# Experimental Study of the Effect of Ultrasound on the Freezing Process of Bo Chinh Ginseng
## 3 Results and Discussions  
### Experimental Effect of Ultrasound Power on Freezing Time of Bo Chinh Ginseng  
The experimental freezing of Bo Chinh ginseng was supported by ultrasound irradiation at a 20 kHz frequency with different ultrasound powers of 0 W, 50 W, 100 W, 150 W and 200 W.  
The test results are shown in the figure below:  
Under the same conditions of ambient temperature, initial material temperature, freezing temperature and air velocity in the freezing chamber, the experimental results show that the ultrasound-assisted freezing mode at a 20 kHz frequency with power levels of 50 W, 100 W, 150 W and 200 W all had a shorter freezing time than the method of freezing without ultrasound support. Previous studies have also shown that ultrasound irradiation traveling through a solid medium causes the solid's molecules to vibrate, causing compression and expansion along with pressure changes, and, ultimately, creating  
Figure 1: Schematic diagram of an ultrasound-assisted freezer: 1—compressor of the 1 st stage; 2—condenser; 3—high-pressure receiver of the 1 st stage; 4—filter-drier of the 1 st stage; 5—sight glass of the 1 st stage; 6—solenoid valve of the 1 st stage; 7—expansion valve of the 1 st stage; 8—heat exchanger; 9—compressor of the 2 nd stage; 10—high-pressure receiver of the 2 nd stage; 11—filter-drier of the 2 nd stage; 12—sight glass of the 2 nd stage; 13—solenoid valve of the 2 nd stage; 14—expansion valve of the 2 nd stage; 15—evaporator; 16—temperature probe; 17—freezing material; 18—emission plate; 19—return air grille; 20—transducer; 21—ultrasound generator; 22—temperature recorder. a \"bubble effect\" which creates microchannels that promote heat and mass transfer and shorten the freezing time [19]. As shown in Figure 2, when the ultrasound power level was increased from 0 to 50 W, the freezing time decreased from 15.1 to 11.6 min (23.2%). Increasing the ultrasound power to 100 W only reduced the freezing time to 0.9 min, which was 7.8% better as compared to the freezing time at 50 W. However, if the ultrasound power continues to increase to 150 W and 200 W, the freezing time tends to increase again as compared to the minimum freezing time corresponding to the 100 W ultrasound power. Specifically, at the power level of 150 W, the freezing time increases by 7.5% as compared to the freezing time at 100 W, and at the ultrasound power of 200 W, the increase is 15% compared to the freezing time at 100 W, which is only a decrease of 18% compared to freezing without ultrasound. The time needed to freeze materials with the help of ultrasound irradiation was lowest at the power level of 100 W Freezing with the help of ultrasound irradiation with a capacity of 100 W saves 29.1% of the freezing time as compared to freezing without ultrasound. However, when the transmitting power is increased, the freezing time also increases due to the thermal effect generated by the ultrasound irradiation [12]. In fact, the total freezing time does not accurately reflect how ultrasound irradiation affects the freezing process of Bo Chhin ginseng. The freezing time is often calculated by obtaining the sum of the time of the three stages: cooling, phase transition and supercooling. Figure 3 shows the freezing curves for the freezing modes with and without the help of ultrasound irradiation, which can be divided into three stages: the cooling phase (from 26 \({}^{\circ}\)C to 0 \({}^{\circ}\)C), phase transition (\(0\div-5\) \({}^{\circ}\)C) and supercooling period (\(-5\div-35\) \({}^{\circ}\)C). From Figure 3, it can be seen that the cooling and phase transition times are faster than the time required for the supercooling stage in all different freezing modes. Figure 4 shows the time at each stage, in which the cooling and phase transition stages in the freezing mode without ultrasound support have shorter times than the other modes. Considering the phase transition, it can be clearly seen that for ultrasound-assisted freezing, the time is much shorter than that of the unassisted freezing mode.