Visualization of boiling structures in high heat–flux pool-boiling

https://doi.org/10.1016/j.ijheatmasstransfer.2003.07.033Get rights and content

Abstract

The present paper presents experimental results of observation of liquid–solid contact and bubble behaviors around the critical heat flux of saturated and subcooled pool boiling on a plate of single crystal sapphire. The observation was conducted from the backside of a rectangular boiling surface and also from the side and backside of a boiling surface with a narrow width. The main results obtained are summarized as follows. The bubble base area is almost dry and lateral coalescence of bubbles forms coalescent dry areas. As the wall superheat increases, liquid–solid contact becomes like a canal meandering through dry areas. The dependency on the surface superheat of the contact-line length density (CLLD) which is defined as the total length of the boundary between wetted and dry areas in a unit area is almost the same as the boiling curve, and the value of CLLD at CHF is not strongly dependent on boiling liquid and subcooling. The relation of the number density of dry areas and their equivalent diameter in the large dry area region at CHF is not dependent on boiling liquid and subcooling, and it is similar to that of dropwise condensation.

Introduction

It is well known that many physical images and models have been proposed for critical heat flux in boiling heat transfer [1], but mechanism triggering the critical heat flux can be grouped into two concepts in general. The first concept assumes that the heat flux itself causes a limit in heat transport and this is named heat–flux governing concept in the present paper. This concept is natural because the heat flux reaches a peak value at the CHF as the word CHF literally means. The types of CHF models based on the heat–flux governing concept are the hydrodynamic instability model proposed by Zuber [2] and the macrolayer depletion model by Haramura and Katto [3]. These models are not directly concerned with the degree of superheat of the boiling surface at the CHF. On the other hand, in the second concept, heat flux is related to the degree of superheat of boiling surface and then the CHF is determined as the peak heat flux. This is named superheat-governing concept. This concept also is natural because the driving force for boiling heat transfer is the degree of superheat. The types of superheat-governing models are the unified-model proposed by Dhir and Liaw [4], the microlayer depletion model of Zhao et al. [5] and the evaporative-meniscus model of Nokolayev et al. [6]. It is known that measurements of CHF can be well described by the empirical equation by Zuber [2]. Although the form of this equation has been successfully derived in the case of the heat–flux-governing models, that is not the case in the case of the superheat-governing models.

Now, if we consider the physical images of vapor bubble and liquid–solid contact structures at the CHF, the hydrodynamic instability model portrays an image, in which: (a) generated vapor escapes from the boiling surface by forming a continuous columnar passage. By contrast, the macrolayer depletion model and the unified-model portray a two-layer structure in which (b) a coalesced bubble is formed on the macrolayer attached to the boiling surface (the term macrolayer is used here to distinguish it from microlayer, which is at the bottom of primary bubbles). On the other hand, evaporative-meniscus model assumes (c) a condition in which a vapor bubble contacting with the boiling surface forms an evaporative meniscus that gradually retreats (namely, base of the vapor bubble dries away). The microlayer depletion model assumes both the conditions (b) and (c).

Many of the above models or physical images address only the case of saturated boiling, and therefore, it will be necessary to discuss the possibility of extending the models to include the CHF of subcooled boiling as well [7]. For example, in the physical image (b), it is a prerequisite that a large coalesced bubble exists. However, as the degree of subcooling increases, coalescence of bubbles becomes to be suppressed and average diameter of vapor bubbles on boiling surface becomes significantly smaller even at the CHF condition [8].

Critical heat flux is of great significance in devices such as those that generate vapor and those that use boiling heat transfer to cool a solid surface, but in the present the mechanism triggering the CHF is explained by many deferent models or physical images as stated above. In our point of view, this is mainly due to the following reason: namely, near the CHF, vapor is generated in large amounts making it difficult for one to observe the state of affairs close to the boiling surface. To address this issue, Nagai and Nishio [9], [10] attempted to directly observe the state of liquid–solid contacts from behind a boiling surface. The boiling surface that they used was a plate made from a single crystal sapphire, which was transparent and had a thermal conductivity higher than that of stainless steel. That work [10] introduced a concept of contact-line length density Γt in which the contact-line length density was defined as the total length of the boundary lines between dry areas and wetted areas (or liquid–solid contact areas) per unit boiling surface area; note that the boundary line can also be called a contact-line or a triple-phase line. The contact-line length density showed a dependency on the degree of superheat, resembling a boiling curve: namely, the contact-line length density attained a peak value at a superheat, which is close to that at the CHF.

Nishio et al. [11] conducted visual observations of vapor bubbles in a pseudo two-dimensional space formed by sandwiching a narrow rectangular horizontal boiling surface between two glass plates. They combined the above results with the visual observations in this pseudo two-dimensional space and proposed a two-layer structure for liquid–vapor structures at the CHF. However, this experiment had some limitations: firstly, structures of liquid–solid contacts and those of vapor bubbles had not been observed simultaneously; secondly, the test liquid was not varied; finally, only the case of saturated boiling was considered.

We believe that the boiling curve is a continuous curve in the high-heat flux boiling region that includes the CHF point; namely, the region that ranges from the high-heat flux nucleate-boiling region to high-heat flux transition-boiling region. In this region, the contact-line length density is a continuous curve resembling the boiling curve, and we believe that this density represents an important index of boiling heat transfer. Incidentally, intensive evaporation takes place near the contact-line or interline [6] formed by vapor bubbles, and we are of the view that the contact-line length density is an index of a spatial density relating to intensive evaporation. The objectives of this work were laid down as follows: collection of experimental data on contact-line length densities by changing the test liquid and liquid temperature as parameters, and making a proposition for the physical image of the CHF phenomenon by simultaneous observations on the liquid–solid contact structures and liquid–vapor structures.

Section snippets

Apparatus for visual observation of liquid–solid contacts on horizontal boiling surface

Fig. 1 shows the experimental set-up used in this work. The boiling surface was a mechano-chemically polished surface of a single crystal sapphire plate that was a square of 40 mm width and 3 mm thickness. The roughness of the boiling surface was not measured in this experiment, but it can be expected that its maximum roughness to be 0.04 μm and that its central-average roughness to be 0.004 μm [12], because we used a polishing procedure identical to our previous works [9], [10]. The sapphire

Results of state of liquid–solid contacts

This section describes the results we could derive regarding the state of liquid–solid contacts, from the images obtained with the experimental apparatus described in Section 2.1.

Results on relationship between state of liquid–solid contacts and state of vapor bubbles

Based on the visual images obtained with the experimental set-up of Section 2.2, we shall present in this section, experimental results to show the relationship between the states of liquid–solid contacts discussed in Section 3 and the states of vapor bubbles.

Conclusions

In this work, visualization experiments were done to observe the state of liquid–solid contacts and the state of vapor bubbles under high-heat flux boiling in saturated and subcooled liquids. The followings are the conclusions drawn.

  • 1.

    In high-heat flux boiling, the base area of each vapor bubble becomes almost dried up, and this base area of a bubble represents a dry area. Therefore, in high-heat flux boiling, liquid–solid contacts are limited to the narrow regions bounded by the vapor bubbles,

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