Characterisation of Monodisperse Regimes of a Droplet Stream Generator

• Autor(a) principal: Cardoso, João Carlos Canhoto
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.6/11686
• topic: Formação de Jato; Gerador de Gotas Piezoelétrcio; Jato de Gotas Contínuas; Jet Fuel e Biocombustível; Rayleigh Breakup; Regime Monodisperso; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Aeronáutica

The interest in studying droplet related phenomena has been increasing over the last decades. In the fluid dispensing equipment industry, a major problem is to minimise droplet diameter and to eject droplets in a controlled manner with a low-cost device. Micro-droplet generation has gained its popularity for its multiple applications, such as biotechnology, manufacturing engineering, and fuel dispensing. Taking all this into account, a new low-cost droplet stream generator was designed and fabricated. The material used to manufacture the stream droplet generator structure was PLA, since it is a 3D printable material, which allowed to minimise the device cost. This structure has three separated components: lid, fluid chamber, and pinhole holder. In order to simplify the disturbance mechanism, it was decided that the disruption waves should be applied directly to the fluid. To achieve that, a piezoelectric cell that vibrates through the variation of waveform parameters was placed above the liquid chamber, creating disturbances directly onto the liquid surface in a parallel direction, making this device a push mode generator. The interchangeable nozzle used was a round stainlesssteel high precision optical pinhole with three different sizes: 100 µm, 150 µm, and 200 µm. Jet attribute properties (droplet diameter, droplet velocity, and distance between droplets) were measured as the different conditions changed (piezoelectric diaphragm frequency, outlet pressure, and nozzle size). The present work studied the spray characteristics for water, jet fuel and a jet fuel and biofuel mixture, and different monodisperse regimes were found. A full characterisation of them is presented and discussed in detail. It was found that the range of droplet diameter for a pinhole size of 200 µm that can be used for all three fluids is 401 µm to 472 µm and a range of velocity of 1.24 m/s to 2.48m/s, while for a pinhole size of 150 µm the droplet diameter range that can be obtained is 287 µm to 340 µm and a range of velocity of 2.04 m/s to 4.38 m/s for all three fluids. For a pinhole size of 100 µm the droplet diameter range for all three fluids is 206 µm to 258 µm and the velocity range is 2.36 m/s to 5.99 m/s. It was found that both droplet diameter and spacing decrease with the increase of inlet flow rate and frequency, and the most important property in the jet formation is the nozzle size. The jet velocity is also highly influenced by the flow rate and nozzle size. When compared, the three fluids behave in a different manner, resulting in different droplet diameter and velocity values. This can be explained by the fluids properties, where the mixture of jet fuel and biofuel presents higher viscosity than the other two fluids.