Proceedings of the 1st Thermal and Fluid Engineering Summer Conference, TFESC August 9-12, 2015, New York City, USA
TFESC-12795
INVESTIGATION OF SUBCOOLED FLOW BOILING HEAT TRANSFER AT 1 ~ 3 BAR Randy Samaroo1, Pervej Rahman1, and Masahiro Kawaji1,2* 1
Department of Mechanical Engineering, City College of New York, New York, USA 2 The CUNY Energy Institute, City University of New York, New York, USA
ABSTRACT Subcooled flow boiling experiments have been performed to obtain a better understanding of vapor bubble behavior under nucleation and entrainment by liquid flow. The main objective of this work is to obtain data to validate 3-D Interface Tracking Models and CFD models which can predict subcooled flow boiling phenomena in fuel assemblies of Pressurized Water Reactors (PWRs). In the experiment, the test section is a vertically oriented annulus of 6.35mm gap width with electrically heated inner rod with thermocouples embedded on the heated surface. Subcooled water at pressures from 1-3bar flowed upward through the test section under turbulent flow conditions. Liquid Reynolds numbers range between 26,000~55,000, with heat fluxes between 15~42kW/m2. The experiments yielded bubble size and directly measured wall temperature data. Measurements were taken using a high speed video camera with simultaneous temperature acquisition. KEY WORDS:Subcooled flow boiling, bubble nucleation, bubble departure, wall temperature, liquid subcooling
1. INTRODUCTION Subcooled flow boiling experiments have been conducted using water at pressures ranging from 1-2 bar in an annular flow channel. These experiments are conducted for a DOE Nuclear HUB project, the Consortium for Advanced Simulation of Light Water Reactors (CASL). One of the CASL project (www.ornl.gov/sci/nsed/docs/CASL_Project_Summary.pdf) goals is the development of CFD models which can predict subcooled flow boiling phenomena in fuel assemblies of Pressurized Water Reactors. To formulate improved closure relations with reduced empiricism, it is necessary to improve our physical understanding of subcooled flow boiling phenomena such as the nucleation, growth and detachment of vapor bubbles from a heated wall, and bubble behavior in highly turbulent subcooled liquid flow including the bubble deformation. In previous studies, subcooled flow boiling has been investigated experimentally but not mesoscopically[16].Recently, Sugrue et al. [7] measured the effects of varying orientation angle, heat flux, mass flux, and pressure on bubble departure diameters in subcooled flow boiling. Their results showed that the bubble departure diameters increase with increasing heat flux and decreasing mass flux, subcooling and pressure. This technical note presents data on the growth and departure of vapor bubbles in water under upward subcooled flow boiling conditions in annular geometry at pressures of 1 and 2 bar. Bubble size and position relative to heated surface as well as wall temperature data are recorded. For this work, four cases are covered, with varying pressures, liquid subcooling, heat fluxes and flow rates. Table 1 lists the cases covered and parameters varied in each case.
2. EXPERIMENTAL APPARATUS AND PROCEDURE 2.1 Experimental Apparatus A schematic of the subcooled flow boiling loop is shown in
Figure 1. The working fluid is degassed water, which is circulated through the loop using a gear pump and is pressurized using a nitrogen gas cylinder. A heat exchanger is used to achieve steady state conditions. The apparatus
*Corresponding Author:
[email protected]
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TFESC-12795 features a stainless steel annular test section that houses a central stainless steel tube, heated by joule heating. The stainless steel heater rod of 12.7mm OD is joined to copper electrodes at the top and bottom to allow for the electrical connection to the power supply. The test section outer housing is stainless steel tubing of 25.4mmID. Additionally, two thermocouples are spot welded internally 180˚ apart at a distance of 825mm from the inlet and190mm below the outlet of the test section. The test section includes a fitted and sealed borosilicate glass window for making visual measurements of key subcooled flow boiling parameters, such as bubble diameter and nucleation site density, using a high-speed video camera. Table 1: Experimental Cases Case
Absolute Mass Flux Pressure (kg/m2s) (Bar)
Subcooling (°C)
Wall Superheat (°C)
P1-01
1
636
3.6
8.4
P1-02
1
1,273
2.9
1.9
P1-03
1
636
5.1
P1-04
1
1,273
P1-05
1
P1-06
Jakob Number 25.4
Average Reynolds Heat Flux Local Nusselt 2 Number (kW/m ) Number 27,787
20.6
36.0
5.7
55,632
20.6
90.1
1.4
4.2
27,240
15.0
49.3
4.1
0.4
1.1
54,849
15.0
67.4
636
8.6
2.4
7.2
26,393
20.5
39.6
1
1,273
3.5
1.4
4.2
55,281
20.5
87.6
P1-07
1
637
17.1
2.4
7.2
24,375
42.2
44.3
P2-01
2
626
5.2
4.8
7.7
32,730
21.8
43.8
P2-02
2
1,253
2.3
4.1
6.6
67,402
21.8
67.3
P2-03
2
626
5.2
1.8
2.9
32,730
23.2
68.6
P2-04
2
626
7.9
10.8
17.3
31,814
41.5
45.2
P3-01
3
619
13.5
2.5
2.7
34,472
24.4
34.2
2.2 Measurements High speed video imaging was employed to acquire magnified images of growing bubbles as shown in; a Photron FASTCAM Mini UX100 camera equipped with a microscopic objective. The microscopic objective makes a significant difference in measuring bubble diameters at elevated pressures and flow rates. Specific bubble properties can then be measured: bubble size and position, and distance from the wall. When measuring the bubble sizes, the bubble diameter is taken as the square root of the product of the horizontal and vertical lengths, similar to the method used by Okawa [10]. With these measurements, additional quantities can be derived, such as aspect ratio and growth rate. Due to the reflections of the growing bubbles on the curved heated surface, bubble contact angles could not be measured with reasonable certainty. A sample image is shown in Figure 2. Using the wall temperature measured, the Jakob and local Nusselt numbers can be calculated:
Figure 1 Schematic diagram of experimental flow loop
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Ja =
!!,! (!! !!!"# )
Nu! (x) =
!!,!
!! !! !! (!! (!)!!! (!))
Figure 2 Typical image of bubbles observed
(P = 1 bar, Re = 55281, Ja = 4.2, Nux = 87.6)
3. EXPERIMENTAL RESULTS From high speed video images of the vapor bubbles, the size, shape and position of the bubbles were determined during growth and after detachment from the heated surface. For Cases P1-05 and P1-06, the bubble size, aspect ratio, growth rate, and distance from wall data are shown in Figures 3-8. During the growth period, larger bubbles that were not immediately sheared away experienced a balance of evaporation and condensation, resulting in a low frequency change in aspect ratio while attached. As the bubble departed further away from the heated surface, the oscillation in shape became more pronounced, and the bubble size rapidly decreased, slowing down as the bubble collapsed. The bubble trajectory changed with Reynolds numbers, however, accelerating away from the wall at higher Reynolds numbers, and decelerating at low Reynolds numbers.
Diameter (mm)
Bubble Diameter 1.2 1 0.8 0.6 0.4 0.2 0
Bubble 1 Bubble 2 Bubble 3 Bubble 4 Bubble 5 Time (s) Figure 3: Bubble Diameters for Case P1-05
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Diameter (mm)
Bubble Diameter 1.2 1 0.8 0.6 0.4 0.2 0
Bubble 1 Bubble 2 Bubble 3 Bubble 4 Time (s) Figure 4: Bubble Diameters for Case P1-06
Bubble Aspect Ra5o 2 1.5
Bubble 1
1
Bubble 2
0.5
Bubble 3
0
Bubble 4 Bubble 5 Time (s) Figure 5: Bubble Aspect Ratios for Case P1-05
Bubble Aspect Ra5o 2 1.5 Bubble 1
1
Bubble 2
0.5
Bubble 3
0
Bubble 4 Time (s) Figure 6: Bubble Aspect Ratios for Case P1-06
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Distance from Wall (mm)
Distance from Wall 2 1.5
Bubble 1
1
Bubble 2
0.5
Bubble 3
0
Bubble 4 Bubble 5 Time (s) Figure 7: Bubble Distances from Heated Surface for Case P1-05
Distance from Wall (mm)
Distance from Wall 2 1.5 Bubble 1
1
Bubble 2
0.5
Bubble 3
0
Bubble 4 Time (s) Figure 8: Bubble Distances from Heated Surface for Case P1-06
For Cases P2-03 and P2-04, the bubble size, aspect ratio, growth rate data, and distance from the wall are shown in Figures 9-11.While Reynolds numbers are roughly the same in both cases, liquid subcooling and applied heat fluxes were varied greatly, resulting in significant differences in bubble dynamics. During the growth period, Case P2-04 with a higher Jakob number experienced growth rate variances roughly fifty times larger than Case P2-04 with a lower Jakob number, but the departure diameters were roughly the same. For Case P2-03, as the bubble departed further away from the heated surface, the aspect ratio settled at a value of about 1.2. The bubble observed in Case P2-04 detached, experienced a change in volume and subsequently reattached to the wall at various times, but had an average aspect ratio around 1 throughout the process.
Diameter (mm)
Bubble Diameter 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0
Case P2-‐03 Case P2-‐04
Time (s) Figure 9: Bubble Diameters for Cases P2-03 and P2-04
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0.5
Case P2-‐04
0
Time (s) Figure 10: Bubble Aspect Ratios for Cases P2-03 and P2-04
Distance from Wall (mm)
Bubble Distance from Wall 0.1 0.08 0.06 0.04
Case P2-‐03
0.02
Case P2-‐04
0
Time (s) Figure 11: Bubble Distances from Wall for Cases P2-03 and P2-04
4. CONCLUSION Bubbles formed under atmospheric pressure experienced shorter growth durations, however, they were seemingly unaffected by the average liquid Reynolds number, with bubble sizes in the same range. Once departed, however, the speed of bubble motion from the heated surface varied, with high Reynolds number flows causing acceleration beyond a threshold distance, and lower Reynolds number flows causing deceleration from the wall. For bubbles formed at 2 bar, bubble diameters were significantly smaller than at atmospheric conditions. Within this set, low Jakob numbers resulted in bubbles that experienced growth times much longer than those seen at higher Jakob numbers.
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TFESC-12795 ACKNOWLEDGEMENT This work was performed in support of the DOE Nuclear Hub project, CASL. The authors would like to acknowledge financial support for this work from Oak Ridge National Laboratory /UT-Battelle, LLC, through a subcontract under the Department of Energy contract DE-AC05-00OR22725.
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