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.     

Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

On the quenching of steel and zircaloy spheres in water-based nanofluids with alumina,silica and diamond nanoparticles

The quenching curves (temperature vs time) for small (∼1 cm) metallic spheres exposed to pure water and water-based nanofluids with alumina, silica and diamond nanoparticles at low concentrations (?0.1 vol%) were acquired experimentally. Both saturated (ΔT_sub = 0 °C) and highly subcooled (ΔT_sub = 70 °C) conditions were explored. The spheres were made of stainless steel and zircaloy, and were quenched from an initial temperature of ∼1000 °C. The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. However, it was found that some nanoparticles accumulate on the sphere surface, which results in destabilization of the vapor film in subsequent tests with the same sphere, thus greatly accelerating the quenching process. The entire boiling curves were obtained from the quenching curves using the inverse heat transfer method, and revealed that alumina and silica nanoparticle deposition on the surface increases the critical heat flux and minimum heat flux temperature, while diamond nanoparticle deposition has a minimal effect on the boiling curve. The possible mechanisms by which the nanoparticles affect the quenching process were analyzed. It appears that surface roughness increase and wettability enhancement due to nanoparticle deposition may be responsible for the premature disruption of film boiling and the acceleration of quenching. The basic results were also confirmed by quench tests with rodlets.     

Droplet evaporation on porous and non-porous ceramic solids heated from top

 This paper presents the results of an experimental investigation, of the effect of radiation heat, on the evaporation of five droplet sizes of pure water, softly deposited on porous and non-porous ceramic solids, at temperature ranging from 75 to 250 °C. Both solids were instrumented with several surface and in-depth thermocouples, and had the same thermal properties. Results show that, the droplet evaporation time, and the surface recovery time for the porous solid were shorter than that of non-porous solid for the same droplet size under identical conditions. Also, smaller droplets were more efficient for cooling both solids. The results were compared with data for the evaporation of water droplets on similar ceramic solids heated from bottom (Abu-Zaid M; Atreya A (1994) J Heat Transfer 116: 694–701). The comparison shows that, the heat radiation has a significant effect of reducing evaporation time, recovery time, and droplet volume of influence for both solids, at the same initial surface temperature. Received on 6 December 1999 / Published online: 29 November 2001     

Experimental study of water droplet boiling on hot, non-porous surfaces

In this paper, the results of a series of experimental tests on single- and multi-droplet boiling systems are presented and discussed. The main objectives of the present study are: a) to investigate experimentally the effect of the boiling onset on the evaporation rate of water droplets; b) to measure the evolution of the solid surface temperature during evaporation; c) to examine the possibility of improving spray cooling efficiencies. The behavior of small water droplets (from 10 to 50 μl) gently deposited on hot, non-porous surfaces is observed. The evaporation of multi-droplet arrays (50 and 100 μl) under the same conditions of the single-droplet tests is analyzed. In particular, the conditions which determine the onset of nucleate and film boiling are stressed out. In the experimental tests, the interaction of different materials with several multi-droplet systems is monitored by infrared thermography. The spray cooling efficiency is related to the solid temperature decrease as a function of the water mass flux. In the present study, the effect of varying the droplet volume and the mass flux is also analyzed and discussed. The results on the droplets evaporation time and on the solid surface transient temperature distribution are also compared with the data obtained by the same authors during the analysis of droplet evaporation in total absence of nucleate and film boiling. In order to analyze the different behavior of the evaporating droplet as a function of the solid surface thermal conductivity, evaporative transients on aluminum, stainless steel and macor (a glass-like, low-conductivity material) are considered. Received on 20 February 1998     

Gas temperature in the trace of water droplets streamlined by hot air flow

In order to obtain the knowledge necessary for developing new effective fire extinguishing technologies, we determined experimentally the gas temperature in the trace of water droplets streamlined by hot air flow. It was important to establish how much the temperature in the droplet trace decreases and how fast it recovery to the initial temperature field after the droplet evaporation. The following parameters were varied: droplet size from 1.3 mm to 1.7 mm, velocity from 1 m/s to 5 m/s, initial airflow temperature from 473 K to 773 K, number of droplets (one or two), and the arrangement of droplets relative to the hot inflow (serial or parallel). The study proves the theoretical hypothesis about a significant influence of evaporation on the temperature in the water droplet trace. When a temperature trace of water droplets is formed, irrespective of their arrangement, the role of the evaporation process strengthens with the gas flow temperature rising. Furthermore, the study specifies typical longitudinal dimensions of the aerodynamic and temperature traces of water droplets. It has been established that when droplets are located in series and in parallel, their combined impact on the temperature and velocity of the gas flow in the medium differs rather considerably.     

煤油液滴直径对两相旋转爆轰发动机流场的影响

为探究煤油液滴不同初始直径对气液两相旋转爆轰发动机流场的影响,假设初始注入的煤油液滴具有均匀直径,考虑雾化破碎、蒸发等过程,建立了非定常两相爆轰的Eulerian-Lagrangian模型,进行了液态煤油/高温空气爆轰的非预混二维数值模拟。结果表明：在初始液滴直径为1～70μm的工况范围,燃烧室内均形成了单个稳定传播的旋转爆轰波;全局当量比为1时,爆轰波前的空气区域大于液滴煤油的蒸气区域,导致波前燃料空气混合不均匀,波前均存在富油区和贫油区,两相速度差导致分离出的空气形成低温条带;当煤油液滴的初始直径较小时,波前的反应物混合过程主要受蒸发的影响,爆轰波可稳定传播;当直径减小至1μm时,煤油液滴在入口处即蒸发,旋转爆轰波表现为气相传播的特性,爆轰波结构平整;当煤油液滴的初始直径较大时,波前的反应物混合过程主要受液滴破碎的影响;对于相同的燃料质量流量,在不同初始煤油液滴直径工况下,煤油液滴最大的停留时间均占爆轰波传播时间尺度的80%以上;爆轰波前燃料预蒸发为气相的占比越高,爆轰波的传播速度越高;初始液滴直径为10～70μm的工况范围内,爆轰波的速度随初始直径的增大先升高后降低。     

Inertia dominated flow and heat transfer in liquid drop spreading on a hot substrate

The present work deals with computational modeling of the fluid flow and heat transfer taking place in the process of impact of a cold liquid drop (T_d = 20-25 °C) onto a dry heated substrate characterized by different thermophysical properties. The computational model, based on the volume-of-fluid method for the free-surface capturing, is validated by simulating the configurations accounting for the conjugate heat transfer. The simulations were performed in a range of impact Reynolds numbers (Re = 2000-4500), Weber numbers (We = 27-110) and substrate temperatures (T_s = 100-120 °C). The considered temperature range of the drop-surface, i.e. liquid-solid system does not account for the phase change, that is boiling and evaporation. The model performances are assessed by contrasting the results to the reference database originating from the experimental and complementary numerical investigations by Pasandideh-Fard et al. [Pasandideh-Fard, M., Aziz, S., Chandra, S., Mostaghimi, J., 2001. Cooling effectiveness of a water drop impinging on a hot surface. International Journal of Heat and Fluid Flow, 22, 201-210] and Healy et al. [Healy, W., Hartley, J., Abdel-Khalik, S., 2001. On the validity of the adiabatic spreading assumption in droplet impact cooling. International Journal of Heat and Mass Transfer, 44, 3869-3881]. In addition, the thermal field obtained is analyzed along with the corresponding asymptotic analytical solution proposed by Roisman [Roisman, I.V., 2010. Fast forced liquid film spreading on a substrate: flow, heat transfer and phase transition. Journal of Fluid Mechanics, 656, 189-204]. Contrary to some previous numerical studies, the present computational model accounts for the air flow surrounding the liquid drop. This model feature enables a small air bubble to be resolved in the region of the impact point. The reported results agree reasonably well with experimental and theoretical findings with respect to the drop spreading pattern and associated heat flux and temperature distribution.     

Measuring evaporation of micro-fuel droplets using magnified DIH and DPIV

This paper details the use of magnified digital in-line holography (MDIH) and digital particle image velocimetry (DPIV) to measure the evaporation rates of fuel micro-droplets undergoing heating. The technique can be used to measure instantaneous evaporation along an individual droplet trajectory, or if applied to a series of droplets, the average evaporation over a number of successive measurement locations. The advantage of this technique over traditional optical techniques is greater spatial resolution and depth of field for the high magnification factors used. An application of the technique to the evaporation measurement of diesel fuel droplets ranging from 10 to 90 μm is presented. Results reveal that similar to larger droplets, temperature plays the dominant role in evaporation processes, with little sensitivity to initial droplet size found for a peak reactor temperature of 660 K.     

On the calculation of wall temperatures in the post dryout heat transfer region

A non-equilibrium post dryout heat transfer model for calculating the wall temperature distribution in vertical upflows is presented in this study. The model is based upon the three path heat transfer formulation developed by MIT researchers (Laverty & Rohsenow 1964, Forslund & Rohsenow 1968, Hynek et al. 1969 and Plummer et al. 1974) that involves heat transfer from wall to vapor, from wall to droplets in contact with the wall and from vapor to liquid droplets in the vapor core. Downstream gradients for the bulk vapor temperature, vapor quality, droplet size and vapor velocities are identical to those used by Hynek et al. (1969) and Plummer et al. (1974). Conditions at the dryout location are calculated using a modified version of a technique developed by Hynek et al. (1969).A procedure for determining an average droplet diameter based on a size distribution is introduced. Migration of droplets through the boundary layer and droplet deposition flux are predicted with the model of Gani? & Rohsenow (1979). Heat transfer from the wall to the impinging liquid droplets is calculated with a correlation by Holman & McGinnis (1969). Mechanisms contributing to wall to droplet heat transfer are identified as (a) droplet-wall contact, (b) intensive droplet evaporation inside the boundary layer, and (c) destruction of the boundary layer due to droplet migration to, and rebound from, the hot surface. The significance of the average droplet size and size distribution is demonstrated through its control over the free stream evaporation and droplet deposition rates.Predicted uniform heat flux wall temperature profiles for water, nitrogen and freon 12 are in good agreement with the data of Era et al. (1966), Bennett et al. (1967), Forslund & Rohsenow (1968), Ling et al. (1971), Groeneveld (1972) and Janssen & Kervinen (1975).     

After fogging process in water injected gas turbine systems

In gas turbine system with after fogging, water droplets are injected after compressor. After fogging could have more significant potential for enhancement of specific power production compared to inlet fogging alone, since a larger water injection rate is possible. Transient analysis of after fogging process is carried out by using a heat and mass transfer modeling on water droplet evaporation. Transient variables such as droplet diameter and air temperature are evaluated as the droplet evaporation proceeds for different values of initial droplet diameter, pressure ratio of compressor, and water injection ratio. The evaporation time for injected droplets are also estimated. Present results show that the evaporation time decreases sensitively with increasing pressure ratio or initial droplet diameter. However, the effect of water injection ratio on evaporation time is relatively insignificant unless water injection ratio is near the critical ratio.     

Influence of heat and mass transfer on the ignition and NOx formation in single droplet combustion

The effect of heat and mass transfer on the ignition, and in a second step on the nitrogen oxide (NO _x ) generation, of single burning droplets is examined in a numerical study. Spherical symmetry with no gravity and no forced convection is presumed; ambient temperature is set at 500 K, below the auto-ignition point. The essentials of a forced droplet ignition by an external energy source are introduced. Two methods are applied: heat introduction at a fixed radial position r and heat introduction at a fixed local equivalence ratio ϕ _r . This study’s distinctiveness compared to previous research is its focus on and its combination of partially pre-vaporized droplets and detailed chemistry, both being technically relevant in kerosene and diesel fuel combustion. The fuel of choice is n-decane (C₁₀H₂₂), and NO _x production is studied exemplarily as a representative group of pollutant emissions. The conducted simulations show a decrease of NO _x formation with an increase of the pre-vaporization rate \Uppsi. \Uppsi. This decrease is generally valid for both methods of heat introduction. However, results on flame stabilization and NO _x production reveal a high sensitivity to parameters of the ignition model. The burning behavior during the initial stages is dominated by the ignition position. Extracting heat from the exhaust gas region of burning droplets shows no impact on the flame position nor on the relative NO _x production. As a consequence, a well-founded modeling of the investigated droplet regime needs to resort to an iterative adaptation of the heat introduction parameters based on the findings of droplet burning and exhaust gas production.     

Large-Eddy Simulation of Interactions Between a Reacting Jet and Evaporating Droplets

Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial droplet sizes (St ₀) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets (St ₀ = 16), smaller droplets (St ₀ = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect depends on St ₀. When the MLR is sufficiently high, the bigger (St ₀ = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures. On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the dissipative droplet dynamic effect.     

Influence of surfactants on pool boiling of aqueous polyacrylamide solutions

The influence of a surfactant, sodium lauryl sulfate (SLS), on the pool boiling behavior of aqueous polyacrylamide solutions has been investigated. The basic test section is a heated horizontal platinum wire submerged in a saturated pool of liquid at atmospheric pressure. Results are reported for six combinations of deionized water, polyacrylamide and surfactant. Measurements of surface tension and steady shear apparent viscosity are reported for each working solution.The boiling heat transfer performance of deionized water containing concentrations of 250 and 1000 (parts per million by weight) wppm of the surfactant did not differ significantly from the values found for deionized water. For the 250 wppm aqueous polyacrylamide solution the boiling performance is poorer than that found for water alone. However, the addition of 250 wppm of SLS to the aqueous polyacrylamide solution resulted in a significant improvement in the boiling heat transfer performance compared to water. An increase in the surfactant concentration to 1000 wppm to the aqueous polymer solution revealed even greater improvement in the boiling performance. At a heat flux of 50 W/cm², increases in the heat transfer coefficient were as much as 50% for the 250 wppm AP-30 — 250 wppm SLS solution and 100% the for 250 wppm AP-30 — 1000 wppm SLS solution as compared to deionized water alone.On the basis of these results, it appears that the influence of surfactants on the boiling heat transfer performance of aqueous polymer solutions may be more significant than in the case where surfactants are added to Newtonian fluids.

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Einfluß von grenzflächenaktiven Stoffen auf das Sieden von Polyacrylamid-Lösungen

Zusammenfassung Es wurde der Einfluß des grenzflächenaktiven Stoffes Sodium-Lauryl-Sulfat (SLS) auf das Verhalten des freien Behältersiedens von Polyacrylamid-Lösungen untersucht. Die Testeinheit besteht aus einem beheizten Platindraht, der in einem mit Flüssigkeit (Umgebungsdruck) gefüllten Behälter eingetaucht ist. Ergebnisse wurden für sechs Kombinationen von vollentsalztem Wasser, Polyacrylamid-Lösungen und grenzflächenaktiven Stoffen untersucht. Messungen der Oberflächenspannung und Viskosität wurden für jedes Gemisch durchgeführt.Der Wärmetransport beim Sieden von vollentsalztem Wasser mit einem Anteil an SLS von 250 bis 1000 wppm (parts per million by weight) unterscheidet sich nicht von dem bei reinem, vollentsalztem Wasser. Bei einer Lösung mit 250 wppm Polyacryl ist das Sieden schwächer als bei reinem Wasser. Bei einer Zugabe von 250 wppm SLS zur Polyacryl-Lösung ist eine signifikante Zunahme des Wärmetransportes beim Sieden festzustellen. Ein Erhöhen der SLS-Konzentration auf 1000 wppm verstärkt nochmals diese Zunahme. Bei einem Wärmefluß von 50 W/cm² erhöht sich der Wärmetransportkoeffizient bei der AP-30 — 250 wppm SLS Lösung um 50% und bei der AP-30 — 1000 wppm SLS Lösung um 100% im Vergleich zum vollentsalzten Wasser.Mit diesen Ergebnissen wird deutlich, daß der Einfluß von grenzflächenaktiven Stoffen auf den Wärmetransport beim Sieden von polymeren Lösungen signifikanter als im Falle von Newtonschen Fluiden ist.

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Nomenclature q heat flux, (W/cm²) - T fluid temperature, (°C) - T _sat saturation temperature of the fluid, (°C) - T _w surface wall temperature, (°C) - T =T _w–T _sat, (°C) - steady shear apparent viscosity, (poise) - shear rate, (sec^–1) - surface tension, (dyne/cm) Dedicated to Prof. Dr.-Ing. U. Grigull's 80th birthday     

Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Turbulent combustion of mono-disperse droplet-mist has been analysed based on three-dimensional Direct Numerical Simulations (DNS) in canonical configuration under decaying turbulence for a range of different values of droplet equivalence ratio (?_d), droplet diameter (a_d) and root-mean-square value of turbulent velocity (u^′). The fuel is supplied in liquid phase and the evaporation of droplets gives rise to gaseous fuel for the flame propagation into the droplet-mist. It has been found that initial droplet diameter, turbulence intensity and droplet equivalence ratio can have significant influences on the volume-integrated burning rate, flame surface area and burning rate per unit area. The droplets are found to evaporate predominantly in the preheat zone, but some droplets penetrate the flame front, reaching the burned gas side where they evaporate and some of the resulting fuel vapour diffuses back towards the flame front. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ?_d > 1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets and this predominantly fuel-lean mode of combustion exhibits the attributes of low Damköhler number combustion and gives rise to thickening of flame with increasing droplet diameter. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion and the relative contribution of non-premixed combustion to overall heat release increases with increasing droplet size. The statistical behaviours of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.     

Dynamics of two-phase bubble-droplets in immiscible liquids

This paper describes an experimental study of two-phase bubble-droplets of butane in distilled water. In the process of direct-contact heat transfer between two immiscible liquids, and with change of phase, evaporation occurs within a liquid droplet to form an interior bubble which ultimately may expand to absorb the entire droplet. The so-called stopped-evaporation two-phase bubble-droplets were formed by such evaporating droplets of butane rising in a column of water by application of pressure on the surface of the water to stop the evaporation. The configurations of such bubble-droplets, which depend on the ratio of mass of the vapor to mass of the liquid, are discussed. Results are also given for the rise velocity, and comparisons are made between the rise velocity of stopped-evaporation droplets and those of evaporating droplets.

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Dynamik zweiphasiger Blasen-Tropfen in unmischbaren Flüssigkeiten

Zusammenfassung Die experimentelle Untersuchung von zweiphasigen Blasen-Tropfen aus Butan in destilliertem Wasser wird beschrieben. Bei direktem Wärmeaustausch mit Phasenänderung zwischen zwei unmischbaren Flüssigkeiten beginnt die Verdampfung innerhalb eines Flüssigkeitstropfens und bildet eine innere Blase, die sich schließlich ausdehnt und den Tropfen absorbiert. Ein Ende der Verdampfung konnte beim Aufsteigen von Butantropfen in eine Wassersäule durch Anlegen von Druck an der Wasseroberfläche erreicht werden. Die Struktur dieser Blasen-Tropfen, die abhängt vom Verhältnis der Massen des Dampfes und der Flüssigkeit, wird diskutiert. Außerdem werden Ergebnisse für die Aufstiegsgeschwindigkeit gegeben und die Aufstiegsgeschwindigkeiten der verdampfenden Tropfen mit denen bei beendeter Verdampfung verglichen.

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Nomenclature d Equivalent spherical diameter of initial droplet (mm) - D Equivalent spherical diameter of bubble-droplet (mm) - m₀₀ Initial mass of droplet (kg) - m _v Mass of vapour (kg) - Re _c Reynolds number based on water properties (Re_c=UD/) - t Time (s) - T _c Temperature of water (°C) - U Rise velocity (m/s) - Z Position of bubble-droplet (mm) - t ₀ Overall temperature difference (C deg) - Density (kg/m³) - Viscosity (kg/m s)     

Heat transfer during transient cooling of high temperature surface with an impinging jet

 An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling of ΔT _sub = 20–80 K, a jet velocity of u _j  = 5–15 m/s, a nozzle diameter of d _j  = 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C and the transient temperatures in the block were measured at eight locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with time was evaluated using a numerical analysis of 2-D heat conduction. Behavior of the wetting front, which is extending the nucleate boiling region outward, is observed with a high-speed video camera. A position of wetting region is measured and it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient cooling. Received on 17 March 2000     

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)     

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.