Measurement of the evaporation mass flow rate in a horizontal liquid layer partly opened into flowing gas

The dynamics of evaporation from a local surface of a horizontal liquid layer under a gas flow is studied experimentally. The average evaporation mass flow rate of a liquid layer (HFE-7100) induced by inert gas (nitrogen) is measured using two independent methods. The influence of the average gas flow rate, gas and liquid temperature, and the layer depth upon the evaporation rate and convection in the liquid layer is investigated. Correlation dependences of the evaporation rate vs. the inert gas flow rate and temperature are obtained. It is found that the average evaporation-rate curve has a local maximum with a growth of the liquid layer depth. With the growth of the gas and liquid temperature, the local maximum in an evaporation flow rate of the liquid layer is shifted to the larger value of the liquid depth.     

Study of Evaporative Convection in an Open Cavity under Shear Stress Flow

When a layer of volatile liquid is exposed to a shear flow of inert gas, thermal patterns, in the form of interfacial ripples and bulk plumes, are created by the combined action of evaporative, shear-driven and surface-tension-driven instabilities. The topology of the interfacial thermal patterns is mainly influenced by the geometry of the evaporating surface, the thickness of the evaporating layer, the intensity of the shear flow and by the physic-chemical properties of the working fluid. In this paper, by means of numerical simulations, we focused our attention on the dynamics of the interfacial thermal patterns for different working fluids and thicknesses of the volatile liquid layer. This study has been performed in the frame of the ESA sponsored Space Program on heat and mass transfer CIMEX-1. The choice of the fluids—ethyl alcohol and FC72 (n-perfluorohexane)—the reference values for the inert gas flow rate, the thickness of the liquid layer as well as the geometrical features of the computational domain correspond exactly to the ones foreseen for the CIMEX-1 experiment. However, the main conclusions can be considered of more general validity.     

Evaporation Rates and Bénard-Marangoni Supercriticality Levels for Liquid Layers Under an Inert Gas Flow

In this work, we propose an approximate model of evaporation-induced Bénard-Marangoni instabilities in a volatile liquid layer with a free surface along which an inert gas flow is externally imposed. This setting corresponds to the configuration foreseen for the ESA—“EVAPORATION PATTERNS” space experiment, which involves HFE-7100 and nitrogen as working fluids. The approximate model consists in replacing the actual flowing gas layer by an “equivalent” gas at rest, with a thickness that is determined in order to yield comparable global evaporation rates. This allows studying the actual system in terms of an equivalent Pearson’s problem (with a suitably defined wavenumber-dependent Biot number at the free surface), allowing to estimate how far above critical the system is for given control parameters. Among these, a parametric analysis is carried out as a function of the liquid-layer thickness, the flow rate of the gas, its relative humidity at the inlet, and the ambient pressure and temperature.     

Analysis of a convective fluid flow with a concurrent gas flow with allowance for evaporation

The paper presents a theoretical analysis of a convective fluid flow with a concurrent gas flow accompanied by evaporation at the interface. The analysis of two-layer flows is based on a mathematical model taking into account evaporation at a thermocapillary boundary and effects of thermal diffusion and diffusion heat conduction in the gas–vapor layer. New exact solutions describing steady two-layer flows in a channel with the interface remaining undeformed and examples of velocity and temperature profiles for the HFE-7100 (liquid)–nitrogen (gas) system are presented. The influence of longitudinal temperature gradients along the channel boundaries, the gas flow rate, and the height of the fluid layer on the flow regime and evaporation rate is studied. A comparison of the calculated data with experimental results is performed.     

SYNTHESIS OF FULLERENES BY HOLLOW CATHODE ARC

The formation of fullerenes in a hollow cathode arc in an inert gas atmosphere is investigated. The effects of the carbon evaporation rate, the inert gas flow rate and the arc temperature on the fullerene yield, production rate and conversion rate are observed.

A magnetic field of 100 Gauss is used to collimate the discharge and to improve the carbon evaporation rate. The agreement among experimental data and theoretical results of the model based on the equations of Smoluchovski is obtained.     

Evaporation heat and mass transfer in natural convection pipe flow

Abstract

A numerical analysis has been performed to examine film evaporation on natural convection heat and mass transfer in a vertical pipe. Coupled governing equations for liquid film and induced gas flow were simultaneously solved by the implicit finite difference method. Results for interfacial heat and mass transfer coefficients are specifically presented for ethanol film and water film vaporization. The predicted results indicate that the heat transfer from gas‐liquid interface to the gas flow is predominated by the transport of latent heat in association with film evaporation. The results are also contrasted with those of zero film thickness and show that the assumption of extremely thin film thickness made by Chang et al. [5] and Yan and Lin [19] is only valid for a system with a low liquid Reynolds number Re _l1. But as the liquid Reynolds number is high, the assumption becomes inappropriate.     

SYNTHESIS OF FULLERENES BY HOLLOW CATHODE ARC

The formation of fullerenes in a hollow cathode arc in an inert gas atmosphere is investigated. The effects of the carbon evaporation rate, the inert gas flow rate and the arc temperature on the fullerene yield, production rate and conversion rate are observed.

A magnetic field of 100 Gauss is used to collimate the discharge and to improve the carbon evaporation rate. The agreement among experimental data and theoretical results of the model based on the equations of Smoluchovski is obtained.     

Indirectly coupled films

A positive coupling has been observed between magnetic films separated by a thin metallic layer. Five methods which have been developed for measuring this coupling and the variation of the coupling with evaporation temperature, measuring temperature, and thickness of the intermediate layer are described. Three mechanisms are proposed for the observed coupling : bulk diffusion of ferromagnetic atoms into the intermediate layer, diffusion of magnetic atoms along grain boundaries, and polarization of conduction electrons. The effect of this coupling on slow switching, pulse switching, and creep is discussed.     

An experimental investigation of modes of flow under conditions of evaporation of liquid on a vertical heating surface

An experimental investigation is performed of the effect of temperature head on the flow of evaporating film of liquid, defined by the wetting line or by ribs, on a vertical heating surface. The experiments are performed under conditions of evaporation of R11 Freon in a medium of own vapor on a vertical copper plate, including the presence of ribbing. The visualization of flow is performed. Analysis is made of the effect of the evaporation intensity in the neighborhood of liquid-vapor-wall contact line on the conditions of film discontinuity and on the pattern of resultant streamer flow. It is demonstrated that, rather than spreading, the liquid in the case of streamer flow on the heating surface contracts downstream even for a close-to-zero equilibrium wetting angle. This is due to intense evaporation of liquid in the region of liquid-vapor-wall contact line, where the liquid film exhibits a minimal thickness, to the variation of curvature of the interface in this region, and to the emergence of thermal contact angle. The dependence of thermal contact angle on temperature head is determined. Dynamic measurements are performed of the local thickness of flowing films of liquid using a capacitance meter, and spectral analysis is performed of waves which arise because of instability of film flow on the evaporating film surface.     

Influence of coating thickness and sand fineness on mold filling in the lost foam casting process

The effects of coating thickness and sand fineness on the mold filling characteristics of aluminum alloy 319 have been investigated. Experiments have been conducted with strip patterns and transparent molds have been used to visually record the flow behavior. The results indicate that a higher fill time is observed in thicker coatings and finer sands. Typical flow velocities are on the order of 80 mm/s and 90 mm/s for sands with AFS numbers of 80 and 20 respectively. By comparison, when a completely impermeable mold is used, the flow velocity is reduced to about 20 mm/s. Under normal casting conditions, the gases formed at the metal front are eliminated rapidly into the sand, so that there is no substantial gas layer ahead of the flowing melt. A gas layer may build up at the metal front if the permeability of the mold medium is not adequate and eventually the gas bubbles may escape by penetration through the liquid metal. Factors affecting polymer degradation may have strong effect on mold filling while parameters associated with the elimination of degradation products may control defect formation.     

Weld Shape Variation and Electrode Oxidation Behavior under Ar-(Ar-CO_2) Double Shielded GTA Welding

Double shielded gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar-CO₂ or CO₂ active gas as the out layer shielding was proposed in this study to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results showed that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidizing the tungsten electrode during the welding process. Active gas, carbon dioxide, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70×10^-6, the surface-tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flow rate on the weld bead morphology and the weld shape was investigated systematically. The results show that when the flow rate of the inner argon shielding gas is too low, the weld bead is easily oxidized and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

Investigation of Ti layer thickness dependent structural, magnetic, and photoemission study of nanometer range Ti/Ni multilayer structures

Structural, magnetic, and electronic properties of Ti/Ni multilayer (ML) samples as a function of Ti layer thickness are studied and reported in this paper. For this purpose [Ti (t nm)/Ni (5 nm)] x 10 ML samples, where t = 3, 5, and 7 nm have been deposited by using electron beam evaporation technique under UHV conditions at room temperature. Structure of ML samples were determined by using XRD (X-ray diffraction) technique and observed that Titanium is deposited mainly in amorphous nature with FCC structure at lower Ti layer thickness of 3 nm, which transform to crystalline HCP structures above than this Ti layer thickness. Corresponding fitted GIXRR (grazing incidence X-ray reflectivity) patterns shows asymmetric nature of Ti-Ni and Ni-Ti interfaces because of heavy intermixing and interdiffusion of Ni and Ti atoms at Ti-Ni interfaces at lower Ti layer thickness. The depth profiling core level and valence band measurements carried out by using XPS (X-ray photoelectron spectroscopy) technique confirms the interdiffusion and intermixing leading to Ti-Ni alloy phase formation at interfaces during deposition, particularly at lower Ti layer thickness of 3 nm. The corresponding magnetization behavior of ML samples has been investigated using Magneto-Optical Kerr Effect (MOKE) technique and observed that, coercitivity decreases while saturation magnetization increases with Ti layer thickness variations. These results are interpreted and discussed in terms of observed micro-structural changes due to Ti layer thickness vitiations in Ti/Ni multilayer samples.     

Experimental Study on the Combined Evaporation Effect and Thermocapillary Convection in a Thin Liquid Layer

The coupling mechanisms and flow characteristics of thermocapillary convection in a thin liquid layer with evaporating interface were studied. The planar liquid layer, with the upper surface open to air, was imposed externally horizontal temperature differences. The measured average evaporating rates and interfacial temperature profiles indicated the relative importance of evaporation effect and thermocapillary convection under different temperature gradients. A temperature jump was found at the interface, which was thought to be related to the influence of evaporation effect. All above mentioned results were repeated in a rarely evaporating liquid to compare the influence of evaporation effect.     

Gas Sensing Properties of Hydrogenated Amorphous Silicon Films

Gas sensing experiments on hydrogenated amorphous silicon (a-Si:H) films have been performed. We show that a-Si:H exhibits a low-temperature gas response that is distinctly different from the more familiar combustive gas response operative on heated metal-oxide surfaces. In particular, we show that at room temperature and above, a-Si:H samples exhibit a dissociative gas response which has first been observed on hydrogenated diamond (HD) samples. Whereas this dissociative gas response disappears at HD surfaces upon evaporation of the adsorbed surface electrolyte layer, a gas response with a similar cross sensitivity profile is observed on a-Si:H surfaces that persists up to the original deposition temperature of the a-Si:H films. We argue that this latter kind of gas response is due to a coordinative gas response that takes place when surface H-atoms of the a-Si:H film enter the coordination sphere of adsorbed analyte gas molecules.     

Retardation of a Heat Front in a Porous Medium Containing an Evaporating Liquid

We have investigated heat transfer in a layer of silica gel impregnated with a liquid (water, aqueous solutions of calcium and magnesium chlorides, formic acid, and carbon tetrachloride). The layer was arranged on a substrate impenetrable for vapor and it was heated from above by a concentrated light flux. It has been found that the evaporation of the liquid contained in the pores of silica gel substantially slows down the propagation of the heat front into the layer so that the effective thermal conductivity of the layer can be reduced to 0.01 W/(m·K); this value is approximately 4–20 times smaller than the values typical of the majority of standard heat-insulating materials. The time of the front lag depends on the layer thickness, density of the incident heat flux, amount of liquid in the pores, and evaporation heat of the liquid. The observed trends in the motion of the front have been described by a simple one-dimensional model that takes into account phase transition (liquid evaporation) in the interior of the porous matrix.     

Evaporation and Condensation of Uranium Dioxide in an Electromagnetic Field

New physical phenomena related to the interaction of laser radiation with high evaporation materials, in particular with uranium dioxide, have been experimentally revealed by a subsecond heating technique. The first phenomenon, interpreted as an absorption flash, was observed in laser heating the sample and was manifested as a sharp drop in temperature on the heating curve. The second phenomenon, interpreted as a threshold condensation of vapor on the sample, was revealed during the cooling stage of the sample initially heated by laser radiation. This was manifested as an exothermal condensation peak on the cooling curve. The study of these phenomena has shown that there was a stable vapor zone above a sample in the field of laser radiation. The size and shape of the small particles produced due to laser evaporation and mainly formed in this vapor zone depended on the power of the laser radiation and the pressure of the inert gas. A change in the parameters of the vapor zone by changing the power of the laser radiation and the pressure of the inert gas gives the potential to produce nanoparticles of targeted sizes. Thus, the study of physical processes and mechanisms of forming nanoparticles of refractory materials at high temperatures motivates further development of the method of laser evaporation for producing nano-dimensional powders of targeted sizes and properties. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

Gas–Liquid Interfacial Friction Factor for the Transition from Stratified to Slug Flow

Calculations based on the slug stability model and simplified stratified flow model provide predictions of the critical liquid height and the critical superficial velocities of a stratified flow for the transition to a slug flow in a horizontal pipe. Since slug flow derives from different interfacial waves patterns, previous interfacial waves model in stratified gas–liquid flows brings about the discrepancy between theoretical prediction and experimental data. A partial analysis for this behavior is given, which recognizes that the values of gas–liquid interfacial friction factor at the onset of slug flow have been underestimated, especially at high gas flows, and they should be obtained indirectly from other measured variables. Modified correlations for the interfacial friction factor are presented and better agreement between predicted and measured critical superficial velocities has been obtained.     

Numerical Modeling of the Tangential Stress Effects on Convective Fluid Flows in an Open Cavity

The study of convective processes caused by impact of various forces on the fluid and gas media is actual nowadays. The increased interest to these problems is determined by preparation of the new experiments on the International Space Station in the frame of the scientific project CIMEX of the European Space Agency. They are the experiments to investigate the convective flows of the fluids with a thermocapillary interface between liquid and gas phases. In the case, when a co-current gas flux generates the tangential stresses on a free boundary the additional characteristics of the convective flows should be detected. In this paper the exact solutions of a stationary problems of convection and of gas flow in the horizontal layers with a free thermocapillary interface are constructed. An evaporation through the fluid-gas flow interface is modeled qualitatively with the help of a heat exchange condition. It is determined that the velocity on the fluid-gas interface can be positive and negative. The equal-zero condition for the interface velocity is found, as well. In the experiments an open horizontal fluid layer is studied so that the closed flux condition is not needed. Although the closed flux requirement is not set explicitly a parameters relation is found, when this condition is fulfilled. The paper presents the velocity and temperature profiles in the conditions, which correspond qualitatively to the CIMEX experiment.     

Gravitational effects on evaporative convection at microscale

Thermocapillary convection induced by phase change (evaporation) has been investigated in confined environment. This paper introduces some insight into the physics of evaporatively-driven thermocapillary convection and emphasizes on the interaction between the observed convection and gravity. Non-equilibrium interfacial conditions lead to temperature/surface tension gradients which drive convective patterns. The latent heat of evaporation leads to an important cooling effect near the triple contact line. Evaporation of volatile liquids in capillary tubes is experimentally investigated to demonstrate the above effects. The size of the capillaries is found to be an important factor in the effect that gravity could have on thermocapillary convection. The oscillatory behaviour observed when buoyancy affects thermocapillary convection could be explained through the coupling between interfacial temperature and the flow within the liquid. The three dimensional nature of the flow structure is found to extend the effect of gravity to the horizontal section of the flow.     

Interfacial Temperature Discontinuities in a Thin Liquid Layer during Evaporation

Recent measurements of the temperature profiles across the liquid-vapor interface of a steady evaporating liquid were performed in a thin planar liquid layer subjected to externally imposed horizontal temperature differences when the interface was open to air. Temperature discontinuities have been found to exist at the interface with an growing tendency as the imposed horizontal temperature difference increasing. Under the co-influence of thermocapillary convection and evaporation effect, a thin layer of 0.5 mm thick with approximate uniform temperature was found just below the liquid-vapor interface. Repeated experiments and further comparisons of the interfacial temperature profiles for different spatial positions along the streamwise center line and varying depths of the liquid layer were also carried out. And the temperature discontinuity was found related to the temperature in liquid phase, which was strongly influenced by the coupling of thermocapillary convection and evaporation effect.