Experimental analysis of the thermal energy storage potential of a phase change material embedded in additively manufactured lattice structures

Recent literature introduced novel additively manufactured porous metallic structures designed to deliver enhancement of the thermal conductivity of organic phase change materials. Among these, so-called lattice structures are of particular interest for application in lightweight components. Originally investigated for their attractive mass-specific mechanical properties, these geometries were recently proposed, in alternative to metallic foams, as a conductive matrix of phase change materials. However, the geometrical parameters of a lattice structure differ from the ones of a metallic foam and no established data exist in the literature about the influence of the lattice cell geometry on the transient heat transfer enhancement of a phase change material. This work presents an experimental comparison of the thermal behaviour of a composite based on an n-Octadecane paraffin wax embedded in four different aluminium lattices with varying unit cell topology (f2ccz, bcc, bccz, f2bcc), but showing the same cell size, aspect ratio and strut diameter. It is noticed that the unit cell topology affects the transient thermal behaviour beyond its direct effect on the cell porosity. To address this, a specific thermal performance parameter is identified. The sample based on the f2ccz topology represents the best candidate. The influence of orientation with respect to gravity on the heat transfer is also investigated. While the samples show relatively low porosity, ranging from 70.7% to 83.4%, the wide pore diameters lead to a high impact of melt convection on the thermal behaviour of most samples. Up to 28% wall temperature variation is evidenced for different heating orientations

Renormalization group approach of itinerant electron systems near the Lifshitz point

Using the renormalization approach proposed by Millis for the itinerant electron systems we calculated the specific heat coefficient $\gamma(T)$ for the magnetic fluctuations with susceptibility $\chi^{-1}\sim |\delta+\omega|^\alpha+f(q)$ near the Lifshitz point. The constant value obtained for $\alpha=4/5$ and the logarithmic temperature dependence, specific for the non-Fermi behavior, have been obtained in agreement with the experimental dat.Comment: 6 pages, Revte

The violent past of Cygnus X-2

Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). Using Mazzitelli's (1989) stellar code we compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions for convective overshooting a case B mass transfer starting with a 3.5 M_sun donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate > 1e-7 - 1e-6 per yr of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (> 4 M_sun) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity < 200 km/s at the birth of the neutron star. The simulations suggests that about 7% of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10% of them reach yet larger Galactocentric distances.Comment: 9 pages, 12 figures, accepted for publication in MNRA

Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275

Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been combined with HST WFPC2 photometry to explore the nature and origin of these objects and discriminate between merger and cooling flow scenarios for globular cluster formation. The objects we have studied are not HII regions, but rather star clusters, yet their integrated spectral properties do not resemble young or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The clusters' Balmer absorption appears to be too strong to be consistent with any of the standard Bruzual & Charlot evolutionary models at any metallicity. If these models are adopted, an IMF which is skewed to high masses provides a better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces the observed Balmer equivalent widths and colors at about 450 Myr. Formation in a continuous cooling flow appears to be ruled out since the age of the clusters is much larger than the cooling time, the spatial scale of the clusters is much smaller than the cooling flow radius, and the deduced star formation rate in the cooling flow favors a steep rather than a flat IMF. A merger would have to produce clusters only in the central few kpc, presumably from gas in the merging galaxies which was channeled rapidly to the center. Widespread shocks in merging galaxies cannot have produced these clusters. If these objects are confirmed to have a relatively flat, or truncated, IMF it is unclear whether or not they will evolve into objects we would regard as bona fide globular clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to appear in the AJ (August issue), also available at http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution Fig.1) Revised Version: previous posted version was an uncorrect ealier iteration, parts of the text, tables and figures changed. The overall conclusions remain unchange

The cooling of atomic and molecular gas in DR21

We present an overview of a high-mass star formation region through the major (sub-)mm, and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud. We used the KOSMA 3m telescope to map the core ($10'\times 14'$) of the Galactic star forming region DR 21/DR 21 (OH) in the Cygnus X region in the two fine structure lines of atomic carbon CI and four mid-$J$ transitions of CO and $^{13}$CO, and CS J=7\TO6. These observations have been combined with FCRAO J=1\TO0 observations of $^{13}$CO and C$^{18}$O. Five positions, including DR21, DR21 (OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the \OI 63 and 145 $\mu$m lines, the \CII 158 $\mu$m line, and four high-$J$ CO lines. We discuss the intensities and line ratios at these positions and apply Local Thermal Equilibrium (LTE) and non-LTE analysis methods in order to derive physical parameters such as masses, densities and temperatures. The CO line emission has been modeled up to J=20. From non-LTE modeling of the low- to high-$J$ CO lines we identify two gas components, a cold one at temperatures of T_\RM{kin}\sim 30-40 K, and one with T_\RM{kin}\sim 80-150 K at a local clump density of about n(H$_2$)$\sim 10^4-10^6$ cm$^{-3}$. While the cold quiescent component is massive containing typically more than 94 % of the mass, the warm, dense, and turbulent gas is dominated by mid- and high-$J$ CO line emission and its large line widths. The medium must be clumpy with a volume-filling of a few percent. The CO lines are found to be important for the cooling of the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the UV-heated source DR21, the gas cooling is dominated by line emission of atomic oxygen and of CO

The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB

We have investigated the dynamical interaction of low- and-intermediate mass stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of 20,000 yr, and consequently do not remain recorded in the density or velocity structure of the gas. The formation of shells that survive at the end of the AGB occurs via two main processes: shocks between the shells formed by two consecutive enhancements of the mass-loss or via continuous accumulation of the material ejected by the star in the interaction region with the ISM. Our models show that the mass of the circumstellar envelope increases appreciably due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar model). We also point out the importance of the ISM on the deceleration and compression of the external shells. According to our simulations, large regions (up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars are expected. These large regions of gas are formed from the mass-loss experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical Journa