An experimental study of vapor bubbles dynamics at water and ethanol pool boiling at low and high heat fluxes
Introduction
Since first papers of Moore and Mesler [1], Labuntsov [2] and Cooper and Lloyd [3], in which authors have proposed the hypothesis of rapidly evaporating thin liquid film at the base of a vapor bubble to explain the sharp decrease in the local temperature at the moment of vapor bubble appearance, the phenomena of the microlayer and influence of it evaporation dynamics on heat transfer at nucleate boiling are widely discussed in the literature [4], [5], [6]. At the same time, the detailed picture, based on the experimental data on the microlayer dynamics, its characteristics, including thickness, evaporation rate, etc., has not been available to date. The complexity of such studies is related to the fact that the characteristic thickness of microlayer is several microns and the process of its formation and development does not exceed few milliseconds.
Nowadays, high-speed video recording from the side of the heater is common and already classical technique of nucleate boiling visualization. This type of recording allows to study the evolution of vapor bubbles shape, to measure their departure diameters and nucleation frequencies. So nowadays there are a lot of works devoted to experimental investigation of the vapor bubbles dynamics at pool boiling of various liquids on surfaces with different orientation, geometry and roughness, at various pressures and subcooling degree [7], [8], [9]. However video recording from the side of a heater has a number of shortcomings. Among them are the impossibility of observing the sizes of microlayer evaporation region and dry spot, formed at the bubble growth stage in a nucleation site, the impossibility of microlayer thickness determination. Moreover, activation of significant number of nucleation sites, even at relatively low heat fluxes, makes it difficult to identify individual vapor bubbles. This significantly increases the measurement error and complicates analysis of the nucleation dynamics at boiling in a wide range of heat fluxes.
The development of modern experimental techniques in the last two decades makes it possible to obtain fundamentally new information on local and integral characteristics of boiling, including heat transfer coefficient and critical heat flux. Nowadays one of the most popular noninvasive methods to measure the non-stationary temperature field of the heating surface is high-speed infrared (IR) thermography. First, who have used this method for boiling process investigation, were Theofanous et al. [10], [11]. In these and subsequent papers [12], [13], [14], [15], it was shown, that the usage of high-speed IR thermography allows to measure bubble nucleation temperature, nucleation site density and frequency, etc. In [16], [17] with the use of this method the evolution of temperature field under single bubble was obtained, on the basis of which the features of the local heat transfer in regions of microlayer and dry spot were studied. Also researchers have actively used laser interferometry technique to study the geometry and dynamics of the liquid microlayer at boiling of various liquids [4], [18], [19], [20], [21]. The laser interferometry technique allows not only to determine the sizes of microlayer and dry spots regions, but also to measure microlayer thickness evolution at the bubble growth stage. Therefore, development and further usage of modern experimental techniques allows to obtain fundamentally new information on the multiscale characteristics of nucleate boiling. However, literature analysis shows that existing experimental data on local boiling characteristics, especially those taking place in the region of triple contact line, are insufficient for theoretical description of the microlayer characteristics, its evolution and evaporation rate at boiling of liquids with various physical properties under different conditions, including pressure change.
In addition to the partially nucleate boiling regime, the nucleation dynamics and evolution of two-phase layer close to heating wall at developed nucleate boiling up to crisis phenomena development (CHF) also has a great interest and importance. Such interest is related both with incomplete understanding of basic heat transfer mechanisms at boiling in the region of high heat fluxes, and with the necessity to determine the main reasons of boiling crisis phenomena development, including modeling safe operation of nuclear reactors [23]. Many researchers have attempted to theoretically describe trigger mechanisms of CHF on the basis of the evolution of liquid-vapor system at developed nucleate boiling. Such approaches can be attributed to the already classic Kutateladze-Zuber hydrodynamic instability model and number of its modifications, as well as subsequent semi-empirical models, which are described in detail in recent reviews [24], [25], [26]. At the same time, existing experimental observations do not yet provide a clear picture and understanding of the characteristics and evolution of the two-phase system close to a heating surface at pool boiling of liquids with various properties in the region of high heat fluxes.
In one of the first papers [27], devoted to an experimental study of vapor bubbles dynamics at developed nucleate boiling, author observed formation of large-scale vapor agglomerates (or “vapor mushrooms”), which were anchored to a heating surface by numerous columnar stems of vapor. However, due to the appearance of a large number of vapor bubbles at high heat fluxes, performed photographic study from the side of a flat heater did not allow author to firmly reiterate this assumption, which was later noted in [11]. Kirby and Westwater [28] and later Van Ouwerkerk [29] with the use of transparent conductive heaters visualized nucleate boiling at various pressures directly from bottom side of the heaters, which made it possible to study in detail the formation and growth of dry spots under single vapor bubbles up to CHF point. As a result, the existence of a thin liquid layer close to a heating wall, which was later called the liquid “macrolayer”, was proposed. In these papers, the hypothesis of the onset of boiling crisis phenomena development as a result of the irreversible dry spots formation and their subsequent lateral growth along a heating surface was also formulated.
Based on the described experimental observations, Katto and Yokoya [30] and further Haramura and Katto [31] proposed a modified hydrodynamic instability model of boiling crisis, which is widely known in the literature as the “macrolayer dryout model”. This model assumes that at developed nucleate boiling, the vapor-liquid system represents stationary columnar stems of vapor, distributed in a thin near-wall liquid macrolayer. The onset of crisis developments corresponds to the moment of complete macrolayer evaporation under massive vapor conglomerate. The hypothesis of liquid rich layer existence close to the heating wall was confirmed by number of researchers [32], [33], [34], [35], [36] during experimental investigations on void fraction distribution close to a heating surface at boiling of water, isopropanol and FC-72. Nevertheless, authors of [34] also pointed out that the existence of stationary vapor stems in the liquid macrolayer is rather unlikely.
Chu et al. [37] demonstrated the structure of large vapor agglomerates, as well as the behavior of dry spots formed under them. The experiments were carried out at saturated water boiling on the surface of narrow transparent heater (2.7 mm wide). With the use of synchronized total reflection technique and video recording from the side of the heater authors observed formation of large-scale coagulated dry patch under vapor agglomerates. Authors pointed out, that this fact also directly contradicts macrolayer dryout model.
Authors of [38], [39], [40] also used total reflection technique to study dry spots dynamics at developed nucleate boiling of well wetting fluids, such as R-113 and ethanol. Based on the data analysis authors of these works suggested that there are always almost dried up surface under single bubbles. With the increase of input heat flux neighboring bubbles coalesce in a lateral direction with each other and form large vapor conglomerate, which in turn forms coagulated dry area on a heating surface. Authors also noted that near the CHF point the dried up surface area can reach 70% of the total heating surface area. However, studies on dry spots dynamics at water boiling performed with the use of high-speed IR recording [11], [41] showed that dry spots occupy a much smaller area in pre-crisis boiling regimes. Crisis phenomena development is a consequence of the appearance of local regions on heating surface with high temperature or irreversible dry spots. In addition, authors of [11] also studied cross-sectional average void fraction as a function of distance from the heater by X-ray radiographic imaging of the boiling liquid volume. Analysis of obtained data showed that void fraction near CHF conditions (q/qCHF = 0.8 ÷ 0.9) at the distances from the heating surface up to 1 mm reaches 90%. Nevertheless, it should be also noted that in this paper the minimum distance from the heating surface, at which void fraction was measured, was 0.5 mm, which in turn, made it impossible to investigate void fraction in the region of liquid rich layer with a thickness of dozens microns [42], [43].
Therefore, nowadays in the literature there is limited number of experimental studies devoted to the processes occurring in the region of the triple contact line at liquid boiling. At the same time, experimental data on the microlayer evaporation dynamics, its dimensions and thickness are highly relevant for the development of new and verification of already existing theoretical approaches for describing both local characteristics of nucleation and the intensity of heat transfer at nucleate boiling. A similar situation is observed with the investigations of the characteristics and evolution of two-phase system at nucleate boiling in the range of high heat fluxes. Depending on experimental information, obtained with the use of various techniques, authors describe physical mechanisms in different ways. This is related to the fact that the liquid-vapor system behavior close to the heating surface depends heavily on a number of parameters, such as liquid microhydrodynamics, local heat transfer rate in the area of vapor bubbles, fluid and wall properties, including wetting, pressure and so on. This indicates the need for further experimental study of the behavior of two-phase system close to a heater at developed nucleate boiling of liquids with different physical properties. In this case, one of the most promising techniques for studying the nucleation dynamics at boiling is performing both high-speed video visualization of boiling process close to a heating surface and investigation of non-stationary temperature field of a heater for further analysis of the local heat transfer characteristics under single vapor bubbles.
In this paper, with the use of above-mentioned high-speed video recording and IR thermography from the bottom side of a transparent heater, evolution of vapor bubbles, microlayer region, as well as the dry spots bounded by the contact line, and thermal characteristics at pool boiling of water and ethanol was studied in detail. Furthermore, behavior of the liquid-vapor system close to the heating surface at boiling was studied in a wide range of input heat fluxes.
Section snippets
Experimental setup and test section
A schematic diagram of setup for pool boiling experiments is presented in Fig. 1. This setup consists of two sealed cylindrical vessels made of stainless steel. To maintain a constant temperature of a working liquid, the boiling chamber was mounted in isothermal bath with two tubular heating elements (pre-heaters) with the total power of 2.4 kW. Water temperature in the isothermal bath was controlled using an electronic temperature controller. The working volume of boiling chamber is 2.5 × 10−3
Boiling curves
To construct boiling curves experimental data on the heating surface temperature obtained using IR-thermography were averaged over recording time of 10 s and surface area at different heat flux density. Corresponding curves for water and ethanol, as well as experimental data of other authors are presented in Fig. 2. Calculations by Rohsenow correlation [45] also are shown in the figure for comparison. Analysis of the results shows that calculated data, as well as experimental data of [46], [47]
Conclusions
In this paper the results of an experimental study of the features of the vapor bubbles dynamics at pool boiling of liquids with different physical properties in a wide range of heat fluxes up to q/qCHF ≈ 0.9 are presented. The usage of high-speed experimental techniques including video macro-visualization and IR-thermography allowed not only to measure the outer diameter of vapor bubbles, but also to study in detail the evolution of the liquid microlayer region and the boundary of dry spots at
Conflict of Interest
None declared.
Acknowledgment
The reported study was funded by the Russian Foundation for Basic Research according to the research project № 17-08-01342. The part of the research was carried out according to the Program of Fundamental Scientific Research of the State Academy of Sciences for 2013–2020 (theme III.18.2.3, AAAA-17-117030310025-3). We express our gratitude to the Novosibirsk State University for giving us an access to the library.
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