Understanding the spatial mass transfer behaviour of a pulsating jet HMV system – Vortex generation and characterisation

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Abstract

The flow patterns generated by a pulsating jet used to study hydrodynamic modulated voltammetry (HMV) are investigated. It is shown that the pronounced edge effect reported previously is the result of the generation of a vortex ring from the pulsating jet. This vortex behaviour of the pulsating jet system is imaged using a number of visualisation techniques. These include a dye system and an electrochemically generated bubble stream. In each case a toroidal vortex ring was observed. Image analysis revealed that the velocity of this motion was of the order of 250 mm s−1 with a corresponding Reynolds number of the order of 1200. This motion, in conjunction with the electrode structure, is used to explain the strong ‘ring and halo’ features detected by electrochemical mapping of the system reported previously.

Highlights

► High-speed images of the flow generated from a pulsating jet used to study HMV are shown. ► Dye solutions or bubble tracers show the flow under different conditions. ► A repetitive vortex is studied as it moves through the fluid and onto an electrode surface. ► This vortex, and its characteristics, produces the spatial mass transfer pattern observed.

Introduction

The investigation of the characteristics of a pulsating jet [1], [2], [3] for hydrodynamic modulated voltammetry (HMV) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17] applications has been reported in a number of previous articles. In brief, the apparatus (an inverted funnel and a narrow (∼2 mm in diameter) jet orifice, produces a reproducible hydrodynamic pulse of liquid which can, in combination with electrochemical apparatus, be useful. For example, this approach has been used to investigate HMV behaviour of a variety of redox systems [2], [3], [18]. In addition, the detection limit of the system [3] and its use to investigate nanostructured [19], [20], [21] electrodes has been demonstrated [1]. However, in the course of previous investigations, mapping of the electrochemical response of the system identified some unusual characteristics [2]. First, the current pulses produced by this apparatus exhibited a strong ‘ring’ like relationship with the edge of the jet mouth [2]. Second, the size of the electrode surround (or support) also affected the mapping data, with a ‘halo’ effect detected. Third, the HMV response of the jet could be detected at a considerable distance (∼1 cm) from the mouth of the jet. While these observations do not affect the operation of the apparatus as a HMV system, they do require further explanation. This is the subject of this manuscript. In order to investigate these observations further, a set of experiments were performed to image the flow patterns produced by the pulsating jet. There are many examples in the literature [22], [23], [24], [25] (see Ref. [25] for general examples) where flow visualisation has been achieved through the use of a variety of different approaches, the experiments described here utilised dye species and gas bubble generation in efforts to visualise the flow of material from the jet into the bulk solution and then onto an electrode support [25], [26]. These two techniques were chosen as they enabled detailed imaging of the first pulse (in the case of the dye system) or an investigation of the repeated fluid flow as a result of the pulsating jet (in the case of the bubble tracer method). In addition it should be noted that the flow generated in this system is periodic in nature. A brief discussion of these techniques and the results of experiments performed on the pulsating jet system will now be presented.

Section snippets

Materials and methods

The experimental arrangement used in this work and specifically its operation to gather useful hydrodynamic modulated voltammetric data has been reported previously. The modulated jet was generated by forced oscillation of a ∼3.5 cm radius membrane attached to the base of an inverted funnel. The neck of the funnel was then pulled into a 2 mm diameter jet orifice. Oscillation of the membrane was driven by a mechanical shaker (a Model V4, Signal Force, Data Physics Ltd.) attached to the centre of a

Results and discussion

Fig. 2 shows a set of images taken, with respect to time, of the neck of the pulsating jet (note the solid black line towards the top of each image in (a) represents the air/liquid interface). In this case the funnel section of the apparatus was filled with water coloured with a red dye while the upper chamber was filled with purified water (see Fig. 1). In this case some loss of the dyed volume is seen at the neck of the jet itself (see Fig. 2a, A1). The mini-shaker was initiated and the

Conclusions

The results shown here clearly demonstrated that the pulsating jet apparatus gives rise to the generation of a repetitive vortex ring which travels through the liquid and impinges on the surface of the electrode. This vortex has a Re value of the order of 1200 in free liquid and is observed to slow on approach to the electrode substrate. On impingement with the electrode/electrode support, further enlargement of the vortex was observed with the consequent generation of secondary/tertiary

Acknowledgements

We thank RSC/EPSRC Analytical studentship for funding JK under grant EP/C011430/1 and the EPSRC (EP/D05849X/1) for funding for the high-speed camera system.

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