Near-field acoustic levitation and applications to bearings:a critical review

The importance to industry of non-contact bearings is growing rapidly as the demand for highspeed and high-precision manufacturing equipment increases.As a recently developed non-contact technology,near-field acoustic levitation(NFAL)has drawn much attention for the advantages it offers,including no requirement for an external pressurized air supply,its compact structure,and its ability to adapt to its environment.In this paper,the working mechanism of NFAL is introduced in detail and compared to all existing non-contact technologies to demonstrate its versatility and potential for practical applications in industry.The fundamental theory of NFAL,including gas film lubrication theory and acoustic radiation pressure theory is presented.Then,the current stateof-the-art of the design and development of squeeze film air bearings based on NFAL is reviewed.Finally,future trends and obstacles to more widespread use are discussed.

STUDY ON DYNAMIC CHARACTERISTICS OF ACOUSTIC LEVITATION

The dynamic characteristics of acoustic levitation were investigated for the purpose of containerless processing of materials. Experimental measurements were accomplished on the variations of sound pressure level with both axial position z (6.5mm≤ z ≤27.5mm) and radial position r (1mm≤ r ≤16mm) in a cylindrical resonant tube(34mm×45mm) of a single axis acoustic levitator. The results showed that the minimum sound pressure levels in both axial and radial directions occur at the centres of two transverse intercepting planes with z =8.5mm and z =21.5mm, which were identified as the sound pressure nodes not only of the axial direction but also of the radial direction, and hence were expected to have good levitation stability. This was further proved by experimental observations.

Parameter Scaling of the Aerodynamic Breakup of the Acoustic Levitated Droplets in an Air Jet Flow

The aerodynamic breakup of the droplet has been intensely studied in this paper.We aim to establish a unified relationship between dimensionless kinematic parameters such as displacement,spreading diameter,Weber num-ber,time,and so on.The breakup characteristics of the acoustic levitated ethanol droplet are experimentally inves-tigated when exposed to an air jetflow.The breakup phenomenons were recorded with a high-speed camera.The breakup characteristics were analyzed,and the physical models of the moving and transforming behaviors were established to explain the breakup mechanisms.We found that the displacement of the windward side of the dro-plet follows free acceleration rule,with the displacement,acceleration,and time in the dimensionless form.The spreading of the diameter during deformation can also be written in a simple equation as a function of Weber number and displacement.We also discussed more details.

Single droplet drying of detergents:Experimentation and modelling

This study evaluates the single droplet drying(SDD)of detergents.Experimental data are used to validate a theoretical multistage model.The experiments are carried out in an acoustic levitator analysing six different detergent formulations at 393 K.The work is completed by developing a model based on conservation equations.The model is composed of the three stages observed in the drying curve.In the first stage,an external surface drying stage occurs until the surface is saturated and the crust is formed.In the next stage,the drying rate is governed by diffusion through the pores.At the same time,the particle heats up until boiling conditions are achieved in the core of the particle.At this point,the third stage begins.In the final stage,the droplet core is in boiling conditions which are governed by the pressure drop through the crust and the concentration of salts.This model has been evaluated for the different formulations being flexible to predict the drying of different mixtures with good accuracy.