Visualization of boiling FC-72 on plate heat exchanger
Kenta Hayashi*, Hirofumi ARIMA**, Takuya Tanaka****

Graduate school of Sci. Eng., Saga Univ.
1-Honjo, Saga-shi, Saga 840-8502, Japan
Email: 21728011@edu.cc.saga-u.ac.jp**
Institute of Ocean Energy, Saga Univ., 1-48, Hirao, Kubara-aza, Yamashiro-cho, Imari-shi, Saga, 849-4256, Japan***
Faculty of Sci. Eng., Saga Univ., 1-Honjo, Saga-shi, Saga 840-8502, Japan

Abstract
To improve the performance of the plate heat exchanger for OTEC, visualization was performed to confirm the boilingbehaver inside the plate heat exchanger. The anodic oxidation aluminum heat plate has been adopted as a heat exchange
plate. However, there are no adoption of the anodic oxidation aluminum plate for the plate heat exchanger. In thisexperiment, flow boiling was visualized using FC-72 as the working fluid using a newly adopted aluminum plate and aplate heat exchanger with a visualization window, which is an improved frame of the plate heat exchanger. The heattransfer surfaces used in the experiments were a smooth surface and a herringbone surface, both of which were anodized
aluminum plates. It is intended for use with ammonia and is coated for corrosion resistance. In this experiment, theexperimental conditions are mass flux G (10, 20, 30, 40kg/m2s), volumetric flow rate of hot water mhw (2.5, 5, 7.5,10L/min), degree of subcooling of working fluid ΔTsub (0, 5, 10K), and the boiling phase was taken a video with a highspeed camera. From this experiment, the following things were found. In case of the flow rate of hot water increases,
the amount of bubbles nucleation increases and the size of the bubbles increases. The void fraction distribution in the flow
direction was obtained by visualization image analysis. The heat transfer coefficient increased with the increase of quality
x. At x > 0.8, the heat transfer coefficient was decreased due to dry-out was observed. It was also confirmed that the heattransfer coefficient of the working fluid increases as the mass flux of the working fluid increases.
KeyWords : Boiling, Visualization, Herringbone plate, Anodic Oxidation Aluminum, FC-72

1. Introduction
In order to improve the performance of plate evaporators for ocean thermal energy conversion, it is essential to clarify
the boiling heat transfer characteristics of plate evaporators. Among them, we have conducted visualization observations
to clarify the boiling behavior in plate heat exchangers. In this study, a sight glass was installed in the plate-type evaporator
frame in order to obtain a wide visualization area. Visualization experiments was performed using it. Boiling heat transfer
was measured and the boiling behaver was visualized.

2. Experiment
2.1 Experimental apparatus
The experiment was a forced convection boiling experiment of FC-72. Fig. 1 shows that the schematic diagram ofexperimental apparatus. The experimental apparatus consists of a test section (plate heat exchanger), a hot water tank, ahot water pump, a working fluid tank, a working fluid pump and a preheater. In addition, the equipment has two circulation
cycles: a working fluid and a hot water circulation system. The test section has a visualization sight glass on the frame ofplate heat exchanger. In addition, the working fluid pump is a magnet pump, the hot water pump is also a magnet pump.
The hot water tank consists of a stainless-steel tank and immersion heaters. The pre-heater has a structure in which aribbon heater is wrapped around the surface of a copper tube.
State quantities were measured with various sensors. The temperature was measured with a K-type thermocouple, theworking fluid flow rate was measured with Coriolis mass flowmeter and volumetric flow rate of hot water was measured
with magnetic flowmeter. These values measured by each sensor were collected and recorded in a data logger.
Fig. 2 shows a schematic diagram of the test section, and Fig. 3 shows the test plates. This test plate is surface-treatedby anodic oxidation specified in JIS H8601 in order to provide corrosion resistance. Also, the film thickness is equivalentto AA15 (20 to 25 μm by actual measurement). Table 1 shows the experimental conditions of this experiment.

Fig. 1 Schematic diagram of experimental apparatus.

Fig. 2 Assembly of test section.

(a) Smooth plate(b) Herringbone plate
Fig.3 Test plates.

Table 1 Experimental conditions.
Mass flux of working fluid G [kg/m2s]
Volumetric flow rate of hot water mhw [L/min]
Degree of subcooling of working fluid ΔTsub [K]10, 20, 30, 40
2.5, 5, 7.5, 10
0, 5, 10

2.2 Data reduction
The overall heat transfer coefficient U, boiling heat transfer coefficient hwf, pressure drop ΔPfr, and duct friction
coefficient ftp were obtained. These values were calculated by following equations:
𝒬𝑈=(1)∆ℎ= 1/( −∆𝑃 = ∆𝑃(2)− )− ∆𝑃 − ∆𝑃− ∆𝑃

𝑓 = ∆𝑃(3)(4)

2.3 Procedure of visualization
The boiling behavers were visualized by the following method. A high-speed camera was arranged in front of the sightglass. A cold lamp was arranged in front of the sight glass as a light source. The visualization movie was recoded periodof 2 seconds at the frame rate (2000 fps) and resolution (1280 × 1028 pixels).

2.4 Analysis visualization data
Visualization data was analyzed by the following method. According to a two seconds length visualization movie wasdivided into each frame, 2000 image files were made. An image file was analyzed by developed software. In the image
analysis, the distribution of bubbles in an image file was extracted by using HOG method with machine learning. The

void fraction was calculated based on the bubble distribution.

3. Result and discussion
3.1 Experiment of visualization
Fig 4 and 5 show visualized images obtained in the experiment. According to Fig. 4, at the flat plate, in case of the massflux of the working fluid and the degree of subcooling are same condition, the amount of bubbles nucleation and the sizeof the bubbles increases with an increase in the volumetric flow rate of hot water.
According to Fig. 5 at the herringbone plate, it can be shown that in case of the mass flux of the working fluid and thedegree of subcooling are equal, the amount of bubbles nucleation and the size of the bubbles also increases with anincrease in the volumetric flow rate of hot water increases with an increase in the volumetric flow rate of hot water.

(a) G = 20 kg/m²s(b) G = 20 kg/m²s(c) G = 20 kg/m²s
mhw = 2.5 L/min
mhw = 5 L/min
mhw = 10 L/min
ΔTsub =10 K
ΔTsub =10 K
ΔTsub =10 K
Fig. 4 Comparison of boiling phenomenon at different flow rate of hotwater on flat plates.

(a) G = 10 kg/m²s(b) G = 10 kg/m²s
mhw = 2.5 L/min
mhw = 3.9 L/min
ΔTsub = 0K
ΔTsub = 0K
Fig. 5 Comparison of boiling phenomenon atdifferent flow rate of hot water on herringbone
plates.

3.2 Analyzed visualization data
The visualized image obtained by the visualization experiment was analyzed with developed software (Tanaka 2021),
and the bubble distribution on the heat transfer surface was obtained. The results are shown in Fig. 6 (a) and (b). Fig 6 (a)and (b) are the results obtained at different mass fluxes ((a) G = 20 kg/m²s, (b) G = 40 kg/m²s). (1) and (2) in Fig.6 (a)and (b) are the original image (1) obtained by visualization, and the bubble distribution image (2) obtained by image
analysis.
In Fig. 6(a)-(1), there are almost no bubbles near the working fluid inlet at the bottom right of the image. On the other
hand, bubbles originate from the lower left area. Furthermore, as the bubbles rise to the top of the figure, their diameterincreases and they coalesce. In the lower right region, no bubbles were observed because the working fluid wassupercooled. In addition, the working fluid is subcooled in the lower left region, which coincides with the hot water side
exit across the heat transfer surface. As shown in Fig. 6(a)-(2), these differences can be clearly seen as differences inbubble distribution between the lower right region and the lower left region by image processing.
On the other hand, under the condition of Fig. 6(b) where the mass flux is large, the distribution of bubbles can be seen
over the entire area as shown in Fig. 6(b)-(1). Also, at the inlet, the bubble diameter is generally larger than his Fig. 6(a)condition. Since the mass flux is larger in Fig. 6(b), the quality at the plate exit is smaller and the distribution of bubbles
is thought to be smaller. From this, as can be seen in Fig. 6(a), there is almost no difference the bubbles distribution
between right and left area at the bottom of the plate. The reason of that the bubbles are distributed over the entire heattransfer surface. Incidentally, the image analysis results are shown in Fig. 6(b)-(2), and bubbles weren’t detected in thelower left area. The reason is that the image analysis program using machine learning misidentified bubbles as noise.
Fig. 7 compares the void fraction distribution in the flow direction under the conditions of Fig. 6(a) and (b). The void
fraction distribution in Fig. 6(a) showed a tendency to increase sharply from near the center of the heat transfer surface
in the height direction (x = 150 mm). On the other hand, the void fraction distribution in Fig. 6(b) was almost constantover the entire heat transfer surface.

3.3 Measurement of overall and boiling heat transfer coefficientFig. 8 shows the overall heat transfer coefficient against to the hot water flow velocity. Both smooth and herringbone

surfaces showed a decrease in overall heat transfer coefficient with an increasing in hot water flow velocity. In addition,
Fig. 9 shows heat transfer coefficient against to the outlet quality. In both cases, it can be seen that at the higher the quality,
the higher the heat transfer coefficient were observed.

(1) Original(2) After analysis(1) Original(2) After analysis(a) Run 1 (G = 20 kg/m²s,(b) Run 2 (G = 40 kg/m²s,mhw = 2.5 L/min, ΔTsub =10 K)
mhw = 2.5 L/min, ΔTsub = 5 K)
Fig. 6 Analysis of boiling phenomenon.
2000

Fig. 7 Distributions of void fraction
against flow direction at different massflux of working fluid.

2000

1000

00

0.5

1

U[W/m2K]

G [kg/m s]
1020
30
40

2

U[W/m K]

2

G [kg/m2s]
102030
40

1000

00

1.5

0.5

Vh[m/s]11.5

Vh[m/s](a) Smooth surface plate, ΔTsub = 0(b) Herringbone plate, ΔTsub = 0Fig. 8 Overall heat transfers against hot water flow velocity.
3000

2G [kg/m s]
1020304022000

hwf[W/m K]

hwf[W/m2K]

3000

1000

000.2

0.4

0.6

0.8

1

2000

1000

0x[-]

2

G [kg/m s]
102030
400.2

0.4

0.6

0.8

1x[-]

(a) Smooth surface plate(b) Herringbone plate
Fig. 9 Heat transfer coefficients of FC-72 against vapor quality.

4. Conclusion(1) As increasing the hot water flow rate, the amount of bubbles nucleation the size of the bubbles increases.
(2) The void fraction distribution in the flow direction was obtained by visualization image analysis.
(3) The heat transfer coefficient increased with the increase in quality x. At x > 0.8, the heat transfer coefficient wasdecreasing due to dry-out.
(4) It was also confirmed that the heat transfer coefficient of the working fluid increases as the mass flux of the working
fluid increases.

References
Takuya Tanaka, Analysis of visualization image of boiling behaver on plate heat exchanger using machine learning, Thesisof under graduate school, Faculty of Sci. Eng., Saga Univ (2021).