Effects of heater size and orientation on pool boiling heat transfer from microporous coated surfaces

Abstract

The present research is an experimental study of pool boiling behavior using flat, microporous-enhanced square heater surfaces immersed in saturated FC-72. Flush-mounted $\text{2}\text{cm}\text{×2}\text{cm}$ and $\text{5}\text{cm}\text{×5}\text{cm}$ copper surfaces were tested and compared to a $\text{1}\text{cm}\text{×1}\text{cm}$ copper surface that was previously investigated. Heater surface orientation and size effects on pool boiling performance were investigated under increasing and decreasing heat-flux conditions for two different surface microgeometries: plain and microporous coated. Results of the plain surface testing showed that the nucleate boiling performance is dependent on heater orientation. The nucleate boiling curves of the microporous coated surfaces were found to collapse to one curve showing insensitivity to heater orientation. The effects of heater size and orientation angle on CHF were found to be significant for both the plain and microporous coated surfaces.

Introduction

Among the first researchers to study the effect of orientation on the nucleate boiling heat transfer was Storr [2], who observed that the heat transfer rate for a given wall superheat increased when rotating its heating surface from horizontal to vertical. Marcus and Dropkin [3] and Githinji and Sabersky [4] have also found that the heat transfer coefficient increases from an inclination angle of 0° (horizontal, upward facing) to 90°. Marcus and Dropkin have described the phenomenon as the result of increased agitation of the superheated boundary region due to the increased path length of the departing bubbles along the surface. Further research indicated that the heat transfer coefficient continues to increase up to an angle between 150° and 175° and then decreases drastically as the angle approaches 180° [5], [6], [7]. Chang and You [8], using a small copper surface in FC-72 saw similar results between 0° and 90°, however, they found that the heat transfer coefficient decreased for angles greater than 90°.

Marcus and Dropkin have additionally hypothesized that the nucleate boiling heat transfer coefficient would become insensitive to inclination angle above a certain heat flux. Nishikawa et al. [5] have confirmed the existence of this transition heat flux. Lienhard [9] has related Nishikawa et al.'s observed transition point to the transition from the isolated bubble regime to the continuous vapor column regime as described by Zuber [10] and Moissis and Berenson [11]. Similar trends were reported by Beduz et al. [6]. Beduz et al. have also found that the heat transfer coefficient was independent of inclination angle for their enhanced surface. Angular independence of the nucleate boiling heat transfer coefficient for other enhanced surfaces has also been shown by Jung et al. [12] and Chang and You [8].

One of the first researchers to study the effects of inclination angle on q_CHF^″ was Githinji and Sabersky [4]. Using a relatively long and thin heating surface $\text{(102}\text{mm}\text{×3.2}\text{mm}\text{)}$ in water, they found that the q_CHF^″ increased from 0° to 90° and then decreased drastically from 90° to 180°. Using a smaller surface (9.9-mm diameter) in liquid helium, Lyon [13] has observed that q_CHF^″ continually decreases from 0° to 180°. This last trend has been the dominant trend seen in the literature. Among the first to correlate the effect of orientation on q_CHF^″ was Vishnev [14]. His correlation was developed for cryogenic fluids based on the helium data of other researchers. Later, El-Genk and Guo [7], using numerous sources of data, have developed separate correlations for water, nitrogen, and helium. More recently, Chang and You [8] have correlated their FC-72 data and checked it against both cryogenic and non-cryogenic fluids from many sources. Howard and Mudawar [15] have developed a CHF model for the near-vertical pool-boiling situation based on their observations in FC-72.

The effects of size on pool boiling heat transfer have been studied previously. At an inclination angle of 0° (horizontal, upward facing), Gogonin and Kutateladze [16] did not see any observable change in q_CHF^″ when they varied the width of their heating surface. Ishigai et al. [17] have found that q_CHF^″ decreased with increasing heater size at an inclination of 180°. Similar trends were reported by Gogonin and Kutateladze [16] and Granovskii et al. [18]. Using 0° oriented flat surfaces with sidewalls, Lienhard, et al. [19] have showed both analytically and experimentally that, in general, q_CHF^″ would decrease with increasing heater size up to a point and then become relatively constant. They related this behavior to the number of “vapor jets” that could be supported by the surface area. Similarly, Saylor et al. [20] have found that q_CHF^″ was relatively constant for large surfaces and increased for decreasing heater size past a certain transition point. Park and Bergles [21] have observed that the nucleate boiling heat transfer coefficient was insensitive to heater size. Additionally, Park and Bergles have found that q_CHF^″ was affected by changes in both height and width in similar trends as those later observed by Saylor et al. for 0° oriented surfaces.

The objective of the present research is to understand the combined effects of heater surface orientation and size on pool boiling behavior of flat, square heater surfaces immersed in saturated FC-72. Two surface conditions were considered: plain and microporous coated. To study the effect of orientation, six different heater orientation angles (0°, 45°, 90°, 135°, 160°, and 180°) were used. To study the effect of heater size, two relatively large surfaces, 4-cm² $\text{(2}\text{cm}\text{×2}\text{cm}\text{)}$ and 25-cm² $\text{(5}\text{cm}\text{×5}\text{cm}\text{)}$, were tested. The results were then compared to the results of Chang and You [8] who studied a 1-cm² $\text{(1}\text{cm}\text{×1}\text{cm}\text{)}$ heater surface. For the plain surface tests, two different heat flux conditions, increasing and decreasing, were also considered.