Hydrodynamics and Local Turbulent Mixing of Submerged, Parallel Liquid Jets: Experiments and CFD Simulations

The hydrodynamics and local turbulent mixing of parallel multiple liquid jets, submerged in liquid, were investigated by means of experiments and computational fluid dynamics (CFD). A renormalization group (RNG) k-ε turbulence model was used to simulate the flow field. The model was validated experimentally by particle image velocimetry (PIV) measurements. In the converging region adjacent to the nozzle exits, the recirculation region disappears, and there is only ambient fluid entrainment. Different jet arrays were compared to evaluate the effects of the jet spatial arrangement on the hydrodynamics and mixing performance. A shorter mixing length in the merging region suggests that mixing is more efficient in the triple-jet system than in other jet systems. Compared with the jet Reynolds number, the jet spacing plays a more significant role in determining the critical mixing regions, while the linear relationship between them is more sensitive than that for multiple parallel plane jets

Investigating Vortex Ring Reconnection in Twin Parallel Pulsed Jets: Influence of Nozzle Spacing and Stroke Ratio

Even though interactions between pulsed jets showed great importance in biological fluid transport, they remain poorly described. An experimental piston/cylinder apparatus is designed to produce twin parallel pulsed jets. Using Particle Image Velocimetry (PIV), we investigate how vortex ring reconnection is influenced by nozzle spacings (S/D_0) varying from 1.49 and 3.20 and stroke ratios (L/D_0) between 2 and 4. Velocity and vorticity visualization suggest there is a critical spacing ratio from which the pulsed jets interact. This value is found to be approximately 3. Below 1.5, the vortex rings are already merged at the jet exit. By implementing a vortex core identification method based on the swirling strength criterion, the reconnection point is then localized. The results highlight how important is the effect of the nozzle spacing compared to the stroke ratio, although the influence of L/D_0 increases with the distance between the jets. Finally, time-frequency analyses confirm the highly fast changes in velocity and vorticity observed during reconnection, and emphasize the importance of the reconnection phase in the newly formed structure

Aproveitamento do efeito ejetor em usina hidrelétrica de baixa queda por meio de condutos laterais em condição submersa

Orientador: Prof. Dr. José Junji OtaCoorientador: Prof. Dr. Tobias BleningerTese (doutorado) - Universidade Federal do Paraná, Setor de Tecnologia, Programa de Pós-Graduação em Engenharia de Recursos Hídricos e Ambiental. Defesa : Curitiba, 27/08/2020Inclui referências: p. 151-161Resumo: O objetivo do presente trabalho de pesquisa é avaliar o efeito de ejeção em uma usina hidrelétrica de baixa queda com descarregadores de fundo laterais. Como abordagem inicial, o estudo inclui dez variantes geométricas, sob condições submersas a jusante usando testes de modelo em escala 1:70. Os resultados do teste modelo são comparados com equações teóricas da literatura científica, e novas equações são propostas para avaliar os efeitos da ejeção. Todas as variantes consistem em três turbinas Kaplan verticais, quatro conduítes laterais inferiores, uma rampa e dois pilares laterais em direção ao canal principal. Vazões constantes de água através das turbinas e condutos laterais, bem como as elevações do escoamento a montante e a jusante da usina, foram calibradas por meio de 122 testes de modelo sob condições submersas a jusante. As alturas de pressão na parte inferior das saídas dos tubos de sucção das turbinas foram medidas usando 10 piezômetros para avaliar o efeito de ejeção total e o ganho de carga das turbinas. As velocidades de escoamento foram medidas em diferentes elevações no modelo por meio de um Velocímetro Doppler Acústico (ADV). As novas equações teóricas calibradas produziram resultados notavelmente consistentes para a avaliação da ejeção efetiva de 537 cenários hipotéticos de escoamento. Como segunda abordagem, três modelos empíricos que relacionam a ejeção efectiva medida com parâmetros adimensionais de escoamento, submersão e turbulência foram obtidos e validados em 28 cenários de escoamento. Os resultados mostram que os parâmetros de turbulência avaliados têm pouca ou nenhuma influência na ejeção efetiva observada. A taxa entre as vazões turbinada e dos condutos laterais, a submersão do escoamento e o número de Froude nas saídas dos dutos laterais são as variáveis relevantes. No estagio final, foram encontradas duas relações empíricas entre o efeito ejetor efetivo e a potência e o rendimento da turbina respeitivamente, para a alternativa com as maiores vazões testadas , incluindo as curvas de colina das turbinas Kaplan no esquema de cálculo do seu modelo teórico, além de uma avaliação do incremento da potencia e rendimento unitários do sistema. Palavras-chave: Efeito ejetor. Usina hidrelétrica de baixa queda. Descargas de fundo. Condição de fluxo submerso. Turbulência. Potência e rendimento.Abstract: The aim of the present research work is to assess the ejection effect in a low-head hydropower plant with lateral conduits. The study includes ten geometrical variants, under submerged downstream condition using 1:70 scale models tests. As an initial approach, the model test results are compared to theoretical equations from the scientific literature, and new equations are proposed to assess ejection effects. All variants consist of three vertical Kaplan turbines, four lateral bottom conduits, an apron and two lateral piers towards the main channel. Steady water discharges through the turbines and lateral conduits, as well as flow elevations upstream and downstream the plant, were calibrated by means of 122 model tests under submerged downstream conditions. Pressure heights at the bottom of the turbine draft tube outlets were measured using 10 piezometers to assess the total ejection effect and the gain of head due to the turbines. Flow velocities were measured at different elevations in the model by means of an Acoustic Doppler Velocimeter (ADV). The new theoretical calibrated equations produced remarkably consistent results for the effective ejection assessment from 537 hypothetical flow scenarios. As a second approach, three empirical models relating the measured ejection effect to flow, submergence and turbulence dimensionless parameters, were obtained and validated from 28 flow scenarios. Results show that the assessed turbulence parameters have little or no influence at all on the observed effective ejection. In this approach, the discharge rate between the turbines and the bottom lateral conduits, flow submergence and the Froude number at the conduits outlets are the key relevant variables, hence, reinforcing the results obtained from the initial approach. As a final step, two empirical relationships between effective ejection effect and turbine power and efficiency were found respectively for the alternative with the highest discharges tested, through the use of the provided power-efficiency curves for the Kaplan turbines, within the calculating scheme for its theoretical model. in addition to an evaluation of the increase of the unitary power and efficiency. Key words: Ejection effect. Low-head hydroelectric powerplant. Lateral conduits. Submerged flow condition. Turbulence. Turbine power and efficienc