Observation of heat transfer mechanisms in saturated pool boiling of water by high-speed infrared thermometry

doi:10.1016/j.ijheatmasstransfer.2021.121006

International Journal of Heat and Mass Transfer

Volume 170, May 2021, 121006

https://doi.org/10.1016/j.ijheatmasstransfer.2021.121006 Get rights and content

Highlights

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The wall temperature distribution in pool saturated boiling of water was measured at 3,000 fps using a high-speed infrared camera.
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The contribution of microlayer evaporation to bubble growth was found to be around 50% for isolated bubbles.
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Interaction between bubbles greatly enhanced convective heat transfer.
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Convective heat transfer dominated wall heat transfer.
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Contribution of microlayer evaporation to wall heat transfer was limited to around 25%, because of its small area.

Abstract

We investigated experimentally the heat transfer mechanisms in saturated pool boiling of water. In the experiment, the temperature of a sapphire heated wall with a titanium thin-film heater was visualized using a high-speed infrared camera with a spatial resolution of 82 μm/pixel and a framing rate of 3,000 fps. Local heat transfer characteristics of the fundamental heat transfer processes, including microlayer evaporation, dry-out, transient heat conduction immediately after rewetting, and convective heat transfer, were investigated based on the surface heat flux distribution obtained by three-dimensional transient heat conduction analysis of the heated wall. The contribution of microlayer evaporation, which shows a high heat flux far exceeding the applied heat flux, to the bubble growth was found to be about 50%, and the heat transfer within the microlayer was dominated by one-dimensional heat conduction in the thickness direction. It was confirmed that the local heat removal immediately after rewetting of the dry patch can be reproduced by the transient heat conduction model. The enhancement of convection by the isolated bubble motion was small, while the interaction between bubbles agitated the liquid strongly and enhanced the convective heat transfer. Via partitioning the heat flux distribution by image analysis, the convective heat transfer was found to be the dominant wall heat transfer mode, and the contribution of the microlayer with an area coverage ratio with respect to the total heat transfer area of less than 10% was small, around 25%.

Introduction

Wall heat transfer during boiling can be divided into fundamental heat transfer processes, such as convective heat transfer induced by bubble motion [1,2], microlayer evaporation [3], contact line heat transfer [4], and rewetting heat transfer [5,6]. To better understand the boiling heat transfer mechanisms and construct a mechanistic heat transfer model, it is necessary to investigate heat transfer characteristics of various fundamental heat transfer processes at the microscale level and their contribution to the overall wall heat transfer. However, since the fundamental processes occur over small spatiotemporal scales, their detailed observation is difficult with conventional thermal measurement techniques. This is a significant barrier to exploring the boiling heat transfer mechanisms.

On the other hand, in the last two decades it has become possible to measure wall heat transfer during boiling in detail by adopting high-resolution techniques, such as MEMS with integrated micro temperature sensors [6], [7], [8], [9], optical thermometry, including high-speed infrared (IR) cameras [10,11] and temperature-sensitive liquid crystals [12]. The MEMS sensors, which are superior in terms of spatiotemporal resolution compared to the optical temperature measurement technologies, have been proposed as a promising research tool to investigate the heat transfer mechanisms in the nucleate boiling at a low heat flux in the isolated bubble region [6], [7], [8], [9] and in flow boiling in a mini/microchannel [13]. However, since only the temperature at the sensor locations can be obtained, this approach is not suitable for measuring boiling in the high heat flux region, where multiple bubbles are generated and the wall temperature field is complex. On the other hand, the spatiotemporal resolution of optical techniques is inferior compared to the thin-film temperature sensors, but they have the advantage of being able to visualize the temperature field. In particular, the high-speed IR thermometry can only be used for heated wall materials that are transparent to IR light, but is presently considered to be the one of most powerful tools in wall heat transport measurement during boiling because of its good spatiotemporal resolution and sensitivity. In the studies using a high-speed IR camera, the boiling temperature field is measured at a framing rate of around 1,000 fps, and a spatial resolution of 100 μm/pixel or less. By using a material for the thin-film heater that is transparent to visible light, such as ITO, it is possible to visualize the liquid-vapor structure on the wall [14] and the microlayer thickness distribution [15] with a visible-light high-speed camera synchronized with an IR camera measuring the wall surface temperature.

The effects of experimental conditions, such as heat flux, subcooling, and flow velocity, on the boiling heat transport characteristics have been investigated in detail for water [16,17] and fluorinated liquids [11,18,19] using the high-speed IR thermometry. Heat flux partitioning through image analysis has made it possible to explore in detail the relationship between bubble behavior (nucleation and dynamics of bubbles) and heat transfer characteristics of fundamental processes. For example, Richenderfer et al. [20] indicated that the liquid phase heat transfer, including forced convection and quenching heat transfer, makes a large contribution to the wall heat transfer up to the critical heat flux (CHF). On the other hand, the contribution of evaporation, which is negligible in the low heat flux region, can increase to a value comparable to the liquid phase heat transfer as the nucleation site density increases with heat flux, during the subcooled flow boiling of water. It is generally difficult to provide roughness on the heated wall surface in the IR thermometry experiments, because it would damage the thin-film heater and caused scattering of IR light, however, Su et al. [21] used a thick electroplated copper film to measure temperature on a rough surface. Their study has contributed to broadening the range of experimental conditions accessible to the IR thermometry of boiling heat transfer.

The IR thermometry is also suitable for experiments in the high heat flux region. The CHF triggering mechanism has been investigated, and the CHF models proposed in the past studies have been verified [18]. Richenderfer et al. [20] and Jung and Kim [22] showed that nucleation around a dry patch inhibits liquid supply and leads to the expansion of the dry patch during saturated boiling of water. Furthermore, Richenderfer et al. [20] pointed out that heat conduction in the substrate also plays an important role in the expansion of dry patches triggering the CHF.

Numerical simulation techniques are not only used for predicting boiling heat transfer, but are also a powerful tool for investigating the heat transfer mechanisms. Urbano et al. [23] and Guion et al. [24] simulated directly the microlayer formed at the bottom of a bubble, and reproduced the experimental results accurately. Sato and Niceno [25] simulated numerically the boiling heat transfer at high heat fluxes in the coalesced bubble region, including very complex bubble behavior, and, by partitioning of the heat flux, showed the contributions of fundamental heat transfer processes to the wall heat transfer.

In this study, the wall surface temperature in the saturated pool boiling of water on a sapphire substrate was measured at 3,000 fps using a high-speed IR camera. The local heat transfer characteristics of various fundamental heat transfer processes and bubble growth characteristics were investigated. Additionally, the contributions of the fundamental processes to the overall wall heat transfer were obtained through the heat transfer partitioning by image analysis, and we discuss the relationship between the results of heat flux partitioning and the behaviors of nucleation and liquid-vapor structure.

Section snippets

Experimental apparatus

Fig. 1 shows schematic drawings of (a) the experimental apparatus and, (b) the sapphire substrate with the thin-film heater. The side wall of the boiling chamber is a Pyrex glass tube with an inner diameter of 80 mm and a length of 100 mm, and the sapphire heated wall attached to the polyetheretherketone (PEEK) jig is located at the bottom of the chamber. The thickness of the mirror-finished single-crystal sapphire substrate is 0.6 mm, the size is 30 mm × 20 mm, the titanium thin-film heater

Analysis of temperature and heat flux

Since the sapphire substrate has a high transmittance in the observed wavelength range of the IR light of 3 μm to 5 μm, in the experiment the IR camera essentially observes the radiation from the titanium thin film. However, to calculate the temperature distributions of the titanium thin-film heater and the sapphire substrate with high accuracy from the observed IR light intensity, it is necessary to consider the absorption, radiation, and reflection that occur in the system comprising the

Local heat transfer in isolated bubble boiling

Fig. 5 shows temperature and heat flux distributions on a line passing through the nucleation site (bubble center) during generation of a typical isolated bubble. The bars in the graphs for bubble growth process (Figs. 5(a) and (c)) show the width of the apparent contact area extracted from the bubble images. The applied heat flux was 0.1 MW/m², and a wall superheat was 14.3 K. The time at bubble nucleation is set to 0 ms. Immediately after nucleation, a microlayer is formed at the bottom of

Microlayer evaporation

The heat and mass transfer characteristics in a microlayer are examined by comparison between the evaporation thickness and initial thickness of the microlayer. The quantity of evaporated liquid in the microlayer that is converted into an equivalent thickness can be calculated by integrating the local heat flux, $\dot{q}$ , from formation to dry-out of the microlayer [7], considering one-dimensional heat conduction within the microlayer in the thickness direction, as follows: $δ_{e v a p} = \int_{0}^{t_{m l}} \dot{q} (r, t) d t / (ρ_{l} h_{l v} -$

Bubble nucleation

In this experiment, two types of bubble generation were observed. One is the bubble generation with a sufficient waiting time, comparable to bubble growth time, and the other is bubble generation that occurs in the rewetting region around the dry patch, with almost no waiting time. Here, the waiting time is defined as the time from the rewetting of the dried nucleation site to the initiation of bubble growth. Therefore, in the bubble generation that occurs near the periphery of the shrinking

Heat flux partitioning

The heat flux distribution was partitioned into microlayer evaporation and liquid phase heat transfer, as shown in Fig. 18(a). The procedure of image analysis through heat flux partitioning is described in Appendix. The convective heat transfer and the rewetting heat transfer could not be separated in the liquid phase heat transfer fully objectively, especially in the coalesced bubble region with a complex liquid-vapor structure. Thus, in this study the liquid phase heat transfer includes both

Conclusions

In this study, the wall temperature in the pool saturated boiling of water at a heat flux up to 0.6 MW/m² was measured with a high-speed IR camera at a framing rate of 3,000 fps. The local heat transfer characteristics of various fundamental heat transfer processes were investigated from the surface heat flux distribution and HTC distribution obtained by the three-dimensional transient heat conduction analysis. In addition, the contribution of various fundamental heat transfer processes to the

CRediT authorship contribution statement

Takanori Tanaka: Investigation, Data curation, Visualization, Software, Writing - original draft, Writing - review & editing. Koji Miyazaki: Resources, Writing - original draft, Writing - review & editing. Tomohide Yabuki: Conceptualization, Methodology, Investigation, Resources, Supervision, Funding acquisition, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The authors report no conflicts of interest.

Acknowledgment

This study was supported by JST PRESTO (JPMJPR17I8). The authors would like to thank Dr. Masatoshi Ito at PHOTRON LIMITED for his help in operating the high-speed IR camera during the boiling experiment.

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Observation of heat transfer mechanisms in saturated pool boiling of water by high-speed infrared thermometry

Highlights

Abstract

Introduction

Section snippets

Experimental apparatus

Analysis of temperature and heat flux

Local heat transfer in isolated bubble boiling

Microlayer evaporation

Bubble nucleation

Heat flux partitioning

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgment

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

Experimental Thermal and Fluid Science

Experimental thermal and fluid science

Int. J. Heat Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

Nuclear Engineering and Design

International Journal of Heat and Mass Transfer