Chaos in thermal pulse combustion

An experimental thermal pulse combustor and a differential equation model of this device are shown to exhibit chaotic behavior under certain conditions. Chaos arises in the model by means of a progression of period-doubling bifurcations that occur when operating parameters such as combustor wall temperature or air/fuel flow are adjusted to push the system toward flameout. Bifurcation sequences have not yet been reproduced experimentally, but similarities are demonstrated between the dynamic features of pressure fluctuations in the model and experiment. Correlation dimension, Kolmogorov entropy, and projections of reconstructed attractors using chaotic time series analysis are demonstrated to be useful in classifying dynamical behavior of the experimental combustor and for comparison of test data to the model results. Ways to improve the model are suggested. (c) 1995 American Institute of Physics.     

Discrete-time stimulation of the oscillatory and excitable forms of a FitzHugh-Nagumo model applied to the pulsatile release of luteinizing hormone releasing hormone

A model for the pulsatile release of luteinizing hormone releasing hormone (LHRH) can be reduced to a FitzHugh-Nagumo model subject to regular and quasiregular (i.e., with slight random variation in the interstimulus interval), discrete-time stimulation. The relationship of output pulse frequency (OPF) to stimulus frequency is compared between the excitable and oscillatory forms of the model and discussed in the context of results from other pulse-driven model systems. Some examples of the changes in OPF caused by quasiregular and purely Poissonian stimuli are given for the excitable case. The unstimulated system frequently interacts with the stimulation in such a complex manner that the OPF bears little resemblance to the frequency of stimulation or of the unstimulated system. Furthermore, the inability of the oscillatory form of the model to allow complete suppression of output pulses for moderate stimulation frequencies suggests that the LHRH system can be more appropriately described by the excitable form of the model. (c) 1995 American Institute of Physics.     

Time-resolved fluorescence relaxation of 3-methyllumiflavin in polar solution

We have studied the fluorescent properties of a well-defined model flavin compound (3-methyllumiflavin) in a relatively polar solvent like propylene glycol or ethanol. Inhomogeneous spectral broadening effects were directly time-resolved by detection at the extreme blue and red edges of the fluorescence band of 3-methyllumiflavin using excitation in the main absorption band. At the high-energy side of the emission band a rapid decay component (tens of picoseconds) was resolved indicative for the disappearance of the initially prepared, nonequilibrium state with a characteristic dipolar relaxation time. At the low-energy side the rise of a solvent relaxed fluorescent species could be time-resolved. The wavelength-dependent effects on the dipolar relaxation were abolished when excitation was at the low-energy side of the absorption band. The experimental decays of the flavin solvate at different energies of fluorescence and excitation are presented as they represent an easy diagnosis for energy dependent solvation dynamics. Wavelength dependent rotation of 3-methyllumiflavin, examined by fluorescence anisotropy decay, turned out to be absent for 3-methyllumiflavin in propylene glycol between 263 and 293 K, probably because of the small change in dipole moment upon flavin excitation.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

Europium isotope separation in the HCl/HDEHP extraction system

The^153/151Eu isotope separation factor (q) for the Eu(II)/Eu(III) chemical exchange in the liquid-liquid extraction system, containing Eu (III) in di (2-ethylhexyl) phosphoric acid (HDEHP) and Eu(II) in water acidified with hydrochloric acid, was found to be 0.9993±0.0002 (2) for the single stage. Some theoretical aspects of separation of the Eu isotopes are discussed. The decisive role of the electron exchange reaction and complexation by the counter-ion in the aqueous phase is emphasized.     

Enzyme and microbial sensors for phosphate, phenols, pesticides and peroxides

Biosensors employing a biocatalyst on a different level of integration have been developed for monitoring environmental pollution. These probes range from laboratory specimen to commercial detectors applied to analyzers. Recent developments on amperometric enzyme and microbial biosensors are presented here. A monoenzymatic bulk-type carbon electrode is described for biosensing organic hydroperoxides in aqueous solutions; peroxidase is immobilized within the electrode body and the direct electron transfer between electrode and enzyme is measured. Both, reversible and irreversible inhibitors of acetylcholinesterase have been quantified by using a kinetically controlled acetylcholine enzyme sequence electrode. The inhibitory effect of pesticides such as butoxycarboxime, dimethoate, and trichlorfon could be quantified within 6 min in molar concentrations. Different multi-enzyme electrodes have been developed for the determination of inorganic phosphate. These sensors represent examples of sequentially acting enzymes in combination with enzymatic analyte recycling. Using this type of amplification nanomolar concentrations can be measured.     

The limit of detection in transversal photothermal deflection spectroscopy with light scattering samples

The limit of detection for the 4,6 dinitro-orthocresol (DNOC) pesticide, including small latex particles as light scattering samples in water as suspending medium, has been determined for the transversal photothermal deflection technique. The results where compared to conventional spectrophotometry. In the photothermal deflection experiments performed with a continuous Ar⁺-excitation beam (=363.8 nm) a LOD of 0.8 g/l is achieved for the DNOC in water.Awarded poster prize     

Error estimates in the determination of rigid body displacements in protein crystals

We report the estimation of random errors in the refinement of the rigid body displacements of the -helices of the enzyme 6-phosphogluconate dehydrogenase. Least-squares refinement of the TLS parameters of the helices has been carried out using X-ray reflection data of 2.1 Å resolution, resulting in anR-factor of 19.5%. Standard deviations were estimated from the normal matrix. The results show that the translational mean-square displacements of nearly all the helices are significant at this resolution. However the libration parameters are only significant when the helices have at least four turns. Screw-rotation tensor values cannot be determined at this resolution.     

Luikov equations applicable to sublimation-drying

Present paper presents a derivation of Luikov equations applicable to sublimation-drying. The physical situation and transfer mechanism are elucidated clearly. The coefficients appearing in Luikov equations are given in a more explicit way. Some formulation mistakes in recent publications are indicated.

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Anwendung der Luikov-Gleichungen auf die Sublimationstrocknung

Zusammenfassung Die Untersuchung bezieht sich auf eine Ableitung der Luikov-Gleichungen, mittels deren sich der Vorgang der Sublimationstrocknung analysieren läßt. Physikalische Anfangssituation und Austauschmechanismen werden klar herausgestellt und die in den Luikov-Gleichungen auftretenden Koeffizienten in expliziter Weise angegeben. Ferner erfolgt Hinweis auf Formulierungsfehler in jüngeren Veröffentlichungen.

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Nomenclature C M _v/V _f, concentration of vapor, kg/m³ - c _pv specific heat of vapor at constant pressure, J/kg K - c _pw specific heat of adsorbed water at constant pressure, J/kg K - c _s specific heat of solid skeleton, J/kg K - C _s M _s/V _f, concentration of solid skeleton, kg/m³ - C _w M _w/V _f, concentration of adsorbed water, kg/m³ - f V _w/V _f, volumetric fraction of adsorbed water - j _F mass flux of vapor by diffusion (Fick) transfer, kg/m² s - j _D mass flux of vapor by filtration (Darcy) transfer, kg/m² s - j _v total mass flux of vapor, kg/m² s - k permeability, m² - M _s mass of solid skeleton, kg - M _v mass of vapor in pores, kg - M _w mass of adsorbed water, kg - P pressure, Pa - q heat flux, W/m² - R gas constant, J/kg K - T temperature, K - V _f volume of the framework of porous medium, m³ - V _v volume of vapor in porous medium, m³ - V _w volume of the absorbed water, m³ Greek symbols /(c _p), effective thermal diffusivity, m²/s - _m effective vapor diffusivity in porous medium, m²/s - _p R T /, Luikov pressure diffusivity, m²/s - +f, porosity of the porous medium - effective thermal conductivity of porous body, W/m K - dynamic viscosity of vapor, kg/m s - kinematic viscosity, m²/s - Ck/=k/, Luikov filtration motion coefficient, s - V _v/V _f, volumetric fraction of vapor - density of absorbed water, kg/m³ - (c _p) M _v c _pv+M _s c _s+M _w c _pw /V _f=Cc _pv+C _s c _s+fc _pw, effective product of density and specific heat of humid porous body, J/m³K     

Diffusion mit überlagerter reibungsbehafteter Strömung

Zusammenfassung Es wird eine allgemeine Differentialgleichung für den Massen- oder Mengenstrom im Fall einer Überlagerung von Diffusion und reibungsbehafteter Strömung, d.h. bei gleichzeitiger Änderung des Gesamtdruckes und der Partialdrücke über den Weg, für den stationären (und quasistationären) Fall abgeleitet. Die sich ergebenden gekoppelten Differentialgleichungen können näherungsweise analytisch gelöst werden.Bei den Überlagerungen wurde allein von Gas-(Fickscher) Diffusion ausgegangen. Unberücksichtigt blieben auch sekundäre Diffusionströme, so durch Druck- u. Thermodiffusion und in anderen Kraftfeldern. Damit wird an zwei Beispielen — einseitige Diffusion (Verdunstung) und Adsorption in porösen Stoffen — gezeigt, daß die Annahme reibungsfreier konvektiver Ausgleichsströmungen bei der Diffusion in porösen Körpern mit engen Poren nicht zulässig ist. Vielmehr treten am Ort der Entstehung des Diffusionsstromes (Verdunstungsspiegel) bzw. in der Adsorptionsfront Änderungen des Gesamtdruckes auf, die zu zusätzlichen Strömungen führen und Rückwirkungen auf den Massenstrom besitzen. Neben der Höhe der Konzentrationen ist für die Größe des Effektes allein das Verhältnis der Transportkoeffizienten für Strömung und Diffusion maßgebend. Der Effekt macht sich bei hohen Konzentrationen der diffundierenden Komponente deutlich bemerkbar, bei niedrigen ist er — wie zu erwarten — zu vernachlässigen.

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Diffusion of superimposed frictional flow

A general differential equation for a mass and volume flow will be derived for the case of diffusion superimposed frictional flow, that means, at simultaneously varying total and partial pressures along the path, for the stationary and quasi-stationary states. The resulting coupled differential equations can be analytically treated for an approximate solution.The conception was based entirely upon (Fick's) gas diffusion. Secondary diffusional flows were neglected, like pressure and thermal diffusions, or those caused by other potential gradients. Two examples, the diffusion through a stagnant gas film (evaporation by diffusion), and the adsorption in porous material exemplify that assuming a frictionless convective arbitration flow is invalid in the case of diffusion within porous structures of narrow pores. Adequately, at the source of the diffusional flow (mirror of diffusional evaporation), respectively within the adsorption front, changes in total pressure arise, causing additional flows acting upon the mass flow. The scope of the effect depends entirely upon the extents of the concentration and the ratio of transport coefficients of the flow and diffusion. The effect increases noticeably at elevated concentration of the diffusing components, while it gets negligible at lower concentration, as to be expected.

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Formelzeichen c Konzentration [kmol/m³] - d _p Porendurchmesser - bezogener Diffusionsstrom - f Querschnittsfläche - bezogener Strom - M relative Molmasse - Mengenstromdichte - P Druck - R _a Radius eines kugelförmigen Partikels - s Diffusionsweg - t Zeit - v Geschwindigkeit des konv. Stroms - w _Fr Frontgeschwindigkeit - x Variable des Weges - X Beladung des Adsorbens [kmol/kg] - Z, z Variable, s. Gl. (23), Gl. (29) Griechische Buchstaben (X) Kapazitätsfaktor - Verhältnis der Transportkoeffizienten _lam/ _D - Diffusions- bzw. äquivalenter Transportkoeffizient - freie Weglänge - dynamische Zähigkeit - Tortuositätsfaktor - Dichte - Lage der Adsorptionsfront - Porosität des Adsorbens Indices o Bezugsgröße bzw. vorgegebener Wert der Größe im Gas - e einseitig - D Dampf, diffundierende Komponente - L Luft, aufnehmendes Gas - lam laminare Strömung - s Feststoff - in der Adsorptionsfront