Lambda Line Research Articles

A numerical study on the effect of thermal and charge stratification on the HCCI natural gas engine

The applications of homogeneous charge compression ignition (HCCI) engine are still limited due to the absence of a definite means to control ignition timing and narrow the operating region. It was found that thermal stratification and charge stratification strategies bring forward good potential to achieve the staged combustion with reduced pressure-rise rates. In this study, a single cylinder engine and a CFD simulation are applied to investigate the combustion processes in the stratified HCCI engine. To obtain thermal and charge stratification on the combustion chamber, exhaust gas recirculation (EGR), glow plug and simulated reformer gas (RG) of CO–H2 mixture are used. Increasing EGR at constant lambda line decreases the maximum achievable power, while at the constant increasing lambda at each constant EGR decreases the indicated mean effective pressure. EGR expands combustion duration hence smoother and quieter operation is achieved. Increasing reformer gas blend fraction considerably expands the operating region toward the lean side. Advancing combustion timing at constant energy flow and intake temperature stem from using glow plug in the combustion chamber, while with increasing mean in-cylinder temperature, combustion duration is prolonged. Abbreviations: 3D: Three-Dimensional; CAD: Crank angle degree; CI: Compression ignition; EGR: Exhaust gas recirculated; HCCI: Homogenous charge compression ignition; NOx: Nitrogen oxides; PM: Particular matter; PRFs: primary reference fuels; RG: Reformer gas; rpm: Revolution per minute; SI: Spark ignition; TB: Third body; THC: total hydrocarbon; UHC: Unburned hydrocarbon

Phase Transition In Liquid 4He by a Mean Field Model

In this work the transition of 4He at the lambda line in presence of a Cattaneo-Maxwell heat flux is studied. A hydrodynamical model is formulated, which chooses as fundamental fields the velocity, the temperature, the heat flux and a phase field function f, for which a time dependent Ginzburg-Landau equation is proposed. Using this model we are able to describe the phase transition and to obtain the pressure-temperature phase diagram which represents the transition, the thermodynamic restrictions and a maximum theorem for the phase field.

Parallel-plate capacitor measurements of the superfluid wall-film thickness in a 3He/4He mixture of 3He mole fraction x = 0.75

A thin 4He-rich superfluid film coats solid surfaces in contact with bulk normal liquid 3He/4He mixtures in a narrow region of the mixture phase diagram at saturated vapor pressure that adjoins the lambda line, the tricritical point, and the 3He-rich branch of the phase-separation curve. Recent arguments suggest that near the tricritical point, the thickness of the film is governed not only by van der Waals forces and gravity but also by Casimir and Helfrich forces that limit the thickness as the phase-separation curve is approached. Experimental evidence has been presented both showing and failing to show this phenomenon.In the present work, parallel-plate capacitor measurements of a mixture having a 3He mole fraction x = 0.75 show a film thickness of from ∼ 14 to ∼ 22 nm as the phase-separation curve is first reached from higher temperature, depending on choice of background subtraction, whereas van der Waals forces limited only by gravity would be expected in our cell to yield a thickness of ∼ 31 nm, rising in the first few mK below the phase-separation temperature to ∼ 43 nm. Despite various uncertainties and anomalies, the results suggest that additional forces may be at work.

The Use of Cryogenic Helium for Classical Turbulence: Promises and Hurdles

Fluid turbulence is a paradigm for non-linear systems with many degrees of freedom and important in numerous applications. Because the analytical understanding of the equations of motion is poor, experiments and, lately, direct numerical simulations of the equations of motion, have been fundamental to making progress. In this vein, a concerted experimental effort has been made to take advantage of the unique properties of liquid and gaseous helium at low temperatures near or below the critical point. We discuss the promise and impact of results from recent helium experiments and identify the current technical barriers which can perhaps be removed by low temperature researchers. We focus mainly on classical flows that utilize helium above the lambda line, but touch on those aspects below that exhibit quasi-classical behavior.

Type-IV phase behavior in fluids with an internal degree of freedom

We have identified a fourth archetype of phase diagram in binary symmetrical mixtures, which is encountered when the ratio of the interaction between the unlike and the like particles is sufficiently small. This type of phase diagram is characterized by the fact that the lambda line (i.e., the line of the second-order demixing transition) intersects the first-order liquid-vapor curve at densities smaller than the liquid-vapor critical density.

Thermodynamic Properties of Superfluid 4He at Negative Pressure

We calculate the thermodynamics of superfluid 4He at negative pressures. We use the Landau theory in which thermodynamic properties are expressed as sums over the thermal distribution of elementary excitations. The excitation dispersion curve is taken from the density functional theory developed by Dalfovo et al. To give a good description of the liquid near to the lambda line, we include the interaction between the elementary excitations using a modification of the roton liquid theory of Bedell, Pines, and Fomin. The calculated quantities include the location of the lambda line, the liquid-vapor spinodal, and lines of constant entropy in the pressure-temperature (P-T) plane. We have also calculated the line of minimum density (zero expansion coefficient) in theP-T plane. This joins the lambda line tangentially at about −5.3 bars. Using the critical properties near the lambda line, we find that the line of maximum density in He I also joins the lambda line tangentially at this pressure. For use in cavitation experiments, we have calculated the states on the lambda line reached by isentropic expansion from He I.

Thermodynamically self-consistent theory for the Blume-Capel model.

We use a self-consistent Ornstein-Zernike approximation to study the Blume-Capel ferromagnet on three-dimensional lattices. The correlation functions and the thermodynamics are obtained from the solution of two coupled partial differential equations. The theory provides a comprehensive and accurate description of the phase diagram in all regions, including the wing boundaries in a nonzero magnetic field. In particular, the coordinates of the tricritical point are in very good agreement with the best estimates from simulation or series expansion. Numerical and analytical analysis strongly suggest that the theory predicts a universal Ising-like critical behavior along the lambda line and the wing critical lines, and a tricritical behavior governed by mean-field exponents.

Superfluid density as a function of temperature and pressure near the lambda line

We report new results for the superfluid density in the region close to the lambda line at pressures up to 25 bars. The data were obtained from second sound velocity measurements made to within 0.9 μK of the transition by a resonance technique. A superconducting pressure gauge was used to allow the measurements to be performed along a constant pressure path. From these data the temperature and pressure dependence of the superfluid density can be derived to test for critical exponent universality. We also report the temperature and pressure dependence of the resonator quality factor.

Lifshitz point in the phase diagram of a ferroelectric liquid crystal in an external magnetic field

The Lifshitz point in the (H,T) phase diagram of a ferroelectric liquid crystal in a transverse magnetic field has been determined using photon correlation spectroscopy and linear electrooptic response measurements. The extrapolated Lifshitz magnetic field is 25(1+/-0.1) T for a smectic-C* material with an unperturbed helical period of approximately 6 &mgr;m, and the Lifshitz point is located approximately 100 mK above the zero-field transition temperature. We have observed the reentrant helical modulated smectic-C* phase just below the lambda line. The width of this phase is 100 mK at the low temperature critical field H(c) and decreases continuously to zero as we approach the Lifshitz point. The phase boundary between the reentrant phase and the smectic-A phase is of second order, whereas the phase boundary with the unwound smectic-&Cmacr;(*) phase is of first order far away from the Lifshitz point and becomes of second order close to the Lifshitz point. The order parameter dynamics is discussed within the Landau theory.

Temperature optimization for superconducting cavities

Since our previous analysis of optimized operating temperature of superconducting cavities in an accelerator a decade ago, significant additional information has been discovered about SRF cavities. The most significant is the Q/sub 0/ (quality factor) shift across the Lambda line at higher gradients as a result of a slope in Q/sub 0/ vs. E/sub acc/ above Lambda. This is a result of the changing heat conduction conditions. We discuss temperature optimizations as a function of gradient and frequency. The refrigeration hardware impacts and changes in cycle efficiency are presented.

Tricritical Point and Superfluid Transition in3He−4HeMixtures in Silica Aerogel

We have ultrasonically studied ${}^{3}\mathrm{He}{\ensuremath{-}}^{4}\mathrm{He}$ mixtures in 87% porosity aerogel. We find only a single transition for ${}^{3}\mathrm{He}$-rich mixtures, in contrast to the detached phase separation curve seen in 98% porosity aerogel. However, it is a continuous transition, much like the superfluid transition seen in ${}^{4}\mathrm{He}$ films on aerogels, rather than a first-order phase separation as seen in bulk mixtures. A kink occurs in the lambda line of the phase diagram at a ${}^{3}\mathrm{He}$ concentration (corrected for the ${}^{4}\mathrm{He}$ inert layer) of ${X}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.51$ and is accompanied by a change in the coupling to the superfluid component, indicating two distinct superfluid morphologies.

Testing critical point universality along the λ-line

We are currently building a prototype for a new test of critical-point universality at the lambda transition in 4He, which is to be performed in microgravity conditions. The flight experiment will measure the second-sound velocity as a function of temperature at pressures from 1 to 30 bars in the region close to the lambda line. The critical exponents and other parameters characterizing the behavior of the superfluid density will be determined from the measurements. The microgravity measurements will be quite extensive, probably taking 30 days to complete. In addition to the superfluid density, some measurements of the specific heat will be made using the low-g simulator at the Jet Propulsion Laboratory. The results of the superfluid density and specific heat measurements will be used to compare the asymptotic exponents and other universal aspects of the superfluid density with the theoretical predictions currently established by renormalization group techniques.

The Tricritical Region for Helium Mixtures in 0.87 Porosity Aerogel

We have used ultrasonic velocity measurements to study 3 He- 4 He mixtures in aerogel with a porosity 0.87. The phase diagram resembles that of bulk mixtures, with a single transition for 3 He-rich mixtures, in contrast to the detached phase separation curve seen in 0.98 porosity aerogel. A kink in the lambda line at a 3He concentration of X C =0.51 suggests that the phase separation line meets it at a tricritical point. We have measured the amount of superfluid which decouples both at low temperature and close to the superfluid transition, as functions of 3 He concentration. Each showed a sudden change at the concentration where the kink appeared in the lambda line, suggesting an abrupt change in the morphology of the superfluid phase in the mixtures. Similar measurements were made for pure 4He films on the same aerogel. We discuss the nature of 3 He-rich mixtures in aerogels based on these experiments.

Study of the minimum refrigeration temperature of regenerative cryocoolers

Based on the principles of thermodynamics, the relation between the isentropic expansion coefficient μ s and the isobaric specific heat C p has been found. The values of μ s at different temperatures and pressures are calculated. From theoretical and experimental values of C p of helium in the supercritical area, the minimum refrigeration temperature which may be reached by traditional regenerative cryocoolers is theoretically demonstrated. It is pointed out that this boundary is just the lambda line of helium. In order to obtain temperatures lower than the lambda line, a new type of regenerative cryocooler which can work in the helium II region is proposed. The working principle, schematic diagram and theoretical refrigeration temperature of this new cryocooler are discussed.

Approaching the 4He lambda line with a liquid nitrogen precooled two-stage pulse tube refrigerator

We report on the design and test of a three-stage refrigerator, consisting of a liquid nitrogen bath with heat exchangers and two subsequent pulse tube stages. The liquid nitrogen bath serves for precooling the oscillating 4He-gas flow. The PTR stages are operated in orifice and double-inlet mode with the orifice valves, reservoirs and second-inlet valves located at ambient temperature. Er 3Ni shot is used for the regenerator matrix in the coldest stage. A minimum temperature of 2.13(1) K has been achieved, which, at the average operating pressure of 19.5 bar, is only 0.19 K above the λ-line for the superfluid phase transition of 4He.

H-alpha emission and absorption from a localized region above the surface of a Be star

view Abstract Citations (2) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS H alpha Emission and Absorption from a Localized Region above the Surface of a Be Star Marlborough, J. M. Abstract The appearance and disappearance of transient spectral features in the He I line lambda(6678) in the Be star lambda Eri is well established. According to one suggestion these transients arise in a local concentration of matter above the stellar photosphere. In this paper a highly simplified model for the localized region is chosen, and the resultant H-alpha profile from the stellar photosphere and the localized region is obtained for a variety of parameters chosen to span the range of these parameters inferred from analysis of the He I lambda(6678) line. For all the cases investigated the additional H-alpha emission and/or absorption is too weak to be detected easily with present techniques. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175271 Bibcode: 1995ApJ...440..303M Keywords: Absorption Spectra; B Stars; Emission Spectra; H Alpha Line; Stellar Envelopes; Stellar Models; Stellar Spectra; Astronomical Spectroscopy; Helium; Mathematical Models; Photosphere; Stellar Atmospheres; Visible Spectrum; Astronomy; STARS: CIRCUMSTELLAR MATTER; STARS: EARLY-TYPE; STARS: EMISSION-LINE; BE full text sources ADS | data products SIMBAD (1)

Vortex creation in a fast adiabatic expansion through the lambda transition

Copious vortex line creation in a fast (∼ 3 ms) adiabatic expansion of a volume of liquid 4He through the lambda transition has been observed. The maximum line density, inferred from measurements of second sound attenuation, is found to be much larger than that in a comparable expansion from just below the lambda line. The phenomenon was predicted by Zurek who proposed it as a model of cosmic string creation in the false vacuum/true vacuum phase transition of the early universe.

A cluster variation approach to the random-anisotropy Blume-Emery-Griffiths model

The random-anisotropy Blume-Emery-Griffiths model, which has been proposed to describe the critical behaviour of 3He-4He mixtures in a porous medium, is studied in the pair approximation of the cluster variation method extended to disordered systems. Several new features, with respect to mean-field theory, are found, including a rich ground state, a non-zero percolation threshold, a reentrant coexistence curve and a miscibility gap on the high-3He-concentration side down to zero temperature. Furthermore, nearest-neighbour correlations are introduced into the random distribution of the anisotropy, and are shown to be responsible for the raising of the critical temperature with respect to the pure and uncorrelated random cases and to contribute to the detachment of the coexistence curve from the lambda line.

Superfluid density from heat-pulse propagation near the lambda line in 4He-aerogel systems.

We determined the superfluid density ${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$ from the speed of heat pulses, using a hydrodynamic model that contains coupling between the aerogel and the helium. To investigate the universality of the exponent of ${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$, measurements of ${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$ were taken from saturated vapor pressure to 29 bars. Fitting ${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$ with a pure power law in reduced temperature leads to a pressure-dependent exponent. Inclusion of a confluent singular term in the fits reveals a pressure-independent exponent \ensuremath{\zeta}=0.755\ifmmode\pm\else\textpm\fi{}0.003 which differs from \ensuremath{\zeta}=0.672 for bulk helium.

State equation of liquid helium-4 from 0.8 to 2.5 K

A state equation for liquid helium is constructed in the range from 0.8 K to about 2.5 K, based on density and temperature as independent parameters. The equation has been fitted to experimental PVT, specific heat, ({partial derivative}P/{partial derivative}T){sub v}, sound velocity, and lambda line properties from 14 different authors to an accuracy comparable with reasonable experimental errors in the measured quantities. Inclusion of logarithmic terms leads to agreement with experimental data as close as 100 {mu}K from the lambda line. It is found that the logarithmic amplitude ratio is not constant as a function of distance along the lambda line. A new determination of lambda line density is also presented. The equation is available in computer form.