Evaporation of acoustically levitated droplets
Evaporation of acoustically levitated droplets
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DOI：
10.1017/s0022112099006266
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发表时间：
1999-11-25
影响因子：
3.7
通讯作者：
Tropea, C
中科院分区：
文献类型：
--
作者：
Yarin, AL;Brenn, G;Tropea, C
在声悬浮纯液滴的表面处的传热和传质的速率的理论预测的情况下，其中的声边界层出现在液滴表面附近，导致在声流。该理论基于通过Yarin，Pfaffenlehner和特罗佩亚（1998）中开发的边界元方法计算声学保持和挤压液滴形状。在已知悬浮液滴周围声场的情况下，计算了悬浮液滴表面附近的声流。这允许计算液滴表面上的舍伍德和努塞尔数分布，以及它们的平均值。然后，利用质量平衡法计算了液滴等效半径随时间的演化，当气体的可压缩性很重要时，该理论适用于相对于声波波长λ的任意尺寸的液滴，包括λ量级的液滴。此外，考虑了声场引起的液滴变形，以及液滴中心从压力节点的位移。估计了由气体中的声流维持的液滴中的液体的内循环的效果。液滴表面的时间平均传热传质速率的分布被发现有一个最大值在液滴赤道和最小值在其极点。舍伍德数的时间和表面平均值被示出为由表达式Sh = KB/根Ω D-0描述，其中B = A（0 e）/（rho（0）rho（0））是声波中的速度的标度（A（0 e）是入射声波的振幅，rho（0）是未扰动的空气密度，c（0）是空气中的声速，ω是超声波范围内的角频率，D-0是液体蒸汽在空气中的质量扩散系数，在计算努塞尔数时，应该用空气的热扩散率代替）。系数K取决于控制参数（声场、液体性质）以及当前等效液滴半径a。
The rate of heat and mass transfer at the surface of acoustically levitated pure liquid droplets is predicted theoretically for the case where an acoustic boundary layer appears near the droplet surface resulting in an acoustic streaming. The theory is based on the computation of the acoustic held and squeezed droplet shape by means of the boundary element method developed in Yarin, Pfaffenlehner & Tropea (1998). Given the acoustic field around the levitated droplet, the acoustic streaming near the droplet surface was calculated. This allowed calculation of the Sherwood and Nusselt number distributions over the droplet surface, as well as their average values. Then, the mass balance was used to calculate the evolution of the equivalent droplet radius in time.The theory is applicable to droplets of arbitrary size relative to the sound wavelength lambda, including those of the order of lambda, when the compressible character of the gas how is important. Also, the deformation of the droplets by the acoustic field is accounted for, as well as a displacement of the droplet centre from the pressure node. The effect of the internal circulation of liquid in the droplet sustained by the acoustic streaming in the gas is estimated. The distribution of the time-average heat and mass transfer rate over the droplet surface is found to have a maximum at the droplet equator and minima at its poles. The time and surface average of the Sherwood number was shown to be described by the expression Sh = KB/root omega D-0, where B = A(0e)/(rho(0)rho(0)) is a scale of the velocity in the sound wave (A(0e) is the amplitude of the incident sound wave, rho(0) is the unperturbed air density, c(0) is the sound velocity in air, omega is the angular frequency in the ultrasonic range, D-0 is the mass diffusion coefficient of liquid vapour in air, which should be replaced by the thermal diffusivity of air in the computation of the Nusselt number). The coefficient K depends on the governing parameters (the acoustic field, the liquid properties), as well as on the current equivalent droplet radius a.For small spherical droplets with a