Rainbow schlieren-based direct visualization of thermal gradients around single vapor bubble during nucleate boiling phenomena of water
Introduction
Boiling heat transfer forms an integral part of the day to day life. Its application ranges from various household appliances to huge thermal power plants. The wide range of applications of the phenomena of boiling heat transfer has generated considerable interest among the researchers for developing a detailed understanding of the process from a fundamental point of view over the past two decades. Since the inception of boiling curve by Nukiyama (1966), the studies of boiling phenomena available in the literature can be divided into nucleate, transition and film boiling regimes. Nucleate boiling is a complex heat transfer mode in which the vapor bubbles generated leave the surface of the heater periodically, thus enhancing the overall heat transfer rates. The inherent capability of nucleate boiling to enhance heat transfer rates is of utmost engineering importance and has been one of the reasons that has always motivated the researchers to explore various methodologies to enhance boiling heat transfer rates and critical heat flux (Kandlikar, 2013) which includes the use of acoustics (Douglas et al., 2007), electric field (Siedel et al., 2011, Tomar et al., 2009) and nanofluids (Barber et al., 2011, Fang et al., 2016). Complex nature of nucleate boiling gets compounded as it is influenced by various sub processes, which include micro-layer phase transfer (Cooper and Lloyd, 1969, Demiray and Kim, 2004, Judd and Hwang, 1976, Voutsinos and Judd, 1975, Wijngaarden and Vossers, 1978), fluctuations in the temperature of the heater surface (Shoji, 2004), and shape, size and density of the nucleation sites (Chatpun et al., 2004, Shoji and Takagi, 2001, Zhang and Shoji, 2003).
A majority of the studies available in the literature focused on understanding the nucleate boiling heat transfer phenomena have primarily employed the use of high speed videography-based imaging techniques to study the bubble dynamics, which includes the study of parameters like contact angle, bubble diameter (Kim et al., 2006), bubble shape and bubble frequency. Consequently, several correlations have been developed with the wide range of experimental data available from various experimental studies (Kandlikar et al., 1992, R. and Lahey, 1992, Tong and Tong, 1997). Conventional studies in nucleate boiling also include the use of detailed numerical models (Dhir and Liaw, 1989, Dhir et al., 2013, Lal et al., 2015, Mukherjee and Kandlikar, 2007, Sato and Niceno, 2015, Tomar et al., 2005) that employ different experimental parameters, such as contact angle, bubble diameter and bubble frequency as some of the input parameters. Experimental studies based on the usage of high speed infrared thermometric cameras for the estimation of surface temperature and subsequent determination of heat transfer coefficients have also been reported in recent years (Hetsroni et al., 2006, Jung and Kim, 2015, Kim, 2009, Kim et al., 2017, Lee et al., 2003).
While the qualitative visualization techniques such as high speed videography (Dhir et al., 2012, Siedel et al., 2008, Siedel et al., 2013) have extensively been employed in the past for understanding the bubble dynamics (e.g., bubble diameter, contact angle etc.), recent advancements in the field of white light sources and lasers coupled with the availability of high quality optical components have highlighted the potential of refractive index-based imaging techniques in the context of nucleate boiling heat transfer phenomena. The inherent advantages lie in the fact that these techniques not only serve as a qualitative tool but also provide whole field quantitative data in a purely non-intrusive and real time mode. Of all the available routes, one may configure these techniques predominantly in three possible ways depending on the parameter that needs to be determined; 1) interferometry, for obtaining the differences in the absolute values of refractive indices (between the test section and the reference medium); 2) schlieren that provides direct information about the gradient of the field of interest, and 3) shadowgraph wherein one obtains the second derivative of the refractive index field. In the context of nucleate boiling phenomena, the limited number of studies that are available in the literature reveal that out of these three techniques, researchers have primarily exploited the potential of interferometry and/or its variants (e.g., optical coherence tomography) for visualizing the bubble dynamics and quantifying the associated heat transfer processes. However, the number of such studies are still quite scarce. Of notable interests are the classical experimental works reported by Chen and Mayinger, 1992, Chen and Mayinger, 1985) and Mayinger and Panknin (1974) wherein the technique of laser interferometry was employed to determine the local variation of heat transfer coefficient around a vapor bubble condensing under various sub-cooled conditions. Lucic et al. (2004) and Manickam and Dhir (2012) made use of holographic interferometry to study and estimate the heat transfer rates around the vapor bubble during flow boiling phenomena. In one of the recent studies presented by Yabuki et al., 2012, Yabuki and Nakabeppu, 2014), visualization of nucleate boiling phenomenon on MEMS sensors was reported. The study employed the technique of holographic interferometry and the results pertaining to the whole field temperature measurements and heat transfer rates were reported.
In recent years, the advancements made in the field of optical instrumentation have resulted into significantly improved levels of dynamic as well as spatial resolutions that may be achieved with refractive index-based imaging techniques. These advancements have enabled the researchers to visualize the small length scale phenomena associated with the real time dynamics of vapor bubbles during nucleate boiling process. Primary interest has been in developing a fundamental understanding of the growth of micro-layer underneath the bubble, a region in which a majority of phase transfer from liquid to vapor is believed to take place. In this direction, Gao et al. (2013) employed a thin film interferometer to investigate the dynamic growth of the micro-layer region. The experiments were conducted using a transparent Indium Tin Oxide (ITO) glass resistance heater. The probing light beam of the interferometer upon reflection from the edges of the micro-layer was made to interfere with the incident reference beam to give rise to a set of fringe patterns, which subsequently were reconstructed to quantify the growth of micro-layer. Meissner et al. (2012) highlighted the potential of optical coherence tomography (OCT) to study nucleate boiling phenomena on an ITO heater and for direct three-dimensional reconstruction of the vapor bubble. Developments in the area of digital holography have also enabled the reconstruction of velocity field from a digital hologram. Bloch et al., 2014, Bloch et al., 2016) used holographic interferometric velocimetry (HIV) to determine the velocity field in sub-cooled flow boiling. Recent study by Duan et al. (2013) employed particle image velocimetry (PIV) for the determination of the velocity field around a vapor bubble during pool boiling. Importance of understanding various physical processes associated with the vapor bubble formation has been emphasized in a recent review article by Prosperetti (2017).
The above presented discussion pertaining to the real-time imaging of the vapor bubble dynamics and heat transfer process in nucleate boiling phenomenon reveals that a majority of researchers have primarily made use of interferometry and/or one of its variants (e.g., holography, OCT etc.). It is pertinent to note here that while interferometry offers relatively superior levels of spatial resolution, it involves stringent alignment of the two arms of the interferometer, is more sensitive to external factors such as minute air currents in the experimental room, flow vibrations etc., and these factors may induce undesired phase changes in the recorded interferograms. Moreover, the quantitative analysis of fringe patterns yields the absolute value of temperature distribution in the field of view, hence one needs to numerically determine the first derivative of temperature (with respect to the spatial co-ordinate) to extract the gradient field information in order to calculate the heat transfer rates (heat transfer coefficient and Nusselt numbers etc.). In this context, one of the other forms of refractive index-based imaging techniques, namely schlieren deflectometry offers unique advantages that include relatively simpler optical configuration, ease of alignment, considerably more cost-effective as it makes use of a white light source instead of monochromatic source (laser) as the light source. Moreover, the intensity patterns recorded in the form of schlieren images provide a direct measure of spatial distribution of gradient information (temperature, density and/or refractive index), thus, one can directly deduce the heat transfer rates from the recorded schlieren images without any further requirement of numerical post processing.
With this background, we report an experimental investigation of nucleate boiling phenomenon using rainbow schlieren deflectometry as the non-intrusive diagnostic technique. Direct visualization of thermal gradients around single vapor bubble generated under saturated conditions in the boiling chamber have been presented in the form of rainbow schlieren images. The optical technique employs a specially-designed color filter to generate the desired contrast in the form of color redistribution depending on the strength of spatial distribution of temperature gradients prevailing in the boiling chamber. Based on the recorded real time schlieren images, phenomena such as development of thermal boundary layer in the vicinity of the heated substrate, inception and further growth of single vapor bubble at the nucleation site, scavenging of superheat layer during the process of bubble departure etc. have been investigated. The work also reports the direct visualization of the vapor trail along the direction of upward moving bubble, development and subsequent shedding of wake vortices from the edges of the condensing bubble and dynamic phenomenon of changes in the shape of the bubble as it moves in the bulk conditions etc. In addition to providing fundamental understanding of various sub-processes associated with nucleate boiling phenomenon, the present work, to the best of the knowledge of authors, reports the first successful demonstration of the potential of rainbow schlieren deflectometry in the context of boiling heat transfer.
Section snippets
Boiling chamber
Nucleate boiling experiments reported in the present work have been performed in a test section of dimensions 50 × 50 × 70 mm3 with its walls made up of 10 mm thick acrylic sheets. For facilitating the undisturbed propagation of the collimated light beam for schlieren visualization, one of the pairs of the two opposite side walls of the boiling chamber has been fitted with high quality optically transparent glass windows (BK-7, flatness: λ/6 ). Straightness and parallelism of the optical
Results and discussion
Primary findings of the direct visualization of various sub-processes associated with nucleate pool boiling heat transfer phenomenon have been presented and discussed in this section. Hue variations recorded in the form of rainbow schlieren images have first been interpreted in qualitative terms to develop a fundamental understanding of the processes that define the entire regime of nucleate boiling heat transfer phenomena, such as the initial development of the superheat layer on the heated
Conclusions
Non-intrusive diagnostics of nucleate pool boiling phenomenon have been presented. Whole-field mapping of temperature gradients around a single vapor bubble have been made in qualitative terms (in the form of hue distributions) using rainbow schlieren deflectometry as a non-intrusive diagnostic tool. The schlieren measurements clearly captured various sub-processes associated with the nucleate boiling heat transfer phenomenon. These sub-processes include the development of superheat layer in
Acknowledgements
This work was supported by Cummins Technology India Ltd., India. Authors acknowledge the support received from Cummins Technology, India.
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