Design parameters of stainless steel plates for maximizing high frequency ultrasound wave transmission Mark Michaud a,b , Thomas Leong a,c , Piotr Swiergon a , Pablo Juliano a , Kai Knoerzer a,⇑ a CSIRO Food and Nutrition Flagship, 671 Sneydes Road, Werribee, VIC 3030, Australia b Institute of Biological and Chemical Engineering, University of Erlangen-Nuremberg, Germany c Mechanical and Product Design Engineering, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Australia article info Article history: Received 30 September 2014 Received in revised form 15 December 2014 Accepted 8 January 2015 Available online xxxx Keywords: Ultrasound Transmission Non-contact Transducer abstract This work validated, in a higher frequency range, the theoretical predictions made by Boyle around 1930, which state that the optimal transmission of sound pressure through a metal plate occurs when the plate thickness equals a multiple of half the wavelength of the sound wave. Several reactor design parameters influencing the transmission of high frequency ultrasonic waves through a stainless steel plate were examined. The transmission properties of steel plates of various thicknesses (1–7 mm) were studied for frequencies ranging from 400 kHz to 2 MHz and at different distances between plates and transduc- ers. It was shown that transmission of sound pressure through a steel plate showed high dependence of the thickness of the plate to the frequency of the sound wave (thickness ratio). Maximum sound pressure transmission of 60% of the incident pressure was observed when the ratio of the plate thickness to the applied frequency was a multiple of a half wavelength (2 MHz, 6 mm stainless steel plate). In contrast, minimal sound pressure transmission (10–20%) was measured for thickness ratios that were not a mul- tiple of a half wavelength. Furthermore, the attenuation of the sound pressure in the transmission region was also investigated. As expected, it was confirmed that higher frequencies have more pronounced sound pressure attenuation than lower frequencies. The spatial distribution of the sound pressure trans- mitted through the plate characterized by sonochemiluminescence measurements using luminol emis- sion, supports the validity of the pressure measurements in this study. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Ultrasound is a versatile processing tool suitable for a range of industrial applications. Low frequency ultrasound (20–100 kHz), also known as power ultrasound is associated and characterized by intense physical phenomena arising from the violent collapse of bubbles due to acoustic cavitation, and is suitable for applica- tions such as homogenization [1], emulsification [2], crystallization [3] and extraction [4]. High frequency ultrasound (400 kHz– 2 MHz) on the other hand, is usually characterized by less violent bubble collapse, making it suitable for cleaning of sensitive compo- nents [5], sonochemical modification [6] and separation of multi- component mixtures [7,8]. In some industrial applications, the process can be at very high temperatures or involve corrosive and/or toxic chemicals that may be hazardous when placed in direct contact with a transducer surface. In such applications, it is desirable for an ultrasonic trans- ducer not to be placed in direct contact with the fluid medium. An example would be in the processing of high temperature fluids like petroleum and palm oil [9,10], as it would reduce the risk of heat damage and enable easier access for periodic cleaning and mainte- nance. Another example would be the treatment of toxic, non-aque- ous solvents, or corrosive materials, where direct contact with maintenance crew or sensitive material has to be avoided. For prod- ucts that are sensitive to harsh treatment (i.e., food, pharmaceuti- cals, etc.), known problems such as generation of high temperatures and release of metal particulates when transducers are positioned in direct contact with the fluid, should also be avoided. A possible solution to these problems is placing the transducer externally to the reactor such that there is a cavity where cooling liquid can be circulated between the ultrasonic transducer and the reactor walls. This however means that the emitted sound wave has to be transmitted through an additional layer of metal (i.e., a transmission plate). Information such as the pressure distribution of transducers positioned in large-scale systems is documented in a few selected http://dx.doi.org/10.1016/j.ultsonch.2015.01.007 1350-4177/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: kai.knoerzer@csiro.au (K. Knoerzer). Ultrasonics Sonochemistry xxx (2015) xxx–xxx Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson Please cite this article in press as: M. Michaud et al., Design parameters of stainless steel plates for maximizing high frequency ultrasound wave transmis- sion, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.01.007