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Visualization study on the bubble behavior on a downward facing hemispherical surface during saturated pool boiling
International Journal of Heat and Mass Transfer 135 (2019) 1013–1022
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Visualization study on the bubble behavior on a downward facing hemispherical surface during saturated pool boiling Fei Qin a, Xiang Zhang b, Deqi Chen a,⇑, Lian Hu a, Fan Bill Cheung c a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China State Nuclear Power Technology Research & Development Center, South Area, Future Science and Technology Park, Beijing 102209, China c Department of Mechanical & Nuclear Engineering, Pennsylvania State University, PA, USA b
a r t i c l e
i n f o
Article history: Received 2 October 2018 Received in revised form 13 February 2019 Accepted 14 February 2019
Keywords: Pool boiling Downward facing Bubble dynamics Critical heat flux
a b s t r a c t Saturated pool boiling experiments were conducted in a scale-down three-dimensional downward facing hemispherical vessel. In this study, a high-speed digital video camera was employed to capture the bubble behavior along the overheated convex surface. The general morphology of coalescent bubble at the bottom center was analyzed based on the observed pictures and videos. The visualization results showed that the bubble behavior along the hemispherical curved surface was quite different from that on the plane and upward surfaces. It was found that the coalescent bubble was cyclical repeatedly forming a stratified vapor layer, and then randomly sliding upward along the convex surface. The downward facing boiling heat flux was directly related to the duration of the boiling cycles, which were obtained based on the visualization by high speed camera and the image processing technique. A Matlab program was developed to recognize and analyze the images, as well as to calculate the characteristic parameters of the coalescent bubble. The boiling cycle was almost constant during the nucleate boiling regime, which was independent with the heat flux level. This study also provided an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that could be even useful for hydrodynamic modeling of the CHF phenomenon. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nuclear power is regarded to be a clean and reliable source of energy. With proper design, nuclear power plants are able to provide safe and consistent power under well-controlled operating conditions. Whereas, it is pretty dangerous if the radioactive materials releasing to the environment under severe accidents. To assure the public of its safety, the issue of Severe Accident Management (SAM) needs to be addressed seriously. That is the so-called In-Vessel Retention (IVR), one key SAM strategy that adopted by some operating nuclear power plants and Advanced Light Water Reactors (ALWRs). During a severe accident, a significant amount of corium can become molten and relocate to the bottom head of the Reactor Pressure Vessel (RPV), as happened in TMI-2 accident. The concept of IVR by passive External Reactor Vessel Cooling (ERVC) in a flooded cavity during severe accidents is a feasible approach to retaining radioactive core melt within the vessel [1]. The reactor cavity flooding is selected as the effective ERVC method because ⇑ Corresponding author. E-mail address: [email protected] (D. Chen). https://doi.org/10.1016/j.ijheatmasstransfer.2019.02.043 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
of the relatively simple installation. As long as the heat flux at the lower head of RPV is lower than the Critical Heat Flux (CHF) limit at the corresponding point for the downward facing boiling process, nucleate boiling will be the prevailing regime [2]. In that case, the temperature of outer surface can be maintained near the saturated temperature of water such that the reactor vessel is able to be sufficiently cooled to sustain its thermal and structural integrity. Therefore, it is important to comprehend the physical phenomena during the IVR-ERVC process. In the past thirty years, there were many experimental and theoretical studies to investigate the ERVC boiling process including the effect of inclination angle on the CHF. Based on those experimental results, there were also many different empirical or semi-empirical correlations developed for CHF by fitting the data. Especially for the engineering application, two-dimensional fullscale test facilities were built to experimentally verify and validate the feasibility of ERVC, such as ULPU series for AP600 and AP1000 in the United States [3,4], KAIST facility for APR1400 in Korea [5,6] and FIRM facility for CAP1400 in China[7]. Whereas, for the veritable physical phenomena of IVR-ERVC under severe accident conditions, there were some representative differences between the 2D simulative facilities and actual 3D boiling phenomenon on the
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outside surface of the convex vessel, for example, the 3D bottom center effect, the upstream two phase flow behavior, and the local CHF variations. Literatures were reported to investigate the pool boiling bubble behavior and CHF limit for the downward facing hemispherical structures. Chu et al. [8] developed the CYBL (Cylindrical Boiling) facility at Sandia National Laboratory which was a full simulation of reactor scale to perform confirmatory tests on the effectiveness of the downward facing pool boiling heat transfer process for the flooded cavity design. The simulated reactor vessel had a cylindrical diameter of 3.7 m and a torispherical bottom head. There were both steady state and quenching experiments carried on the CYBL test matrix. The visualizations of boiling process exhibited a cyclic nature of vapor dynamics in the nucleate downward facing pool boiling and two-phase flow along the heated surface. The results showed that the AP600 flooded cavity should be capable of cooling the reactor vessel in central region under the prototypical heat load and heat flux distributions. Additional quenching experiments of downward facing pool boiling with a copper substrate were performed by El-Genk et al. [9] in saturated water using two test sections of different thicknesses. Boiling curves were derived at six locations on the bottom center region and then at a local inclination angle of approximately 8° along the outer surface of the test section. The heat flux increased with increasing h in the lower heat flux region and decreased with increasing values of h at very high heat fluxes. Interestingly, the thickness of the copper heater had no effect on the observed CHF values. CHF occurred sequentially from the lower most to the highest positions in that order. As the thickness of the test section increased, the difference in time between subsequent CHF occurrences also increased. Kim et al. [10] experimentally measured and analyzed the film boiling heat transfer coefficients for a downward facing hemispherical surface from the quenching tests using DELTA facility in Seoul National University. Two test sections were made of copper to maintain low Biot numbers, and the outer diameters of the hemispheres were 120 and 294 mm, respectively. The effect of diameter on film boiling heat transfer was quantified utilizing results obtained from the test sections. The measured heat transfer coefficients were found to be greater than those predicted by the conventional laminar flow theory on account of the interfacial wavy motion incurred by the Helmholtz instability. The visualization study depicted clear-cut quenching process on the downward facing hemisphere, and was utilized to facilitate theorizing the film boiling on the curved surface. Suh [11] constructed CASA (Corium Ablation Stopper Apparatus) facility in Seoul National University, which had a downward facing hemispherical vessel and geometrically asymmetric thermal insulator of the APR1400 scaled linearly down by 1/10, to investigate the CHF with saturated deionized water at the atmospheric pressure and three-dimensional random flow effect during the severe accident. The heated vessel played a pivotal role in CASA depending on the configuration of the oxide pool and metal layer to bring about the focusing effect expected of a molten pool in the lower head during a severe accident. The results showed that CHF was in the vicinity of 1.5 MW/m2 in the metal layer with the focusing effect, which was found to be lower than that in the ULPU-2400 configuration V data. Cheung et al. [12,13] studied the downward facing boiling and CHF phenomena on the outer surface of a hemispherical vessel using SBLB (Sub-scale Boundary Layer Boiling) test facility at Pennsylvania State University. The facility provided a three-dimensional simulation of the pool boiling process on the outer surface of a reactor vessel with and without surrounding insulation structure. A significant spatial variation of the CHF, with the local CHF limit increasing monotonically from the bottom center to the equator
of the vessel, was measured for both saturated and subcooled boiling. For the insulation configurations of AP600 and APR1400, an internal upward co-current two-phase flow was both observed in the annular channel between the insulation structure and the test vessel [14]. At the same heat flux level, more flow was induced along the vessel outer surface for the case with insulation compared to the case without. As a result, the nucleate boiling heat transfer and CHF for the case with insulation were found to be consistently higher than those for the corresponding case without. These differences in the nucleate boiling heat transfer and the CHF were attributed to the buoyancy-driven two-phase flow effect. Dizon et al. [15] investigated two different approaches to enhance the cooling capacity of APR1400 reactor vessel with insulation at SBLB facility. The first method involved the usage of an enhanced insulation structure to improve the steam venting through the annular bottleneck channel. The second one involved the usage of a micro-porous media coating on the outer surface of the vessel to promote the downward facing pool boiling heat transfer. A substantial increase on the local CHF was both observed by the two approached. Subsequently, Sohag et al. [16,17] developed a new and versatile coating technique called ‘‘cold spray” which did not require heat treatment and could be used in the existing and new sites to form a micro-porous coating on a hemispherical test vessel made of stainless steel. Both quenching and steady state experiments were performed under simulated IVRERVC conditions using the cold-sprayed coated vessel to quantitatively determine the amount of downward facing boiling heat transfer and CHF enhancement over the corresponding case without surface coating. Visualization of the quenching process for bare and coated vessels, under the identical working conditions (i.e., same initial vessel temperature and saturated water temperature) were performed to identify the differences in the downward facing boiling and CHF variations. The experimental results showed that micro-porous coatings evidently enhanced the heat transfer and CHF values than the bare surface. Moreover, the coatings showed a relatively high degree of durability and fatigue resistance after exposing to multiple thermal shocks experienced during the CHF boiling tests. Lu et al. [18] built a three-dimensional visually experimental apparatus to investigate the downward facing boiling phenomena and CHF limits of IVR-ERVC for CAP1400. The vapor morphology at different heat flux levels and inclination angles of the hemispherical downward-facing surface was carefully studied. The visualized results displayed significant differences in vapor morphology between 3-D and 2-D slice ERVC experiments in the bottom central region. Vapor motion characteristics corresponding to the occurrence of CHF were examined by determining the boiling cycle and maximum vapor height. The effect of inclination angle on the vapor morphology was identified and local variation of CHF along the vessel outer surface was also measured. The effect of bottom heating on the downstream CHF behavior was experimentally investigated, and the mechanism responsible for the influence that was revealed by the vapor morphology was determined. Zhong et al. [19] designed a heating system with liquid metal as the intermediate heat transfer medium to simulate a scaled-down three-dimensional reactor vessel. The liquid metal was heated by DC power and then circulated inside the simulative hemispherical vessel. The objective of the study was to investigate the boiling regimes and heat fluxes on the outer surface of the hemispherical lower head. The boiling heat transfer was experimentally studied on a hemispherical plain surface and interconnected grooves with triangular cavities surface using saturated deionized water at atmospheric pressure. The results showed that CHF on the plain surface at the inclination angle of 85° was 857.3 kW/m2, with no boiling crisis observed on the structured surface up to the highest heat flux of 1366.9 kW/m2. The CHF enhancement on the
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mocouple data. In their study, wall temperature before immersion was well above the minimum film boiling temperature to investigate the effects of liquid sub-cooling and angular location on film boiling. The vapor film thickness was varied between 1 and 2 mm at the saturated pool boiling of the downward facing hemisphere. All of the aforementioned test facilities provided a simulation of the downward facing boiling and experimental study on the CHF limits during the ERVC conditions. There were also some visualized
Fig. 1. Schematic diagram of the experimental apparatus.
Fig. 3. Schematic diagram of the heating blocks.
Fig. 2. Pictorial view of the water tank and windows assembly.
structured surface was more than 59% greater than the one on the plain surface at the inclination angle of 85°. The analytical results showed that the structured surface formed a liquid-vapor conversion path with the cavities as stable nucleation sites and the interconnected grooves as cooling water supply pathways. Thus, the structured surface significantly enhanced the boiling heat transfer and the CHF. Most recently, Beck et al. [20,21] performed quenching experiments on SBLB facility and observed the film boiling phenomena using high-speed video. HTC was calculated by making use of the visualizations. An image processing technique utilizing thresholding and edge detection was useful for extracting the interfacial vapor film thickness. With the modeled HTC, the heat flux in film boiling was compared using a traditional lumped capacitance method, and a new method that utilized both the video and ther-
Fig. 4. Steady-state experimental procedure.
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(a) 0.22MW/m2
(b) 0.36MW/m2
(e) 0.58MW/m2
(f) 0.73MW/m2
(c) 0.51MW/m2
(g) 0.87MW/m2
Fig. 5. Vapor behavior at different heat flux levels.
Fig. 6. Observation of transition boiling after reaching CHF limit.
Fig. 7. Temperature history of typical boiling curves at the bottom center.
investigations in both quenching and steady state experiments. Those observations mostly qualitatively described the boiling phenomena during IVR-ERVC conditions. However, quantitatively analysis should be utilized to discuss the two-phase flow and bubble dynamic behaviors, especially for steady state experiments. In addition, few studies were reported in the downward facing hemispherical structure using the image processing technique. Only Beck et al. investigated the vapor film dynamics and heat transfer during the quenching experiments, not steady-state ones. Whereas, the steady-state experiments played a key role in understanding IVR-ERVC conditions. In this paper, a scale-down three-dimensional downward facing pool boiling facility was conducted to investigate the two-phase natural circulation flow characteristics. The general morphology of the coalescent bubbles was quantitatively analyzed based on the visualizations. A Matlab program was developed to recognize and analyze the digital pictures by using the advanced image processing technique. This study also provided an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that could be even useful for hydrodynamic modeling of the CHF phenomenon.
2. Experimental method 2.1. Experimental setup Steady state experiments were performed in the threedimensional test facility to simulate the downward facing pool boiling phenomena on the outer surface of a scaled-down hemispherical vessel. Cheung [12–15] had illustrated the scaling analysis and structures of SBLB facility very carefully, and the design mentality of current experimental setup was referred to those literatures. The test facility shown schematically in Fig. 1 consisted of a water tank, condenser, hemispherical test vessel, immersion heaters, simulated insulation structure, liquidometer, data acquisition system, DC power system, and a high-speed video camera. The water tank was cylindrical in shape, 3 m in height and 1.5 m in diameter. The height of 3 m was able to simulate the liquid head effect on the natural circulation and boiling heat transfer phenomena. Four large viewing windows were located axialsymmetrically on each side of the tank for visualization of boiling
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(a) t = 20ms
(c) t = 60ms
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(b) t = 40ms
(d) t = 80ms
(e) t = 100ms
(f) t = 120ms
Fig. 8. Typical boiling process observed in the bottom center region.
process on the vessel outer surface by the high-speed camera. The visualizations were recorded by a FR-Stream 625 high speed video camera. The frame rate of this camera could reach 500fps at the resolution of 1280 1024 pixels. The test facility was able to be operated with and without insulation structure. This paper described the visualized results obtained from the later situation. Fig. 2 showed the pictorial view of the water tank and windows assembly. The tank was equipped with four immersion heaters, which were capable of supplying a total of 40 kW to preheat the water inside the tank. Several resistance thermometers were located in the tank to measure the temperature of the water. In addition, there was a reflux condenser assembly located on the top of the tank. The test vessel was comprised of two main parts: the lower part consisting of a heated bottom-head vessel and the upper part consisting of a non-heated cylindrical wall. The vessel with the diameter of 297 mm was fastened to the upper cylinder through the threaded connection. The boiling surface was made of oxygenfree copper, which was very smooth after polishing with the roughness of 6.3 lm. There were two circular heating zones for
the bottom region and three rectangular ones for the upper region. The three upper heating zones were identical at each lateral side. Within each heating zone, 4-mm grooves were cut in order to reduce the heat conduction between different zones. Fig. 3 showed the schematic diagram of the heating zones. There were totally 185 cartridge heaters of 600 W each inserted in the vessel, leading to the heat flux levels up to 1.5 MW/m2 of the bottom region and 2.0 MW/m2 of the upper region. K-type thermocouples were embedded in the copper to monitor the temperatures. 2.2. Experimental procedure Prior to the steady-state experiments, the water tank was filled with deionized water and preheated to the saturated temperature by four 10 kW immersion heaters. Heating usually required between 4 and 5 h to reach saturation. The temperature would be maintained for a few minutes to degas the water. The steadystate experimental procedure was shown in Fig. 4. Once the water was heated to the desired temperature, the power system was then turned on. For each heat flux level of reaching the steady state, the camera recorded the visualized results for at least five minutes.
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Heating power in the test section was gradually increased by slowly increasing the voltage. Every time the voltage was increased, it was necessary to check the stability of temperature fluctuations and the boiling phenomena. At least two consecutive runs were conducted for each condition. The step increment of heating power was gradually decreased when approaching CHF in order to catch the boiling crisis more precisely. It was at the CHF onset point that a sudden increase (over 50 °C/min) of wall temperature on the test section happened. Once CHF was approached, the power was switched off immediately to protect the safety of test section. From a conservative-point-of-view, the last power step before CHF occurrence was used to calculate the resulted CHF values. 2.3. Measuring error and uncertainty analysis The spot where the occurrence of CHF seemed to be random. In measuring the temperatures of the test section, K-type thermocouples were used and each thermocouple was calibrated before inserting in the vessel. The error on the temperature measuring was lower than ±1 °C. The relative error on the voltage, current and test heater size values were less than ±1%, ±1%, and ±2%. In this study, the heat flux was calculated from the inverse heat conduction problem. The uncertainty of the vapor layer thickness depended on two factors: one was the accuracy of the dimension calibration based pixel size of the picture by high-speed camera, the other one was the determination of liquid-vapor interface. According to the two references [16,17], the resulting in an uncertainty was less than 5% on the heat flux, and the total uncertainty of the vapor layer thickness was less than 10%. 3. Results and discussion 3.1. Visualization of the steady-state experiments Steady state downward-facing pool boiling experiments were performed under the simulated IVR-ERVC conditions to observe the characteristics of the two-phase flow behaviors. The recorded vapor dynamics at different heat flux levels at the bottom center were shown in Fig. 5. At low heat flux level, the flow regime of bubbly flow characterized by small bubbles that did not coalesce was usually observed at the bottom center region. With the increase of heat flux, the bubbles tended to combine together with a vapor slug due to the effects of buoyancy and downward orientation of the vessel outer surface. The vapor mass grew outward in the form of an expanding pancake or cap and eventually broke up into several individual irregular shape slugs. At high heat flux level, the pancake-like vapor mass grew and expanded very quickly, two-phase morphology downstream appeared to be in the churn-turbulent flow regime, which exhibited strong turbulent mixing with threedimensional swirls and chaotic behaviors. As the heat flux increased, the nucleate boiling became more and more vigorous, producing the largest vapor mass and bubbles. Once the CHF limit was reached, the surface temperature increased rapidly, leading to the flow regime transited to the transition boiling due to the mechanism of departure of nucleate pool boiling, as shown in Fig. 6. Based on the micro-layer evaporation theory, the CHF occurrence mechanism was composed of two parts, the evaporation of thin liquid film underneath the elongated bubble adhering to the lower head outer surface, and the depletion of supplement of liquid due to the relative motion of vapor bubbles along with the downward facing curved surface. The former adopted the Kelvin-Helmholtz instability analysis of vapor-liquid interface of the vapor jets. When the heat flux was close to CHF,
the vapor-liquid interface became highly distorted, which blocked liquid to feed the thin liquid film and the thin liquid film would dry out gradually. While the latter considered that the vapor bubbles moved along with the downward facing curved surface, the liquid in two-phase boundary layer entered the liquid film that would be exhausted when the CHF occurred. Fig. 7 displayed the temperature history of typical boiling curves after reaching the CHF limit at the bottom center region. The temperatures recorded in this curve were measured by the inserted thermocouples. During the steady-state experimental procedure, when heat flux was increased to approach the CHF limit, the power was cut off immediately to avoid the burnout of the simulated vessel. Afterwards, the boiling phenomena went similar to a quenching process following the over-heating. The temperature curves appeared as sudden temperature jumping and then decreased immediately. These temperature-time curves showed the time evolution from the transition or film boiling to CHF limit, then propagation to nucleate boiling, finally cooling down to the saturated temperature. It was worth noting that the location of local CHF limit at a given angular position corresponded to the boiling curve where the slope reached a maximum during the quenching process at the same wall superheat. Visual observations indicated that the boiling process near the bottom center was cyclic in nature and had four distinct phases: direct liquid solid contact, bubble nucleation and growth, coalescence, and vapor mass dispersion (ejection). Because of the asymmetrical configuration, the flow patterns were physically asymmetrical. The general direction of two-phase flow was in the radial direction. This radial flow gave rise to an orderly progression of the four phases as a function of the radius from the bottom center on the test vessel. The progression of flow patterns had the appearance of waves of vapor emanating from the bottom center of the test vessel, although vapor was actually produced over the entire surface. The medium heat flux level of 0.51 MW/m2 was chosen as a typical boiling process to discuss in the study because of the typical phenomenon was observed. At low heat flux level, small bubbles did not coalesce at the bottom center region, while at high heat flux level, the pancake-like vapor mass grew and expanded too quickly with 3D swirls and chaotic behaviors. Therefore, the typical sequences of video frames that illustrated this cyclic progression for medium heat flux of 0.51 MW/m2 were shown in Fig. 8. It was depicted that the vapor slugs in the morphology of a cap or pancake were distinctly observed at the medium heat flux level. Driven by the buoyancy, the vapor bubbles tended to coalesce in the bottom center region and grew into a pancake-like vapor mass. Subsequently, the vapor mass kept on growing outward to the mature size in the form of an expanding annular ring, and eventually broke up into several individual irregular bubbles as they slipped upward along the vessel outer surface. Liquid was then able to rewet the heating surface, causing a revival of bubble nucleation and initiating a new vapor formation, coalescence, growth and departure cycle. The two-phase motions downstream appeared to be in the churn-turbulent flow regime, which exhibited strong turbulent mixing with three-dimensional swirls and chaotic flow behaviors.
3.2. Advanced image processing technique A Matlab program named IMGPROCS was developed to identify and analyze the digital photos, as well as to calculate the flow characteristic parameters, such as the vapor thickness and normal velocity. The step-by-step procedures of the image process were as follows, and the flow diagram was shown in Fig. 9:
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Fig. 9. Advanced image processing procedure.
(a) Import background and target images in batches. (b) Obtain image’s pixel location coordinates; define the geometry and scale of the image, as well as the time sequence between the two images. (c) Acquire the image gray-scale information; enhance the image contrast and filter the noise. (d) Segment the target and background image with the subregion method; remove the background from the target image with the subtraction-silhouette method, as well as the other irrelevant information. (e) Restore the interfacial boundary and fill the internal space for the bubble or steam film by the self-developed mixed filling algorithm. (f) Label the connectivity domain. (g) Defined the center of the scaled-down hemispherical vessel as the coordinate origin of the polar coordinates, and the thickness of the steam film is obtained by capturing the vapor–liquid interface in the radial direction. (h) The normal velocity (radial velocity) is obtained according to the following formula:
u¼
DL Dt
u: the normal velocity DL: the difference of steam thickness in Dt time interval Dt: time interval Fig. 10 showed the binary images based on the visualized results in Fig. 8 through the IMGPROCS program. In this paper, the center of outer vessel was defined as origin of the coordinate due to the symmetry in the lower head. In other words, the bottom center was 0°, the equator position of the left and right sides were 90 and 90°, respectively. The definition of coordinate position was mainly for the convenience of processing in the Matlab program.
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The pixel retrieval algorithm was employed to capture the vapor-liquid interface in the program. As mentioned above, the nucleate boiling showed cyclical changes, 8 s were sufficient to reflect the characteristics of cyclical changes. While reducing the number of processed data, so that it was convenient and accurate to select 8 s as the length of time. Figs. 11 and 12 showed the 3D surface diagram and contour chart from 45 to 45° in the outer vessel at 0.51 MW/m2 (the typical condition). The figures also depicted the spatial variation of the vapor thickness in different locations. It was obvious that the boiling process exhibited typical cyclic characteristics in the bottom center region, the thickness of vapor film varied periodically at different angles. Driven by the buoyancy force, the vapor phase constantly coalesced and slipped upward along the convex vessel surface during the process of rising, eventually forming the mature vapor mass at the adjacent region apart from the bottom center. At this moment, the thickness of vapor film grew the maximum. That was also the reason why the maximum vapor film thickness took place on both lateral sides of the outer vessel instead of the bottom center region, as shown in Fig. 11. It was crucial to the success of external reactor vessel cooling, which also seriously affected the local occurrence and location of CHF during the severe accidents. Fig. 12 showed that the thickness of the vapor film exhibited a discontinuity phenomenon at a high angle due to the cause of the buoyant force. In addition, there was a sudden collapse in the thickness of the vapor film at the local angular position or at a certain time in Fig. 11. This phenomenon could be explained by the rapid changes in the vapor film caused by the vapor phase separation from the vessel surface and the insufficient restore on the interface of the vapor film during the image processing. The restore algorithm in the interface of the vapor film would be further developed in the future. Correspondingly, the periodic coalescence and departure of vapor mass led to the maximum velocity of vapor film, which was consistent with the variation of vapor film thickness. Whereas, the boiling phenomenon was relatively steady at the bottom center region nearby. As a consequence, the slip and rising velocity was lower than that in the region downstream. Furthermore, due to the stretch and expand of the vapor slug departure from the vessel surface to the surrounding liquid, the normal velocity was similarly lower at the bottom center region. The variations of the vapor film thickness at the annular angles 0 and 45° were shown in Figs. 13 and 14, respectively. It was noted that these two angular locations were chosen as the characteristic positions due to the motion features and morphology of the vapor slug. As shown in Fig. 13, the thickness of vapor film was always beyond zero, the peak value could reach as high as 20 mm during the boiling process. Moreover, the boiling cycle exhibited quite uniformly periodic characteristics, which was approximately 0.2 s. In other words, there were about 5 cycles per second. Nevertheless, the thickness of vapor film occasionally had a value of zero at the location 45° as shown in Fig. 14. That was because when the vapor slug slipped away from the location, there was no vapor covered on the surface any more. The maximum height of the vapor film was close to 35 mm, which was nearly twice of the bottom center. In addition, due to the occasional growth and departure of the vapor slug, the boiling cycle appeared a little bit oscillated especially when there was no vapor covered. Essentially the boiling cycle was indicated by the vapor morphology at the bottom center. Similarly, the distributions of vapor film normal velocity with time at the annular angles 0 and 45° were shown in Figs. 15 and 16, respectively. In this paper, the normal velocity of vapor slug was defined as the interfacial velocity between the liquid and vapor phase. Due to the vector property, the velocity departed from the vessel surface was positive; the contrary direction was negative. As shown in Fig. 15, the distribution of vapor film normal
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(a) t = 20ms
(b) t = 40ms
(c) t = 60ms
(d) t = 80ms
(e) t = 100ms
(f) t = 120ms
Fig. 10. Binary images based on the visualized results.
velocity was periodically fluctuated in accordance with the variation on the thickness, which had a value at a magnitude of 0.5 m/s. The rapid increase of the velocity was due to the collapse or breakup of the vapor mass after it grew to the mature size. Whereas, the normal velocity of vapor film occasionally remained zero at the location 45° as shown in Fig. 16, which was contributed to the same aforementioned reason. Besides, the average velocity was nearly 2 m/s rejecting the sudden peak value, which was faster than those at the bottom center. The differences on the velocity between the two angular locations suggested that the stretch and expand of the vapor slug was more intense and vigorous at the location 45° than the boiling phenomenon at the bottom center. In this paper, statistical analysis was carried out on the boiling cycle at the bottom center region by the spectrum analysis method. Fig. 17 showed the variation of boiling cycle and vapor
thickness with different heat flux levels at the bottom center region. It was shown that vapor thickness increased almost linearly from 6 to 16 mm at the heat flux level from 0.22 to 0.87 MW/m2 during the nucleate boiling regime, whereas the vapor thickness decreased rapidly to 3 mm and formed a stable vapor film after the occurrence of CHF. Since the boiling phenomena was pretty turbulent, the liquid/vapor interface kept fluctuant and wavy during the transition or film boiling. That was also the reason for the vapor film thickness decreased slightly as the heat flux increased from about 0.7 MW/m2 to about 0.9 MW/m2. During the experimental study, we repeated those test matrix for more than 5 times to confirm this special phenomenon. In addition, according to Beck’s research, the vapor film thickness was varied between 1 and 2 mm at the saturated pool boiling of downward facing hemisphere during the quenching experiments, which was quite close to our results between 2 or 3 mm.
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Fig. 11. Variation of vapor film thickness with time and angles.
Fig. 12. Contour chart of the vapor film thickness.
Fig. 15. Distribution of vapor film normal velocity at the angle 0°.
Fig. 13. Variation of vapor film thickness at the angle 0°.
Fig. 16. Distribution of vapor film normal velocity at the angle 45°.
Fig. 14. Variation of vapor film thickness at the angle 45°.
On the contrary, the boiling cycle was almost constant with the time 0.2 s during the nucleate boiling regime though there were some fluctuations. It was noted that due to the randomness and uncertainty of the flow boiling phenomena, the boiling cycle obtained in this study was a statistical average value based on the abundant visualized pictures. In addition, with the transition from nucleate boiling to film boiling, the growth of the vapor blanket had no obvious periodicity, and the determined frequency of the vapor motion could not be counted by the image processing technique.
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Acknowledgement The authors are grateful for the support of National Science and Technology Major Project of the Ministry of Science and Technology of China (NO. 2010ZX06002-004), and the project No. 106112017CDJQJ148806 supported by the Fundamental Research Funds for the Central Universities. References
Fig. 17. Variation of boiling cycle and vapor thickness with different heat flux levels.
4. Conclusion In the present study, steady state pool boiling experiments were performed in the iSBLB facility on a downward-facing hemispherical vessel to visually investigate the characteristics of two-phase flow behavior. Vapor mass morphology of the boiling process at the bottom center was observed and recorded using the highspeed camera. A Matlab program was developed to identify and analyze the digital images. The conclusions were summarized as follows: (1) Vapor behaviors along the hemispherical curved surface at different heat flux levels were observed and discussed through visualizations, even approaching the CHF limits. Nucleate pool boiling on the downward-facing hemespherical surface was a typically cyclic process. As the heat flux increased, vapor bubbles tended to coalesce rigorously in the bottom center region, which lead to the flow regime transited from the bubbly flow to transition or film boiling flow due to the mechanism of departure of nucleate boiling. (2) The vapor dynamic parameters, such as vapor thickness, boiling cycle, and normal velocity were quantitatively calculated and analyzed by the advanced image processing technique. With the heat flux level increased, vapor thickness varied periodically at the bottom center and increased almost linearly from 6 to 16 mm at the heat flux levels from 0.22 to 0.87 MW/m2 during the nucleate boiling regime, whereas the vapor thickness decreased rapidly to 3 mm and formed a stable vapor film after the occurrence of CHF. The boiling cycle was almost constant with the time 0.2 s during the nucleate boiling regime, which was independent with the heat flux levels. Similarly, the vapor film normal velocity was also periodically fluctuated in accordance with the variation on the thickness, which had a value at a magnitude of 0.5 m/s. Conflict of interest The authors declared that there is no conflict of interest.
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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Visualization study on the bubble behavior on a downward facing hemispherical surface during saturated pool boiling Fei Qin a, Xiang Zhang b, Deqi Chen a,⇑, Lian Hu a, Fan Bill Cheung c a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China State Nuclear Power Technology Research & Development Center, South Area, Future Science and Technology Park, Beijing 102209, China c Department of Mechanical & Nuclear Engineering, Pennsylvania State University, PA, USA b
a r t i c l e
i n f o
Article history: Received 2 October 2018 Received in revised form 13 February 2019 Accepted 14 February 2019
Keywords: Pool boiling Downward facing Bubble dynamics Critical heat flux
a b s t r a c t Saturated pool boiling experiments were conducted in a scale-down three-dimensional downward facing hemispherical vessel. In this study, a high-speed digital video camera was employed to capture the bubble behavior along the overheated convex surface. The general morphology of coalescent bubble at the bottom center was analyzed based on the observed pictures and videos. The visualization results showed that the bubble behavior along the hemispherical curved surface was quite different from that on the plane and upward surfaces. It was found that the coalescent bubble was cyclical repeatedly forming a stratified vapor layer, and then randomly sliding upward along the convex surface. The downward facing boiling heat flux was directly related to the duration of the boiling cycles, which were obtained based on the visualization by high speed camera and the image processing technique. A Matlab program was developed to recognize and analyze the images, as well as to calculate the characteristic parameters of the coalescent bubble. The boiling cycle was almost constant during the nucleate boiling regime, which was independent with the heat flux level. This study also provided an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that could be even useful for hydrodynamic modeling of the CHF phenomenon. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nuclear power is regarded to be a clean and reliable source of energy. With proper design, nuclear power plants are able to provide safe and consistent power under well-controlled operating conditions. Whereas, it is pretty dangerous if the radioactive materials releasing to the environment under severe accidents. To assure the public of its safety, the issue of Severe Accident Management (SAM) needs to be addressed seriously. That is the so-called In-Vessel Retention (IVR), one key SAM strategy that adopted by some operating nuclear power plants and Advanced Light Water Reactors (ALWRs). During a severe accident, a significant amount of corium can become molten and relocate to the bottom head of the Reactor Pressure Vessel (RPV), as happened in TMI-2 accident. The concept of IVR by passive External Reactor Vessel Cooling (ERVC) in a flooded cavity during severe accidents is a feasible approach to retaining radioactive core melt within the vessel [1]. The reactor cavity flooding is selected as the effective ERVC method because ⇑ Corresponding author. E-mail address: [email protected] (D. Chen). https://doi.org/10.1016/j.ijheatmasstransfer.2019.02.043 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
of the relatively simple installation. As long as the heat flux at the lower head of RPV is lower than the Critical Heat Flux (CHF) limit at the corresponding point for the downward facing boiling process, nucleate boiling will be the prevailing regime [2]. In that case, the temperature of outer surface can be maintained near the saturated temperature of water such that the reactor vessel is able to be sufficiently cooled to sustain its thermal and structural integrity. Therefore, it is important to comprehend the physical phenomena during the IVR-ERVC process. In the past thirty years, there were many experimental and theoretical studies to investigate the ERVC boiling process including the effect of inclination angle on the CHF. Based on those experimental results, there were also many different empirical or semi-empirical correlations developed for CHF by fitting the data. Especially for the engineering application, two-dimensional fullscale test facilities were built to experimentally verify and validate the feasibility of ERVC, such as ULPU series for AP600 and AP1000 in the United States [3,4], KAIST facility for APR1400 in Korea [5,6] and FIRM facility for CAP1400 in China[7]. Whereas, for the veritable physical phenomena of IVR-ERVC under severe accident conditions, there were some representative differences between the 2D simulative facilities and actual 3D boiling phenomenon on the
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outside surface of the convex vessel, for example, the 3D bottom center effect, the upstream two phase flow behavior, and the local CHF variations. Literatures were reported to investigate the pool boiling bubble behavior and CHF limit for the downward facing hemispherical structures. Chu et al. [8] developed the CYBL (Cylindrical Boiling) facility at Sandia National Laboratory which was a full simulation of reactor scale to perform confirmatory tests on the effectiveness of the downward facing pool boiling heat transfer process for the flooded cavity design. The simulated reactor vessel had a cylindrical diameter of 3.7 m and a torispherical bottom head. There were both steady state and quenching experiments carried on the CYBL test matrix. The visualizations of boiling process exhibited a cyclic nature of vapor dynamics in the nucleate downward facing pool boiling and two-phase flow along the heated surface. The results showed that the AP600 flooded cavity should be capable of cooling the reactor vessel in central region under the prototypical heat load and heat flux distributions. Additional quenching experiments of downward facing pool boiling with a copper substrate were performed by El-Genk et al. [9] in saturated water using two test sections of different thicknesses. Boiling curves were derived at six locations on the bottom center region and then at a local inclination angle of approximately 8° along the outer surface of the test section. The heat flux increased with increasing h in the lower heat flux region and decreased with increasing values of h at very high heat fluxes. Interestingly, the thickness of the copper heater had no effect on the observed CHF values. CHF occurred sequentially from the lower most to the highest positions in that order. As the thickness of the test section increased, the difference in time between subsequent CHF occurrences also increased. Kim et al. [10] experimentally measured and analyzed the film boiling heat transfer coefficients for a downward facing hemispherical surface from the quenching tests using DELTA facility in Seoul National University. Two test sections were made of copper to maintain low Biot numbers, and the outer diameters of the hemispheres were 120 and 294 mm, respectively. The effect of diameter on film boiling heat transfer was quantified utilizing results obtained from the test sections. The measured heat transfer coefficients were found to be greater than those predicted by the conventional laminar flow theory on account of the interfacial wavy motion incurred by the Helmholtz instability. The visualization study depicted clear-cut quenching process on the downward facing hemisphere, and was utilized to facilitate theorizing the film boiling on the curved surface. Suh [11] constructed CASA (Corium Ablation Stopper Apparatus) facility in Seoul National University, which had a downward facing hemispherical vessel and geometrically asymmetric thermal insulator of the APR1400 scaled linearly down by 1/10, to investigate the CHF with saturated deionized water at the atmospheric pressure and three-dimensional random flow effect during the severe accident. The heated vessel played a pivotal role in CASA depending on the configuration of the oxide pool and metal layer to bring about the focusing effect expected of a molten pool in the lower head during a severe accident. The results showed that CHF was in the vicinity of 1.5 MW/m2 in the metal layer with the focusing effect, which was found to be lower than that in the ULPU-2400 configuration V data. Cheung et al. [12,13] studied the downward facing boiling and CHF phenomena on the outer surface of a hemispherical vessel using SBLB (Sub-scale Boundary Layer Boiling) test facility at Pennsylvania State University. The facility provided a three-dimensional simulation of the pool boiling process on the outer surface of a reactor vessel with and without surrounding insulation structure. A significant spatial variation of the CHF, with the local CHF limit increasing monotonically from the bottom center to the equator
of the vessel, was measured for both saturated and subcooled boiling. For the insulation configurations of AP600 and APR1400, an internal upward co-current two-phase flow was both observed in the annular channel between the insulation structure and the test vessel [14]. At the same heat flux level, more flow was induced along the vessel outer surface for the case with insulation compared to the case without. As a result, the nucleate boiling heat transfer and CHF for the case with insulation were found to be consistently higher than those for the corresponding case without. These differences in the nucleate boiling heat transfer and the CHF were attributed to the buoyancy-driven two-phase flow effect. Dizon et al. [15] investigated two different approaches to enhance the cooling capacity of APR1400 reactor vessel with insulation at SBLB facility. The first method involved the usage of an enhanced insulation structure to improve the steam venting through the annular bottleneck channel. The second one involved the usage of a micro-porous media coating on the outer surface of the vessel to promote the downward facing pool boiling heat transfer. A substantial increase on the local CHF was both observed by the two approached. Subsequently, Sohag et al. [16,17] developed a new and versatile coating technique called ‘‘cold spray” which did not require heat treatment and could be used in the existing and new sites to form a micro-porous coating on a hemispherical test vessel made of stainless steel. Both quenching and steady state experiments were performed under simulated IVRERVC conditions using the cold-sprayed coated vessel to quantitatively determine the amount of downward facing boiling heat transfer and CHF enhancement over the corresponding case without surface coating. Visualization of the quenching process for bare and coated vessels, under the identical working conditions (i.e., same initial vessel temperature and saturated water temperature) were performed to identify the differences in the downward facing boiling and CHF variations. The experimental results showed that micro-porous coatings evidently enhanced the heat transfer and CHF values than the bare surface. Moreover, the coatings showed a relatively high degree of durability and fatigue resistance after exposing to multiple thermal shocks experienced during the CHF boiling tests. Lu et al. [18] built a three-dimensional visually experimental apparatus to investigate the downward facing boiling phenomena and CHF limits of IVR-ERVC for CAP1400. The vapor morphology at different heat flux levels and inclination angles of the hemispherical downward-facing surface was carefully studied. The visualized results displayed significant differences in vapor morphology between 3-D and 2-D slice ERVC experiments in the bottom central region. Vapor motion characteristics corresponding to the occurrence of CHF were examined by determining the boiling cycle and maximum vapor height. The effect of inclination angle on the vapor morphology was identified and local variation of CHF along the vessel outer surface was also measured. The effect of bottom heating on the downstream CHF behavior was experimentally investigated, and the mechanism responsible for the influence that was revealed by the vapor morphology was determined. Zhong et al. [19] designed a heating system with liquid metal as the intermediate heat transfer medium to simulate a scaled-down three-dimensional reactor vessel. The liquid metal was heated by DC power and then circulated inside the simulative hemispherical vessel. The objective of the study was to investigate the boiling regimes and heat fluxes on the outer surface of the hemispherical lower head. The boiling heat transfer was experimentally studied on a hemispherical plain surface and interconnected grooves with triangular cavities surface using saturated deionized water at atmospheric pressure. The results showed that CHF on the plain surface at the inclination angle of 85° was 857.3 kW/m2, with no boiling crisis observed on the structured surface up to the highest heat flux of 1366.9 kW/m2. The CHF enhancement on the
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mocouple data. In their study, wall temperature before immersion was well above the minimum film boiling temperature to investigate the effects of liquid sub-cooling and angular location on film boiling. The vapor film thickness was varied between 1 and 2 mm at the saturated pool boiling of the downward facing hemisphere. All of the aforementioned test facilities provided a simulation of the downward facing boiling and experimental study on the CHF limits during the ERVC conditions. There were also some visualized
Fig. 1. Schematic diagram of the experimental apparatus.
Fig. 3. Schematic diagram of the heating blocks.
Fig. 2. Pictorial view of the water tank and windows assembly.
structured surface was more than 59% greater than the one on the plain surface at the inclination angle of 85°. The analytical results showed that the structured surface formed a liquid-vapor conversion path with the cavities as stable nucleation sites and the interconnected grooves as cooling water supply pathways. Thus, the structured surface significantly enhanced the boiling heat transfer and the CHF. Most recently, Beck et al. [20,21] performed quenching experiments on SBLB facility and observed the film boiling phenomena using high-speed video. HTC was calculated by making use of the visualizations. An image processing technique utilizing thresholding and edge detection was useful for extracting the interfacial vapor film thickness. With the modeled HTC, the heat flux in film boiling was compared using a traditional lumped capacitance method, and a new method that utilized both the video and ther-
Fig. 4. Steady-state experimental procedure.
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(a) 0.22MW/m2
(b) 0.36MW/m2
(e) 0.58MW/m2
(f) 0.73MW/m2
(c) 0.51MW/m2
(g) 0.87MW/m2
Fig. 5. Vapor behavior at different heat flux levels.
Fig. 6. Observation of transition boiling after reaching CHF limit.
Fig. 7. Temperature history of typical boiling curves at the bottom center.
investigations in both quenching and steady state experiments. Those observations mostly qualitatively described the boiling phenomena during IVR-ERVC conditions. However, quantitatively analysis should be utilized to discuss the two-phase flow and bubble dynamic behaviors, especially for steady state experiments. In addition, few studies were reported in the downward facing hemispherical structure using the image processing technique. Only Beck et al. investigated the vapor film dynamics and heat transfer during the quenching experiments, not steady-state ones. Whereas, the steady-state experiments played a key role in understanding IVR-ERVC conditions. In this paper, a scale-down three-dimensional downward facing pool boiling facility was conducted to investigate the two-phase natural circulation flow characteristics. The general morphology of the coalescent bubbles was quantitatively analyzed based on the visualizations. A Matlab program was developed to recognize and analyze the digital pictures by using the advanced image processing technique. This study also provided an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that could be even useful for hydrodynamic modeling of the CHF phenomenon.
2. Experimental method 2.1. Experimental setup Steady state experiments were performed in the threedimensional test facility to simulate the downward facing pool boiling phenomena on the outer surface of a scaled-down hemispherical vessel. Cheung [12–15] had illustrated the scaling analysis and structures of SBLB facility very carefully, and the design mentality of current experimental setup was referred to those literatures. The test facility shown schematically in Fig. 1 consisted of a water tank, condenser, hemispherical test vessel, immersion heaters, simulated insulation structure, liquidometer, data acquisition system, DC power system, and a high-speed video camera. The water tank was cylindrical in shape, 3 m in height and 1.5 m in diameter. The height of 3 m was able to simulate the liquid head effect on the natural circulation and boiling heat transfer phenomena. Four large viewing windows were located axialsymmetrically on each side of the tank for visualization of boiling
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(a) t = 20ms
(c) t = 60ms
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(b) t = 40ms
(d) t = 80ms
(e) t = 100ms
(f) t = 120ms
Fig. 8. Typical boiling process observed in the bottom center region.
process on the vessel outer surface by the high-speed camera. The visualizations were recorded by a FR-Stream 625 high speed video camera. The frame rate of this camera could reach 500fps at the resolution of 1280 1024 pixels. The test facility was able to be operated with and without insulation structure. This paper described the visualized results obtained from the later situation. Fig. 2 showed the pictorial view of the water tank and windows assembly. The tank was equipped with four immersion heaters, which were capable of supplying a total of 40 kW to preheat the water inside the tank. Several resistance thermometers were located in the tank to measure the temperature of the water. In addition, there was a reflux condenser assembly located on the top of the tank. The test vessel was comprised of two main parts: the lower part consisting of a heated bottom-head vessel and the upper part consisting of a non-heated cylindrical wall. The vessel with the diameter of 297 mm was fastened to the upper cylinder through the threaded connection. The boiling surface was made of oxygenfree copper, which was very smooth after polishing with the roughness of 6.3 lm. There were two circular heating zones for
the bottom region and three rectangular ones for the upper region. The three upper heating zones were identical at each lateral side. Within each heating zone, 4-mm grooves were cut in order to reduce the heat conduction between different zones. Fig. 3 showed the schematic diagram of the heating zones. There were totally 185 cartridge heaters of 600 W each inserted in the vessel, leading to the heat flux levels up to 1.5 MW/m2 of the bottom region and 2.0 MW/m2 of the upper region. K-type thermocouples were embedded in the copper to monitor the temperatures. 2.2. Experimental procedure Prior to the steady-state experiments, the water tank was filled with deionized water and preheated to the saturated temperature by four 10 kW immersion heaters. Heating usually required between 4 and 5 h to reach saturation. The temperature would be maintained for a few minutes to degas the water. The steadystate experimental procedure was shown in Fig. 4. Once the water was heated to the desired temperature, the power system was then turned on. For each heat flux level of reaching the steady state, the camera recorded the visualized results for at least five minutes.
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Heating power in the test section was gradually increased by slowly increasing the voltage. Every time the voltage was increased, it was necessary to check the stability of temperature fluctuations and the boiling phenomena. At least two consecutive runs were conducted for each condition. The step increment of heating power was gradually decreased when approaching CHF in order to catch the boiling crisis more precisely. It was at the CHF onset point that a sudden increase (over 50 °C/min) of wall temperature on the test section happened. Once CHF was approached, the power was switched off immediately to protect the safety of test section. From a conservative-point-of-view, the last power step before CHF occurrence was used to calculate the resulted CHF values. 2.3. Measuring error and uncertainty analysis The spot where the occurrence of CHF seemed to be random. In measuring the temperatures of the test section, K-type thermocouples were used and each thermocouple was calibrated before inserting in the vessel. The error on the temperature measuring was lower than ±1 °C. The relative error on the voltage, current and test heater size values were less than ±1%, ±1%, and ±2%. In this study, the heat flux was calculated from the inverse heat conduction problem. The uncertainty of the vapor layer thickness depended on two factors: one was the accuracy of the dimension calibration based pixel size of the picture by high-speed camera, the other one was the determination of liquid-vapor interface. According to the two references [16,17], the resulting in an uncertainty was less than 5% on the heat flux, and the total uncertainty of the vapor layer thickness was less than 10%. 3. Results and discussion 3.1. Visualization of the steady-state experiments Steady state downward-facing pool boiling experiments were performed under the simulated IVR-ERVC conditions to observe the characteristics of the two-phase flow behaviors. The recorded vapor dynamics at different heat flux levels at the bottom center were shown in Fig. 5. At low heat flux level, the flow regime of bubbly flow characterized by small bubbles that did not coalesce was usually observed at the bottom center region. With the increase of heat flux, the bubbles tended to combine together with a vapor slug due to the effects of buoyancy and downward orientation of the vessel outer surface. The vapor mass grew outward in the form of an expanding pancake or cap and eventually broke up into several individual irregular shape slugs. At high heat flux level, the pancake-like vapor mass grew and expanded very quickly, two-phase morphology downstream appeared to be in the churn-turbulent flow regime, which exhibited strong turbulent mixing with threedimensional swirls and chaotic behaviors. As the heat flux increased, the nucleate boiling became more and more vigorous, producing the largest vapor mass and bubbles. Once the CHF limit was reached, the surface temperature increased rapidly, leading to the flow regime transited to the transition boiling due to the mechanism of departure of nucleate pool boiling, as shown in Fig. 6. Based on the micro-layer evaporation theory, the CHF occurrence mechanism was composed of two parts, the evaporation of thin liquid film underneath the elongated bubble adhering to the lower head outer surface, and the depletion of supplement of liquid due to the relative motion of vapor bubbles along with the downward facing curved surface. The former adopted the Kelvin-Helmholtz instability analysis of vapor-liquid interface of the vapor jets. When the heat flux was close to CHF,
the vapor-liquid interface became highly distorted, which blocked liquid to feed the thin liquid film and the thin liquid film would dry out gradually. While the latter considered that the vapor bubbles moved along with the downward facing curved surface, the liquid in two-phase boundary layer entered the liquid film that would be exhausted when the CHF occurred. Fig. 7 displayed the temperature history of typical boiling curves after reaching the CHF limit at the bottom center region. The temperatures recorded in this curve were measured by the inserted thermocouples. During the steady-state experimental procedure, when heat flux was increased to approach the CHF limit, the power was cut off immediately to avoid the burnout of the simulated vessel. Afterwards, the boiling phenomena went similar to a quenching process following the over-heating. The temperature curves appeared as sudden temperature jumping and then decreased immediately. These temperature-time curves showed the time evolution from the transition or film boiling to CHF limit, then propagation to nucleate boiling, finally cooling down to the saturated temperature. It was worth noting that the location of local CHF limit at a given angular position corresponded to the boiling curve where the slope reached a maximum during the quenching process at the same wall superheat. Visual observations indicated that the boiling process near the bottom center was cyclic in nature and had four distinct phases: direct liquid solid contact, bubble nucleation and growth, coalescence, and vapor mass dispersion (ejection). Because of the asymmetrical configuration, the flow patterns were physically asymmetrical. The general direction of two-phase flow was in the radial direction. This radial flow gave rise to an orderly progression of the four phases as a function of the radius from the bottom center on the test vessel. The progression of flow patterns had the appearance of waves of vapor emanating from the bottom center of the test vessel, although vapor was actually produced over the entire surface. The medium heat flux level of 0.51 MW/m2 was chosen as a typical boiling process to discuss in the study because of the typical phenomenon was observed. At low heat flux level, small bubbles did not coalesce at the bottom center region, while at high heat flux level, the pancake-like vapor mass grew and expanded too quickly with 3D swirls and chaotic behaviors. Therefore, the typical sequences of video frames that illustrated this cyclic progression for medium heat flux of 0.51 MW/m2 were shown in Fig. 8. It was depicted that the vapor slugs in the morphology of a cap or pancake were distinctly observed at the medium heat flux level. Driven by the buoyancy, the vapor bubbles tended to coalesce in the bottom center region and grew into a pancake-like vapor mass. Subsequently, the vapor mass kept on growing outward to the mature size in the form of an expanding annular ring, and eventually broke up into several individual irregular bubbles as they slipped upward along the vessel outer surface. Liquid was then able to rewet the heating surface, causing a revival of bubble nucleation and initiating a new vapor formation, coalescence, growth and departure cycle. The two-phase motions downstream appeared to be in the churn-turbulent flow regime, which exhibited strong turbulent mixing with three-dimensional swirls and chaotic flow behaviors.
3.2. Advanced image processing technique A Matlab program named IMGPROCS was developed to identify and analyze the digital photos, as well as to calculate the flow characteristic parameters, such as the vapor thickness and normal velocity. The step-by-step procedures of the image process were as follows, and the flow diagram was shown in Fig. 9:
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Fig. 9. Advanced image processing procedure.
(a) Import background and target images in batches. (b) Obtain image’s pixel location coordinates; define the geometry and scale of the image, as well as the time sequence between the two images. (c) Acquire the image gray-scale information; enhance the image contrast and filter the noise. (d) Segment the target and background image with the subregion method; remove the background from the target image with the subtraction-silhouette method, as well as the other irrelevant information. (e) Restore the interfacial boundary and fill the internal space for the bubble or steam film by the self-developed mixed filling algorithm. (f) Label the connectivity domain. (g) Defined the center of the scaled-down hemispherical vessel as the coordinate origin of the polar coordinates, and the thickness of the steam film is obtained by capturing the vapor–liquid interface in the radial direction. (h) The normal velocity (radial velocity) is obtained according to the following formula:
u¼
DL Dt
u: the normal velocity DL: the difference of steam thickness in Dt time interval Dt: time interval Fig. 10 showed the binary images based on the visualized results in Fig. 8 through the IMGPROCS program. In this paper, the center of outer vessel was defined as origin of the coordinate due to the symmetry in the lower head. In other words, the bottom center was 0°, the equator position of the left and right sides were 90 and 90°, respectively. The definition of coordinate position was mainly for the convenience of processing in the Matlab program.
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The pixel retrieval algorithm was employed to capture the vapor-liquid interface in the program. As mentioned above, the nucleate boiling showed cyclical changes, 8 s were sufficient to reflect the characteristics of cyclical changes. While reducing the number of processed data, so that it was convenient and accurate to select 8 s as the length of time. Figs. 11 and 12 showed the 3D surface diagram and contour chart from 45 to 45° in the outer vessel at 0.51 MW/m2 (the typical condition). The figures also depicted the spatial variation of the vapor thickness in different locations. It was obvious that the boiling process exhibited typical cyclic characteristics in the bottom center region, the thickness of vapor film varied periodically at different angles. Driven by the buoyancy force, the vapor phase constantly coalesced and slipped upward along the convex vessel surface during the process of rising, eventually forming the mature vapor mass at the adjacent region apart from the bottom center. At this moment, the thickness of vapor film grew the maximum. That was also the reason why the maximum vapor film thickness took place on both lateral sides of the outer vessel instead of the bottom center region, as shown in Fig. 11. It was crucial to the success of external reactor vessel cooling, which also seriously affected the local occurrence and location of CHF during the severe accidents. Fig. 12 showed that the thickness of the vapor film exhibited a discontinuity phenomenon at a high angle due to the cause of the buoyant force. In addition, there was a sudden collapse in the thickness of the vapor film at the local angular position or at a certain time in Fig. 11. This phenomenon could be explained by the rapid changes in the vapor film caused by the vapor phase separation from the vessel surface and the insufficient restore on the interface of the vapor film during the image processing. The restore algorithm in the interface of the vapor film would be further developed in the future. Correspondingly, the periodic coalescence and departure of vapor mass led to the maximum velocity of vapor film, which was consistent with the variation of vapor film thickness. Whereas, the boiling phenomenon was relatively steady at the bottom center region nearby. As a consequence, the slip and rising velocity was lower than that in the region downstream. Furthermore, due to the stretch and expand of the vapor slug departure from the vessel surface to the surrounding liquid, the normal velocity was similarly lower at the bottom center region. The variations of the vapor film thickness at the annular angles 0 and 45° were shown in Figs. 13 and 14, respectively. It was noted that these two angular locations were chosen as the characteristic positions due to the motion features and morphology of the vapor slug. As shown in Fig. 13, the thickness of vapor film was always beyond zero, the peak value could reach as high as 20 mm during the boiling process. Moreover, the boiling cycle exhibited quite uniformly periodic characteristics, which was approximately 0.2 s. In other words, there were about 5 cycles per second. Nevertheless, the thickness of vapor film occasionally had a value of zero at the location 45° as shown in Fig. 14. That was because when the vapor slug slipped away from the location, there was no vapor covered on the surface any more. The maximum height of the vapor film was close to 35 mm, which was nearly twice of the bottom center. In addition, due to the occasional growth and departure of the vapor slug, the boiling cycle appeared a little bit oscillated especially when there was no vapor covered. Essentially the boiling cycle was indicated by the vapor morphology at the bottom center. Similarly, the distributions of vapor film normal velocity with time at the annular angles 0 and 45° were shown in Figs. 15 and 16, respectively. In this paper, the normal velocity of vapor slug was defined as the interfacial velocity between the liquid and vapor phase. Due to the vector property, the velocity departed from the vessel surface was positive; the contrary direction was negative. As shown in Fig. 15, the distribution of vapor film normal
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(a) t = 20ms
(b) t = 40ms
(c) t = 60ms
(d) t = 80ms
(e) t = 100ms
(f) t = 120ms
Fig. 10. Binary images based on the visualized results.
velocity was periodically fluctuated in accordance with the variation on the thickness, which had a value at a magnitude of 0.5 m/s. The rapid increase of the velocity was due to the collapse or breakup of the vapor mass after it grew to the mature size. Whereas, the normal velocity of vapor film occasionally remained zero at the location 45° as shown in Fig. 16, which was contributed to the same aforementioned reason. Besides, the average velocity was nearly 2 m/s rejecting the sudden peak value, which was faster than those at the bottom center. The differences on the velocity between the two angular locations suggested that the stretch and expand of the vapor slug was more intense and vigorous at the location 45° than the boiling phenomenon at the bottom center. In this paper, statistical analysis was carried out on the boiling cycle at the bottom center region by the spectrum analysis method. Fig. 17 showed the variation of boiling cycle and vapor
thickness with different heat flux levels at the bottom center region. It was shown that vapor thickness increased almost linearly from 6 to 16 mm at the heat flux level from 0.22 to 0.87 MW/m2 during the nucleate boiling regime, whereas the vapor thickness decreased rapidly to 3 mm and formed a stable vapor film after the occurrence of CHF. Since the boiling phenomena was pretty turbulent, the liquid/vapor interface kept fluctuant and wavy during the transition or film boiling. That was also the reason for the vapor film thickness decreased slightly as the heat flux increased from about 0.7 MW/m2 to about 0.9 MW/m2. During the experimental study, we repeated those test matrix for more than 5 times to confirm this special phenomenon. In addition, according to Beck’s research, the vapor film thickness was varied between 1 and 2 mm at the saturated pool boiling of downward facing hemisphere during the quenching experiments, which was quite close to our results between 2 or 3 mm.
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Fig. 11. Variation of vapor film thickness with time and angles.
Fig. 12. Contour chart of the vapor film thickness.
Fig. 15. Distribution of vapor film normal velocity at the angle 0°.
Fig. 13. Variation of vapor film thickness at the angle 0°.
Fig. 16. Distribution of vapor film normal velocity at the angle 45°.
Fig. 14. Variation of vapor film thickness at the angle 45°.
On the contrary, the boiling cycle was almost constant with the time 0.2 s during the nucleate boiling regime though there were some fluctuations. It was noted that due to the randomness and uncertainty of the flow boiling phenomena, the boiling cycle obtained in this study was a statistical average value based on the abundant visualized pictures. In addition, with the transition from nucleate boiling to film boiling, the growth of the vapor blanket had no obvious periodicity, and the determined frequency of the vapor motion could not be counted by the image processing technique.
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Acknowledgement The authors are grateful for the support of National Science and Technology Major Project of the Ministry of Science and Technology of China (NO. 2010ZX06002-004), and the project No. 106112017CDJQJ148806 supported by the Fundamental Research Funds for the Central Universities. References
Fig. 17. Variation of boiling cycle and vapor thickness with different heat flux levels.
4. Conclusion In the present study, steady state pool boiling experiments were performed in the iSBLB facility on a downward-facing hemispherical vessel to visually investigate the characteristics of two-phase flow behavior. Vapor mass morphology of the boiling process at the bottom center was observed and recorded using the highspeed camera. A Matlab program was developed to identify and analyze the digital images. The conclusions were summarized as follows: (1) Vapor behaviors along the hemispherical curved surface at different heat flux levels were observed and discussed through visualizations, even approaching the CHF limits. Nucleate pool boiling on the downward-facing hemespherical surface was a typically cyclic process. As the heat flux increased, vapor bubbles tended to coalesce rigorously in the bottom center region, which lead to the flow regime transited from the bubbly flow to transition or film boiling flow due to the mechanism of departure of nucleate boiling. (2) The vapor dynamic parameters, such as vapor thickness, boiling cycle, and normal velocity were quantitatively calculated and analyzed by the advanced image processing technique. With the heat flux level increased, vapor thickness varied periodically at the bottom center and increased almost linearly from 6 to 16 mm at the heat flux levels from 0.22 to 0.87 MW/m2 during the nucleate boiling regime, whereas the vapor thickness decreased rapidly to 3 mm and formed a stable vapor film after the occurrence of CHF. The boiling cycle was almost constant with the time 0.2 s during the nucleate boiling regime, which was independent with the heat flux levels. Similarly, the vapor film normal velocity was also periodically fluctuated in accordance with the variation on the thickness, which had a value at a magnitude of 0.5 m/s. Conflict of interest The authors declared that there is no conflict of interest.
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