Low-temperature Branch Research Articles

Global kinetics for n-heptane ignition at high pressures

A kinetic mechanism of 1011 elementary reactions with 171 chemical species for n-heptane ignition is analysed and reduced to 4 global steps with adjusted rate coefficients to describe ignition at pressures around 40 atm. Two of these steps account for the high temperature branch and the other two for the low temperature branch of the ignition mechanism. The ignition delay time passes through a negative temperature dependence during the transition between the two branches. This is accounted for by the reversible third reaction step, which models the first and second O2-addition in the degenerated chain branching mechanism at low temperatures. Ignition delay times calculated with the adjusted 4-step model are compared to those from the detailed kinetics and experimental data. Finally the 4-step mechanism is analysed by asymptotic methods and explicit ignition delay time formulas are derived.

Relaxation processes in ionic conductive chalcogenide glasses studied by electrical and NMR spectroscopies

The frequency dependence of conductivity has been studied over a large temperature range in Ag + and Li + ionic conductive glasses. The activation energies E′ ac deduced from the Arrhenius plot of σ ac are independent of the frequency. They are comparable with the activation energies deduced from the low-temperature branch of NMR curves log 1/ T 1 = f (1/ T). The empirical factor β = E′ ac/ E dc has been deduced from these results. Fitting the curves to log σ ac = f (log ω) leads to values of S that are dependent on the temperature.

Nonadiabaticity, stoichiometry and mass diffusion effects on supercritical combustion in a tubular reactor

The high-pressure oxidation of dense organic fluids, which are completely miscible in water at supercritical temperatures can be achieved by burning a supercritical combustible mixture in a flow-through tubular reactor. We extend a previous model of this process by specifically accounting for the effects of stoichiometry, mass diffusion and radiative, heat losses on the multiplicity and stability of supercritical premixed flames within the reactor. Our model for the supercritical tubular reactor differs from those for conventional chemical reactors in that the Lewis numbers are assumed to be large and conductive/radiative heat losses occur at the inlet, which behaves much like a flameholder. In general, we show that for sufficiently small heat losses, there are both high-speed, high-temperature and low-speed, low-temperature solution branches for a given steady, planar flame location within the reactor. In addition, the otherwise stable high-temperature flame loses stability to disturbances corresponding to nonsteady, nonplanar modes of supercritical combustion for sufficiently large values of the activation energy and/or sufficiently small or large values of the flow velocity. In the limit of weak stoichiometry and small mass diffusivity, the flame speeds are shown to decrease as the former decreases further and the latter increases, whereas local radiative heat losses, which cause the flame to be located closer to the inlet, are destabilizing with respect to steady, planar burning.

A model of the solar chromosphere-corona transition region based on classical thermal conduction

view Abstract Citations (40) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Model of the Solar Chromosphere-Corona Transition Region Based on Classical Thermal Conduction Athay, R. Grant Abstract A hot corona cooled by inward thermal conduction very likely results in a transition region whose isotherms follow a highly roughened surface. Assuming a checkerboard of cool peaks and hot valleys in surfaces of constant temperature and using the observed emission measure at T = 60,000 K, the ratio of the mean height of the cool peaks to the mean separation of peaks and valleys has a value of 160. By allowing nearly all of the parallel conduction flux of 1 million ergs/sq cm per sec at 1 million K in the hot valleys to flow across magnetic field lines into the cool peaks, a predicted emission measure in very good agreement with the low-temperature branch of the observed emission measure, including the Ly-alpha region, is found. This purely conductive model has the advantages of explaining both the observed emission measure curve and the wide vertical extent of the transition region at the limb without introducing ad hoc secondary heat sources. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169272 Bibcode: 1990ApJ...362..364A Keywords: Chromosphere; Solar Atmosphere; Solar Corona; Surface Roughness; Thermal Conductivity; Heat Sources; Lyman Alpha Radiation; Magnetic Field Configurations; Solar Magnetic Field; Solar Physics; SUN: ATMOSPHERIC MOTIONS; SUN: TRANSITION REGION full text sources ADS |

The Combination Reaction of CH3 and C6H5O

The kinetics and mechanism of the CH3+C6H5O reaction have been examined by means of the RRKM theory and kinetic modelling . Both the high temperature shock tube data for the production of cresols and the low temperature branching ratios (for production of cresols against methylcyclohexadienones , CH3C6H5O) derived from Mulcahy and Williams's data1 on the pyrolysis of di -t-butyl peroxide and phenol mixtures could be reasonably accounted for by the mechanism: ���������� CH3+C6H5O → CH3C6H5O → o- and p-CH3C6H4OH+M→ CH3C6H5OThe energy barrier for the thermal isomerization of CH3C6H5O to cresols was estimated to be ~ 132 kJ mol-1.

P‐V‐T and Bubble‐point Measurements in the System HBr – H2O and its Critical Behaviour

Abstractp‐V‐T measurements on HBr/H2O mixtures up to 820 K and 1500 bar in the complete range between pure H2O and pure HBr are reported. The bubble‐point line has been determined by visual observation up to about 690 K and 500 bar. – For the vapour phase an equation of state (Dieterici‐type) with mixing rules is applied. The experimental data including literature values are analyzed by integration of the Gibbs‐Duhem equation. – The system HBr/H2O is azeotropic even at elevated pressures. The azeotropic line intersects the high temperature branch of the critical curve at about xHBr = 0.045, 684 K and 335 bar. The low temperature branch of the critical curve is nearly identical to the critical point of HBr.

Three-phase solid-liquid-vapor equilibriums in the binary hydrocarbon systems methane-n-hexane and methane-benzene

Pressure, temperature, liquid-phase compositions, and liquid-phase molar volumes are presented along the solid-liquid-vapor (SLV) loci of the binary systems methane-n-hexane and methane-benzene. The data were taken by using cryoscopic techniques and are compared to the solid-liquid (SL) data at elevated pressures of Kuebler and McKinley. The standard deviations of the smoothed SLV data for liquid-phase solute composition presented herein from the raw SLV data are 3.7% for the methane-n-hexane system and 8.6% for the low-temperature branch of the methane-benzene system. The corresponding standard deviation for the high-temperature (high solute composition) branch for the system methane-benzene is less than 0.7%. 8 refs.

Self diffusion in nonstoichiometric compounds†

A formalism for tracer diffusion in nonstoichiometric compounds is developed at the Bragg-Williams approximation along parallel lines with the Anderson theory of nonstoichiometry. It is specifically assumed that both components of a Frenkel pair may contribute independently to the diffusion coefficient. The equation for the tracer diffusion coefficient leads to a “transition” region in a In D ∗ vs 1 T plot for one side of stoichiometry. In this region the slope may attain the value of the sum of the high and low temperature branches. It is also shown that under certain conditions the compositional behavior of the activation energy of diffusion should parallel the compositional behavior of the heat of solution. The success of the mass action law approach, as manifested in ln D ∗/ln po 2 plots in the case of nonstoichiometric oxides, is also shown to be due to a self cancellation of terms involving defect interactions.

Instability phenomena during the RF heating and melting of ceramics

Solutions to the coupled nonlinear differential equations that govern the electromagnetic field and the temperature for the RF induction heating of infinitely long samples of poor electrical conductors whose electrical conductivity is an activated function of temperature can display multiple steady states. The surface temperature of the sample can be an S-shaped function of the applied RF power for certain values of the process parameters. For an RF skin depth that is large compared to the radius of a cylindrical sample, the relevant process parameters can be taken as measures of the applied RF power, the surface heat loss, and the ambient temperature. The low-temperature branch of the S represents a state in which there is hardly any electrical conduction while the high-temperature branch corresponds to a state in which the sample couples significantly to the RF field. The turning points of the S are characterized by critical values of temperature and applied RF field. The case of heat loss by radiation is analyzed in detail, but the instability phenomenon occurs for other loss mechanisms as well. For samples with sufficiently large thermal conductivities, the multiple steady states occur for ambient temperatures below a critical temperature equal to 0.134 times the temperature that characterizes the assumed activated increase of electrical conductivity. Samples with smaller values of thermal conductivity are nonisothermal and have S-curves that shift toward lower values of applied RF power as thermal conductivity is decreased. The case of skin depths comparable to or smaller than the sample radius can be analyzed by numerical methods and displays similar S-curves in the solid state as well as additional cusped S-curves related to the abrupt change, characteristic of ceramics, of electrical conductivity on melting. Experimental measurements on RF heating of silicon at 450 kHz confirm the existence of multiple steady states that correspond to the upper and lower branches of the S-curve; measurements place the upper knee of the S at 950 ± 50°C, in reasonable agreement with a calculated temperature of 850°C. Stable operation of crystal growth processes such as internal centrifugal zone growth and skull melting requires operation on the upper branch of the S-curve; this requires the use of high RF frequencies to attain small skin depths in most ceramic materials.

Low-temperature electron transfer in bacterial photosynthesis

The role of a continuous medium frequency distribution of the Debye form in low-temperature thermal electron transfer processes is investigated. When the electrons are strongly coupled to the medium (the medium reorganization energy is 0.2–0.4 eV) a temperature-independent rate constant is predicted from zero temperature up to over 100 K and with far-infrared Debye frequencies. This is compatible with the experimentally observed low-temperature branch in the Arrhenius relationship of the electron transfer from cytochrome c to excited bacteriochlorophyll, Bch +, in bacterial photosynthesis. However, in order to reproduce the high-temperature branch, of finite activation energy, additional activation of intramolecular modes of higher frequencies must be assumed. An intramolecular mode of a coupling constant of 8–9 and a frequency of about 400 cm −1 can account for this branch. Finally, the estimated electron exchange integral is about 10 −4 eV, indicating that the process is strongly a none diabetic.

Molecular beam study of the mechanism of catalyzed hydrogen–deuterium exchange on platinum single crystal surfaces

The hydrogen–deuterium exchange reaction was studied by molecular beam scattering on low and high Miller index crystal faces of platinum in the surface temperature range of 300–1300°K. Under the condition of the experiments which put strict limitation on the residence time of the detected molecules, the reaction product, HD, was readily detectable from the high Miller index, stepped surfaces (integrated reaction probability, defined as total desorbed HD flux divided by D2 flux, is ∼10−1) while HD formation was below the limit of detectability on the Pt(111) low Miller index surface (reaction probability <10−5). Atomic steps at the platinum surface must play a controlling role in dissociating the diatomic molecules. The exchange reaction is first-order in D2 beam pressure and half-order in H2 background pressure. This observation indicates that an atom–molecule reaction or possibly an atom–atom reaction is the rate-limiting step in the exchange. The absence of beam kinetic energy dependence of the rate indicates that the molecular adsorption does not require activation energy. The surface is able to store a sufficiently large concentration of atoms which react with the molecules by a two-branch mechanism. The rate constants for this two-branch mechanism were determined under conditions of constant H atom coverage, reducing the bimolecular reaction to a pseudo-first-order reaction. At lower temperatures (<600°K) the rate constant for the exchange is k1 = (2±1) ×105 exp(−4.5±0.5 kcal/RT) sec−1. The rate determining step appears to be the diffusion of the D2 molecule on the surface to a step site where HD is formed via a three-center (atom–molecule) reaction, or via a two-center (atom–atom) reaction subsequent to D2 dissociation at the step. At higher temperatures (≳600°K) the reaction between an adsorbed H atom and an incident D2 gas molecule competes with the low temperature branch. The rate constant for this branch is k2 = (1±2) ×102 exp(−0.6±0.3 kcal/RT) sec−1. The catalyst action of the platinum surface for the exchange reaction is due to its ability to adsorb and dissociate hydrogen molecules with low activation energy and to store atomic hydrogen on the surface thereby converting the gas phase molecule–molecule reaction to an atom–molecule or atom–atom reaction of low activation energy.

The Mesomorphic Behavior of S-Cholesteryl ω-Phenylalkanethioates

Abstract The first 16 homologous S-cholesteryl ω-phenylalkanethioates were synthesized from 3β-mercaptocholest-5-ene and the imidazolides of the corresponding ω-phenylalkanoic acids. The mesomorphic properties of these compounds were determined by microscopic inspection and by differential scanning calorimetry. With the exception of the phenylthioacetate, the 4-phenylbutanethioate, and the 6-phenylhexanethioate, all homologs are cholesteric. S-Cholesteryl 8-phenyloctanethioate and its higher homologs are monotropic smectic. Both the cholesteric-isotropic and the smectic-cholesteric transition temperatures fall on two hyperbolic curves: a high temperature branch for homologs of odd and a low temperature branch for homologs of even acyl chain length. Some of the cholesteric members, of this series exhibit the visible spectrum of selectively reflected light within a narrow temperature range.

THERMAL AND STIMULATED MOLECULAR SCATTERING OF LIGHT IN LIQUIDS

The temperature dependence of the spectrum of the light scattered by anisotropy fluctuations is studied in liquids (salol and benzophenone). Two branches are observed in the temperature dependence of the splitting between the components of the doublet induced by shear waves. There is a low temperature branch, in which the splitting between thecompoments decreases when temperature increases, and there is a high temperature branch, in which this splitting decreases when temperature decreases. In the Rayleigh line wing of the liquids that were studied one can distinguish two lorentzians the width of which differed by two orders. Two relaxation times and their temperature dependence are defined from these data. With the use of these measurements and data of the temperature dependence of an other parameter it is shown that Rytov's theory with two times of anisotropy relaxation describes very well the temperature behaviour of the splitting between components of Rayleigh wing fine structure. The gain of stimulated Rayleigh line wing light scattering is measured as well. The phenomenon of stimulated temperature light scattering due to an electrocalorical effect is observed in the liquids that were studied. Laser application in the investigations of physical optics led to the discovery of several new phenomena. Particularly, many good results have been obtained these recent years in the study of thermal and stimulated molecular scattering of light [I], [4]. The number of works in this field is ever growing. Some recent results of the authors and their colleagues will be under discussion in this paper concerning the studies of spectra of thermal depolarized light scattering, stimulated light scattering in Rayleigh line wing, and stimulated entropy or temperature scattering of light. Fine structure of Rayleigh line wing. The fine structure in the spectrum of thermal depolarized scattering of light (fine structure of Rayleigh line wing (FSW)) was first observed and interpreted by Tiganov and the authors [5]. The new phenomenon implies that a doublet can be observed for certain polarizations of exciting and scattered light in the spectrum of the Rayleigh line wing. This phenomenon occurs due to scattering of light by anisotropy fluctuations produced by the fluctuations of deformations (shear waves) [5]. This interpretation was proved by the studies of polarization, angular and light frequency dependence of FSW component shift [5] , [6], [7], [S], ~91. In the first temperature investigations it was found unexpectedly that when the temperature decreases the interval between the FSW components does not increase and even decreases [5], [6]. The further experiments made by Sabirov and the authors [8], [9] revealed the whole picture of somewhat extraordinary temperature dependence of the component shift produced by transverse hypersound. The diagram of the dependence of intervals between the components of the transverse doublet 2 Av, vs temperature in salol is given in figure 1 for the case when its viscosity changed from 10' to poise. In the temperature range of 500C to 3 OC the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972137 CI-216 1. L. FABELINSIUI AND V. S. STARUNOV interval between the transverse components is decreasing when the temperature increases, as it was expected from the Maxwell scheme of viscosity and the Leontovich theory with one relaxation time [I]. FIG. 1. Temperature dependence of splittings between the FSW components in liquid salol. theory, 0 experiment. In the temperature range of 3 OC to 45 OC the doublet was not observed. In the temperature range of 45 OC to 120 OC FSW was observed, the distance between the FSW components was increasing when the temperature increases. The same dependence of Av, upon t was observed by Sabirov and the authors for the case of benzophenone. We showed in [8] that the described character of Av, dependence vs t could be qualitatively explained if one supposed the existence of two processes of fluctuation dissipation with different relaxation times 7, and z2 (2, > 2,). The 'theoretical calculation of light scattering anisotropy fluctuations with the account of two relaxation times has been recently performed by Volterra [lo], Rytov [Ill and Romanov and Solovyev [12]. If in the general Rytov's theory one assumes the same dispersion law for optoelastic coefficient X(o) as for shear modulus i (o) the following equation for the intensity distribution can be obtained [13] : Here X,, p, are the optoelastic coefficient and the shear modulus for o -+ co. 490, p are the shear viscosity and the density, z, = yo/p,, (2, 2 z, 2 z2), 0 is the angle of scattering. q = 2 K sin 912 where K is wave-number of the exciting light. T and R are represented as following : The first term in eq. (1) is responsible for arising of the shear components in the light scattering. Let us introduce the symbol y = (52; T:)-~, where

The measurements of temperature dependence of thermal conductivity of silver for different fixed values of heat flux density (liquid helium temperature range)

Measurements are made of the temperature dependence of the thermal conductivity coefficient for seven fixed values of heat flux density (q) flowing along a silver sample of technical purity in the helium temperature range. The temperature dependence of the thermal conductivity coefficient is determined using a difference method, Kr(ΔT), and a graphical differential method, Krg(T). The existence of a family of curves with q as a parameter is established in the temperature region of pronounced temperature dependence of the thermal conductivity. It is concluded that the thermal conductivity coefficient for silver investigated is also a function of the heat flow density in the temperature range of the pronounced temperature dependence of the thermal conductivity. In the range of the low temperature branch of K(T) the analytical dependence of the thermal conductivity coefficient on the density of heat flux, is found to be in agreement with theoretical considerations. [Russian Text Ignored].

Mesomorphism of Homologous Series. II. Odd-Even Effect

Abstract The determination of mesomorphic behavior was extended to cover the first sixteen members of the cholesteryl w-phenylakanoates. Microscopy and scanning calorimetry were used to identify the mesophases and to measure transition temperatures and associated latent heats. The clearing temperatures fall on two distinct curves: a high temperature branch occurring for odd number of C-atoms of the acid chain and a low temperature branch occurring for even numbers of C-atoms. The difference in clearing points of neighboring homologues decreases from about 100°C for low members to a few degrees for high members of the series. Contrary to this, the smectic-cholesteric transition temperatures show no such odd-even effect, although their enthalpies and entropies do exhibit a pronounced odd-even behavior. The enthalpies and entropies do exhibit a pronounced odd-even behavior. The enthalpies and entropies of the cholesteric-isotropic transitions show an odd-even effect where the odd chain length branch has the ...

Mechanical properties of bismuth at different temperature and strain rates

1. The plasticity of bismuth (99.999% Bi) at low temperatures (to 0\dgC or 0.5Tm,oK) is equal to zero and then increases sharply, particularly in upsetting, and reaches a peak at 50\2-100\dgC (0.6\2-0.7Tm,oK), particularly a lower strain rates. With further increase of the temperature the plasticity again decreases. 2. The temperature-rate dependence of the resistance to deformation can be divided into low-temperature ( t1) branches; evidently temperature t1 is the initial recrystallization temperature-it increases with the strain rate. 3. The plasticity and resistance to deformation of bismuth are sensitive to changes in the strain rate. 4. With t\s>t1 the temperature-rate dependence of the resistance to deformation has a monotonic character and is more evident at higher degrees of strain.

Cool-flame oxidation of ketones

The cool flames of three ketones (acetone, methyl ethyl ketone, and diethyl ketone) have been studied in a static system. The pressure, light emission, and temperature of the reacting gases have been measured and recorded simultaneously. The light emission appears as a pale-blue glow to the naked eye, and occurs not only in the cool flame, but also in the slow reaction before and after the passage of the flame. The maximum intensity of light occurs just after the rate of pressure change has reached its maximum value. Even in the slow reaction there are significant temperature increases. In the cool-flame region, the self-heating is even more marked-temperature rises of 20°C before the cool flame, and 120°C in the flame itself frequently being observed. The temperature rise begins at the center of the reaction vessel, and spreads out to the walls. The temperature changes observed in the flames at the critical condition are just sufficient to take the system into the region of negative temperature coefficient. Chemical analysis has shown that there is a considerable build-up of organic peroxides in the cool flame, followed by a very sharp fall as the flame temperature reaches its maximum. These facts combine to support the theory of cool flames put forward by Knox and Norrish and others. In this theory, the cool flame is regarded simply as a “low-temperature” slow reaction in which the rate of heat evolution is greater than the maximum rate of heat loss. Consequently, the reaction accelerates, but when the temperature reaches the negative temperature coefficient region this ceases. At the high temperatures reached, the low-temperature branching intermediate is rapidly destroyed, and so the rate falls. This sequence may be repeated a number of times thus leading to the phenomenon of multiple cool flames.

Evidence for Logarithmic Singularities in the Specific Heats of the Ferroelectric Crystals KH2PO4and KH2AsO4

By drawing interpolation curves between the high- and low-temperature branches of the specific heats of K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ and K${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$ (measured by Stephenson and co-workers) we have decomposed each specific-heat function into the sum of a monotonic term plus an anomolous term, peaked about the transition temperature. In the region of the transition temperature ${\ensuremath{\theta}}_{c}$ we find that the paraelectric and ferroelectric branches of the anomalous specific heat of each crystal have the limiting behavior ${C}^{a}=\ensuremath{-}A\mathrm{ln}|\ensuremath{\theta}\ensuremath{-}{\ensuremath{\theta}}_{c}|+B,$ where $A=1.2\ifmmode\pm\else\textpm\fi{}0.3$ cal/mole deg, $B=0.95\ifmmode\pm\else\textpm\fi{}0.07$ cal/mole deg for $\ensuremath{\theta}g{\ensuremath{\theta}}_{c}$, and $A=14.2\ifmmode\pm\else\textpm\fi{}1.4$ cal/mole deg, $B=14.4\ifmmode\pm\else\textpm\fi{}0.3$ cal/mole deg for $\ensuremath{\theta}l{\ensuremath{\theta}}_{c}$. These constants have the same values for both crystals.

Characteristics of thermal softening of alloyed austenite

1. From a study of the effect of alloying elements on the thermal softening of austenite, it is shown that the temperature dependence of the hardness is exponential (Shishokin equation) and is characterized by the initial hardness Ho and the thermal softening coefficienta. 2. Alloying slightly changes Hol, the initial hardness for the low temperature branch of the curve. Chromium, tungsten and molybdenum increase the initial hardness Ho2; cobalt and nickel either do not change, or slightly lower, it for high temperatures. The effect of alloying elements on the value of Ho2 is in accordance with their effect on recrystallization during heating of strain-hardened austenite. 3. Alloying elements do not affect the thermal softening coefficient which, below the critical temperature of physical softening, Tps, is 0.0006 and approximately 0.00250 above it. 4. The relation between the reduced hardness H/Ho and temperature is governed by a single equation independently of the nature or percentage of the alloying element. Below Tps,log H/Hol=−0.0006 T, and above it, log H/Ho2 = −0.0025 T. This conclusion is supported by plotting 160 points obtained from 1920 hardness measurements on 16 different alloys. 5. Chromium, tungsten and molybdenum considerably increase the critical temperature of physical softening. Introduction of nickel and particularly of cobalt somewhat lowers this temperature. The differences in the effects of alloying elements are probably connected with their action on the bond energy in the austenite crystal lattice. 6. For differently alloyed austenite, a quantitative connection has been found between the value of T and the temperature of recrystallization softening.