Nuclear Science and Engineering Research

8-2

Research on Boiling Heat Transfer Mechanism=Development of Simultaneous Measurement Technology for Surface Heat Flux and Surface Temperature=
several µmjunctions (measuring points)
common metal film (copper)

1

2

heated surface
heated block

(1)

(3)

(4)

(1) bubble
generation(3)(4) bubble
detachment

3
heatedsurface

heated(2)1 mmbubble

block(copper)heated block

Fig.8-6 Principle of new temperature sensors
New temperature sensors with multiple individual metal
wires (constantan) and a common metal film (copper)
were developed.
(a) outer appearance(b) Microscope photograph of the surface center(before the common copper film was formed.)

nucleustemperature
sensors

sensor diameter
50 µm

500 µm

Fig.8-7 Boiling test piece with new temperature
sensors installed
Manufacturing technology was developed for the newsensors. A technology for placing the sensors in a highdensity of up to six sensors per 1 mm at a depth of lessthan 2 μm from the surface has been established.

Boiling has an extremely high heat transfer coefficient andis widely used in equipments such as heat exchangers,boilers, and boiling water reactors. It has been studied
extensively, but its mechanism still remains unknown. This
research developed a measuring technology for the surface
heat flux and surface temperature. Pool boiling experiments
were performed to elucidate the heat transfer mechanism.
The following technologies are required for the measurementof the surface heat flux and temperature below a boiling bubble.
(1) A non-contact measurement technique without theinstallation of sensors on the surface(2) A high spatial resolution technique to make measurements
at least to three points per 1 mm, considering that the contact
area of a boiling bubble with its heated surface has a radius ofonly about 1 mm(3) A rapid response technique to trace boiling, which is afast phenomenon with the highest frequency component at
more than 500 Hz
To achieve (1), we introduced an analytical approach toheat conduction in which the surface temperature and surface
heat flux are calculated simultaneously using the measured
body temperature. To achieve (2), we need to installtemperature sensors inside the body at a high density of threeor more points per 1 mm because the spatial and time
resolutions of the calculated values depend on those of thedetachingprocess

growing
process

1084bubble(air)

2104
13

three phase line
temp. sensors

1100(2)05water(liquid)

2 3 4heated surface (solid)

ｰ V1 +

1.6 µmindividual
metal wire(constantan)

Measured temperature (℃)

112
ｰ V3 +ｰ V2 +

1 r(mm)0nucleus Boiling test piece

1015

2025time (ms)Fig.8-8 Experimental results of temperature distribution
just below a boiling bubble
We succeeded in deriving the temperature distribution justbelow a boiling bubble from generation to detachment.

measured temperatures. Moreover, the sensors must belocated in close proximity to the heated surface to achieve (3).
To meet these requirements, we developed a newtemperature sensor. The concept is shown in Fig.8-6.
Multiple temperature junctions are formed in a copper block.
Each junction has an insulated constantan wire as its negativepole, but shares a common positive pole made of copper. The
common pole can be formed by sputtering technology, so thejunctions can be located in close proximity beneath thesurface. Fig.8-7 shows a boiling test piece with the newtemperature sensors installed. We established the manufacturing
technology for placing the new sensors at a high density ofup to six points per 1 mm and in close proximity to thesurface (less than 2 µm beneath the surface).
We performed pool boiling experiments. Here, a test piecewith four temperature sensors installed within 1 mm at adepth of 1.6 µm beneath the surface was used. As shown inFig.8-8, we succeeded in deriving the temperature distribution
just below a bubble. The sensors show their lowest
temperatures when the solid-gas-liquid three-phase line
reaches their positions, and this confirms the reasonablenessof the hypothesis that heat transfer near the three-phase line
dominates the boiling heat transfer.
The present study was entrusted by the Ministry of Education,Culture, Sports, Science and Technology of Japan (MEXT).

Reference
Liu, W. et al., Development of Measurement Technology for Surface Heat Fluxes and Temperatures, Nuclear Engineering and Design, vol.249, 2012, p.166-171.

88 JAEA R&D Review 2012