A study of acoustic cavitation and hydrogen production

Abstract

This thesis presents a study of acoustic cavitation generated in an ultrasonic reactor, with the particular aim of enhancing hydrogen gas production and release. The stabilisation of cavitation clusters formed in a set of ultrasonic reactors is demonstrated. The highly stable cluster is induced by the positioning of a rod at the antinode of the sound field employed. These sound fields were characterised with a new technique based on particle imaging. Here adding rheoscopic particles within such reactors revealed a novel and useful method for the characterisation of standing wave fields. This observation was supported by pressure measurements using a hydrophone. In addition the stabilised cluster was investigated using an electrochemical method to monitor the erosion of the surface directly above the cluster, at short (10’s of ?s) and long (100’s of s) timescales. Both timescales indicate changes in the stability and nature of the cluster, which in turn is dependent on the local surface conditions (roughness) of the rod/electrode assembly.

Low light level imaging of the stabilised cavitation cluster demonstrates the occurrence of sonochemiluminescence (SCL). It is shown that the spatial extent of light emitted via SCL is correlated with the pressure amplitude of the sound field. A visual ‘shimmer’ effect is also shown to be emanating from stabilised cavitation clusters. This is attributed to local heating which in turn induces refractive index changes, which are enhanced through the use of Schlieren imaging. This local cluster induced-heating of the liquid is quantified using a variety of physical measurements. Investigation into the ultrasonic enhancement of the production of molecular hydrogen from aluminium corrosion is made. This study showed that the sonochemical enhancement was insignificant compared to local heating effects associated with the sound field. Analysis of the performance of an electrolysis system, designed and manufactured by the project sponsors (HTOGO Ltd.), is reported. Measurement of the hydrogen gas produced by the system highlights a low Faradaic efficiency and long response time for gas release. An innovative method for the rapid release of gas via ultrasonic outgassing of a liquid reservoir, containing hydrogen and oxygen gas bubbles, is demonstrated. A novel optically isolated Coulter counter system for the in-situ determination of the size distribution of bubbles in a bubbly liquid reservoir is reported. This thesis illustrates the underpinning principles of this technique and the determination of the best calculation method for successful calibration and accurate measurement of the bubble size distributions generated in an electrochemical reactor. The knowledge gained and the new technology developed in this project is expected to accelerate and improve the development of the

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ORCID for Peter Birkin: orcid.org/0000-0002-6656-4074

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