Adjacent Gas Phase Research Articles

Generation and Observation of Long-Lasting and Self-Sustaining Marangoni Flow.

When solute molecules in a liquid evaporate at the surface, concentration gradients can lead to surface tension gradients and provoke fluid convection at the interface, a phenomenon commonly known as the Marangoni effect. Here, we demonstrate that minute quantities of ethanol in concentrated sodium hydroxide solution can induce pronounced and long-lasting Marangoni flow upon evaporation at room temperature. By employing particle image velocimetry and gravimetric analysis, we show that the mean interfacial speed of the evaporating solution sensitively increases with the evaporation rate for ethanol concentrations lower than 0.5 mol %. Placing impermeable objects near the liquid-gas interface enforces steady concentration gradients, thereby promoting the formation of stationary flows. This allows for contact-free control of the flow pattern as well as its modification by altering the objects shape. Analysis of bulk flows reveals that the energy of evaporation in the case of stationary flows is converted to kinetic fluid energy with high efficiency, but reducing the sodium hydroxide concentration drastically suppresses the observed effect to the point where flows become entirely absent. Investigating the properties of concentrated sodium hydroxide solution suggests that ethanol dissolution in the bulk is strongly limited. At the surface, however, the co-solvent is efficiently stored, enabling rapid adsorption or desorption of the alcohol depending on its concentration in the adjacent gas phase. This facilitates the generation of large surface tension gradients and, in combination with the perpetual replenishment of the surface ethanol concentration by bulk convection, to the generation of long-lasting, self-sustaining flows.

INSTABILITY OF A GRAVITY-DRIVEN LIQUID FILM CONFINED IN AN INCLINED RECTANGULAR DUCT

This study deals with the confinement effects due to top and side walls on the stability of a falling liquid film flowing down an incline. To this aim, we consider a liquid film flowing between two infinite plates and in rectangular ducts in the presence of an adjacent gas phase, focusing on the case of zero net gas flow rate. A rigorous linear stability analysis is conducted for three gas-liquid systems of particular interest in applications. This enables us to demonstrate the effect of the gas-liquid density ratio and viscosity ratio in small channels/ducts and shallow downward inclinations. Although the analysis considers all wave numbers, the neutral stability boundary is found to be defined mainly by long-wave perturbations in the two-plate (TP) geometry, as well as in the rectangular ducts. The decrease of the channel height for the considered liquid-gas systems always stabilizes the liquid film. However, the Kapitza criterion for the onset of Kapitza waves on the surface of a falling liquid film can either overpredict or underpredict the stability limits of the channel/duct flows, depending on the channel height, its aspect ratio, and the viscosity and density ratios of the phases. The falling liquid film, bounded by the side walls, becomes unstable at larger flow rates with the decrease of the duct aspect ratio. The presence of recirculating gas in the channel/duct is found to affect the stability through the dynamic shear stress components at the gas-liquid interface. Therefore, its presence must be taken into account.

Stability of gravity-driven thin-film flow in the presence of an adjacent gas phase

The stability of a falling liquid film flowing down an inclined channel is revisited. The classical Kapitza criterion for the onset of long wave instability ignored the presence of the adjacent gas phase and provided the critical liquid Reynolds number, Recr=5/6cot(β) for the flow destabilization. In the current study, the impact of the adjacent gas on Recr is studied via solution of the Orr-Sommerfeld equations in both the liquid and gas phases. The particular case of zero net (recirculating) gas flow is investigated, though this case is shown to be relevant to a wider range of concurrent and countercurrent gas flows. The results obtained confirm the recent finding that Kapitza instability may be fully suppressed in sufficiently small channels. However, in large channels, where the critical perturbation for the flow destabilization is long wave, the Kapitza criterion largely over predicts the critical Reynolds number. This is shown to be related to the increasing impact of the dynamic interactions between the gas and liquid at the interface, whereas the role of the liquid inertia diminishes. A revised analytical expression for Recr is derived, which accounts for those interactions in gravity driven thin film flows.

An experimental study of the behavior of intumescent coatings under localized fires

This paper presents an experimental study of the behavior of intumescent coatings exposed to localized fires. The main objective is to assess the extent to which thermal properties of intumescent coatings obtained under uniform heating can be applied for localized fire exposures. The experiments used two types of specimens representing protected steel beams and columns. Localized pool fires were placed underneath horizontal specimens or beside vertical specimens. Solvent-based and waterborne intumescent coatings were applied to the specimens. Measurements were made of temperatures of the steel specimens and the adjacent gas phase, as was coating expanded thicknesses at several locations. Depending on their location relative to fires, the intumescent coatings were variably unreacted, melted, partially expanded, or fully expanded (corresponding to steel temperatures of about less than 100 °C, 100–300 °C, 300–400 °C, and above 400 °C, respectively). The appearances and expansion ratios for various regions of coatings under localized fires were consistent with those under uniform heating from previously obtained furnace test results. Steel temperatures of the specimens were calculated using thermal conductivities of coatings derived from furnace tests, and were found to agree reasonably with the experimental results, thus indicating that it may be feasible to apply thermal conductivities derived from furnace tests to predict steel temperatures under localized heating scenarios.

Acceleration of chemical reaction fronts

The propagation of reaction-diffusion fronts in an open liquid solution layer is critically affected by mass transfer between the liquid solution and the adjacent gas phase. This is the case in the iodate–arsenous acid (IAA) reaction when run under stoichiometric excess of iodate. Here, iodine is the reaction product, which has a low solubility in the liquid phase, hence, excess iodine rapidly evaporates. In the gas phase, it diffuses and overtakes the reaction front propagating in the liquid medium because its diffusion coefficient in the gas phase is considerably larger than that in aqueous solution. Evaporated iodine is re-dissolved into the reaction medium ahead of the reaction front. Since iodine is the autocatalytic species of the IAA reaction, this additional gas-phase transport may lead to an acceleration of the propagating reaction front.

Experimental Observation of Hypergolic Ignition of Superbase-Derived Ionic Liquids

The hypergolic ignition behaviors of four newly synthesized ionic liquids with dicyanamide anion (ionic liquids 1 and 3) and cyanoborohydride anion (ionic liquids 2 and 4) were experimentally investigated. The results showed that successful hypergolic ignition of both group ionic liquids were achieved with white fuming nitric acid and red fuming nitric acid. Two distinct flame initiation processes illustrated by several stages were observed for each group of ionic liquids. Specifically, the ionic liquid 1 drop showed immerging, exploding, and ignition: the drop immerged and reacted with the oxidizers underneath the liquid surface, producing gas vapor and white foam like intermediates on the surface; then, the surface exploded by fast accumulation of heat and vapor underneath, followed by flame kernels formation in the adjacent gas phase. The ionic liquid 2 drop showed bouncing and igniting: it just created a crater on the oxidizer surface and then sat on the Leidenfrost vapor layer generated by impaction and reaction. Ionic liquids 3 and 4 showed a similar flame initiation process, respectively, to that of ionic liquids 1 and 2. Furthermore, the explosion delay time and ignition delay time of the four ionic liquids were recorded, both the previous and present experimental repeatabilities were analyzed, and it was shown that heat loss control was important to the experimental repeatability.

Thermo-hydrodynamic transport phenomena in partially wetting liquid plugs moving inside micro-channels

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas–liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a ‘unit cell’ flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated.

Determination of absolute adsorption for argon on flat surfaces under sub- and supercritical conditions

A new method, employing computer simulation, is proposed for the determination of the location of the interface separating the adsorbed phase from the adjacent gas phase, giving a means to calculate the “absolute” amount adsorbed. The method involves monitoring the fraction of successful insertions of molecules into differential volumes in the simulation box. By applying the concept of equal areas, as implemented in the determination of the Gibbs dividing surface, for the profile of the fraction of success versus distance, we are able to determine the location of the interface bounding the adsorbed phase. This allows us to find (1) the thickness of the adsorbed phase, (2) the absolute surface density (absolute loading) and (3) the volumetric density of the adsorbed phase, as functions of pressure. Knowing the absolute surface density as a function of pressure at different temperatures, we are able to calculate the heat of adsorption as a function of loading, using the Clausius–Clapeyron equation and to show that this is consistent with the heat obtained from the fluctuation formula in grand canonical Monte Carlo simulations.

Filament seasoning and its effect on the chemistry prevailing in hot filament activated gas mixtures used in diamond chemical vapour deposition

Tantalum hot filaments (HFs) find frequent use as a means of activating hydrocarbon/hydrogen mixtures used for chemical vapour deposition of thin film diamond. This contribution reports systematic studies of the power consumed by a tantalum HF, and companion laser based measurements of the relative H atom number densities in the gas phase adjacent to the HF surface, in pure H 2 and in dilute CH 4/H 2 gas mixtures, as a function of process conditions (filament temperature, gas pressure, extent of HF carburisation). The measurements serve to highlight the way in which the adjacent gas phase chemistry and composition affects the HF surface chemistry, and vice versa.

Thin-Film Formation on a Rotating Disk.

Thin-film formation on a rotating disk during the spin-coating process is analyzed. In the experiments for a solution of PMMA (polymethyl methacrylate) solute and chlorobenzene solvent, the final film thickness of dry film formed on silicon wafers for integrated circuits is measured with an ellipse thickness meter. Also, the transient film thickness during the entire film formation process on a rotating optical glass is measured using a laser displacement meter. A numerical calculation considering heat and mass transfer such as solvent evaporation is performed using a one-dimensional complete model proposed by the authors for each phase of liquid film and adjacent gas phase. Satisfactory agreement is obtained between these numerical and experimental results and the effect of various process parameters, e.g., spinning speed and initial film thickness, is clarified by this study.

Importance of surface reactions during radiative ignition of composite solid propellants

A composite propellant subjected to radiant heating at the surface is theoretically investigated. Numerical solutions of the mathematical model which considers ignition as due to runaway chemical reactions at the propellant surface or in the adjacent gas-phase indicate that although inclusion of the surface reactions alter the ignition behavior slightly, the process is dominated by gas-phase reactions.

Gas Phase Ignition of a Composite Solid Propellant Subjected to Radiant Heating

Abstract —Ignition characteristics of a composite solid propellant subjected to radiative heating a: the surface has been theoretically investigated. Consequent to irradiation, the propellant pyrolizes producing fuel and oxidizer vapours which diffuse into the adjacent gas phase. The pyrolyzed fuel undergoes homogeneous gas phase reactions with the external oxidizer as well as the oxidizer evolved at the surface leading to ignition. The partial differential equations describing the processes of diffusion and chemical reaction have been numerically solved on a computer. The model predicts physically realistic behaviour.

Breaking bubbles and the water-to-air transport of particulate matter

Bubbles collapsing at an air-liquid interface eject small, high-speed droplets into the adjacent gas phase. The composition of these aersol-sized droplets may be markedly different from the bulk solution in which the bubbles originate. This bubble-breaking/fractionation phenomenon has major implications for a variety of environmental and industrial processes. In preliminary work reported here we have measured the transfer of micron-size latex particles to jet drops produced by the bursting of one-millimeter diameter bubbles. Jet drop concentration was measured as a function of: bulk particle concentration, bulk ionic strength, and depth of bubble release. Results show the droplet concentration to be relatively insensitive to bulk concentration but strongly dependent on ionic strength and trace contaminants.

Ellipsometric Determination of the Spectra of Adsorbed Molecules

This paper presents a detailed analysis of the application of ellipsometry to obtaining the optical spectra (principally infrared (i.r.)) of molecules adsorbed on reflecting surfaces. Both external and total internal reflections are considered and the conditions for optimum sensitivity examined. A new empirical quantity, the relative complex optical density, is defined which exhibits thin film properties well, particularly in the case of multiple reflection measurements. An explicit expression is derived for this density function (relating it to the optical constants of the media and other system parameters), which is both reasonably simple and correct to second order terms in the film thickness. It is shown that for thin films, no higher order terms need be included, but that in general the second order term must be retained. Various limiting cases are examined to gain insight into the optical behavior of thin films, and to the same end, model calculations performed for CCl4 physically adsorbed on Ag, Ni, Sb, and Ge. In relation to conventional reflection spectroscopy, ellipsometric spectroscopy is shown to have three major advantages: (1) in general, higher sensitivity to adsorbate properties; (2) very much lower sensitivity to absorption of radiation by the adjacent gas phase; (3) more information, permitting the optical constants and film thickness to be determined. Finally, the practicability of the technique is demonstrated by presenting preliminary results for CH3OH reversibly adsorbed on Ag, showing clearly the C—H stretching bands.

Flame spread over fuel beds: solid-phase energy considerations

This paper reports on an analytical and experimental investigation of flame spread over the surface of a solid-fuel bed in an oxygen-inert environment. It is postulated that the processes controlling the flame-spreading phenomenon occur in a very small “ignition region” at the leading edge of the spreading flame adjacent to the surface, and attention is focused on this small region. The solid-phase energy equation is uncoupled from the gas-phase conservation equations and solved separately, retaining as a boundary condition the heat flux into the surface from the adjacent gas phase. The resulting solutions, in the form of simple algebraic equations, show clearly the different relationships between the physically important parameters associated with flame propagation over: (1) a “thick” fuel bed (thermal wave penetration into the solid beneath the ignition region much less than the fuel-bed depth), and (2) a “thin” fuel bed (subsurface temperature gradients negligible). The “critical thickness” criterion for separating the regions of thin and thick fuel-bed flame-spreading characteristics evolves as a naturally occurring parameter of the problem. The main dependence of heat flux on environmental parameters is obtained from consideration of the gas-phase conservation equations in a quiescent environment. By combining the results from the gas-phase and solid-phase analyses, simple algebraic relationships are obtained between flame-spreading velocity and parameters of theoretical and practical interest. Experimentally, using the techniques described in Ref. 1, the dependence of flame-spreading velocity on pressure level, oxidant mole fraction, diluent gas, initial temperature and fuel-bed depth is determined for polymeric and cellulosic materials. In addition, the influence of forced convective motion of the environment on flame spreading velocity is investigated experimentally. The analytical results provide excellent correlation for all data obtained in a quiescent environment, with the exception of the influence of initial temperature. Data obtained by other investigators are shown to be correlated as well. It is concluded that the role played by the solid phase in the over-all flame-spreading mechanism is generally understood, but the gas-phase processes require further definition.