Effect of subatmospheric pressures on heat transfer, vapor bubbles and dry spots evolution during water boiling

https://doi.org/10.1016/j.expthermflusci.2019.109974Get rights and content

Highlights

  • The subatmospheric boiling was studied through high-speed IR thermography and visualization.

  • The influence of pressure on evolution of dry spots under bubbles was determined.

  • Multiscale boiling characteristics were plotted for pressure range of 8.8–103 kPa.

  • A discussion was held on the applicability of some theoretical approaches.

Abstract

The present paper reports the results of the comprehensive experimental investigation of an influence of subatmospheric pressures on multiscale heat transfer characteristics during liquid pool boiling. Experiments were carried out in the pressure range of 8.8–103 kPa at saturated water boiling using high-speed IR thermography, high-speed visualization from different sides and the specially designed transparent ITO heater. This made it possible to obtain simultaneously extensive data set on the effect of reduced pressure on main characteristics of boiling, including heat transfer coefficients, nucleation site density, growth rate and departure diameter of vapor bubbles. High-speed visualization from a bottom side of transparent heater allowed to investigate an evolution of dry spots bounded by triple contact line depending on pressure for the first time. It was demonstrated that the growth rate of dry spots is constant in time and has a non-monotonic dependence on pressure.

Introduction

Being one of the most effective heat transfer regimes boiling is quite often used in practice. But despite numerous studies there are still questions related to the description of dynamics of two-phase flows, the theory of heat transfer and crisis phenomena development during nucleate boiling [1], [2]. Commonly, dimensionless correlations presented in the literature were obtained for certain fluids and are valid only in certain pressure range. For example, at pressures range of p/pcr < 0.002 the well-known hydrodynamic theory of pool boiling crisis [3], [4] shows significantly overestimated results than the experiments [5], [6]. The complexity of the theoretical description of the boiling process is primarily due to the fact that this is conjugate task, which requires taking into account the influence of the physical and chemical surface properties, including its geometry, morphology, wetting properties, etc. Secondly, boiling is a multiscale non-stationary process and for its description it is necessary to consider the effects that occur on different spatial and temporal scales. These features of the boiling also create additional complexity for the experimental study of this process.

It is well known that the system pressure is one of the most important parameters which has the complex effect on the nucleation, the heat transfer rate and critical heat fluxes at nucleate boiling. In the second half of the last century various authors [7], [8], [9], [10], [11], [12], [13], [14], [15] showed that with pressure reduction, the sharp decrease in the density of nucleation sites and the emission frequency of vapor bubbles, as well as the increase in the growth rate and departure diameters of bubbles are observed. This reflects the fact, that with pressure reduction vapor density and surface tension dramatically change, which leads to the increase in the critical radius of the vapor bubble and wall superheating corresponding to boiling incipience, and as a result to the increase in the Jakob number. The change in the nucleation site density and the emission frequency of vapor bubbles leads to significant surface temperature fluctuations. A significant change in the local boiling characteristics and in the dynamics of two-phase flows near a heated wall at subatmospheric pressures has a negative effect on the intensity of heat transfer and the value of the critical heat flux.

In the literature, a lot of attention is paid to an investigation of the dynamics of vapor bubbles during boiling of various liquids at subatmospheric pressures. In particular, authors of [7], [8], [13], [16], [17], [18], [19], [20] analyzed a growth rate of vapor bubbles at pool boiling down to p = 1 kPa with the use of high-speed video recording from the side of heating surface. It was shown that a growth rate of vapor bubbles at subatmospheric pressures boiling cannot be described in frame of heat diffusion-controlled scheme of bubble growth, at which the interfacial heat transfer is the only limiting factor. Bubble growth curves obtained for different reduced pressures are characterized by different exponents n in power law Req(t) ~ tn, which demonstrates the manifestation of different mechanisms of bubble growth with pressure change [21]. The pressure reduction also has a significant effect on the bubbles shape - in particular, massive vapor bubbles formed at very low pressures have specific, so-called “mushroom” shape. The departure of such bubbles is accompanied by the appearance of high-velocity liquid jet, penetrated into its flattened base. Also recently, Rullière et al. [17] noted the “cyclic boiling regime” at water boiling at 1.2 kPa, which is characterized by the appearance of numerous small bubbles on a surface after massive bubble liftoff. The nucleation cycle can restart as soon as this “bubble crisis” ends. A detailed review of the experimental studies devoted to the analysis of vapor bubbles dynamics at subatmospheric pressures boiling is presented in the recent paper of Rullière et al. [20].

However, despite the fact that evolution of vapor bubbles at subatmospheric pressures boiling has been studied in detail, nowadays this direction of researches continues to actively develop [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Primarily this is due to the expansion of the field of application of boiling regimes at reduced pressures. For example, subatmospheric pressures boiling is realized in liquid desiccant dehumidification systems and absorption chillers, which are used to remove excess heat and to maintain an optimal thermal regime during operation of various types of equipment. In addition, boiling at low pressures is a promising method for cooling microelectronic devices that require maintaining a given low operating temperature [22], [23], including space applications.

The development of new experimental techniques in the last two decades, including high-speed IR thermography [26], [27], [28], [29], [30], [31], [32], [33], laser interferometry [28], [34], [35], [36], [37], rainbow schlieren technique [38], [39] and so forth, has made it possible today to obtain fundamentally new information on the bubble dynamics and multiscale characteristics of boiling including local heat transfer in the area of nucleation site, the thickness and evaporation rate of liquid microlayer, the evolution of dry spots formed under vapor bubbles, etc. In particular, Jung and Kim [28] and Serdyukov et al. [32] based on the data of high-speed IR thermography showed that the local heat flux density at the moment of bubble appearance and at the initial stage of its growth during water boiling at atmospheric pressure can be as high as 1 MW/m2. This is due to the formation of so-called liquid microlayer under the vapor bubble. Its small thickness of several microns provides the extremely high values of heat transfer coefficient in this region. The presence of a rapidly evaporating thin liquid film at the base of vapor bubble was supposed back in the 1960s [40], [41], [42]. However, only the development of high-speed laser interferometry techniques has made it possible to investigate in recent years the evolution of the microlayer region and its thickness with high temporal and spatial resolutions [28], [34], [35], [36], [37].

An equally important local characteristic of boiling is the evolution of dry spots under vapor bubbles, which is closely related to the microlayer evaporation rate. The description of the dry spots dynamics is important not only to determine local heat transfer rate in the area of a single nucleation site during the vapor bubble growth and departure, but also to describe the crisis phenomena development at boiling, especially at subatmospheric pressures. Recently Surtaev et al. [31] have demonstrated that the high-speed video recording from the bottom side of a transparent heater allows to analyse in detail the evolution of microlayer region and dry spots under vapor bubbles. In particular, it was shown, that at water and ethanol pool boiling the dry spots growth rate is constant in time in a wide range of heat fluxes.

Despite the fact that the usage of above described modern experimental techniques with high temporal and spatial resolutions allows to obtain fundamentally new information on the boiling process, the vast majority of experimental data, including local heat transfer in the region of the nucleation site, the evolution of liquid microlayer and dry spots were obtained at water boiling only at atmospheric pressure. Of course, this fact does not allow to fully test existing theoretical models to describe local boiling characteristics, and also limits the possibility of creating new correlations that would be applicable to describe multiscale heat transfer characteristics and crisis phenomena during boiling of liquids in a wide range of pressures. The aim of this work is the experimental study of heat transfer, nucleation site density, the evolution of vapor bubbles, as well as dry spots during water boiling at subatmospheric pressures using both high-speed visualization and infrared thermography and specially designed transparent ITO heater.

Section snippets

Experimental setup and test section

The scheme of experimental setup for the investigation of local and integral characteristics of heat transfer during pool boiling at subatmospheric pressures is shown in Fig. 1a. The setup consists of two sealed cylindrical stainless steel vessels inserted one inside an other. To observe boiling in the inner chamber, the setup has two sealed windows located on one optical axis. The setup is vacuumed according to DIN 28400-3:1992-06 standard. Deionized water (MiliQ by «Merck») on the saturation

Heat transfer rate at subatmospheric boiling

This subsection presents the results obtained from a data analysis of high-speed infrared thermography. In Fig. 2 boiling curves for saturated water obtained in the experiments at different pressures are shown. The averaged temperature of the heating surface was calculated by averaging the temperature field over the heater area and time for 10 s, taking into account the temperature drop across the sapphire substrate in the stationary approximation. As can be seen in the figure, with the

Conclusions

In the present study an experimental data on the main characteristics of water boiling depending on pressure in the range of 8.8–103 kPa were obtained using the complex of experimental techniques, including high-speed IR thermography and high-speed visualization. It was shown, that:

  • -

    Heat transfer coefficient, as well as the nucleation site density and emission bubble frequency decreases with pressure reduction;

  • -

    The vapor bubbles growth rate and departure diameter of vapor bubbles increase with

Declaration of Competing Interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

The reported study was funded by the Russian Science Foundation (Project № 18-79-00078).

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