Conductor destruction in electrical explosion

Conductor damage has been examined when the current is switched off at various stages in electrical explosion. If the switching is fairly rapid, kink instability growth is accompanied by formation of vapor bubbles at the kinks within the volume of the liquid metal. Later switching results in vapor bubbles uniformly distributed along the conductor. Cumulative ejection occurs under certain conditions. A qualitative interpretation is given.     

Kinetic theory analysis of explosive boiling of a liquid droplet

The process of rapid phase transition from highly superheated liquid to vapor is frequently so fast and violent that it is called explosive boiling. The paper uses the kinetic theory of evaporation to study growth of an internal vapor bubble produced by homogeneous nucleation within a highly superheated liquid droplet boiling explosively in a hot medium. Evaporation/condensation coefficient is estimated by comparing the predictions of the theory with available experimental data. We show that the value of the evaporation coefficient can be very low for high reduced temperatures (0.06 for butane at 378 K), in agreement with recent molecular dynamic simulations.     

Transient behavior of boiling bubbles generated on the small heater of a thermal ink jet printhead

The fundamental behavior of boiling bubbles generated on a small film heater used for thermal ink jet (TIJ) printers is investigated experimentally under the condition of a single pulse heating in a pool of water. The pulse power and the pulse width are varied in wide ranges that include the printing conditions. As the pulse power is increased or the pulse width increased at a fixed high pulse power, numerous fine bubbles appear simultaneously on the heater and then coalesce into a thin vapor film to grow to a vapor bubble, before collapsing at the center of the heater. For a long pulse width sequence, the coalesced bubble repeats the growth and collapse. Bubble behavior is also studied in the same heat flux range using a platinum film heater enabling surface temperature measurement. From a comparison of the two heaters, the dominant mechanism of nucleation on the TIJ heater is believed to be spontaneous nucleation at around the heating rate for printing. The dependence of the size and lifetime of the coalesced bubble on pulse power and pulse width are examined. Based on the analytical model presented by Asai [J. Heat Transfer 113 (1991) 973], the pressure impulse arising during the rapid evaporation of the superheated liquid, presumed to dominate the subsequent growth of the coalesced bubble, is estimated from the measured size of the coalesced bubble. The relationship between the pressure impulse and the superheat energy in the liquid is discussed.     

Heterogeneous nucleation on ultra smooth surfaces

An experimental investigation is presented with heterogeneous nucleate boiling on ultra smooth metallic surfaces (30-365 nm RMS roughness), including brass, unpolished stainless steel, and electropolished stainless steel. The fluids used for the investigation are highly wetting pentane and butane. It is observed that the incipient superheat is low for all cases considered, despite the fact that no vapor trapping cavities are available for incipience at low superheat. These data provide further evidence that in addition to vapor trapping, another mechanism must be available for heterogeneous nucleation in boiling systems. The boiling curves are presented for the different surface/fluid combinations. It is found that the heat transfer rate on the brass surface is considerably better than that on the stainless steel surfaces due to the ease with which nucleation sites are formed. In contrast, nucleation site formation on stainless steel is considerably more sparse than that on brass.     

On the dynamics and instability of bubbles formed during underwater explosions

Dynamics of explosion bubbles formed during underwater detonations are studied experimentally by exploding fuel (hydrogen and/or carbon monoxide)–oxygen mixture in a laboratory water tank. Sub-scale explosions are instrumented to provide detailed histories of bubble shape and pressure. Using geometric and dynamic scaling analyses it has been shown that these sub-scale bubbles are reasonable approximations of bubbles formed during deep sea underwater explosions. The explosion bubble undergoes pulsation and loses energy in each oscillation cycle. The observed energy loss, which cannot be fully explained by acoustic losses, is shown here to be partly due to the excitation of instability at the interface between the gaseous bubble and the surrounding water. Various possible mechanisms for the dissipation of bubble energy are addressed. The analysis of the experimental data gives quantitative evidence (confirmed by recent numerical studies) that the Rayleigh–Taylor instability is excited near the bubble minimum. The dynamics of the bubble oscillation observed in these experiments are in good agreement with experimental data obtained from deep sea explosions     

Velocity fields around spheres and bubbles investigated by laser-doppler anemometry

An experimental investigation is presented in which the velocity fields around sheres and bubbles moving in a cylinder have been measured by laser-Doppler anemometry (LDA). Instabilities in the flow field at rather low Deborah numbers have been discovered and these instabilities are damped by inertia forces. It is shown that the wall correction factor K is a rapidly decreasing function of the Deborah number. The experimental measurements have been compared with numerical simulations, and on the basis of this comparison it has been possible to identify a time constant and a zero-shear-rate viscosity for the test liquid.     

Initiation of explosive boiling of a droplet with a shock wave

The role of incident shock waves in the initiation of vapor explosions in volatile liquid hydrocarbons has been investigated. Experiments were carried out on single droplets (1–2 mm diameter) immersed in a host fluid and heated to temperatures at or near the limit of superheat. Shocks generated by spark discharge were directed at previously nonevaporating drops as well as at drops boiling stably at high pressure. Explosive boiling is triggered in previously nonevaporating drops only if the drop temperature is above a threshold temperature that is near the superheat limit. Interaction of a shock with a stably boiling drop immediately causes a transition to violent unstable boiling in which fine droplets are torn from the evaporating interface, generating a two-phase flow downstream. On the previously nonevaporating interface between the drop and the host liquid, multiple nucleation sites appear which grow rapidly and coalesce. Overpressures generated in the surrounding fluid during bubble collapse may reach values on the same level as the pressure jump across the shock wave that initiated the explosive boiling. A simple calculation is given, which suggests that shock focusing may influence the location at which unstable boiling is initiated.     

Visualization of capillary boiling

Capillary boiling has been experimentally analyzed using an optical technique, based on the extraction of gray levels at specific locations from video frames. Boiling is achieved by placing a small capillary in a large container filled with water at boiling temperature. A heating wire located inside the capillary provides the additional heat that triggers the evaporation. The phenomenon is videotaped and digitized. A small region of the filmed field is chosen and the gray level of all the digitized frames is stored in a file which is then analyzed. This constitutes in essence the possibility of using a large number of non-intrusive, fixed (Eulerian) virtual sensors. The information extracted concerns the global properties of the phenomenon such as the time between the departure of two subsequent bubbles, and more detailed properties such as the shape and volume of the departing bubbles or even the motion of liquid packets inside the capillary. Moreover, this technique permits the study of time and space correlations. It is observed that the bubbles depart from the capillary tip with a variable frequency that can be approximately grouped around two values. Clear evidence that this is due to two different mechanisms is given. The time between the departure of two bubbles is correlated with the volume of the departing bubbles. Received: 21 April 1999/Accepted: 18 July 2000     

Flow boiling heat transfer characteristics of R123 and R134a in a micro-channel

Flow boiling heat transfer in a single circular micro-channel of 0.19 mm ID has been experimentally investigated with R123 and R134a for various experimental conditions: heat fluxes (10, 15, 20 kW/m²), mass velocities (314, 392, 470 kg/m² s), vapor qualities (0.2–0.85) and different saturation pressures (158, 208 kPa for R123; 900, 1100 kPa for R134a). The heat transfer trends between R123 and R134a are clearly distinguished. Whether nucleate boiling is suppressed at low vapor quality or not determines the heat transfer trend and mechanism in the flow boiling of micro-channels. High convective heat transfer coefficients in the two-phase flow of micro-channels enables nucleate boiling to be suppressed even at low vapor quality, depending on the wall superheat requirement for nucleate boiling. In the case of early suppression of nucleate boiling, specifically R123, heat transfer is dominated by evaporation of thin liquid films around elongated bubbles. In the contrary case, namely R134a, nucleate boiling is dominant heat transfer mechanism until its suppression at high vapor quality and then two-phase forced convection heat transfer becomes dominant. It is similar to the heat transfer characteristic of macro-channels except the enhancement of nucleate boiling and the short forced convection region.     

Influence of bubble expansion and relative velocity on the performance and stability of an airlift pump

An airlift pump which raises liquid by means of compressed air introduced near the lower end of the eduction pipe is an example of a self-control system. It has been shown by Hjalmars (1973) that an occasionally observed breakdown in the self-control mechanism, which leads to instability, is due to the fact that the control mechanism is delayed, with the effect that a small, time-dependent perturbation of the stationary flow satisfies a differential equation with delayed argument. This investigation was carried out with the assumption of a single-phase flow of an ideal incompressible liquid. The aim of the present study is to consider the stability conditions of an airlift pump within the frame of a more general flow model, namely a separate two-phase flow of compressible gas and incompressible liquid, which takes into account the effects of the expasion of gas bubbles during their lift and of the relative velocity, i.e. the difference in the velocity of gas bubbles and the liquid.     

An experimental investigation of droplet evaporation and coalescence in a simple jet flow

An experimental study of a simple jet flow, which contains a dispersion of fine droplets, has been carried out in order to investigate the effect of turbulence, evaporation and coalescence on the droplet size distributions within the jet. Very little evaporation occurs in the potential core of the jet, while in the far-field, where the potential core has vanished and the droplets disperse more readily, evaporation occurs predominantly in the outer portions of the spray. Evidently, turbulence enhances the evaporation rate of droplets at the edges of the spray, and fresh air entrained from the outer regions increases the evaporative driving force. Coalescence has also been observed within the spray, although this effect is rather subtle compared to the evaporation effect in the dilute jets investigated here. Nevertheless, sufficient measurements have been taken to validate, at least partially, any coalescence models, in addition to any turbulence and evaporation models for dilute poly-disperse sprays.     

Instability waves and the origin of bubbles in fluidized beds: Part 1: Experiments

Statistically significant measurements of the propagation properties of instability waves in a two-dimensional liquid fluidized bed are reported. Visual and quantitave measurements show that although the waves experience an initially exponential growth in amplitude, the ultimate state of motion exhibited is that of the complicated formation and destruction of cylindrical bubble-like structures. Expected values of their amplitude, frequency, and velocity are measured, and preliminary scaling laws are proposed. The implication is that bubbles in gas-fluidized beds are a result of the same instability of the state of uniform fluidization.     

Evaporation of pure liquids with increased viscosity in a falling film evaporator

The present study investigated fluid dynamics and heat transfer of viscous pure liquids in a falling film evaporator. This is of special benefit as it avoids mass transfer effects on the evaporation behaviour. Experiments at a single-tube glass falling film evaporator were conducted. It allowed a full-length optical film observation with a high-speed camera. Additionally the evaporator was equipped with a slotted weir distribution device. Test fluids provided viscosities ranging from μ = 0.3 to 41 mPa s. The Reynolds number was between 0.7 and 1,930. Surface evaporation and the transition to nucleate boiling were studied to gain information about the film stability at maximum wall superheat. A reliable database for laminar and laminar-wavy viscous single component films was created. The experimental results show a significant enhancement in the wave development due to the film distribution. A wavy flow with different wave velocities was superposed to the film in each liquid load configuration without causing a film breakdown or dry spots on the evaporator tube. It was found that nucleate boiling can be allowed without causing film instabilities over a significant range of wall superheat.     

Control of vortex shedding by thermal effect at low Reynolds numbers

An experimental study has been made of the control of vortex shedding in the wake of two two-dimensional bluff bodies, a circular cylinder and a flat ribbon. The study has shown that this control, easily realized by heating the bluff body, depends on the nature of the fluid. In the absence of buoyancy effects, related to the temperature dependence of the dynamic viscosity, the heating is found to stabilize the wake in air while the opposite result is obtained in water. Detailed measurements of the velocity fields in air, in isothermal and in heated body, show that this control is linked to slight modifications of the flow in the near wake and can be taken into account by the effective Reynolds number approach. The measurements also show that the degree of instability can be related to the level of interaction between the two initial shear layers at the end of the recirculation zone.     

Motion of bubbles and bubble-droplets in an immiscible liquid

A method is presented by which the movement of bubbles is recorded using still photography. Data can be obtained such as path, velocity, indications of surface instabilities, variation of size like growth and collapse.Results are presented for rising small and medium size air bubbles, spherical cap-shape butane bubbles, evaporating butane droplets and condensing butane bubbles in distilled water.

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Bewegungen von Blasen und Blasen-Tropfen in nichtmischbarer Flüssigkeit

Zusammenfassung Es wird eine Methode zur Wiedergabe der Blasenbewegung unter Benutzung der Standphotographie mitgeteilt. Man erhält damit Informationen über den Weg, die Geschwindigkeit, Anzeichen von Instabilitäten der Oberfläche und eine Änderung der Größe wie durch Wachstum und Zusammenbruch.Ergebnisse werden für den Aufstieg kleiner und mittlerer Luftblasen, kugeliger hutförmiger Butanblasen, verdampfender Butantropfen und kondensierender Butanblasen in destilliertem Wasser mitgeteilt.

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Nomenclature ATMP Atmospheric pressure (mm Hg) - f Number of bubbles per second - H Water head above the nozzle tip (mm) - T_n Temperature in the nozzle (°C) - T_s Butane saturation temperature at the nozzle tip (°C) - T_w Water temperature (°C) - U Rise velocity (mm/s) - Z Height above the nozzle tip (mm) - T Temperature difference - t Time between every consecutive shot(s)     

Evaporator critical heat flux in the double-wall artery heat pipe

The fact that heat is transferred into a heat pipe through the liquid-saturated evaporator wick gives rise to the so-called boiling limit on the heat pipe capacity. The composite nature of the double-wall artery heat pipe (DWAHP) wick structure makes the prediction of the evaporator superheat (Δ T_crit) and the critical radial heat flux (q_r) very difficult. The effective thermal conductivity of the wick, the effective radius of critical nucleation cavity, and the nucleation superheat, which are important parameters for double-wall wick evaporator heat transfer, have been evaluated based on the available theoretical models. Empirical correlations are used to corroborate the experimental results of the 2 m DWAHP. A heat choke mounted on the evaporator made it possible to measure the evaporator external temperatures, which were not measured in the previous tests. The high values of the measured evaporator wall temperatures are explainable with the assumption of a thin layer of vapor blanket at the inner heating surface. It has been observed that partial saturation of the wick (lean evaporator) causes the capillary limit to drop even though it may be good for efficient convective heat transfer through the wick. The 2 m long copper-water heat pipe had a peak performance of 1850 W at 23 W/cm² with a horizontal orientation.     

Limit of superheat in uniformly heated fluid

The homogeneous nucleation limit is investigated in pure liquid subject to intense uniform heating at constant pressure. The energy equation is solved in conjunction with a new non-equilibrium vapor formation model in order to predict the maximum attainable liquid superheat as a function of the heating rate. It is shown that for uniformly heated liquids, conditions related to the local temperature in a critical vapor embryo and to the local heat consumption for vapor generation on the homogeneous fluctuation centers, must be satisfied simultaneously in order to initiate explosive boiling. The effect of heating rate on the maximum attainable superheat temperature could be as high as 10 K.     

Numerical investigation of static flow instability in a low-pressure subcooled boiling channel

A three-dimensional two-fluid model to predict subcooled boiling flow at low pressure is presented. The model is adopted to investigate the two-phase flow and heat transfer characteristics in a heated channel. The presence of bubbles as a consequence of heating flow through a vertical rectangular channel has a significant effect on the overall pressure drop along the channel. Numerical results were compared against a series experimental data performed at various conditions – mass flux, heat flux, inlet temperature and exit pressure. Good agreement on the overall pressure drop was achieved. The onset of flow instability velocity was also accurately determined when compared against measurements. Predicted results of void fraction provided useful information towards a more fundamental understanding of the occurrence of onset of nucleate boiling, onset of significant voiding and onset of flow instability. The phenomenon of boiling onset oscillations was also predicted through the use of the two-fluid model.     

An experimental investigation of combined turbulent free and forced evaporation

The wide variation in correlations available in the literature for predicting water evaporation rates in a moving air stream necessitated a new investigation to determine which correlations can be considered reliable. Water evaporation measurements were made from a heated pool (a class-A pan) into a low speed wind tunnel. The evaporation regime examined combined turbulent free and turbulent forced convection over the range 0.1 < Gr_m/Re² < 10.0. The data includes the range in which combined convection modes are important, as well as the limits where either free or forced convection effects may dominate. The data are compared to several evaporation correlations based on laboratory wind tunnel data. These historical correlations do not produce consistent estimates in predicting evaporation rates. It is believed that the apparent inconsistencies arise because many correlations do not adequately describe the appropriate evaporation regimes for which they are valid. A new correlation using the combined free/forced convection Sherwood number has been developed to predict evaporation rates for a moving air stream. This correlation allows the results of this study to be extended to other evaporating conditions (i.e. variation in surface geometry and air turbulence levels) than those described here. For a 95% confidence limit, the Sherwood number correlation matches the data within ±7.9%.     

Dye-bubble interactions in an open channel flow

An innovative technique has been developed to visualize the effect that a localized surface reaction has in an open channel flow field. The working fluid is hexanoic acid mixed with mineral oil, and it flows over an aluminum plate embedded with sodium metal. Hexanoic acid and sodium metal react to form hydrogen gas and hexanoic salt. The hydrogen gas forms bubbles that rise to the surface and are convected downstream by the fluid. The rising bubbles induce the formation of counter-rotating vortices that straddle the reaction site. Bubble entrainment stretches and bends the dye filaments, and buoyancy transports the bubbles away from the reaction. The products of the reaction introduce velocity fluctuations into an otherwise laminar flow, inducing what has been described by some researchers as pseudoturbulence. Downstream of the reaction, far away from the disturbances caused by the buoyant bubbles, the velocity fluctuations dampen out and the flow relaminarizes.