Nucleate-boiling Regime Research Articles

Experiments to Understand Bubble Base Growth Mechanism(S) on Hydrophobic Surfaces Under the Influence of Bulk Flow Inertia During Nucleate Boiling Regime

Experiments to Understand Bubble Base Growth Mechanism(S) on Hydrophobic Surfaces Under the Influence of Bulk Flow Inertia During Nucleate Boiling Regime

Experimental study on transient heat transfer characteristics of intermittent spray cooling

ABSTRACT Experiments were performed to investigate the transient cooling characteristics of intermittent spray cooling (ISC) in single-phase and nucleate-boiling regimes. The consumption of residual liquid in the non-injection period was confirmed via observations of the dynamic behavior through visualization experiments. The time variation of the surface heat transfer is relatively small in the single-phase regime and minimal in the initial nucleate-boiling regime. Further, ISC was considered as a process where the transient mass flow rate was specified by the characteristic time. This idea was successfully evaluated in the prediction of the critical heat flux in cryogen spray cooling. Abbreviations: CHF: critical heat flux; CSC: cryogen spray cooling; DC: duty cycle; ISC: intermittent spray cooling; SFSM: sequential function specification method

Developing a Mathematical Model for Nucleate Boiling Regime at High Heat Flux

A mathematical model has been developed for heat exchange in nucleate boiling at high flux applying an energy balance on a macrolayer. The wall superheat, macrolayer thickness, and time are the parameters considered for predicting the heat flux. The influence of the wall superheat and macrolayer thickness on average heat flux has been predicted. The outcomes of the current model have been compared with Bhat’s constant macrolayer model, and it was found that these models are in close agreement corresponding to the nucleate pool boiling regime. It was concluded that the wall superheat and macrolayer thickness contributed significantly to conduction heat transfer. The average conduction heat fluxes predicted by the current model and by Bhat’s model are in close agreement for a thinner macrolayer of approximately 50 µm. For higher values of the wall superheat, which corresponds to the nucleate pool boiling condition, the predicted results strongly agree with the results of Bhat’s model. The findings also validate the claim that conduction across the macrolayer accounts for the main heat transfer mode from the heater surface to boiling liquid at high heat flux in nucleate pool boiling.

Cavity formation during water entry of heated spheres**Project supported by the National Natural Science Foundation of China (Grant No. 11672094).

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.

Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces

In 1756, Leidenfrost observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants. The presence of these vapour films can also reduce liquid-solid drag. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling--by heat transfer--the liquid-gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating.

Development of empirical correlations to predict the secondary droplet size of impacting droplets onto heated surfaces

The present article reports an experimental analysis of the mechanisms of secondary atomization which occur at the impact of individual droplets onto heated targets. The experiments follow those reported in a previous article (Moreira et al. 2007) and encompass the use of different liquids and impact conditions. An image analysis system is combined with a phase Doppler interferometer to measure extended size distributions, which cover the full range of diameters generated at all heat transfer regimes. The results evidence that disintegration mechanisms depend on the heat transfer regimes; therefore, a universal relation cannot be devised for the outcome of droplet impact. Analysis shows that droplets impacting within the nucleate-boiling regime break-up by a thermal-induced mechanism associated with the vapour pressure at bubble nucleation sites, combined with liquid surface tension. On the other hand, within the film-boiling regime, disintegration is associated with radial disruption of the rim at the early instants after impact, as in non-heated targets, and with the rupture of the ligaments of the cellular structures. Functional relations available at the literature, mostly developed for impacts onto non-heated surfaces, are well fitted to the experimental results obtained within the film-boiling regime, since the break-up mechanisms are qualitatively similar. On the other hand, such relations cannot predict the secondary atomization occurring within the nucleate-boiling regime, as the break-up mechanisms within this regime have significantly different characteristics. In this context, the present article recognizes the relevance of the relations devised for ‘cold impacts’, to fit the size of secondary droplets within the film-boiling regime, as the correlation formulated here has a similar form: SMD/D 0 = f(We, Re) ~ A 1 We −0.6 Re −0.23 and proposes a new correlation for impacts within the nucleate-boiling regime: SMD/D 0 = f(We, Re, Ja) ~ A 2 We −0.14 Re −011 Ja −03. These correlations are observed to hold for impacts onto rough surfaces with dimensionless roughness R a/D 0 smaller than 2E-3, but not for larger roughness amplitudes, for which the data are quite scattered.

A compensation electrothermograph for liquids and gases

A follow-up system of an electrothermograph intended for conducting physicochemical studies of processes, such as boiling, in liquids and gases is described. A new bridge device, in which a common-mode signal that unbalances the bridge does not appear, is used in the instrument. The advantages of the follow-up systems built according the scheme proposed are shown. The measurement technique and the main metrological characteristics of the instrument are described. An example of actual studies of a superintense nucleate-boiling regime is given.

Explosive regime of the development of instability leading to vapor-film collapse on a heated solid hemispherical surface

We investigated the local processes proceeding in the case of changing the boiling regime on a heated cylindrical rod with a hemispherical end immersed in water ( T = 285 K) at a depth equal to the radius of the hemisphere. The rod was heated in ambient air up to temperature T ≤ 1000 K. Vapor-film escape was observed visually using a microscope and two video cameras installed below and aside from the hemisphere immersed in the liquid. The pressure arising in the liquid and the vapor-film thickness were measured using fiber-optical sensors [1]. In our experiments, we used metallic rods made of steel, pure copper, and copper covered by PSr62 silver solder. After immersing the hemisphere in a liquid, freeconvection flow formed near the heated surface. Then, wave formations appeared on the vapor‐fluid interface. The amplitude and characteristic length of the surface waves were on the order of magnitude of a vapor-film thickness. The process of vapor-film escape and passing to bubble boiling proceeded following two different scenarios. According to one of them, the wave perturbances having the form of isolated solitons or wave trains were enhanced with time and enveloped the entire hemisphere immersed in the water. Afterwards, a vapor-film explosion occurred and the nucleate-boiling regime was established. In a number of cases, the explosive boiling of the film was accompanied by the formation of a jet flow directed from the lower end of the hemisphere into the fluid. The explosive escape of the vapor film accompanied by the formation of the jet flow can be repeated (up to 30 times) in intervals of 0.3 to 1 s. In the other regime, the wave perturbances, once arisen, attenuated gradually and passed relatively smoothly to the nucleate-boiling regime. Smooth passage to nucleate boiling was observed only at the first immersion in water for a new hemisphere or for that freshly cleaned from oxides. In repeated experiments with surfaces having the oxide film, the vapor escaped explosively. In Figs. 1‐3, we show photographs of a vapor film before explosion and with various types of its escape. The experiments were carried out under normal pressure, and the temperatures of the cooling water and heated surface were 293 and 795 K, respectively. The characteristic values of the vapor‐fluid jet velocity (Fig. 3) attained 0.3 m/s. Vibrations of the vapor-film surface were detected both at the second boiling crisis with quiet vapor-film escape and with vapor explosion. The characteristic longitudinal dimension of the wave structures attained

Aircraft Experiments on Microgravity Pool Boiling. Vapor-Liquid Behavior and Heat Transfer Characteristics in Boiling of n-Pentane, CFC-113 and Water.

Boiling of n-pentane, CFC-113 and water under microgravity were studied, utilizing parabolic flight maneuvers with a Caravelle aircraft. The experimental apparatus was constructed so as to permit simultaneous video recording of the side view of vapor bubbles, generated on a Joule-heated, transparent indium-oxide film plated on a glass substrate, and the backside view through the substrate. The heat transfer to n-pentane or CFC-113 in the nucleate-boiling regime deteriorated slightly under microgravity, while the critical heat flux was lowered to two-fifths of the corresponding terrestrial value. In contrast, the heat transfer to water significantly deteriorated under microgravity. The difference in the degree of heat transfer deterioration thus observed is presumably ascribed to a considerable difference, between the former two liquids and water, in the bubble behavior in the vicinity of the heater surface, which in turn must depend on the surface tension of each liquid and the wettability of the heater surface with the liquid.