Surtaev A.S.   Serdyukov V.S.   Pavlenko A.N.   Moiseev M.I.  

Synchronized high-speed visible- and infrared-based experimental techniques for investigation of the pool boiling heat transfer.

Reporter: Surtaev A.S.

A.S. Surtaev, V.S. Serdyukov, M.I. Moiseev, A.N. Pavlenko
Kutateladze Institute of Thermophysics, 630090, Novosibirsk, Russia
Novosibirsk State University, 630090, Novosibirsk, Russia

The ability of nucleate boiling to facilitate heat absorption due to significant heat of va-porization is widely used in numerous heat transfer applications. However, despite of numerous studies, there are a number of unresolved fundamental issues related to the basic mechanisms of heat transfer during boiling process. In recent years, the development of new experimental methods with high temporal and spatial resolution allowed to obtain fundamentally new infor-mation, which is necessary for a better understanding of the process of heat transfer at boiling, and to create new theoretical models [1, 2]. This paper presents the results of an experimental study of local and integral characteristics of heat transfer at pool boiling obtained by using syn-chronized high-speed thermography (IR) and video recording (HSV).
Pool boiling tests were conducted at saturation and subcooling conditions at atmospheric pressure. Thin film (δ = 500 nm) made of Indium-Tin-Oxide (ITO) was used as a heater. The ITO layer was vacuum-deposited onto 0.4 mm thick sapphire substrates. The high transmit-tance of the substrate, coupled with poor transmittance of the ITO, ensures that temperature measurements correspond to the bottom of the ITO substrate. That simplifies thermal analysis of the heater.  The samples were resistively heated by a DC power supply via thin silver elec-trodes. Ethanol was used as a working fluid.
To measure the temperature field of the heating element, high-speed thermographic cam-era with recording frequency up to 1 kHz was used. Visualization of boiling, as well measure-ment of the local characteristics of vapor bubbles were carried out by using high-speed digital video camera. The record of the heater voltage and current values, IR data of the heater surface and digital high-speed video were synchronized by means of ADC and LabView. This experimental technique has been successfully applied by authors to investigate the local characteristics, heat transfer and crisis phenomena development at boiling in falling subcooled liquid films [3].
In the first stage, the dynamics of boiling in dependence on heat flux and initial tempera-ture of the working fluid was investigated. In the range of low heat fluxes and high degrees of subcooling (ΔTsub), the experiments show the regime of “sitting” bubbles existing on the heating surface for the long time. Typically, collapse of the sitting vapor bubbles occurs as a result of interaction and merging of multiple vapor bubbles. Based on the analysis of experimental data, a significant effect on the heat transfer at low heat fluxes is provided by natural convection. It has also been found that under certain conditions the vapor phase is removed periodically from the heater, which is accompanied by periodic variations of the surface temperature. As follows from the transient thermal field data analysis, at heat flux q = 25 W/cm2 and subcooling degree ΔT = 10 °C the temperature, and hence the heat transfer coefficient as averaging over the surface, varies with a frequency of about 9 Hz.
At saturation, the first vapor bubble appears on the surface already at q = 4.7 W/sm2. In contrast to the boiling of subcooled liquid, at saturation the bubbles detach from the surface after the growth stage and emerge in the core of the pool by the buoyancy force. Typical frames of the high-speed video and IR-video are presented in Fig. 1. Intense heat removal due to evaporation of the liquid into vapor bubbles results in rapid cooling of the surface. Thus, at the moment of bubble nucleation the cold spot appears on the heater surface as shown on the IR-frame.
The experimental data was used to derive the boiling curve for ethanol which is in a good agreement with the prior experimental studies [4, 5] and the theoretical model. In addition to the integral characteristics of heat transfer, the microfeatures of the boiling such as departure bubble diameter, frequency of nucleation, and density of nucleation sites, were also investigated depending on the heat flux and the initial temperature of the liquid. Experimental IR-data with high spatial resolution allowed us to analyze the transient temperature field under the separate vapor bubble and to determine the distribution of heat flux and the local heat transfer coefficient in the region of the triple contact line. It facilitates evaluation of the integral heat transfer by evaporation microlayer and defines the basic mechanisms of boiling heat transfer.

It is shown that the use of high-speed video recording synchronized with infrared ther-mography is very promising technic for the study of integral and local characteristics of boiling liquids including the use of micro- and nano-structured surfaces.

The work was financially supported by RFBR № 14-08-00635a.

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