In our experiment, there were four schemes. They were
performed as follows: (a) seven test tubes containing water
were placed inside the vacuum chamber. Water used in the
experiment was first boiled and allowed to cool to 32 ± 0.5 C.
Each test tube was filled with 100 mL volume of water; (b) into
one of the test tubes, a thermocouple was embedded at 1 cm
below the water surface to measure the temperature change
during the cooling process; (c) the chamber door was closed
tightly; (d) images were taken at a frame rate of 100 frames s1
by the high speed video camera during the whole vacuum
cooling process; (e) when the cold trap was cooled to - 6 C,
both the vacuum pump and frequency convertor were
switched on, and the pressure of vacuum chamber was
reduced to 1000 Pa and maintained at this range until the
completion of experiment. The volumetric displacement of
vacuum pump was 0.002 m3 s1 at stages of “before boiling”
and “after boiling”. However, different volumetric displacements
of vacuum pump were used (0.002m3 s1, 0.0016m3 s1,
0.0012 m3 s1 and 0.0008 m3 s1 corresponding to Schemes.
1e4 respectively) at the stage of “during boiling” (Fig. 2); (f)
when the temperature measured by the thermocouple
reached 10 C, the vacuum cooling process was stopped