•Experimentally and theoretically examined hydrodynamics of microlayer formation.•Simultaneously measured microlayer thickness and macroscopic bubble shape in water.•Developed an advanced model with ...surface tension, bubble geometry and residual flow.•Validated the model with experimental data obtained for various fluids and pressures.
The hydrodynamic formation of a microlayer at the base of a vapor bubble growing on a heated wall was experimentally and theoretically studied. Single bubble nucleate boiling experiments were conducted in a pool of saturated water under atmospheric pressure. A complete picture of the bubble geometry was obtained, including the three-phase contact line, microlayer and macroscopic bubble. This was performed using integrated laser interferometry, infrared thermometry and shadowgraph techniques. Existing models of the initial microlayer thickness that use an idealized hemispherical bubble shape and neglect surface tension force significantly overestimate microlayer thickness measured in experiment. The visualization results revealed that the non-hemispherical shape and surface tension of the bubble play a critical role in determining initial microlayer thickness. Theoretical analysis also indicated that the short-lived residual flow could play a key role in microlayer formation. Finally, a sophisticated model of initial microlayer thickness, developed to take into account the three identified mechanisms, provides predictions in agreement with experiments for different fluids.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
On a visible-transparent boiling surface, the detailed geometry of a microlayer can be detected using a total reflection technique combined with laser interferometry. On an infrared-opaque boiling ...surface, the surface temperature and heat flux distribution can be obtained using a high-speed infrared thermometry technique. In the present study, an experimental technique to study heat transfer in the microlayer is described that permits the simultaneous use of the total reflection combined with laser interferometry and infrared thermometry. Boiling takes place from an infrared-opaque and visible-transparent indium-tin-oxide thin-film heater deposited on an infrared- and visible-transparent calcium fluoride substrate. Details of microlayer geometry and the associated surface temperature and heat flux distribution are obtained using an integrated experimental technique. Heat transfer mechanisms in the microlayer are quantitatively discussed.
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BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Direct experimental measurement of wall heat flux partitioning was performed.•Bubble area of influence factor significantly affected the wall heat flux partitioning.•Optimum value of bubble area of ...influence factor was found to be 0.5.•Correction for overlapping area of influence of merging bubbles was proposed.
Heat transfer models in liquid-vapor two-phase flow with wall boiling rely on the wall heat flux partitioning to quantify heat transfer to liquid and vapor separately. Several wall heat flux partitioning models have been proposed over the years based on variety of heat transfer mechanisms, but the three basic mechanisms that form the core of these models are liquid convection, surface quenching and evaporation heat transfer. A key parameter commonly used to determine the relative contribution made by each mechanism is area fraction of influence of bubble which is determined by multiplying maximum bubble projected area fraction with bubble area of influence factor (K). In classic wall heat flux partitioning models, K accounts for the area within which heat is transferred to liquid that moves in towards the heated wall as bubbles lift-off. The value of K has been a subject of controversy over the years with no unanimous conclusion among researchers. Therefore, in this paper, advanced diagnostic approach involving the combination of infrared thermometry and total reflection principle was employed to experimentally study nucleate flow boiling. Rigorous data analyses was performed to partition the wall heat flux into the aforementioned three basic heat transfer mechanisms using different values of K. All three heat transfer mechanisms were significantly sensitive to varying values of K, but setting K = 0.5 with percentage uncertainties of −60%/+50% closely predicted the experimental measurements. In addition, overlapping area of influence due to merging bubbles was observed to be significant in the model at high heat flux condition and must be discounted to get the true bubble area of influence. A correction method for the overlapping area of influence was therefore proposed to enhance accuracy of the predictive model.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•The influence of evaporative thermal resistance on microscopic bubble growth is investigated.•Evaporative resistance is found to be important in the understanding of micro-layer evaporation.•An ...approach to the measurement of evaporative resistance is proposed.•Anomalies in earlier published measurements are explained.
Simulations of the formation of small steam bubbles indicate that the rate of growth of bubbles is very sensitive to the rate of evaporation of the micro-layer of liquid beneath the bubble. Such evaporation is rapid, and is modelled as being driven by the large heat flux through the thin liquid layer caused by the difference in temperature between the solid–liquid interface, and the saturation temperature in the interior of the bubble. However, application of this approach to recent experimental measurements of Jung and Kim generated anomalous results. In this paper we demonstrate that a model of the micro-layer heat flux that includes an allowance for the finite evaporative thermal resistance is able to eliminate these anomalies. This evaporative thermal resistance is a consequence of near-interface molecular dynamics, characterised by a quantity termed ‘evaporation coefficient’. Whilst in most engineering applications evaporative thermal resistance is small compared to conductive resistance, here, with the micro-layer thickness ranging from a few microns down to zero, it becomes of considerable importance. Selection of a molecular ‘evaporation coefficient’ to restore consistency to the anomalous measurements allows a plausible numerical value to be inferred. For the several times and multiple locations studied, a fairly consistent value of between 0.02 and 0.1 is indicated, (for saturated water in laboratory conditions), which itself is consistent with earlier literature values of this rather difficult quantity. It is shown that the evaporative resistance always represents a large fraction of the conductive resistance, and for important phases of the process dominates it. The need for inclusion of this phenomenon in the micro-layer models used in bubble analysis is clear.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The heat transfer mechanisms of nucleate boiling are associated with how the liquid–vapor phase and the surface temperature are distributed and interact beneath a single bubble on a heated surface. A ...comparative analysis of the hydrodynamic and thermal behavior of a single bubble may contribute greatly to the understanding of nucleate boiling heat transfer. In this paper, a technique to simultaneously measure the liquid–vapor phase boundary, temperature distribution, and heat transfer distribution at a boiling surface is described. The technique is fully synchronized in time and spatially resolved, and is applied to explore single-bubble nucleate boiling phenomena in a pool of water subcooled by 3°C under atmospheric pressure. The temperature and heat flux distributions at the boiling surface are quantitatively interpreted in relation to the distribution and dynamics of the dry and wet areas, the triple contact line, and the microlayer underneath the single bubble. The results show that intensive wall heat transfer during single-bubble nucleate boiling exactly corresponds to the extended microlayer region. However, the overall contribution of the microlayer evaporation to the growth of a bubble is relatively small, and amounts to less than 17% of the total heat transport.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•We made simultaneous observations of the dynamics and thermal behavior of dry patches.•The critical heat flux phenomenon occurred during the rewetting of a large dry patch.•The bubbles nucleating at ...the advancing liquid front played a key role in the failure of the rewetting.•We determined an empirical value for the critical line density of the bubbles at the triple contact line.
We observed the dynamics of dry patches underneath massive bubbles during the pool boiling of saturated water under atmospheric pressure, and measured the associated temperature distribution. We synchronized the observations both spatially and temporally using high-speed total reflection and infrared thermometry techniques. The observations presented in this paper provide evidence that the critical heat flux phenomenon is triggered during the rewetting of large dry patches with periphery temperatures that are much lower than minimum film boiling temperature, so-called Leidenfrost point. As the liquid meniscus advanced toward the dry patch, numerous secondary bubbles nucleated and impeded the flow of liquid toward the dry patch. This prevented the liquid from rewetting the dry patch. The key physical mechanism for triggering CHF is initiated when the line density of the secondary bubbles nucleating at the periphery of a shrinking dry patch underneath a departing mushroom bubble exceeds a critical value. These bubbles completely block the liquid inflow into the dry patch and prevent rewetting, eventually causing the dry patch to expand irreversibly. We used our experimental data to determine an empirical value of the critical line nucleation site density.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Effects of surface inclination on pool boiling were experimentally examined.•Heat transfer and major bubble parameters were simultaneously measured.•A modified wall boiling model considering bubble ...merging was developed.•The presented model reasonably predicted pool boiling heat transfer on inclined surfaces.
The basic wall boiling model widely used in computation fluid dynamics codes gives no regard to influences of surface orientation upon boiling mechanism. This study aims at examining the effects of surface orientation on wall heat flux and bubble parameters in pool nucleate boiling and incorporating those into the wall boiling model. Boiling experiments on a flat plate heater submerged in a pool of saturated water were conducted under atmospheric pressure. Relevant bubble parameters as well as boiling heat transfer characteristics were simultaneously measured using a unique optical setup integrating shadowgraph, total reflection and infrared thermometry techniques. It was observed that as an upward-facing heater surface with a constant wall superheat of 7.5°C inclines from horizontal towards vertical, the heat flux significantly increased; nucleation site density increased intensively at the upper part of the heater surface where thermal boundary layer might become thickened; isolated boiling bubbles tend to slide up due to buoyancy and coalesce with each other, thus forming one single large bubble. Such observations on the wall heat flux and bubble parameters according to surface orientation could not be predicted by the present basic wall boiling model only centered with isolated bubbles. A modified wall boiling model incorporating the effects of merging of isolated bubbles on an inclined surface was proposed. The model reasonably predicted the experimental data on various orientation angles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
This paper presents analysis of the evaporation of the microlayer beneath a steam bubble growing at a heated surface. The microlayer depletion rate, and the associated evaporative heat flux, have ...been assessed using optical measurements of its thickness, and infrared measurements of the temperature of the solid surface beneath the bubble. Depletion of the liquid film is modelled from a molecular point of view, and comparison with measurements is attempted. A model of evaporation based on a simple kinetic theory representation of the molecular fluxes impinging on and emitted from a liquid surface is studied. From this, strong support is obtained for the hypothesis that only a fraction of impinging vapour molecules are absorbed by a liquid surface. The measurements suggest a value for the ‘accommodation coefficient’, of approximately 0.03 and a corresponding ‘effective evaporative heat transfer coefficient’, that are reasonably consistent with estimates from other workers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Visual images of mesh-shaped wetting structure near quench front.•Two-dimensional profile of wall temperature and heat flux at heated surface.•Comparison between reflooding velocity and quench front ...velocity.•Temperature and heat flux changewoo with time when quench front passes.
A detailed rewetting phenomenon is observed using refrigerant R141b as the working fluid. Rewetting phenomenon is visualized based on total reflection images and high-speed images. Furthermore, two-dimensional profiles of wall temperature and heat flux at the heated surface are obtained using a high-speed infrared thermometry camera. These images were temporally and spatially synchronized, so that the relationship between the hydraulic and thermal behaviors of the rewetting phenomenon could be examined in detail. The results show that a mesh-shaped wetting structure is formed near the quench front and the heated surface above the quench front remain in a dry state. The time variations of the temperature and heat flux at a fixed point change abruptly, and the oscillations of the temperature and heat flux increase as the quench front approaches the point. The relationship between the wall temperature and heat flux exhibits a similar trend for different flow rate conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Fast transient heat transfer by sinusoidal pulse power Kam, Dong Hoon; Jung, Satbyoul; Kang, Jun-young ...
International communications in heat and mass transfer,
January 2023, 2023-01-00, Volume:
140
Journal Article
Peer reviewed
In nuclear reactors, sudden ejection of control rods, reactivity-initiated accident (RIA), can incur pulse power and threaten the reactor safety. On this basis, the effect of pulse power shape has ...been thoroughly assessed in this study considering rapid negative feedbacks in reactor cores. Overall tendency dramatically changes when the power supply rate becomes very fast. Especially for the high amplitude with long sustenance under fast transient conditions, heat transfer performance can be enormously enhanced. A synchronized high-speed camera has been used to visualize the nucleation and vapor dynamics. The fast transient condition shows highly increased nucleation sites and even nucleation-induced vapor blanket along the wire. Fast transient input power shows receding behavior of the merged blanket generating stable films, and heat transfer further enhances when the high amplitude condition is sustained. Some conditions show insufficiency of power input, which leads to low heat transfer performance. The amplitude of input power plays a crucial role, while both the amplitude and the sustenance period determine the overall trends under fast transient conditions. Enhanced heat transfer under fast transient conditions may effectively secure the safety margin, even though local surface temperature, where stable vapor film is located, could cause some negative results on the integrity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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