Maximum Central Temperature Research Articles

A robust, compatible, and effective core-shell structured phase change materials for controlling the temperature rise caused by cement hydration

Temperature cracks caused by the internal and external temperature gradient pose a threat to the durability of concrete structures. Phase change materials (PCM) offer a feasible solution by absorbing the released heat of cement hydration, yet of which the applications are limited by potential leakage and weak combination with cement matrix. Herein, this study employed silica as a protective shell to inhibit core PCM (n-octadecane, n-OD) from leaking, and enhance the compatibility with Portland cement. The surveyed core-to-shell mass ratios of PCM were chosen as 1:1, 1.5:1, and 2:1. The encapsulation of n-OD was successfully demonstrated by characterization results of Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microcapsules labeled M3 possessed the highest melting enthalpy of 183.4 J/g due to the highest core-to-shell ratio of 2:1 and exhibited good physico-chemical stability that pertained integrity after the mechanical mixing into cement paste. The effect of complexing M3 into cement paste was further investigated. The increasing dosage of M3 evidently remediated temperature rise that with highest dosage of 30.0 vol%, the maximum central temperature of cement paste decreased by 16.9 °C compared to the reference group. Moreover, it was unravelled that M3 components promote cement hydration and pore filling, thus with such high dosage of 30.0 vol%, the cement sample strength mildly decreased by 22.0%, which is dramatically lower compared with previous studies, i.e., at least ∼58.4% decrease in strength with similar dosage. Hence, the as-prepared M3 microcapsules can serve as promising functional additives for controlling the internal temperature rise of cement-based materials to mitigate the formation of temperature cracks.

Temperature development and cracking characteristics of high strength concrete slab at early age

High-strength concrete (HSC) generally is made with high amount of cement which may release large amount of hydration heat at early age. The hydration heat will increase the internal temperature of slab and may cause potential cracking. In this study, slab specimens with a dimension of 600 ✕ 600 ✕ 100 mm were cast with concrete incorporating silica fume for test. The thermistors were embedded in the slabs therein to investigate the interior temperature development. The test variables include water-to-binder ratio (0.25, 0.35, 0.40), the cement replacement ratio of silica fume (RSF; 5 %, 10 %, 15 %) and fly ash (RFA; 10 %, 20 %, 30 %). Test results show that reducing the W/B ratio of HSC will enhance the temperature of first heat peak by hydration. The increase of W/B decrease the appearance time of second heat peak, but increase the corresponding maximum temperature. Increase the RSF or decrease the RFA may decrease the appearance time of second heat peak and increase the maximum central temperature of slab. HSC slab with the range of W/B ratio of 0.25 to 0.40 may occur cracking within 4 hours after casting. Reducing W/B may lead to intensive cracking damage, such as more crack number, and larger crack width and length.

Production of nanocrystalline Y 2O 3:Eu powder by microwave plasma-torch and its characterization

Y 2O 3:Eu red-emitting nanophosphors were synthesized directly by dissolving Y 2O 3 and Eu 2O 3 in HNO 3 and by using an atmospheric microwave-plasma torch for a direct continuous preparation and for mass production of Y 2O 3:Eu powders. Atmospheric microwave plasmas produce a high-temperature plasma flame due to ohmic heating by high frequency electric fields, revealing a maximum central temperature of 6500 ± 350 K. Therefore, the synthesized Y 2O 3:Eu nanophosphors from instantaneous evaporation of dissolved Y 2O 3 and Eu 2O 3 in HNO 3 have favorable characteristics such as regular size and cubic equilibrium structure. The nanophosphors were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL).

Assessing `Bing' Sweet Cherry Tolerance to a Heated Controlled Atmosphere for Insect Pest Control

Sweet cherries (Prunus avium `Bing') exposed to 113 or 117 °F (45 or 47 °C) in an atmosphere of 1% oxygen with 15% carbon dioxide (balance nitrogen) were heated to a maximum center temperature of 112 or 115 °F (44 or 46 °C) in 41 or 27 min, respectively. Heated cherries had similar incidence of pitting and decay, and similar preference ratings after 14 days of storage at 34 °F (1 °C) as nonheated or methyl bromide fumigated fruit. Heated cherries and methyl bromide fumigated cherries were less firm after 14 days of cold storage than nonheated, control fruit. The stems of methyl bromide fumigated cherries were less green than heated or nonheated cherries. Cherries exposed to 113 °F had lower titratable acidity than nonheated cherries, fumigated cherries, or cherries exposed to 117 °F. Cherry quality after 14 days of cold storage was not affected by hydrocooling before heating (5 min in water at 34 °F) or by method of cooling after heating (hydrocooling, forced air cooling, or static air cooling). Cherries stored for 14 days at 34 °F in 6% oxygen with 17% carbon dioxide (balance nitrogen) had similar market quality as cherries stored in air at 34 °F. Results suggest that `Bing' sweet cherry can tolerate heating in an atmosphere of low oxygen containing elevated carbon dioxide at doses that may provide quarantine security against codling moth (Cydia pomonella) and western cherry fruit fly (Rhagoletis cingulata).

Some constraints on quasar progenitors and quasar evolution

There is strong evidence that quasars are powered by the gravitational energy released by in fall of matter into a compact supermassive object which is most likely a black hole with mass M>106 to 109MO. The question then arises as to how such massive black holes came to be formed. What sort of objects were the progenitors? While looking for an answer we have to bear in mind another bit of intriguing evidence that even high Z quasars show lines of heavier elements (Oxygen, magnesium, etc) with solar abundances, strongly suggesting that matter had already undergone nuclear processing at even earlier epochs. Again even the oldest stars in our galaxy show evidence of metal content. All this suggests that atleast part of the metal content could have been generated by pregalactic supermassive stars, sometimes referred to as Pop.III objects. Such stars with masses ranging from 102–106 M⊙ could have formed in clusters consisting of about a few hundred or thousand of these objects at Z ⋍ 10. Now objects >106 M⊙ would have central temperatures not high enough for nuclear reactions the reason being a well known instability of general relativistic origin which is reached when GM/R⋍9c2/16 (M⊙/M)½; MO ⋍M⊙ giving rise to a maximum central temperature Tc reached at the instability, i.e. just before collapse. This is given by Tc = 2×1013/(M/MO) giving Tmax = 107K (necessary for CNO reactions) for M ⋍ 106M⊙ and for M> 106 M⊙, Tc is too low. It is believed that 10−5 of the primordial material might have been processed in Pop III objects which would have undergone nuclear reactions and evolved over some fraction of a Salpeter time, cσT/4πGmp ⋍ 108 yrs; (σT = Thompson cross section, mp is proton mass)., producing heavier elements in the process. It turns out that objects of mass M <300 M⊙ can explode and scatter the heavy elements. For instance a 116 M⊙ oxgen core of a 200MO object can completely disrupt and the evolution of such objects can explain anomalies like the O/Fe enrichment in metal poor stars and the G dwarf problem. Again there can be substantial mass loss from these superstars (a typical empirical relation like M ⋍ (tD/tKH)¼. L/GM/R (where tD is the dynamical time, and tKH is the Kelvin-Helmholtz scale, L is the luminosity) predicting · ≈ 10−3MO/yr) again leading to enrichment of the medium. Stars >300 MO do not explode but would collapse. The relaxation time scale for a cluster of these objects is again 5×108 to 109 yrs, so that they would all collapse to form a central black hole of M ∼ 108 −109 MO, the surrounding earlier ejected matter enriched in heavier elements being now accreted onto it. There would have been atleast a few times 108 of these black holes formed. Assuming near Eddington luminosity the rate of accretion given by ·(t) ⋍ 8πGM(t)/kc⋍16πG2M2(t)X ρ(t)V/c4, ρ(t) being a function of Z) would put a constraint on when Pop III stars formed and accreted enough matter to trigger quasar activity as quasars do not seem to be present before z = 4. This gives the result that they could not have formed before z ⋍8–10. Another possibility of forming a central black hole is by the collapse of a dense star cluster (consisting mainly of neutron stars or white dwarfs), the central relaxation time is tR∼VC3 / mc21n(0.4 Nc) ∼109yr. One obtains the result that for a core of compact stars to evolve to a relativistic state in a Hubble time it should have a minimal velocity dispersion given by Vc (min)⋍ 103(mc) (1-A/B) (7-3A/B)km/s, A, B are constants. This corresponds to clusters roughly having 107–109 compact stars and the gravitational collapse of such a cluster would then lead to black holes in the required mass range. An observed example is the central 3.5 pc core of NGC 4151 which has a mean stellar density of 2×108 M pc−3 corresponding to V ⋍ 103 km/s. As the central black hole grows the relative tidal force decreases and quasar activity can stop when the hole can no longer break up stars. (Tidal break up is also producing and scattering heavy elements.) For a central black hole accreting white dwarfs M <106MO and for it to break up neutron stars M <2 · 103MO. The break up of a neutron star by such a hole would give a burst of energy ∼1052ergs/sec which has so far not been recorded in any quasar or AGN, the upper limit to the luminosity being ∼1049 ergs/s. This may be indirect evidence that the mass of the central black hole is well above 104 MO in all cases. It is a pleasure to thank Professor Martin Rees for valuable discussions.

COOKING METHODS FOR ELIMINATION OF Salmonella typhimurium EXPERIMENTAL SURFACE CONTAMINANT FROM RARE DRY‐ROASTED BEEF ROASTS

ABSTRACTThree cooking procedures were tested for effectiveness in eliminating an experimental Salmonella typhimurium contaminant from surfaces of rare dry‐roasted beef roasts. Dipping roasted and cooled roasts in cooking oil at 160°C (320 F) or 180°C (365°F) for a minimum of 60 set was effective, but submerging similar roasts contained in plastic bags in 89.4–93.3°C (193–200°F) water for 3 min was not effective in eliminating surface survivors. Injection of steam into the oven during part of the roasting period also was effective. We found that a minimum of 10 min of steam injection was necessary to eliminate the contaminant. Experiments with steam injection at the beginning or end of roasting led to the conclusion that survivors on surfaces of dry‐roasted beef roasts were probably on the surface at the beginning of the roasting process. Subjective evaluation of the degree of rareness of center slices of roasts reaching maximum center temperatures between 54.4–64.1°C (130–147.5°F) indicated the rare area decreased about 2% for each degree increase in temperature. Roast center temperatures at time of removal from the oven correlated significantly with the maximum center temperature.

ALUMINUM‐26 AS A PLANETOID HEAT SOURCE IN THE EARLY SOLAR SYSTEM

The results of thermal model calculations which assume 26Al as a heat source are presented. The relation between 26Al content, the maximum central temperature obtained, and the time interval after formation until central cooling commences is elucidated. Because of the heating times required, these results constrain maximum permissible planetoid dimensions more severely than do previous calculations which assume a high initial temperature.

Effects of temperature and pressure gradients on catalyst pellet effectiveness factors—I

The dusty gas model is used to establish the effects of temperature and pressure gradients on catalyst pellet effectiveness factors for reaction systems in which species molecular weights and transport coefficients are indistinguishable and Σν i = 0. For this class of reactions, the total molar flux of species i is shown to be expressible simply as N i = − cD ∇ϰ i in terms of the molar concentration, the Bosanguet diffusivity, and the mole fraction gradient. The effects of temperature and pressure gradients are reflected only in variations in molar concentration and diffusivity. Furthermore, the temperature-pressure relationship is shown to be given by the thermal transpiration equation for a pure gas. Typical numerical results are reported for first order reactions in spherical pellets under diffusive conditions ranging from the Knudsen through the bulk diffusion regimes. The variation in diffusion regime is shown to be controlled by an additional parameter α, the Knudsen to bulk diffusion ratio. Comparisons are made with the classical Weisz-Hicks nonisothermal pellet solutions based on N i = − D eff∇ c i. For highly exothermic reactions, effectiveness factors are 18% lower in the Knudsen regime and 30% higher in the bulk diffusion regime than are the Weisz—Hicks values. For highly endothermic reactions with a significant diffusion limitation, the effectiveness factors are 30% lower than the Weisz—Hicks values. The classical Damköehler relationship for pellet temperature rise is shown to apply in the Knudsen regime, with the maximum dimensionless center temperature given by (1 + β), where β is the heat of reaction parameter. This temperature is accompanied by a maximum dimensionless center pressure of (1 + β) 1 2 . In the bulk diffusion regime, the maximum center temperature is shown to be increased by the additional term β 2/4. This additional temperature rise accounts for the 42% increased bulk diffusion effectiveness for highly exothermic reactions.

Temperature distribution inside a burning cigarette

THE study reported here has been undertaken to resolve the large discrepancies between reports of the temperature distribution in the combustion coal of a cigarette. Studies using bare thermocouples (refs 1–4 and R. G. Hook, paper presented at Twentieth Tobacco Chemists' Conference, Winston-Salem, North Carolina, November 1966) indicate that the highest temperatures occur on the central axis of the coal (the actual reported values vary considerably but are usually in the range 800–900° C during a puff, 700–800° C during the natural smoulder between puffs, and 800–850° C under steady state continuous draw conditions). Under all smoking regimes, the thermocouple-measured temperature decreases by up to 300° C along a radius from the maximum central temperature to the periphery of the coal, although one thermocouple study2 has reported the occurrence of occasional peripheral hot spots, with temperatures higher than the centre of the coal. (Ref. 2 contains a comprehensive compilation of more than forty studies of temperature measurements inside burning cigarettes prior to 1968.) In contrast, temperature measurements by X-ray observations of the melting of small metal particles placed within the cigarette5 indicate that the highest temperatures (900° C) occur at the periphery of the coal. Furthermore, measurements of the coal's surface temperature by radiation methods (ref. 5 and A. T. Lendvay, F. M. Watson III, and T. S. Laszlo, paper presented at the CORESTA/Tobacco Chemists' Joint Conference, Williamsburg, Virginia, October 1972) have recorded temperatures of about 850–920° C during a puff, 700° C (below the surface ash) during smoulder, and short-lived hot spots on the coal's surface as high as 1200° C (ref. 5).

EVOLUTION OF STARS OF LOW MASS

Evolutionary sequences of models in the early and main sequence stages have been constructed for stars in the mass range [Formula: see text] with a composition X = 0.739, Z = 0.021. The general behavior of the evolutionary track in the Hertzsprung–Russell diagram is similar to that obtained for medium-mass stars. The [Formula: see text] star reaches the main sequence in 4 × 108 years, with a radiative core covering about half of the mass. The [Formula: see text] star develops a radiative core before reaching the main sequence in 8.5 × 108 years, but with the onset of thermonuclear reactions the energy generation is strong enough to cause the central radiative region of this star to become convective again. The [Formula: see text] star reaches the main sequence as a wholly convective star in 1.7 × 109 years. During the evolution of the [Formula: see text] star, the maximum central temperature attained is 4 × 106 °K, which is not sufficient to stabilize the star on the hydrogen-burning main sequence. Therefore, as contraction continues, the star contracts towards the white dwarf configuration.

Sur la masse minimum des étoiles de la séquence principale

The maximum central temperature of low mass stars and the minimum mass of main-sequence stars are calculated as functions of the chemical composition. The results are in good agreement with those of Kumar but diverge slightly from those of Hayashi.

Early and Main Sequence Evolution of Low-Mass Stars.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Early and Main Sequence Evolution of Low-Mass Stars. Ezer, D. ; Cameron, A. G. W. Abstract Evolutionary sequences of models have been constructed for stars of composition X = 0.739 and Z=0.021, with masses 0.1, 0.2, 0.3 and 0.4Mn. Our stellar structure program has been modified to include the effects of pressure ionization in the Saha equation (Rouse, Astrophys. J. 139, 339, 1964) and the electron screening factor in the thermonuclear reaction rates. The evolutionary study starts from the threshold of energy stability and the energy sources are assumed to be due to gravitational contraction and to the H (p,p+v) 2D, 2D (p,y) 5He and 3He (3He,2p) 4He reactions. The general behavior of the evolutionary track in the H-R diagram is similar to the ones obtained for medium-mass stars which have been previously described. (Ezer and Cameron, Astron. J. 70, 139, 1965). The star of 0.4Mo. reaches the main sequence in 4 x 10~ yr with a radiative core covering about half of the mass. We confirm the previous conclusions of Hayashi and Nakano (Progr. Theoret. Phys. 30, 460, 1963) that the 0.3Mn star develops a radiative core before reaching the main sequence in 8.5 x 10~ yr, but the 0.2Mo. star reaches the main sequence as a wholly convective star in 1.7 x 10~ yr. But at the onset of thermonuclear reactions, the energy generation is strong enough to cause the central radiative region of the 0.3 Me star to become convective again. Therefore, the stars in the mass range 0.2-0.3M are the only stars which evolve with complete mixing of material. During the evolution of the 0. 1Mn star the maximum central temperature attained is 4 X 106 0K, which is not sufficient to stabilize the star on the hydrogen-burning main sequence. Further contraction increases electron degeneracy; therefore, the central temperature decreases as contraction continues, and the star contracts towards the white dwarf configuration. Publication: The Astronomical Journal Pub Date: February 1966 DOI: 10.1086/110102 Bibcode: 1966AJ.....71R.384E full text sources ADS |

Helium Flash in Less Massive Stars

The structure of a stellar core of condensed type is generally discussed to show that physical variables at the center can be determined almost completely as functions of central entropy, if the polytropic index is known in the relatively central region. Based on this relation, the thermal runaway through the helium flash in less massive stars is investigated, treating the core mass as a parameter. Taking into account the finite thermal conductivity, a convection core is shown to develop in the degenerate region. If we assume a sufficiently large efficiency of convective heat-transport, the maximum central temperature in the course of the helium flash is too low to lead the core of 0.53 M⊙ to explosion, in contrast to the model sequence calculated by Schwarzschild and Härm. The effect of the finite efficiency of convective heat-transport is investigated, using the mixing length theory of convection and treating the ratio of the mixing length to the scale-height of pressure as a parameter. Structures of cores including this effect are solved numerically. It is found that, if this ratio is smaller than a critical value which depends on the core mass, an instability occurs; the heat energy liberated by helium burning is blocked and the central temperature reaches a value, above which the effect of inertia leads the core into explosion.

Study of Degeneracy in Very Light Stars.

view Abstract Citations (65) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Study of Degeneracy in Very Light Stars. Kumar, Shiv S. Abstract Convective models have been constructed for stars of mass M<0.1M0.. For a given mass, the internal structure of the star is computed at several radii, taking into account nonrelativistic degeneracy of the stellar material. It is found that for these stars the central temperature has a maximum value at one particular radius and this value decreases when the radius is greater or smaller. For example, for a Population I star of mass 0.05Mo. the maximum temperature of two million degrees occurs at a radius of 0.12R0., and the temperature decreases if we increase or decrease the radius. For Population I stars with mass less than 0.05 M0. the central temperatures and densities are too low for energy production by the proton-proton cycle and even for a star of mass 0.07Mo. this process may not produce sufficient energy. Application of these models is made to contracting stars and it is found that Population I stars of mass less than 0.05M0. become degenerate objects and that they do not reach the main-sequence stage. The limiting mass for such an evolution may be as high as 0.07M . For a star of mass 0.05Mo., the time scale for contraction to the stage of maximum central temperature is estimated to be approximately one billion years, which is small as compared with the age of the galaxy. After this stage the star begins to cool slowly and evolves towards a completely degenerate configuration. Thus it becomes a "black" dwarf without ever going through normal stellar evolution. Publication: The Astronomical Journal Pub Date: 1962 DOI: 10.1086/108658 Bibcode: 1962AJ.....67S.579K full text sources ADS |