Model of explosive pulsed vaporization of dielectric liquids by intense laser irradiation of their surfaces

The mechanism of explosive vaporization interaction of laser radiation with matter is studied theoretically. It is shown that, in dielectric liquids with a free surface, periodic explosive boiling is possible if the laser radiation intensity exceeds the rate of heat transfer from the region of laser radiation absorption. Analytical expressions are obtained to estimate the pulsating boiling period and the thickness of the surface liquid layer dispersed by fluctuation vapor bubbles during each boiling. The degree of absorption of laser radiation by the aerosol formed above the liquid surface is estimated. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 17–24, November–December, 2008.     

高温熔融铝液与水接触爆炸过程实验研究

为探究高温熔融铝液与水相互作用的爆炸机理,设计了一套高温熔融铝液与水接触爆炸实验测试系统,应用红外热像仪、高速摄像机、压力传感器等设备监测高温熔融铝液与水相互作用过程,并对高温熔融铝液与水接触爆炸过程的能量转化规律进行了分析.实验结果表明:高温熔融铝液与水接触瞬间发生膜态沸腾,水剧烈汽化产生的冲击在水中产生扰动并快速扩...     

Microscope activation near the wall during explosive boiling

The inception process of nucleation in explosive boiling systems is theoretically investigated. With the effect of pulse heating or sudden cooling, the temperature distribution near the surface during explosive boiling is calculated. The liquid near the wall can maintain a stable layer induced by strong attractive force, and there exists maximum supersaturation beyond this stable layer. As the surface temperature and temperature gradient are high enough, the critical distance of maximum supersaturation can be larger than the radius of critical bubble, and the homogeneous nucleation will dominate the inception boiling process. For explosive boiling induced by pulse heating, homogeneous nucleation forms after a short time; while homogeneous nucleation can dominate the initial explosive boiling induced by sudden cooling.     

Nonlinear vibrations of a vapor film in the presence of intense heat fluxes

Waves propagating along the interface between a thin vapor film and a liquid layer in the presence of a heat flux are investigated. The boundary conditions on the vapor-liquid phase surface take into account the temperature dependence of the pressure and the possibilities of formation of the metastable state of the superheated liquid and mass flow. Variations in the saturation pressure as functions of the temperature and mass flux lead to generation of weakly damped periodic waves of low amplitude whose velocity can be much higher than the velocity of the gravity waves. The waves ensure stability of the vapor film beneath the liquid layer in the gravity field. The finite-amplitude waves on the surface of the vapor film differ from the Stokes surface waves on the free surface of isothermal fluid. Instability regimes related with superheating of the liquid ant its explosive boiling when the amplitude of an initially small wave increases to infinity in a finite time can develop in a certain working-parameter regime.     

Explosive boiling of a liquid droplet

A mathematical model describing growth of an internal vapor bubble produced by homogeneous nucleation within a liquid droplet during explosive boiling is presented. Existing experimental results for explosive boiling of superheated droplets confirm the predictions of the model. The difference between the present model and the classical theories of bubble growth is discussed.     

Destabilization of film boiling by means of a thermal resistance

Described is the type of vaporisation which takes place when a thermal resistance, consisting in a film of a substance of low heat conductivity, is placed between the surface of a quenched sample and the cooling liquid. This type of vaporisation, larvate boiling, is characterised by an alternate wetting/non-wetting of the solid surface.Two conditions are necessary for larvate boiling: thermal resistance and surface effusivity.Substituting larvate boiling for film boiling allows the heat flux between a solid surface at high temperature and the cooling liquid to be greatly increased.     

Initiation of explosive boiling of a droplet with a shock wave

The role of incident shock waves in the initiation of vapor explosions in volatile liquid hydrocarbons has been investigated. Experiments were carried out on single droplets (1–2 mm diameter) immersed in a host fluid and heated to temperatures at or near the limit of superheat. Shocks generated by spark discharge were directed at previously nonevaporating drops as well as at drops boiling stably at high pressure. Explosive boiling is triggered in previously nonevaporating drops only if the drop temperature is above a threshold temperature that is near the superheat limit. Interaction of a shock with a stably boiling drop immediately causes a transition to violent unstable boiling in which fine droplets are torn from the evaporating interface, generating a two-phase flow downstream. On the previously nonevaporating interface between the drop and the host liquid, multiple nucleation sites appear which grow rapidly and coalesce. Overpressures generated in the surrounding fluid during bubble collapse may reach values on the same level as the pressure jump across the shock wave that initiated the explosive boiling. A simple calculation is given, which suggests that shock focusing may influence the location at which unstable boiling is initiated.     

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m²s, and heat flux 80–330 kW/m².The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout.In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality.This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.     

Kinetic theory analysis of explosive boiling of a liquid droplet

The process of rapid phase transition from highly superheated liquid to vapor is frequently so fast and violent that it is called explosive boiling. The paper uses the kinetic theory of evaporation to study growth of an internal vapor bubble produced by homogeneous nucleation within a highly superheated liquid droplet boiling explosively in a hot medium. Evaporation/condensation coefficient is estimated by comparing the predictions of the theory with available experimental data. We show that the value of the evaporation coefficient can be very low for high reduced temperatures (0.06 for butane at 378 K), in agreement with recent molecular dynamic simulations.     

Prediction of film boiling heat transfer coefficients for binary mixtures

Film boiling of binary liquid mixtures may be significantly different from that of single-component liquids due to the mass diffusion effect. A theoretical analysis is performed to outline the effects of mass diffusion phenomena on film boiling heat transfer process from a horizontal cylinder heating surface to the binary liquid mixtures of ethylene oxide/water and ethanol/benzene over whole range of compositions. These two binary systems are chosen for illustrating the strong and weak mass diffusion effects, respectively, on film boiling. Furthermore, a simple correlation for predicting heat transfer coefficient is proposed to demonstrate the idea that the dimensionless F factor can satisfactorily account for the mass diffusion effect on film boiling heat transfer of binary mixtures.     

Mixed convection in turbulent film boiling on a vertical ellipsoid under high and low velocity liquid

The theoretical study researched into heat transfer of turbulent film boiling on an isothermal ellipsoid under high and low velocity liquid. The flowing velocity of the saturated liquid at the boundary layer is determined by potential flow theory. The larger the eccentricity parameter is the smaller the mean Nusselt number will be. Besides, for the cases of turbulent film boiling under the flowing liquid, the increase in the Froude number will bring out an increase in the mean Nusselt number.     

An updated three-zone heat transfer model for slug flow boiling in microchannels

This work proposes a novel physics-based model for the fluid mechanics and heat transfer associated with slug flow boiling in horizontal circular microchannels to update the widely used three-zone model of Thome et al. (2004). The heat transfer model has a convective boiling nature and predicts the time-dependent variation of the local heat transfer coefficient during the cyclic passage of a liquid slug, an evaporating elongated bubble and a vapor plug. The capillary flow theory, extended to incorporate evaporation effects, is applied to estimate the bubble velocity along the channel. A liquid film thickness prediction method also considering bubble proximity effects, which may limit the radial extension of the film, is included. The minimum liquid film thickness at dryout is set to the channel wall roughness. Theoretical heat transfer models accounting for the thermal inertia of the liquid film and for the recirculating flow within the liquid slug are utilized. The heat transfer model is compared to experimental data taken from three independent studies. The 833 slug flow boiling data points cover the fluids R134a, R245fa and R236fa, and channel diameters below 1 mm. The proposed evaporation model predicts more than 80% of the database to within ±30%. It demonstrates a stronger contribution to heat transfer by the liquid slugs and correspondingly less by the thin film evaporation process compared to the original three-zone model. This model represents a new step towards a complete physics-based modelling of the bubble dynamics and heat transfer within microchannels under evaporating flow conditions.     

Two phase boundary layers in laminar film boiling

The laminar film boiling is analyzed by means of an integral procedure. The method treats the film boiling as a two phase boundary layer problem; thereby the effect of the interfacial shear on the heat transfer rate can be investigated. The problem is attacked by simultaneously solving the vapor and liquid boundary layer equations. An extensive comparison of the predicted results with the exact solutions substantiates the validity of the present integral procedure. Even the details of the velocity and temperature profiles turn out to be in close agreement with the exact solutions.     

Limit of superheat in uniformly heated fluid

The homogeneous nucleation limit is investigated in pure liquid subject to intense uniform heating at constant pressure. The energy equation is solved in conjunction with a new non-equilibrium vapor formation model in order to predict the maximum attainable liquid superheat as a function of the heating rate. It is shown that for uniformly heated liquids, conditions related to the local temperature in a critical vapor embryo and to the local heat consumption for vapor generation on the homogeneous fluctuation centers, must be satisfied simultaneously in order to initiate explosive boiling. The effect of heating rate on the maximum attainable superheat temperature could be as high as 10 K.     

A mechanistic IR calibration technique for boiling heat transfer investigations

This paper presents a new calibration technique to improve the accuracy of infrared thermometry in boiling heat transfer investigations.The technique is suitable for heaters consisting of a thin, infrared (IR) opaque conductive film coated on one side of a flat and IR semi-transparent substrate. The conductive film is in contact with the liquid and acts as the boiling surface. The IR camera sees the boiling surface through the substrate. If the substrate is not completely transparent, the radiation emitted by the IR opaque film is partially absorbed and contaminated by the radiation emitted by the substrate itself. Therefore, the correlation between the IR radiation measured by the IR camera and the temperature of the boiling surface (IR opaque film) is not unique, but depends on the temperature distribution in the substrate.To solve this issue, we developed a model that solves the coupled conduction/radiation inverse problem in the heater. The problem is inverse because the boundary condition for the conduction problem (the boiling surface temperature) is not known. The IR camera measures the combined radiation emitted by the boiling surface, emitted by the substrate and also the reflection of the background radiation; from that information one has to reconstruct the boiling surface temperature.The technique is unique in that it takes into account the spectral dependence of optical properties in the optical materials. For this reason, it is particularly suitable for heaters where the optical properties of the conductive film and the substrate materials depend on the wavelength of the IR radiation.Using this technique, we can measure with improved accuracy the time-dependent 3D temperature distribution in the heater, as well as local temperature and local heat flux distributions on the boiling surface. The validation of the technique was carried out using transient conduction experiments. Then, the technique was applied to transient pool boiling experiments to prove its feasibility and show the potential applications.     

Flow boiling behaviors in hydrophilic and hydrophobic microchannels

Surface wettability is a critical parameter in small scale phenomena, especially two-phase flow, since the surface force becomes dominant as size decreases. In present study, experiments of water flow boiling in hydrophilic and hydrophobic rectangular microchannels were conducted to investigate the wettability effect on flow boiling in rectangular microchannels. The rectangular microchannels were fabricated with a photosensitive glass to visualize flow pattern. The hydrophilic bare photosensitive glass microchannel was chemically treated to obtain a hydrophobic microchannel. And, visualization of flow patterns was carried out. And boiling heat transfer and two-phase pressure drop was analyzed with visualization results. The boiling heat transfer coefficient in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with nucleation site density and liquid film motion. And the pressure drop in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with unstable motions of bubble and liquid film. Finally, we find out the wettability is important parameter on the flow pattern, which were highly related with two-phase heat and mass transfer.     

Mass transport effect on the heat transfer coefficient during boiling of multicomponent mixture

In this work a simplified calculation method taking into account the effect of mass transport on the heat transfer coefficient (HTC) during boiling of multicomponent mixture has been elaborated. The calculation results were compared with own experimental data for ternary system methanol–isopropanol–water and Grigoriev data [1] (acetone–methanol–water). The experiments were performed in different hydrodynamic conditions such as: pool boiling and liquid evaporation at the free surface of the falling film. The experimental data covered wide range of heat fluxes from 6 to 30 kW/m² in the case of liquid evaporation from the falling film and from 30 to 240 kW/m² for pool boiling. The analysis of the results indicates that the mass transfer resistance in the liquid phase caused a significant reduction of experimental value HTC in comparison to so-called ideal HTC.     

A theoretical study of free convection in turbulent film boiling on a horizontal elliptical tube

The present theoretical study investigates turbulent film boiling on an isothermal elliptical tube under quiescent liquid. The effect of radiation is included in the present analysis. The results of the boiling heat transfer under the turbulent vapor show both the temperature and velocity present the non-linear distribution. Besides, under the free convection turbulent film boiling with higher Rayleigh values, the elliptical tube can get a better heat transfer efficiency than a circular tube. However, when Rayleigh values are low, the eccentricity of an elliptical tube seldom influences the heat transfer. Finally, a comparison between the results of the present study and those reported in a previous theoretical and experimental data is provided.     

纳秒脉冲激光烧蚀非晶合金的研究进展

非晶合金是一类原子排列处于长程无序状态的新型结构材料，具有一系列优异的力学和物理性能，从而有望应用于国防、空天等关键领域。在这些领域的应用中，非晶合金易受到强激光或空间等离子体的作用而失效。同时，非晶合金在高能激光辐照下的结构响应本身也极具科学意义。因此，近年来这方面的研究得到了越来越多的关注。本文将重点关注纳秒脉冲激光对非晶合金在大气和水环境下的烧蚀，针对熔化、流体动力学失稳、爆炸沸腾、气泡动力学等烧蚀过程中的几种典型现象，简要介绍相关的实验和理论研究进展。最后，对未来值得进一步研究的方向进行了概述。