生活水箱内封装相变材料超声波强化传热研究

Study on heat transfer enhancement of phase change material encapsulated in domestic water tank using ultrasound wave

现有生活水箱中封装相变材料强化换热方法主要包括添加翅片或石墨等,这些方法会导致封装结构蓄能密度降低,且石墨等高导热剂会在蓄能过程中发生沉降等,为此,提出将超声波技术应用于生活水箱中封装相变材料,在不降低封装结构蓄能密度的同时,利用超声波在液态介质中会发生空化和声流效应的特性实现相变材料熔化过程的强化换热,并以当前应用最广泛的圆柱型结构封装相变材料为例,通过可视化实验分析其在不同热水温度和不同频率超声波下的强化传热性能。研究结果表明:超声波的应用可显著加快相变材料的熔化,且随着加热时间推移,超声波的强化作用更加明显;当加热温度为60℃时,与无超声波作用相比,28 kHz超声波作用下相变材料液相体积分数的强化效率可由10 min时的12.7%提升至95min时的40.8%;就超声波频率而言,受空化作用发生频率和声压衰减作用的影响,单纯增加超声波频率并不会持续增加相变材料的熔化性能;当超声波频率从20 kHz增大为28 kHz和40 kHz时,相变材料完全熔化时间先减少再增大;热水温度升高会使液相相变材料温度升高和空化阈值降低,从而增强超声波的空化作用;当热水温度为55,58和60℃时,与无超声波作用相比,28 kHz超声波作用下相变材料完全熔化时熔化速率的强化效率分别为42%,44%和48%。

Existing methods for enhancing heat transfer of phase change material(PCM)encapsulated in domestic water tanks mainly include the addition of fins or graphite,but these methods reduce the energy storage density of the encapsulation,and sedimentation may occur during the melting process of the PCM for the high thermal conductivity additions such as graphite.In order to solve these problems,the ultrasonic wave which could generate effects of cavitation and acoustic streaming in the liquid medium was proposed to enhance the heat transfer of cylindrically encapsulated PCM in domestic water tanks without reducing its energy storage density.In order to evaluate its effect,a visualization experiment with different ultrasonic frequencies and different heating temperatures were conducted.The results show that the ultrasound wave significantly accelerates the melting rate of PCM,and its effect increases with the increase of the heating time.At the heating temperature 60℃,the enhancement efficiency of 28 kHz ultrasound wave on the liquid phase volume fraction of PCM is 12.7%at 10 min and 40.8%at 95 min compared to those without ultrasound.As for the ultrasonic frequency,under the influence of cavitation frequency and attenuation effect of sound pressure,the increase of the ultrasonic frequency does not necessarily result in the increase of the enhancement.As the ultrasonic frequency increases from 20 kHz to 28 kHz and 40 kHz,the complete melting time of PCM first decreases and then increases.In addition,the increase of heating temperature can increase the temperature of the liquid phase change material and decrease the effect of cavitation,benefiting the usage of the ultrasonic wave.When the heating temperatures are 55,58 and 60℃,compared to the efficiency without ultrasonic wave,the enhancement efficiency of melting rate with 28 kHz ultrasonic wave are 42%,44%and 48%,respectively,when the PCM is completely melted.