声波作用下铜球传热特性实验

Experimental study of acoustically enhanced heat-transfer from copper sphere

针对频率为500 Hz～3000 Hz和声压级为110 d B～133 d B的声场作用对铜球在空气中自然冷却的传热特性的影响,通过热电偶测温的方法,分析铜球温度梯度的分布与声场声压级、频率以及铜球直径的关系。结果表面,当频率f一定时,随着声压级的增加,铜球的传热效果得到明显增强,对于直径为5 mm的铜球,在133 d B声场中传热系数最大增加了25%。当声压级一定时,在频率范围中存在某一频率,此时铜球的传热系数最大,此特殊频率随着声压级的增大而增大。当铜球的直径为5 mm时,可以在低频段观测到声流效应的影响,而当铜球的直径为10 mm、15mm时,很难在低频段辨别出声流效应的影响。所得结论为声波应用于电站锅炉中,强化煤颗粒燃烧提供了依据。

The effects of acoustic frequency(500 Hz～3000 Hz) and sound pressure level(110 d B～133 d B) on the nature-convective heat transfer rate during cooling of a stationary sphere were investigated experimentally.The spheres were immersed in the air(27?C). The temperature of the sphere was recorded using a data logger equipped with a T-type thermocouple in the center of the spheres. The relationship between sound pressure levels, frequency and the diameter of the sphere with the temperature gradient of the sphere were analyzed.The results show that higher sound pressure levels cause higher heat-transfer rate. When the diameter of the copper sphere is 5 mm, the heat transfer rate could be increased up to 1.25 times for SPL = 133 d B.However, there is a special frequency, at which Nu reaches a maximum, due to the flow separation caused by flow acceleration. This special frequency increases with increasing SPLs. When the sphere size is 5 mm, at low frequencies(500 Hz～700 Hz), acoustic streaming is the dominant factor. It results in an increasing Nu with decreasing frequency. However, when the sphere size is 10 mm or 15 mm, the effect of acoustic streaming is hardly detectable in the low frequency band. The conclusions highlight a good application prospect in combustion for the boiler.