- Email: [email protected]
Nucleation site interaction between artificial cavities during nucleate pool boiling on silicon with integrated micro-heater and temperature micro-sensors
International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Nucleation site interaction between artificial cavities during nucleate pool boiling on silicon with integrated micro-heater and temperature micro-sensors C. Hutter a,1, K. Sefiane a,⇑, T.G. Karayiannis b, A.J. Walton a, R.A. Nelson c, D.B.R. Kenning b a
University of Edinburgh, School of Engineering, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, UK Brunel University, School of Engineering and Design, Uxbridge UB8 3PH, Middlesex, UK c Los Alamos National Laboratory, Los Alamos, NM 87545, USA b
a r t i c l e
i n f o
Article history: Received 11 April 2011 Accepted 3 February 2012 Available online 6 March 2012 Keywords: Nucleate pool boiling Bubble interaction Horizontal coalescence Artificial cavity
a b s t r a c t Nucleate boiling is commonly characterised as a very complex and elusive process. Many involved mechanisms are still not fully understood and more detailed consideration is needed. In this study, bubble growth from micro-fabricated artificial cavities with varied spacing on a horizontal 380 lm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using 16 integrated micro-sensors situated on the boiling surface, each with an artificial cavity located in its geometrical centre. Experiments with three different spacings 1.5, 1.2 and 0.84 mm in between cavities with a nominal mouth diameter of 10 lm and a depth of 80 lm were undertaken. To conduct pool boiling experiments, the test section was mounted inside a closed stainless steel boiling chamber with optical access and completely immersed in degassed fluorinert FC-72. Bubble nucleation, growth and detachment at 0.5 and 1 bar absolute pressure were investigated using high-speed imaging. The effect of decreasing inter-site distance on bubble nucleation frequency, bubble departure frequency and diameter with increasing wall superheat is presented. Furthermore, the frequency of horizontal bubble coalescence was determined. The regions of influence on the measured frequencies and bubble departure diameter were compared with recently published findings. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nucleate boiling is one of the most effective heat transfer processes at low temperature differences with a wide range of cooling applications in industry. However, many mechanisms involved in nucleate boiling are still not fully understood and more investigations are required. Chekanov [1] was the first to conduct experiments with two artificial nucleation sites spaced close enough to make interactions between nucleation sites visible. The artificial nucleation sites were created by two heated copper rods in contact with a perm-alloy plate immersed in water. The time between two departed bubbles from neighbouring nucleation sites followed a gamma distribution, commonly used as probability model for waiting times. He introduced three regions of interaction determined by the dimensionless cavity spacing S/Dd where S is the distance between two cavity centres and Dd the average bubble departure diameter. For S/Dd < 3 the formation of a bubble at one nucleation site inhibits the formation of a vapour bubble at the ⇑ Corresponding author. E-mail address: K.Sefi[email protected] (K. Sefiane). Current address: Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. 1
0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2012.02.014
neighbouring nucleation site. For S/Dd > 3 the formation of a bubble at one nucleation site promotes the formation of a vapour bubble at the neighbouring nucleation site and if S/Dd 3 no interaction between the two nucleation sites takes place. Chekanov suggested acoustic and hydrodynamic effects as responsible mechanisms for the observed inhibition and promotion of bubble growth from closely spaced nucleation sites. Judd et al. [2–5] studied in their experiments the interaction phenomena between adjacent nucleation sites. They used a transparent glass surface 3.6 mm thick, coated with a 0.3 lm stannic oxide layer acting as the heater. The system pressure was reduced in order to decrease the number of active nucleation sites, to increase the bubble size and to prevent any damage to the boiling surface. Like Chekanov they introduced three regions of interaction between nucleation sites determined by the dimensionless cavity spacing S/Dd where, Dd is the average bubble departure diameter for the observed time period. S/Dd < 1 is called a ‘‘promotive’’ region, where a nucleation site that is unable to capture vapour nuclei is within the area influenced by a continually active nucleation site that can deposit vapour nuclei in it, also called ‘‘seeding’’. Bubbles will form at the adjacent nucleation site more frequently than would otherwise be the case. 1 < S/Dd < 3 is an ‘‘inhibitive’’ region, where a nucleation site that is unable to capture vapour nuclei is
2770
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
Nomenclature Dd p pl pg rb
average bubble departure diameter (m) pressure (bar) liquid pressure (bar) vapour pressure (bar) nuclei curvature radius (m)
within the area influenced by an intervening active nucleation site capable of depositing/displacing nuclei in it. The nucleation site itself is under the influence of a continually active nucleation site and bubbles will form at the adjacent nucleation site less frequently than would otherwise be the case. The third region is the ‘‘independent’’ region with S/Dd > 3, where bubble formation at one nucleation site is not influenced by the formation of bubbles at a neighbouring nucleation site. Judd and co-authors suggested ‘‘site seeding’’ to be responsible for these phenomena. Kenning and Yan [6] investigated temperature fluctuations in the heated wall during bubble growth. A direct electrically heated stainless steel plate was immersed in water. The plate, 0.13 mm thick and 46 103 mm2 in area, was carefully cleaned and the surface was well wetting with a contact angle less than 20°. The water was degassed in a separate tank, but since the boiling chamber was opened to the atmosphere, the amount of dissolved gas was uncertain. On the back of the plate, a thermochromic liquid crystal layer, which had a visual colourplay from 104 °C to 123 °C, was deposited. The importance of wall temperature variations for bubble growth and activity of nucleation sites was confirmed. The cooling effect on the wall surface was considerable and limited to the maximum contact area of the bubble on the heated surface. The cooling effect decreased closer to the outer limit of the ‘‘area of influence’’. They concluded that nucleation sites inside the maximum occurring diameter of a growing bubble influence each other through thermal fluctuations on the wall surface. This interaction caused intermittence that had a large influence on the production of bubbles at the sites. Golobicˇ and Gjerkeš [7] activated nucleation sites with a laser beam (diameters of heated spot area between 1.66 and 5.23 cm) on a 25 lm thin copper or titanium foil immersed in saturated water. The applied heat flux was up to 560 kW/m2. Spacings between the three or four simultaneously active nucleation sites were ranging from 2.6 to 4.1 mm. They concluded that interactions between two artificially activated sites decrease the overall activity of both sites. The activity at both sites may decrease or it is increased at one site and simultaneously decreased at the other. If there is more than one active site in its vicinity, they influence each others activity. Zhang and Shoji [8] conducted experiments on a silicon surface, which was 0.2 mm thick and had a surface area of a diameter of 15 mm and cylindrical artificial cavities. The cavities are 10 lm in diameter and 80 lm deep. They were arranged as single or twin cavities with different spacing of 1, 2, 3, 4, 6 and 8 mm. The boiling liquid was distilled water and the silicon surface was heated by Nd:YAG laser irradiation. Temperature fluctuations were measured beneath and around cavities with radiation thermometers (spatial resolution 120 lm, temperature resolution 0.08 K and temporal resolution 3 ms). All pictures were captured with a high-speed camera at 1297 frames per second (fps). In their experiments, Zhang and Shoji [8] observed three crucial effects; effect ‘‘A’’ is the hydrodynamic interaction between bubbles, ‘‘B’’ the thermal interaction between nucleation sites and ‘‘C’’ the horizontal and declining bubble coalescence. In the region S/Dd > 3 none of the three interactions can be observed, meaning that the two artificial
S
spacing between two cavities (m) surface tension (N/m)
r
cavities are independent and behave like single cavities would. The region 2 < S/Dd 6 3 is influenced by effect ‘‘A’’ and has a higher bubble departure frequency than region S/Dd > 3. In the region 1.5 < S/ Dd 6 2 effect ‘‘A’’ and ‘‘B’’ are both dominant. The dominance of effect ‘‘B’’ leads to a lower bubble departure frequency. In the region where S/Dd 6 1.5 all three effects have an influence on the behaviour of the process resulting in an increased bubble departure frequency. However, the authors point out that the influence range of thermal interactions is also related to the thermal properties and the thickness of the boiling substrate. Direct bubble/bubble interaction or bubble coalescence was classified by Williamson and El-Genk [9] and later Buyevich and Webbon [10] into three different types, 1) Lateral coalescence far away from the heated wall, which has no effect on boiling heat transfer, 2) Vertical coalescence between consecutive bubbles near the wall and 3) Lateral coalescence between adjacent bubbles near the wall. Yang et al. [11] confirmed the three types in numerical simulations and Zhang and Shoji [8] proposed to add a fourth type, 4) Lateral coalescence between adjacent declining bubbles near the wall. During vertical coalescence a bubble, previously departed from a nucleation site, is merging with the successive bubble growing from the same nucleation site, see Fig. 1(a). Lateral coalescence, also called horizontal, occurs between two bubbles simultaneously growing from two adjacent nucleation sites, see Fig. 1(b). The event of a bubble which has already departed from one nucleation site merging with a bubble still growing from a neighbouring nucleation site is called declining coalescence, see Fig. 1(c). The types 3) and 4) mentioned above are important for the heat transfer process, but only occur when the distance between two nucleation sites is less than 1.5 times the bubble departure diameter. Bonjour et al. [12] characterised the thermal effect of coalescence by conducting experiments with artificial nucleation sites. A vertical duraluminium (AU4G) heated boiling surface was immersed in either pentane for the study of the influence of
(a)
(b)
(c)
Fig. 1. Three types of direct bubble/bubble interaction or bubble coalescence: (a) vertical coalescence, (b) horizontal coalescence and (c) declining coalescence [8,9,11].
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
2771
Fig. 2. Schematic overview of the experimental setup including boiling chamber, condenser, data acquisition, light source and high-speed camera.
coalescence on the boiling curve and bubble departure diameter as a function of the wall superheat, or R-113 to study the influence of coalescence on bubble frequency. Heat fluxes between 103 and 104 kW/m2 or wall superheats between 5 and 35 K were applied. For moderate heat fluxes, the bubble frequency decreased, but for low or high heat fluxes it increased. Coalescence improved the heat transfer coefficient, which was attributed to supplementary micro-layer evaporation, forming below two merging bubbles. However, the highest heat transfer coefficient was measured for an optimal site distance (1.05 mm < S < 1.5 mm), for which no coalescence took place. It is very clear that there is a lack of experimental data in a controlled environment. Therefore the determination of the influence of spacing between two nucleation sites requires well defined and exactly positioned artificial cavities. Currently the best way to produce such artificial cavities is by means of making use of micro-fabrication. In this study the influence of spacing between neighbouring cavities on bubble nucleation frequency, bubble departure frequency, horizontal bubble coalescence frequency and bubble departure diameter were investigated with the help of micro-fabricated silicon substrates.
Fig. 3. Location of the 16 sensors on the top surface of the silicon device, including the connections to the 64 pads on the left and right side of the chip. A single cavity was at the geometrical centre of each sensor. A close-up of the section detail A can be found in Fig. 4.
2. Experimental setup and procedures Pool boiling experiments were conducted in a stainless steel boiling chamber with connected external condenser, Fig. 2. The
Fig. 4. Detail A of Fig. 3 with distances between artificial cavities (drawing not to scale).
Fig. 5. (a) The image shows a part of the square layout (84 84 lm) of the Ti/Ni sensor with an artificial cavity in its centre. (b) Detail B (a) shows an artificial cavity with a nominal mouth diameter of 10 lm and a depth of 80 lm in the geometrical centre of a micro-sensor.
2772
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
Fig. 6. Cross-sectional view of the jig designed to hold the silicon device in place and to physically connect the micro-sensors and the heater with embedded spring probes.
chamber had a support heating system consisting of four cartridge heaters implemented in the bottom and two flexible silicone heaters wrapped around the wall of the chamber (not indicated in Fig. 2). This was used to heat up and keep the boiling liquid, fluorinert FC-72, at saturation temperature for the set pressure. The support heating system was also used to degas the liquid prior to
(a)
each experiment for at least two hours. The vapour generated during the degassing and boiling experiments was directed to an external condenser, which was connected to a chilled bath. The cooling water flow rate and temperature were used to control the system pressure. The boiling substrate was a silicon wafer with a micro-fabricated heater on the back and 16 temperature micro-
(b)
(c)
Fig. 7. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.5 mm between the two cavities S5 and S6 at 0.5 bar.
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2773
(b)
(c)
Fig. 8. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.5 mm between the two cavities S5 and S6 at 1 bar.
sensors, with an artificial cavity etched in the geometrical centre of each sensor layout, distributed in two rows on the front side, Figs. 3 and 4 [13]. The chip size was 50 50 mm2, whereas the heater covered an area of 40 37 mm2. The heater had a serpentine layout with 8 turns. Heat flux up to 37.8 kW/m2 was used to start saturated nucleate boiling on the substrate. After the onset of boiling, the power was decreased to 21.3 kW/m2 and kept at this power for 30 min to degas the test section. The power supply was then switched off and after a waiting time of 15 min, boiling was initiated again. The power was then immediately reduced to maintain the chosen wall temperature superheat. Heat fluxes from 0.5 to 5.7 kW/m2 with a measurement error of 1% were applied during these experiments. Bubble growth was now limited to the location of the artificial cavities. The cylindrical cavities had a nominal diameter of 10 lm and were 80 lm deep. They were arranged in pairs, in which the spacing between two cavities was varied between 0.84, 1.2 and 1.5 mm. In this study only cavities at S1-S6 were used, Figs. 3 and 4. The distance between any two pairs of artificial cavities was 3 mm, which, for bubble departure diameters of 1 mm, was exactly at the limit beyond which two neighbouring nucleation sites do not interact with each other, following Judd et al. [2–5] and Zhang and Shoji [8]. For the presented experiments only a few measured bubble diameters exceeded 1 mm. The first and second row of nucleation sites had a distance of 10 mm in between and did not interact. Thus, any interaction between sites other than the investigated pairs could be neglected. The square temperature micro-sensors had a width of 0.84 mm, a nominal resistance of 0.6 X and each had four connections, Fig. 5 (a) and
(b). Two were used to send a constant current through and two to measure voltage across each sensor with a data acquisition at a frequency of 1 kHz. The resistance and accordingly the voltage changed with temperature. More details of the silicon device micro-fabrication can be found in Hutter et al. [13]. The micro-sensors were calibrated using T-type thermocouples and had an accuracy of ±0.5 K. The T-type thermocouples were calibrated with a high precision thermometer (F250MkII, Automatic Systems Laboratories), which had an accuracy of ±0.01 K between 20 °C and 85 °C. Before the test section was positioned in its holder, Fig. 6, it was always cleaned for 30 min in an ultrasonic bath, rinsed with deionised water for 10 min and dried with pure nitrogen. Bubble growth was recorded with a high-speed camera (IDT Nanosense MkIII, 1280 1024 pixel at 1040 fps) working at 1000 fps [13]. The bubble nucleation frequency, bubble departure frequency, horizontal coalescence frequency and bubble departure diameter were determined from 0.815 s long high-speed image sequences. The bubble departure frequency was the bubble nucleation frequency minus the frequency of vertical coalescence. For frequency measurements, the measurement error was estimated to be ±2 events and for the departure diameter ±2 px, whereas the standard deviation of the average bubble diameter was indicated for the cases where it was larger than the estimated measurement error. The estimated measurement error represents the maximum deviation measured during analysing the same highspeed image sequence 100 times. The distance between the camera and the observed nucleation site may vary slightly for each experiment, hence the calibration will as well. Therefore, the mea-
2774
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
(b)
(c)
Fig. 9. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.2 mm between the two cavities S3 and S4 at 0.5 bar.
surement error is given here in pixel. The boiling substrate was illuminated by a continuously working coherent backlight. Due to the design of the jig, that was used to hold the test section in position, the high speed camera had to be inclined at an angle of approximately 20°. This did not influence the measured diameters as they represent for all measurements the apparent diameter, where the bubbles were widest. All experiments were conducted at saturated conditions. Saturated boiling was taking place when the T-type thermocouples, indicating the vapour and liquid temperature, showed the same temperature. Simultaneously to the imaging, the wall superheat temperature was measured with the integrated temperature micro-sensors and the heat flux applied to the boiling substrate calculated from the power supplied to the integrated heater. After one sequence was recorded, the integrated heater was switched off for 15 min, before a new wall superheat was set for further measurements and recording.
place for the closest spaced nucleation sites at the lowest pressure of 0.5 bar. 3.1. Two adjacent active artificial nucleation sites In Figs. 7–12(a)–(c) the bubble departure diameter, the bubble nucleation frequency and the bubble departure frequency are plotted as a function of the wall superheat for a cavity spacing of 1.5, 1.2 and 0.84 mm at 0.5 and 1 bar. The wall superheat represents the average temperature during the captured time period from the sensor at the corresponding cavity. The limit for the lowest wall superheat was the temperature to keep the cavities active, which is in good agreement with the theoretical minimum temperature for a cavity to trap vapour nuclei calculated from the Laplace–Young equation
pg pl ¼ 3. Results and discussion The experiments presented in this paper were conducted at 0.5 and 1 bar. Decreasing pressure increases the boiling inception temperature and leads to larger bubble departure diameters. Reducing the pressure increases the possibility of bubble interactions and is one way to reduce the number of 10 micro-fabricated test sections. The observed interactions in this paper are vertical and horizontal coalescence, Fig. 1(a) and (b). Vertical coalescence may appear at one of two adjacent sites or simultaneously at both adjacent sites. Vertical coalescence was observed for all experimental conditions and cavity spacings. However, horizontal coalescence only took
2r rb
ð1Þ
where pg is the vapour pressure, pl the pressure of the liquid, r the surface tension and rb the nuclei curvature radius which equals the cavity mouth radius rc. In order for the nucleus not to shrink, its internal temperature must equal the saturation temperature for the pressure of the vapour phase calculated from Eq. (1). The theoretical minimum temperature for 0.5 and 1 bar are 1.96 and 0.93 K, respectively. The wall superheat was increased until unwanted (other than the ones under observance) new nucleation sites became active. The measured parameters for both of the neighbouring cavities behave very similarly at 0.5 and 1 bar for a cavity spacing of 1.5 mm. The following discussion focuses first
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2775
(b)
(c)
Fig. 10. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.2 mm between the two cavities S3 and S4 at 1 bar.
on the bubble departure diameter and then on nucleation and departure frequency. The bubble departure diameter is found to increase in a nearlinear fashion with increasing superheat for 0.5 and 1 bar for all considered cavity spacings of 1.5, 1.2 and 0.84 mm, Figs. 7–12(a). The departure diameters for the same wall superheats are slightly larger at lower pressure for cavities with a distance of 1.5 and 1.2 mm in between. However, this is only apparent when the linear trend lines are compared, Figs. 7–10(a). Unlike for larger spacings, for the 0.84 mm spacing no significant difference in size is detectable, Figs. 11 and 12(a). 3.2. Departure diameter For the departure diameter there is a small increase for the two widest spacings between the two cavities. For cavity spacings of 0.84 and 1.2 mm the difference in departure diameter is very small and can be treated as insignificant. The difference between the two closest spaced cases and the widest spacings is increasing with wall superheat and ranges from 0.05 to 0.24 mm for 0.5 bar and from 0.07 to 0.16 mm for 1 bar. The departure diameter reaches slightly higher values for the same superheat at 0.5 bar. All these differences are very small and are only valid for the comparison of the linear trend lines of the departure diameter measurement with increasing wall superheat for each cavity. The linear trend lines for the departure diameter do not intercept the origin. The first bubbles appear slightly above the earlier mentioned theoretical minimum temperature. Many of the expressions reported in the literature for the bubble departure diameter
obtained empirically or analytically are not consistent. In some sources the bubble departure diameter is increasing with wall superheat, in others it is insensitive to or it even decreases with increasing wall superheat. In Cole and Rohsenow [14], Kutateladze and Gogonin [15], Jenson and Memmel [16] and Gorenflo et al. [17] all expressions are based on the Jacob number. In all these studies the departure diameter is increasing with increasing wall temperature. The reason for the use of micro-fabricated artificial cavities is the control of the location and to increase the consistency of the performed experiments. To up-scale the results to a real surface is not yet possible due the complexity of the geometry of those sites. A thorough discussion may be found in Shoji [18]. At the closest spacing between two artificial cavities the wall superheat is lowest for all considered dimensionless spacings, S/ Dd. At 0.5 bar the wall superheat may be influenced by the spacing S. For a distance of 1.2 mm the wall superheat is higher than at 1.5 and 0.84 mm. This seems not to be true at 1.0 bar and S/Dd > 3.0, see Table 1. This observation suggests interactions between sites may be influencing the departure diameter, in which case the S/ Dd approach as proposed in the literature becomes complicated and is inefficient to describe the phenomenon as other effects interfere. The interaction regions suggested by Zhang and Shoji [8] are included in the bubble departure diameter plots, Figs. 7–12(a). From the intersection points between the limits of these regions (horizontal lines in the bubble departure diameter plots) and the linear trend lines for bubble departure diameter for each cavity, the two corresponding wall superheats for the two cavities were determined and averaged (wherever there was a discrepancy).
2776
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
(b)
(c)
Fig. 11. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 0.5 bar.
3.3. Interaction between neighbouring sites The difference between the two superheats for the two cavities was always below 1 K. From this it was possible to determine the interaction region limits (vertical lines) due to spacing on the bubble nucleation frequency and the bubble departure frequency at corresponding temperatures, Figs. 7–12(b) and (c). These regions of interaction are based on the general assumption that interactions should scale with S/Dd, which will be tested in this paper. The average departure diameter Dd represents the departure diameter of 5 bubbles for each set wall superheat and pressure for each observed artificial nucleation site. The departure diameter represents only bubbles where no vertical or horizontal coalescence took place previously. As observed in previous studies, Hutter et al. [13] the bubble nucleation frequency for a cavity spacing of 1.5 mm initially increases rapidly, then appears to level off, before it slightly drops for the highest wall superheats. This behaviour is more pronounced at 0.5 bar and the small drop of frequency only noted at the same pressure, Figs. 7 and 8(b). The initial rapid increase, which flattens with higher superheats, can also be seen for the bubble departure frequency at 0.5 bar, Fig. 7(c). Frequency measurements are less scattered if vertical coalescence is not considered. In Figs. 9 and 10(b) and (c) the bubble nucleation frequency and the bubble departure frequency for two cavities, spaced 1.2 mm from each other, are plotted for the same two pressures with increasing wall superheat. The two neighbouring cavities, again, behave very similarly. After the sharp initial increase, the bubble
nucleation frequency at 0.5 bar reaches a maximum, before it starts to decrease for superheats above 8 K, Fig. 9(b). The slight decrease for higher wall superheats is still noticeable if the vertical coalescence is not considered, Fig. 9(c). The departure frequency appears constant for superheats above 4 K at 1 bar, Fig. 10(c). In Figs. 11 and 12(b) and (c) the same parameters are presented for two neighbouring cavities with a spacing of 0.84 mm for 0.5 and 1 bar. The bubble nucleation frequency seems to drop above 9 K, however, if the vertical coalescence is not considered, the frequency appears to be almost constant above 4 K wall superheat at 0.5 bar, Figs. 11 and 12(b). Fig. 12(b) and (c) are very similar to Fig. 10(b) and (c) and no influence of the spacing is apparent. At 0.5 bar, all frequency plots look similar in Figs. 7, 9 and 11 with a break point around 3 K of wall superheat but different values of S/Dd. At 1 bar, all frequency plots in Figs. 8, 10 and 12 look similar. The notable feature is the large scatter in nucleation frequency with increasing wall superheat but not in departure frequency. This indicates that the dependency of the vertical coalescence on wall superheat is less clear and adds uncertainty to the experimental observations. Experimental bubble frequencies measured by Zhang and Shoji [8] were strongly influenced by the variation of S/Dd at the applied heat fluxes. The heat fluxes applied in this study are typically one order of magnitude smaller as compared to those of Zhang and Shoji, but the latent heat of evaporation at the normal boiling temperature is around 24 times smaller for FC-72 as compared to that of water. For S/Dd equal to 0.5 the frequency was about three times higher than for 1.5 and around 1.75 times higher than at 2.5 for the highest applied heat flux of 37.2 kW/m2. The measured nucleation
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2777
(b)
(c)
Fig. 12. (a) Bubble nucleation frequency, (b) bubble departure frequency, (c) bubble departure diameter as a function of wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 1 bar.
Table 1 Averaged wall superheats at which the linear trend lines intersect the departure diameters confining the interaction regions suggested by Zhang and Shoji [8]. S/Dd
S = 1.5 mm
S = 1.2 mm
S = 0.84 mm
p = 0.5 bar 1.5 2.0 3.0
10.4 6.6 2.7
11.3 7.5 3.6
6.7 3.2 <1
p = 1.0 bar 1.5 2.0 3.0
>14 9.9 4.9
>14 9.8 5.0
8.9 5.5 2.1
and departure frequency seem not to be influenced by decreasing spacing, or any interactions for the applied heat flux are minor and hidden in the rather scattered data. 3.4. Horizontal coalescence In this paper the occurrence of horizontal coalescence was also investigated. Horizontal coalescence took place only for the two closest spaced cavities S1 and S2 at 0.5 bar. In Fig. 13 the frequency of horizontal coalescence is presented as a function of increasing wall superheat. Bubbles growing from S1 and S2 only merged above a wall superheat of 8 K and showed no clear pattern with further increasing superheat. At the highest measured superheat the frequency reached its maximum, slightly above 60 Hz. Vertical
Fig. 13. Frequency of horizontal coalescence as a function of increasing wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 0.5 bar. Regions of influence, following Zhang and Shoji [8], are indicated.
coalescence occurred in some cases for each nucleation site prior to horizontal coalescence. After two bubbles coalesced horizontally no further vertical coalescence occurred before departure. Zhang and Shoji [8] reported horizontal coalescence for spacing S/Dd smaller than 1.5. Measured frequencies for S/Dd equal to 1 are high-
2778
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
er than frequencies at 12 K, the corresponding temperature for S/Dd equal to 1.1 for these experiments. However, Zhang and Shoji do not report any reached wall superheats nor distinguish between horizontal and declining coalescence, and the latter was not considered here. 4. Conclusions Experimentally measured bubble nucleation frequency, departure frequency, frequency of horizontal coalescence and bubble departure diameter of bubble growth from pairs of artificial cavities with three different values of spacing were measured with increasing wall superheat at pressures of 0.5 and 1 bar. Interaction between neighbouring cavities was investigated by considering the bubble departure frequencies. Bubble nucleation frequency, departure frequency and bubble departure diameter were very similar for the pairs of artificial cavities. The lower pressure shifted slightly the regions of influence suggested by Zhang and Shoji [8] to lower superheat, due to the small increase in departure diameter at 0.5 bar. The strong ‘‘inhibitive’’ and ‘‘promotive’’ regions for the departure frequency with decreasing spacing between cavities have not been observed in the present study. An increase in departure diameter with increasing spacing was observed. However, it is difficult to compare the results of Zhang and Shoji with different applied heat fluxes. Also the differences between liquid properties might influence the boundaries of the region of influence. Horizontal coalescence of bubbles from neighbouring cavities was only observed for the closest spacing at 0.5 bar for superheats above 8 K. Thermal interaction through the substrate has not been studied, however, we believe this might play an important role. Furthermore, the criterion used for bubbles interactions in the literature, namely the dimensionless spacing may not be sufficient. A more complete criterion must include the thermal properties of the substrate. Acknowledgements This work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) by grant EP/C532813/1. The authors are grateful to Dr. H. Lin and Dr. G. Cummins for the silicon device fabrication.
References [1] V.V. Chekanov, Interaction of centers in nucleate boiling, High Temp. 15 (1) (1977) 101–106. [2] R.L. Judd, C.H. Lavdas, The nature of nucleation site interaction, J. Heat Transfer – Trans. ASME 102 (3) (1980) 461–464. 384. [3] A. Calka, R.L. Judd, Some aspects of the interaction among nucleation sites during saturated nucleate boiling, Int. J. Heat Mass Transfer 28 (12) (1985) 2331–2342. [4] R.L. Judd, On nucleation site interaction, J. Heat Transfer – Trans. ASME 110 (2) (1988) 475–478. [5] R.L. Judd, A. Chopra, Interaction of the nucleation processes occurring at adjacent nucleation sites, J. Heat Transfer – Trans. ASME 115 (4) (1993) 955– 962. [6] D.B.R. Kenning, Y. Yan, Pool boiling heat transfer 393 on a thin plate: features revealed by liquid crystal thermography, Int. J. Heat Mass Transfer 39 (15) (1996) 3117–3137. [7] I. Golobicˇ, H. Gjerkeš, Interactions between laser-activated nucleation sites in pool boiling, Int. J. Heat Mass Transfer 44 (1) (2001) 143–153. [8] L. Zhang, M. Shoji, Nucleation site interaction in pool boiling on the artificial surface, Int. J. Heat Mass Transfer 46 (3) (2003) 513–522. [9] C.R. Williamson, M.S. El-Genk, High-speed photographic analysis of saturated nucleate pool boiling at low heat flux, ASME Winter Annual Meeting, Paper 91WA-HT-8, 1991. [10] Y.A. Buyevich, B.W. Webbon, The isolated bubble regime in pool nucleate boiling, Int. J. Heat Mass Transfer 40 (2) (1997) 365–377. [11] Z.L. Yang, T.N. Dinh, R.R. Nourgaliev, B.R. Sehgal, Numerical investigation of bubble growth and detachment by the Lattice–Boltzmann method, Int. J. Heat Mass Transfer 44 (1) (2001) 195–206. [12] J. Bonjour, M. Clausse, M. Lallemand, Experimental study of the coalescence phenomenon during nucleate pool boiling, Exp. Therm. Fluid Sci. 20 (2000) 180–187. [13] C. Hutter, D.B.R. Kenning, K. Sefiane, T.G. Karayiannis, H. Lin, G. Cummins, A.J. Walton, Experimental pool boiling investigations of FC-72 on silicon with artificial cavities and integrated temperature microsensors, Exp. Therm. Fluid Sci. 34 (4) (2010) 422–433. [14] R. Cole, W. Rohsenow, Correlations for bubble diameters for boiling of saturated liquids, Chem. Eng. Progress, Symp. Ser. 65 (1969) 211–213. [15] S.S. Kutateladze, I.I. Gogonin, Growth rate and detachment diameter of a vapour bubble in free convection boiling of a saturated liquid, High Temp. 17 (1979) 667–671. [16] M.K. Jenson, G.J. Memmel, Evaluation of bubble departure diameter correlations, Proceedings of the eigth International Heat Transfer Conference, San Francisco, USA, vol. 4, 1986, pp. 1907-1912. [17] D. Gorenflo, V. Knabe, V. Bieling, Bubble density in surfaces with nucleate boiling – its influence on heat transfer and burnout heat flux at elevated saturation process, in: Proceedings of the 8th International Heat Transfer Conference, San Francisco, USA, 1986, vol. 4, pp. 1995–2000. [18] M. Shoji, Studies of boiling chaos: a review, Int. J. Heat Mass Transfer 47 (2004) 11051128.
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Nucleation site interaction between artificial cavities during nucleate pool boiling on silicon with integrated micro-heater and temperature micro-sensors C. Hutter a,1, K. Sefiane a,⇑, T.G. Karayiannis b, A.J. Walton a, R.A. Nelson c, D.B.R. Kenning b a
University of Edinburgh, School of Engineering, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, UK Brunel University, School of Engineering and Design, Uxbridge UB8 3PH, Middlesex, UK c Los Alamos National Laboratory, Los Alamos, NM 87545, USA b
a r t i c l e
i n f o
Article history: Received 11 April 2011 Accepted 3 February 2012 Available online 6 March 2012 Keywords: Nucleate pool boiling Bubble interaction Horizontal coalescence Artificial cavity
a b s t r a c t Nucleate boiling is commonly characterised as a very complex and elusive process. Many involved mechanisms are still not fully understood and more detailed consideration is needed. In this study, bubble growth from micro-fabricated artificial cavities with varied spacing on a horizontal 380 lm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using 16 integrated micro-sensors situated on the boiling surface, each with an artificial cavity located in its geometrical centre. Experiments with three different spacings 1.5, 1.2 and 0.84 mm in between cavities with a nominal mouth diameter of 10 lm and a depth of 80 lm were undertaken. To conduct pool boiling experiments, the test section was mounted inside a closed stainless steel boiling chamber with optical access and completely immersed in degassed fluorinert FC-72. Bubble nucleation, growth and detachment at 0.5 and 1 bar absolute pressure were investigated using high-speed imaging. The effect of decreasing inter-site distance on bubble nucleation frequency, bubble departure frequency and diameter with increasing wall superheat is presented. Furthermore, the frequency of horizontal bubble coalescence was determined. The regions of influence on the measured frequencies and bubble departure diameter were compared with recently published findings. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nucleate boiling is one of the most effective heat transfer processes at low temperature differences with a wide range of cooling applications in industry. However, many mechanisms involved in nucleate boiling are still not fully understood and more investigations are required. Chekanov [1] was the first to conduct experiments with two artificial nucleation sites spaced close enough to make interactions between nucleation sites visible. The artificial nucleation sites were created by two heated copper rods in contact with a perm-alloy plate immersed in water. The time between two departed bubbles from neighbouring nucleation sites followed a gamma distribution, commonly used as probability model for waiting times. He introduced three regions of interaction determined by the dimensionless cavity spacing S/Dd where S is the distance between two cavity centres and Dd the average bubble departure diameter. For S/Dd < 3 the formation of a bubble at one nucleation site inhibits the formation of a vapour bubble at the ⇑ Corresponding author. E-mail address: K.Sefi[email protected] (K. Sefiane). Current address: Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. 1
0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2012.02.014
neighbouring nucleation site. For S/Dd > 3 the formation of a bubble at one nucleation site promotes the formation of a vapour bubble at the neighbouring nucleation site and if S/Dd 3 no interaction between the two nucleation sites takes place. Chekanov suggested acoustic and hydrodynamic effects as responsible mechanisms for the observed inhibition and promotion of bubble growth from closely spaced nucleation sites. Judd et al. [2–5] studied in their experiments the interaction phenomena between adjacent nucleation sites. They used a transparent glass surface 3.6 mm thick, coated with a 0.3 lm stannic oxide layer acting as the heater. The system pressure was reduced in order to decrease the number of active nucleation sites, to increase the bubble size and to prevent any damage to the boiling surface. Like Chekanov they introduced three regions of interaction between nucleation sites determined by the dimensionless cavity spacing S/Dd where, Dd is the average bubble departure diameter for the observed time period. S/Dd < 1 is called a ‘‘promotive’’ region, where a nucleation site that is unable to capture vapour nuclei is within the area influenced by a continually active nucleation site that can deposit vapour nuclei in it, also called ‘‘seeding’’. Bubbles will form at the adjacent nucleation site more frequently than would otherwise be the case. 1 < S/Dd < 3 is an ‘‘inhibitive’’ region, where a nucleation site that is unable to capture vapour nuclei is
2770
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
Nomenclature Dd p pl pg rb
average bubble departure diameter (m) pressure (bar) liquid pressure (bar) vapour pressure (bar) nuclei curvature radius (m)
within the area influenced by an intervening active nucleation site capable of depositing/displacing nuclei in it. The nucleation site itself is under the influence of a continually active nucleation site and bubbles will form at the adjacent nucleation site less frequently than would otherwise be the case. The third region is the ‘‘independent’’ region with S/Dd > 3, where bubble formation at one nucleation site is not influenced by the formation of bubbles at a neighbouring nucleation site. Judd and co-authors suggested ‘‘site seeding’’ to be responsible for these phenomena. Kenning and Yan [6] investigated temperature fluctuations in the heated wall during bubble growth. A direct electrically heated stainless steel plate was immersed in water. The plate, 0.13 mm thick and 46 103 mm2 in area, was carefully cleaned and the surface was well wetting with a contact angle less than 20°. The water was degassed in a separate tank, but since the boiling chamber was opened to the atmosphere, the amount of dissolved gas was uncertain. On the back of the plate, a thermochromic liquid crystal layer, which had a visual colourplay from 104 °C to 123 °C, was deposited. The importance of wall temperature variations for bubble growth and activity of nucleation sites was confirmed. The cooling effect on the wall surface was considerable and limited to the maximum contact area of the bubble on the heated surface. The cooling effect decreased closer to the outer limit of the ‘‘area of influence’’. They concluded that nucleation sites inside the maximum occurring diameter of a growing bubble influence each other through thermal fluctuations on the wall surface. This interaction caused intermittence that had a large influence on the production of bubbles at the sites. Golobicˇ and Gjerkeš [7] activated nucleation sites with a laser beam (diameters of heated spot area between 1.66 and 5.23 cm) on a 25 lm thin copper or titanium foil immersed in saturated water. The applied heat flux was up to 560 kW/m2. Spacings between the three or four simultaneously active nucleation sites were ranging from 2.6 to 4.1 mm. They concluded that interactions between two artificially activated sites decrease the overall activity of both sites. The activity at both sites may decrease or it is increased at one site and simultaneously decreased at the other. If there is more than one active site in its vicinity, they influence each others activity. Zhang and Shoji [8] conducted experiments on a silicon surface, which was 0.2 mm thick and had a surface area of a diameter of 15 mm and cylindrical artificial cavities. The cavities are 10 lm in diameter and 80 lm deep. They were arranged as single or twin cavities with different spacing of 1, 2, 3, 4, 6 and 8 mm. The boiling liquid was distilled water and the silicon surface was heated by Nd:YAG laser irradiation. Temperature fluctuations were measured beneath and around cavities with radiation thermometers (spatial resolution 120 lm, temperature resolution 0.08 K and temporal resolution 3 ms). All pictures were captured with a high-speed camera at 1297 frames per second (fps). In their experiments, Zhang and Shoji [8] observed three crucial effects; effect ‘‘A’’ is the hydrodynamic interaction between bubbles, ‘‘B’’ the thermal interaction between nucleation sites and ‘‘C’’ the horizontal and declining bubble coalescence. In the region S/Dd > 3 none of the three interactions can be observed, meaning that the two artificial
S
spacing between two cavities (m) surface tension (N/m)
r
cavities are independent and behave like single cavities would. The region 2 < S/Dd 6 3 is influenced by effect ‘‘A’’ and has a higher bubble departure frequency than region S/Dd > 3. In the region 1.5 < S/ Dd 6 2 effect ‘‘A’’ and ‘‘B’’ are both dominant. The dominance of effect ‘‘B’’ leads to a lower bubble departure frequency. In the region where S/Dd 6 1.5 all three effects have an influence on the behaviour of the process resulting in an increased bubble departure frequency. However, the authors point out that the influence range of thermal interactions is also related to the thermal properties and the thickness of the boiling substrate. Direct bubble/bubble interaction or bubble coalescence was classified by Williamson and El-Genk [9] and later Buyevich and Webbon [10] into three different types, 1) Lateral coalescence far away from the heated wall, which has no effect on boiling heat transfer, 2) Vertical coalescence between consecutive bubbles near the wall and 3) Lateral coalescence between adjacent bubbles near the wall. Yang et al. [11] confirmed the three types in numerical simulations and Zhang and Shoji [8] proposed to add a fourth type, 4) Lateral coalescence between adjacent declining bubbles near the wall. During vertical coalescence a bubble, previously departed from a nucleation site, is merging with the successive bubble growing from the same nucleation site, see Fig. 1(a). Lateral coalescence, also called horizontal, occurs between two bubbles simultaneously growing from two adjacent nucleation sites, see Fig. 1(b). The event of a bubble which has already departed from one nucleation site merging with a bubble still growing from a neighbouring nucleation site is called declining coalescence, see Fig. 1(c). The types 3) and 4) mentioned above are important for the heat transfer process, but only occur when the distance between two nucleation sites is less than 1.5 times the bubble departure diameter. Bonjour et al. [12] characterised the thermal effect of coalescence by conducting experiments with artificial nucleation sites. A vertical duraluminium (AU4G) heated boiling surface was immersed in either pentane for the study of the influence of
(a)
(b)
(c)
Fig. 1. Three types of direct bubble/bubble interaction or bubble coalescence: (a) vertical coalescence, (b) horizontal coalescence and (c) declining coalescence [8,9,11].
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
2771
Fig. 2. Schematic overview of the experimental setup including boiling chamber, condenser, data acquisition, light source and high-speed camera.
coalescence on the boiling curve and bubble departure diameter as a function of the wall superheat, or R-113 to study the influence of coalescence on bubble frequency. Heat fluxes between 103 and 104 kW/m2 or wall superheats between 5 and 35 K were applied. For moderate heat fluxes, the bubble frequency decreased, but for low or high heat fluxes it increased. Coalescence improved the heat transfer coefficient, which was attributed to supplementary micro-layer evaporation, forming below two merging bubbles. However, the highest heat transfer coefficient was measured for an optimal site distance (1.05 mm < S < 1.5 mm), for which no coalescence took place. It is very clear that there is a lack of experimental data in a controlled environment. Therefore the determination of the influence of spacing between two nucleation sites requires well defined and exactly positioned artificial cavities. Currently the best way to produce such artificial cavities is by means of making use of micro-fabrication. In this study the influence of spacing between neighbouring cavities on bubble nucleation frequency, bubble departure frequency, horizontal bubble coalescence frequency and bubble departure diameter were investigated with the help of micro-fabricated silicon substrates.
Fig. 3. Location of the 16 sensors on the top surface of the silicon device, including the connections to the 64 pads on the left and right side of the chip. A single cavity was at the geometrical centre of each sensor. A close-up of the section detail A can be found in Fig. 4.
2. Experimental setup and procedures Pool boiling experiments were conducted in a stainless steel boiling chamber with connected external condenser, Fig. 2. The
Fig. 4. Detail A of Fig. 3 with distances between artificial cavities (drawing not to scale).
Fig. 5. (a) The image shows a part of the square layout (84 84 lm) of the Ti/Ni sensor with an artificial cavity in its centre. (b) Detail B (a) shows an artificial cavity with a nominal mouth diameter of 10 lm and a depth of 80 lm in the geometrical centre of a micro-sensor.
2772
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
Fig. 6. Cross-sectional view of the jig designed to hold the silicon device in place and to physically connect the micro-sensors and the heater with embedded spring probes.
chamber had a support heating system consisting of four cartridge heaters implemented in the bottom and two flexible silicone heaters wrapped around the wall of the chamber (not indicated in Fig. 2). This was used to heat up and keep the boiling liquid, fluorinert FC-72, at saturation temperature for the set pressure. The support heating system was also used to degas the liquid prior to
(a)
each experiment for at least two hours. The vapour generated during the degassing and boiling experiments was directed to an external condenser, which was connected to a chilled bath. The cooling water flow rate and temperature were used to control the system pressure. The boiling substrate was a silicon wafer with a micro-fabricated heater on the back and 16 temperature micro-
(b)
(c)
Fig. 7. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.5 mm between the two cavities S5 and S6 at 0.5 bar.
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2773
(b)
(c)
Fig. 8. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.5 mm between the two cavities S5 and S6 at 1 bar.
sensors, with an artificial cavity etched in the geometrical centre of each sensor layout, distributed in two rows on the front side, Figs. 3 and 4 [13]. The chip size was 50 50 mm2, whereas the heater covered an area of 40 37 mm2. The heater had a serpentine layout with 8 turns. Heat flux up to 37.8 kW/m2 was used to start saturated nucleate boiling on the substrate. After the onset of boiling, the power was decreased to 21.3 kW/m2 and kept at this power for 30 min to degas the test section. The power supply was then switched off and after a waiting time of 15 min, boiling was initiated again. The power was then immediately reduced to maintain the chosen wall temperature superheat. Heat fluxes from 0.5 to 5.7 kW/m2 with a measurement error of 1% were applied during these experiments. Bubble growth was now limited to the location of the artificial cavities. The cylindrical cavities had a nominal diameter of 10 lm and were 80 lm deep. They were arranged in pairs, in which the spacing between two cavities was varied between 0.84, 1.2 and 1.5 mm. In this study only cavities at S1-S6 were used, Figs. 3 and 4. The distance between any two pairs of artificial cavities was 3 mm, which, for bubble departure diameters of 1 mm, was exactly at the limit beyond which two neighbouring nucleation sites do not interact with each other, following Judd et al. [2–5] and Zhang and Shoji [8]. For the presented experiments only a few measured bubble diameters exceeded 1 mm. The first and second row of nucleation sites had a distance of 10 mm in between and did not interact. Thus, any interaction between sites other than the investigated pairs could be neglected. The square temperature micro-sensors had a width of 0.84 mm, a nominal resistance of 0.6 X and each had four connections, Fig. 5 (a) and
(b). Two were used to send a constant current through and two to measure voltage across each sensor with a data acquisition at a frequency of 1 kHz. The resistance and accordingly the voltage changed with temperature. More details of the silicon device micro-fabrication can be found in Hutter et al. [13]. The micro-sensors were calibrated using T-type thermocouples and had an accuracy of ±0.5 K. The T-type thermocouples were calibrated with a high precision thermometer (F250MkII, Automatic Systems Laboratories), which had an accuracy of ±0.01 K between 20 °C and 85 °C. Before the test section was positioned in its holder, Fig. 6, it was always cleaned for 30 min in an ultrasonic bath, rinsed with deionised water for 10 min and dried with pure nitrogen. Bubble growth was recorded with a high-speed camera (IDT Nanosense MkIII, 1280 1024 pixel at 1040 fps) working at 1000 fps [13]. The bubble nucleation frequency, bubble departure frequency, horizontal coalescence frequency and bubble departure diameter were determined from 0.815 s long high-speed image sequences. The bubble departure frequency was the bubble nucleation frequency minus the frequency of vertical coalescence. For frequency measurements, the measurement error was estimated to be ±2 events and for the departure diameter ±2 px, whereas the standard deviation of the average bubble diameter was indicated for the cases where it was larger than the estimated measurement error. The estimated measurement error represents the maximum deviation measured during analysing the same highspeed image sequence 100 times. The distance between the camera and the observed nucleation site may vary slightly for each experiment, hence the calibration will as well. Therefore, the mea-
2774
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
(b)
(c)
Fig. 9. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.2 mm between the two cavities S3 and S4 at 0.5 bar.
surement error is given here in pixel. The boiling substrate was illuminated by a continuously working coherent backlight. Due to the design of the jig, that was used to hold the test section in position, the high speed camera had to be inclined at an angle of approximately 20°. This did not influence the measured diameters as they represent for all measurements the apparent diameter, where the bubbles were widest. All experiments were conducted at saturated conditions. Saturated boiling was taking place when the T-type thermocouples, indicating the vapour and liquid temperature, showed the same temperature. Simultaneously to the imaging, the wall superheat temperature was measured with the integrated temperature micro-sensors and the heat flux applied to the boiling substrate calculated from the power supplied to the integrated heater. After one sequence was recorded, the integrated heater was switched off for 15 min, before a new wall superheat was set for further measurements and recording.
place for the closest spaced nucleation sites at the lowest pressure of 0.5 bar. 3.1. Two adjacent active artificial nucleation sites In Figs. 7–12(a)–(c) the bubble departure diameter, the bubble nucleation frequency and the bubble departure frequency are plotted as a function of the wall superheat for a cavity spacing of 1.5, 1.2 and 0.84 mm at 0.5 and 1 bar. The wall superheat represents the average temperature during the captured time period from the sensor at the corresponding cavity. The limit for the lowest wall superheat was the temperature to keep the cavities active, which is in good agreement with the theoretical minimum temperature for a cavity to trap vapour nuclei calculated from the Laplace–Young equation
pg pl ¼ 3. Results and discussion The experiments presented in this paper were conducted at 0.5 and 1 bar. Decreasing pressure increases the boiling inception temperature and leads to larger bubble departure diameters. Reducing the pressure increases the possibility of bubble interactions and is one way to reduce the number of 10 micro-fabricated test sections. The observed interactions in this paper are vertical and horizontal coalescence, Fig. 1(a) and (b). Vertical coalescence may appear at one of two adjacent sites or simultaneously at both adjacent sites. Vertical coalescence was observed for all experimental conditions and cavity spacings. However, horizontal coalescence only took
2r rb
ð1Þ
where pg is the vapour pressure, pl the pressure of the liquid, r the surface tension and rb the nuclei curvature radius which equals the cavity mouth radius rc. In order for the nucleus not to shrink, its internal temperature must equal the saturation temperature for the pressure of the vapour phase calculated from Eq. (1). The theoretical minimum temperature for 0.5 and 1 bar are 1.96 and 0.93 K, respectively. The wall superheat was increased until unwanted (other than the ones under observance) new nucleation sites became active. The measured parameters for both of the neighbouring cavities behave very similarly at 0.5 and 1 bar for a cavity spacing of 1.5 mm. The following discussion focuses first
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2775
(b)
(c)
Fig. 10. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 1.2 mm between the two cavities S3 and S4 at 1 bar.
on the bubble departure diameter and then on nucleation and departure frequency. The bubble departure diameter is found to increase in a nearlinear fashion with increasing superheat for 0.5 and 1 bar for all considered cavity spacings of 1.5, 1.2 and 0.84 mm, Figs. 7–12(a). The departure diameters for the same wall superheats are slightly larger at lower pressure for cavities with a distance of 1.5 and 1.2 mm in between. However, this is only apparent when the linear trend lines are compared, Figs. 7–10(a). Unlike for larger spacings, for the 0.84 mm spacing no significant difference in size is detectable, Figs. 11 and 12(a). 3.2. Departure diameter For the departure diameter there is a small increase for the two widest spacings between the two cavities. For cavity spacings of 0.84 and 1.2 mm the difference in departure diameter is very small and can be treated as insignificant. The difference between the two closest spaced cases and the widest spacings is increasing with wall superheat and ranges from 0.05 to 0.24 mm for 0.5 bar and from 0.07 to 0.16 mm for 1 bar. The departure diameter reaches slightly higher values for the same superheat at 0.5 bar. All these differences are very small and are only valid for the comparison of the linear trend lines of the departure diameter measurement with increasing wall superheat for each cavity. The linear trend lines for the departure diameter do not intercept the origin. The first bubbles appear slightly above the earlier mentioned theoretical minimum temperature. Many of the expressions reported in the literature for the bubble departure diameter
obtained empirically or analytically are not consistent. In some sources the bubble departure diameter is increasing with wall superheat, in others it is insensitive to or it even decreases with increasing wall superheat. In Cole and Rohsenow [14], Kutateladze and Gogonin [15], Jenson and Memmel [16] and Gorenflo et al. [17] all expressions are based on the Jacob number. In all these studies the departure diameter is increasing with increasing wall temperature. The reason for the use of micro-fabricated artificial cavities is the control of the location and to increase the consistency of the performed experiments. To up-scale the results to a real surface is not yet possible due the complexity of the geometry of those sites. A thorough discussion may be found in Shoji [18]. At the closest spacing between two artificial cavities the wall superheat is lowest for all considered dimensionless spacings, S/ Dd. At 0.5 bar the wall superheat may be influenced by the spacing S. For a distance of 1.2 mm the wall superheat is higher than at 1.5 and 0.84 mm. This seems not to be true at 1.0 bar and S/Dd > 3.0, see Table 1. This observation suggests interactions between sites may be influencing the departure diameter, in which case the S/ Dd approach as proposed in the literature becomes complicated and is inefficient to describe the phenomenon as other effects interfere. The interaction regions suggested by Zhang and Shoji [8] are included in the bubble departure diameter plots, Figs. 7–12(a). From the intersection points between the limits of these regions (horizontal lines in the bubble departure diameter plots) and the linear trend lines for bubble departure diameter for each cavity, the two corresponding wall superheats for the two cavities were determined and averaged (wherever there was a discrepancy).
2776
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
(b)
(c)
Fig. 11. (a) Bubble departure diameter, (b) bubble nucleation frequency and (c) bubble departure frequency as a function of wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 0.5 bar.
3.3. Interaction between neighbouring sites The difference between the two superheats for the two cavities was always below 1 K. From this it was possible to determine the interaction region limits (vertical lines) due to spacing on the bubble nucleation frequency and the bubble departure frequency at corresponding temperatures, Figs. 7–12(b) and (c). These regions of interaction are based on the general assumption that interactions should scale with S/Dd, which will be tested in this paper. The average departure diameter Dd represents the departure diameter of 5 bubbles for each set wall superheat and pressure for each observed artificial nucleation site. The departure diameter represents only bubbles where no vertical or horizontal coalescence took place previously. As observed in previous studies, Hutter et al. [13] the bubble nucleation frequency for a cavity spacing of 1.5 mm initially increases rapidly, then appears to level off, before it slightly drops for the highest wall superheats. This behaviour is more pronounced at 0.5 bar and the small drop of frequency only noted at the same pressure, Figs. 7 and 8(b). The initial rapid increase, which flattens with higher superheats, can also be seen for the bubble departure frequency at 0.5 bar, Fig. 7(c). Frequency measurements are less scattered if vertical coalescence is not considered. In Figs. 9 and 10(b) and (c) the bubble nucleation frequency and the bubble departure frequency for two cavities, spaced 1.2 mm from each other, are plotted for the same two pressures with increasing wall superheat. The two neighbouring cavities, again, behave very similarly. After the sharp initial increase, the bubble
nucleation frequency at 0.5 bar reaches a maximum, before it starts to decrease for superheats above 8 K, Fig. 9(b). The slight decrease for higher wall superheats is still noticeable if the vertical coalescence is not considered, Fig. 9(c). The departure frequency appears constant for superheats above 4 K at 1 bar, Fig. 10(c). In Figs. 11 and 12(b) and (c) the same parameters are presented for two neighbouring cavities with a spacing of 0.84 mm for 0.5 and 1 bar. The bubble nucleation frequency seems to drop above 9 K, however, if the vertical coalescence is not considered, the frequency appears to be almost constant above 4 K wall superheat at 0.5 bar, Figs. 11 and 12(b). Fig. 12(b) and (c) are very similar to Fig. 10(b) and (c) and no influence of the spacing is apparent. At 0.5 bar, all frequency plots look similar in Figs. 7, 9 and 11 with a break point around 3 K of wall superheat but different values of S/Dd. At 1 bar, all frequency plots in Figs. 8, 10 and 12 look similar. The notable feature is the large scatter in nucleation frequency with increasing wall superheat but not in departure frequency. This indicates that the dependency of the vertical coalescence on wall superheat is less clear and adds uncertainty to the experimental observations. Experimental bubble frequencies measured by Zhang and Shoji [8] were strongly influenced by the variation of S/Dd at the applied heat fluxes. The heat fluxes applied in this study are typically one order of magnitude smaller as compared to those of Zhang and Shoji, but the latent heat of evaporation at the normal boiling temperature is around 24 times smaller for FC-72 as compared to that of water. For S/Dd equal to 0.5 the frequency was about three times higher than for 1.5 and around 1.75 times higher than at 2.5 for the highest applied heat flux of 37.2 kW/m2. The measured nucleation
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
(a)
2777
(b)
(c)
Fig. 12. (a) Bubble nucleation frequency, (b) bubble departure frequency, (c) bubble departure diameter as a function of wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 1 bar.
Table 1 Averaged wall superheats at which the linear trend lines intersect the departure diameters confining the interaction regions suggested by Zhang and Shoji [8]. S/Dd
S = 1.5 mm
S = 1.2 mm
S = 0.84 mm
p = 0.5 bar 1.5 2.0 3.0
10.4 6.6 2.7
11.3 7.5 3.6
6.7 3.2 <1
p = 1.0 bar 1.5 2.0 3.0
>14 9.9 4.9
>14 9.8 5.0
8.9 5.5 2.1
and departure frequency seem not to be influenced by decreasing spacing, or any interactions for the applied heat flux are minor and hidden in the rather scattered data. 3.4. Horizontal coalescence In this paper the occurrence of horizontal coalescence was also investigated. Horizontal coalescence took place only for the two closest spaced cavities S1 and S2 at 0.5 bar. In Fig. 13 the frequency of horizontal coalescence is presented as a function of increasing wall superheat. Bubbles growing from S1 and S2 only merged above a wall superheat of 8 K and showed no clear pattern with further increasing superheat. At the highest measured superheat the frequency reached its maximum, slightly above 60 Hz. Vertical
Fig. 13. Frequency of horizontal coalescence as a function of increasing wall superheat with a spacing of 0.84 mm between the two cavities S1 and S2 at 0.5 bar. Regions of influence, following Zhang and Shoji [8], are indicated.
coalescence occurred in some cases for each nucleation site prior to horizontal coalescence. After two bubbles coalesced horizontally no further vertical coalescence occurred before departure. Zhang and Shoji [8] reported horizontal coalescence for spacing S/Dd smaller than 1.5. Measured frequencies for S/Dd equal to 1 are high-
2778
C. Hutter et al. / International Journal of Heat and Mass Transfer 55 (2012) 2769–2778
er than frequencies at 12 K, the corresponding temperature for S/Dd equal to 1.1 for these experiments. However, Zhang and Shoji do not report any reached wall superheats nor distinguish between horizontal and declining coalescence, and the latter was not considered here. 4. Conclusions Experimentally measured bubble nucleation frequency, departure frequency, frequency of horizontal coalescence and bubble departure diameter of bubble growth from pairs of artificial cavities with three different values of spacing were measured with increasing wall superheat at pressures of 0.5 and 1 bar. Interaction between neighbouring cavities was investigated by considering the bubble departure frequencies. Bubble nucleation frequency, departure frequency and bubble departure diameter were very similar for the pairs of artificial cavities. The lower pressure shifted slightly the regions of influence suggested by Zhang and Shoji [8] to lower superheat, due to the small increase in departure diameter at 0.5 bar. The strong ‘‘inhibitive’’ and ‘‘promotive’’ regions for the departure frequency with decreasing spacing between cavities have not been observed in the present study. An increase in departure diameter with increasing spacing was observed. However, it is difficult to compare the results of Zhang and Shoji with different applied heat fluxes. Also the differences between liquid properties might influence the boundaries of the region of influence. Horizontal coalescence of bubbles from neighbouring cavities was only observed for the closest spacing at 0.5 bar for superheats above 8 K. Thermal interaction through the substrate has not been studied, however, we believe this might play an important role. Furthermore, the criterion used for bubbles interactions in the literature, namely the dimensionless spacing may not be sufficient. A more complete criterion must include the thermal properties of the substrate. Acknowledgements This work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) by grant EP/C532813/1. The authors are grateful to Dr. H. Lin and Dr. G. Cummins for the silicon device fabrication.
References [1] V.V. Chekanov, Interaction of centers in nucleate boiling, High Temp. 15 (1) (1977) 101–106. [2] R.L. Judd, C.H. Lavdas, The nature of nucleation site interaction, J. Heat Transfer – Trans. ASME 102 (3) (1980) 461–464. 384. [3] A. Calka, R.L. Judd, Some aspects of the interaction among nucleation sites during saturated nucleate boiling, Int. J. Heat Mass Transfer 28 (12) (1985) 2331–2342. [4] R.L. Judd, On nucleation site interaction, J. Heat Transfer – Trans. ASME 110 (2) (1988) 475–478. [5] R.L. Judd, A. Chopra, Interaction of the nucleation processes occurring at adjacent nucleation sites, J. Heat Transfer – Trans. ASME 115 (4) (1993) 955– 962. [6] D.B.R. Kenning, Y. Yan, Pool boiling heat transfer 393 on a thin plate: features revealed by liquid crystal thermography, Int. J. Heat Mass Transfer 39 (15) (1996) 3117–3137. [7] I. Golobicˇ, H. Gjerkeš, Interactions between laser-activated nucleation sites in pool boiling, Int. J. Heat Mass Transfer 44 (1) (2001) 143–153. [8] L. Zhang, M. Shoji, Nucleation site interaction in pool boiling on the artificial surface, Int. J. Heat Mass Transfer 46 (3) (2003) 513–522. [9] C.R. Williamson, M.S. El-Genk, High-speed photographic analysis of saturated nucleate pool boiling at low heat flux, ASME Winter Annual Meeting, Paper 91WA-HT-8, 1991. [10] Y.A. Buyevich, B.W. Webbon, The isolated bubble regime in pool nucleate boiling, Int. J. Heat Mass Transfer 40 (2) (1997) 365–377. [11] Z.L. Yang, T.N. Dinh, R.R. Nourgaliev, B.R. Sehgal, Numerical investigation of bubble growth and detachment by the Lattice–Boltzmann method, Int. J. Heat Mass Transfer 44 (1) (2001) 195–206. [12] J. Bonjour, M. Clausse, M. Lallemand, Experimental study of the coalescence phenomenon during nucleate pool boiling, Exp. Therm. Fluid Sci. 20 (2000) 180–187. [13] C. Hutter, D.B.R. Kenning, K. Sefiane, T.G. Karayiannis, H. Lin, G. Cummins, A.J. Walton, Experimental pool boiling investigations of FC-72 on silicon with artificial cavities and integrated temperature microsensors, Exp. Therm. Fluid Sci. 34 (4) (2010) 422–433. [14] R. Cole, W. Rohsenow, Correlations for bubble diameters for boiling of saturated liquids, Chem. Eng. Progress, Symp. Ser. 65 (1969) 211–213. [15] S.S. Kutateladze, I.I. Gogonin, Growth rate and detachment diameter of a vapour bubble in free convection boiling of a saturated liquid, High Temp. 17 (1979) 667–671. [16] M.K. Jenson, G.J. Memmel, Evaluation of bubble departure diameter correlations, Proceedings of the eigth International Heat Transfer Conference, San Francisco, USA, vol. 4, 1986, pp. 1907-1912. [17] D. Gorenflo, V. Knabe, V. Bieling, Bubble density in surfaces with nucleate boiling – its influence on heat transfer and burnout heat flux at elevated saturation process, in: Proceedings of the 8th International Heat Transfer Conference, San Francisco, USA, 1986, vol. 4, pp. 1995–2000. [18] M. Shoji, Studies of boiling chaos: a review, Int. J. Heat Mass Transfer 47 (2004) 11051128.