Influence Of Acoustic Excitation Research Articles

Effect of Acoustic Excitation on R134a/Al2O3 Nanolubricant Mixture Boiling on a Reentrant Cavity Surface

This paper quantifies the influence of acoustic excitation of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a commercial (Turbo-BII-HP) boiling surface. A nanolubricant with 10 nm diameter Al2O3 nanoparticles at a 5.1% volume fraction in the base polyolester lubricant was mixed with R134a at a 1% mass fraction. The study showed that high-frequency ultrasound at 1 MHz can improve R134a/nanolubricant boiling on a reentrant cavity surface by as much as 44%. This maximum enhancement occurred for an applied power level to the fluid of approximately 6 W and a heat flux of approximately 6.9 kW/m2. Applied power levels larger and smaller than 6 W resulted in smaller boiling heat transfer enhancements. In total, five different applied power levels were studied: 0 W, 4 W, 6 W, 8 W, and 12 W. The largest and smallest enhancement averaged over the tested heat flux range were approximately 12% and 2% for the applied power levels of 6 W and 4 W, respectively. In situ insonation at 1 MHz resulted in an improved dispersion of the nanolubricant on the test surface. An existing pool-boiling model for refrigerant/nanolubricant mixtures was modified to include the effect of acoustic excitation. For heat fluxes greater than 25 kW m−2, the model was within 4.5% of the measured heat flux ratios for mixtures, and the average agreement between measurements and predictions was approximately 1% for all power levels.

Phase-averaged characteristics of flow around a circular cylinder under acoustic excitation control

The influence of internal acoustic excitation on the aerodynamic performance of a circular cylinder in a uniform flow is studied by the phase-averaged measurements of fluid forces and flow fields around the cylinder. The experimental results indicate that the mean drag as well as the fluctuating lift are reduced by the acoustic excitation control from a slit on a circular cylinder. The phase-averaged PIV measurements at the vortex shedding frequency show that the periodic and fluctuating velocities are suppressed in the wake of the controlled cylinder, which indicates the influence of acoustic excitation on the vortex shedding and the corresponding reduction in the fluid forces on the circular cylinder. On the other hand, the phase-averaged flow field at the excitation frequency of unstable shear layer, which is four times larger than the vortex shedding frequency, indicates the formation of discrete vortices along the shear layer of the cylinder wake. The interaction of these vortices with the cylinder wake causes the reduction in the cross-correlations of velocity fluctuations in the flow field, which results in the attenuation of the vortex shedding phenomenon in the cylinder wake.

Flow control around a circular cylinder by internal acoustic excitation

The possible drag reduction and the corresponding variation of the flow field around a circular cylinder are studied in a wind tunnel using flow control with acoustic excitation, which is supplied internally through a slit to the flow over the cylinder. The drag and the lift force acting on the cylinder are evaluated from the measurement of pressure distributions over the cylinder. The results indicate that the drag is reduced about 30% in comparison with a stationary cylinder, when the control parameters are optimized, such as the slit angle, the forcing Strouhal number and the excitation amplitude. The corresponding flow field around the cylinder with and without control is measured by particle image velocimetry. It is found that the wake of the circular cylinder is elongated downstream and the streamwise mean velocity is accelerated on both sides of the cylinder by the influence of acoustic excitation. On the contrary, the velocity fluctuations and the Reynolds stress in the near wake of the cylinder are strongly diminished under acoustic excitation. These variations of the wake flow support the drag reduction observed in the present measurements.

Streaming of a particulate aerosol due to acoustic excitation

The acoustic equations pertaining to an aerosol containing small solid particles in suspension are developed. In order to study the acoustic streaming of this medium, that is, the steady unidirectional motion of particles which is superimposed upon the oscillatory acoustic motion, it it necessary to carry the analysis to so-called second order acoustic variables. It is shown that the inclusion of the dynamic and thermal properties of the suspended particles significantly influences the attenuation of sound and the streaming of the medium. Since the well known but presently unexploited process of sonic agglomeration of particulate aerosols depends to a large extent upon the acoustic streaming phenomenon, it is suggested that this work may be useful in further development of this process. A numerical example describing the streaming of an aerosol in a duct under the influence of acoustic excitation is developed in some detail.