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To gain insight into shock–droplet interaction and detonation wave–droplet interaction in the real physical environment, two solvers named interTwoPhaseLSCentralFoam and interTwoPhaseLSReacCentralFoam are developed under the OpenFOAM® framework. Computations were conducted on the primary breakup of a droplet when subjected to a shock wave and detonation wave. The droplet breakup in shock–droplet interaction divides into two distinct stages when the shock wave is involved. The first stage is a shear-force dominating stage. In the latter stage, there is a high-pressure gradient, which is similar to a discontinuity inside the droplet, leading to the catastrophic breakup. The detonation wave-induced droplet breakup undergoes three stages: initial stage, stretching stage, and balance stage. In the initial stage, the droplet was impacted by the intense compression. Under the impact of the reflected wave on the windward surface, the droplet is continuously stretched, marking the stretching stage. Then, the structure of the parent droplet remains relatively unchanged throughout the detonation, known as the balance stage. During the detonation wave–kerosene droplet interaction, there is a decoupling of detonation wave in certain regions. As kerosene vapor initiates a reaction with the atmosphere and prevails, the detonation wave undergoes a localized re-initiation process behind the incident shock, creating a dual-wave configuration in its entirety. The detonation wave also experienced a shift of mode from hydrogen–air driven to kerosene–air driven. [ABSTRACT FROM AUTHOR]

Anti-condensation is important in various fields, e.g., transportation, power transmission, internal drying of precision instruments. Vapor sink is an important manner to suppress the happening of water vapor condensation, and shows potentials in some special scenarios. However, the mechanism of anti-condensation by vapor sink is not well understood up until now. In this article, the effect of substrate temperature on the dry zone generated by vapor sink were systematically studied with experiments and theoretical analysis. First, the effect of hygroscopic solutions on the local humidity were measured. The results indicated that the local humidity level near the hygroscopic solutions was effectively suppressed, reducing the water vapor condensation. Second, the variation of the dry zone with the cooling substrate temperature was systematically studied. The results indicated that the dry zone exhibits a hyperbolic relationship with the substrate temperature. The dry zone changing under different humidity conditions and cooling time were also studied. The dry zone remained approximately unchanged over time in a short period. Finally, a simple yet effective model between the dry zone and substrate temperature was deduced. The theoretical results are in good agreement with the experimental data. Our study may deeper the understanding of condensation suppression via vapor sink and offer guidance to the design of anti-condensation or anti-icing surfaces. [ABSTRACT FROM AUTHOR]

Equilibrium molecular dynamics (MD) simulations are used to investigate the liquid/vapor interface where particle exchange between the liquid and vapor phase is quantified in terms of the evaporation and condensation coefficient. The coefficients are extracted from MD simulations via a particle counting procedure. This requires defining a vapor boundary position for which we introduce an accurate and robust method and present a comparative study with existing methods from the literature. This novel method relies on the behavior of the flux coefficient within the interphase region by scanning the position of a particle sink boundary from the liquid toward the vapor phase. We find a distinct local maxima is attained on the vapor side of the interphase that is identified as the vapor boundary position based on an interpretation of transmission probability theory and the Kullback–Leibler divergence. The ratio of the evaporation flux to the outgoing flux at this location is defined as the evaporation coefficient. This method retains the simplicity of existing methods but eliminates several disadvantages. We apply this method to MD simulations of monatomic fluids neon, argon, krypton, and xenon. We observe a correlation between the molecular transport parameter appearing in the transmission probability theory and the characteristic interface fluctuation length scale from the capillary wave theory. This gives an expression for the evaporation coefficient that agrees well with values extracted from MD using the particle counting procedure. Compared to existing methods, the evaporation/condensation coefficient is determined more accurately for temperatures between the triple and critical points. [ABSTRACT FROM AUTHOR]

Boiling of the coolant at the hot surface provides relatively better cooling by absorbing latent heat along with convection heat transfer as compared to heat transfer under single-phase conditions. In boiling, the orientation of heated surface also plays a crucial role. Downward facing boiling is complex than upward facing boiling, as the detachment of the bubble inhibited due to the heater surface orientation. Consequently, the bubble residence time and interaction with other bubbles are different in such boiling conditions. Our experiments on a large downward facing flat surface (100 × 400 mm2) revealed unexplored boiling phenomena. The boiling process is dominated by a complex engulfing phenomenon, which is rarely reported in the past. The engulfing phenomena have been captured using high-speed photography, wherein, at low heat fluxes, it is observed that larger bubbles engulf small bubbles by opening their mouth and swallowing the small bubbles. However, at higher heat fluxes, this phenomenon disappears. A larger vapor blanket is formed due to engulfing of bubbles, which may lead to departure from nucleate boiling. This engulfing behavior depends on the heat flux and subcooling. With the increase in heat flux, it is found that the rate of vapor engulfing increases. We have attempted to explain the science behind such engulfing phenomenon based on the capillary pressure difference. These results are consistent at various subcooling. This research provides new insights into nucleate boiling and may help in developing advanced mathematical models for accurate heat transfer prediction on downward facing nucleate boiling. [ABSTRACT FROM AUTHOR]

Vapor shearing is a common issue encountered in the operations of falling film heat exchangers. The vapor stream effect depends on its orientation. This study investigates liquid film hydrodynamics and heat transfer performance under the influence of vapor streams from different orientations. The results indicate that both orientation and velocity of vapor determine the encountering time and position of the films on the tube's two sides. The liquid film thickness uniformity and the liquid column deflection vary significantly depending on the orientation and velocity of the vapor. Zones of accelerated liquid film, climbing liquid film, liquid stagnation, and transition of liquid film flow pattern are observed. The gradient of film thickness along the tube axis and the deflection in time-averaged peripheral film thickness increase as the vapor orientation varies from 0° to 90° and subsequently decrease as the vapor orientation varies from 90° to 180°. Vapor streams have more pronounced effects on time-averaged peripheral film thickness in regions close to the liquid inlet and outlet. Vapor streams result in changes in peripheral heat transfer coefficients toward the downstream side depending on the orientation and velocity of the vapor. The impact of vapor streams on the overall heat transfer coefficient does not directly correlate with the velocity of the vapor when maintaining the same orientation. [ABSTRACT FROM AUTHOR]

The composition of the gaseous phase of cavitation bubbles and its role on the collapse remains to date poorly understood. In this work, experiments of single cavitation bubbles in aqueous ammonia serve as a novel approach to investigate the effect of the vapor contained in a bubble on its collapse. We find that the higher vapor pressure of more concentrated aqueous ammonia acts as a resistance to the collapse, reducing the total energy dissipation. In line with visual observation, acoustic measurements, and luminescence recordings, it is also observed that higher vapor pressures contribute to a more spherical collapse, likely hindering the growth of interface instabilities by decreasing the collapse velocities and accelerations. Remarkably, we evidence a strong difference between the effective damping and the energy of the shock emission, suggesting that the latter is not the dominant dissipation mechanism at collapse as predicted from classical correction models accounting for slightly compressible liquids. Furthermore, our results suggest that the vapor inside collapsing bubbles gets compressed, consistently with previous studies performed in the context of single bubble sonoluminescence, addressing the question about the ability of vapors to readily condense during a bubble collapse in similar regimes. These findings provide insight into the identification of the influence of the bubble content and the energy exchanges of the bubble with its surrounding media, eventually paving the way to a more efficient use of cavitation in engineering and biomedical applications. [ABSTRACT FROM AUTHOR]

This study investigates the dynamic modeling of a spherical vapor bubble in an infinite liquid. Initially, the liquid is in a state of saturation, but then, there is a change in ambient pressure. Unlike previous studies that assumed vapor saturation, our model fully considers the metastable fluid state of subcooled vapor and superheated liquid. The theoretical model is based on the state equation of vapor and the kinetic theory of gases, allowing the visualization of the effects of superheating and subcooling on the bubble pressure and the phase transition rate. The accuracy of the numerical solutions for the bubble collapse and growth is confirmed by experimental results using water as the working fluid. Two approaches of heat transfer simplification are validated in the numerical solution, and an adhesion coefficient within the range of 0.1–1 is recommended for the calculation. Additionally, this study provides insights into how the metastable fluid state affects the bubble pressure and phase transition, especially in the early stages of bubble collapse. Furthermore, the conservation of vapor mass inside the bubble is also demonstrated. [ABSTRACT FROM AUTHOR]

This study investigates the influence of viscoelasticity on the collapse of aspherical vapor bubbles near a solid boundary through numerical simulations. A fully compressible three-dimensional finite volume method is employed, incorporating a single-fluid homogeneous mixture cavitation model and the simplified linear Phan-Thien Tanner viscoelastic constitutive model. The collapse dynamics, liquid jetting, shock wave formation, and associated pressure impact are analyzed, and the viscous and viscoelastic stress fields are presented. A comparison of viscoelastic to Newtonian dynamics reveals significant differences in collapse behavior and shock wave formation due to viscoelasticity. Viscoelasticity can induce jet piercing, which is not observed in the Newtonian collapse, and increases vapor re-evaporation after the first collapse. The effect of changing the initial standoff distance is examined for both viscoelastic and Newtonian fluids, where a second jet formation is present only for the viscoelastic collapse, and the second collapse's intensity is increased due to increased vapor production during rebound. Additionally, the variation of elasticity in the viscoelastic case demonstrates a correlation between the amount of vapor produced during rebound and the relaxation time for the investigated cases. [ABSTRACT FROM AUTHOR]

The present work investigates the plausible modes of interaction between two adjacently located vapor bubbles growing on high wettability surface during subcooled nucleate pool boiling. The study becomes important as the phenomenon of bubble interaction has a significant impact on the resulting bubble dynamic parameters as well as on microlayer dynamics of the respective vapor bubbles. As against the symmetric growth of microlayer in the case of isolated pool boiling, microlayers of mutually interacting vapor bubbles no longer remain symmetric and tend to deplete relatively faster, thereby influencing the resulting wall heat transfer rates. Toward this, the microlayers and dynamics of adjacently located interacting vapor bubbles have been mapped in a simultaneous manner using thin-film interferometry and high-speed rainbow schlieren deflectometry as a function of varying subcooling levels. Results of the experiments revealed that parameters such as non-dimensional spacing between the two vapor bubbles, supplied heat flux, and degree of subcooling have a strong impact on the mechanism of bubbles' interaction, which, in turn, influence the bubble departure frequency, bubble diameter, and microlayer dynamics. Three dominant modes of bubble interaction, hydrodynamic interaction (HI), thermal interaction (TI), and coalescence (C), were identified as a function of the level of subcooling. Quantification of the experimental data showed that the equivalent diameter of the vapor bubble increases with an increase in the level of heat flux, while it decreases with an increase in the subcooling level. Additionally, the secondary nucleation affects the liftoff time and movement of the primary bubble and can hydrodynamically deplete the microlayer of the first bubble even when the bubbles are far apart, with the depletion rates showing a strong dependence on the growth rate and location of the secondary nucleation. [ABSTRACT FROM AUTHOR]

A pulsating heat pipe has shown promising results in heat transfer enhancement for several industrial applications. Despite having multiple advantages in the design and physics-based parameters, its applications are still limited due to the lack of understanding of the dynamics of the chaotic interaction of the liquid slug and vapor bubble. The randomly distributed liquid slug and vapor bubble start oscillating upon applying the heat at the evaporator section. A transition from the self-sustained oscillatory to chaotic operation in the pulsating heat pipe has been observed in various experimental studies. However, there are only a few works explaining the chaotic interaction of liquid slugs and vapor bubbles. We present the analysis of the oscillatory behavior of the position of the liquid slug in the pulsating heat pipe using a nonlinear mathematical model. The identification of various operating regions in the parameter space using the bifurcation analysis reveals the presence of a route to chaos by the period-doubling bifurcation. The fast Fourier transform of the temporal evolutions confirms the presence of the period doubling phenomena. Furthermore, the Lyapunov exponent and the correlation dimension are used to detect chaos and quantify the dimension of the chaotic attractor, respectively. Moreover, the Hurst exponent is used to determine the persistency of the oscillations, which indicates that the oscillations are weakly persistent in the chaotic regime. The transition to the chaotic regime is analyzed by period doubling, and a change in the frequency beyond period-doubling shows a gradual shift to the chaotic operation. [ABSTRACT FROM AUTHOR]

Dynamic gas–liquid mass-transfer processes are extensively encountered in gas–liquid mixture transport systems, where mechanical pumps pressurize the mixture and are accompanied by flow and mass-transfer instabilities. Herein, our proposed gaseous cavitation model was innovatively developed to revolutionize the independent unidirectional absorbed or evolved mass transfers. Complex gas–liquid behaviors under the synergetic effects of gaseous and vapor cavitations were achieved for the first time in an on-orbit refueling mechanical pump. Four coupled mass-transfer processes, namely, evolution, evaporation, absorption, condensation, and gas–liquid distribution, were investigated through numerical calculations. The results indicated that when the solution was close to critical saturation and conversion of the mass-transfer direction, a surge in the mass-transfer rate, and more intense hydrodynamic instability occurred. The vapor drove the accumulation of the evolved gas along the edge of the vapor in the impeller, where the evolved-dominated mass-transfer bands existed on the suction surfaces of the long blade, exhibiting the degassing characteristics of the vapor cavity, and other regions belonged to absorption-dominated region. Continuous dissolution induced by significant positive pressure gradient led to the maximum absorbed oxygen concentration at the impeller outlet. The maximal increments of absorbed oxygen in the suction chamber, impeller, and volute were 98%, 447%, and 694%, respectively, and the volume fractions were attenuated by 18.3%, 12.5%, and 5.0%, respectively. Notably, an increase in the gas volume fraction was the dominant reason for exacerbating the instability of the impeller forces, and the range of the radial force tended to be narrow and concentrated as the concentration increased. [ABSTRACT FROM AUTHOR]

Droplet impact on a heated substrate is an important area of study in spray cooling applications. On substrates significantly hotter than the saturation temperature, droplets immediately hover on its vapor cushion, exhibiting the Leidenfrost phenomenon. Here, we report the phenomena wherein addition of Al2O3 nanoparticles to water significantly increases the onset of dynamic Leidenfrost temperature (TDL) and suppresses the overall Leidenfrost regime. We experimentally revealed that the onset of TDL delays with increasing the nanoparticle concentration of the colloidal dispersions at a particular Weber number (We). But, for a constant concentration, the onset of TDL decreases with an increase in impact We. In contrast to water droplets, the colloid droplets exhibit vigorous spraying behavior due to the nanoparticulate residue deposition during the spreading and retraction stages. Further, the residue on the heated substrate changes the departure diameter of the vapor bubbles during boiling, prevents bubble coalescence and vapor layer formation, and reduces the propensity to attain dynamic Leidenfrost regime. With the aid of scaling analysis of TDL with impact We, we have explored the thermo-hydrodynamic behavior of impacting colloid droplets on a superheated substrate. Finally, we have also segregated the different boiling regimes of colloid droplets over various impact We. [ABSTRACT FROM AUTHOR]

A multi-relaxation-time pseudopotential lattice Boltzmann (LB) model was developed to simulate multiphase flow with a large density ratio (1000) and wide viscosity ratio (15), which are highly close to the realistic cavitation phenomenon. The model is validated by Laplace law and is implemented to simulate the collapse process of single and multiple cavitation bubbles. The simulation results are in accordance with the experimental results. By comparing to other LB models and numerical methods, the stability and superiority of the present model are morphologically demonstrated. The collapse pressure and the impact jet velocity on the wall are investigated, and the jet velocity on the wall shows a typical bimodal distribution, while only one perk is observed on the pressure distribution for a single bubble collapse in near-wall region. Moreover, the results indicated that the wall–bubble distance has a greater influence on the wall pressure than the bubble–bubble distance for double bubble interactions. Finally, the simulation of the bubble cluster indicated that the inner bubble has stronger collapse intensity than the outer layer bubble, while the side bubble collapse intensity is stronger than the corner bubbles. The simulation of a series of cases proves that the proposed model is a reliable tool to investigate realistic cavitation bubble dynamics. [ABSTRACT FROM AUTHOR]

Falling film technology has been widely applied in various industrial processes, but vapor shearing effects cause significant problems. The mechanism by which vapor affects the hydrodynamics of a falling film has not yet been fully understood, particularly for cross vapor streams. In this paper, a three-dimensional model is developed to investigate the hydrodynamics of a falling film on a horizontal tube with a cross vapor stream. The results indicate that (1) there are valley and peak zones during the spreading of the liquid film; (2) the effect of cross vapor flow on the hydrodynamics of the falling film depends on the film flow rate, the cross vapor velocity, and the position on the tube; (3) with increasing vapor velocity, the peripheral film thickness increases on both the upwind and downwind sides for values of z* (the dimensionless axial coordinate) from 0 to 0.1, while decreasing on the upwind side but increasing on the downwind side for values of z* from 0.2 to 0.5; and (4) under the action of the cross vapor stream, liquid film offset occurs, characterized by the formation of new stagnation and detachment zones on the downwind side of the tube. [ABSTRACT FROM AUTHOR]

We present Rb vapor transport through micro-scale capillaries on the impact of temperature and capillary inner diameters within the limits of the Knudsen number K n ≫ 1. Daily absorption spectral measurements were taken over several months to evaluate the dynamics of transport. We provide new insight into the diffusion mechanism and observe a quasi-single-layer coating on the surface based on the analysis of a slowly increasing absorption signal. The dwell time of the atom on the glass is directly derived from the diffusion dynamics at different temperatures. According to the mass flow rate, high vapor temperatures caused a faster transport speed, indicating rapid loading in microchannels. We provide a valuable model for future quantum device implementation through the use of miniaturized structures, such as photonic crystal fibers and optical waveguides. [ABSTRACT FROM AUTHOR]

The dynamics of saturated vapor between two intersecting walls is examined. It is shown that, if the angle ϕ between the walls is sufficiently small, the vapor becomes unstable, and spontaneous condensation occurs in the corner, similar to the so-called capillary condensation of vapor into a porous medium. As a result, an ever-growing liquid meniscus develops near the corner. The diffuse-interface model and the lubrication approximation are used to demonstrate that the meniscus grows if and only if ϕ + 2 θ < π , where θ is the contact angle corresponding to the fluid/solid combination under consideration. This criterion has a simple physical explanation: if it holds, the meniscus surface is concave—hence, the Kelvin effect causes condensation. Once the thickness of the condensate exceeds by an order of magnitude the characteristic interfacial thickness, the volume of the meniscus starts to grow linearly with time. If the near-vertex region of the corner is smoothed, the instability can be triggered off only by finite-size perturbations, such that it includes enough liquid to cover the smoothed area by a microscopically thin liquid film. [ABSTRACT FROM AUTHOR]

Evaporation of a liquid drop surrounded by either vapor of the same fluid, or vapor and air, is usually attributed to vapor diffusion, which, however, does not apply to the former setting, as pure fluids do not diffuse. The present paper puts forward an additional mechanism, one that applies to both settings. It is shown that disparities between the drop and vapor in terms of their pressure and chemical potential give rise to a flow. Its direction depends on the vapor density and the drop's size. In undersaturated or saturated vapor, all drops evaporate, but in oversaturated (yet thermodynamically stable) vapor, there exists a critical radius: smaller drops evaporate, whereas larger drops act as centers of condensation and grow. The developed model is used to estimate the evaporation time of a drop floating in saturated vapor. It is shown that, if the vapor-to-liquid density ratio is small, so is the evaporative flux; as a result, millimeter-sized water drops at temperatures lower than 70 ° C survive for days. If, however, the temperature is comparable (but not necessarily close) to its critical value, such drops evaporate within minutes. Micron-sized drops, in turn, evaporate within seconds for all temperatures between the triple and critical points. [ABSTRACT FROM AUTHOR]

To investigate the vapor kinetic boundary condition (which is the boundary condition for the Boltzmann equation) in the presence of a non-condensable (NC) gas at a non-equilibrium liquid interface, we performed numerical simulations of non-equilibrium vapor (condensable gas) and NC gas mixture flows. The Enskog–Vlasov direct simulation Monte Carlo method (EVDSMC method) was utilized for this two-surface problem to obtain the evaporation and condensation coefficients, which represent the vapor–molecule evaporation and condensation rates, respectively. These coefficients are incorporated in the kinetic boundary condition. The simulation results showed that the evaporation and condensation coefficients decrease with increasing numbers of NC-gas molecules at the liquid interface with the same tendency. To investigate the validity of these coefficients, we also utilized the obtained evaporation and condensation coefficients for Boltzmann equation analysis. Hence, we concluded that these coefficients simply depend on the NC-gas number density at the liquid interface if the liquid temperature is constant. Thus, they are independent of the non-equilibrium or equilibrium state, and therefore, the coefficient values obtained for the equilibrium state can be used in the analysis of a non-equilibrium state. This finding will aid future predictions of non-equilibrium vapor flows with NC gas. [ABSTRACT FROM AUTHOR]

A vapor bubble's footprint on a heated surface is composed of dry and wet regions, and their temporal evolution is governed by several forces, such as gravity, vapor inertia, viscous friction, and capillarity. In this Letter, we derive three types of relations that depict the dynamic variations in the length scales for the bubble base, dry area, and thin liquid film under a single vapor bubble. The proposed relations are validated using measurement results from clear images of the phase structure captured by high-speed visualization. Here, we reveal that the viscous friction is a dominant force acting on the bubble expansion process; however, it has been previously ignored for simplicity in dealing with the classical Rayleigh equation. Additionally, the critical role of the liquid wetting character in the vicinity of the moving triple phase line is illuminated to quantify the dry area dynamics. Finally, we find an analytic expression to obtain the thickness profile of the thin liquid film under a bubble based on the well-known Landau and Levich liquid coating theory. [ABSTRACT FROM AUTHOR]

Flows of fluids made of complex organic molecules exhibit unconventional fluid dynamic behavior in the vapor phase if their thermodynamic state is close to that of the vapor–liquid critical point. If the molecule is sufficiently complex, this thermodynamic domain is characterized by negative values of the fundamental derivative of gasdynamics Γ, the fluid is called Bethe–Zel'dovich–Thompson (BZT) fluid, and atypical phenomena, such as rarefaction shockwaves, are theoretically admissible. The nature of the steepening of nonlinear waves in dense vapor flows of organic fluids evolving in this thermodynamic region can be significantly affected by the presence of temperature gradients in the flow. This study investigates the evolution of finite-amplitude acoustic waves in these conditions. The steepening of the wavefront is analyzed using the wavefront expansion technique, and the deformation of the wavefront is simulated numerically by solving the Westervelt equation. The results of simulations of wave propagation in dense vapors indicate that, though Γ governs the nature of steepening waves, local gradients in sound speed and density can alter the rate of steepening and can enhance or delay shock formation in the medium, a result relevant also to the envisaged experiments aimed at proving the existence of nonclassical gasdynamics phenomena in BZT vapors. [ABSTRACT FROM AUTHOR]

We numerically investigate the effect of non-condensable gas inside a vapor bubble on bubble dynamics, collapse pressure, and pressure impact of spherical and aspherical bubble collapses. Free gas inside a vapor bubble has a damping effect that can weaken the pressure wave and enhance the bubble rebound. To estimate this effect numerically, we derive and validate a multi-component model for vapor bubbles containing gas. For the cavitating liquid and the non-condensable gas, we employ a homogeneous mixture model with a coupled equation of state for all components. The cavitation model for the cavitating liquid is a barotropic thermodynamic equilibrium model. Compressibility of all phases is considered in order to capture the shock wave of the bubble collapse. After validating the model with an analytical energy partitioning model, simulations of collapsing wall-attached bubbles with different stand-off distances are performed. The effect of the non-condensable gas on rebound and damping of the emitted shock wave is well captured. [ABSTRACT FROM AUTHOR]

An experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen, and carbon dioxide) at reduced pressure (from 40 to 1000 mbars) using four different substrates (namely, aluminum, titanium, Macor, and polytetrafluoroethylene) with a wide range of thermal conductivities. Reducing the atmospheric pressure increases the diffusion coefficient of water vapor in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in encouraging agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The half-space problem of the temperature, pressure, and concentration jumps for a binary mixture of vapors is investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules with the complete condensation condition. First, the problem is shown to be reduced to three elemental ones: the problem of the jumps caused by the net evaporation or condensation, that caused by the gradient of temperature, and that caused by the gradient of concentration. Then, the latter two are investigated numerically in the present contribution because the first problem has already been studied [Yasuda, Takata, and Aoki, Phys. Fluids 17, 047105 (2005)]. The numerical method is a finite-difference one, in which the complicated collision integrals are computed by the extension of the method proposed by Sone, Ohwada, and Aoki [Phys. Fluids A 1, 363 (1989)] to the case of a gas mixture. As a result, the behavior of the mixture is clarified not only at the level of the macroscopic quantities but also at the level of the velocity distribution function. In addition, accurate formulas of the temperature, pressure, and concentration jumps are constructed for arbitrary values of the concentration of the background reference state by the use of the Chebyshev polynomial approximation. The solution of the corresponding problem of a vapor-gas mixture and that of the temperature-jump problem on a simple solid wall are also obtained as special cases of the present problem. [ABSTRACT FROM AUTHOR]

The van der Waals polytropic gas model is used to investigate the role of attractive and repulsive intermolecular forces and the influence of molecular complexity on the possible nonclassical gas dynamic behavior of vapors near the liquid-vapor saturation curve. The decrease of the sound speed upon isothermal compression is due to the well-known action of the van der Waals attractive forces and this effect is shown here to be comparatively larger for more complex molecules with a large number of active vibrational modes; for these fluids isentropic flows are in fact almost isothermal. Contributions to the speed of sound resulting from intermolecular forces and the role of molecular complexity are analyzed in details for both isothermal and isentropic transformations. Results of the exact solution to the problem of a finite pressure perturbation traveling in a still fluid are presented in three exemplary cases: ideal gas, dense gas and nonclassical gas behavior. A classification scheme of fluids based on the possibility of exhibiting different gas dynamic behaviors is also proposed. [ABSTRACT FROM AUTHOR]

This paper provides the theoretical basis for energetic vapor bubble implosions induced by a standing acoustic wave. Its primary goal is to describe, explain, and demonstrate the plausibility of the experimental observations by Taleyarkhan et al. [Science 295, 1868 (2002); Phys. Rev. E 69, 036109 (2004)] of thermonuclear fusion for imploding cavitation bubbles in chilled deuterated acetone. A detailed description and analysis of these data, including a resolution of the criticisms that have been raised, together with some preliminary HYDRO code simulations, has been given by Nigmatulin et al. [Vestnik ANRB (Ufa, Russia) 4, 3 (2002); J. Power Energy 218-A, 345 (2004)] and Lahey et al. [Adv. Heat Transfer (to be published)]. In this paper a hydrodynamic shock (i.e., HYDRO) code model of the spherically symmetric motion for a vapor bubble in an acoustically forced liquid is presented. This model describes cavitation bubble cluster growth during the expansion period, followed by a violent implosion during the compression period of the acoustic cycle. There are two stages of the bubble dynamics process. The first, low Mach number stage, comprises almost all the time of the acoustic cycle. During this stage, the radial velocities are much less than the sound speeds in the vapor and liquid, the vapor pressure is very close to uniform, and the liquid is practically incompressible. This process is characterized by the inertia of the liquid, heat conduction, and the evaporation or condensation of the vapor. The second, very short, high Mach number stage is when the radial velocities are the same order, or higher, than the sound speeds in the vapor and liquid. In this stage high temperatures, pressures, and densities of the vapor and liquid take place. The model presented herein has realistic equations of state for the compressible liquid and vapor phases, and accounts for nonequilibrium evaporation/condensation kinetics at the liquid/vapor interface. There are interacting shock waves in both phases, which converge toward and reflect from the center of the bubble, causing dissociation, ionization, and other related plasma physics phenomena during the final stage of bubble collapse. For a vapor bubble in a deuterated organic liquid (e.g., acetone), during the final stage of collapse there is a nanoscale region (diameter ∼100 nm) near the center of the bubble in which, for a fraction of a picosecond, the temperatures and densities are extremely high (∼108 K and ∼10 g/cm3, respectively) such that thermonuclear fusion may take place. To quantify this, the kinetics of the local deuterium/deuterium (D/D) nuclear fusion reactions was used in the HYDRO code to determine the intensity of the fusion reactions. Numerical HYDRO code simulations of the bubble implosion process have been carried out for the experimental conditions used by Taleyarkhan et al. [Science 295, 1868 (2002); Phys. Rev. E 69, 036109 (2004)] at Oak Ridge National Laboratory. The results show good agreement with the experimental data on bubble fusion that was measured in chilled deuterated acetone. [ABSTRACT FROM AUTHOR]

Half-space problem of evaporation and condensation of a binary mixture of vapors is investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules with the complete condensation condition. The problem is analyzed numerically by a finite-difference method, in which the complicated collision integrals are computed by the extension of the method proposed by Y. Sone, T. Ohwada, and K. Aoki [“Temperature jump and Knudsen layer in a rarefied gas over a plane wall: Numerical analysis of the linearized Boltzmann equation for hard-sphere molecules,” Phys. Fluids A 1, 363 (1989)] to the case of a gas mixture. As a result, the behavior of the mixture is clarified not only at the level of the macroscopic quantities but also at the level of the velocity distribution function. In addition, accurate formulas of the temperature, pressure, and concentration jumps caused by the evaporation and condensation are constructed for arbitrary values of the concentration of the background reference state by the use of the Chebyshev polynomial approximation. [ABSTRACT FROM AUTHOR]

Steady flows of a vapor around its condensed phase of arbitrary shape, on the surface of which evaporation and condensation of the vapor may take place, are considered in the presence of a small amount of a noncondensable gas. By a systematic asymptotic analysis of the Boltzmann system, the present authors have derived the fluid-dynamic system describing such flows in the continuum limit in the case where the amount of the noncondensable gas is infinitesimal compared with that of the vapor [K. Aoki, S. Takata, and S. Taguchi, Eur. J. Mech. B/Fluids 22, 51 (2003)]. In the present study, the fluid-dynamic system is applied to some physical problems, and it is demonstrated with concrete examples that such a tiny amount of the noncondensable gas causes dramatic changes of the overall vapor flows. [ABSTRACT FROM AUTHOR]

The evaporation and condensation at an interface of vapor and its condensed phase is considered. The validity of kinetic boundary condition for the Boltzmann equation, which prescribes the velocity distribution function of molecules outgoing from the interface, is investigated by the numerical method of molecular dynamics for argon. From the simulations of evaporation into vacuum, the spontaneous-evaporation flux determined by the temperature of condensed phase is discovered. Condensation coefficient in equilibrium states can then be determined without any ambiguity. It is found that the condensation coefficient is close to unity below the triple-point temperature and decreases gradually as the temperature rises. The velocity distribution of spontaneously evaporating molecules is found to be nearly a half-Maxwellian at a low temperature. This fact supports the kinetic boundary condition widely used so far. At high temperatures, on the other hand, the velocity distribution deviates from the half-Maxwellian. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A droplet growth model is formulated that describes the growth of homogeneous multi-component droplets suspended in a dilute gas–vapor mixture. The droplet is suspended in a real carrier gas, in which the vapors are diluted. The model is applicable for all Knudsen numbers. It is shown that the model can easily be extended to describe the growth of layered droplets. The model calculations are compared to the results of droplet growth experiments of supersaturated water and/or supersaturated n-nonane in methane. The model performs well for all the cases, except for the case when supersaturated n-nonane vapor is added to supersaturated water vapor in methane gas. In the latter case it is proposed that the discrepancies originate from the incomplete wetting of n-nonane on liquid water at the given conditions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Droplet growth rates of droplets suspended in methane gas and supersaturated water and/or n-nonane vapor are experimentally determined. The experiments are performed by applying the nucleation pulse principle using a modified shock tube. The droplets are optically detected using a combination of constant angle Mie scattering and light extinction measurements. From the analysis of the droplet growth rates in the binary systems the diffusion coefficients of water in methane and n-nonane in methane have been determined at two different conditions, being 11 bar and 242 K, and 44 bar and 247 K. The droplet growth rates in the ternary system have been determined at the same two conditions. From the analysis of these experiments it is evident that supersaturated water vapor does not condense onto n-nonane droplets while supersaturated n-nonane vapor does condense onto water droplets. This can be related to the wetting properties of liquid water on liquid n-nonane and vice versa. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper is the second part of the study of a steady flow of a vapor in a half space condensing onto a plane condensed phase of the vapor at incidence in the presence of a noncondensable gas near the condensed phase. The aim of the study is to clarify the behavior of the vapor and noncondensable gas on the basis of kinetic theory under the assumption that the molecules of the noncondensable gas are mechanically identical with those of the vapor. In the first part [S. Taguchi et al., Phys. Fluids 15, 689 (2003)], the case of subsonic condensation, where the Mach number corresponding to the flow-velocity component perpendicular to the condensed phase at infinity is less than unity, is considered. In the present second part, the case of supersonic condensation is investigated in detail on the same lines as the first part.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A steady flow of a vapor in a half space condensing onto a plane condensed phase of the vapor at incidence is considered in the case where another gas that neither evaporates nor condenses (the noncondensable gas) is present near the condensed phase. The behavior of the vapor and noncondensable gas is investigated on the basis of kinetic theory under the assumption that the molecules of the noncondensable gas are mechanically identical with those of the vapor. In particular, the relation, among the parameters of the vapor at infinity (the pressure, temperature, and flow velocity of the vapor), those related to the condensed phase (the temperature of the condensed phase and the corresponding saturation pressure of the vapor), and the amount of the noncondensable gas, that admits a steady solution is obtained numerically by the use of a model Boltzmann equation proposed by Garzó et al. [Phys. Fluids A 1, 380 (1989)]. The present analysis is the continuation of an earlier work by Sone et al. [Transp. Theory Stat. Phys. 21, 297 (1992)], where the case in which the vapor flow is condensing perpendicularly onto the condensed phase is investigated exclusively. The case with subsonic condensation is discussed in the present paper (the case with supersonic condensation is left to the subsequent paper). [ABSTRACT FROM AUTHOR]

The steady two-dimensional laminar flow of a stream of saturated vapor flowing over a tube that is kept at a uniform temperature, below the saturation temperature, is examined. Owing to the temperature difference between the vapor stream and the solid surface a film of condensate is generated that flows along the surface due to shear, pressure-drop, and gravity. In the limit as the boundary layer and film thickness remain smaller than the radius of curvature of the surface a simplified lubrication-type formulation describes the temperature and flow fields in the film, whereas the usual boundary layer formulation is applied in the vapor boundary layer. The case of flow past a horizontal cylinder of radius R is investigated numerically with the oncoming stream aligned with gravity. The parameters that control momentum and heat transfer in this problem are the viscosity ratio, μ[sub cs] / μ[sub ss], the density ratio, ρ[sub cs] / ρ[sub ss], the Prandtl number, Pr = C[sub pcs]μ[sub cs] / k[sub cs], the Froude number, Fr = U²[sub ∞] / (g π R), and finally the thickness ratio between the condensate and the vapor boundary layer, &epsilon, which is also a measure of the temperature difference between the vapor stream and the tube wall. Then, the Nusselt number and the skin friction coefficient, averaged over the upper half of the cylinder, are calculated for a wide parameter range. When Fr is very small and relatively large the flow remains attached until the trailing stagnation point of the cylinder. As the effect of adverse pressure drop becomes more pronounced (Fr increases or ε decreases) it is shown that the solution exhibits two different types of singularity in the rear part of the cylinder. The first one is a typical Goldstein singularity because it appears at the tube wall and it is associated with vanishing skin friction (wall shear) and rapidly increasing film thickness. The second one takes place near the interface... [ABSTRACT FROM AUTHOR]

Investigates the steady behavior of a multicomponent mixture of vapors and noncondensable gases between two parallel plane condensed phases for small Knudsen numbers in light of kinetic theory. Formulation of problem and basic equations; Asymptotic analysis for small Knudsen numbers.

Examines numerically the validity of some asymptotic-theory predictions such as the existence of two resonant radii and a limit size of oscillating vapor bubbles for a given sound amplitude and frequency. Unique characteristics of oscillating vapor bubbles; Role played by resonance phenomena in the size of vapor bubbles; Thermal penetration length.

Investigates the two-surface problem of evaporation and condensation for a mixture of a vapor and a noncondensable gas in nonlinear situations on the basis of the Boltzmann equation. Asymptotic analysis of the problem for small values of the Knudsen number; Continuum limit; Use of direct simulation Monte Carlo method for a wide range of the Knudsen number.

A dispersion equation describing the effect of temperature differences on the stability of three-dimensional cylindrical liquid jets in the presence of vapor cavities is presented by the use of linear stability analysis. The mathematical model and its solving method are verified by comparing them with the data in the literature, and then the effect of temperature differences between jet and surrounding gas on the spatial stability of liquid jet is investigated. Some conclusions can be drawn from the results of this investigation: (1) the temperature difference destabilizes the liquid jet when the jet liquid is cooler than the surrounding gas, (2) the smallest atomized droplet without taking into account the effect of temperature differences is significantly larger than that when the effect of temperature differences is taken into account, (3) the effect of temperature differences on the stability of liquid jet has little relationship with azimuthal wave modes, (4) cavitation destabilizes the liquid jet when the value of the bubble volume fraction is not greater than 0.1 (0 ≤ α ≤ 0.1), and the temperature difference can weaken this effect of cavitation on the stability of liquid jet, and (5) cavitation is responsible for generating smaller droplets, the effect of cavitation on the critical wave number with and without taking into account the effect of temperature differences is quite different, and temperature difference is likely to fully restrain the effect of cavitation on the critical wave number; however, cavitation is again responsible for generating smaller droplets despite the effect of temperature differences when the bubble volume fraction α = 0.1. These findings may explain some observations of practical atomizer performance. [ABSTRACT FROM AUTHOR]

Particle image velocimetry (PIV) and high-speed photography are used to measure the flow structure at the closure region and downstream of sheet cavitation. The experiments are performed in a water tunnel of cross section 6.35x5.08 cm[sup 2] whose test area contains transparent nozzles with a prescribed pressure distribution. This study presents data on instantaneous and averaged velocity, vorticity and turbulence when the ambient pressure is reduced slightly below the cavitation inception level. The results demonstrate that the collapse of the vapor cavities in the closure region is the primary mechanism of vorticity production. When the cavity is thin there is no reverse flow downstream and below the cavitation, i.e., a reentrant flow does not occur. Instead, the cavities collapse as the vapor condenses, creating in the process hairpin-like vortices with microscopic bubbles in their cores. These hairpin vortices, some of which have sizes as much as three times the height of the stable cavity, dominate the flow downstream of the cavitating region. The averaged velocity distributions show that the unsteady collapse of the cavities in the closure region involves substantial increase in turbulence, momentum, and displacement thickness. Two series of tests performed at the same velocity and pressure, i.e., at the same hydrodynamic conditions, but at different water temperatures, 35 °C and 45 °C, show the effect of small changes in the cavitation index (σ=4.69 vs. σ=4.41). This small decrease causes only a slight increase in the size of the cavity, but has a significant impact on the turbulence level and momentum deficit in the boundary layer downstream. Ensemble averaging of the measured instantaneous velocity distributions is used for estimating the liquid void fraction, average velocities, Reynolds stresses, turbulent kinetic energy and pressure distributions. The results are used to examine the mass and momentum balance downstream of th... [ABSTRACT FROM AUTHOR]