-streak, annular-streak-droplet, churn, injection, droplet-injection and bubble flow patterns are observed in the vapor mixtures condensation under different inlet ethanol mass concentration (60%, 31%, 20%, 6%, 4% and 2%), which is obviously different from pure steam condensation. The equivalent surface free energy differences under various vapor conditions and vapor–surface temperature differences are calculated quantitatively. The experimental results show that flow patterns are closely related to the equivalent surface free energy differences. A correlation based on the critical quality is proposed to indicate the injection two-phase flow patterns transition.

Numerical Simulation of Laminar Film Condensation in a Horizontal Minitube with and Without Non-Condensable Gas by the VOF Method

Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without non-condensable gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a non-condensable gas will lead to the generation of a gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a non-condensable gas drops sharply compared with that of pure vapor condensation. Meanwhile, the non-condensable gas with a smaller thermal conductivity would cause a stronger negative effect on heat flux as a result of a higher thermal resistance of heat conduction in the non-condensable gas layer.

Fast production of Bose-Einstein condensates of metastable helium

We report on the Bose-Einstein condensation of metastable Helium-4 atoms using a hybrid approach, consisting of a magnetic quadrupole and a crossed optical dipole trap. In our setup we cross the phase transition with 2x10^6 atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This novel approach to cooling Helium-4 provides enhanced cycle stability, large optical access to the atoms and results in production of a condensate every 6 seconds - a factor 3 faster than the state-of-the-art. This speed-up will dramatically reduce the data acquisition time needed for the measurement of many particle correlations, made possible by the ability of metastable Helium to be detected individually.

Experimental investigation of condensation heat transfer on hybrid wettability finned tube with large amount of noncondensable gas

Condensation heat transfer outside horizontal plain and finned tubes with different surface wettability is experimentally studied. The methods of self-assembled monolayer coatings of n-octadecyl mercaptan with oxidation and etching treatments are employed to create the hydrophobic surfaces or superhydrophobic surfaces with nanostructures. The hydrophilic–hydrophobic (HH) hybrid and the hydrophilic–superhydrophobic (HS) hybrid surfaces based on the finned tubes are fabricated as well. The experimental results show that the HS hybrid finned tube achieves the highest condensation heat transfer performance in the presence of a large amount of noncondensable gas while it does not exhibit superior performance for the pure vapor condensation. The fraction of water vapor in the nitrogen–vapor mixture has significant influence on the condensation heat transfer coefficient. The dropwise condensation transforms gradually to the film condensation on both the superhydrophobic and the HS hybrid finned tubes with the increase of water vapor fraction.

Dynamics of a Bose-Einstein Condensate on Changing Speeds of an Atomchip Trap Potential

We report experimental behaviors of condensed $^{87}Rb$ atoms responding to changes in the trap potential of the atomchip. The two-types of adiabatic and non-adiabatic overall changes were implemented by changing the ramp-down speed of the chip-wire current, which can dominantly modify the one-axis magnetic field gradient. Under the adiabatic process, a pure condensate stayed in the initial spin state and collectively oscillated with both monopole and dipole modes, while an atomic cloud above the critical temperature exhibited sound waves in a dense ultracold gas. On the other hand, Bose-Einstein condensate atoms with non-adiabatic perturbation were split into spatially different positions by spin states through spin-flip. We investigated the split ratio among spin states depending on final evaporation frequency. Potential changes, of course, cause collective oscillations regardless of the changing process.

Decay of graviton condensates and their generalizations in arbitrary dimensions

Classicalons are self-bound classical field configurations, which include black holes in general relativity. In quantum theory, they are described by condensates of many soft quanta. In this work, their decay properties are studied in arbitrary dimensions. It is found that generically the decays of other classicalons are enhanced compared to pure graviton condensates, i.e. black holes. The evaporation of higher dimensional graviton condensates turns out to match Hawking radiation solely due to nonlinearites captured by the classicalon picture. Although less stable than black holes, all self-bound condensates are shown to be stable in the limit of large mass. Like for black holes, the effective coupling always scales as the inverse of the number of constituents, indicating that these systems are at critical points of quantum phase transitions. Consequences for cosmology, astro-and collider physics are briefly discussed.

Creation of 174Yb Bose—Einstein Condensates in a Crossed FORT

Quantum degenerate gases of alkaline-earth-like atoms are unique systems used for quantum simulation, quantum computing and studies of quantum phase transitions. We report an all-optical formation of Bose—Einstein condensates of ytterbium atoms. About 106 atoms of 174Yb are transferred to a far-off-resonance optical trap (FORT) and then cooled by evaporative cooling. Phase transition occurs at the critical temperature of 520 nK. A pure condensate containing approximately 2 × 104 atoms has been obtained in the crossed FORT, with an atomic peak density of ∼8 × 1014 cm−3. The condensate lifetime exceeds 1 s.

Vacuum Stability of the Wrong Sign (−ϕ 6) Scalar Field Theory

We apply the effective potential method to study the vacuum stability of the bounded from above (−ϕ 6) (unstable) quantum field potential. The stability (∂E/∂b = 0) and the mass renormalization (∂ 2 E/∂b 2 = M 2) conditions force the effective potential of this theory to be bounded from below (stable). Since bounded from below potentials are always associated with localized wave functions, the algorithm we use replaces the boundary condition applied to the wave functions in the complex contour method by two stability conditions on the effective potential obtained. To test the validity of our calculations, we show that our variational predictions can reproduce exactly the results in the literature for the \(\mathcal {PT}\)-symmetric ϕ 4 theory. We then extend the applications of the algorithm to the unstudied stability problem of the bounded from above (−ϕ 6) scalar field theory where classical analysis prohibits the existence of a stable spectrum. Concerning this, we calculated the effective potential up to first order in the couplings in d space-time dimensions. We find that a Hermitian effective theory is instable while a non-Hermitian but \(\mathcal {PT}\)-symmetric effective theory characterized by a pure imaginary vacuum condensate is stable (bounded from below) which is against the classical predictions of the instability of the theory. We assert that the work presented here represents the first calculations that advocates the stability of the (−ϕ 6) scalar potential.

Dropwise condensation on superhydrophobic nanostructured surfaces: literature review and experimental analysis

It is well established that the dropwise condensation (DWC) mode can lead up to significant enhancement in heat transfer coefficients as compared to the filmwise mode (FWC). Typically, hydrophobic surfaces are expected to promote DWC, while hydrophilic ones induce FWC. To this end, superhydrophobic surfaces, where a combination of low surface energy and surface texturing is used to enhance the hydrophobicity, have recently been proposed as a promising approach to promote dropwise condensation. An attractive feature of using superhydrophobic surfaces is to facilitate easy roll-off of the droplets as they form during condensation, thus leading to a significant improvement in the heat transfer associated with the condensation process.High droplet mobility can be obtained acting on the surface chemistry, decreasing the surface energy, and on the surface structure, obtaining a micro- or nano- superficial roughness. The first part of this paper will present a literature review of the most relevant works about DWC on superhydrophobic nanotextured substrates, with particular attention on the fabrication processes. In the second part, experimental data about DWC on superhydrophobic nanotextured samples will be analyzed. Particular attention will be paid to the effect of vapour velocity on the heat transfer. Results clearly highlight the excellent potential of nanostructured surfaces for application in flow condensation applications. However, they highlight the need to perform flow condensation experiments at realistic high temperature and saturation conditions in order to evaluate the efficacy of superhydrophobic surfaces for practically relevant pure vapor condensation applications.

Condensation of quasiparticles and density modulation beyond the superfluid critical velocity

We extend our earlier study of the ground state of a bosonic quasiparticle Hamiltonian by investigating the effect of a constant external velocity field. Below a critical velocity the ground state is a quasiparticle vacuum, corresponding to a pure superfluid phase at zero temperature. Beyond the critical velocity energy minimization leads to a macroscopic condensation of quasiparticles at a nonzero wave vector ${\mathbf{k}}_{\mathbf{v}}$ parallel to the velocity $\mathbf{v}$. Simultaneously, physical particles also undergo a condensation at ${\mathbf{k}}_{\mathbf{v}}$ and, to a smaller extent, at $\ensuremath{-}{\mathbf{k}}_{\mathbf{v}}$. Together with the Bose-Einstein condensation at $\mathbf{k}=\mathbf{0}$, the three coexisting condensates give rise to density modulations of wave vectors ${\mathbf{k}}_{\mathbf{v}}$ and $2{\mathbf{k}}_{\mathbf{v}}$. For larger $|\mathbf{v}|$ our model predicts a bifurcation of ${\mathbf{k}}_{\mathbf{v}}$ with corresponding two pure condensates and no density modulation.

Manipulation and coherence of ultra-cold atoms on a superconducting atom chip

The coherence of quantum systems is crucial to quantum information processing. Although superconducting qubits can process quantum information at microelectronics rates, it remains a challenge to preserve the coherence and therefore the quantum character of the information in these systems. An alternative is to share the tasks between different quantum platforms, for example, cold atoms storing the quantum information processed by superconducting circuits. Here we characterize the coherence of superposition states of (87)Rb atoms magnetically trapped on a superconducting atom chip. We load atoms into a persistent-current trap engineered next to a coplanar microwave resonator structure, and observe that the coherence of hyperfine ground states is preserved for several seconds. We show that large ensembles of a million of thermal atoms below 350 nK temperature and pure Bose-Einstein condensates with 3.5 × 10(5) atoms can be prepared and manipulated at the superconducting interface. This opens the path towards the rich dynamics of strong collective coupling regimes.

Generation of ultracold paraexcitons in cuprous oxide: A path toward a stable Bose-Einstein condensate

Cooling of a gas of trapped paraexcitons in Cu${}_{2}$O to 100 mK, close to the lattice temperature, is reported. We show that the activation of a transverse acoustic scattering mechanism by the applied stress is responsible for the production of ultracold excitons. This low temperature is indispensable to achieving a stable and pure Bose-Einstein condensate of three-dimensional excitons in a strongly inelastic environment.

A numerical study of the ground state and dynamics of atomic–molecular Bose–Einstein condensates

In this paper, we numerically investigate the ground-state structure and dynamics of atomic–molecular Bose–Einstein condensates at zero temperature, which are modeled by coupled Gross–Pitaevskii equations (GPEs). To get the ground state, we evolve a gradient flow with discrete normalization numerically. To study the dynamics, we employ an efficient numerical method—the time-splitting Fourier pseudospectral method for solving the coupled GPEs. The proposed numerical methods have been numerically tested and employed in studying the mechanism on how an atomic condensate can be converted into an atomic–molecular mixture or a pure molecular condensate from an atomic condensate either in equilibrium or dynamically.

Compact high-flux source of cold sodium atoms

We present a compact source of cold sodium atoms suitable for the production of quantum degenerate gases and versatile for a multi-species experiment. The magnetic field produced by permanent magnets allows to simultaneously realize a Zeeman slower and a two-dimensional magneto-optical trap (MOT) within an order of magnitude smaller length than standard sodium sources. We achieve an atomic flux exceeding 4 × 10(9) atoms/s loaded in a MOT, with a most probable longitudinal velocity of 20 m/s, and a brightness larger than 2.5 × 10(12) atoms/s/sr. This atomic source allows us to produce pure Bose-Einstein condensates with more than 10(7) atoms and a background pressure limited lifetime of 5 min.

Optimizing the efficiency of evaporative cooling in optical dipole traps

We present a combined computational and experimental study to optimize the efficiency of evaporative cooling for atoms in optical dipole traps. By employing a kinetic model of evaporation, we provide a strategy for determining the optimal relation between atom temperature, trap depth, and average trap frequency during evaporation given experimental initial conditions. We then experimentally implement a highly efficient evaporation process in an optical dipole trap, showing excellent agreement between the theory and experiment. This method has allowed the creation of pure Bose-Einstein condensates of ${}^{87}$Rb with $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms starting from only $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms initially loaded in the optical dipole trap, achieving an evaporation efficiency ${\ensuremath{\gamma}}_{\mathrm{eff}}$ of 4.0 during evaporation.

Condensation on downward-facing surfaces subjected to upstream flow of air–vapor mixture

This paper reports experimental results for film condensation on vertical and downward inclined smooth and grooved surfaces subjected to ascending streams of a vapor–air mixture. An experimental facility was built in order to evaluate the heat fluxes for different surface inclinations, surface sub-cooling temperatures and air to vapor ratios in the mixture. Two experimental procedures were employed to evaluate the heat absorption rate during the condensation process. The employment of grooved surfaces resulted in a 10% enhancement of the condensation rate in the case of the pure vapor condensation and a negligible effect when noncondensable gases (NCGs) were present in the mixture.

Mass accommodation of water: bridging the gap between molecular dynamics simulations and kinetic condensation models.

The condensational growth of submicrometer aerosol particles to climate relevant sizes is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. We have simulated the mass accommodation and evaporation processes of water using molecular dynamics, and the results are compared to the condensation equations derived from the kinetic gas theory to shed light on the compatibility of the two. Molecular dynamics simulations were performed for a planar TIP4P-Ew water surface at four temperatures in the range 268–300 K as well as two droplets, with radii of 1.92 and 4.14 nm at T = 273.15 K. The evaporation flux from molecular dynamics was found to be in good qualitative agreement with that predicted by the simple kinetic condensation equations. Water droplet growth was also modeled with the kinetic multilayer model KM-GAP of Shiraiwa et al. [Atmos. Chem. Phys.2012, 117, 2777]. It was found that, due to the fast transport across the interface, the growth of a pure water droplet is controlled by gas phase diffusion. These facts indicate that the simple kinetic treatment is sufficient in describing pure water condensation and evaporation. The droplet size was found to have minimal effect on the value of the mass accommodation coefficient. The mass accommodation coefficient was found to be unity (within 0.004) for all studied surfaces, which is in agreement with previous simulation work. Additionally, the simulated evaporation fluxes imply that the evaporation coefficient is also unity. Comparing the evaporation rates of the mass accommodation and evaporation simulations indicated that the high collision flux, corresponding to high supersaturation, present in typical molecular dynamics mass accommodation simulations can under certain conditions lead to an increase in the evaporation rate. Consequently, in such situations the mass accommodation coefficient can be overestimated, but in the present cases the corrected values were still close to unity with the lowest value at ≈0.99.

New volume translated PR equation of state for pure compounds and gas condensate systems

The purpose of this paper is to improve the Peng–Robinson equation of state (PR EOS 1978) prediction of saturated liquid density of pure compounds and gas condensate systems by volume translation method. Thus, a temperature-dependent volume translation factor has been proposed. Also, a new mixing rule for volume translation factor of mixtures has been proposed. The new model requires only adjusted critical compressibility factor for each compound which has been determined and reported for 29 compounds. No binary interaction coefficients kij are used.The average absolute percent deviation (AAPD) of predicted liquid densities has been calculated in the reduced temperature range (0.3–0.99) for the pure compounds of the alkanes (methane to eicosane), ethylene, propylene, carbon dioxide, carbon monoxide, hydrogen sulfide, nitrogen, and ammonia. The AAPD of PR EOS for aforementioned pure compounds is 8.40%. By using the proposed volume translation model, the AAPD reduced to 1.11 and 1.29 for the optimized parameters and the generalized correlations, respectively. Moreover, the AAPD for 62 different systems of binary, ternary, and up to eight-component systems has been calculated. The AAPD of PR EOS for these mixtures is 7.26%; however, using the proposed model reduced this value to 1.17 and 1.19 for the optimized parameters and the generalized correlations, respectively.

Electrical properties, structure, and surface morphology of poly(p-xylylene)–silver nanocomposites synthesized by low-temperature vapor deposition polymerization

The crystalline structure, surface morphology, electrical, and optical properties of thin films of nanocomposites consisting of silver nanoparticles embedded in poly(p-xylylene) matrix prepared by low-temperature vapor deposition polymerization were studied. Depending on the filler content, the average size of silver nanoparticles varied from 2 to 5 nm for nanocomposites with 2 and 12 vol.% of silver, correspondingly. The optical adsorption in the visible region due to surface plasmon resonance also exhibited a clear correlation from silver content, revealing a red shift of the adsorption peak with the increase of the metal concentration. The temperature dependences of the dc resistance of pure p-xylylene condensate and p-xylylene–silver cocondensates during polymerization as well as temperature dependences of the formed poly(p-xylylene)–silver nanocomposites were examined. The observed variation of the temperature dependences of electrical resistance as a function of silver concentration are attributed to different conduction mechanisms and correlated with the structure of the composites. The wide-angle X-ray scattering and AFM measurements consistently show a strong effect of silver content on the nanocomposite structure. The evolution of the size of silver nanoparticles by thermal annealing was demonstrated.