Thermal patterns in evaporating liquid

In this paper, some of the preparatory experiments of the ESA sponsored space program CIMEX-1 are presented. A liquid layer of variable thickness is subject to a flow of inert gas. The non-uniform evaporation induced by the gas flow creates a temperature gradient parallel to the interface triggering in that way thermocapillary convection. The combined action of evaporation, thermocapillarity and gravity has been not completely clarified both theoretically and experimentally. The experiment presented in this work concerns a liquid layer of ethanol of 2.2 mm thickness in presence of a mass flow of Nitrogen whose intensity varies in the range of hundreds of milliliter per minute. The experiments were performed at an initial liquid temperature of 21°C. The patterns observed are strongly dependent on the flow rate of inert gas. A change in the instability patterns has been observed for a gas flow of about 1.7 l/min.     

Influence of Boundaries on Shear-driven Flow of Liquids in Open Cavities

In recent years, the development of two-phase flow cooling systems in micro-electronic as well as surface coating/patterning technologies, often based on thin films and liquid filled micro-enclosures, has renewed the interest in the classical problem of the cavity flow driven by a shear stress imposed at the gas-liquid boundary. In this paper, we study, numerically, the influence of the cavity geometry and boundaries on the three dimensional velocity field driven by a shear flow of inert gas. This study has been performed in the frame of the ESA sponsored Space Program on heat and mass transfer CIMEX-1 and it has to be considered as a contribution in the preparation of the Space experiment. The reference values for the flow parameters as well as the geometrical features encountered in the present paper target the main features of the CIMEX-1 experiment, although the main conclusions can be considered of 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.     

Study of Marangoni instability of an evaporating liquid layer

Abstract

The onset of Marangoni instability of an evaporating liquid layer is studied theoretically. By assuming the surface regression of the liquid layer is negligibly small and the surrounding gas phase is asymptotically steady, similarity solutions are obtained prior to the onset of instability. Linear stability analysis is then applied to obtain the critical Marangoni number for the onset of instability. The results indicate that : (1) The onset condition is a strong function of the initial temperature of the liquid layer with which the critical Marangoni number decreases. (2) As time proceeds, the thermal boundary layer thickness near the free surface becomes larger, and the liquid layer becomes more unstable. (3) For a liquid layer with a higher initial temperature (closer to the boiling point) the most unstable mode of the disturbance shifts from a lower wave number to a higher wave number.     

Study on Mechanical Property in Steel-Aluminum Solid to Liquid Bonding

The bonding of solid steel plate to liquid Al was conducted using rapid solidification.The influence of thickness of FeAl compound layer at the interface on interfacial shear strength of bonding plate was studied.The results show that the relationship between thickness of Fe-Al compound layer and interfacial shear strength is S=30.4 8.51 h-0.51h^2 0.007h^3(where h is thickness of Fe-Al compound layer,S is interfacial shear strength).When thickness of Fe-Al compound layer is 10.7μm,the largest interfacial shear strength is 71.6MPa。     

Interfacially engineered liquid-phase-sintered Cu–In composite solders for thermal interface material applications

Cu particle-containing In-matrix composites for thermal interface material (TIM) applications were prepared via liquid phase sintering, following chemical modification of the Cu–In interfaces. The optimized composite TIM possessed 1.5 times the thermal conductivity, and twice the yield strength, of pure In. Joints of the composite TIM between pairs of cylindrical Cu rods were used to measure shear behavior and thermal resistance as functions of three parameters: (i) joint thickness, (ii) thermal excursion history, and (iii) type of interfacial layers between Cu and In. The composite joints showed good shear compliance, with a shear yield strength of 2.7 MPa, as well as substantially lower joint thermal resistance (0.021 cm² K W^?1) than pure In joints, which are commercially used in high-end TIM applications. The thermal resistance of the joints was found to be a sensitive function of the interfacial contact resistance between the Cu particles and In within the TIM, as well as between the TIM and the Cu substrates. The TIM–substrate interfaces, in particular, play an increasingly important role as the joint becomes thinner, limiting the joint thermal resistance. To reduce the interfacial contact resistance, a diffusion barrier of 1–2-nm-thick Al₂O₃ was applied by atomic layer deposition on both the Cu particles and the Cu substrates, followed by a 20-nm-thick Au layer, which served as a wetting enhancer. The engineered interfaces also improved the stability of the composite TIM joints under aging conditions.     

Measurement and analysis of hydroluminescence spectrum

The spectrum of hydrodynamic luminescence (HL)—that is, of light emitted from a liquid flow under strong mechanical action—has been studied. It is shown that the light-emitting gas has low rotational and high vibrational temperatures, which is evidence in favor of an electrical, rather than thermal, nature of HL excitation.     

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.     

Non-unique solution for combined-convection assisting flow over vertical flat plate

Non-unique solutions of flow and temperature field are reported here for the first time for non-similar flows given by the laminar boundary layer equations for combined-convection flow past a vertical flat plate. The solution of the boundary layer equation for natural convection constitutes the self-similar solution whose perturbation with respect to the small parameter (ε), which is inversely proportional to the square root of the Richardson number (G _x ), provides the non-similar solution. Solutions obtained by the shooting method indicate two sets for the self-similar solution (ε = 0) — one of them showing positive velocity everywhere inside the shear layer (well-known oft-reported physical result). The other self-similar solution shows that recirculation in the outer part of the shear layer may not be physical — as it has not been experimentally demonstrated so far. In contrast, the perturbative part of the non-similar solution (ε ⊋ 0) is seen to be either convergent or divergent depending upon the choice of integration domain of the shear layer equations — bringing forth the question of the validity of such perturbation procedures and possible stability of the basic solution itself.     

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.     

Conversion of methane to hydrogen in a reversible flow superadiabatic inert porous medium reactor

Using the analysis of the experimental data on partial oxidation of methane as an example, we have shown that the chemical processes in the inert medium of a reciprocating flow reactor can be modeled with good accuracy by the standard kinetic scheme for homogeneous processes due to the fact that the gas flow in the region of combustion is described by two temperatures — the gas and framework temperatures. Such a modification of the chemical model requires neither changing the recognized mechanism of homogeneous chemistry nor correcting the volume heat transfer coefficient.     

A model of interfacial waves in annular two-phase flow at different gravity levels

Microgravity provides an ideal environment to study interfacial waves (small scale phenomena) without the dominating effects of gravity. In this paper, conductance probes were used to measure film thickness and liquid velocities. Physical models of interfacial waves are presented for vertical upward flow (+1g), vertical downward flow (?1g), and microgravity (μg), at different gas and liquid flow rates. Based on these studies, the predominant parameters and the features of the wave structure are discussed. A preliminary mathematical model was proposed for the interfacial waves.     

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.     

Effects of bondline thickness on Mode-II interfacial laws of bonded laminated composite plate

For a variety of modern industries, interfacial delamination is a critical issue for the design and application of laminated composite structures. Numerous global experimental studies have been conducted to characterize the toughness of adhesively bonded composite structures. In the recent two decades, cohesive zone models (CZMs) have been receiving intensive attentions. The local interfacial traction–separation laws as the fundamental input are crucial for the successful applications of CZMs. Several local tests have also been conducted to determine the interfacial traction–separation laws in adhesively bonded joints. However, very few tests have been employed to investigate the dependency of the local interfacial traction–separation laws on the bondline thickness, particularly, for the laminated composite joints under Mode-II loading conditions. In this work, the effects of bondline thickness on the interfacial behavior have been systematically investigated at various typical bondline thicknesses (from 0.1 to 0.8 mm). The effects of adhesive thickness on the interfacial toughness, interfacial strength, and shapes of the local interfacial traction–separation laws have also been evaluated. The test results indicated that the measured Mode-II (shear test) interfacial shear strength of the composite joints increases as the adhesive layer becomes thicker. It was found a significant dependency of the measured shapes of the Mode-II interfacial laws on the bondline thickness. Several other interesting issues were also reported in this work. This work may provide valuable baseline test data for analytical and numerical modeling of fracture and failure of laminated composite structures.     

Formation of stainless steel layer on mild steel by welding arc cladding

The discrete microstructural characterization and the formation of stainless steel layer on mild steel where produced in cladding deposits, and fusion boundary region were investigated using tungsten inert gas (TIG) arc, high current pulsed arc and constricted plasma arc. The experimental procedure involved making bead-on-plate method for controlled travel speed, employing filler metal by using tungsten inert gas arc, pulsed current gas tungsten arc and transferred plasma arc, with subsequent sectioning and examination of the reaction interface. For TIG arc cladding, using filler metal of small diameter the deposit does not become stainless steel, but on using 3.2 mm diameter filler metal it becomes stainless steel with less than 50% dilution. For pulsed arc cladding, the complete stainless steel is not obtained on account of the existence of an incomplete mixture, particularly at the fusion boundary region. However, on using a large diameter filler metal at a pulse frequency of 500 Hz, the complete stainless steel microstructure has been accomplished. The plasma arc cladding can be achieved in such a way that the conversion into stainless steel on the mild steel surface — which is the microstructures of cellular austenite in cladding deposit and cellular dendritic austenite containing δ or σ-phase in fusion boundary region — is possible irrespective of the melt penetration and the dilution. The following conditions were found to be beneficial for the formation of stainless steel microstructure layer on the mild steel: using large diameter filler metal, below 50% dilution, and further rendering arc localized and constricted.     

Coupled heat and mass transfer during nonisothermal absorption by falling droplet with internal circulation

We considered mass and heat transfer during nonisothermal absorption of a gas by a falling droplet with internal circulation. Gas phase is assumed to be free of inert admixtures and mass transfer is liquid phase controlled. Mass flux is directed from a gaseous phase to a droplet, and the interfacial shear stress causes a fluid flow inside the droplet. Droplet deformation under the influence of interface shear stress is neglected. Absorbate accumulation and temperature increase in the bulk of liquid phase are taken into account. The problem is solved in the approximations of a thin concentration and temperature boundary layers in the liquid phase. The thermodynamic parameters of the system are assumed constant. The system of transient partial parabolic differential equations of convective diffusion and energy balance with time-dependent boundary conditions is solved by combining the similarity transformation method with Duhamel's theorem, and the solution is obtained in a form of Volterra integral equation of the second kind which is solved numerically. Theoretical results are compared with available experimental data for water vapor absorption by falling droplets of aqueous solution of LiBr.     

Asymptotic analysis of the small-scale structure of the laminar boundary layer with suction

The features of the gas flow in the working section of a transonic wind tunnel have been considered. On the basis of the method of joining asymptotic expansions and the theory of detached zones mathematical models of the flow are proposed. The flow over a perforation and transverse and longitudinal slots has been investigated. In the latter case, a nonstationary and a stationary analogy with a two-dimensional flow are stated. A deflector — a new device for reflecting shocks — is proposed. The problems are discussed.     

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.     

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.     

Interfacial stress transfer of fiber pullout for carbon nanotubes with a composite coating

An analytical approach has been established to evaluate the interfacial stress transfer characteristics of single- and multi-walled carbon nanotubes (CNTs) with composite coatings by means of fiber pullout model. According to the present model, the effects of several parameters such as coating thickness, layer numbers and dimension of CNTs on interfacial stress transfers were investigated and analyzed. The results suggested that the maximum interfacial shear stress occurred at the pullout end of CNTs and decreased with increasing coating thickness as well as CNT wall thickness (layer numbers). Moreover, the distribution of the interfacial shear and coating axial stress along the CNT length was found to be largely affected by the friction coefficient in the interface between the CNT and the coating layer.