Boiling heat transfer enhancement over copper tube via electrolytic and electrostatic effects

Highlights

• An active technique was used to study pool boiling heat transfer over a copper tube.

• Higher HTC and earlier ONB were achieved with the electrolytic boiling and the assistance of attached surfactants.

• More nucleation sites and higher bubble detachment rate were obtained by the active method.

• An optimal electrolytic current was observed to achieve the best boiling heat transfer performance.

Abstract

The effects of electrolytic pool boiling with and without the application of charged surfactants on heat transfer performance were experimentally investigated. The experiments were conducted at the saturated state under atmospheric pressure. This is the first study of the heat transfer mechanism in pool boiling using electrolysis with charged surfactants on copper tubes. The results indicate that heat transfer performance is not proportional to the applied electrolytic current. An optimal current was obtained for the enhancement of the heat transfer coefficient during electrolytic boiling. Supported by high-speed images, an analysis of bubble behaviors under different conditions was conducted. Compared with when water was used as the working fluid without the application of an electric field, the use of charged surfactants in electrolytic boiling with a current of 3 mA resulted in a 1.68 times higher heat transfer coefficient due to the number of bubble nucleation sites and the bubble departure frequency. Furthermore, the circumferential surface temperature distributions during boiling under different experimental conditions were elucidated by the analysis of bubble dynamics.

Introduction

Boiling heat transfer is effective in heat transfer applications. The main mechanism involved is the liquid–vapor phase change, which allows the transport of large amounts of heat with the assistance of latent heat. The boiling heat transfer mechanism improves the thermal performance of power plants, [1], [2], [3] batteries, [4], [5] spacecrafts, [6], [7], [8] cooling systems, [9], [10], [11], [12], [13] inkjet printers, [14], [15], [16] and other systems.

Surface roughness and wettability play key roles in boiling heat transfer. A higher heat transfer coefficient (HTC) can be achieved with rougher surfaces because of the increase in the number of bubble nucleation sites. [17], [18], [19] Regarding surface wettability, the HTC in low heat flux regimes increases with the degree of hydrophobicity of a surface due to earlier bubble nucleation. However, at high heat flux intervals, bubble aggregation results in the film boiling phenomenon, which reduces the critical heat flux (CHF). [20], [21] By contrast, although the HTC of a hydrophilic material is low in low heat flux regimes owing to the delay in bubble nucleation, a higher CHF is obtained with high heat fluxes because of the rewetting ability of the material, which results in the departure of bubbles at a high frequency. [21], [22]

Current methods for changing surface roughness and wettability to investigate boiling heat transfer are categorized as passive and active techniques. Passive techniques include chemical deposition, [23], [24], [25], [26], [27], [28] machining, [29], [30], [31] and ultrafast laser texturing. [32], [33], [34], [35], [36] Active techniques, such as light-induced methods [37], [38] and electric field control, [39], [40], [41] have been developed in recent years. Liu et al. [37] prepared a TiO₂-coated surface and employed UV light irradiation to alter surface wettability. Thus, the effect of light-induced surfaces on boiling heat transfer was experimentally studied. Tanaka et al. [39] proposed an electrolytic bubble nucleation activation method on a flat copper surface to improve boiling heat transfer. Theirs was the first study to experimentally investigate the electrolytic effect on pool boiling. With the assistance of electrolysis, earlier onset of nucleate boiling (ONB) occurred and a higher HTC was achieved. Cho et al. [41] investigated boiling performance by using charged surfactants as a working fluid and applying an electric field. The bubbles could be controlled, which indicated that bubbles can be generated or eliminated to adjust boiling performance.

Studies on the use of a tubular surface in pool boiling using active techniques are few or nonexistent. Pool boiling heat transfer enhancement over a copper tube may increase the thermal performance of boiling tubes in a power plant or coil–shell heat exchanger in industrial applications. In this study, we investigated pool boiling heat transfer over a copper tube with or without using charged surfactants as working fluids under the condition of an additional electric field applied at different electrolytic current strengths. We also analyzed boiling heat transfer characteristics and bubble dynamics under different conditions by using high-speed visualization.