Effect of ultrasonic on freezing state and heat transfer of droplet

doi:10.11949/0438-1157.20200301

Abstract

Abstract:

Ultrasonic -assisted freezing has been widely used in food, medical and biochemical fields. To clarify the effect of ultrasonic wave on the variations of liquid-solid ratio and void ratio in freezing state, and to reveal heat transfer law in the process of freezing transformation, the mathematical model of ultrasonic droplet freezing was established which was combined with the theory of acoustics and heat & mass transfer based on the experiment. The droplet freezing state under the action of ultrasound was analyzed, and the heat dissipation caused by ultrasonic cavitation effect and heat production caused by ultrasonic thermal effect was compared. The result shows that the ultrasonic wave is helpful to bubble overflow on surface of the droplet, and the larger the overflow rate is, the smaller the liquid-solid ratio is in the freezing process; when the bubble overflows is certain, the smaller the droplet is, the larger the porosity is; when the ultrasonic wave is certain, the smaller the overflow rate is, the larger the porosity is in the freezing process; for the cooling and freezing process, there is a reasonable ultrasonic loading time for different ultrasound frequencies and intensities.

Key words: ultrasound, freezing state, heat and mass transfer

摘要：

超声波辅助冻结在食品、医疗及生物化学等领域得到了广泛的应用，为了明确超声波对溶液冻结过程中液固比例、孔隙率的影响，揭示冻结过程中热量的传递规律，在液滴冻结状态实验观测的基础上，结合声学及热质传递理论，建立了超声波液滴冻结理论模型，分析了超声波对液滴冻结状态的影响，比较了超声空化效应所引起的传质散热量和超声热效应引起的产热量的大小关系。结果表明：超声波有助于液滴表面的气泡逸出，气泡逸出率越大冻结过程中液固比例越小；当气泡逸出率一定时，液滴越小冻结过程中的孔隙率越大；超声波一定时，逸出率越小冻结过程中的孔隙率越大；对于冷却冻结过程，不同频率和强度的超声波存在合理的超声波加载时间。

关键词: 超声波, 冻结状态, 传热传质

CLC Number:

TQ 025.3

Jian CONG,Penghui GAO,Donghai ZHANG,Jinpeng ZHOU,Zhenghan ZHANG. Effect of ultrasonic on freezing state and heat transfer of droplet[J]. CIESC Journal, 2020, 71(11): 5117-5128.

丛健,高蓬辉,张东海,周晋鹏,张正函. 超声波对液滴冻结状态及传热的影响[J]. 化工学报, 2020, 71(11): 5117-5128.

Figures/Tables 13

Fig.1 Experimental system of liquid drop freezing state with ultrasonic

Fig.2 Schematic diagram of droplet freezing

Fig.3 Variation of droplet porosity during freezing

Fig.4 Droplet porosity variation with time

Fig.5 State of droplet in the process of freezing

Table 1 Parameters of the model

Parameter	Symbol	Unit	Value
ultrasonic frequency	f	Hz	30000/35000/40000
ultrasonic intensity	I	W?m^-2	600/800/1000
sound velocity	c	m?s^-1	1480
bubble overflow rate	φ	1	0.10/0.05/0.01
ambient temperature	T_∞	K	258.15
droplet’s initial temperature	T₁	K	288.15
droplet’s diameter	D	mm	2.0/2.5/3.0
diffusion coefficient	D_V	m²?s^-1	2.55×10^-5
ambient air velocity	u	m?s^-1	0.1

Fig.6 Variation of droplet’s diameter in different initial size and bubble flow rate

Fig.7 Variation of droplet proportion with different initial parameters

Fig.8 Effect of ultrasonic frequency and intensity on the porosity of droplet

Fig.9 Liquid proportion of droplet in the process of freezing

Fig.10 Heat dissipation induced by mass transfer and heat generated by thermal effect in different ultrasonic intensity

Fig.11 Heat dissipation induced by ultrasonic cavitation and heat generated by thermal effect in different ultrasonic frequency

Fig.12 Heat dissipation induced by ultrasonic cavitation and heat generated by thermal effect in different bubble overflow rate

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