Molecular dynamics simulation for homogeneous nucleation of water and liquid nitrogen in explosive boiling

Molecular dynamics simulations are carried out to study the energy conversion in the homogeneous nucleation processes of the explosive boiling induced by laser heating. Liquid nitrogen and water are investigated as the working fluid. Velocity scaling method is applied to realize the heating process. Three influencing factors, the initial equilibrium temperature of the liquid, the area of the heating zone and the heat quantity are analyzed. It is found that the conversion ratio of energy between heat quantity and potential energy is from 66% to 78% in the process of heating. The influence of the heat quantity on the energy conversion of liquid nitrogen is the same in trend as that of water. The influence of the initial equilibrium temperature and the area of the heating zone on the liquid nitrogen is less than that of water. The difference of energy conversion between water and liquid nitrogen is pretty dramatic, which is because of the hydrogen bond formed by the Coulombic interaction among water molecules.     

Mechanical nonequilibrium considerations in homogeneous bubble nucleation for unsteady-state boiling

With thermal and mechanical nonequilbrium taken into consideration, the classical kinetic theory of boiling is modified to study unsteady-state homogeneous nucleation processes. Based on this newly developed model, the degree of superheat and the maximum nucleation rate corresponding to different rates of temperature rise in water are calculated and presented. For the first time, the initial nonequilibrium vapor pressure and the initial growth rate of bubble nuclei with different initial embryo sizes and different rates of temperature rise are accurately modeled. The resulting algorithm provides a method by which the details of bubble nucleation in a superheated liquid can be predicted, leading to a better understanding of the kinetics of boiling. Model validation, accuracy and application are also presented and discussed.     

Model for boiling explosion during rapid liquid heating

The process of rapid liquid heating with a linearly increasing boundary temperature condition has been simulated by applying the analytical solution of 1D semi-infinite heat conduction in association with the molecular theory of homogeneous nucleation boiling. A control volume having the size of a characteristic critical cluster at the liquid boundary is considered, and the corresponding energy balance equation is obtained by considering two parallel competing processes that take place inside the control volume, namely, transient external energy deposition and internal energy consumption due to bubble nucleation and subsequent growth. Depending on the instantaneous rate of external energy deposition and boiling heat consumption within the control volume, a particular state is defined as the boiling explosion condition in which bubble generation and growth cause the liquid sensible energy to decrease. The obtained results are presented in terms of the average liquid temperature rise within the control volume, maximum attainable liquid temperature before boiling explosion and the time required to achieve the condition of boiling explosion. The model is applied for the case of water heating at atmospheric pressure with initial and boundary conditions identical to those reported in the literature. Model predictions concerning boiling explosion are found to be in good agreement with the experimental observations. The boiling explosion condition as predicted by the present model is verified by comparing the heat flux across the liquid–vapor interface with the corresponding limit of maximum possible heat flux, q_max,max, at the time of boiling explosion. A comparative study between the actual heat flux and the limit of maximum heat flux, q_max,max, at the time of boiling explosion for different rates of boundary heating indicates that, with much higher boundary heating rates, it is possible to heat the liquid to a much higher temperature before theoretical instantaneous boiling explosion occurs.     

表面微结构对流动沸腾中核沸腾抑制因子的影响

用两根内表面微结构不同的水平光滑管环状流区流动沸腾换热实验数据，采用叠加模型分别建立了流动沸腾换热关系式，并比较它们的抑制因子。结果表明，表面微结构对抑制因子有显的影响；当表面的平均凹腔半径较大时，抑制因子明显增大。表明表面微结构改变对流动沸腾换热能起到较好的强化作用。     

Active nucleation site density in boiling systems

Realizing the significance of the active nucleation site density as an important parameter for predicting the interfacial area concentration in a two-fluid model formulation, the active nucleation site density has been modeled mechanistically by knowledge of the size and cone angle distributions of cavities that are actually present on the surface. The newly developed model has been validated by various active nucleation site density data taken in pool boiling and convective flow boiling systems. The newly developed model clearly shows that the active nucleation site density is a function of the critical cavity size and the contact angle, and the model can explain the dependence of the active nucleation site density on the wall superheat reported by various investigators. The newly developed model can give fairly good predictions over rather wide range of the flow conditions (0 kg/m² s ? mass velocity ? 886 kg/m² s; 0.101 MPa ? pressure ? 19.8 MPa; 5° ? contact angle ? 90°; 1.00×10⁴ sites/m² ? active nucleation site density ? 1.51×10¹⁰ sites/m²).     

Availability analyses for heterogeneous nucleation under steady heating in pool boiling

Onset of heterogeneous nucleation is analyzed in this paper based on the criterion that change in the derivative of the availability function with respect to bubble radius is equal to zero. Comparisons of the predicted nucleation parameters obtained based on this criterion with those obtained previously based on the criterion that the change in Gibbs function equal to zero are made. It is found that the Gibbs function nucleation criterion is the necessary condition for onset of bubble nucleation while the availability nucleation criterion is the sufficient condition for onset of nucleation. Comparing with the Gibbs function criterion, the availability criterion predicts a larger nucleation radius at the same wall temperature and requires a higher nucleation temperature at the same heat flux although the differences in values are small.     

A thermodynamic analysis for heterogeneous boiling nucleation on a superheated wall

A thermodynamic model based on Gibbs free energy and availability is developed for onset of heterogeneous nucleation on heated surfaces with different wettabilities in pool boiling. Different from classical nucleation theory, this model takes into consideration the temperature gradient in the superheated liquid layer adjacent to the wall as well as the contact angle between the liquid and the wall. Using Gibbs free energy equilibrium condition, a closed form solution is obtained on the critical radius for onset of heterogeneous boiling nucleation on walls with different wettabilities. Effects of contact angles and wall temperatures on the critical radius, the wall temperature gradient of the superheated liquid layer and the heat flux at onset of heterogeneous nucleate boiling are illustrated. These effects on the change of availability during the heterogeneous nucleation process, representing the energy barrier for the occurrence of the first-order phase transition, are also discussed.     

Modeling of multicomponent homogeneous nucleation using continuous thermodynamics

A theory of homogeneous nucleation in a multicomponent vapor is developed by combining classic nucleation and continuous thermodynamics concepts. The perfect gas equation of state is used in conjunction with this theory to obtain a model valid at low vapor pressures. The theory is applied to kerosenes used for fuels in aeronautics (Jet A, JP-7, and RP-1) at temperatures from 220 to 360 K and at vapor pressures up to 1 bar. The results show that although the overall nucleation trends regarding the dependency on vapor pressure and temperature are similar for all kerosenes, Jet A has a distinct behavior compared with JP-7 and RP-1. For all kerosenes, as pressure increases, the nucleus size rapidly decreases and is smallest at low temperatures.     

A transient model for heterogeneous nucleation under pulse heating in pool boiling

A transient analysis on heterogeneous nucleation under pulse heating in pool boiling is developed based on changes in Gibbs free energy and availability, with temperature distributions determined from one-dimensional transient heat conduction theory. Numerical solutions are obtained for critical radius and nucleation heat fluxes under various pulse widths in different working fluids. It is found that both the critical radius and nucleation work increase with pulse width while the nucleation heat flux decreases with pulse width. To verify the proposed theory, experiments are carried out for onset of nucleation in water, alcohol and R113 under pulse heating at different pulse widths. It is found that the experimental data for nucleation heat flux at short pulse widths agree well with the proposed transient model, and those at long pulse widths approach the previous steady model. Predicted results of critical radius, nucleation heat flux and the change in availability at various pulse widths for water, alcohol and R113 are presented and compared.     

Superheating in nucleate boiling calculated by the heterogeneous nucleation theory

With the heterogeneous nucleation theory the superheating of the liquid boundary layer in nucleate boiling is described not only for the onset of nuclear boiling but also for the boiling crisis. The rate of superheat depends on the thermodynamic stability of the metastable liquid, which is influenced by the statistical fluctuations in the liquid and the nucleation at the solid surface. Because of the fact that the cavities acting as nuclei are too small for microscopic observation, the size and distribution function of the nuclei on the surface necessary for the determination of the probability of bubble formation cannot be detected by measuring techniques. The work of bubble formation reduced by the nuclei can be represented by a simple empirical function, whose coefficients are determined from boiling experiments. With that the heterogeneous nucleation theory describes the superheating of the liquid, which was checked on several fluids like refrigerants, liquid gases, organic liquids and water.     

Quasi-homogeneous boiling nucleation on a small spherical heater in microgravity

Pool boiling experiments were conducted in the European Space Agency (ESA) multi-user facility, the bubble, drop, particle unit (BDPU) in the microgravity environment of space. A part of the study involved the heating of a small sphere immersed in R-123 to the onset of nucleate boiling. An analysis of the nucleation process is presented, based on a prior work for so-called quasi-homogeneous nucleation with a flat heater surface in microgravity. Reasonably good qualitative agreement exists between the analysis and measurements.     

Thermal interaction effect on nucleation site distribution in subcooled boiling

An experimental work on subcooled boiling of refrigerant, R134a, to examine nucleation site distributions on both copper and stainless steel heating surfaces was performed. In order to obtain high fidelity active nucleation site density and distribution data, a high-speed digital camera was utilized to record bubble emission images from a view normal to heating surfaces. Statistical analyses on nucleation site data were done and their statistical distributions were obtained. Those experimentally observed nucleation site distributions were compared to the random spatial Poisson distribution. The comparisons showed that, rather than purely random, active nucleation site distributions on boiling surfaces are relatively more uniform. Experimental results also showed that on the copper heating surface, nucleation site distributions are slightly more uniform than on the stainless steel surface. This was concluded as the results of thermal interactions between nucleation sites with different solid thermal conductivities. A two dimensional thermal interaction model was then developed to quantitatively examine the thermal interactions between nucleation sites. The results give a reasonable explanation to the experimental observation on nucleation site distributions.     

An analysis of surface-microstructures effects on heterogeneous nucleation in pool boiling

The interaction of surface microstructures and wettability effects on heterogeneous nucleation in pool boiling is analyzed in this paper based on the changes of free energy and availability. It is shown that the bubble is most easily formed on a concave surface in comparison with a convex surface or a plane surface at the same wettability and the same wall temperature. It is found that the effect of microstructures greatly enhances nucleation of bubbles when the curvature radius of these microstructures is in the range of 5–100 times less than the bubble radius. Larger than this limit, the surface roughness effect is negligible and the wettability effect predominates. Closed form analytical solutions for the critical radius and change in availability are obtained for the special case of homogeneous nucleation where no wall temperature gradient exists on surfaces with microstructures. Under this simplified assumption, it is found that the microstructures have no effect on critical nucleation radius and their effect on the change in availability is underestimated.     

1D plane numerical model for boiling liquid expanding vapor explosion (BLEVE)

The depressurization of a vessel containing saturated or subcooled liquid may occur in a variety of industrial processes and often poses a potentially hazardous situation. A 1D plane numerical model was developed for estimating the thermodynamic and the dynamic state of the boiling liquid during a boiling liquid expanding vapor explosion (BLEVE) event. Based on the choice of the initial nucleation sites density, the model predicts, simultaneously, the bubble growth processes in the liquid at the superheat-limit state, the front velocity of the expanding liquid, and the shock wave pressure formed by the liquid expansion through the air.Conditions of shock formation were found to be normally associated with high initial temperatures that can bring the liquid to its superheat-limit state during the initial depressurization. Furthermore, the high initial temperature also induces a generation of higher vapor pressures that forces a rapid mixture expansion.Model predictions of the shock wave strengths, in terms of TNT equivalence, were compared against those obtained by simple energy models. As expected, the simple energy models over predicts the shock wave strength. However, the simple model which accounts for the expansion irreversibility, produces results which are closer to current model predictions.     

Some aspects of the interaction among nucleation sites during saturated nucleate boiling

The results of an original experimental investigation of the interaction between nucleation phenomena occurring at adjacent nucleation sites during saturated boiling of dichloromethane are presented and compared with the results of Chekanov's investigation [Teplofiz. Vysok. Temp.15, 121–128 (1977)]. The interaction was found to be positive for distances between nucleation sites less than or equal to the mean bubble departure diameter in that bubble formation at one site tended to promote bubble formation at adjacent sites and negative for distances between sites greater than the mean bubble departure diameter in that bubble formation at one site tended to inhibit bubble formation at adjacent sites. No interaction was detected for distances between nucleation sites greater than three mean bubble departure diameters.     

Dynamics of boiling succeeding spontaneous nucleation on a rapidly heated small surface

The dynamics of boiling succeeding spontaneous nucleation on a small film heater immersed in ethyl alcohol are investigated at heating rates ranging from 10⁷ K/s to approximately 10⁹ K/s, under which spontaneous nucleation is dominant for the inception of boiling. Immediately after the concurrent generation of a large number of fine bubbles, a vapor film that covers the entire surface is formed by coalescence and rapidly expands to a single bubble. As the heating rate is increased, the coalesced bubble flattens and only a thin vapor film grows before cavitation collapse. Similar behaviors are also observed for water. Based on the observed results, a theoretical model of the dynamic bubble growth due to the self-evaporation of the superheated liquid layer, which develops before boiling incipience, is presented. The calculated results are compared with the observed results.     

Effects of geometrical parameters on the boiling limit of bi-porous wicks

In this paper, the boiling limit of bi-porous sintered metal wicks is considered. The effects of the particle size (45 ? d ? 200 μm) and the cluster size (250 ? D ? 675 μm) on the boiling limit are investigated experimentally and analytically. In order to gain understanding of the boiling phenomena in bi-porous wicks, the bubble movements in the bi-porous wicks are visualized using glass particle wicks and a high speed camera. Based on the visualization results, an analytic model for predicting the boiling limit of the bi-porous wicks is developed. The model predicts the experimental data well within the error of 11%, and indicates that the permeabilities of liquid and vapor are key parameters for determining the boiling limits of wicks. The bi-porous wicks have higher boiling limits than the mono-porous wicks by up to 67%, since they have much higher liquid/vapor permeabilities, by having separate flow paths for each phase. From the model, an optimal configuration of bi-porous wick is found to be 4 < D/d < 6.     

An experimental investigation of bubble nucleation of a refrigerant in pressurized boiling flows

An experimental investigation is performed to determine the effect of system pressure and heat flux on flow boiling and associated bubble characteristics of a refrigerant in a narrow vertical duct. A high-pressure flow boiling test loop was built and TLC (thermo-chromic liquid crystal) was applied to the back of the heater foil for high resolution and accurate measurement of heater surface temperature. Refrigerant R-134a is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify their influence in bubble characteristics such as bubble nucleation, growth, departure, and coalescence. Two synchronized high resolution and high-speed cameras are used to simultaneously capture TLC images as well as bubbling activities at high frame rates. By varying flow rate and system pressure, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure from 690 to 827 kPa increased the bubble frequency and size by about 32 Hz and 20 μm, respectively. Bubble coalescence was also observed after departure from the nucleation site.     

Microscale heterogeneous boiling on smooth surfaces—from bubble nucleation to bubble dynamics

In this investigation, boiling incipience and bubble dynamics on a microheater with a geometry of 100 μm × 100 μm fabricated with MEMS technology are evaluated using a high-speed digital camera. For the purpose of comparison with conventional boiling heat transfer, boiling incipience and bubble dynamics are also studied on a carefully selected microheater with a fabricated defect (i.e., a microcavity on the heater surface). Of industrial interest are the effects of dissolved gases on boiling incipience and bubble dynamics, which are also discussed in detail. The possible nucleation temperature (or incipience temperature) is analyzed and discussed from the perspective of the measured bulk temperature of the microheater and a 3D heat conduction numerical model. The time-resolved bubble dynamics (i.e., the bubble size evolution, interface velocity and interface acceleration) are all presented along with high-speed digital images. Based upon this investigation, it is clear that explosive boiling can take place on a smooth surface no matter how slow the heating rate, and dissolved gases have a significant influence on the incipience temperature and bubble behavior. Furthermore, this study illustrates that the classical kinetics of boiling can explain the explosive boiling occurring on a smooth surface in principle and can provide a useful guide for the design of microscale heat transfer and/or MEMS devices. Although unexpected, due to the gravitational effects, Marangoni flow on the vapor–liquid interface induced by the temperature gradient was also observed.