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.     

Numerical simulation of pool boiling of a Lennard-Jones liquid

We performed a numerical simulation of pool boiling by a molecular dynamics model. In the simulation, a liquid composed of Lennard-Jones particles in a uniform gravitational field is heated by a heat source at the bottom of the system. The model successfully reproduces the change in regimes of boiling from nucleate boiling to film boiling with the increase of the heat source temperature. We present the pool boiling curve by the model, whose general behavior is consistent with those observed in experiments of pool boiling.     

Possible Mechanism for Formation of Nonwettable “Dry Spots” on a Heated Surface during Nucleate Pool Boiling: II. Feedwater Stop Regime

The problems of simulation of heterogeneous nucleate pool boiling on a horizontal surface on the ascending branch of the boiling curve from the formation of a steam lens (SL) to the boiling crisis are considered. The proposed hypothesis provides in a number of cases a logically consistent interpretation of experiments and outlines the organizational principle of transferring the wall–liquid–steam system into the regime of nonwettable “dry spot” formation. The model includes the following types of nucleate boiling: (a) cyclic boiling with the contact line reverse to the bubble bottom center and bubble departure from the surface (at low heat flux q and the contact angle θ < 90°); (b) single steam bubble conversion into a steam lens, i.e., local film boiling with the possibility of spreading of a single “dry spot” at the variation of the contact angle θ ≥ 90°, and substantial growth of the departure diameter D_d and SL lifetime τd; (c) formation of a single steam cluster of four SLs at a given pressure, the liquid underheating, and the average wall overheating.     

Interpreting the influence of fuel spray impact on mixture preparation for HCCI combustion with port-fuel injection

This paper addresses the influence of fuel spray impact on fuel/air mixture for combustion in port-fuel injection engines. The experiments include time resolved measurements of surface temperature synchronized with PDA measurements of droplet dynamics at impact and were conducted to quantify the effects of interactions between successive injections on the mixture preparation for combustion in homogeneous charge compression ignition (HCCI) engines. Analysis shows that, during engine warm up, the heat transfer over the entire valve surface occurs within the vaporization-nucleate-boiling regime and the local instantaneous surface temperature correlates with the dynamics of droplets impacting at the same point. A functional relation is found for the heat transfer coefficient, which also describes other experiments reported in the literature. Similarity does not hold after the engine warms up because heat transfer and droplet vaporization at the surface are dominated by multiple interactions between droplets arisen from diverse heat transfer regimes. However, results evidence the existence of a critical surface temperature which sets a transition between overall heat transfer regimes dominated by local nucleate boiling at lower temperatures and by local intermittent transition regimes at higher temperatures. The heat transfer within the overall nucleate boiling regime is shown to be due to a thin film boiling mechanism leading to breakdown of the liquid-film at a nearly constant surface temperature, regardless of injection frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid film. The results further evidence the important role of spray impingement for mixture preparation as required for HCCI.     

ELECTRIC FIELD EFFECTS ON FILM BOILING ON A WIRE

Experimental results concerning pool film boiling on a wire under the action of an externally imposed electric field are reported. The working fluid was saturated R113 at atmospheric pressure, the heaters were platinum wires of 0.2, 0.3, and 0.6 mm diameter. An electrostatic field of cylindrical geometry was created around the wire by means of a 60-mm-diameter cylindrical cage in order to assess electrohydrodynamic (EHD) effects on boiling. Voltages up to 15 kV d.c. were applied. The results showed that two different film boiling regimes, separated by an additional boiling crisis, can exist in the presence of an electric field. The first regime, at low wire superheat, is strongly influenced by the electric field, showing a remarkable heat transfer enhancement with increasing voltage. The second one, at higher superheat, is weakly dependent on the field strength and almost coincident with the zero field one. The two film boiling regimes are separated by a transition characterized by a hysteresis cycle, similar to the one between nucleate boiling and first film boiling regime.     

Transition to Film Boiling in Microgravity： Influence of Subcooling

The transition process to film pool boiling in microgravity is studied experimentally aboard the Chinese recoverable satellite S J-8. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Small, primary bubbles are formed and slid on the surface, which coalesce with each other to form a large coalesced bubble. Two ways are observed for the transition from nucleate to film boiling at different subcoolings. At high subcooling, the coalesced bubble with a smooth surface grows slowly. It is then difficult for the coalesced bubble to cover the whole heater surface, resulting in a special region of transition boiling in which nucleate boiling and local dry areas can coexist. In contrast, strong oscillation of the coalesced bubble surface at low subcooling may cause rewetting of local dry areas and activation of more nucleate sites, resulting in an abrupt transition to film boiling.     

LASER DOPPLER ANEMOMETRY MEASUREMENTS OF BUBBLE RISE VELOCITY AND DEPARTURE FREQUENCY

Previous experimental investigations have employed photographic techniques to quantify the rise velocity and departure frequency of bubbles departing from a boiling surface, a method that requires a significant time investment to provide accurate time-averaged results. The current study focuses on applying laser Doppler anemometry (LDA) techniques to quantify these parameters. Initially, the LDA system is employed to measure the rise velocity and departure frequency of air bubbles in FC-72, R-113, and water. These air bubble tests verify the ability of the LDA measurement technique to quantify the bubble parameters. The LDA technique is then used to quantify the bubble parameters from a wire during isolated nucleate boiling conditions in saturated FC-72. Highly time-dependent departure frequencies (7 to 242 Hz) are measured in the isolated nucleate boiling regime. LDA signals representing departure frequencies in the fully developed nucleate boiling regime are discussed.     

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract

A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly reduced as a result. The effects of diameter, length and surface roughness of the cylindrical attachment were also investigated.     

Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel

Accurate models for the onset of nucleate boiling, density of active nucleation sites (N_a), bubble departure size (D_d), and departure frequency (f_d) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m²s), inlet subcooling (ΔT_sub = 4.7–33.3?C), and heat flux (q″ = 1.7–28.9 W/cm²). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, N_a, D_d, and f_d for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.     

Eulerian–Eulerian simulation of non-uniform magnetic field effects on the ferrofluid nucleate pool boiling

The nucleate pool boiling heat transfer of ferrofluid on a horizontal plate in the presence of a non-uniform magnetic field has been studied numerically using Eulerian–Eulerian approach. Also, the wall partitioning model was extended to consider the boiling surface modification by the nanoparticles deposition on the heated surface. Adding nanoparticles causes deterioration in the boiling heat transfer coefficient and void fraction. Moreover, applying the magnetic field intensifies these reductions.     

A fractal study for nucleate pool boiling heat transfer of nanofluids

In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r...     

Experimental Study of the Influence of Structured Capillary-Porous Coatings on the Dynamics of Development of Transient Processes and the Crisis Phenomena at Stepwise Heat Release

The effect of structured plasma-sprayed capillary-porous coatings on transient processes and the development of crisis phenomena at boiling under pulsed heat release was studied. The working fluid was liquid nitrogen on the saturation line at atmospheric pressure. It is shown that under unsteady heat release, there is a degeneration of the development of the boiling crisis on heaters with structured capillary-porous coatings at q &lt; q_CHF (critical heat flux at steady heat release). Under unsteady pulsed heat release, no rapid transition to the film boiling regime (without passing through the nucleate boiling stage) is observed on heaters with such coatings until the thermal load is more than two times higher than the critical heat flux for steady heat release. This significantly increases the times of transition to post-critical heat transfer. Analysis of synchronized measurements of surface temperature of heaters and high-speed video recording of transient processes shows that the degeneration of the heat transfer crisis at q &lt; q_CHF on samples with coatings occurs due to significantly lower liquid boiling temperature differences and specific features of the dynamics of propagation of self-sustaining evaporation fronts in comparison with a smooth heater.     

低温液体核态流动沸腾传热的机理模型

对核态沸腾表面上的各种传热机理进行了分析和量化,建立了低温液体核态流动沸腾传热的机理模型,并将该模型纳入双流体模型实现了数值求解,数值预测的结果详细地反映出了壁面上各参数随流动的变化情况。该机理模型的计算结果表明,气泡挣脱后液体与过热壁面间的激冷效应是导致壁面上各参数在OSV处突变的根本原因。     

水喷射淬冷高温壁面的传热实验研究

本文采用直接表面温度测量的方法对水喷射高温壁面的传热过程进行实验研究．系统地研究了射流出口速度、冷却水过冷度、喷嘴至加热面的间距、初始壁温以及水喷雾的流量密度等参数对淬冷表面的膜态沸腾、最小临界点、过渡沸腾、最大临界点等传热过程的影响，给出了有关实验结果，分析了它们的特点。     

厚度不均对纳米多孔镀层发热极限及熔毁失效的影响

采用纳米多孔膜可以实现新型的具有极高热流密度的薄液膜沸腾相变传热。在薄液膜沸腾的基础研究中,通过在纳米多孔膜表面加工纳米级别厚度的铂镀层实现加热和测温。通过扫描电镜观察,发现实验样品残骸表面有“河流”状形貌形成,结合元素分析推断铂镀层局部发生热熔。本文对铂镀层进行简化并建立电网络模型,计算并分析了铂镀层局部厚度不均对整体发热极限及熔毁失效的影响。分析结果表明,镀层厚度的不均,将会使镀层在达到极限热流密度后,极易出现“河流”状熔毁,使镀层永久失效;而厚度更加均匀的铂镀层,有助于获得更高的极限热流密度。     

液氮中导线加热丝的沸腾传热特性研究

本文以50μm的磷青铜丝作为加热丝和测温元件,采用控制热流密度的方式测量了0°-90°倾角下加热丝在液氮中的沸腾曲线,结果表明:核态沸腾在增加热流密度时存在滞后现象;Bromley公式能准确的预测出膜态沸腾换热曲线的斜率,Zuber模型和Kutateladze公式预测水平细加热丝的CHF误差在15%以内;对于Leidenfrost热流密度的预测,常温流体的计算模型并不适用;CHF随倾角的变化较大,且大于加热平面在相同倾角下的变化幅度。     

Refrigerant-oil mixtures boiling in a planar confined space

In the frame of the JOULE 1 R&D programme of the Commission of the European Communities a project has been carried out on enhanced evaporation heat transfer surfaces. A specific investigation has been realised on refrigerant oil mixtures boiling in a planar confined space.An experimental investigation of the boiling phenomenon in the confined space between a 30 mm wide × 120 mm high, heated plate and an opposing, adiabatic plate was carried out. The heated surface was made of a copper-aluminium-nickel alloy (Ra = 1 micrometer) and a saturated R-113/SUNISO 3GS oil mixture at atmospheric pressure was used as the boiling fluid.The maximum heat flux tested was approximately 90% of CHF. The parameters investigated were the gap size (1–5 mm) and the oil concentration (1–7% by weight). It was again observed that confinement does not improve the nucleate boiling performance of pure R-113 in any significant way, whereas the CHF decreases with decreasing gap size. In addition, while the presence of oil was observed to have only a relatively minor effect on low flux nucleate boiling, it caused a serious degradation of the high flux boiling performance. This deterioration increased with increasing oil concentration and was more severe for smaller gap sizes. However, for a given gap size, the CHF increased with increasing oil concentration accompanied by increasingly larger surface superheats.     

NUCLEATE POOL BOILING UNDER DC ELECTRIC FIELD

This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results.     

低温混合工质池核沸腾换热研究

随着 J- T(焦耳汤姆逊效应 )节流制冷机的发展 ,需要对混合工质进一步研究。文中介绍了几种主要的沸腾换热机理 ,指出沸腾换热研究的难点和发展方向 ;并分析了混合工质沸腾换热系数低于相应纯质的原因 ;最后列举了几种通用的混合工质沸腾换热关联式     

Bistable heating of solid samples immersed in liquid helium

A bistable heating of samples is found in liquid helium. Using the strong temperature dependence of the no-phonon doublet of the ⁴ T ₁(G) ⁶ A ₁(S) emission of the ZnSe:Cr⁺ centre, the temperature can be determined in the excited volume. Temperature jumps of about 5 K are recorded. The cause for the bistable crystal heating lies in the transition from the nucleate boiling to the film boiling state of the helium. In this process, the heat transfer is drastically reduced and thus the sample is further heated.