A Possible Mechanism for the Formation of Unwettable Dry Spots on the Heated Surface under Nucleate Boiling: Part I. Basic Models and Characteristics of Heterogeneous Nucleate Pool Boiling at a Low Pressure

The problems associated with the physics of heterogeneous pool boiling at a low pressure on a flat horizontal surface are considered. The examples of parametric mismatch between the trends are connected with the thermophysical properties and wall surface microgeometry, with the size of the working area, with the affected zone of a growing bubble, and with the contact angle at the interface between the liquid, solid, and vapor phases. A conclusion is drawn concerning the possible causes of ambiguity in the results of the simulation of nucleate pool boiling modes under calculation of the individual contribution of each of the heat transfer mechanisms (convection, liquid microlayer evaporation, and rearrangement of the thermal boundary layer after bubble detachment). It is emphasized that the problem should be solved in the 3D conjugate formulation.     

About lack of fragmentation for hot droplets at low subcooling of coolant

It was confirmed in experiments that during contact between cool and hot liquids, the lower sub-cooling of the cool liquid below the saturation temperature changes the characteristics of a vapor layer covering the fragments of hot liquid. This factor also decreases the probability of spontaneous direct contact between two kinds of liquid, explosive incipience of the cool liquid, and pressure pulse generation (the latter triggers fine fragmentation of hot coolant and vapor explosion). The mechanism that describes this trend in vapor layer behavior has been described.     

AN EXPERIMENTAL INVESTIGATION OF TEMPERATURE AND PRESSURE OSCILLATION IN THE BOILING OF LIQUID HELIUM

Boiling phenomena in liquid helium II (He II) and liquid helium I (He I) were experimentally investigated. The temperature oscillations during boiling in He II are the result of the propagation of the thermal boundary layer and/or the expansion of a vapor bubble to the location of the superconductor temperature sensor. The pressure oscillations are caused by the direct contact of liquid He II with the higher-temperature heater surface. The pressure oscillations are very periodic, and there is a strong correlation between the temperature and the pressure oscillations. In the boiling of He I, bubbles detach from the heater surface and are detected by the superconductor temperature sensor. He I boiling is different from the boiling of He II in that there is no correlation between the temperature and the pressure oscillations.     

Drop impact on superheated surfaces

At the impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surface ("contact boiling"), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back ("gentle film boiling"), or both forms the Leidenfrost layer and ejects tiny droplets upward ("spraying film boiling"). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ~We(1/4)). We also interferometrically measure the vapor thickness under the droplet.     

高温壁面上静止液滴的Leidenfrost温度模型研究

本文以高温壁面上的静止液滴为研究对象,对其蒸发特性开展了理论研究.不同计算工况下得到的液滴蒸发过程中半径和蒸汽膜厚度变化与实验值吻合良好.结果表明随着壁面温度的降低,蒸汽膜厚度逐渐减小.结合表面粗糙度的影响,研究中提出了 Leidenfrost温度的触发机制:当蒸汽膜厚度足够小时,会极易被加热表面的不平整突起贯穿,蒸汽...     

Boundary layer theory modified for analysis of wave flow on the surface of a viscous liquid finite-thickness layer resting on a hard bottom

The theory of a boundary layer that is adjacent to the surface of an indefinitely deep viscous liquid and caused by its periodic motion is modified for analysis of finite-amplitude flow motion on the charged surface of a viscous conductive finite-thickness liquid layer resting on a hard bottom (the thickness of the layer is comparable to the wavelength). With the aim of adequately describing the viscous liquid flow, two boundary layers are considered: one at the free surface and the other at the hard bottom. The thicknesses of the boundary layers are estimated for which the difference between an exact solution and a solution to a model problem (stated in terms of the modified theory) may be set with a desired accuracy in the low-viscosity approximation. It is shown that the presence of the lower (bottom) boundary layer should be taken into account (with a relative computational error no more than 0.001) only if the thickness of the viscous layer does not exceed two wavelengths. For thicker layers, the bottom flow may be considered potential. In shallow liquids (with a thickness of two tenths of the wavelength or less), the upper (near-surface) and bottom layers overlap and the eddy flow entirely occupies the liquid volume. As the surface charge approaches a value that is critical for the onset of instability against the electric field negative pressure, the thicknesses of both layers sharply grow.     

Modeling of the vaporization front on a heater surface

The boiling-up of a metastable liquid with appearing vaporization fronts is theoretically considered. The boiling-up occurs usually on the surface of a heater. At the initial stage, growth of a spherical vapor bubble is observed. If the temperature of the liquid exceeds a threshold value, the vaporization fronts develop near the line of contact of a vapor bubble and the heater. The vaporization fronts extend along the heater with a constant speed. A model of steady propagation of the vaporization front is developed. The temperature and propagation velocity of the interface are determined from the balance equations of mass, momentum, and energy in the neighborhood of the vaporization front and from the stability condition of motion of the interface. It is shown that a solution of these equations exists only if the liquid is heated above a threshold value. The propagation velocity of the vaporization front also has the threshold value. The calculated velocity of interface motion and the threshold value of temperature are in reasonable agreement with available experimental data for various liquids within wide ranges of saturation pressures and temperatures of the overheated liquid.     

Comparative Study of Wetting and Cooling Performance of Polymer–Salt Hybrid Quench Medium With Conventional Quench Media

Wetting kinetics, kinematics, and cooling performance of a polymer–salt hybrid quenchant were investigated. The rewetting phenomenon for brine, water, polymer, and polymer–salt hybrid solutions was characterized as rapid uniform, fast non-uniform, slow uniform, and fast uniform processes, respectively. A dimensionless rewetting time was proposed to assess the nature of the wetting front. The hybrid quenchant showed higher heat transfer during vapor and transition boiling and lower heat transfer during nucleate boiling and convective cooling. The presence of salt in the hybrid solution resulted in early destabilization of the vapor film and an increase in wetting front velocity and rewetting temperature. The polymer constituent delayed the rewetting phenomenon.     

Stability of active systems with a spatially periodic activity: Analysis of a simple model and application to the boiling crisis problem

We investigate theoretically the possibility to control the transition between two metastable states in reactive systems by imposing a spatial modulation. In particular, we consider the technologically very important case of the transition between the low temperature (nucleate boiling) and the high temperature (film boiling) phases of boiling of a liquid over a heat generating element, also known as the boiling crisis. With the help of a simplified model, we demonstrate that the dangerous regime where the high temperature phase invades the whole system requires a larger heat power in a periodically spatially modulated system, than in a uniform system. The possibility that a local perturbation, such as a small gas bubble, may induce locally a transition to the film boiling state is also considered. We show that the transition to the film boiling regime is hindered in a spatially periodic system. (c) 2002 American Institute of Physics.     

Self-propelled Leidenfrost droplets

We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchetlike) topology. The pumping effect is observed when the liquid is in the Leidenfrost regime (the film-boiling regime), for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid.     

On diffusion mixing of vapor and gas

A self-similar solution is obtained for a one-dimensional problem of diffusion mixing of vapor and gas, which is followed by condensation process. Two limiting cases of mixing are considered: in the first one, the vapor and gas occupy in their initial states the volumes of semi-infinite extension, in the second case, the vapor has a semi-infinite extension, and constant values of temperature and gas concentration are maintained at its boundary. The peculiarities of the temperature and concentration fields are analysed versus the vapor and gas temperatures, and the temperature values are found, at which the mixing occurs with the condensate formation.     

Stochastic dynamics of boiling on the heating element surface

Dynamics of a liquid boiling on a heating element surface is investigated for the equivalent problem of dynamics of temperature of the fuel cell obtained in the concept of boiling curve. It has been shown that the structural insta-bility of the potential of boiling curve leads to the need of studying the problem of temperature fluctuations of the surface on which boiling occurs. A theoretical analysis of the Itô's equation for the considered system model has shown that at a certain intensity of external random factors, the system passes from one state of self-organized criticali-ty to another. These processes are accompanied by 1/f^α–noise (flicker-noise), which is regarded as an objective indica-tor of the boiling crisis. The theoretical results of the work have been confirmed by experimental data of other authors.     

On the form of interfacial surface at evaporation front propagation along the heater in the layer of metastable liquid

Within the framework of approximate physical and mathematical model, we considered the stationary problem of propagation of evaporation front in superheated liquid along the flat heater. The analytical dependence of the vapor layer thickness on the coordinate and physical parameters has been obtained. The solution is presented in invariant dimensionless form. Satisfactory agreement between theoretical results and new experimental data is shown.     

Investigation of evaporation and boiling of liquids

It is shown experimentally that a minimum in the evaporation curve of water drops, a 5% aqueous solution of ethanol, and a 5% aqueous solution of n-butanol, i.e., the maximal evaporation rate of these liquids, is observed at a critical thermal load under boiling. Based on this finding, a method to determine the temperature of onset of the boiling crisis is suggested. It is found that the evaporation stages of drops of a liquid correlate with boiling regimes for this liquid.     

Pulsed laser evaporation: equation-of-state effects

Theoretical study of laser ablation is usually based on the assumption that the vapor is an ideal gas. Its flow is described by gas dynamics equations [1, 2]. The boundary conditions at vaporization front are derived from the solution of the Boltzmann equation that describes the vapor flow in the immediate vicinity of the vaporizing surface (so-called Knudsen layer) [1]. This model is applicable within the range of temperatures much lower than the critical temperature of target material. In the present work, a general case is considered when the temperature of the condensed phase is comparable to or higher than the critical temperature. The dynamics of both condensed and gaseous phases can be described in this case by the equations of hydrodynamics. The dynamics of vaporization of a metal heated by an ultrashort laser pulse is studied both analytically and numerically. The analysis reveals that the flow consists of two domains: thin liquid shell moving with constant velocity, and thick low-density layer of material in two-phase state. Received: 2 March 1999 / Accepted: 28 May 1999 / Published online: 21 October 1999     

Thermocapillary motion of a drop near a plane interface between liquids

The thermocapillary motion of a liquid drop immersed into another liquid near an infinite plane interface between two liquids is theoretically analyzed. The motion is considered under the conditions of a constant temperature gradient normal to the interface at infinity and small Reynolds and Peclet numbers. The problem is solved in bispherical coordinates. The analysis takes into consideration the thermal conductivity of the liquids and the thermocapillary motion of the liquids due to a nonuniform temperature distribution over the plane interface.     

Cavity formation during water entry of heated spheres

We experimentally study the cavity formation when heated spheres impact onto water at low and high subcooling.The observations present that the formation and appearance of the cavity are affected by the boiling modes and the heat transfer intensity. In the nucleate-boiling regime, a rough cavity can be formed at a rather low impact velocity, while at the same velocity, the cavity formed in the film-boiling regime may have a very smooth interface with a stable vapor layer around the sphere. We discuss the effects of the impact speed, water and sphere temperatures on the stability of the vapor layer. For low subcooled water, the stable vapor layer will be disturbed when increasing the impact velocity, leading to a disturbed cavity. For high subcooled water, the film boiling has a particular boiling model in which the vapor layer around the sphere cannot keep its stability. In this particular film-boiling regime, no cavities can be formed at low impact velocities and only broken cavities can be formed at high impact velocities.     

On the structure of the velocity field due to wave motion at a uniformly charged interface between two viscous immiscible liquids

An analytical solution to the problem of evolution of a capillary-gravitational wave on the uniformly charged interface between viscous immiscible liquids is found. It is shown that, away from the interface, both the total liquid flows and their wave-related eddy components on both sides of the interface decrease rapidly. The amplitude of the velocity field curl changes stepwise in going through the interface. The ratio between the amplitudes of the velocity field eddy components in the media being considered depends on the charge density at the interface, ratio of the kinematic viscosities, and densities of the upper and lower liquids.     

The thermal contact problem in nano- and micro-scale photothermal measurements

Thermal contact resistance between two solids is discussed with regard to its influence on the measurements of temperature and thermo-physical parameters in micro- and nano-structures. Two important applications are considered: thin film coatings on substrates and local measurements with a nano-probe in scanning thermal microscopy. The mechanical contact of a copper layer on carbon is measured by adhesion strength experiments and correlated to the thermal transport across the interface deduced from infrared radiometric measurements. A novel quantity the thermal wave contrast is introduced which takes into account the interface resistance and modifications of the coating and substrates at the interface. With regard to scanning thermal microscopy the contact resistance problem is discussed for 3ω-measurements in the active mode and for temperature measurements in the passive mode. It is shown that the thermo-elastic response can offer a means to avoid the influence of the thermal contact resistance on local temperature measurements.     

浓度边界层中成长汽泡的界面特性

对于双组分池内核态沸腾，热边界层中生成的汽泡，同时也处于浓度边界层中．本文建立了边界展中汽泡表面张力模型，对温度和浓度引起的表面张力变化进行分析．结果表明，表面张力从汽泡顶部到底部是递增的，从而形成液体沿界面自上而下的Marangoni流．通过汽泡底部微层相界面方程的求解，探讨了微层变形特征，由此分析汽泡脱离机理．微层变形与文献[1]中指出的接触线的变化从本质上是一致的．