Effects of the anomalous temperature dependence of water density on surface gravity current

This paper reports experimental results on the propagation of a plane water jet at a temperature above the maximum-density temperature (4° C) along the free surface of initially quiescent water at a temperature of about 0°C. For comparison, experiments were performed in which the temperatures of the lower and higher layers were more than 4°C, other conditions being equal. The experiments revealed a number of new hydrodynamic effects, including peculiar flow instability and a fine structure of the density field at large times.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 63–69, January–February, 2005     

Experimental studies of a steam jet refrigeration cycle: Effect of the primary nozzle geometries to system performance

This paper describes an experimental investigation of a steam jet refrigeration. A 1 kW cooling capacity experimental refrigerator was constructed and tested. The system was tested with various operating temperatures and various primary nozzles. The boiler saturation temperature ranked from 110 to 150 °C. The evaporator temperature was fixed at 7.5 °C. Eight primary nozzles with difference geometries were used. Six nozzles have throat diameters ranked from 1.4 to 2.6 mm with exit Mach number of 4.0. Two remained nozzles have equal throat diameter of 1.4 mm but difference exit Mach number, 3.0 and 5.5. The experimental results show that the geometry of the primary nozzle has strong effects to the ejector performance and therefore the system COP.     

An experimental study of the evaporation characteristics of emulsified liquid droplets

This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.List of Symbols h_fg latent heat of vaporization, KJ/kg - m mass of the droplet, gm - T_b boiling temperature, °C - T_c critical temperature, °C - T_L Leidenfrost temperature, °C - T_s initial surface temperature of the hot surface, °C     

Molecular tagging thermometry with adjustable temperature sensitivity

We report an extension to the technique of molecular tagging thermometry which allows for adjustable temperature sensitivity. The temperature dependence of laser-induced phosphorescence of the water-soluble phosphorescent triplex (1-BrNp•Mβ-CD•ROH) is used to conduct temperature measurements in aqueous flows. It is shown that the temperature sensitivity of phosphorescence intensity can be adjusted by changing the time delay between the laser excitation pulse and the start of the phosphorescence emission acquisition. For example, for a phosphorescence integration period of 1 ms, the temperature sensitivity of the measured phosphorescence intensity varies in the range 8.15–18.2% per °C at 25°C as the time delay changes from 1 to7 ms. This temperature sensitivity is much higher than that of most fluorescent dyes used for temperature measurements (e.g. less than about 2% per °C for Rhodamine B). The implementation and application of this new approach are demonstrated by conducting temperature measurements in the wake of a heated cylinder.     

Comparison of the thermal performances of two nanofluids at low temperature in a plate heat exchanger

The objective of this study is to compare experimentally the thermal performances of two types of commercial nanofluids. The first is composed of oxides of alumina (γAl₂O₃) dispersed in water and the second one is aqueous suspensions of nanotubes of carbons (CNTs). The viscosity of the nanofluids is measured as a function of the temperature between 2 and 10 °C. An experimental device, containing three thermal buckles controlled in temperature and greatly instrumented permits to study the thermal convective transfers. The evolution of the convective coefficient permits to study the convective thermal transfers. The evolution of the convective coefficient is presented according to the Reynolds number, at low temperature from 0 to 10 °C and for the two aforementioned nanofluids. An assessment of the pressure drops in the circuit as well as of the powers of the circulator and outputs is dealt with.     

A note on the rubber-like properties of gelatin

Summary The shape of the load-elongation curve and the temperature coefficient of the elastic modulus have been examined for the unidirectional extension of cross-linked gelatin films swollen in an alcohol-water mixture. The load-elongation curve for extensions up to 300% was of the form predicted by the kinetic theory of elasticity. The temperature coefficient of the stress at constant extension was positive and over the temperature range 50° to 70°C was proportional to absolute temperature. Between 50°C and 0°C the temperature coefficient of the stress was much greater. Over this temperature range the elastic modulus, as measured by extending a film at different temperatures, increased greatly as the temperature was lowered. It is suggested that the large temperature coefficient of the stress at temperatures below 50 °C is due to the formation of gelling cross-links causing straightening of the gelatin chains.

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Zusammenfassung Die Form der Spannungs-Dehnungskurve und der Temperaturkoeffizient des Elastizitätsmoduls wurde für lineare Streckung von vernetzten Gelatine-Filmen bestimmt, die in Alkohol-Wasser-Mischungen gequollen waren. Bis zu einer Dehnung von 300% hatte die Kurve die aus der kinetischen Elastizitätstheorie zu erwartende Form. Der Temperaturkoeffizient der Spannung für konstante Dehnung war positiv und im Temperaturbereich 50–70° C proportional der absoluten Temperatur; zwischen 0 und 50° C war er viel größer. In diesem Temperaturbereich vergrößert sich der Elastizitätsmodul, gemessen durch Dehnung des Films bei verschiedenen Temperaturen, beträchtlich mit sinkender Temperatur. Es wird angenommen, daß der große Temperaturkoeffizient der Spannung bei Temperaturen unterhalb 50° C auf die Bildung gelierender Querbindungen zurückzuführen ist, die die Gelatineketten ausrichten.

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Communication No. 1955 H from the Kodak Research Laboratories.     

Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

The collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170 °C to 500 °C. When a droplet impacted onto the solid at a temperature of 170 °C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300 °C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500 °C at the low-impact Weber numbers (∼30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300 °C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500 °C, the two dimensionless deformation behaviors of the droplets were very similar to each other.     

Experiments on the effect of low temperature on some plastic properties of metals

Summary The yield value, breaking strength, elongation and hardness of a number of pure polycrystalline metals were measured at room temperature and at –183°C. The yield value is always higher at –183°C than at 20°, the difference being relatively small (<75%) for the cubic facecentered and a number of hexagonal metals (e.g. Mg). This difference is large for the body-centered metals and for Zn, Cd and Sn. The former metals have a larger elongation at lower temperature, the latter, however, become more or less brittle at low temperature. The breaking strength and the hardness increase generally with decreasing temperature.This research was carried out during the war in the physics laboratory of the N.V. Philips, Eindhoven. The author resides now in Delft.     

Effect of the Nonmonotonic Temperature Dependence of Water Density on the Propagation of a Vertical Plane Jet

This paper gives results of two experiments in which warmer water in the form of a vertical plane jet was let into colder water. In one experiment, the temperature of inflowing and initially quiescent water exceeded 4°C and warm water propagated only along the free surface. In the other experiment, the temperature of inflowing water was above 4°C and that of quiescent water was below 4°C. In this case, two jets — surface and bottom — first formed, and then the inflowing liquid was entirely concentrated in the bottom jet. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 2, pp. 23–29, March–April, 2006.     

Rheological aspects of the rejuvenation of aged bitumen

Rheological techniques are used to investigate the rejuvenation of aged bitumen. The thermal transition associated with the collapse of the compact structure constituted by asphaltene is determined by Dynamic Mechanical Thermal Analysis. For aged bitumen, this transition shifts to a higher temperature but when rejuvenating agents are added, the transition returns to its original value. The “rutting factor,” G*/sin δ allows to define the maximum temperature the binder can reach without permanent deformation. The employed rejuvenating agents are suitable because permanent deformation is postponed. Viscosity results reveal that aged bitumen needs a high mixing temperature (>200°C) to behave like a fluid material able to wet, adhere, and envelop aggregates. The addition of rejuvenating agents considerably reduces mixing and compaction temperatures. The mixture of 80% aged bitumen—20% recycled motor oil, obtained exclusively from waste materials is an apt binder that can compete satisfactorily with new 60/70 bitumen.Paper presented at the AERC2005. This paper will be part of a special issue of AERC2005.     

A cryostat for use with a thermal conductivity hot wire system

Summary A cryostat for use with a thermal conductivity hot wire system is described in which temperatures between –70°C and –130°C can be maintained constant to within (0.01°C). Control of temperature is made by adjusting the vacuum between a five component mixture and liquid oxygen. Extra control is made by adjusting the current through a heating coil immersed in the cryostat mixture. The temperature indicator is a platinum wire surrounding the tubes.     

Laser-Raman remote temperature sensing in liquids

A feasibility study has been conducted on the use of laser-Raman spectroscopy as a remote temperature sensing technique for liquids. Empirical relations between the temperature and parameters describing Raman band intensities were determined over a temperature range of 15 to 65 °C in carbon tetrachloride, benzene, ethylene glycol, aqueous sodium nitrate (5 M), and water. Using a 2-W argon ion laser and two 0.25-m monochromators in tandem, it was possible to measure temperatures in water to within 2 °C and, in ethylene glycol, to within 4 °C.     

A torsional creep instrument and a torsional pendulum,both with accurate temperature control

Summary This paper describes a torsional pendulum and a torsional creep instrument. With the pendulum shear moduli between 10⁶ and 10¹⁰ N/m² can be measured at frequencies from 0.1 to 20 Hz. The creep instrument is suitable for measurement of shear compliances lower than 10^–7 m²/N in the time range from 1 to 10⁵ seconds. In both instruments, specimens are kept at the right temperature by blowing heated nitrogen gas through a surrounding thermostatic chamber. The signal of a platinum resistance thermometer, provided in each chamber, automatically controls the heating of the gas. Temperatures from –180 to +300 °C can be maintained with an absolute accuracy of ±1 °C and a long term stability of ±0.05 °C. It is shown that one cannot directly compare one and the same shear property, calculated from the shear modulus as measured with the pendulum as well as from the shear compliance as measured with the creep instrument. This is due to differences in the temperature of one thermostatic chamber over against the other. Finally, the paper presents a method to reduce these differences to ±0.1 °C, although the absolute accuracy of temperature control remains ±1 °C.

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Zusammenfassung Die Arbeit beschreibt ein Torsionspendel und eine Torsions-Kriechapparatur. Mit dem Pendel können Schermoduli zwischen 10⁶ und 10¹⁰ N/m² bei Frequenzen zwischen 0,1 und 20 °C gemessen werden. Die Kriechapparatur ist geeignet für die Messung von Scherkomplianzen kleiner als 10^–7 m²/N in Zeiten zwischen 1 bis 10⁵ sec. In beiden Geräten werden die Proben durch das Einblasen von erhitztem Stickstoff durch eine umgebende thermostatische Kammer bei der richtigen Temperatur gehalten. Die Anzeige eines Platin-Widerstandsthermometers, das in jeder Kammer angebracht ist, kontrolliert automatisch die Erwärmung des Gases. Es können Temperaturen zwischen –180 und +300 °C mit einer absoluten Genauigkeit von ±1 °C und einer Langzeitstabilität von ±0,05 °C eingestellt werden. Es wird gezeigt, daß ein direkter Vergleich der gleichen Schereigenschaft, die zum einen aus dem mit dem Pendel gemessenen Schermodul und zum anderen aus der mit der Kriechapparatur ermittelten Kriechkomplianz errechnet wird, nicht möglich ist. Das beruht auf Temperaturdifferenzen zwischen den thermostatisierten Kammern.Abschließend stellt die Arbeit eine Methode vor, um diese Differenzen auf ±0,1 °C zu senken. Die absolute Genauigkeit der Temperaturkontrolle bleibt bei ±1 °C.

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Paper presented at the Conference on Experimental Rheology, University of Bradford, April 17–19, 1968.     

The peculiar rheo-optical behavior of bisphenol A-polycarbonate and polymethylmethacrylate

The rheological and stress-optical behavior of the melts of several grades ob bisphenol-A-polycarbonate (PC) and polymethylmethacrylate (PMMA) is investigated. Pertinent flow birefringence measurements are carried out in a remodelled cone-plate apparatus [1]. The shear stress in the polymer melt is calculated from the dynamic moduli, which are determined separately. It is shown that the linear stress optical rule is obeyed. In this way, the stress-optical coefficient C of the melt can be determined. The low-Mw polycarbonates all behave as Maxwellian fluids. The main stress direction does not deviate significantly from 45°. In the temperature range from 160° to 260°C the stress-optical coefficients of the different grades lie between 3 and 4×10^–9 Pa^–1 and show a weak temperature dependence. The stress-optical coefficient of PMMA is about a factor of 100 lower and shows a peculiar temperature-dependence, changing its sign at 144°C. The results are discussed in terms of the anisotropy of the polarizability of the polymer chain.     

Hot-wire calibration over a large temperature range

The hot-wire calibration method as proposed by Cimbala and Park (1990) has been showen to be accurate within a temperature range of 20–45°C. This is a significant extension of the range used by Cimbala and Park (27.5– 34.5°C). The accuracy of the calibration is not affected by the ambient temperature. The calibration curve obtained seems to hold over a long period of time, thus reducing the need for frequent calibrations. Due to contamination the accuracy eventually decreases and the probe has to be re-calibrated.     

An experiment on the breakup of impinging droplets on a hot surface

Breakup characteristics of liquid droplets impinging on a hot surface are investigated experimentally with the wall temperatures in the Leidenfrost temperature range of 220–330°C for n-decane fuel. Factors influencing droplet breakup are wall temperature, impinging velocity, droplet diameter and impinging angle. The 50% breakup probability shows that the impinging velocity decreases linearly with the droplet diameter increase and there exists an optimum impinging angle near 80° having the minimum value in the impinging velocity for given wall temperature and droplet size. Near the wall temperature of 250°C corresponding to the Leidenfrost temperature, a peculiar nonlinear behavior in the breakup probability is observed.This work was supported by the Turbo and Power Machinery Research Center, Seoul National University.     

Experimental study of gas deflagration temperature distribution and its measurement

Explosion temperature is one of the main factors in combustible gas explosion accidents. Despite all this, this problem has not yet received considerable attention, especially few fundamental data related to the temperature distribution of gas explosions in closed vessels in literatures. According to characteristics of gas deflagrations, this work developed a gas explosion temperature measurement system whose response time to temperature is approximate 10 μs. By using this system, an experimental study was carried out which is concerned with the deflagration temperature distribution of premixed methane-air mixtures in the 20 L spherical vessel with a diameter of 168 mm. Experimental results show that temperatures on or near the wall are obviously lower than those in the center part of the vessel and there is a conspicuous gradient from the wall to the center part of the vessel. In the inside of the vessel, the deflagration temperature of premixed methane-air mixtures near the ignition spot at the center of the vessel can approximately reach 1200 °C, while near the wall, only 300 °C. This result throws a light on the specific regularity of gas temperature distribution near the boundary. It is possible to provide an important basis for understanding the general characteristics of gas deflagrations in closed vessels as well as choosing good measurement designs. Otherwise, if the ignition is located in the geometrical center of the spherical vessel, velocity of the flame increases with the distance away from the center inside the vessel, and when the flame arrives at the inner wall, this velocity descend sharply.     

Thermo-mechanical response of Al 6061 with and without equal channel angular pressing (ECAP)

The thermo-mechanical responses of Al 6061 before and after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. Al 6061 was solution heat treated before ECAP pressing at room temperature and subjected to up to three passes. After pressing, the billets were aged at 100 °C for 2 days. An as-received Al 6061-T651 was studied similarly to investigate the differences between processed and non-processed specimens. The responses of ECAP material were determined at −30, 22, 125 and 250 °C, and at strain rates from 10⁻⁵ to 2530 s⁻¹; the 6061-T651 specimens were subjected to uniaxial compressive loading at −31, 22, 85, 150, 230 and 315 °C, and strain rates ranging from 10⁻⁵ to 2200 s⁻¹. It was found that, the ECAP process increases the strength versus the T651 condition. Additionally, the Al 6061 ECAP is not sensitive to strain rate at room and lower temperatures, but the sensitivity increases as the number of passes and/or temperature are increased and this is the same for the non-processed material. Increasing the number of passes increases the flow stress at room and lower temperatures, has almost no effect at 125 °C and decreases at 250 °C. For both materials, the dynamic flow stress is higher than the stress at quasi-static strain rates even when the quasi-static strain rate regime is insensitive to strain rate. The Al 6061 has strong texture after one pass but steadily increases as the number of passes are increased. This is the first study that reports on the thermo-mechanical responses of ECAP and non-ECAP Al 6061 at such a wide range of strain rates, including dynamic, and temperatures.     

A low frequency torsional rheometer

Summary An instrument has been developed for measuring the viscoelastic behaviour of polymer melts at low frequencies, in the range 10^–3 to 50 Hz. The sample is contained between a cone and a fixed plate, or between parallel plates. The moving member is driven in torsional oscillation through a torsion wire. The amplitude of the resulting oscillation is compared in amplitude and phase with the driven end of the torsion wire. The amplitudes are measured digitally using optical diffraction gratings, and either an oscilloscope or a high-speed ultra-violet recorder is used to determine the phase angle between the two signals. The moving member is supported on an air bearing, which provides a very low friction support with a high degree of positional control thus giving a well defined sample geometry. The torsion wire is driven using a vibrator with a d.c. drive amplifier fed from a very low frequency oscillator. The sample temperature is controlled to better than 0.01 °C, with temperature gradients across the sample of a similar order of magnitude. The temperature range of the instrument is from –50 °C to +200 °C.The angular resolution of the measuring system is 3 × 10^–5 radius, so that an accuracy of better than ±1% in the amplitude measurements can be obtained with the amplitude of shear in the sample kept sufficiently low that a linear stress-strain relation is maintained.With 3 figures     

Thermal contraction and volume relaxation of amorphous polymers

Two experimental techniques are described for the determination of the change of specific volume of polymers with temperature and aging time, which allow measurements between – 160 °C and + 200 °C. Four technical amorphous polymers, PS, PVC, PMMA and PC have been investigated. Volume-temperature curves under constant rate of cooling are presented and interpreted with respect to relaxation processes known from other physical investigations. The rate dependence of dilatometric glass transition temperatures is compared with the time dependence of rheometric glass transition temperatures from shear creep data. Volume relaxation data at constant aging temperature are presented. Aging is found to proceed until very low temperatures in the glassy state for e.g. PMMA.For polystyrene, a comparison is made between the predictions of a very simple theory of volume relaxation due to Kovacs with experimental data, using additional information from volume temperature curves and the time temperature shift of the shear creep transition. The theory predicts a rate of volume relaxation which is much lower than that found by experiment.